dynamic stall

[time builder]

Reading in the DA40 POH: Approved maneuvers: 2) Stalling (with the exception of dynamic stalling).

What exactly are they referring to when they say "dynamic stalling."

 

[FL420]

Probably mean "accelerated stall."

 

[AC560]

It is fancy word for whip stalls I believe (not allowed for aircraft certified under part 23 in normal category). I could be entirely wrong though.

 

[The_Russian]

Whip stalls do not exist. See FL420's post.

 

[AC560]

Whip stalls do not exist.

14CFR23.3(a)(2) Stalls (except whip stalls); and

It is also referenced in the DA 20 manual

I always understood a whip stall to come from a tail slide where it causes the ends to flip when you lose all energy.

 

[The_Russian]

Bad venacular.

 

[ackattacker]

Probably mean "accelerated stall."

Accelerated stalls and dynamic stalls are not quite the same thing. An accelerated stall is when you stall the airplane by pulling "g's" and loading the wing to the point that it stalls. It can occur in a steep turn or when recovering from a dive or any number of other scenarios.

A dynamic stall is when you use the aircraft's rotational inertia to achieve an angle of attack much greater than you could obtain normally, because the inertia overcomes the natural or artificially induced "nose drop" response. In some types, particularly certain T-tails and canard designs, this can lead to a "deep stall" which may be unrecoverable.

 

[FL420]

An accelerated stall is when you stall the airplane by pulling "g's" and loading the wing to the point that it stalls. It can occur in a steep turn or when recovering from a dive or any number of other scenarios.

I agree

A dynamic stall is when you use the aircraft's rotational inertia to achieve an angle of attack much greater than you could obtain normally, because the inertia overcomes the natural or artificially induced "nose drop" response. In some types, particularly certain T-tails and canard designs, this can lead to a "deep stall" which may be unrecoverable.

I'm having a little trouble picturing this. Are you talking about a situation, like in a flat spin, where the angle of attack is ≈90º?

 

[ackattacker]

I'm having a little trouble picturing this. Are you talking about a situation, like in a flat spin, where the angle of attack is ≈90º?

Well, that would be pretty extreme.

Let's say a wing normally stalls at, say, 15 degrees angle of attack.

Normally you can get to 15 degrees and no further, because as soon as you get there the nose drops and the airplane recovers.

But if we aggressively "yank" the nose up, the airplane will continue to rotate past 15 degrees due to rotational inertia.

In certain aircraft, you will reach an angle of attack in which the aircraft will no longer have any nose drop tendency. It will be stable in a stalled condition. This is a "deep stall" or "super stall". The angle of attack is large, but not 90 degrees.

There are two basic types which are susceptible to this. The first is a canard design. The second is certain T-tail aircraft, particularly rear-engined jet T-tails.

The canard design is the easiest to explain. In the canard design the main wing is behind the center of gravity, so if the main wing is EVER allowed to stall the airplane will pitch nose up. This is prevented by setting the angle of incidence of the canard to always stall first, creating a nose drop and preventing the main wing from stalling. If we "dynamically stall" the aircraft, we may be able to stall the main wing and enter a deep stall.

The second instance of a T-tail jet is a byproduct of two factors. A) the basic pitching tendency at the stall; and B) reduced elevator effectiveness.

A) is caused by a combination of wing planform characteristics, and contributions from the fuselage.

In a swept and tapered wing, it is easier to stall the tips than the inboard sections. In a full stall the tips will be very deeply stalled, causing a shift in the center of lift forward, and creating a nose-up tendency. Also, the fuselage acts as an airfoil and this lift is generally forwards of the center of gravity, aggravating the nose up tendency.

B) in T-tail the tail can become trapped in the disturbed wash from the stalled wing, severely reducing elevator effectiveness and preventing recovery.

 

[time builder]

B) in T-tail the tail can become trapped in the disturbed wash from the stalled wing, severely reducing elevator effectiveness and preventing recovery.

Which would apply to the DA40 where it has the T-tail. Thanks Ackattacker, by the way, why the change in Avatar? I liked the old one.