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Twisting Under the Risers

Any force that can twist the pilot under the risers will redirect the thrust. This becomes VERY powerful and can quickly render the craft uncontrollable!

There are several forces that cause this. 

Lean-Back: Anytime the motor is leaning back (not hanging vertical) there are two forces at work; P-Factor, a very minor force, and the horizontal component of torque - a very powerful force.

1. "P-Factor" happens whenever the propeller is not hitting the air head-on. When angled back, the descending blade is taking a bigger "bite" of faster-moving air and so produces more lift. It is a very small force for PPG's because it is based on the speed difference created by forward motion. We just don't have much speed to give much difference.

2. The Horizontal component of torque is just the motor's torque that acts on the horizontal plane. Hang the motor so that the thrust line is pointing straight down (like the pilot at right) - and the spinning prop will try to twist the pilot under the risers. As you angle the motor up, this becomes less of a factor until it goes away completely on an upright motor.

Asymmetric: Another powerful force that can arise is Asymmetric thrust. It happens when the motor's thrustline is not centered on the pilot's back. Every time he powers up it's like someone shoving him on a shoulder blade - he wants to twist. While powerful, it is usually the easiest to fix.

Gyroscopic: This is almost negligible but it can play a small part during the take off run. The odd trait of spinning bodies that keep a top top upright works on our spinning prop too. If you apply a force perpendicular to the plane of rotation (like pushing down on the edge a spinning bicycle wheel) the force will act 90 degrees away in the direction of rotation. If, when launching, you go from being upright to leaning back, the motor will want to twist due to this force. But once leaned back, this force has no beaing.

Fixing It

In flight, the only fix is to reduce power. Reduce it gradually then hold it for to let any swings stabilize. Modulating the power can aggravate the problem, causing you to swing all the way around so that the thrust is pointing backwards. Oops.

Better yet, fix it on the ground. If you hang back too much, adjust. If the motor is not centered adjust. Note that some machines might hang fine with centered thrust but then the torque moves the motor over. Hang in a simulator and have someone physically twist the motor to see if it moves. If so, secure it, usually an easy operation that can be done with the straps or wire ties.