PROPELLER TORQUE EFFECT (PTE)
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 prop is to a small extend being "held back", so that force is absorbed by the tendency of the motor to rotate clockwise. The amount of Propeller Torque Effect, depends on prop mass, prop diameter, prop pitch, and prop RPM (engine RPM over Redrive ratio), but is basically directly related to available power. More power, means (unfortunately) more Propeller Torque Effect. 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. You can counter this a number of ways, which we split into two categories:
a)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.
b)Active counters: Those the pilot can induce in flight: including leftside weightshift, left counter-steer, asymmetric trim setting (slower on left side), or differential speedbar (more weight on left side), or any combination of these.
Warning!!!! Any amount of brake you use in flight, especially on full power (e.g. takeoff and climbout), is too much brake! This is due to the thrust being several meters below the wing, resulting in a very positive angle of attack on the wing. 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! We try to takeoff allowing for the lots of spare space to the right, and allow the wing to slowly turn to the right during climbout, slowly circling the takeoff field, until safe cruise height is reached, then backoff power and resume straight and level flight.
The other two effects 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. This rotation can be one of two things: 1) Gyroscopic Precession, or 2) Asymmetric Blade Thrust
ASYMMETRIC BLADE THRUST (ABT)
Assymetric Blade Thrust can also cause the pilot/motor to turn away from the direction the wing is flying. It is caused by the blade disk not being vertical in flight: If, as is often the case, when doing a static hang-check, you find a prop in a vertical position tilts backward toward the top (i.e. the pilot and motor are tilting 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, their 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 on the left side (downwards), less on the right side (upwards). 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 the blades while on the left side, relative to the airflow, is increased (relative to a vertical prop disk), similarly the blade on the right-side 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, 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 thrustline aimed to the right of the flightline. This causes the wing to bank to the left, even though the motor is trying to turn to the right.
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.
ABT can lead to a continuos oscillation, with the wing rolling to the one side, until the pendular stability kicks in when you go "over-the-top", then rolling to the other side as your inertia takes you through the bottom of the "swing", then ABT pushes you back into the roll, which seems to get worse if the pilot attempts to do anything to counteract with the brakes.
Both are quickly counteracted 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.
GYROSCOPIC PRECESSION (GP)
A spinning propeller (also your crankshaft and your flywheel) 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.
If you try to push the bottom of the prop disk forward, 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 left 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 wingtip!!! This is dangerous, as your thrust is no longer forward (direction of wing's flight), but toward that wingtip. The right wingtip 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.
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.
The change in pitch (which leads to the precession or left yaw) could be from 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 siophic 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 catastrophe!!!!!
Another instructor had a student spin 16 times under his wing in under 2 seconds. The wing carried on 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 long 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... 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 flypast 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?
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