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I recently got into paragliding, but I am quite confused on how their controls work.

I am quite aware of how the control surfaces work on fixed-wing aircraft. Let's use for example an aileron. Lowering the aileron, increases the angle of attack of this wing, increasing lift, making the aircraft to roll.

On paragliders, things seem very confusing for me.

The theory

The control acts as a "brake". By pulling one brake there is an asymmetry in drag, causing the wing to yaw. Due to the yawing motion a difference in airspeed in the two sections of the wing causes a difference in lift, having as a side-effect the aircraft to roll too.

Thus pulling the right brake, makes the aircraft yaw to the right, and then roll to the right.

My thoughts

While one of the controls is pulled, the trailing edge gets lowered. This also lowers the aft-portion of the wing cord, increasing the angle of attack. I would expect that this would also increase the lift of this portion of the wing causing the to raise.

Thus pulling the right brake, makes the aircraft roll to the left.

For me, there seems to be nothing special with brakes. For example, in case of emergency (brake line failure), I can use the C-risers to alter the angle of attack of one part of the wing to turn. But then again the same paradox happens. I pull the right C-riser, the right wing has increased angle of attack, but still it dives!

How exactly do paraglider "brakes" work? What makes them behave as a brake, and not as an aileron?

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  • $\begingroup$ Moving the aileron doesn’t change AoA. Downward aileron increases the camber and upward aileron works as a spoiler, slowing air flow over the wing. $\endgroup$ – TomMcW Jan 17 '18 at 4:43
  • $\begingroup$ Related answer that addresses the paraglider issue (see below). To understand the relation between paraglider stability and paraglider control, you have to understand that an uncommanded turn involves some sideslip, and an intentional roll input also causes some sideslip. aviation.stackexchange.com/questions/53437/… . $\endgroup$ – quiet flyer Jul 8 at 19:12
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I think the answers so far neglect the curvature of the paraglider sail. Changing the lift at the tips creates an imbalance of the side force caused by the downward pointing tips. This side force will pull the sail sideways and cause the sail-pilot combination to roll.

To illustrate my point, I shamelessly copied this picture and added vectors perpendicular to the sail surface, like this:

Paraglider with force vectors

Now you will see why I used a photo: The tips of the sail are almost vertical so the side force caused by pulling the local trailing edge down has a sizeable lever arm with the center of gravity (which I assume to be near the pilot's head). @Ken's graphical representation hides this detail and fails to show the lever arm.

This side force will do two things:

  1. It pulls the sail sideways so it will shift away from a position vertical over the center of gravity.
  2. It causes the whole sail-pilot-arrangement to roll, in this case clockwise (from the perspective of the observer).

Next, the increased drag on the pulled-down side will retard this part of the sail, adding a yawing moment and yawing rotation which will now stabilize the bank angle. The whole paraglider turns. With the banked paraglider the side force component of the lift force compensates the centrifugal acceleration (rotating system of reference) rsp. creates the centripetal acceleration (fixed system of reference) which keeps the yaw rotation alive.

Also here the rolling motion is started by a lateral shift of the lift vector, but the vertical component of lift counteracts the desired rolling motion. Only the lateral force, which dominates due to the inclination of the sail, is responsible for the rolling moment.

Exiting the turn is achieved by doing the same, only in reverse.

Now for pitch control: Pulling the trailing edge down on both sides increases camber and lift, so the paraglider rises and the added drag retards the sail. This causes a pitch-up motion around the center of gravity and slows the paraglider down further. Relieving tension on the risers reduces camber, lift and drag so the sail accelerates and the whole paraglider pitches down around its center of gravity.

Therefore, any rotation is around the center of gravity. This does not need any hypothetical pendulum movement or "pendulum effect", which does not exist and is a fallacy anyways.

Oh, and one last word for the believers in the cult of the pendulum effect: I don't care that you downvote what you don't understand. Please try and argue logically and while observing the laws of physics. This, however, will involve the clear danger that you will lose your faith. Apologies for any loss of direction in your lives. That direction was wrong, anyway.

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  • $\begingroup$ OK, how about considering another mechanism? Pilot and parachute steady state flight (same velocity), pilot pulls left line, left side slows, pilot continues forward, right wing (initially less lift from lower AOA, now more lift from greater speed), paraglider rolls and pivots around draggier left wing. The trick is to make the dragger a poor lifter to avoid the initial opposite roll. Almost a spoiler? Cheers, from the "cult". $\endgroup$ – Robert DiGiovanni Jul 8 at 1:13
  • $\begingroup$ Very nice description of one version of the "pendulum effect" in action. Sideforce acting high above the CG will certainly create a roll torque. In hang gliders (and high wing airplanes) something similar exists, but has more to do with the wing's drag vector than a true sideforce vector. Anyway, nice explanation, and welcome to the cult. $\endgroup$ – quiet flyer Jul 8 at 3:05
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    $\begingroup$ @PeterKämpf I feel your frustration. Please don't give up, who else will fight the pendulum and winglet cult? $\endgroup$ – AEhere supports Monica Jul 8 at 16:23
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    $\begingroup$ @quietflyer I'm 90% certain you are trolling at this point, you keep twisting the meaning of pendulum to mean who knows what. " dihedral-like pendulum effect in sideslip"? Please. $\endgroup$ – AEhere supports Monica Jul 8 at 16:27
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    $\begingroup$ @quietflyer Wikipedia has been known to be outright wrong, so I invite you to check the history of that article and see the same arguments over meaning, lack of citations of any major literature in the field and overall simplistic reasoning without any analytical backup. Also a number of contributors challenging the use of the pendulum wording in literature, I know I have never encountered it in my career. "dihedral-like pendulum effect in sideslip" so a dihedral effect then? Or simple lateral stability due to tail volume, coupled with roll as it often is? Where's the pendulum? $\endgroup$ – AEhere supports Monica Jul 8 at 20:15
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While one of the controls is pulled, the trailing edge gets lowered. This also lowers the aft-portion of the wing cord, increasing the angle of attack. I would expect that this would also increase the lift of this portion of the wing causing the to raise.

