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6 Answers

Aircraft stability

Asked by: 2795 views Aerodynamics

Here is the question:

With anhedral and high angle of sweepback, when low speed and high AOA, the aircraft is easy to go into

A. Dutch roll

B. Sprial

C. Spin

Why the answer is A?

The sweepback is helpful for directional stability and anhedral reduces the lateral stability. With strong directional stability and weak lateral stability, I think it should go into sprial instead of dutch roll.


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6 Answers

  1. Bill Trussell on Mar 08, 2013

    The correct way to think of this situation is maneuverability rather than pure stability. It would seem that the aircraft in question was built for speed and maneuverability and would likely be a candidate for stability augmentation systems to prevent the adverse reactions which might result from strictly manual inputs. Dutch rolling is both an entry and a recovery from high angle of attack on the downward moving wing in my mind.

    Not sure from the description what type of aircraft you might be describing or the intended mission but anytime anhedral is involved it is intended for quick rolling inputs like a fighter jet.

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  2. Wes Beard on Mar 08, 2013

    A swept back wing with anhedral indicates a KC-12 type airplane. Heavy jet airplane that can go fairly fast once at altitude. Perhaps a tanker. The use of anhedral would indicate the aircraft design is too stable and the designers needed more maneuverability. the use of swept back wings indicate an aircraft that flies in the transonic region or above .73ish mach it so depending on the design.

    Dutch rolls are a result of the sweptback wing design and the fact anhedral is used makes the condition more pronounced. It happens in high speed flight where yaw dampers are not used.

    A spiral or spin can occur but is not a result of the type of airplane you mentioned.

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  3. Brian on Mar 08, 2013

    A swept wing at a high Cl exhibits and exorbinat amount of laterall (roll) stability. Not directional stability. That’s the break in the chain of your thinking.

    Thinking stability or control leads to the same conclusion. I stick to stability. High roll stability relative to directional stability you get Dutch roll. As it appears you are already aware, the opposite relationship results in spiral divergence.

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  4. Nathan Parker on Mar 09, 2013

    The key point in the question is “when low speed and high AOA”. For swept wing aircraft, the dihedral effect is magnified at high AoA, so the aircraft becomes more laterally stable at low speeds, which as the OP pointed out, contributes to the Dutch Roll phenomenon.

    The anhedral is a distraction. Anhedral is normally used on high wing, swept wing aircraft, because the high wing location also magnifies the dihedral effect, The resulting lateral stability of such aircraft will therefore fall into the normal range, and not be reduced, as you might be led to believe.

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  5. ccwebb on Mar 29, 2013

    To spin, you must be stalled first. The dutch roll will occur no matter what, while the spin will occur ONLY if stalled.

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  6. NAVEEN SINHA on Aug 26, 2013

    The aircraft has a sweepback which is akin to dihedral, this fact remains clear as you yourself have said that. Now anhedral is given so that a sweepback does not increase the lateral stability of the aircraft beyond the controllable regimes of the aircraft. However, in this nowhere is the thing mentioned about the vertical fin which provides directional stability to the aircraft. An aircraft with poor directional stability i.e small vertical surfaces or fin and having a sweepback will certainly exhibit dutch rolling and not spiral. The reason is pretty simple which is any change in the lateral plane, the aircraft due to sweep back lifts the wing back due to increase in lift of the down going wing, however due to poor directional stability it goes past beyond the mean and this cycle continues.

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