This document is Copyright © 1996-2007 by Capable
Computing, Inc.
All rights reserved. Unauthorized duplication or publication is
prohibited.
This document may be freely downloaded and printed for personal, non-commercial use only, provided that this copyright notice is included.
Getting Started
What's New
The MotoWizard
Making a Performance Prediction
Specifying Battery Information
Examining the Results
Saving or Printing the Results
Graphing the Results
Projects
The Wiring Wizard
Using the Database Browsers
Obtaining Database Updates
Tips and Tricks
Finding the Maximum Efficiency Point of a Motor
Gearing and Propping for Maximum Efficiency
Caveats
All Propellers are not Created Equal
Motor Characteristics can Vary
Static Thrust can be Misleading
Registration Information
Registering the Evaluation Version
More Information
Glossary
MotoCalc is a program for predicting the performance of an electric model aircraft power system, based on the characteristics of the motor, battery, gearbox, propeller or ducted fan, and speed control. You can specify a range for the number of cells, gear ratio, propeller diameter, and propeller pitch, and MotoCalc will produce a table of predictions for each combination.
MotoCalc will predict weight, current, voltage at the motor terminals, input power, output power, power loss, motor efficiency, motor RPM, power-loading, electrical efficiency, motor RPM, propeller or fan RPM, static thrust, pitch speed, and run time. By producing a table of predictions, MotoCalc lets you determine the optimum propeller size and/or gear ratio for your particular application.
MotoCalc can also do an in-flight analysis for a particular combination of components, predicting lift, drag, current, voltage, power, motor and electrical efficiency, RPM, thrust, pitch speed, propeller and overall efficiency, and run time at various flight speeds. It will also predict stall speed, hands-off level flight speed, throttle, and motor temperature, optimal level flight speed, throttle, and motor temperature, maximum level flight speed, rate of climb, and power-off rate of sink.
MotoCalc's graphing facility can plot any two parameters against any other (for example, lift and drag vs. airspeed).
If you have particular requirements, such as a minimum run time, maximum current, or maximum power loss (which is dissipated as heat), you can use MotoCalc's filter facility to filter out the unacceptable combinations.
To reduce the amount of information you have to deal with, MotoCalc comes with a database of motors, cell types, gearboxes and propellers, speed controls, and filters. For example, the database contains over 1900 motors (including the Astro, Aveox, Graupner, Keller, Kontronik, MaxCim, Plettenberg, and Robbe lines), and over 170 different cell types (including the entire Sanyo and SR lines, and many NiMH, LiPo, and LiNP R/C cells).
If the motor you are using is not included in the database, MotoCalc will help you figure out its parameters from test data, catalog information, or from the specs of another similar motor. And if you don't know the aerodynamic characteristics of your plane, MotoCalc's lift and drag coefficient estimator will make short work of determining them.
If you are a newcomer to electric flight, MotoCalc's MotoWizard will ask you a few simple questions about your model and your preferences (such as brand of motor), and will then make suggestions as to the ideal power system. You can then use the rest of MotoCalc to investigate one or more of the suggestions in detail, and then use MotoCalc's MotOpinion feature to get a plain-English analysis telling you how the power system, and your plane, will perform.
If you are a bit more experienced, and know some of the components you want to use to power your model (for example, a particular motor or battery), you can tell the MotoWizard this and it will try to choose a suitable power system using those components.
Printing This Manual
If you prefer a printed version of this manual, we
suggested printing the Adobe PDF version, which can be found on
the MotoCalc CD (as of version 8.03) in the file "MotoCalc.pdf".
A copy can also be found on the MotoCalc web site at
http://www.motocalc.com/motocalc.pdf.
The manual is about 115 pages. If you don't wish to print it on
your own printer, it can be printed at most copy shops. They may
ask for proof that you have the right to print it, in which case
you can refer them to this sentence in the PDF file: You are hereby
granted permission to print a single copy of the PDF version of
this document for personal use.
Getting Started with MotoCalc
There are many different ways to use MotoCalc. This brief tutorial assumes that you have a model that you are intending to power with an electric motor (or more than one), whether that be a designed-for-electric model, or a conversion of a glow model. (For a more in-depth tutorial, please visit http://www.motocalc.com/tutorial.htm.)
The following is an overview of the steps that will get you well on your way to using MotoCalc to help choose a power system for your model:
1. Ask the MotoWizard to suggest several different power systems that meet your requirements in terms of performance and flying time, and preferences (if any) as to the type of equipment to use.
2. Select one of the MotoWizard's suggestions, and copy it to the MotoCalc Workbench. Save this as a project.
3. Click the Compute Report... button to generate a detailed in-flight analysis of the suggested power system.
4. Click the Opinion button to generate a MotOpinion report to help you interpret the in-flight analysis.
5. Based on the results above, either go back to step 1 and select a different suggestion, return to step 2 and modify some of the suggested components, or install the recommended power system in your plane and go fly!
These five steps are described in more detail below.
The MotoWizard is described in great detail in its own section of this manual, so we won't repeat it all here. Instead, we'll give a brief overview.
The MotoWizard consists of a number of pages on which you answer some basic questions about your model, it's intended performance, information about where you fly, and any preferences you may have. The pieces of information you need to supply are:
Based on these specification, the MotoWizard will consider hundreds or thousands of possible electric power systems, and suggest those that it deems most suitable.
Note: If you already have a good idea of the power system components you want to use in your model, you may want to skip using the MotoWizard and go straight to the MotoCalc Workbench.
Examine the suggestions made by the MotoWizard. If you are unhappy with them (for example, suggestions involving a very large number of cells, or only brushless motors when you would prefer brushed), return to the MotoWizard's option pages and narrow your preferences.
At this point, you can also print the MotoWizard's suggestions by clicking the Print... button, save them to a text, HTML, or comma-separated-values (CSV) file by clicking Save..., or copy one or more of the suggestions to the Windows clipboard (to paste into an e-mail, for example) by clicking with the secondary mouse button and selecting Copy One Result or Copy All Results.
When you are happy with the MotoWizard's suggestions, select one (click on it with the mouse), and then click the Accept button. This will fill in the MotoCalc Workbench window with settings corresponding to the selected suggestion.
At this point, you can modify any of the parameters that you wish. For example, if the MotoWizard is suggesting a 4:1 gearbox and 12x8 propeller, but you know you can only obtain a 3.7:1 gearbox, change the gear ratio (and perhaps decrease the propeller size slightly). You may also want to specify ranges for some parameters (for example, if the MotoWizard suggested 14 cells, you might want to try 12 to 16 cells).
Select Save As... from the Project menu, type in a name for your project, and click OK.
Click the Compute Report... button to generate an in-flight analysis (if you specified any ranges in step 2 above, you will get a static analysis, from which you can pick one result and click the In-flight... button to get an in-flight analysis).
A summary of the project (motor, battery, speed control, airframe, etc.) appears at the top of the analysis. The last line of the summary, labeled "Stats", summarizes the predicted performance of the model.
Below this you will see a table of predictions at different airspeeds, estimating things like current consumption, input and output power, and so on. Using color coding, the table will also indicate the model's predicted stalling speed, optimal flying speed, and hands-off flying speed (these speeds are generally at less than full throttle).
Click the Opinion... button at the bottom of the in-flight analysis to display the MotOpinion page, which will give you a plane-English description of how your plane will fly, and any potential problems with the selected power system.
Based on the information in the in-flight analysis and MotOpinion report, you may wish to revise or refine your power system. One way to do this is to return to the MotoWizard (select MotoWizard... from the Project menu in the MotoCalc Workbench window) and either select another one of the MotoWizard's suggestions (step 1), or change some of the options and have the MotoWizard generate new suggestions.
Alternatively, you might just slightly revise the current suggestion by making changes in the MotoCalc Workbench window, perhaps trying different propeller sizes, more or fewer cells, or a different gear ratio (step 2). Or, if you had specified any ranges in step 2, you can select a different result from the static analysis and generate a new in-flight analysis (step 3).
After a few iterations, you should have narrowed down your choices to a few power systems suitable for your model and your requirements. At this point, it's time to install some equipment, and go flying. Because of variations between simulation and reality, predictions are never 100% accurate, so you may want to make some further refinements after a few test flights (perhaps a different brand or slightly different size of propeller for example).
For a much more detailed tutorial, please visit http://www.motocalc.com/tutorial.htm. If you purchased MotoCalc on a CD, a copy of this in-depth tutorial can be found there, in the file "Tutorial.pdf".
MotoCalc 8 continues the tradition of being the most popular, capable, accurate, and easy-to-use electric flight performance prediction system for the hobbyist. Here is a brief summary of the new features in this release:
A More Versatile MotoWizard
The MotoWizard, introduced in MotoCalc 6, made it easy to choose
a power system starting with nothing other than some basic airframe
information and the desired level of performance. In MotoCalc
7, the MotoWizard became both smarter and faster, looking at more
possibilities in less time.
Now in MotoCalc 8, it has become more versatile by letting you specify specific components that you want to use to power your model. For example, you might want to use that trusty 7x3300 NiMH battery and Astro Cobalt 05G motor you have on hand. Now you can tell the MotoWizard this, and it will try to find the right propeller to use with these components in your model.
Or perhaps you have just put the finishing touches on a beautiful scale model, and want to use a propeller with the correct scale diameter. Tell the MotoWizard the desired diameter, and it will work with this restriction to find a suitable power system.
The list of cell types used by the MotoWizard has also been updated to reflect the changing times. There are more Lithium-Polymer cells, and newer NiMH cells. You also have the option of specifying exactly which type of cell you want to use.
New Information for Cells
MotoCalc 8 makes use of a cell's "C" rating, if known.
This additional information is used to warn you if the predicted
current is too high, and is used by the MotoWizard to avoid choosing
power systems where that would be the case.
The type of the cell (NiCd, NiMH, LiPo, LiNP) is now also recorded in the database.
Ranges for Parallel Cells and Batteries
MotoCalc 7 introduced support for parallel cells, but you could
only specify one number of parallel cells at a time. Now you can
specify a range for this quantity, just like you always could
for series cells. So if you want to compute predictions for everything
from a 2S2P to 4S3P battery, you can.
Gearbox Weight
A new Gearbox Weight field in the Drive System section lets MotoCalc
add the weight of the gearbox to that of the model when computing
the total weight, resulting in more accurate weight estimates.
