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I read somewhere that conventional aircraft are designed so that the main wing always stalls before the tailplane. Why is that?

My concern is that if the wing stalls, suddenly the tail will push the aircraft into a higher AoA, and maybe stall itself, though the pilot would still have control for some time to avoid that. On the other hand, if the tail stalls, the plane will (violently?) pitch down, but would be able to recover afterward.

So what am I missing?

Edit: Assume the center of gravity of the plane is in front of the main wing

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  • $\begingroup$ this question doesn't make any sense without disclosing where the CG is. $\endgroup$ – rbp Dec 5 '15 at 20:17
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You are missing the weight force. The wing should stall first because then it will produce less lift and the weight will make the aircraft pitch down.

In attached flow, the lift from wing and tail is balanced such that the combined resulting force is acting exactly at the longitudinal position of the center of gravity. If the wing stalls, the balance of lift is shifted backwards (regardless of the tail producing lift or a downforce), because now the forward part of the wing-tail-combination will produce proportionally less lift than it did in attached flow. The center of gravity will then be ahead of the resulting lift force and will pull the nose of the aircraft down.

A tail stall is bad news for the pilot and should be avoided:

  • A stalled tail will have much less elevator effectivity, reducing the control power available to the pilot
  • A stalled tail at high angle of attack is stalling at maximum lift, so it produced positive lift before. If it stalls, its lift will shrink and make the aircraft pitch up.

In a conventional configuration the tail flies in the flow field of the wing. Because the wing's downwash increases with angle of attack, the angle of attack variations at the tail are reduced. This helps to keep the flow at the tail attached over the whole useable angle of attack range of the wing. A tail stall is either caused by a wrong location of the center of gravity (CG) or by a very poor transsonic design which causes shock-induced stalling of the tail when the wing is still doing fine. Very rarely can you stall the tail by a wrong trim setting when a moveable stabilizer is used for trimming, or in a deep stall.

Some supersonic aircraft compensate for the backward shift of the center of lift in supersonic flow by pumping fuel from forward to rear. If an airplane with such a rear CG location slows down to subsonic speed, it will need to produce proportionally more lift with the tail than with the wing. If it now pitches up (say, for a tight turn) it will risk a tail stall. Note that such a configuration is aerodynamically unstable at subsonic speed.


EDIT:

Now you define the center of gravity as ahead of the wing, which makes for a very stable configuration. The tail is producing a downforce to compensate the pitch-down action of the wing's lift.

If the wing stalls first, lift will become less than weight and the aircraft will accelerate downwards. Now the angle of attack will increase at both the tail and the wing; at the tail even for two reasons:

  1. The downward acceleration will change the local flow direction to a larger angle of attack, and
  2. The diminished downwash from the wing will cause an angle of attack increase of its own at the tail. This effect takes a little longer to manifest itself: The delay is the distance between the quarter points of tail and wing divided by the airspeed.

Both effects will quickly create a dominant pitch-down moment for the aircraft because the tail will contribute much less downforce than before the stall, and the diminished pitch-down moment of the wing will become insignificant.

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  • $\begingroup$ I edited my question so that you guys know what I was imagining (c.g. is in front of the wing). So wing produces negative pitching moment, as soon as it stalls there is a net positive moment and the aircraft pitches up more. $\endgroup$ – Jacob Dec 5 '15 at 21:50
  • $\begingroup$ @Jacob: Stall does not equal zero pitching moment. Normally, in a stall the forward part of the airfoil loses more lift and the center of pressure moves back, increasing the negative pitching moment. However, the wing's pitching moment is small when compared to that of the tail. It will dominate the pitch moment in a stall regardless of CG position. $\endgroup$ – Peter Kämpf Dec 5 '15 at 22:01
  • $\begingroup$ @Jacob: Now I get what you mean: The wing will produce less lift and less pitch-down moment for the full aircraft. But in a stall lift is lower than weight, and the whole aircraft will accelerate downwards. Now the tail will produce even less downforce and pitch the aircraft down. $\endgroup$ – Peter Kämpf Dec 5 '15 at 22:16
  • $\begingroup$ Yup that's what I was looking for! Neglected the downward acceleration. Thanks! $\endgroup$ – Jacob Dec 5 '15 at 22:31

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