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Electric motor power selection

Old 03-20-2018, 07:18 PM
  #1  
Windyboy
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Default Electric motor power selection

Hello Everyone,

I am new to this forum and to RC modeling airplanes in general. I am planning to build a small RC plane and studying the needed theory.

I have a basic question about motor power. Here my hypothetical example:

Lets' assume my plane has a weight of 4 LB. I would like to be able to to load it with 2LB extra. That means the total weight the aerodynamic forces have to lift is W=6 LB.

Wings: I can choose different types of wings (swept, rectangular, etc.) but the important parameter is wing loading (LB/ft^2). If the total area of both wings is A, the wing loading wold be W/A.

Now it is time to talk about the electric motor (brushed or brushless) rated in Watts. I read that the motor power would depend on the wing loading.

I know that power = torque times speed

The motor will be attached to a propeller of a certain diameter D and pitch.

How do I choose the right type of motor? Could anyone put me on the right track with the basic concepts to connect airplane weight W, wing loading and motor power?

Thanks,
Windyboy
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Old 03-21-2018, 12:33 AM
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quorneng
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Windyboy
There is no magic link between Watts weight and wing loading. It all depends on how you want the plane to fly.
This links gives you the sort of range for Watts/lb
https://www.rc-airplane-world.com/watts-per-pound.html
And this link for the range of wing loading.
http://www.flyrc.com/wing-load-calculator/
There is no single formula. It all depends on what sort of pane and what you are trying to do with it.
I hope this helps.
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Old 03-21-2018, 04:00 AM
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Stevephoon
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Hi Windyboy,

quorneng is right, there are many, many factors in selecting a power system for a plane. But when starting out you may not need to evaluate everything...

The best resource I found when starting out was Ed Anderson's "Everything You Wanted To Know About Electric Power Flight" E-Book. You can find here on WattFlyer.

https://www.wattflyer.com/forums/showthread.php?t=31368


For my smaller parkflyer type planes I've started by just looking mainly at the thrust, voltage and weight of the motor by looking at the specs on motors over on HeadsUpHobby.com. They usually have a lot of information on their motors using different batteries and props. Initially a 1 to 1 thrust to weight ratio seemed like a good starting place for small sport planes. Another rule of thumb is to use about 100 watts per lb for sport planes.

As you begin to understand more and start looking for different types and sizes of planes, I have liked some of the information that Ken Meyers has posted on The Ampeer. http://www.theampeer.org/

Good Luck and keep asking questions!

Steve
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Old 03-21-2018, 04:09 AM
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ron_van_sommeren
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Originally Posted by Windyboy View Post
... I am planning to build a small RC plane and studying the needed theory. ...
Start with a proven design/kit for beginners: straight wings, no frills. And get experienced help/club. Let them check your plane before covering it, or several times earlier.

Originally Posted by Windyboy View Post
... Now it is time to talk about the electric motor (brushed or brushless) rated in Watts. ...
Brushed motors are obsolete. Way more choice in brushless motors and brushless controllers, more power, higher efficiency.
Max. current (in ampère) rating is more important to motor than max. power (in watt).

Originally Posted by Windyboy View Post
... How do I choose the right type of motor? Could anyone put me on the right track ...
Several excellent messages in this thread today
Power versus weight and Kv - RCG
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Old 03-21-2018, 04:24 AM
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ron_van_sommeren
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Note that Kv (in rpm/volt) says nothing/nada/zero/niente/niks about max.motor power, Kv is not a rating, not a figure of merit.
Kv is about what the motor wants to do, Imax and Pmax are about what the motor can do ...
  1. Kv and voltage determine how fast motor wants to run.
    (rpm_noload = voltage × Kv)
  2. Rpm and prop determine torque, torque in turn determines current.
    (current = torque × Kv (in SI units!))
  3. Max.current Imax and, to a lesser extent, max.power Pmax determine whether that voltage/motor/rpm/prop combo can run without going up in smoke.
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Old 03-21-2018, 04:27 AM
  #6  
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Without a watt-meter you are in the dark, until something starts to glow
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Old 03-23-2018, 03:21 PM
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Windyboy
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Thanks everyone.

I am pondering about the minimum take off speed and its seems like stall speed is important in that regard. Stall speed is the minimum speed at which the plane can fly. It is calculated using C_Lmax, which occurs close to stall when the wing is at a large angle of attack. But when the plane is lifting itself off the ground and still moving horizontally on the runway its angle of attack is not the stall angle of attack. Flaps help increase C_L even when the angle of attack is not large. While speeding up on the runway the angle of attack of a cambered wing is not zero since the wing is attached to the fuselage at a small angle. But that angle is not the stall angle of attack.

So How are the stall speed and the minimum take off speed related? How does the knowledge of CLmax for the airfoil, which happens at stall, help us determine the minimum take off speed?

Thanks!
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Old 03-23-2018, 05:36 PM
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quorneng
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Windyboy
For a model plane I fear you are rather over thinking the problem.
Compared to full size models are considerably over powered, they have to be simply because the characteristics of air does not favour small er sizes. As a result in the majority of cases if it flies it will likely take off easily enough.
The big issue is the nature of the runway, Even fairly close cut grass is like a full size taking off on a stubble field. Even tarmac is quite rough for a model plane.
Of course you must exceed the stall speed to take off at all so if it can be calculated it will give an idea of the minimum take off speed but most will exceed this by quite a margin to ensure the plane gets away cleanly with little danger of stalling.
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Old 03-23-2018, 10:08 PM
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ron_van_sommeren
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Advice for Getting Into Flying Radio Controlled (RC) Airplanes - The Ampeer Newsletter


Some well-structured reading and handy e-tools for rainy/windy days.
Will save you, and us a lot of questions. Notably the 'what went wrong?' kind of questions
Will also prevent you from burning up several controllers and/or motors and/or battery:
E-flight primer and tools

And please, do your RC equipment, wallet, ego, battery, controller, motor, house/garage/car a big favour ... get a watt-meter. It will more than pay for itself, will save you at least one fried motor and one fried controller. Will also help you finding the best setup.
And keep watt-/multi-meter wires short
too long wires batteryside will kill ESC over time: precautions, solutions & workarounds

Last edited by ron_van_sommeren; 03-24-2018 at 02:56 AM.
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