Flat and Curved Plate Wings
Both the MotoWizard and the MotoCalc workbench's Drag and Lift
Coefficient Estimator now support flat and curved plate wings
such as those found in many foam park-flyers and some balsa models.
(This feature requires MotoCalc 8.02 or higher.)
Direct Link to Technical Support
If you are having a technical problem with MotoCalc 8, you can
contact our technical support team directly from within MotoCalc.
You can also automatically attach the project that you currently
have open to your support request, so that we can work with exactly
the data that you are working with.
Tip of the Day
Whenever MotoCalc starts, it will display a helpful tip of the
day to help you learn about MotoCalc's many powerful features,
and new ways to make use of this versatile tool.
Internet Proxy Support
If your computer is behind a firewall, or must communicate through
an Internet proxy for some other reason, you can now configure
MotoCalc to direct its communications (e.g. downloading updates
and contributing data) through the proxy.
Improved Look and Feel
No, we didn't completely change the way
MotoCalc feels, but we did make a few changes to improve the way
it works and looks. The main window (now known as the MotoCalc
Workbench) has been made slightly less cluttered. The selection
of propeller versus fan drive systems is now done by selecting
the appropriate tab in the Drive System section. For mouse wheel
fans, all the various windows that have scrollable areas now support
the mouse wheel.
MotoCalc is a powerful tool for choosing an electric power system for your model. It lets you vary almost any parameter of the motor, battery, propeller and gearbox (or ducted fan), speed control, and airframe, and make a detail prediction of how it will all perform.
However, for the beginning electric modeler, or for the more experienced modeler who's not interested in numbers, MotoCalc can require too many parameters, and provide too much information. The MotoWizard is there to do all of that for you. All you have to do is answer a few simple questions about your model and your preferences, and the MotoWizard will provide some power system suggestions.
Note: Currently, the MotoWizard only supports propeller driven models. A future version of MotoCalc may have a Fan Wizard as well.
Note: In order for the MotoWizard to consider using a particular motor, the weight must be known for that motor. A few of the motors in MotoCalc's motor table do not have weight information, therefore these motors will never appear in the MotoWizard's suggestions.
Please see also the MotoWizard Limitations.
When you start MotoCalc, the MotoWizard appears automatically, unless you have turned off this feature by un-checking the box in the bottom left hand corner. To start the MotoWizard manually, select MotoWizard... from the Project menu.
The MotoWizard is divided into eight pages, indicated by the tabs at the top of the MotoWizard window. You can go directly from page to page, or preferably, go through them in sequence using the Next button at the bottom of each page.
The first page asks you for your model's name, and the number of motors it uses. When the MotoWizard first starts up, this will already be filled in with the information from the current project. To work with a new plane, press the Clear button. If you start up the wizard again after starting a new project, and wish to use that project in the wizard, click the Copy Plane from MotoCalc Window button.
After filling in the plane's name and the number of motors, press the Next button to go to the Performance page...
The performance page is where you tell the MotoWizard what you are expecting from this plane.
Select which level of performance you are interested in, ranging from sedate to hotliner, and then select the approximate flying time you wish to achieve. The different performance levels are not just a measure of required power, but imply other desirable model characteristics as well:
Sedate - low power and a very low wing loading.
Trainer - moderate power and a low wing loading.
Sport - moderately high power, and a moderate wing loading.
Pylon - moderately high power, high wing loading, high pitch speed.
Aerobatic - high power, moderately high wing loading.
3D Aerobatic - very high power, very low wing loading, very high static thrust, low pitch speed.
Sailplane - moderately high power, very low wing loading, low pitch speed.
Hotliner - very high power, moderately high wing loading, high static thrust.
Click the Next button to go on to the Model page...
Tell the MotoWizard the dimensions of your model on the Model page. For each measurement, click the radio button next to the units you are using for that measurement.
The dimensions needed are the wing span (the distance from one wing tip to the other), the wing area (including the area where the wing passes through, over, or under the fuselage), and the empty weight.
The empty weight includes the weight of the airframe, receiver, receiver battery (if used), and servos. It does not include the weight of the motor, motor battery, and speed control.
If you are thinking about converting a glow model, use the published weight, minus the weight of the recommended glow engine and a tank full of fuel. As a rough guide, an .049 weighs about 2oz (60g), a .15 weighs about 5oz (140g), a .25 weighs about 6.5oz (190g), a .40 weighs about 8.8oz (250g), and a .60 weighs about 18oz (500g).
After filling in the measurements, press the Next button to go to the Airfoil page...
This page is used to tell the MotoWizard what kind of airfoil your plane has.
Click the airfoil that most closely matches the shape (ignoring the thickness for the moment) of your model's airfoil. Often, advertisements or catalog descriptions of models will indicate whether the airfoil is undercambered, flat bottomed, semi-symmetrical, or symmetrical.
If the wing is a simple flat or curved sheet of foam or balsa wood, select Flat Plate or Curved Plate respectively.
When you've selected the airfoil shape, click the Next button to go to the Thickness page...
In addition to the shape of your model's airfoil, the MotoWizard needs to know approximately how thick it is.
On this page, click the airfoil that most closely matches the thickness (disregard the shape) of your model's airfoil. Advertisements and catalog descriptions often draw attention to the fact that model has a particularly thick or thin airfoil. If you happen to know the actual percentage thickness of the airfoil, select the thinnest airfoil if it's less than 7.5%, the second thinnest if it's less than 10%, the second thickest if it's less than 14.5%, and the thickest otherwise.
For a flat or curved sheet wing, select the thinnest airfoil since most such wings are no more than 2% to 3% thick.
Click the Next button to go on to the Field page...
The elevation above or below sea level, and optionally pressure and temperature, at the field you plan to fly from.
On this page, enter the elevation and click the radio button for the units you are using. The elevation must be between -1000 and +36000 feet or -304 and +10972 metres. If you leave this field blank, MotoCalc will assume sea level.
Air pressure and temperature also affect performance. However, both pressure and temperature will vary from day to day, so you may not want to specify them and just use averages instead.
If you do want to specify pressure and temperature, click the Show Advanced Settings button and two additional fields will appear. Enter the appropriate values and select the units you are using, just like you did for the elevation.
The pressure must be between 28 and 32 inches of mercury (inHg), or between 94.8 and 108.3 kiloPascals (kPa). The temperature must be between -58 and +122°F, or -50 and +50°C. You can leave these fields blank, in which case the MotoWizard will assume 29.92 inHg (101.3 kPa) and 68°F (20°C) respectively. Note that the pressure should be the reported sea-level pressure, not the pressure you would read directly from a barometer.
Click the Next button to go to the Motor options page...
The MotoWizard will try to find the best possible power systems for your model, which might not be ones that are readily available, are out of your desired price range, or are otherwise undesirable. There are three pages of options where you can, if you wish, force the MotoWizard to narrow down the search. The first of these is for Motor restrictions.
On the left side, click the appropriate radio button to specify which category of motors you want to use (brushed, brushless, or either) or indicate that you want to use a specific motor.
If you select one of the three categories, then on the right side of the page you can narrow the choice down to a particular manufacturer by selecting a name from the list.
If you indicated that you want to use a specific motor, then the right side of the page will change to let you choose a motor from MotoCalc's extensive database (which you can add to later).
Click the Select Motor... button to open the Motor Browser and choose a motor.
Click the Next button to go to the Gearing and Propeller options page...
The Gear & Prop options page lets you restrict the type of drive system (direct or geared), gear ratio, and propeller sizes that the MotoWizard will consider.
On the left side of the page, you can narrow down the type of drive system. Normally, the MotoWizard will consider both direct drive and geared systems, but you might have a preference for one or the other. If so, select the desired type of drive system.
Alternatively, you can indicate that you want to use a specific gear ratio. If you click the Specific Gear Ratio radio button, a data field will appear where you can type the gear ratio (don't type the ":1" part though).
On the right side of the page, indicate whether you want to allow the MotoWizard to choose any size propeller, use a specific size, or be restricted to a maximum size.
If you choose Specific Size or Maximum Size, two fields appear where you can enter the diameter and pitch of the propeller. Two radio buttons to the right of these fields indicate whether the measurements are in inches or centimetres.
You can fill in one or both of these fields. If you fill in both, the MotoWizard will consider only exactly that size propeller. If you fill in only one field, that measurement will be restricted but the other will not. For example, if you select Specific Size and select a diameter of 10 inches but leave the Pitch field blank, the MotoWizard can consider various 10 inch diameter propellers such as 10x5, 10x6, 10x7, and so on (this can be handy when you're trying to choose a propeller that looks right on a scale model).
Click the Next button to go to the Battery options page...
On the Battery options page, you can choose the type of batteries that you want the MotoWizard to consider, or specify exactly the battery you want to use.
If you select any of the first five choices in the list on the left side of the page, the MotoWizard will only consider the selected categories of batteries. You can then also limit the number of cells in the battery to 7, 8, 10, 12, 14, 18, or 24 NiCd or NiMH cells (or 2, 3, 4, 5, 6, or 8 LiPo or LiNP cells in series). If you don't select one of these limits, the MotoWizard can come up with power systems requiring up to 36 NiCd or NiMH cells, or 12 LiPo or LiNP cells (assuming such a power system would be suitable for your model). If your charger can't handle that many, select a suitable maximum number of cells.
If you select Specific Battery on the left side of the page, the right side will change so you can choose exactly the type of cells, and how many you wish to use.
Click the Select Cell Type... button to open the Cell Browser and choose the type of cells you want to use.
The Series and Parallel fields let you specify the number of cells in series and/or parallel that you want in your battery. You can fill in neither, one, or both of these. If you fill in neither, the MotoWizard will consider any viable combination of series and parallel wiring. If you fill in either or both of the fields, the choices will be constrained accordingly (however, the MotoWizard will never choose paralleled NiCd or NiMH cells on its own). For example, if you've selected a particular kind of LiPo cell and typed "3" in the Series field and left the Parallel field blank, the MotoWizard will consider 3S1P, 3S2P, 3S3P, and so on as possible battery configurations.
When you're done setting Battery options, click the Finish button to get to the Results...
When you click to go to the Results page, the MotoWizard will analyze thousands (sometimes hundreds of thousands) of possible power systems to come up with the best ones for your model. Depending on the speed of your computer, the options you've selected, this can take anywhere from a few seconds to a minute. A progress indicator will show how far along the wizard is.
