...as pertaining to powered paragliding.

There are three Primary Propeller Forces that all pilots of powered-paragliders should have an in-depth understanding of. These are the primary causes of accidents among new pilots. All PPG training courses should cover these in detail, both at theoretical level and to consider the practical application of these in powered paragliders.
Let me say this: without a thorough understanding and appreciation of these propeller effects, powered paragliding may be dangerous. Yet, with the proper mindset toward powered flying and sufficient training (practical AND theory), it may well be the safest form of powered flight.

The Three Primary Propeller Forces Are:

  1. Propeller Torque Effect (PTE)
  2. Gyroscopic Precession (GP)
  3. Asymmetric Blade Thrust (ABT)

All three forces produce strong forces on the motor, pilot and wing which must be considered at all times. These three forces (and others) also affect all propeller-driven aircraft, however these forces are particularly noticeable in powered paragliders, due to their compact nature. Conventional aircraft have large, rigid wings and fuselages, both firmly fixed to the prop via the engine, which offer great resistance to the effects of these forces in the form of mechanical moments, or leverage. A powered paraglider wing is NOT rigidly fixed to the motor. The maximum extent of the resisting components is only a few dozen inches, being the pilot's elbow to elbow laterally, and kneecaps (or toes) to propeller in length.

1. Propeller Torque effect

As the prop spins anti-clockwise (as viewed from behind) on most pusher prop configurations, the engine tends to want to spin in the opposite direction, due to the drag induced by the blades, as well as the inertia of prop, crankshaft and flywheel, during acceleration.
Imagine if you held the blades still (resisted their rotation) and pulled the starter rope slowly. Because the prop cannot turn, the engine will turn instead, albeit in the opposite direction. In a similar fashion, the spinning prop is to a small extend being "held back" by the drag it experiences with the air, so that force is absorbed by the tendency of the motor to rotate clockwise to compensate.
This results in the motor not being vertical, but will appear to be rolled over to one side by a certain amount.
If, as in most cases, the propeller spins anti-clockwise as viewed from behind, the engine will attempt to spin clockwise.This has the effect of lowering your right-shoulder, and lifting your left shoulder, which in turn produces a right-roll or banking effect on your wing, which then tends to turn to the right.

· The amount of Propeller Torque Effect, depends on prop mass, prop diameter, prop pitch, and prop RPM (engine RPM over Reduction-drive ratio), but is basically directly related to available power.
·More power, means (unfortunately) more Propeller Torque Effect, and more tendency for the wing to bank and turn to the right. However, there are ways to reduce excessive right rolls.
· The extent to which the motor banks in response to the torque effect, depends on how much resistance there is to this. Some resistance is a natural result of the risers trying to hold the engine level.
·Additional resistance to torque-induced roll can be built-in to the design, and there are certain measures the pilot can utilize. These "Torque Counter-Measures" come in two types:
· Passive counters: Those you design in, or set before take-off. e.g. set the right carabiner higher than the left, so that in flight, the two are more equal (or longer carab on right side); or pack any extra gear (tools, oil, camera, etc) on the left side; or your motor and/or harness may have some asymmetry built-in; or you can increase any cross-overs or cross-bracing present.
· Active counters: Those the pilot can induce in flight, including: left-side weight shift, left counter-steer, asymmetric trim setting (slower on left side), or differential speed bar (more weight on left side), reducing the power level; or any combination of these.

Warning!!!! Any amount of brake you use in flight, especially on full power (e.g. takeoff and climb out), is too much brake input!!! The only exception is just enough steering input for direction control. A powered paraglider flies best with ZERO brake!!!!! So, if your torque induced turn is excessive and you try to counter with just left brake, you will very likely stall the left wing with disastrous results!
Try to launch allowing for plenty of open space to the right, and allow the wing to slowly turn to the right during climb out, slowly circling the takeoff field, until safe cruise height is reached, then back off power and resume straight and level flight.
Primarily due to the Propeller Torque Effect, it is much easier to execute right-hand turns, than left-hand turns in a powered paraglider, unless the power is at a low setting. In some cases, it may be close to impossible to execute a sharp left turn without stalling the left wing.
Always plan low passes with an escape to the right! To summarize, Propeller Torque Effect causes the motor and wing to "Roll" or "Bank" to the right, inducing a right-hand turn. The other two Propeller Forces are often confused and mistaken for each other: If you rotate in the yaw axis until you are not facing the same way as the wing, you may be in serious trouble, especially if this happens just after launch.

