If you do what you've always done, you get what you always get.

In this video a Pilot takes an asymetric collapse and comes in hard. Fortunately without serious injury. But does he blame it on airfield turbulence or does he understand the real reason?

One simple thing that the pilot does causes the collapse to occur by introducing three changes to the forces involved in flight.
1. Gyroscopic Precession.
2. Asymetric Blade Thrust
3. Thrust Vector.

All three are involved when he brings up his knees and changes the direction of the thrust and unloads the wing asymetrically whilst lowering the angle of attack.

The following are extracted from an article originally appearing in "Skywings" magazine, June 1997.
Authored by Michel Carnet, Silver medallist in the 1995 FAI European Paramotor Championships

Gyroscopic Precession:

A spinning propeller (also your crankshaft and your flywheel) acts as a gyroscope, which tends to initially resist any forces attempting to change its 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, 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 this can 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 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!!!!!

[b]Asymetric Blade Thrust[/b]
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. [/quote]

[b]Thrust Vector[/b]
Your prop can be thrusting in line with your [url=http://paramotorclub.org/forum/viewtopic.php?p=3668#3668]trajectory[/url] or you can angle it to thrust up or down by up to 30 degrees. This changes the [url=http://paramotorclub.org/forum/viewtopic.php?p=3668#3668]Aerodynamic Load[/url] This force [url=http://paramotorclub.org/forum/viewtopic.php?p=3668#3668](See "Glossary - Forces")[/url] is what determines the airspeed of the craft. If you keep the thrust (throttle setting) the same and angle the prop downwards (as in this video) you slow down. When you slow down the wing (under its own inertia) moves ahead of you a little and this reduces the [url=http://paramotorclub.org/forum/viewtopic.php?p=3668#3668]angle of attack[/url] momentarily. A low angle of attack makes the wing more vunerable to collapse.

<img src="http://www.paraglide.uk.com/Images/parajet/ThrustVectorForces.jpg[/img]

The last prop effect is the well known "Torque problem". To an extent this is eliminated in a swinging arm machine when in a steady state (flying on a straight trajectory at constant speed) but can still have effects when increasing and decreasing thrust.

[quote][b]Propeller Torqe effect.[/b]
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.