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Dec 15, 2007, 12:56 AM
Registered User

The basic way to choose a power sytem

The following steps help you choose a power system quickly without going into details. This is very helpful not only to those who are still confused with it, but also to those who want to convert a glow powered model into an electric powered one.

Problem: You have a 6 pound-sport model and you want to find out which motor/Battery/ESC/Prop you should use.

Solution: step 1- Formula: As a rule of thumb, a 165 watt- brushless motor is needed to make a 1.25lb-sport model fly and perform some basic aerobatics (132 watts/Lb).

step 2- By applying the above formula, a 792 watt- brushless motor is then needed to make your 6lb-sport model perform some stunts. 6 lbs x 165/1.25lbs = 792 watts ; (132 watts/Lb).

step 3- Then, you shop online or go to your LHS to pick up a 800 watt- outrunner motor. Because you have a sport model, but not a pylon racer, you may need a 800 watt-motor that has a low kv. A lower kv motor can spin a bigger prop to produce thrust that has a large airstream ; most likely, you need that type of thrust to enjoy your 6 lb-sport model. Once you get the motor, you will figure out the right battery/Esc/ prop that you should use. Usually, the prop size is specified by the manufacturer in the motor manual depending on the type of the model.

step 4- Now, you have the right motor, so which esc and pack do you have to use? By using this formula ( Power= Volts x Amps), a 800 watt-electric motor that spins a manufacturer recommended prop should deliver approximately 800 watts of power or a little bit more when you hook it up into a 14.4 volt-battery or a 4 cell-lipo. If you do the maths, (Amps= Power/volts), the current draw should be around 58 amps. Therefore, with the right prop attached to its shaft, this motor needs a 4 cell-lipo and a 65A to 70 amp brushless esc in order to produce 800 watts, which is the power required to fly the model. Even if the motor draws 58 amps at full throttle, we need to use a 65 to 70 amp ESC to be on a safe side. As a reminder, an ESC controls the amount of amps going to the motor depending on the throttle stick position, but it DOES NOT RESTRICT the motor FROM drawing the amount of amps, which may exceed its rating. The max or the average amps drawn by the motor are in function of the diameter and pitch of the prop that its shaft is spinning (load). So, we want to make sure that the amps drawn by the motor do not exceed the ESC rating. Many modelers are still baffled with it.

step 5- Finally, let's find out the capacity and the discharge rate of the pack. Because the motor draws 58 amps at full throttle, the capacity of the 4 cell-lipo (14.4V) should be at least 6000 mah and it should be able to handle a discharge rate of 10C to 12C. Why is that? As a reminder, the amount of amps drawn by the motor tells you how much capacity your battery SHOULD HAVE in order to supply enough power to the motor. In other words, if you know the amps drawn by the motor, you can calculate the capacity and the C rating of the pack that you SHOULD use. Why is it better to use 6000 mah 12C rather than 2100 mah 30C pack to power up the 800 watt -motor? That because the motor pulls 58 amps at full throttle and by doing the maths, the motor running time will be only around 2.25 minutes if you choose to use the 2100 mah pack. On the other hand, the motor running time should be around 10 to 12 minutes before the BEC shuts it off if you use the 6000 mah 12C pack. The battery that you should use therefore depends on the amps drawn, but not on your preference which is based on your subjective analysis of the power system. Many modelers are still confused with it

So, you need the following parts in order to fly your 6lb- sport model :

- A 800 Watt-outrunner motor with the manufacturer recommended prop (low kv)

-65 or 70 amps brushless ESC

-4 cell 6000mah 12C lipo

The hobbico Electristar EP that weights about 5.5lbs is another good example of this set up except the fact that it is powered by 14.4V nimh pack. The hobbico technical team did some calculations before powering up this model.
Last edited by Hasina75; Feb 04, 2008 at 03:40 PM.
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Dec 31, 2007, 10:28 AM
Registered User
Thanks for the Guide. I'm a 66 year old beginner and needed this kind of info to power up my J-3 Piper Cub with a 83" wing span. I hope I know what I'm getting into.
Thanks Again
Dec 31, 2007, 05:17 PM
Registered User
Good luck on your project. I have the World Model .25 size J3-cub. It is powered by a OS.25 LA.
Another method to choose a power system is to compute the thrust to weight ratio. If you know the weight of your model and the amount of thrust that can be produced by the powerplant you have chosen, then you can tell whether or not that power system is good for the model by computing the thrust to weight ratio. However, this method can not be done without a thrustmeter. Some people just use a scale to measure the thrust of a motor with the right prop attached to its shaft.

