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I have an RC Flying Wing that uses elevons for control. To go up both elevons move upwards and the wing pitches up. Both go down and the wing pitches down. I’m OK with this.

However, if I want to roll right, looking from the rear of the wing, the left hand elevon goes down and the right hand one goes up. I can see this when I fly it that it rolls right but don’t understand why?

If an upward pitch angle produces lift to go up, (like when the elevons act as elevators) then why doesn’t the right wing produce more lift and thus the wing roll left? Either a flap produces lift or doesn’t?

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    $\begingroup$ Related: How is pitch controled in a flying wing? $\endgroup$ – fooot Jan 21 '18 at 21:36
  • $\begingroup$ I can't explain, but I was provided with some good reference books: 'Tailless Aircraft in Theory & Practice', K Nickel, M Wohlfahrt. ISBN 0 340 6142 .1; 'Only the Wing', R E Lee, ISBN 978-9-135623-03-8; 'Nurflügel', R Horten, P Sellinger, ISBN 3-900310-09-2 (In German) $\endgroup$ – Urquiola Jan 29 '18 at 21:29
  • $\begingroup$ Your question would only make sense if each wing was free to change its angle-of-incidence independent of the other wing, so that the elevon acted more like a trim tab than an aileron. $\endgroup$ – quiet flyer Sep 9 at 15:31
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Good question! There's a bit of a misconception: when the elevon moves up, it actually decreases lift. It pushes air up which pushes the wing down.

This explains the roll behaviour, but how does decreasing the lift make the plane go up? The key here is that the lift is reduced only at the rear of the plane. In other words, the rear of the plane is pushed down, but only the rear. This makes the nose point up (just put the plane on the floor and push down on the tail, the nose will be up).

In this new orientation, the angle of attack (the angle of the wing relative to the airflow) is increased, and consequently the lift increases (the little bit of downforce at the rear of the wing doesn't really make a big difference). The plane will start to climb, which will reduce the angle of attack (the wing will be pointed the same way as the direction of travel), until the plane is in a steady climb.

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  • $\begingroup$ Thank you, I know it’s simplified and aero dynamics is complicated but that’s good enough for me! $\endgroup$ – hydev Jan 21 '18 at 22:03
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    $\begingroup$ @hydev For most aerodynamic principles, you don't need complicated mathematics to understand them. The most important point is that the wings generate the majority of the lift, and the controls just point the wing (and consequently, the lift force) in a certain direction. $\endgroup$ – Sanchises Jan 22 '18 at 8:22
  • $\begingroup$ "and consequently the lift" - did you mean "and consequently the lift increases"? is a word missing here? It might also help to explain why increasing the angle of attack increases the lift. $\endgroup$ – D.W. Jan 22 '18 at 18:46
  • $\begingroup$ @D.W. Yes, that's what I meant. But answering how lift works is way outside the scope of this question. $\endgroup$ – Sanchises Jan 22 '18 at 19:01
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A somewhat simplified answer.

When both elevons go up, they will push the back of the plane down. This makes the nose start to point upwards, or, as we say, the plane pitches up. This increases lift, and so the plane goes up. So the sequence really is: elevons up --> back end pushed down = pitch up --> increased lift --> increasing height.

If one elevon goes up, and one down: the one going up will create a pressure downwards, the one going down will create a pressure upwards --> roll towards the elevon going up.

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A downward deflected elevon creates an upward force, an upward deflected elevon creates a downward force. This pair of forces thus produce a rolling moment, making the aircraft start to roll.

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  • $\begingroup$ "Elevons are installed on each side of the aircraft at the trailing edge of the wing. When moved in the same direction (up or down) they will cause a pitching force (nose up or nose down) to be applied to the airframe. When moved differentially, (one up, one down) they will cause a rolling force to be applied." Either a flap produces lift or it doesn’t? A little either way sure, because it messes with the wing's camber, but that's not its real job; it's a control surface to provide torque. (Everyone's taking about lift, but isn't it really: drag?) $\endgroup$ – Mazura Jan 22 '18 at 20:13

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