[smark]

Ok, another question about electrics.

I've noticed in the magazines and online that out runner types of electric motors are fairly common. Yet there are numerous references to installing a gearbox and swinging large high thrust props.

1: For what type of aircraft/ flying would it be better to gear the motor rather than leave it as a direct drive and'

2: How do you choose a gear ratio.

Okay that's two questions, but the second would have come up sooner or later.

[FaceDeAce]

I am no expert, but I look at it this way.

Prop choice depends on the performance you are looking for. Fast runs once spooled up or moderate speed that pulls like a tractor. I always run the largest diameter prop possible - I like tractors.

Take the kV of the motor and multply it by the voltage of the battery you are going to use. Then multiply by 0.70 to simulate a loaded motor. If the result is greater than 12,000 rpm then you are entering the realm where you should consider putting a gear box on it.

The gear ratio chosen is dependent on the results of the above calc.

[Mike Anderson]

Another way to look at it is the RPM limit of the three most common motor types

1 - DC (Brushed) (all inrunners) Max practical RPM is about 20,000
2 - AC (Brushless) Inrunners - typical max RPM is 50k to 60k
3 - AC outrunners - typical max rpm at 15k

Now consider that every motor's efficiency increases with RPM and you will see that you want to spin a motor as fast as you can. But that may be too fast for the pitch speed you want or just plain crazy (60k!) so you will need to gear it down.

The ratio would be the high-as-you-dare motor RPM divided by the desired prop RPM that gives the best combo of thrust & pitch speed and that is a mission-related thing.

Starting to see why performance calculation programs like MotoCalc are so popular?.

[moo]

Originally Posted by Mike Anderson

Now consider that every motor's efficiency increases with RPM

But propeller efficiency decreases with speed.

Consider also that gears and extra bearings waste power too, so you would tend towards a low kV outrunner for best efficiency (and flight times), and a smaller battery for better overall performance (lighter weight).

[tommy321]

Originally Posted by moo

But propeller efficiency decreases with speed.

I disagree.

Any fixed pitch propeller will have a peak efficiency at a single advance ratio. Advance radio is V/(n*D) where V is forward velocity, n is RPM, and D is diameter. If you think of a propeller as a spinning airfoil, it will be most efficient at the Angle of attach that provides best L/D. Propeller blade angle of attack on a spinning propeller depends on forward speed and rotational speed.

So as you increase RPM, you can maintain peak propeller efficiency by increasing forward speed as well. (i.e. Efficiency will be reduced if you increase or decrease RPM away from the peak efficiency advance ratio.).

http://selair.selkirk.bc.ca/aerodyna...nce/Page8.html

Scroll down to the propeller efficiency section.

So an inrunner can be much more efficient than an outrunner if you setup your gearbox to allow the motor to spin near its maximum speed, and you pick the best propeller for your speed and application.

Tom

[tommy321]

Back to your original question though...

When would you go with a gearbox? It's a little like asking when would you go with a 4-stroke, or when whould you go gasoline. It's mostly personal preference. But...

1) If there isn't an outrunner that fits your application.

2) If you are a real stickler and want to gain a couple percent efficiency by doing math and getting your motor and propeller spinning at the perfect speed.

3) If you have a drawer full of weird sized propellers that you want to use.

4) If you're a reasonable pilot so that you're not always crashing and stripping the gearbox.

If you answered yes to any of the above conditions, then an inrunner might be for you. Note that it doesn't really have anything to do with what kind of flying you're doing. The air doesn't know what is driving the spinning propeller. So if an inrunner/gearbox and an outrunner spin the same prop at the same RPM, the plane would fly the same.

I agree that additional weight and energy loss in the gears might be an issue if you're using a cheap gearbox (like a GWS, or the ones that come with Himmax inrunners - these are both cr*p IMHO). If you get yourself something like a Brute or Cobri gearbox the power lost to the bearings and gears will be negligible (again MHO).

Now, the increased efficiency will probably only result in a couple additional seconds of airtime on most airframes. I say, experiment till you find something you enjoy flying.... and fly it.

Tom

[smark]

Thanks guys for all your responses.

Given the universal axiom that I never get what I really want, I think the never ending quest for the last percentage increase in efficiency is for some one else and I will stick to ±5%.