As a rule of thumb, you need at least 1lb of thrust to make a 3lb model fly. In other words, the thrust to weight ratio should be at least (1/3= 0.33 )to make a model fly. 0.5 is needed for acceptable performance and 0.57 is needed for agressive flying. A model aircraft can not be flown if the thrust to weight ratio is less than 0.33. I think you need 0.57 for your J3 cub because it is a scale.

As we discussed earlier, 792 watts of power is needed to fly a 6 lb model.
Now, by using the thrust to weight ratio formula, we need 2 lbs of thrust to fly that 6lb model.
Therefore, the relationship between thrust to weight ratio and watts is that a 792 watt-motor that delivers approximately 2 lbs of thrust is needed to fly a 6 lb model
Last edited by Hasina75; Jan 20, 2008 at 02:03 PM.
Jan 15, 2008, 08:53 PM
Crash test pilot trainee
Ok. This may have been covered somewhere already but, I have really been looking. Once u have your motor and esc for said motor...What is an allowable battery amp wise that you could use? Is a system built around a 54 amp motor with a 65 amp esc for example be damaged by a battery with the capability of 100amps? If I understand this right, only the esc can determine how much of the amps are delivered to the motor? If not, what am I missing? Besides weight factors what is to much amp from a battery if there is one on any given build?
Jan 15, 2008, 10:48 PM
The Revegetator
Chris F's Avatar
Hi MUTDOG, the main part of the power system that dertermines the Amp draw is the prop size, because it is the prop that creates the load on the system. To determine the battery for your model you should have some idea of which motor you will be using. The specs for the motor will tell you the maximum Amp load and possibly the maximun efficiency. Your ESC should be determined by the Max Amp load of the motor (or the maximum Amp draw you intend using) plus a safety margin of 5 to 10 Amps. The battery requirements for your power system will be determined by the C rating and it's Capacity (mAh). Read through the C Rating and mAh sections again and it will show you how to work it out. A battery capable of 100 Amps will only deliver what the power system needs up to 100 Amps and no more so no damage will be done to the power system. You will damage a lipo pack if you exceed the C rating and/or over discharge the pack.
Jan 16, 2008, 12:24 AM
Crash test pilot trainee
Thank you sir. That last sentence put it all together for me. Now its very clear.
Jan 24, 2008, 10:03 PM
Registered User
I am understanding how to figure out the sizes and types of motors and how batteries and motor create different amounts of power, but I am still confused on why you would need to use a gear box when there are many motors that seem like they would produce enough power for even large models. I just annot understand or "picture" why you would gear a motor.

For instance. I am converting a GP Super Sportster from a 550 brushed geared motor to a brushless with 3 cell lipo.

Why did they use a gearbox to decrease the rpms when useing the 550 can motor?

Why would I NOT gear an inrunner?

I have decided to use a Eflite Power 25 outruner to power it, but I am still uncertain why. I just followed the advice from many different sourses, and I would like to learn WHY?

Just tryng to learn as much as possible

Super sport weighs 3.5 lbs
Low wing semi-symetrical
42" wing span

Thanks for any help.I am trying to keep up. the more I read and in different ways the more I learn.
Jan 25, 2008, 06:22 AM
Proud member of LISF and ESL

When And Why Would You Use A Gearbox?

We are going to discuss why we would consider adding a gearbox to a brushed
electric motor.


I am going to get real loose with the words "gear ratio" for a moment, but try
to follow me. Think of gear and gear ratio as the way we adjust the load on the
motor. I can adjust the "gear ratio" on my motor/propeller set-up in one of two

1) change the propeller
2) add a gear box and change the propeller

The goal is to get the motor spinning, at full power, at its optimum watt range
so that we do not over burden it, but so that we get the power to the propeller
efficiently. We are trying to get the best balance between pitch speed, thrust
and current draw.

If I increase the diameter of the propeller while holding the pitch constant I
put a greater load on the motor. A 10X6 prop puts a greater load on the motor
than a 9X6 prop. It will cause the motor to draw more power, more amps. At the
same time, it may load it enough that it causes it to slow down. Its peak RPM
may will be less. This is similar to changing gear ratios on your bicycle.
You can feel the effect in your legs.

If I deepen the pitch on the propeller while holding the diameter constant, I
also increase the load on the motor. A 9X6 going to a 9X7 going to a 9X8. In
this case I am increasing the "pitch speed". Again, this is similar to changing
the gear ratio. As I go to a deeper pitch the current draw will increase, the
watts increase and we may again load the motor enough to decrease its top rpms.

If I go too wide, or too deep, I can overload the motor and burn it out.

So, on a direct drive set-up, no gearbox, I tune my propeller between pitch and
diameter to get the motor to the power range I want. Again, this is EXACTLY the
same as changing gear ratios, in practical application.