The results are displayed in five columns, the first of which is the name of a motor. If you indicated on the MotoWizard start page that your plane is to be powered by two or more motors, the motor name will be preceded by "P", "S", or "X", indicating that the motors are to be wired in parallel, series, or series-parallel (or parallel-series) respectively. "X" can only appear if you've specified four motors.
The second column describes the battery, giving the number of cells, and the type of cells. Unless you specified a specific cell type, the MotoWizard will only consider about 15 different types of cells that are representative of the hundreds of types available. Thus, the cells you see in the results may not be exactly what is available to you. You can use the MotoCalc Workbench later to experiment with substitutions (or go back to the Battery options page and choose the closest specific cell type and then regenerate the results for that).
The third column gives the gear ratio, which will be either a numeric ratio like "2.5:1", or blank for direct drive. Please note that to keep the display readable, the numbers are rounded to the nearest tenth. If you specifed a specific gear ratio of 2.38 for example, it will be displayed as 2.4.
The propeller is listed in the fourth column, giving the diameter and pitch. For example, "8x4.5" means an 8 inch diameter and 4.5 inch pitch (if you have MotoCalc set up to use metric propeller sizes, the diameter and pitch will be in centimetres).
The last column is a relative rating for this power system. The first power system in the result list will always have a rating of 1.000. The rating is based on a number of factors which are used to judge the suitability of the power system to your model and the desired performance. The MotoWizard shows you only the best possible power systems, so even those with a lower rating will still be a good choice to provide the level of performance that you indicated.
In some cases, the MotoWizard cannot find a suitable power system for your model. If that happens, you will see a message indicating the reasons none were found. Please refer to the information on limitations below.
Initially, the MotoWizard results are sorted by rating, with the highest rated power system first. You can click on any of the column headings to sort by that column instead. Clicking on an already sorted column, or clicking on a column with the secondary mouse button, will sort that column in reverse order.
To analyze a given power system further, select it from the list by clicking on it, and then click the Accept button (alternatively, you can just double-click the selected power system to accomplish the same thing).
The MotoWizard will copy the selected motor and battery onto the MotoCalc Workbench. The filter will be left blank. The drive system and speed control sections will be filled in with the requirements for a generic speed control that is suitable for the predicted performance. The airframe parameters will be filled in based on the information you specified in the Model, Airfoil, and Thickness pages.
The MotoWizard will also set your MotoCalc flying field options to correspond to the information you specified on the Field page. Finally, the Performance and Options settings you specified are remembered as well, so if you re-run the MotoWizard later on the same project, your settings will still be as you left them.
Once stored as a project, you can use the full power of MotoCalc to experiment with variations on the MotoWizard's suggestion and refine your power system choice.
Clicking the In-flight... button will generate an in-flight analysis of the selected suggested power system. This button is just a short cut, and will produce the same results as clicking Accept, followed by clicking Compute Report... in the MotoCalc Workbench window. Please refer to Examining the Output for a detailed description of the in-flight analysis.
Note: Clicking the In-flight... button will copy the selected motor and battery onto the MotoCalc Workbench before generating the in-flight analysis.
The Opinion... button will generate a MotOpinion report, describing your model's power system performance, flight performance, and any potential problems. You can also get this report by clicking the Opinion... button on the in-flight analysis.
Note: Clicking the Opinion... button will copy the selected motor and battery onto the MotoCalc Workbench before generating the MotOpinion report.
You can save or print the MotoWizard's recommendations by clicking the Save... or Print... button respectively. When saving, you have your choice of formats: plain text, HTML (suitable for publishing on the web), or CSV (comma-separated-values, suitable for importing into a spreadsheet program).
Both the saved and printed output begin with a summary of all the settings that you specified in the MotoWizard, followed by the table of results (in the order they were sorted when you clicked the Save... or Print... button).
The MotoWizard is designed to choose an appropriate power system for a specified model, level of performance, and flying time. In order to analyze the millions of possible combinations, it has to take some shortcuts to narrow down the choices quickly, or you would have to wait hours for the results.
For example, the MotoWizard does not try every cell type in the database. Instead, it tries a representative sample of types covering the range of cells available to the modeler. There is always the possibility that a particular model falls between the cracks, and that it would work if only the MotoWizard had considered the brand X cell, which is 10% lighter and of 10% lower capacity than the one it did consider.
The MotoWizard also does not try every speed control. As a matter of fact, it doesn't try any speed control in particular, but rather just uses a generic one suitable for the predicted performance level. You can then choose a speed control, either from MotoCalc's database, or from a catalog, that is similar to the generic one the MotoWizard came up with.
The MotoWizard does try every motor in the database (except those for which the weight is not known). But it does not try all the drive systems. Like the speed controls, the MotoWizard tries generic drive systems, based on a range of propeller sizes and gear ratios. No particular propeller brands are considered, nor any particular gearbox. The MotoWizard merely chooses an appropriate gear ratio and propeller size, and it is up to you to choose a corresponding real-world propeller or gearbox.
Making use of the options on the options pages can speed up the search for a power system tremendously. If you know you want to use a direct-drive brushed motor and a 7-cell NiCd battery, you can save the MotoWizard (and yourself) a lot of time by specifying this.
When the MotoWizard simply cannot find an answer, it might be impossible to achieve the desired results (i.e. a given performance level, for a specified time) for that model. For example, if a model is quite heavy, there may be no way to achieve "Sedate" performance; the model is simply too heavy to fly in such a manner. In such cases, choosing a higher level of performance rather than lower may yield results.
Finally, the MotoWizard currently does not supported ducted fans, or power systems involving more than four motors.
All the data that you must supply to make a performance prediction is entered in the MotoCalc Workbench window.
The first time you run MotoCalc, some sample information is already filled in. You can modify this, select New from the File menu to clear everything, or select Open to retrieve a project. If you saved a project before last exiting MotoCalc, that project will have been automatically reloaded when you started MotoCalc. If you just came to the MotoCalc Workbench window by selecting a result from the MotoWizard, parameters from that result will be filled into the MotoCalc Workbench window.
The window is divided into six panes, each of which contains fields into which to enter data about a different component of the power system configuration (which together make up a project):
Some of the names of the data entry fields are in color, indicating certain aspects of that field:
Fields labeled in blue are not part of the component whose section they appear in. For example, the Number of ESCs field, although it appears in the Speed Control section, is an attribute of the project as a whole, instead of as part of the speed control description.
For ducted fan projects only, fields labeled in green are stored as part of the Airframe component, even though some of these fields appear in the Drive System section. For example, Intake Diameter, although relevant to a fan drive system, is an attribute of the airframe in which the fan is installed.
The steps to making a performance prediction are:
Specify the motor characteristics (or select a motor from MotoCalc's extensive database).
Specify the cell characteristics (or choose a cell type from MotoCalc's database).
Indicate the range of cell counts you're interested in (for instance, 7 to 10 cells).
Specify the range of gear ratios you're interested in (or select a drive system from MotoCalc's database).
Specify the range of propeller pitches and diameters you're interested in.
Specify the characteristics of the speed control (or select from MotoCalc's database).
Specify the airframe characteristics (or select from the database).
Specify any restrictions you want (such as maximum current).
Press the Compute Report... button.
This will produce a report in a new window, giving the predictions for each combination of cell count, gear ratio, propeller diameter, and propeller pitch. You can sort this report by any column just by clicking on the column heading. You can also save the report (as an ASCII text file), or print it to any Windows compatible printer.
Note:
Each time you press the Compute Report... button, MotoCalc
will produce a new report window. MotoCalc can display as many
simultaneous report windows as the memory on your computer will
allow. This makes it easy to produce predictions for several different
combinations of components, and compare them side by side (assuming
your screen is sufficiently large).
Specifying Motor Information
Motor information is specified in the Motor section of MotoCalc's Workbench window. MotoCalc uses this information to determine how the motor will perform under different loads at various speeds. This is the core of MotoCalc's predictions, so it is important to get accurate motor information.
If the motor you are interested in is a commonly available one, it is probably already included in MotoCalc's extensive database. To select a motor from the database, click the Open button in the Motor section (or the Open... item on the Motor menu) to browse the motor table. Once you've selected a motor, the motor characteristics will automatically be filled into the appropriate fields. Refer to Using the Database Browsers for more information.
If the motor you are interested in is not in MotoCalc's database, you can enter the motor characteristics yourself. The three pieces of information that MotoCalc needs to perform its calculations are the motor constant (RPM/V), no-load current (A), and armature resistance (Ohms). If you have this information about a motor, you can enter it directly. If you do not have this information, you can either perform some tests and enter the information into the Test Data Input window, or see if you have enough other information to fill in the Catalog Data Input window.
The Motor field is used to give a name to a motor. This name will appear on the performance report, and this name is also used to refer to the motor in the database; no two motors can have the same name. A motor name can have up to 40 characters.
The motor constant specifies the free-running RPM per Volt. This information is usually available from the manufacturer of the motor. The motor constant must be specified.
The no-load current is the current, in Amps, consumed by the motor when it is free-running. This information is usually available from the manufacturer of the motor. The no-load current must be specified.
This is the armature resistance, in Ohms, of the motor. This information is usually available from the manufacturer of the motor. The armature resistance must be specified.
The weight of the motor, in ounces or grams. This information is used by MotoCalc when computing total aircraft weight, and in determining the ability of the motor to dissipate heat.
This check box indicates whether or not the motor is brushless. The MotoWizard uses this information to narrow down motor choices, and the MotOpinion report uses this to ensure you've selected the right kind of speed control.
This check box indicates whether or not the motor is an out-runner brushless motor (also known as rotating can or washing machine). This information is relevant in the calculation of motor heating (out-runner motors don't have the heat dissipation ability of ordinary brushless motors), and in limiting RPM in the MotoWizard.
The New button in the Motor section (or the
New item on the Motor menu) clears all the motor
information fields (they're all set back to blank).
Adding a Motor to the Database
Once you've computed or entered the characteristics of a motor, you may want to save it for future reference. To do this, simply make sure that you've entered a unique name for the motor, and click the Save button in the Motor section (or the Save item on the Motor menu).
Warning: If you Open an existing motor, change the characteristics, and Save it again without first changing the name, the new characteristics will replace the existing ones for that motor. Don't do this unless you're sure that that's what you want to do.
