From what it appears to me, flight control surfaces seem to be taken for granted. In terms of how they actually work and what kind of physical outputs they give in terms of forces and such.

Take for example the elevators of a Boeing 777, if the pilot wanted to pitch up, the elevators would move up. What are the physics behind this? Is it air being deflected thus a change in momentum occurring to pivot the aircraft's tail down or is the angle of attack changing due to the moving nature of the trailing edge? or both?

I am quite interested to know because I want to simulate Aerodynamical forces in a game I plan to make soon. Diagrams would also be useful but not required. Thanks in advance.

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    $\begingroup$ Welcome to aviation.stackexchange! These are all great questions and very much on topic here. Generally there is a "one question per post" policy here so you may want to split these apart into 3 posts to get better answers. $\endgroup$ – Dave Jan 21 '20 at 22:33
  • $\begingroup$ Alright, I understand. Thanks. $\endgroup$ – Kiyo Jan 21 '20 at 22:36
  • $\begingroup$ I hope you learn quickly because you are going to have to move pretty fast from grasping the basic physics of how a control surface changes pitch, to understanding the mathematical equations well enough to write accurate simulation code. $\endgroup$ – Michael Hall Jan 21 '20 at 23:17
  • $\begingroup$ You may transform the title in a question (this is a Q&A website) as specific as possible to help navigate through questions about aerodynamic or surface control without opening each question. $\endgroup$ – Manu H Jan 22 '20 at 8:47

Thankfully, aerodynamics in the usual flight range is linear.

Therefore, there is a gradient of lift over angle of attack and another one over the flap deflection angle. Both are constant over a range of maybe ±15° and can be combined. The angle of attack is referenced to the fixed part of the flight surface and the deflection angle to the moving part relative to the fixed part.

Another parameter which influences lift is the camber of the flight surface. Positive camber produces more lift at the same angle of attack. Deflecting a flap changes this camber, and its effect can be linearly added to that of the angle of attack.

Is it air being deflected thus a change in momentum occurring to pivot the aircraft's tail down?

Yes, when a flap moves, the angle of attack stays constant but the camber of the flight surface changes, thus producing a change in lift. In your example, a trailing-edge up deflection decreases camber, the surface produces less or negative lift which produces a moment around the center of gravity.

And yes, lift is produced by deflecting a stream of air.

Things get more complicated and interesting when the aircraft leaves the angle of attack range in which aerodynamics is linear.

  • $\begingroup$ Ah thanks, if it were flaps on the main wing for landing lets say, would the direction lift produced by the main win change to be more opposing to forward movement but still enough to maintain stable flight? And by deflecting you do mean that air is deflected over and under the wings right? $\endgroup$ – Kiyo Jan 22 '20 at 12:08
  • $\begingroup$ Lift is not linear. Especially once it reaches the stall angle of attack. $\endgroup$ – Mark Jones Jr. Jan 22 '20 at 12:08
  • $\begingroup$ @MarkJonesJr. Yes, as this answer clearly says. What is wrong with that? $\endgroup$ – Peter Kämpf Jan 22 '20 at 14:30
  • $\begingroup$ @PeterKämpf the first sentence in your answer says "Aerodynamics is linear." Clearly. $\endgroup$ – Mark Jones Jr. Jan 22 '20 at 14:52
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    $\begingroup$ @MarkJonesJr. No, it doesn't. Please read the full sentence. $\endgroup$ – Peter Kämpf Jan 22 '20 at 22:30

A trailing-edge control surface, when it deflects, changes the camber of the overall airfoil. More camber means more lift, in whatever direction that airfoil is mounted.

In your example, adding up elevator increases the horizontal stabilizer's camber, which increases the downward force it applies.

Philosophically, "why" it does this is just, well, that's how air behaves when you push it past something with that shape.


Well, both.

Lift can be described as a moving wing colliding with air molecules at an angle, the result of the collision is the wing moves one way and the air mass the other, as per momentum physics.

Moving the trailing edge, or the entire surface, increases the angle of attack, resulting in more lift at a given speed $V$:

$Lift$ = 1/2 × Lift Coefficient x Air Density x $V^2$ x Wing Area

Deflection of the control surface produces a linear response to lift, as Peter Kampf says. A graph of angle of attack vs lift coefficient generally shows a linear relationship through most of the unstalled AOA range.

Further inspection of the effect on AOA, of deflecting a trailing edge flap down, as compared with "drooping" a leading edge, does indeed show doing this changes the angle of attack of the lifting surface!

Flaps are usually near the wing roots, and leading edge slats near the wing tips, for this reason. We want the root to stall first.

Yes, deflecting a control surface changes the camber of the wing, which also factors into the lift coefficient, but, relative to the original AOA of the wing/fuselage, the flapped portion of the wing will stall at a lower AOA than the slatted portion. Therefor, deflecting the control surface also changes AOA.

The importance of this concept is highlighted in a slow flight coordinated turn. Use of ailerons without coordinating rudder may result in a sharp roll in the opposite direction of the intended roll, because the AOA of the "down" aileron wing now exceeds its stall AOA.

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    $\begingroup$ A wing colliding with molecules is a (useful) simplification, like an angled steel plate encountering a fusillade of bullets. But those molecules colliding with all the other molecules is what causes lift. Avoid the bullet imagery. Safer to not mention molecules at all, and instead stick to pressure differences. $\endgroup$ – Camille Goudeseune Jan 22 '20 at 4:32
  • $\begingroup$ Momentum theory describes "surfing" the air (bottom lift), but not "Bernoulli" pressure differences (top lift). Bernoulli is limited at both very high and very low Reynolds numbers, but would be applicable for most aircraft control surfaces such as horizontal stabilizer/elevators. Momentum theory need not involve bullets, it is useful to describe the properties of collision, and is part of "lift". But what works for the bird does not work for the bee, or the blue whale. There is no ubiquitous definition for lift. $\endgroup$ – Robert DiGiovanni Jan 22 '20 at 8:59
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    $\begingroup$ Momentum theory only gives useful results at hypersonic speed when inertial energy dominates the flow. As Camille says, better avoid it for describing subsonic flow. $\endgroup$ – Peter Kämpf Jan 22 '20 at 22:37
  • $\begingroup$ @Peter Kampf your "constant over a range" link clearly shows the "Bernoulli lift spike" (with much lower drag) in the Coefficient of Lift vs AOA graph until stall. Post stall, lift drops with higher AOA, but then rises again to a second maximum at 45 degrees (with hugely more drag). Can not this second maximum, like a water ski, be entirely explained by momentum theory? $\endgroup$ – Robert DiGiovanni Jan 23 '20 at 0:06
  • $\begingroup$ But, most importantly, we all should be aware that a control surface, flap, or slat deflection can locally change AOA. $\endgroup$ – Robert DiGiovanni Jan 23 '20 at 0:08

From an old uni book on stability & control of aeroplanes, showing the pressure distribution over the horizontal tail:

enter image description here

  1. First picture: no elevator or trim tab deflection, pressure peaks at the stabiliser nose at two different Angles of Attack $\alpha_h$.
  2. Two different elevator deflections $\delta_e$: pressure peaks at the elevator profile nose.
  3. Two different trim tab deflections $\delta_{te}$: pressure peaks at the trim tab nose.

The magnitude of the pressure peaks per degree deflection is a function of the surface area. Location of the peaks is a function of which surface has AoA ≄ 0.

  • $\begingroup$ What is this graph against? $\endgroup$ – Kiyo Feb 12 '20 at 20:20

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