To some extent I can trade pitch for diameter and vice versa. So you will see
motors listed as accepting a range of propellers. Typically as diameter goes
up, pitch goes down.


For this sample motor, each of these props will probably produce a similar watt
output but they do it with different results.

The wider prop will provide more thrust but the lower pitch will produce less
speed. So I can tune for the application. Sailplanes typically want more
thrust for steeper climb but are not as concerned about speed. Pylon racers
are less concerned about climb or acceleration as they are about top speed.
Hopefully you get the idea. I am tuning the "gear ratio" by changing the prop.

If you are not with me up till now, then ask because what comes next depends on
your understanding what is above.


Now, suppose I have a given motor, say a brushed 550, and my prop choices don't
give me the thrust I want to take my 2 meter sailplane up at a steep enough
angle to make me happy. It takes too long to get to soaring height. Or,
suppose I want to fly a larger, heavier plane with the motor I have. My prop
choices don't give me enough thrust to handle the heavier plane. What do I do?

I can put in a gear box. The gearbox will have two effects. It will reduce the
top speed to the prop, but it will increase the torque available to turn the
This allows me to go to a wider propeller but my top speed will be reduced. Now
I can get an steeper climb, or perhaps I can fly a larger or heavier plane. I
am going to stay with the sailplane for the rest of the discussion, but it
applies equally to any kind of aircraft. We are talking gear ratios.

Again, using the bicycle example, you shift to a lower gear to go up the hill.
You can
get up the hill in first but if you were to go to third you might not have
enough power in your legs to turn the pedals. So you tune the gear ratio to
match the available power.

A typical prop on a 550 motor in a sailplane, like a Goldberg Electra would be
an 8X4 prop. That is the widest prop, the highest thrust prop that this motor
can comfortably turn and provide enough speed to fly the glider. The motor will
likely pull about 18 amps on an 8.4V pack. It will fly the plane but the climb
angle might only be 25 degrees. So it might take me 2 minutes to fly up the
height I want to reach. This plane isn't really made for speed, so going to a
7X6 prop, trying to get more speed, won't help.

But if I put a gear box on, say a 3:1 ratio, I can go to an 11X8 or a 12X7 prop.
Now I get a lot more thrust and the plane will climb at a 50 degree angle. Now
I get to height in less than a minute and the motor might only be pulling 16
amps. I climb in less time AND I may be drawing fewer watts to do it.

That is why we go to a gear box. Usually it is to allow us to swing a wider
prop at a slower speed in order to get more thrust at the sacrifice of speed.

I hope that helps.


Because we have two motor types in the brushless world we add flexibility and
complexity. More choices means more to decide.

The gearbox discussion with a brushless inrunner is exactly the same as for the
brushed motor above, so I won't repeat it.

However if we look at outrunners vs. inrunners we see that outrunners tend to
spin slower/volt with more torque. this has a similar effect to having a
gearbox on an inrunner. So how do you decide?

1) Personal Preference
2) Mounting restrictions
3) Available motor choices

Some people don't like gearboxes. It is another thing to maintain and another
thing to break. Also gearboxes tend to make noise and some people don't like
that. however there is nothing spinning around inside the plane with a gearbox.
So you can mount the motor/gearbox without regard to clearance as long as you
have adequate air flow. You can just clamp a gearbox/inrunner to the frame of
the plane and you are done. I have seen motor/gearboxes left loose in the nose
of the plane. The Multiplex Easy Glider is set-up this way. No mount at all,
it just sits there.

Outrunners need space. You have a spinning can that must be protected from
contacting another surface, lose parts, wires, etc. Grass, string, stuff can
get caught on that spinning can. In some cases this could be a problem, so a
gearbox might be preferred.

I have read that brushless inrunners are typically more efficient than
outrunners. Even with the gearbox losses I have read that inrunners are still
more efficient at turning those bigger props. So, if that is true, and if that
matters, it could shape your decisions.

Again, I hope that helps.
Last edited by aeajr; Jan 25, 2008 at 06:42 AM.
Jan 25, 2008, 03:55 PM
Registered User
Thanks for helping me visualize the situation. It realy helps to hear it as it pertains to you own personal questions. I understand gearing/proping much better. Now I just need to use it in real life situations. Thanks for the time you put into your response.

The follow up question I have is

Are thrust and RPM equal? It seems that you can spin a smaller prop faster and achieve the same amount of thrust compared to spinning a larger prop slower.