3. This rotation can be one of two things:

1) Gyroscopic Precession, or 2) Asymmetric Blade Thrust

2) Gyroscopic Precession (GP)

A spinning propeller (also your crankshaft, flywheel and reduction pulleys) acts as a gyroscope, which tends to initially resist any forces attempting to change it axis of rotation.
If such a force persists or is strong enough, the spinning prop will NOT be deflected in the direction the force is applied, but in a direction 90 degrees further around its rotation.
Example: If you try to push the bottom of the prop disk forward (as in tilting it over backward), and the prop is spinning anti-clock as viewed from behind, the force will be effected 90 degrees anti-clockwise from the bottom, i.e. on the Right-hand side of the prop disk. So, trying to tilt the prop-disk over backwards, will result in the prop disk deflecting to the Left!
In paramotors, this happens mostly when the pitch of the motor changes. If your prop rotates counter-clockwise (as viewed from behind) as most pusher props do, then if you (and your motor) pitch upwards (i.e. lean backwards), then the motor will yaw sharply to the left, such that you may end up facing your left wing tip!!!
This is dangerous, as your thrust is no longer forward (direction of wing's flight), but toward that wing tip. The right wing tip will dip down sharply (being pulled by the thrust), and the wing will roll over to the right (starboard) and start turning to the right, but you are facing (and being pushed) to the left! This leads to a disastrous situation.
This is similar to the "Lock-out" problem experienced by pilots under Tow or Winch launches. I have seen this happen to many pilots without them being aware of its cause, nor its remedies. Most survive purely due to instinctively releasing the throttle.

Change in pitch (which leads to the precession or left yaw) could be caused by any of:
· Sharply applying brakes - the wing slows down, the pilot/motor swings forward of the wing and, having high attachments, leans over backwards, i.e. pitching upwards.
· Going suddenly from cruise (or glide) to full power - same effect as above.
· From a sitting upright (or leaning forward) position, to suddenly leaning back and lifting the knees to a more comfy position.

Unfortunately, sometimes pilots do all three in one motion, which can lead to a catastrophic situation. One instructor had a student spin 16 times under his wing in under two seconds! The wing continued flying out to sea, and the lines had 16 twists; the pilot had no input to the wing. He killed the engine and started unwinding, the wing carved a gradual curve back to shore, but he struck a building before regaining control of the wing, while flying downwind. A hospital stay and some serious injuries later, plus a motor almost written off, due to Gyroscopic Precession!

Here is something every paramotor pilot MUST do:

  • Take a bicycle wheel and hold it by the axle
  • Ask someone to get the wheel spinning quite fast
  • Then try to tilt the wheel over in one direction
  • You will be surprised by the forces and the inevitable result
  • You will find it IMPOSSIBLE to tilt the wheel in the direction intended - the stronger force you apply, the more powerful the deflection in another direction
  • Try to predict which way the wheel will deflect with various forces
  • Then try the same with the wheel spinning in the other direction

Bear in mind that a propeller has much more rotational energy than your experimental bicycle wheel and will have much more precession forces.

The only thing a pilot can do when precession kicks in, is to gradually back off the power and wait for recovery and level flight, then slowly re-apply power. Just hope this does not happen near the ground. Unfortunately, it is most likely to happen just after launch when you least can afford to back off the power.
POP QUIZ: just to see if you understand the cause and effect of GP: Imagine doing a low fly past and the left side of your propeller-cage brushes against a bush on the ground (heaven forbid this really happening), and so tends to swing the motor and pilot sharply to the left. What will the resulting effect be from Gyroscopic Precession?

