francis rich wrote:There is another propeller effect that may be a factor in this; "asymetric blade thrust". The prop disc is tilted back and the right side of the prop disc on the upsweep has a higher angle of attack (to the incident airstream) than the left side on the downsweep. The right side is thrusting more strongly than the left. As the pilot's body shields the left side more and more from the airstream the effect increases.
Interesting comment Francis, care to elaborate on this asymetric blade thrust, something I know nothing about, but would like a better understanding of it, I did look it up- seems it doesnt come into play much on a paramotor:
'Torque is what happens when you spin a prop in the air. The engine/pilot tries to spin opposite. Just like when drilling into a chunk of wood and your hand wants to twist opposite the drill bit. In this analogy, the wood is air, the drill bit is your prop and the drill is you.
Note that the closer the ropes (risers) are together, the less force it takes to twist up in them.
The PPG Bible covers this topic thoroughly but here's a summary. Torque acts in two planes, around the vertical axis (but only if the motor is tilted back) and around the propeller axis.
Lay a paramotor flat on its back, hanging by ropes attached to the carabiners, so the propeller plane is horizontal with the floor. Now throttle up. You'll spin like a twister. Angle the motor up a bit. You'll still spin, just not quite as much. Angle it up further, you'll still spin, just not quite as much. You get the idea. That's the effect of a motor hanging back and its called the Horizontal Component of Torque (HCoT).
Now hang from bungee chords with the propeller plane vertical. Throttle up. It wants to tilt you right. That's the riser shift or weight shift component of torque (WSCoT). HCoT is twisting your body left while WSCoT is tilting you right. Both are causing a right turn, though.
The real villain of twist is that thrust gets redirected, causing a bank and not propelling forward any more. Otherwise the wing wouldn't really care all that much.
What Matters
Here are the forces that matter in order of importance.
1. The most powerful twisting force for most pilots is the Horizontal Component of Torque (HCoT). It's only present on machines that tilt back, though. It's easy to minimize by decreasing the tilt back. On low hook-in machines, move the hang points aft. On others, make whatever adjustments are necessary to get the prop plane more vertical.
2. Offset thrust (OTh) is where the thrust line pushes on one shoulder or another. This may be caused by torque if it twists the thrust line over to one shoulder. What matters is where the thrust line is relative to the center of the risers. That's why numerous manufacturers of low hook-in machines move one riser out using a metal piece on their right swing arm.
The significance of this force is that it matters less how much power you have. If the thrust line is too far off, you'll twist wildly. Solve it by making sure that, even at full power, your thrust line stays centered.
The above forces account for nearly all torque related accidents, usually in combination with a harness setup that doesn't resist the twist.
3. Weight shift component of torque (WSCoT) has limited effect based on how much riser shift happens. Go up and do a maximum weight shift without brake input. How much turn do you get? On everything I've flown its easy to counter with brakes.
4. Gyroscopic Precession is barely relevant, mostly because it's so fleeting. Gyroscopic action will make you twist left as you tilt back. It only happens during the tilting. So as the wing lifts and tilts you back (if your motor is normally tilted back in flight), it will cause a brief twist to the left. But once you're leaned back, the effect goes completely away.
5. P-Factor, or Asymmetric Blade Thrust is essentially irrelevant. Not only is the force small for our craft, but on most machines it's actually trying to counter the other forces that cause problems.
P-Factor is where the descending blade takes a bigger bite of air and so pulls a little harder. Gyroscopic action reduces the impact of the force which is made even smaller by our slow speed. On airplanes with high horsepower, it's a big deal, for us, it's not.
6. Rotational Mass Acceleration is present during throttle up only. It's the least relevant of all forces since it's only present during that brief time that the prop is accelerating.
Its the force touted by geared redrive users who claim that torque is countered by having the motor's rotating mass run opposite to the prop's—a partially true statement that's nonsense since the force is fleeting and small. Plus, the prop has far more rotational inertia than do the motor's spinning parts so the pilot still feels a twisting during prop acceleration. '
Dean
Sun Sep 06, 2009 6:24 pm 