I might be killing a dead horse!, but it just seems like the "same difference"
Jan 25, 2008, 04:30 PM
Proud member of LISF and ESL
I don't know if I can answer that perfectly. There certainly is a relationship. However the issue may be, can you spin the prop fast enough. Air is a fluid, like water, and can only move so fast. You can spin a prop so fast that the air can not keep up.

I the world of water propellers, we call this cavitation. I am not sure if the same term applies. When a prop cavitates, it loses bite. A vacuum forms around the prop and thrust drops off rapidly.

It is much more efficent to spin a bigger prop slower to generate thrust than to spin a smaller prop faster.

Hope that helps.
Jan 26, 2008, 10:53 AM
Registered User
Once again I have not heard this discription before, and Now I feel like a moron because after hearing you explain it I realized the dumb question I just asked. The same principals apply to Nitro motors also, and as youi said Boats, and any other propeler. Thanks
Jan 26, 2008, 04:16 PM
If it flies, I will crash it
mxspode's Avatar
Well, I have been slowly poring over these forums for a while now and I have managed to gather allot of info. This thread may however be the best yet. Not by the amount of info in it but by the amount that I was able to understand after reading it. aeajr last comments really helped clear up a bunch of confusion and specifically as it regards props and gear boxes. Pitch and diameter and pitch speed all make sense now. I also finally understand the motor charts at the GWS web site and I am feeling a little less stupid for it.

I now at least have some confidence that the random assortment of parts that I have can be assembled with the proper finishing parts to yield a working and flyable plane. Feeling better about this hobby all of the time.

Thanks to all of the contributors in this thread. It was a great read.

Jan 28, 2008, 02:41 PM
Registered User
The follow up question I have is

Are thrust and RPM equal? It seems that you can spin a smaller prop faster and achieve the same amount of thrust compared to spinning a larger prop slower.

Poolpatrick> You asked a good question. I will give you an example, which is based on a real life situation in order to answer your question clearly.

The amount of thrust produced by a 10 x7 " APC-EP prop that spins at 10200 rpm is the SAME AS the amount of thrust produced by a 5 x 3.5" APC-EP prop that spins at 20400 rpm. HOWEVER, the diameter of the airstream produced by the 10 x7" prop is larger than that of the 5 x3.5 " prop even if both props props produced the THE SAME AMOUNT OF THRUST. So, you may think that you can use both spinning props for a pylon racer and you'll achieve the same amount of speed , not really :-). A pylon racer will fly faster with the 5x3.5" than the 10X 7" prop EVEN IF both props produce the same amount of thrust . That because the diameter of the airstream produced by the 5x3" prop is SMALLER than that of the 10 x7" .

In other words, for the same amount of thrust at different RPM, a small airstream produced by a small prop is needed for speed while large airstream produced by a large prop is needed for higher climbrates and to hover and to fly around. That's why, we use small props for pylon racers and speed wings; and, we use big props for slow flyers and 3D models. So, when we talk about thrust, we have to refer to the size of the airstream, which is in function of the prop diameter, but not to the amount of thrust itself. The question that we should ask is, " does this amount of thrust has a small or a large airstream? Then, the size of that airstream tells us for which type of model we have to use that thrust. Often, we are meaningless when we say a higher thrust means a higher speed without specifying the prop size and the type of model.
Last edited by Hasina75; Jan 28, 2008 at 04:06 PM.
Jan 29, 2008, 01:22 AM
<- Balsa flies better ->
wolw's Avatar
A small note,
The discussion here is about static thrust, its not the same once airborne.

When choosing a prop I only give thrust a quick glance to see if it looks resonable, then I mock around until I find a nice prop with the right pitch speed compared to thrust suitable to the models weight and the specifications of the motor.

Playing with numbers can be fun but they're no that exact.
I would say that the 132W/lb power/weight recommendation is "too much" for a regular sports model, personally I like lots of power but to say that it's a "rule" and "needed" I can't comply with. Going for higher W/lb ratio makes your plane heavier too.

My experience tells me that the 100W/lb is a quite correct recommendation for a sports model, more is fun

I prefer to power after wingload instead, my old PC-9 had about 18oz/ft which puts it between a trainer and a sports model.

To illustrate: My PC-9 @ 92W/lb (never mind the tree I got a proper airfield now).

Seagull Pilatus PC-9 flight 2 (4 min 15 sec)

Last edited by wolw; Jan 29, 2008 at 09:33 AM.
Jan 29, 2008, 04:28 AM
Proud member of LISF and ESL
Originally Posted by wolw
My experience tells me that the 100W/lb is a quite correct recommendation for a sports model, more is fun

I prefer to power after wingload instead, my old PC-9 had about 18oz/ft which puts it between a trainer and a sports model.

I have never heard this measure before. Peter, can you expand on this "power after wingload"?

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