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Why do gliders have bungee cords in the control systems and what do they do?

Are they on all control surfaces?

Why don't ultralights have them?

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    $\begingroup$ Could you add some more information about what you mean by bungee? A quick Google search finds Bungee Launch and Bungee Springs for trimming. $\endgroup$ – Bianfable Aug 23 at 15:23
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    $\begingroup$ You should include a more accurate description of the bungee (maybe a picture?) $\endgroup$ – Manu H Aug 24 at 8:23
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The bungee is just a bidirectional spring that tends to hold the elevator at position x, and if you move the stick you are stretching the spring in one direction or the other. They are used in the elevator control circuit to provide an adjustable centering force for trim purposes, and on gliders with all flying tails, a measure of stick free static stability (you don't get a lot - many gliders are mildly divergent in pitch if you aren't holding the stick, but this isn't a big deal since you are almost never flying hands off). The glider's pitch trim control adjusts the "at rest" or relaxed position of the spring so it will tend to fix the elevator at this position or that unless overcome by a control input.

Normally you would have a trim tab on an elevator/stabilizer tail or a trim/anti-servo tab on an all flying tail, as regular airplanes do to provide speed-seeking trim and strong stick-free stability, but gliders operate with much lower loading and forces on the tail and can get away with a bungee spring that acts like a little man under the floor pushing on the stick, this way or that to try to hold it at a given position, so you don't have to. This does away with all the linkage, tab surface etc, which is a big deal when you are shooting for a 40:1 or more L/D.

You can put bungees in any control circuit if you want to be able to trim it with an adjustable centering force, or want to increase the forces. Some light aircraft use bungee springs in the rudder control circuit to provide rudder trim. The spring just replaces your foot by exerting a spring force so you don't have to. The downside is the spring loads are present when you move the controls out of the trimmed position, adding to the perceived rudder forces when maneuvering.

I'm pretty sure there are ultralights out there that use elevator bungees for trim purposes instead of a tab. However, driving the surface directly aerodynamically using a tab on the surface provides a more sensitive and robust solution and if I was designing an ultralight, where drag isn't that big a consideration, I'd use a tab for the elevator, and perhaps a bungee in the rudder and aileron circuits if I wanted rudder and roll trim.

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JohnK is right, they are helping to trim the aircraft. But there is more to them.

Ideally, if you want to modify the hinge moment of a control surface, you would add a Flettner tab (in order to decrease control forces, like done on the SB-10 rudder) or an Anti-Flettner tab (in order to increase them, like on the Diamond DA20 Katana elevator.). No bungee or spring needed, and works equally well over the full speed range.

But that is the point – the correcting action of a tab grows with airspeed. On the other hand, if you add a spring in the control linkage, its force only depends on the stick position. Speed does not influence its contribution to control forces.

Did you ever notice that those bungees or springs are coupled with a cambered trailing edge? On elevators only? They are – always! And that is the point to their existence: That camber also will add a hinge moment contribution, and one which grows proportionally to dynamic pressure. Now you have two elements which add their own hinge moment contribution, one constant and one proportional to dynamic pressure. And that is key to their function: Together they increase stick travel and force over speed.

The trailing edge camber will try to push the elevator trailing edge up, which decreases the elevator's deflection angle. The bungee or spring pushes against that so the elevator stays at the desired angle. If the aircraft speeds up, the camber contribution now overpowers the bungee or spring and the elevator moves to pull the aircraft's nose up. When the aircraft slows down, now it is the spring which overpowers the trailing edge camber and the elevator is pushed trailing edge down in order to lower the aircraft's nose.

Diagam stick force over speed

Stick force variation over speed decreases with speed, without that trick the stick force response to speed changes would be too small at high speed. By adding the camber-spring combination, gliders achieve proper speed stability over their whole flight speed range.

Why do gliders have bungee cords in the control systems and what do they do?

Coupled with trailing edge camber, bungees or springs in the elevator control linkage help to increase the speed stability of aircraft.

Are they on all control surfaces?

No, elevators only.

Why don't ultralights have them?

Some do, but most don't need them due to their limited speed range.

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  • $\begingroup$ So if I understand correctly, this mechanism compensates for the decrease in aerodynamic pitch stability at high speeds and thus allows having less of it and the associated trim drag, right? $\endgroup$ – Jan Hudec Aug 24 at 8:15
  • $\begingroup$ @JanHudec: Less of it at low speed, yes. It is mainly for comfort, though. $\endgroup$ – Peter Kämpf Aug 24 at 10:27
  • $\begingroup$ I've been flying a Junior lately, which has a moulded in tab extension on each elevator with a bit of down deflection to drive the elevator up and preload the bungee. It works quite well and has very effective speed seeking when moving the trim control. Being neutral in roll, I can bank into a thermal, pull the trim knob full aft to command minimum stick free speed, around 40kt, and cross my arms, and it will hold the turn quite nicely for a couple times around until the bumps upset things and it slowly starts diverging, while holding the turn. $\endgroup$ – John K Aug 24 at 16:05
  • $\begingroup$ If you want to custom tailor the stick force relative to position (and indirectly, speed) you can replace the bungee acting directly on the control circuit with a profile cam with a spring loaded roller that wants to nest in the middle of the cam profile. When you move the stick you move the cam and force the roller up the profile in either direction, providing a return to center force. You can very the cam profile to make the force buildup non-linear if you want. Make the roller arm movable to allow the neutral "nest" position to be changed, and there's your pitch trim. $\endgroup$ – John K Aug 24 at 16:14

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