There are number of ways to determine the parameters for a motor, including deriving them from motor tests, or computing them from values published in catalogs, or from the constants of a known motor. However, the probably the best way is to measure the constants directly, which can be done as described below (follow the instructions in the section for the appropriate type of motor):
Brushed Motors
Measuring the Motor Constant
This can be determined by chucking the motor shaft into a drill press (or some other drill with an exactly known RPM). If the motor has adjustable timing, set it to neutral first. Connect a volt meter across the motor terminals, and run the drill while holding the motor (be careful; if the motor is not chucked in straight, it will wobble violently). Divide the RPM by the measured voltage, and you will get the value for the Motor Constant.
Measuring the No-load Current
Connect the motor to a variable power supply, with an ammeter in series with one of the leads. Slowly increase the voltage. You will notice the current go up, and then start to level off at some point. This is the no-load current. This depends on the timing of the motor, and should also be measured at neutral timing.
Measuring the Armature Resistance
Connect the motor to a variable voltage power supply, with an ammeter in series with one of the leads, and a voltmeter across the motor terminals. Keep the motor shaft from turning by holding it with pliers or clamping it in a vice. Slowly increase the voltage until the current reaches 5A or so (or half the rated safe operating current of the motor, whichever is less). Divide the measured motor terminal voltage by the measured current to get the armature resistance.
Brushless Motors (Including Out-runner Motors)
Measuring the Motor Constant
Like a brushed motor, this can be determined by driving the motor shaft at a known RPM using a drill press. However, a brushless motor does not produce a DC voltage on its terminals. Instead, it produces a three-phase AC voltage. To measure this requires a three-phase full-wave rectifier, which can be constructed from six 1N4001 diodes, a 47 kOhm resistor, and a 0.1 microFarad capacitor. The diagram below is a schematic of such a circuit:
The three input terminals of the rectifier should be connected to the motor, and the output terminals to a DC voltmeter. Run the drill while holding the motor (be careful; if the motor is not chucked in straight, it will wobble violently). Measure the voltage, and add a correction factor to account for the voltage loss in the diodes (see below). Divide the RPM by the corrected voltage, and you will get the value for the Motor Constant.
Note: To calculate the diode voltage loss, connect an approximately 3V DC power source (for example, two AA alkaline cells wired in series) to any two of the three input terminals of the rectifier (without the motor connected). Measure the voltage at the two input terminals, and again at the two output terminals, using a digital voltmeter. The difference between these two voltages is the required correction factor.
Measuring the No-load Current
Unlike a brushed motor, a brushless motor's no-load operating current can't be measured without using a speed control. Connect a battery, appropriate brushless speed control, a receiver (or a servo tester) and the motor as you would in an aircraft, except connect an ammeter in series with one of the battery leads. Do not install a propeller on the motor.
Turn everything on, and slowly increase the throttle. The current will start to rise, and eventually level off. This is the no-load current. Different speed controls have different internal resistances, and operate the motor with differing timing, so if possible, test with more than one speed control and take an average of the readings. Also, be sure the speed controls are set to their most conservative timing modes (i.e. as close to neutral as possible).
Warning: Do not use full throttle. Be careful not to run the motor beyond its rated maximum RPM, or you may damage the motor.
Measuring the Armature Resistance
Connect two terminals of the motor to a variable voltage power supply, with an ammeter in series with one of the leads, and a voltmeter across the two motor terminals. Leave the third motor terminal unconnected.
Keep the motor shaft from turning by holding it with pliers or clamping it in a vice. Slowly increase the voltage until the current reaches 5A or so (or half the rated safe operating current of the motor, whichever is less). Divide the measured voltage by the measured current, and then multiply that by 0.875 to get the armature resistance.
If you have the equipment necessary to measure motor terminal voltage, current, and RPM, or you have published test data containing this information, you can use the Test Data Input window to automatically compute the motor parameters and fill them into the Motor section of the MotoCalc Workbench window. To open the Test Data Input window, click on the Tests button in the Motor section (or the Enter Test Data... item on the Motor menu). The Test Data Input window will appear.
You need to have test results from two different motor runs:
One free running (i.e. with no load) and one under load, or.
One at the best efficiency point and one other.
Specify the type of test data you have in the Type of Data section of the Test Data Input window. For each run, you need the following data:
Terminal Voltage - the voltage at the motor terminals.
Current - the current through the motor (or any part of the circuit).
RPM - the rate at which the motor is turning.
It is important that all three measurements for a given motor run be taken simultaneously, because they can change over time (due to motor heating, the battery running down, etc.). The three measurements must correspond to one another.
Once you have entered a set of measurements into the Test Data Input window, click OK to compute the motor characteristics, automatically fill them into the Motor section of the MotoCalc Workbench window, and close the Test Data Input window. If you want to experiment with different sets of measurements, you can click Apply instead. This does the same thing, but the Test Data Input window remains open.
If the message "Warning! Data Points Too Close" appears in the Test Data Input window, it means you have taken measurements that are too similar to one another to compute accurate motor characteristics from.
You may also see the message "Error! Inconsistent Data", which indicates that the input data is implausible. This generally occurs when the test data points are too close, and the errors inherent in taking measurements throw the data off enough to result in an impossible situation (such as a negative armature resistance or motor constant).
To avoid either of these messages (and get meaningful
results), it is best to have test data points that are widely
separated in at least two of the three parameters (Voltage, Current,
and RPM). For example, when using free running and loaded data,
the loaded tests should be done with sufficient load on the motor
to slow it down to about 50% to 75% of its free-running RPM and
significantly increase the current. Using closely spaced data
points (such as a loaded test done at 95% of free-running RPM)
will result in the computation of very inaccurate motor characteristics.
Entering Motor Data from a Catalog
If all you have available to you is the data from a manufacturer's or mail order company's catalog, you may still have the information that MotoCalc needs to determine the motor characteristics. The Catalog Data Input window is designed to make sense of this information. To open the Catalog Data Input window, click on the Catalog button in the Motor section (or the Enter Catalog Data... item on the Motor menu) of the MotoCalc Workbench window. The Catalog Data Input window will appear.
There are four pieces of information that you need to find out from the catalog. The first two are:
Nominal Voltage - the voltage that the motor was intended for.
Free Running RPM - the RPM of the motor, with no load, at the nominal voltage.
Any two of the following are required:
Current at Maximum Efficiency - the current, in Amps, at which the motor is most efficient.
Stall Current - the current, in Amps, that the motor draws if the armature is prevented from turning when being fed the Nominal Voltage.
No-load Current - the current, in Amps, that the motor draws at the Nominal Voltage when there is no load on the motor shaft.
As you fill in these fields, MotoCalc will calculate the motor characteristics as soon as it has enough information for each one. The grayed-out fields of the Catalog Data window show the computed characteristics as they are being calculated. Note that as soon as you type values into two of the three current fields, the third becomes Grey, because it is now being calculated by MotoCalc instead of filled in by you.
For those of you who are mathematically inclined, the relationship between No-load Current, Maximum Efficiency Current, and Stall Current is:
Note that data from a catalog can be notoriously inaccurate. See the Caveats section for more details.
Once you have entered a set of catalog data into the Catalog Data Input window, click OK to automatically fill them into the Motor section of the MotoCalc Workbench window, and close the Catalog Data Input window. If you want to experiment with different sets of measurements, you can click Apply instead. This does the same thing, but the Catalog Data Input window remains open.
If you are in the habit of winding (or rewinding) your own motors, or you have a motor for which you don't know the motor parameters, and also have a similar motor with a different number of winds for which you do know the parameters, MotoCalc's motor designer will let you estimate the parameters based on a known motor and the change in windings.
To open the Motor Designer, click on the Design button in the Motor section (or the Motor Designer... item on the Motor menu) of the MotoCalc Workbench window. The Motor Designer window will appear.
The top left pane of the Motor Designer is where you specify the known Baseline Motor. The fields here are identical to those in the Motor section of the MotoCalc Workbench window. The Open button can be used to select a motor from the database.
The lower left pane is where you specify information about the Baseline Motor Windings:
Turns per Pole - the number of turns of wire around each pole of the motor's armature (or stator in the case of a brushless motor).
Wire Gauge - the size of wire (AWG) used to wind the armature or stator. If your baseline motor is a multi-wind motor (i.e. wound with multiple parallel thinner wires), use the wire gauge of the equivalent single thicker wire.
The lower right pane is where you specify the corresponding information about the New Motor Windings. If you've specified a wire gauge for the baseline motor, MotoCalc will recommend a wire gauge for the new motor, based on the ratio of turns per pole. If you leave the new motor's wire gauge field blank, the recommended gauge will be used in the calculations.
The upper right pane is where the computed parameters of the Motor to Build will appear. The Motor field can be filled in with the name you wish to give the new motor.
MotoCalc uses a very simple model to compute the effect of varying the number of turns in a motor. The further that the number of turns deviates from the baseline motor, the less accurate the prediction will be. If the ratio of turns between the new motor and baseline motor is too high for the prediction to be usefully accurate, you will see the message "Warning! High Turns Ratio" in the Motor to Build pane.
In order for the predictions to be valid at all, the baseline and new motors must be identical except for the number of turns and the wire gauge. They must be of the same size, shape, and construction, and have the same kind of magnets. If you are rewinding an existing motor, this will obviously be the case.
Once you have entered a set of parameters into the
Motor Designer window, click OK to automatically fill them
into the Motor section of the MotoCalc Workbench window, and close
the Motor Designer window. If you want to experiment with different
sets of measurements, you can click Apply instead. This
does the same thing, but the Motor Designer window remains open.
Specifying Battery Information
Battery information is entered into the Battery section of the MotoCalc Workbench window. The characteristics of the cells within the battery affect the amount of current the cells can deliver, how long they can deliver it for, and how much voltage is lost internally. The number of cells in the battery defines the maximum voltage that can be delivered (assumed to be 1.2V per cell under no load).
By experimenting with different cell types, one can see what effects the different characteristics of a cell have. For example, switching from Sanyo 1700SCR cells to 2000SCE cells will reduce power output due to the higher internal resistance, and increase run time (there will also be an increase in weight).
If the battery you are interested in is built from commonly available cells, these cells are probably already included in MotoCalc's database. To select a cell type from the database, click the Open button in the Battery section (or the Open... item on the Battery menu) to browse the cell table. Once you've selected a cell type, the cell characteristics will automatically be filled into the appropriate fields. Refer to Using the Database Browsers for more information.
