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Torque and its related minions have contributed to a number of
crashes. The pilot starts twisting under the risers, usually to the left
which pushes him left and decreases forward thrust. The wing goes right
in an unwanted right turn. At this point, the pilot is confused—he's
pointed left, the wing is right, he's slowing down and not climbing. The
lack of climb makes him stay on the throttle—exactly the wrong reaction.
In extreme cases, the pilot pulls so much left brake to stop the turn
that he spins the wing. Ouch.
Why? There are two powerful forces and a bunch of nearly irrelevant
ones. It's not terribly important to know what they are but it is
important to 1) recognize when it's happening and react accordingly, and
2) set up your machine to minimize the effect.
We'll assume your prop spins counter-clockwise as viewed from the
rear. Nearly all belt driven machines, which is nearly all of the large
ones, spin this way.
Quick overview
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.
What to Do
First and foremost practice what to do if it starts: throttle back!
Reduce power, reduce brakes. It's better to land straight ahead or with
minor turns than it is to crash on your side at full power. That's
expensive at best.
Second is to make sure your machine is adjusted to minimze all
twisting effects. Find an instructor or experienced pilot who
understands these concepts and implement the suggestions.
Good luck and fly straight. Well, mostly straight.
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