Secondary stalls

[zerozero]

I know that a given airfoil will always stall at the same angle of attack but I'm under the impression that if you experience a 'secondary stall' then the airfoil will, in fact, stall at a lower angle of attack.

Am I incorrect?

I've looked through my books and cannot find any information. I would appreciate it if you could cite a source.

Thanks.

[Edited to ask for source]

[john_tullamarine]

I am not sure what you mean by a "secondary" stall.

The problems usually fall into the following boxes -

(a) the normal 1g stall as in certification data in the AFM

(b) accelerated (greater than 1g) stall when the stall speed increases due to the increased load factor. This is relevant to stalls in turning flight and during ham-fisted recoveries from any stall - this latter may be what you are thinking of ?

(c) a rather different phenomenon at very high pitch rates where the initial separation re-attaches due to a vortex field and the wing continues to generate significant lift forces at angles considerably higher than the "normal" stalling angle. This is rarely relevant to fixed wing operations due to the inability to generate the required pitch rates ... but may be a consideration in some rotary wing situations.

[zerozero]

Thanks for the quick reply.

I was thinking of the secondary stall that sometimes results from a rushed recovery.

I can remember demonstrating these to my students. The lower deck angle is obvious (and if I remember correctly a higher IAS too) but I'm wondering if the wing actually stalled at a lower angle of attack than normal.

I'd appreciate any elaboration.

[Tinstaafl]

The stalling angle remains the same as long as the configuration remains the same ie flap setting not altered etc.

There would typically be a descending flight path in the circumstance you describe.

AoA is relative to the flight path, not the horizon. With a descending flight path then the 'deck angle', as you put it, relative to the horizon will be lower for a given AoA.

If you pull more than one 'g' during the recovery then any subsequent stall will be at a faster speed. This can lead to a false asumption of a lower AoA if one uses the common 1'g' AoA vs. IAS relationship.

[Genghis the Engineer]

The steady-state aerodynamic stall, much beloved of textbooks, is always at the same angle of attack. No dispute about that. Except of-course, that in a real aeroplane, as opposed to a wind tunnel, there's rarely such as thing as a steady state.

The piloting stall also may not be a true aerodynamic stall. The stall, as seen by the pilot can involve elevator authority (and hence CG position, also power setting), wing drop (potentially a function of residual aileron deflection, differential amounts of dead flies on the wing, slight mis-setting in flaps), sideslip (rudder input, rudder trim setting), lateral CG (fuel imbalance).

If you've ever had an aeroplane "mush" on you for example, this is a pitch limitation, and really very little to do with the aerodynamic stall of the wing, which is probably still flying fine, just in a very high-drag regime, causing a large rate of descent.

Also if you decelerate harder into the stall, you can enter a dynamic situation and get a higher AoA before the aircraft responds. If you study unsteady aerodynamics (and I wouldn't recommend it unless you are very fond of higher maths), then you'll see that pitch rate actually will affect the stalling AoA, and with a high deceleration rate (actually rate of increase of AoA), you'll get some very low stall speeds.

Not sure of a any other book on the subject, but I'd start with Darrol Stinton's "Flying qualities and flight testing of the aeroplane", very expensive but unbeatable in the English language for it's subject coverage.

G

[Genghis the Engineer]

If it helps, the following is cut-and-pasted from something I'm working on at the moment and hoping that I might publish sometime late next year under the working title "airworthiness evaluation techniques for small light aeroplanes".


3.2. Defining the stall and stall warning for a pilot.
3.2.1. It is important to appreciate that the stall, as seen by the pilot, is not identical to the stall as would be understood classically by an aerodynamicist. The following definition, which is extracted from BCAR Section S, is typical of the definitions contained in any civil certification standard:-
3.2.2. (From S201(a)) Stall demonstrations must be conducted by reducing the speed by approximately 1kn/s from straight and level flight until either a stall results as evidenced by a downward pitching motion or downward pitching and rolling motion not immediately controllable or until the longitudinal control reaches the stop.
3.2.3. A more simple definition, which is a variation upon that taught in the military test pilots schools such as ETPS, is that a stall is the point following deceleration at which the pilot ceases to have full control over the aeroplane. This is compatible with the definition above, since an uncontrolled motion or the longitudinal control being on the stop are clear indicators that the pilot does not have full control over the aircraft in all axes – however wing rocking or other low-speed departures from controlled flight may also be included.
3.2.4. During a test programme, the test team would normally define the stall for a specific aircraft. Notwithstanding that other definitions may be useful in certain circumstances, the three most common definitions are:-
3.2.4.1. The longitudinal control being on the stop (often termed “mush” by pilots). This is most common at forward CG / hangpoint states where insufficient nose-up control authority exists to fully aerodynamically stall the wing.
3.2.4.2. A downward pitching motion (often termed a “pitch break”). This is most common at aft CG/hangpoint states, where there is sufficient nose-up control authority to fully aerodynamically stall the wing.
3.2.4.3. A wing drop, usually accompanying a pitch break. This occurs where the two sides of the mainplane do not stall simultaneously.
3.2.5. The stall warning is those characteristics of the aircraft which indicate to a pilot that he or she is flying at conditions close to the stall and caution may be needed. Stall warning characteristics will vary between aircraft and are invariably noted in the operators manual. The following are typical stall warnings:-
3.2.5.1. Airframe buffet, as localised airflow starts to detach.
3.2.5.2. Stick buffet, as localised airflow, usually over the wing root in a conventional 3-axis/tailplane aircraft, detaches and strikes the tail control surfaces.
3.2.5.3. Artificial stall warning devices, normally either based upon an AoA sensor [ ]or a localised airflow pressure sensor.[ ], [ ].
3.2.5.4. Unusually nose-high aircraft pitch attitude.
3.2.5.5. Unusually nose-up longitudinal control position.
3.2.5.6. Lack of control responsiveness.
3.2.6. During the airworthiness evaluation process for any aircraft, the following need to be determined:-
3.2.6.1. What are the stalling characteristics at representative deceleration rates? Are they acceptable?
3.2.6.2. What are the stall warning clues? Are they adequate?
3.2.6.3. Is the aircraft fully controllable during deceleration down to the point of stall?
3.2.7. Based upon the above assessment, the acceptability of the aircraft may be determined, and the contents of any advice concerning stalling that is to be included in the operators manual.

(Then follows about 10 pages of maths, which I won't inflict).

G

[zerozero]

Thanks for all the responses.