If your battery is composed of cells that are not in MotoCalc's database, you can enter the cell characteristics yourself. The two pieces of information that MotoCalc needs are the cell's capacity, and the cell's internal resistance.
The Cell field is used to give a name to a cell type. This name will appear on the performance report, and this name is also used to refer to the cell type in the database; no two cell types can have the same name. A cell type name can have up to 40 characters.
This unlabeled field appears just to the right of the cell name field described above and specifies the cell's "C" rating if known. The "C" rating, multiplied by the cell capacity (in Ah or mAh), gives the maximum current (in A or mA respectively) that the cell is able to deliver without damage. For example, a 2300mAh cell with a "C" rating of 20 can deliver 46000mA (or 46A). MotoCalc uses the "C" rating for a number of things.
Filtering - if the Use Filter checkbox is checked, power system combinations that exceed the maximum current allowed by the cells are filtered out (if the "C" rating is known).
MotoWizard - the MotoWizard will not suggest any power system combinations that cause the "C" rating to be exceeded. If you specified a specific cell type in the MotoWizard's Battery page and the "C" rating is not known, it will estimate a "C" rating based on the cell's internal resistance and weight.
MotOpinion - the MotOpinion report will warn you if the "C" rating is exceeded (if the "C" rating is known).
Note that the "C" rating is not known for all types of cells (the concept really only came into common use with the introduction of Lithium-Polymer cells). All the Lithium-Polymer cells in MotoCalc's database (as shipped) have a "C" rating, as do some of the more commonly used Nickel-Cadmium and Nickel-Metal-Hydride cells.
This is the capacity of the cell, in mAh (milliAmp-hours). This information is usually written on the cell, or on the battery shrink-wrap. If no cell capacity is specified, 1700mAh is assumed.
This is the voltage of the cell, in V (Volts). Nickel-Cadmium (NiCd) and Nickel-Metal-Hydride (NiMH) cells have a nominal voltage of 1.2V. Rechargeable Lithium cells have a nominal voltage of 3.0V. Lithium-Ion and Lithium-Polymer cells are 3.7V. If you leave this field blank, MotoCalc looks in the cell Chemistry field, and uses the appropriate voltage. In fact, if the Chemistry field is filled in, the voltage field turns gray and cannot be edited.
This is the internal resistance of the cell, in Ohms. This information is generally not included on either the cell or the battery shrink-wrap, and it requires rather sophisticated techniques to measure accurately. If you do not know this value, you'll have to guess. This value is generally around 0.004 to 0.007 for C-sized cells, 0.006 to 0.008 for AR sized cells, and 0.008 to 0.012 for AA sized cells. Generally, the fatter a cell is, the lower is its resistance. If no cell impedance is specified, 0.004 Ohms is assumed.
The weight of an individual cell, in ounces or grams. This information is used by MotoCalc when computing total aircraft weight. If no cell weight is specified, 1.89oz or 53.5g (the weight of a Sanyo 1700SCR cell) is assumed.
This the type of cell chemistry, which is one of NiCd (Nickel-Cadmium), NiMH (Nickel-Metal-Hydride), LiPo (Lithium-Polymer), or LiNP (Lithium-Nano-Phosphate). If the cell is of some other chemistry, this field can be left blank, and the appropriate voltage should be filled into the Voltage field.
The New button in the Battery section (or the New item on the Battery menu) clears all the battery information fields (they're all set back to blank).
There are two fields for specifying the cell count (specifically, the number of cells wired in series). The first specifies the minimum number of cells, and the second specifies the maximum number of cells. When MotoCalc produces its report, it will make predictions for each cell count in the range. If no number is specified as a minimum, 7 is assumed. If no number is specified as a maximum, the number specified for the minimum is assumed.
Note: The series cell counts are not stored in the database's cell table, since the number of cells you'll want to use is independent of the characteristics of the individual cells. Instead, this information is stored with the project.
With the increasing use of Lithium-Polymer cells for electric flight, and their relatively low current capability compared to NiCd cells, the practice of wiring one or more battery packs in parallel is becoming common (this is not a good idea to do with NiCd or NiMH packs).
The Parallel Cells fields are used to indicate the number of paralleled battery packs (or potentially, the number of paralleled cells within a pack if it is so constructed; electrically the two are equivalent). If no numbers are specified, a single non-paralleled battery is assumed.
As with the Series Cells fields, there are two fields for paralleled cells. The first specifies the minimum number, and the second specifies the maximum. MotoCalc will make predictions for each number in the range. If no number is specified for the minimum, 1 is assumed. If the maximum is omitted, it is assumed to be the same as the minimum.
If you make use of the Wiring Wizard, the Parallel Cells fields are filled in for you automatically when you exit from the Wizard window.
Note: Like the series cell counts, the parallel cell/battery counts are also not stored in the database's cell table, since these too are independent of the characteristics of the individual cells. This information is stored with the project.
Once you've entered the characteristics of a cell, you may want to save it for future reference. To do this, simply make sure that you've entered a unique name for the cell type, and click the Save button in the Battery section (or the Save item on the Battery menu).
Warning:
If you Open an existing cell type, change the characteristics,
and Save it again without first changing the name, the
new characteristics will replace the existing ones for that cell
type. Don't do this unless you're sure that that's what you want
to do.
Measuring Cell Impedance
The capacity, voltage, and weight of a cell are generally known, since they are specified by the manufacturer (although actual capacity under load can differ from the rated capacity, it will generally be within about 10% or so). Cell impedance (internal resistance) on the other hand is not always published.
To determine the impedance of a cell, fully charge a battery pack using your normal charger. Then, run it about half way down (perhaps by flying a shorter-than-usual flight with it, or using a discharger).
Next, prepare two loads with differing resistances (for packs of 6 to 10 cells, three or five #1157 automotive light bulbs wired in parallel make suitable loads). Measure both the current (I1 and I2) and the voltage (V1 and V2) with the battery connected to each load in turn (one of the many brands of hobbyist watt-meters this easy). The battery's impedance, R, is then given by the formula:
In other words, the difference in the voltages divided by the difference in the currents. Divide this by the number of cells in the battery to determine the impedance for a single cell. Be sure to use high quality connectors for these tests, or the connection resistance will introduce significant errors into the result.
The choice of gear ratio and propeller size, or ducted fan unit, is probably the largest factor influencing the performance of an electric flight system. Unlike those messy glow motors, which work best with a propeller or fan of a given size, the optimal propeller or fan size for an electric motor depends on the voltage at which it is run, and the amount of current we're willing to supply.
Using MotoCalc to "experiment" with different sizes and gear ratios can take some of the guesswork out of designing an electric flight system.
Gear and propeller or fan information is entered into the Drive System section of the MotoCalc Workbench window.
If you are using an off-the-shelf gearbox/propeller combination or ducted fan (drive system), it may already be included in MotoCalc's database. To select a drive system from the database, select the appropriate drive system type (propeller or ducted fan, see below) and click the Open button in the Drive System section (or the Open... item on the Drive System menu) to browse the drive system table. Once you've selected a drive system, the system characteristics will automatically be filled into the appropriate fields. Refer to Using the Database Browsers for more information.
MotoCalc supports two kinds of drive systems: propeller (with optional gearbox) and ducted fan. The drive system information required depends on the type of drive system.
For propeller drive systems, you can specify any combination of gear ratio and propeller sizes you wish. By specifying a range of ratios and/or sizes, you can generate a table of predictions, from which you can choose the optimum combination.
For ducted fan drive systems, you can specify the characteristics of the fan rotor, the hub, the intake and exhaust ducting, and a fan efficiency factor.
The Description field is used to give a name to a drive system. This name will appear on the performance report, and this name is also used to refer to the drive system in the database; no two drive systems can have the same name. A drive system description can have up to 40 characters.
Two tabs labeled Propeller and Ducted Fan appear near the bottom of the Drive System section. They are used to select the drive system type. When you click on one of these, the appropriate set of drive system parameter fields is displayed. You can also switch between propeller and fan drive systems using the Propeller and Ducted Fan items on the Drive System menu.
Note: The drive system type (propeller vs. fan) is not stored in the database's drive system table, since the type of drive system you'll want to use is a function of the aircraft. Therefore, this information is stored as part of the current project.
This is the ratio of the motor RPM (gearbox input RPM) to propeller RPM (gearbox output RPM). This generally varies from about 1.5 to about 10, depending on the gearbox. If you are not using a gearbox, use a ratio of 1.0, or leave this field blank. If left blank, 1.0 (direct drive) is assumed.
You can specify a whole range of ratios, and the increment you want to use. For example, you can specify 2.5 to 3.5 by 0.5, which will cause MotoCalc to try 2.5:1, 3.0:1, and 3.5:1 gear ratios. If you only want to specify a single ratio, leave the to and by fields blank, or enter the same values for both ends of the range, and enter 1 for the increment. For example: 2.38 to 2.38 by 1.
Note: The word "to" between the fields refers to the range, as in "from ... to ...". It does not refer to the ratio (as in "3 to 1", more commonly written "3:1"). The ":1" is implied, and should not be typed.
The approximate efficiency of the gearbox, expressed as a percentage. This is the fraction of motor shaft power reaching the propeller shaft. The remainder (generally a few percent) is lost in the gearbox in the form of heat and noise. There is little published data on gearbox efficiency, so for most gearboxes, this is an estimate.
The Gearbox Effic field has a drop-down list which will yield typical efficiencies for various types of gearboxes. If you don't know the actual efficiency of a particular gearbox, select the appropriate type from this list.
If you leave this field blank, the default gearbox efficiency value specified in the Options window is assumed. If this too is blank, an efficiency of 92% is assumed.
The weight of the gearbox, in ounces or grams. This information is used by MotoCalc when computing total aircraft weight. If no gearbox weight is specified, MotoCalc does not add the weight of the gearbox into the total aircraft weight. If the gear ratio is 1:1, the gearbox weight field is ignored.
This is the diameter of the propeller, in inches or centimetres. You can specify a range of diameters you wish to try, and the increment you want to use. For example, you can specify 10 to 13 by 1.5, which will cause MotoCalc to try 10, 11.5, and 13 inch diameter propellers. If you only want to specify a single diameter, only enter a value into the first field, or enter the same values for both ends of the range, and enter 1 for the increment. For example: 10 to 10 by 1.
