Deterrent Buffet Definition

[FCeng84]

Stall Definition and Stall ID

I think the discussion of Deterrent Buffet will benefit from a bit of background regarding what is it that is to be deterred - namely stall. Another concept that must be part of the mix is Stall Identification - i.e., characteristics of the airplane response that the crew will recognize as indicating approach to or onset of stall and thus prompt pilot action to back away from that condition.

First of all, the definition of stall. Normally with a given wing configuration (i.e., flap and speedbrake setting) increasing angle-of-attack will result in increased lift. The slope of the lift curve (plot of CL vs. Alpha) is the key aerodynamic parameter, CL-alpha. For most wings the lift curve will exhibit positive CL-alpha up to the AOA where significant flow separation occurs. AOA for CLmax is the angle of attach for which lift is maximum. Beyond that point further increase in AOA will not result in increased lift. For wings that exhibit a sharp stall break lift will actually decrease rapidly as AOA increases beyond AOA for CLmax.
In conjunction with the variation in lift that results from increasing AOA, the variation in pitching moment must also be considered. A wing that has positive pitch stability will generate more and more nose down pitching moment as AOA increases. This requires more and more nose up command to achieve flight at higher and higher AOA. For a wing with this charateristic, removing the nose up control input results in nose down pitch returning the wing to lower AOA. Some wings (particularly at higher AOA approaching or exceeding AOA for CLmax) will exhibit negative pitch stability whereby their tendency is to pitch up to even higher AOA as a result of an AOA increase. For a wing with this tendency, significant nose down control power may be needed to return from high AOA to low AOA.
A third consideration regarding high AOA is drag which generally increases with higher AOA. Of key concern is the ratio of lift over drag that is an indication of wing performance. At high AOA lift decreases while drag increases resulting in lower L/D and thus lower performance.

Now the question: What it is that needs to be deterred with regard to flight at or beyond stall AOA (i.e., AOA for CLmax)? Two considerations here. First is the loss of wing performance as measured by lift or the ratio of lift-over-drag. Second is the prospect of pitch-up instability and potential loss of pitch control if the aerodynamic pitch up is strong enough to overwhelm the available nose down pitch control power. (This is often referred to as "locked in stall".) The first of these is not desirable and, if close enough to the ground, is a real threat to continued safe flight and landing. The second is a potential short term disaster as it may result in loss of controlled flight.

In order to avoid the pitfalls of stall the FAA regulations call for airplane response characteristics that give the flight crew positive indication of the onset of stall. There are three means of providing Stall ID:
1. Positive nose down pitch break that cannot be arrested by the pilot. In this situation the airplane naturally recovers from high AOA by itself.
2. Deterrent Buffet - the airplane exhibits sufficient buffet that the flight crew will be deterred from commanding even higher AOA.
3. Full nose up pitch control input from the pilot with no further increase in pitch attitude. The airplane will not go to extreme AOA where either significant loss of performance or pitch stability may occur.

As a closing thought it must be recognized and remembered that there is a very wide range of airplane response characteristics at high AOA. For some wings approaching AOA for CLmax will be very smooth and controlled. For others it will involve lots of buffet. The most concerning situation is an airplane that exhibits a sharp pitch-up instability without much buffet to alert the crew as AOA increases. If an airplane does have a significant pitch up at high AOA that risks locked in stall there must be provisions built into the system to either prevent the pilot from go to that level of AOA or there must be sufficient stall buffet to deter the crew from commanding the airplane into that dangerous situation.