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Does anybody know what will happen to an aircraft, if it is stable (in longitude) for let's say: -8° > angle of attack > 8° ?

So for angle of attack 4° the aircraft is not stable (centre of pressure is in front of centre of gravity), so the angle of attack will further increase until 8° where it becomes stable (centre of pressure behind centre of gravity).

So except that my aircraft will oscillate a bit, it should always return to a stable position, right?

Pondlife
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Helmut K.
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  • This question is being discussed on meta: https://aviation.meta.stackexchange.com/q/4193/1467 – Federico Apr 20 '21 at 17:01
  • NB: The question originally included the second part about whether statically unstable aircraft can be flown. I've removed it in an edit to leave only the first, unique part. – Therac Apr 20 '21 at 18:09

2 Answers2

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I find it hard to come up with an actual example that behaves like postulated. Normally, airplanes are most stable at low angles of attack and lose this stability at higher angles.

But there is indeed one case where angle of attack stabilizes the airplane: A swept flying wing without a vertical tail. Like the Horten designs. Those would enjoy increasing directional stability with increasing angle of attack. Your limits are a bit extreme, but a Horten IV was directionally indifferent at high speed when the typical angle of attack of the root airfoil would be between 2° and 0°. At lower speeds the induced drag would help to pull the airplane out of a sideslip as explained here.

Due to the high inertial moment around the vertical axis this behavior can be controlled by the pilot because sideslip angles will build up slowly. But it is not pleasant since it needs constant control inputs and puts an unnecessary workload on the pilot which might interfere with other tasks like navigation and collision avoidance.

Of course, this example is not quite what you wanted, since a higher sideslip will not stabilize the airplane. Only pulling up will.

If we hypothesize that an airplane behaves as you postulated, it will tend to leave the unstable region, but this might happen both in positive or negative AoA direction. An excursion into negative angles of attack is rather unpleasant (have you ever flown an outside loop?) and will probably make the pilot pull up hard. What happens next? The airplane flies through the unstable AoA region and picks up more pitch speed, only to arrive in the stable region with such a force that it overshoots the AoA limit and either stalls or, given enough flight speed, breaks up.

The Saab JAS-39 Grippen had such an unexpected overshoot at the most unfortunate of times, namely during a flight display on the occasion of the yearly Wattenfestival in Stockholm in 1993.

Any instability in the regular operating region is a bad idea and an invitation for mishaps that could be easily avoided.

Peter Kämpf
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  • If we are going to include directional stability/instability in the answer-- at low angles-of-attack some hang gliders are prone to a dynamic yaw-roll oscillation mode that has a resemblance to Dutch Roll, but may be fundamentally different in some ways, possibly with different driving forces and different relationships between sideslip and yaw rate and bank angle at any given point in the oscillation. The downforce developed by the wingtips which have significant washout, and the relationship between this downforce and local airspeed which is influenced by yaw rate, may play a key role. – quiet flyer Apr 19 '21 at 18:25
  • This oscillation is mentioned in at least one book on stability and control but in my opinion is not really correctly described-- hang gliders have both sweep and anhedral, and this dynamic yaw-roll oscillation happens in the part of the flight envelope where the net "effective dihedral" due to the combined effects of sweep and anhedral can be demonstrated to be negative rather than positive-- but better open a chat window to say much more about that-- – quiet flyer Apr 19 '21 at 18:35
  • Would the B-2 also fall into this category of planes in Q? It, of course, has computers stabilizing it's flight and thus does not need pilot focus on maintaining steady flight. – Jpe61 Apr 20 '21 at 09:52
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    @Jpe61 … and it flies with split ailerons open. This alone gives it sufficient stability. The Hortens also had wing tip spoilers but left them closed until landing. – Peter Kämpf Apr 20 '21 at 16:04
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Answering your second question as originally asked:

Is it possible to fly an aircraft which is not statically stable? I think with a really good control system this should be possible?

The F-16 was designed not only to not be statically stable but to actually have NEGATIVE stability. This means that the aircraft is designed intentionally to move out of its original position (and not return) upon application of some external force, and will continue to move even after that force has been removed. This is a feature of most modern combat aircraft, and it's very possible to fly them, if the aircraft is equipped with a flight computer. Without a computer, it would be very difficult.

MD88Fan
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    Not without a computer. Downvoted for the implied ease of flying an unstable airplane, and for it being just a partial answer. – Michael Hall Apr 19 '21 at 14:16
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    @MichealHall partial answers are fine. But with the new edit to the question, his answer is no longer relevant. – Abdullah is not an Amalekite Apr 19 '21 at 15:15
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    @Abdullah: Question revisions should not make any answer obsolete. Having checked the edit history, this answer addresses the question as originally asked. PS if a question has more than one question, the question should be closed until it's narrowed down. For now, +1 and Looks OK - From Review –  Apr 20 '21 at 14:28
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    @ymb1, I removed the DV for my primary objection, but since it is still a partial answer, and because the question now excludes the quoted portion, I don't think it warrants an upvote... – Michael Hall Apr 20 '21 at 14:39
  • Notice: The question originally included the second part about whether statically unstable aircraft can be flown. I've edited it to leave only the first, unique part. So no fault on the answerer's part for answering the no-longer-existing part. – Therac Apr 20 '21 at 18:10