This is the pitch of the propeller, which is defined as the distance the propeller would move forward if turned one revolution into a solid material. You can specify a range of pitches you wish to try, and the increment you want to use. For example, you can specify 5 to 6 by 0.5, which will cause MotoCalc to try 5, 5.5, and 6 inch pitch propellers. If you only want to specify a single pitch, only enter a value into the first field, or enter the same values for both ends of the range, and enter 1 for the increment. For example: 5 to 5 by 1.
Note that the total number of propellers that will be tried is equal to the number diameters times the number of pitches. For example, if you specified 10 to 13 by 1.5 for the diameter, and 5 to 6 by 0.5 for the pitch, MotoCalc will try 10x5, 10x5.5, 10x6, 11.5x5, 11.5x5.5, 11.5x6, 13x5, 13x5.5, and 13x6 propellers (nine sizes in all).
If you are trying a very wide range of propellers (e.g. 8 to 12 by 1 diameter and 4 to 10 by 1 pitch), you can use a pitch ratio restriction to prevent MotoCalc from trying ridiculously high-pitch propellers such an 8x10.
This specifies a fudge factor used in computing the power absorbed by a propeller.
You can either type in a power constant, or select a propeller brand from the drop-down list (by clicking on the downward-pointing arrow next to the field).
Power constants tend to vary somewhat from one propeller to the next in the same series, so the values provided are averages. The cause of the variation is generally the propeller's pitch, which often is not exactly the value stamped on the propeller. However, using an average propeller constant for a series of propellers will still produce acceptable accuracy.
If you do not know the power constant for the propeller you intend to use, either select a similar propeller from the list, or click the Const... button to use the Propeller Constant Estimator to determine the constant experimentally.
Note: The power constant for the Rev Up propellers depends on the pitch, so MotoCalc cannot compute the constant unless you have specified the pitch. Furthermore, if you've specified a range of pitches, MotoCalc will warn you, and will compute the power constant based on the average pitch. Therefore, you will get better results with Rev Up propellers if you only select a single pitch (and its corresponding constant) at a time.
If you leave this value blank, a default of 1.31 is used (this was the value used in older versions of MotoCalc that didn't have this field, and is an average for typical propellers).
This specifies the efficiency of the propeller at producing thrust. It is not an absolute efficiency, but merely a measure relative to other propellers.
You can either type in a thrust constant, or select a propeller brand from the drop-down list (by clicking on the downward-pointing arrow next to the field).
If you do not know the thrust constant for the propeller you intend to use, either select a similar propeller from the list, or click the Const... button to use the Propeller Constant Estimator to determine the constant experimentally.
If you leave this field blank, a default of 0.95 is used, which is an average for typical propellers.
This specifies the number of blades on each propeller. If you leave this blank, a standard two-bladed propeller is assumed.
This field is used to specify the number of propellers used on the aircraft. If no number is specified, one propeller per motor is assumed (see Motors - Series and Parallel below).
The number of propellers can be either more than, less than, or the same as the number of motors. If more, then the number of propellers must be a multiple of the number of motors. If less, the number of motors must be a multiple of the number of propellers.
If there are more propellers than motors, it is assumed that each motor is driving more than one propeller (most likely through some sort of belt drive). If there are fewer propellers than motors, several motors are driving a single propeller (through a gear or belt arrangement).
Note: The number of propellers is not stored in the database's drive system table. Instead, this information is stored with the aircraft project.
This specifies the diameter of the fan rotor.
This specifies the pitch of the fan, which is defined as the distance the fan would move forward if turned one revolution into a solid material. You can use the Fan Pitch Measurements window to compute the pitch from measurements, or you can use the Fan Coefficient Estimator window to estimate the pitch, thrust coefficient, and power coefficient all at once.
The latter is preferable, because it will compute a pitch and coefficients which are consistent with actual test results, and hence produce more accurate performance predictions.
The weight of the fan unit (not including the motor), in ounces or grams. This information is used by MotoCalc when computing total aircraft weight. If no fan weight is specified, MotoCalc does not add the weight of the fan unit into the total aircraft weight.
This specifies the diameter of the fan hub and motor.
This specifies the relative efficiency of the fan at producing thrust, relative to the thrust it could produce at a given RPM if it were perfect. Due to the simplified mathematical model used by MotoCalc to predict fan performance, the thrust coefficient can even be larger than 1.0. This does not mean that the fan is more than 100% efficient, just that it is better than MotoCalc would otherwise predict.
If you leave this field blank, a default value of 0.96 is used.
The only way to determine the thrust coefficient is experimentally (either from your own tests, or from published test data). You can use the Fan Coefficient Estimator window to enter the experimental data, from which MotoCalc can compute the thrust coefficient.
This specifies a fudge factor used in predicting the amount of power absorbed by the fan at a specified RPM. This factor compensates for inaccuracies that would otherwise appear in the simplified mathematical model used by MotoCalc to predict fan power absorption.
If you leave this field blank, a default value of 0.82 is used.
The only way to determine the power coefficient is experimentally (either from your own tests, or from published test data). You can use the Fan Coefficient Estimator window to enter the experimental data, from which MotoCalc can compute the power coefficient.
This specifies the intake duct diameter and length. The diameter is the inside diameter at the intake lip, and the length should be measured from the lip to the front of the fan blades. If you leave the diameter blank, a diameter is chosen so that the intake area equals the swept area of the fan. If you leave the length blank, a length is chosen such that the intake has a 5 degree divergence half-angle.
This specifies the exhaust duct diameter and length. The diameter is the inside diameter at the end of the tail cone, and the length should be measured from the tail cone to the rear of the fan blades (it is assumed that the ducting remains at the fan diameter for the length of the motor). If you leave the diameter blank, a diameter is chosen so that the exhaust area equals the swept area of the fan. If you leave the length blank, an exhaust length equal to the fan diameter is chosen.
Note: The intake diameter and length, and exhaust diameter and length are considered part of the airframe, not part of the particular ducted fan. As such, they are saved with the airframe information instead of the drive system information. To remind you of this, these fields, along with the fields in the Airframe pane, are all labeled in green when the drive system section is in ducted fan mode.
Use these fields for multi-motored aircraft. MotoCalc supports any regular combination of series and/or parallel wired motors. In the Series field, specify the number of motors wired in series. In the Parallel field, specify the number in parallel. For example, if you have a six-motored aircraft, with two parallel strings of three motors each, specify 3 in the Series field, and 2 in the Parallel field. Note that if the plane is wired with a series-connected group of three sets of two parallel motors each, the same parameters would be used. From a performance point of view, there is no difference between the two configurations.
If you have a single string of motors in series, or a single set in parallel, you can leave the other field (i.e. Parallel and Series respectively) blank.
When working with more than one motor, the predictions that MotoCalc produces are for all the motors together. To determine values for each motor, do the following:
Divide the voltage (Volts) prediction by the number of motors in series.
Divide the current (Amps) predictions by the number of motors in parallel.
Divide input and output power (Watts) predictions by the total number of motors.
On the other hand, the restrictions you specify in a filter are on a per-motor basis. This is because such restrictions are generally based on the limits of the motor. (The only restrictions affected by multiple motors are current and power loss; the thrust restriction is on total thrust.)
If you make use of the Wiring Wizard, the Series Motors and Parallel Motors fields are filled in for you automatically when you exit from the Wizard window.
Note: The numbers of series and parallel motors are not stored in the database's drive system table, since the number of drive systems you'll want to use is independent of the characteristics of the individual drive systems. Instead, this information is stored with the project.
The New button in the Drive System section (or the New item on the Drive System menu) clears all the drive system information fields (they're all set back to blank).
The propeller's power constant and thrust constant can be estimated using the Propeller Constant Estimator window, which is accessed by clicking the Const... button in the Drive System section, or the Constant Estimator... item on the Drive System menu.
To use the estimator, you need to have test data specifying the motor used, the motor terminal voltage, motor current, the gear ratio, propeller diameter and pitch, and the measured RPM and thrust.
Electrical Measurements
Start by selecting the appropriate motor by clicking the Select... button and choosing the motor using the database browser. Then enter the measured motor terminal voltage (i.e. the voltage right at the motor terminals, to minimize any error due to speed control or wiring losses), and the measured current.
Given these two pieces of information, MotoCalc will predict the propeller RPM and display it in the RPM field (unless the RPM field already has a value in it, in which case you can click the Estimate RPM button to force the estimate anyway). This value can be used if the available test data doesn't include the RPM, although the propeller constant estimates will be much less accurate if this is the case.
Propeller and Gear Measurements
Enter the gear ratio (if a gearbox is used), propeller dimensions, and the measured RPM and thrust in the appropriate fields. Use the radio buttons to the right of each field to specify the units of measurement.
As you enter the information, MotoCalc will estimate the power and thrust constants. The estimated values appear in the top right pane of the estimator window. If at any point it is not possible to compute the power or thrust constants, the message Inconsistent Input will appear. This can happen if, for example, the thrust and RPM are such that a motor efficiency of over 100% would be required. A situation like this would indicate either a mistake in measurement, a typographical error, or perhaps a propeller with a pitch and diameter very different from the values printed on it.
If the test data includes RPM, and this RPM is dramatically different than that predicted when you first entered the motor terminal voltage and current, be suspicious of either the test data, or the motor parameters for the selected motor.
Assumed Test Conditions
MotoCalc indicates the conditions under which it assumes the tests were done in the lower pane of the estimator window. Since all tests are assumed to be of short duration, motor heating is not taken into account. For the best possible accuracy in estimating the propeller constants, you should set the conditions to match those in which the tests were performed. Clicking the Change... button will display the Options window, where you can set these conditions.
Using the Estimated Constants
Once you've entered the test data, click OK to copy the propeller constants into the corresponding fields in the Drive System section.
If you have more than one set of test data (for example, with different numbers of cells, or different motors), you might want to use the estimator several times, record the computed constants, and then use an average. In this case, be sure to discard any test cases where the estimated RPM differs significantly (more than 10%) from the measured RPM.
Adding the Constants to the Propeller List
If you are confident in the accuracy of the propeller constants, and have determined that the constants are similar for different sizes of the same propeller brand and type, you may want to add that propeller type to the list of known propeller power and thrust constants (the drop-down list for the Power Constant and Thrust Constant fields). To do this, type the propeller name into the Propeller Constant Estimator's Name to Use field, and click the Update Lists button.
The Fan Pitch can be computed using the Fan Pitch Calculator window, which is accessed by clicking the Pitch... button in the Drive System section or the Pitch Calculator... item on the Drive System menu.
