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Old 02-23-2008, 05:30 AM
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AEAJR
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WHY CONSIDER A GEARBOX?
by Ed Anderson

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
ways:

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.

9X7
10X6
11X5

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.

ADD A GEARBOX

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
propeller. 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 about 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.


WHAT ABOUT BRUSHLESS INRUNNER VS OUTRUNNER?

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.

Summary

We can tune our power system by adjusting the "gear ratio". This can be done by
changing props to some degree. After that we go to gearbox systems to tune our
power systems to give us the performance we want.
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