That is correct.

But with the lift also the drag raises. This causes the parachute to turn into the direction where the control is pulled.

Since the pilots body does not want to turn (yet) it "swings" out in the former direction forcing the other side of the parachute to raise over the side to turn to despite the lift is less here...


But still... Shouldn't the "braked" wing raise, at least briefly and slightly, if there is more lift? – user3634713

Yes, but it is outweighed by the force of the pilots "swinging body" into the other direction. This is because while "swinging out" the pilots weight drags more on the inner "wing" (where the turn is heading to) and the "outer wing" the parachute is less supporting, therefore the ratio of supported weight to lift gets better then on the "outer wing". enter image description here

Also why is this not experienced in other aircraft with conventional ailerons? What is different in paragliders? – user3634713

A convertional aircraft has its "reference point" (where all the forces virtually impact) is at most at the same level (in height) where the wings are mounted. Therefore "swing effects" do not matter that much.

A para gliders "reference point" is more than one wingspan below the "wing level".

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  • $\begingroup$ But still... Shouldn't the "braked" wing raise, at least briefly and slightly, if there is more lift? Also why is this not experienced in other aircraft with conventional ailerons? What is different in paragliders? $\endgroup$ – user3634713 Jan 14 '18 at 16:22
  • $\begingroup$ The main difference in my mind is the flexibility of the paraglider. The weight of the pilot can pull differently on different strings while the pilot swings around. In an aircraft the pilot position would be fixed and the "strings" could also be loaded in compression. $\endgroup$ – Peter Kämpf Jan 14 '18 at 19:06
  • $\begingroup$ @PeterKämpf: I think the main difference is the distance of the "reference point" and the actual "turning center" when the object changes the bank angle. On an regular plane (even a Trike) they are close together, on a paraglider they are far away from each other which causes the swing effects and therefore changing forces on the "strings" supporting the pilot. $\endgroup$ – Timothy Truckle Jan 14 '18 at 22:41
  • $\begingroup$ Of course, the correct way to turn a paraglider is to weightshift in the direction you want to turn before you pull down the brake... $\endgroup$ – Dave Gremlin Jan 15 '18 at 22:09
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    $\begingroup$ @TimothyTruckle - No, no. There's no pulling involved. The standard technique for turning a paraglider is to first look in the direction you are turning to ensure it is clear, second shift your weight in your harness in that direction, finally tighten the turn with the brake on that side. You can actually turn a paraglider with both arms folded in front of you and just leaning into the turn - I'm sure we've all done it for our instructors during training $\endgroup$ – Dave Gremlin Jan 17 '18 at 20:56
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I've also recently started paragliding, and with some background in aerodynamics have convinced myself that the reason paragliders experience this counter-intuitive reaction (right turn from what would be called a left aileron deflection), is that the direction of the lift change is radial from the total vehicle CG, which is about at the pilots chest. Expanding on some of the comments above, imagine the lifting surface as an arc of constant distance from the CG (I realize that's not completely true, especially for high performance gliders, but it's close), with outward lift components distributed throughout its span, aligned radially from the CG. If you increase the magnitude of the lift components on the right side, due to right trailing edge pulled down by a right brake input, they still are pointing directly away from the pilot/CG, so they apply NO NET ROLL MOMENT relative to the CG. The lift components of an airplane wing are essentially tangential, i.e. normal to their arm from the CG, so in addition to generating lift due to right aileron down deflection, they also generate a lot of left roll moment. The attached cartoon shows why the primary aileron roll effect on a conventional aircraft is negligible on a paraglider, leaving the drag/yaw effect to do most of the work. The yaw moment yaws the glider to the right, which develops left sideslip. This sideslip causes a right force on the wing, which now has a long arm above the CG, and rolls the glider to the right, beginning our pendulum-style turn. Comparison of Aileron Effect on Paraglider vs Conventional Aircraft

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I think by pulling down gently on one side the pilot affects a chain of behaviors that cause the turn.

1- pulling the brake on one side, say right side, causes reduction in lift on that side even thought it adds to angle of attak, for it forces that side to function more like a parachute then a wing, with immediate increase in drag and more importantly loweres the same side giving better unbalance lift to the other side.

2- Then because of the extra lift on the left side it swings the pilot up and to the left causing the whole parachute to bank left and turn.

Anybody who has had a heavy bike knows that if you want to turn left you push the left side of the handle forward, or pull right side in, totally counter intuitive.

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