Note: It is preferable to use the Fan Coefficient Estimator window to estimate the pitch and fan coefficients all together based on real test data, because the measured pitch of a fan is not always indicative of its effective pitch.
The left side of this window is used to specify characteristics of the fan.
Make the following measurements from the fan rotor. If you measure the blade angle, you do not need to measure the blade width and height. Likewise, if you measure the blade width and height, you do not need to measure the blade angle (MotoCalc will compute it):
Rotor diameter - Indicated by ROTOR DIAM in the diagram above, the diameter of the fan rotor..
Blade angle - Indicated by ANGLE in the diagram above, the angle of attack of the blade.
Blade width at perimeter - Indicated by WIDTH in the diagram above, the width of the blade at its tip, as viewed from below (mathematically, the width of the vertical projection of the blade tip).
Blade height at perimeter - Indicated by HEIGHT in the diagram above, the height of the blade at its tip, as viewed from the side (mathematically, the height of the horizontal projection of the blade tip).
Make the specified measurements, and enter them into the appropriate fields. Depending on the equipment you have, it may be easier to measure the angle, or it may be easier to measure the width and height.
As soon as you've entered enough information, MotoCalc will compute and display the pitch in the top right pane of the window.
Press OK to accept the measurements and transfer the computed pitch to the Fan Pitch field of the Drive System section.
The fan's thrust coefficient, power coefficient, and pitch can be estimated using the Fan Coefficient Estimator window, which is accessed by clicking the Coeff... button in the Drive System pane, or the Coefficient Estimator... item on the Drive System menu.
To use the estimator, you need to have test data specifying the motor used, the motor terminal voltage, motor current, the fan rotor and hub diameter, and the measured RPM, thrust, and efflux velocity.
Electrical Measurements
Start by selecting the appropriate motor by clicking the Select... button and choosing the motor using the database browser. Then enter the measured motor terminal voltage (i.e. the voltage right at the motor terminals, to minimize any error due to speed control or wiring losses), and the measured current.
Given these two pieces of information, MotoCalc will predict the fan RPM and display it in the RPM field (unless the RPM field already has a value in it, in which case you can click the Estimate RPM button to force the estimate anyway). This value can be used if the available test data doesn't include the RPM, although the coefficient estimates will be less accurate if this is the case.
Fan Measurements
Enter the fan dimensions, and the measured RPM, thrust, and efflux velocity in the appropriate fields. Use the radio buttons to the right of each field to specify the units of measurement.
As you enter the information, MotoCalc will estimate each coefficient, and also the fan pitch. The estimated values appear in the top right pane of the estimator window.
If the test data includes RPM, and this RPM is dramatically different than that predicted when you first entered the motor terminal voltage and current, be suspicious of either the test data, or the motor parameters for the selected motor.
If the estimated fan pitch differs significantly from the measured fan pitch, this is nothing to worry about. It just means that the fan blades have extra twist to them, and that the measured pitch at the blade tips is not an accurate reflection of the effective pitch of the fan.
Assumed Test Conditions
MotoCalc indicates the conditions under which it assumes the tests were done, in the lower pane of the estimator window. Since all tests are assumed to be of short duration, motor heating is not taken into account. For the best possible accuracy in estimating the fan coefficients, you should set the conditions to match those in which the tests were performed. Clicking the Change... button will display the Options window, where you can set these conditions.
Using the Estimated Coefficients
Once you've entered the test data, click OK to copy the pitch and coefficients into the corresponding fields in the Drive System section.
If you have more than one set of test data (for example, with different numbers of cells, or different motors), you might want to use the estimator window several times, write down the computed pitch and coefficients, and then use an average. In this case, be sure to discard any test cases where the estimated RPM differs significantly (more than 10%) from the measured RPM.
Once you've entered the characteristics of a drive system, you may want to save it for future reference. To do this, simply make sure that you've entered a unique name for the drive system, and click the Save button in the Drive System section (or the Save item on the Drive System menu).
Warning:
If you Open an existing drive system, change the characteristics,
and Save it again without first changing the name, the
new characteristics will replace the existing ones for that drive
system. Don't do this unless you're sure that that's what you
want to do.
Specifying a Speed Control
The speed control (we use the term here to include on/off controllers as well) controls the amount of power that gets to the motor. Although MotoCalc only deals with full-throttle calculations, the characteristics of the speed control are still important, because even at full throttle, some power is lost in the speed control. Furthermore, speed controls have a maximum current rating that one must not exceed.
Speed control information is entered into the Speed Control section of the MotoCalc Workbench window.
If you are using an off-the-shelf speed control, it may already be included in MotoCalc's database. To select a speed control from the database, click the Open button in the Speed Control section (or the Open... item on the Speed Control menu) to browse the speed control table. Once you've selected a speed control, its characteristics will automatically be filled into the appropriate fields. Refer to Using the Database Browsers for more information.
There is only one piece of information that MotoCalc needs to know about a speed control, and that is its resistance. You can also specify a maximum current, which will restrict MotoCalc's output to include only those combinations of components that don't exceed this current (but only if the Use Filter check box in the Filter section is checked).
The Name field is used to give a name to a speed control. This name will appear on the performance report, and this name is also used to refer to the speed control in the database; no two speed controls can have the same name. A speed control name can have up to 40 characters.
This specifies the resistance, in Ohms, that the speed control has when it is completely on (full throttle). For relay based on/off controllers, this will typically be around 0.003 to 0.005 Ohms. For MOSFET based speed controls and soft-start on/off controllers, this will depend on the type and number of MOSFETs in the circuit, and could range anywhere from 0.028 Ohms for a single IRFZ40 MOSFET, down to 0.001 Ohms. Consult the speed control's documentation for this information. If you do not specify the ESC resistance, 0.01 Ohms is assumed.
This is the maximum current, in Amps, that the speed control can handle. This restriction is only applied if the Use Filter check box is checked.
Note: If the Maximum Current field in the Filter section is filled in, and that value is less than the maximum current specified in the Speed Control section, the Filter's maximum is used (i.e. the lower of the two maximums applies).
The weight of the speed control, in ounces or grams, not including the motor wiring. This information is used by MotoCalc when computing total aircraft weight.
This specifies if the speed control is a high-rate model (switches at 1kHz or higher), or a low-rate (also known as frame-rate) one. This affects the efficiency of the speed control at partial throttle settings.
This check box indicates whether or not the speed control is for brushless motors. The MotOpinion report uses this to ensure you've selected the right kind of speed control to suit the motor.
The Number of ESCs field is used to specify power systems having more than one speed control. For example, one might build a twin-motored model with a separate speed control for each motor, both operating from a single battery. Or, one might use a separate battery for each speed control. In both cases, this is specified by putting "2" in the Number of ESCs field (in the latter case, one would also put "2" in the Parallel Cells field).
If you specify more than one speed control via the Number of ESCs field, you must specify at least that many motors in the Parallel Motors field. Motors (or sets of motors) connected to multiple speed controls are electrically equivalent to being wired in parallel with each other. Furthermore, the number of parallel motors must be a multiple of the number of speed controls (for example, you can't have a total of three motors operating from two speed controls).
If you make use of the Wiring Wizard, the Number of ESCs field is filled in for you automatically when you exit from the Wizard window.
Note: The multiple speed control setting is not stored in the database's speed control table, since this is independent of the characteristics of the individual speed control. This information is stored with the project instead.
The New button in the Speed Control section (or the New item on the Speed Control menu) clears all the speed control information fields (they're all set back to blank).
Once you've entered the characteristics of a speed control, you may want to save it for future reference. To do this, simply make sure that you've entered a unique name for the speed control, and click the Save button in the Speed Control section (or the Save item on the Speed Control menu).
Warning:
If you Open an existing speed control, change the characteristics,
and Save it again without first changing the name, the
new characteristics will replace the existing ones for that speed
control. Don't do this unless you're sure that that's what you
want to do.
Specifying Airframe Information
The airframe, from MotoCalc's point of view, is that part of the plane not related directly to the power system. MotoCalc uses airframe information to predict flight characteristics such as stall speed, hands-off level flight speed, and maximum level flight speed. MotoCalc uses very rudimentary aerodynamic formulae to give you approximate figures for these characteristics; it is not able to do things like compare the relative performance of different airfoils, for example.
Airframe information is entered into the Airframe section of the MotoCalc Workbench window.
There are a few airframes already included in MotoCalc's database (you can also add your own; see Adding an Airframe to the Database). To select an airframe from the database, click the Open button in the Airframe section (or the Open... item on the Airframe menu) to browse the airframe table. Once you've selected an airframe, its characteristics will automatically be filled into the appropriate fields. Refer to Using the Database Browsers for more information.
There are several pieces of information about the airframe that MotoCalc uses to make predictions about flight performance. Three of these, wing area, empty weight, and maximum lift coefficient, are used to predict stall speed (the speed below which the aircraft will cease to fly). Wing area and drag coefficient are used to compute maximum level flight speed. Wing area, empty weight, and lift coefficient are used to calculate hands-off level flight speed (the speed at which the plane will neither gain nor lose altitude with the throttle set appropriately, and the control surfaces at neutral). The wing span is used to compute some of the coefficients, rate of climb, and rate of sink.
The Name field is used to give a name to an airframe. This name will appear on the performance report, and this name is also used to refer to the airframe in the database; no two airframes can have the same name. An airframe name can have up to 40 characters.
This specifies the wing span, in inches or centimetres. The span is the measure from wing tip to wing tip, including the part that passes over, under, or through the fuselage.
If no wing span is specified, MotoCalc will assume a span such that the wing's aspect ratio (span:chord) is 8:1, which is a compromise, being half way between a typical sport plane and a typical sailplane.
This specifies the wing area, in square inches or square decimetres. If the aircraft has a lifting tail, include its area in the wing area. Also include the part of the wing that passes through the fuselage.
If no wing area is specified, MotoCalc will not be able to predict wing loading, stall speed, hands-off level flight speed, maximum speed, rate of climb, or rate of sink.
This is the combined weight of all components of the aircraft except the motor, battery, and speed control. When making flight performance predictions, MotoCalc will add to this the motor weight (as specified in the Motor section), the battery weight (number of cells times cell weight, as specified in the Battery section), and speed control weight (as specified in the Speed Control section).
Be sure to include the weight of the gearbox and propeller in the empty weight.
