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Weight shift arms have opposite bend for a belt-drive motor?


fuzzybabybunny
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I don't really understand why weight-shift arms need to have an opposite bend for motors that are belt-driven. Correct me if I'm wrong:

- motors, by themselves, spin counterclockwise.

- a reduction drive that uses a gear reverses the direction of spin of the prop, so the prop spins clockwise

- a reduction drive that uses a belt doesn't change the direction of spin of the prop, so the prop spins counterclockwise

So looking from the back:

- a prop that spins clockwise will induce a counterclockwise yaw in the cockpit (more pull on the left riser)

- a prop that spins counterclockwise will induce a clockwise yaw in the cockpit (more pull on the right riser)

The Miniplane uses a geared reduction drive for their Top 80 motor and the Minari motor uses a belted reduction drive.

http://www.miniplane-usa.com/pages/models.htm

It says: 

"Because the Minari is a belt driven engine, the ABM arms have the opposite bend to them, compared to the standard ABM to counteract torque produced by the motor."

What "bend" are they referring to? The S-curve bend?

Here's are the S-curve arms on the Top 80:

blackframe1.jpg

Here are the arms on the Minari:

beach1_640x.jpg

Ummm... they look the same to me? What opposite bend are they talking about?

Or is the "bend" they're talking about a bend towards the left or right, so that one riser gets more weight as the other?

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I'm going to think aloud on this - point out any mistakes.

With a belt drive the prop spins the same direction as the motor output shaft. With a gear driven motor it spins the opposite way.

Both the motor and the prop will create a torque. The motor against it's mountings as it works to spin the output shaft and the prop as it works against the air.

Now the motor and it's torque is contained by the frame structure, the only force acting externally to the paramotor is the prop against the air.

So a counter-clockwise prop (viewed from the rear) will try to spin the frame clockwise and will put more force downwards on the right hand riser. This is why the risers on the arms are offset to the right (looking down) in order to counter this by using my weight to put a bit more force on the other riser.

I'm guessing that the "bend" in the arms is to the left or right (looking down) to achieve the same thing.

NB I am totally ignoring angular momentum and gyroscopic precession..

Edited by Hodders
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its to off set the weight of the pilot. Side shift.

 

example. on the S arm of my belt drive machine. the top of the S that attaches to the wing is leaning right and the end of the S that connects to my harness is 1" to the left. Both arms are like this effectively sitting me 1" to the left to counter the right twist created by the anti/counter clockwise belt driven prop.

 

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