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Consider an aircraft where-

Center of gravity lies forward of Center of Pressure in CRUISE.

Now as angle of attack increases,

CP moves forward,

then for particular angle, it must move forward of CG.

In that case, wouldn't longitudinal stability be lost? As , now the CP would be making nose pitch up moment, the tail plane will also make nose pitch up moment ( as it acts downwards)

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Longitudinal stability is related to Neutral Point (NP) rather than CP. Exactly because CP moves around, it is not a convenient reference point.

There is no strict requirement for the tail to create downforce for a stable airplane, even though this is a common arrangement.

It may well happen that CP may move ahead of CG and the aircraft remains stable. But it won't keep doing it indefinitely: at some point the wing will stall, CP will move back (this will depend on the wing plan form though), and NP will cease to exist as such. Stability analysis is difficult in such regimes.

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  • $\begingroup$ if CP moves ahead of CG , and the aircraft is in stall, then in which direction will the tail create force? $\endgroup$
    – Sachin Chaudhary
    Nov 20, 2018 at 6:17
  • $\begingroup$ As always in stall, it depends. Is the aircraft still in balance? Such question rarely arises (outside of aerobatics), given that the stalled condition is not meant for flight. But if it is, then by definition the tail must be creating positive lift. If not, there is nothing to talk about, it can be anything. That said, being stalled with CP ahead of CG is rather unusual situation, given that on most wings CP will move back after stall. $\endgroup$
    – Zeus
    Nov 20, 2018 at 7:07
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Assuming a "classical" planform with CG near the first 1/3 of the wing chord and a horizontal stabilizer in back of the aircraft, yes, the center of pressure of the wing could move forward of the CG when angle of attack increases, resulting in a pitch up force.

If you ever scratch build a wing it is fun to watch it tumble end over end if you try to "fly" it by it self. The forward shift of CP is normal as AoA increases. This is mainly due to breakdown of lift producing airflow over the top back part of the wing that gets progressively worse until stall. The wing will tumble over backwards over and over again.

What to do? Notice the horizontal stabilizer also pitches up as AoA increases, and has a much longer lever arm to control pitch compared with the wing CP. So you try to "right size" your H stab to exactly match the torque of the wing CP moving forward. This helps keep the NET CP in the same spot.

Also, notice Hstabs are either delta shaped or "chunkier" lower aspect design that wings. This means the wing stalls first! The H stab then helps pitch the nose down to restore proper air flow over the wing. (No, Burt Rutan did not invent this, but it was applied very well to his canard designs).

Finally, even if the elevator trim is set to provide down force to the tail at cruise, as AoA increases, tail lift force will go from down to nuetral to up on its own, or the alert pilot will apply down elevator to avert the stall!

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