3. Asymmetric Blade Thrust (ABT)

Asymmetric Blade Thrust can also cause the pilot/motor to turn away from the direction the wing is flying. It is caused by the spinning propeller disk not being vertical in flight:
If, as is usually the case in flight, your propeller is generally tilted backwards (i.e. the pilot and motor are leaning backwards), then we have Three contributing factors: Assume the prop turns in the same direction as above.

1. First Factor: Each blade, as it travels from the top downward, will sweep forward (relative to the engine). Equally, on its upward travel, will sweep backwards. In flight, the airspeed of the blades will differ as follows: As the blades travel downwards (and sweeps forward) on the left-side, the blade-airspeed will be the Flying Airspeed, plus Rotation Velocity, PLUS forward sweep speed. On the right-side, as the blades travel upwards, their airspeed is Flying Airspeed, plus Rotation Velocity, MINUS rearward sweep speed. So, each blade has higher airspeed while descending on the left side, but less when ascending on the right side. This induces more thrust on the Left Side, which tends to cause the motor to yaw to the right.

2. Second Factor: As the motor is tilted over backwards at the top, the Angle-of-Attack (AOA) of each blade, while descending on the left side, is increased (relative to a vertical prop disk); similarly while ascending on the right-side, each blade decreases in AOA.This also causes more thrust on the left side of the prop disk, and less on the right, causing a right-yaw, adding to the first factor.

3. Third Factor: Usually in a paramotor, the pilot's body shields some of the airflow to the prop. As the motor (and pilot) yaw to the right as a result of the above two effects, the airflow comes increasingly from the left of the pilot's forward line, exposing more of the left side to cleaner air (less shielding on left) and causing more of the right-side to be shielded from clean air. This also causes increased thrust on the left side, and reduced thrust on the right side, further adding to the first two factors. The result, is a constant thrust line aimed to the right of the flight line. This causes the wing to bank to the left, even though the motor is trying to turn to the right.

Typical symptoms of this problem, is where the pilot experiences regular and continuous oscillations from right to left and back and over again, without any control input or turbulence. Often, attempting to prevent these oscillations with brake input just worsens the situation. This is caused by the ABT pushing the motor over to the right of the flight line, the wing initiates a left bank without any left brake, until the natural lateral pendular-stability takes over and the pilot swings back to the bottom of the wing, but inertia causes him to overshoot to the other side inducing a right-bank, starts recovering, but then the ABT pushes him over again, leading to a continuous side to side swinging action. The only remedy is to remove the cause of the ABT by ensuring the propeller disk is closer to vertical in flight. This is usually just a case of adjusting the trim of the harness or moving the attachment points further backwards.

The difference between the two major effects, GP and ABT, is that the first is a momentary force, which disappears after the changing pitch, whereas the second is an almost constant force, with the pilot/motor combination almost continuously facing to the one side of the wing's flight direction. The first, GP, can suddenly appear sometimes unexpectedly, and can surprise the pilot in its extent and severity. The second, ABT, is almost constant, depending on the extent to which the prop disk is tilted relative to the vertical. Both are quickly contracted by reducing power. Read that line again!

GP can be prevented by a healthy understanding of precession and preventing sudden changes in pitch. ABT can be prevented by a proper static hang-check, and setting the attachments points and/or Centre of Gravity to ensure the motor (and prop) is as close to vertical as possible. Remember that even a perfectly vertical static prop will tend to lean over backwards by as much as 15 degrees due to the thrust being 6 or 7 meters below the drag (length of lines separating motor/thrust from wing/drag). Thorough understanding of the three major propeller effects (Torque Effect, Precession and ABT) should be part of every powered paragliding training course. They account for the majority of incidents and accidents. I hope this helps prevents a few incidents; and assists the general knowledge, safety and enjoyment in this fledgling recreation aviation sport of powered paragliding.