If no empty weight is specified, MotoCalc will not be able to predict wing loading, stall speed, hands-off level flight speed, rate of climb, or rate of sink.
A lift coefficient is a measure of the wing's ability to produce lift. Factors that affect the lift coefficient are the angle of attack and type of airfoil. For a typical semi-symmetrical airfoil (e.g. Eppler 205) at 2 to 3 degrees angle of attack, the lift coefficient will be around 0.4 to 0.5. At 10 degrees, it would be around 1.1. Most such airfoils will stall at a lift coefficient around 1.1 to 1.3.
MotoCalc uses three different lift coefficients:
Cl is the lift coefficient when the wing is flying at the angle of attack equal to its angle of incidence relative to the horizontal stabilizer. MotoCalc uses this to compute the hands-off level flight speed, which is indicated in green on the In-flight Analysis. This is the speed that the aircraft will fly with the throttle set appropriately (to maintain that speed), and the control surfaces at neutral.
Clopt is the lift coefficient at the angle of attack that gives the best lift-to-drag ratio (LDmax). MotoCalc indicates the level flight speed at this angle of attack in turquoise on the In-flight Analysis. The optimal lift coefficient is used in computation of the rates of climb and sink (with the throttle set at maximum and zero respectively).
Clmax is the lift coefficient at the maximum angle of attack, above which the wing will stall. The wing must be at this angle of attack when flying at the stall speed, indicated in yellow on the In-flight Analysis. This is the slowest speed that the aircraft will fly (with the throttle set appropriately to maintain that speed).
MotoCalc also uses three different drag coefficients:
Cdp is the wing's profile drag coefficient, which is a measure of the aerodynamic cleanliness of the wing. This is affected primarily by angle of attack, and also somewhat by the camber.
Cdi is the wing's induced drag coefficient, which is a measure of the drag caused by lift. This is affected by angle of attack, camber, and aspect ratio (span-to-chord ratio).
Cdo is a measure of drag caused by other features of the plane, such as landing gear, surface finish, and other similar factors.
These three drag coefficients are summed up into one overall drag coefficient, Cd, which is used in the hands-off level flight calculation. For Clopt level flight, Clopt / LDmax is used.
Note that the size of the aircraft does not affect the drag coefficient, but only the total amount of drag. MotoCalc automatically takes the size (based on the wing area) into account.
Total drag coefficients typically range from about 0.02 (for a sleek pylon racer) to 0.12 (for a biplane with lots of drag producing features such as bracing wires and posts). Typical values for a sport plane or glider are around 0.04 to 0.06.
The lift and drag coefficient cannot be edited directly. Instead, they are set by varying the airfoil and airframe parameters in the Lift and Drag Coefficient Estimator window, accessed by clicking the Coeff... button. Changes in the aspect ratio (span-to-chord ratio) will also change the coefficients.
The New button in the Airframe section (or the New item on the Airframe menu) clears all the airframe information fields (they're all set back to blank).
The Lift and Drag Coefficients are determined using the Lift and Drag Coefficient Estimator window, which is accessed by clicking the Coeff... button in the Airframe section, or the Coefficient Calculator... item on the Airframe menu.
The left side of this window is used to specify characteristics of the airfoil being used, and how it is rigged relative to the horizontal stabilizer.
If you know the characteristics of the airfoil, use the three sliders to set the thickness (as a percentage of the chord), camber (also as a percentage of the chord), and angle of attack (angle of the chord line relative to the horizontal stabilizer). As you adjust the sliders, the airfoil diagram changes to illustrate the parameters you have selected.
If your plane's wing consists of a flat or slightly curved sheet of material (such as foam or balsa wood), click the Flat or curved plate with square edges check box, and adjust the sliders appropriately.
If you do not know the airfoil characteristics, use the Wing Airfoil Measurements or Common Airfoils windows.
Note: For non-plate airfoils, the airfoil diagram illustrates a generic airfoil (it actually corresponds to the NACA x4xx series, with the point of maximum camber at 40%). When you specify a specific camber and thickness, the airfoil may not look exactly like the one you are using (for example, most flat bottomed airfoils will appear as slightly undercambered), but it will have approximately the same aerodynamic characteristics.
If you don't know the thickness and camber of your model's airfoil, but it uses a well know airfoil and you know it's name, click the Choose from List... button to display a list of common airfoils.
Selecting one of the airfoils from this list and clicking the OK button will set the thickness and camber sliders to match the thickness and camber of the selected airfoil.
Note: Only the thickness and camber of these airfoils are used. Some of these airfoils have special features, such as reflexed trailing edges, and what appears in the airfoil window of the Lift and Drag Coefficient Estimator may not resemble the actual airfoil. However, it will be close enough for MotoCalc to make its predictions.
If you do not know the either the name or the characteristics of the airfoil, click the Compute from Measurements... button to display the Wing Airfoil Measurements window.
MotoCalc needs six measurements to compute the airfoil parameters. Before making the measurements, draw two lines on the side-view plan. One line, the datum line, should be drawn through or parallel to the horizontal stabilizer, and extended forward past the leading edge of the wing. The other line, the chord line, should be drawn from the leading edge to the trailing edge.
Take the following measurements off the plan:
Height of center of wing leading edge - Indicated by LE HEIGHT in the diagram above, the vertical distance from the datum line to the point where the chord line intersects the leading edge. If this point is below the datum line, enter a negative measurement.
Height of center of wing trailing edge - Indicated by TE HEIGHT in the diagram above, the vertical distance from the datum line to the point where the chord line exits the trailing edge. If this point is below the datum line, enter a negative measurement.
Leading edge to trailing edge (chord) - Indicated by CHORD in the diagram above, the distance from the leading edge to the trailing edge as measured along the chord line.
Height of upper surface at thickest point - Indicated by UH in the diagram above, the distance from the datum line to the upper surface of the wing, measured at the point where the wing is the thickest. For maximum accuracy, this should be measured along a line drawn perpendicular to the chord line (the vertical green line in the diagram), instead of perpendicular to the datum line. If this point is below the datum line, enter a negative measurement.
Height of lower surface at thickest point - Indicated by LH in the diagram above, the distance from the datum line to the lower surface of the wing, measured at the point where the wing is the thickest. For maximum accuracy, this should be measured along a line drawn perpendicular to the chord line (the vertical green line in the diagram), instead of perpendicular to the datum line. If this point is below the datum line, enter a negative measurement.
Height of chord line at thickest point - Indicated by CH in the diagram above, the distance from the datum line to the chord line, measured at the point where the wing is the thickest. For maximum accuracy, this should be measured along a line drawn perpendicular to the chord line (the vertical green line in the diagram), instead of perpendicular to the datum line. If this point is below the datum line, enter a negative measurement.
Make the specified measurements on the plan, and enter them into the appropriate fields. It doesn't matter what units you use (inches, centimetres, or whatever you prefer), so long as you use the same units for all the measurements. MotoCalc will compute the thickness (to the nearest 0.5%), camber (to the nearest 0.5%), and angle of attack (to the nearest 0.5°) from these measurements.
Press OK to transfer the computed airfoil parameters to the Lift and Drag Coefficient Estimator window and close the measurements window.
The right side of the Lift and Drag Coefficient Estimator is used to specify characteristics of the rest of the airframe. There are four categories of characteristics: Fuselage Cross Section, Surface Finish, Landing Gear, and Protrusions. Select the appropriate choice for your model in each category.
As you adjust the airfoil sliders, and change the airframe characteristics, the eight coefficients displayed in the airfoil illustration will change:
Cl - the lift coefficient of the airfoil with the selected thickness, camber, and angle of attack.
Clmax - the maximum lift coefficient of the airfoil, beyond which it will stall. As the angle of attack is increased, the airfoil illustration will turn from blue to olive as it nears the stall, and to red beyond the stall.
Clopt - the lift coefficient at which the airfoil has the best lift-to-drag ratio.
LDmax - the best lift-to-drag ratio for the airfoil.
Cdp - the profile drag coefficient of the airfoil with the selected thickness, camber, and angle of attack.
Cdi - the induced drag coefficient of the airfoil with the selected thickness, camber, and angle of attack.
Cdo - the drag coefficient of the rest of the airframe.
Cd - the total drag coefficient (the sum of Cdp, Cdi, and Cdo).
Warning: The lift and drag coefficient estimates made by MotoCalc are just approximations. There is a lot more to airfoil performance than just thickness, camber, and angle of attack. MotoCalc uses approximations based on common airfoils, such as the NACA x4xx series. These approximations are most accurate for Reynolds numbers between about 200,000 and 500,000, which covers most model aircraft applications. See the suggested reading section for more information.
Once you've entered the characteristics of an airframe, you may want to save it for future reference. To do this, simply make sure that you've entered a unique name for the airframe, and click the Save button in the Airframe section (or the Save item on the Airframe menu).
Warning: If you Open an existing airframe, change the characteristics, and Save it again without first changing the name, the new characteristics will replace the existing ones for that airframe. Don't do this unless you're sure that that's what you want to do.
Real life power systems have various restrictions placed on them. For example, most "can" type motors are limited to about 25A current due to their inexpensive brushes. If you are designing a plane for a limited motor run (LMR) event, you may have chosen a rather small battery (e.g. 7 x Sanyo 270AA), and only want MotoCalc to show you combinations that will run for at least the specified time.
Restrictions are entered into the Filter section of the MotoCalc Workbench window.
MotoCalc comes with a number of predefined filters that you can use. For example, the Speed 600 filter includes the restrictions generally applied to Speed 600 motors (limited current due to the brushes, and limited power loss since the excess heat will ruin the magnets). To select a filter from the database, click the Open button in the Filter section (or the Open... item on the Filter menu) to browse the filter table. Once you've selected a filter, its characteristics will automatically be filled into the appropriate fields. Refer to Using the Database Browsers for more information.
There are a number of things you can restrict in a filter. You can specify any or all of the restrictions. You can also temporarily turn the filter off to see what would happen if you lift the restrictions.
The Filter field is used to give a name to a filter. This name will appear on the performance report, and this name is also used to refer to the filter in the database; no two filters can have the same name. A filter name can have up to 40 characters.
This indicates the maximum current that you want the motor to draw. Typically, this is limited by the abilities of the motor's brushes. If you are using low cost cells (such as 600AA), these also restrict current because they will overheat at high currents.
Note: When working with a multi-motor aircraft, this restriction is on a per-motor basis.