Abnormal Procedures (Procedural)
Figure: Approach to stall, from G450 Aircraft Operating Manual, Historic, §06-03-30, figure 2.
How Wing Sweep Affects Stall Onset
Figure: Straight wing versus swept wing CL, from Eddie's notes.
[Dole, pgs. 125-126] Throughout the history of aviation, the slow speed region of flight has been the most hazardous. The reasons for this are quite different for propeller aircraft and for turbojet aircraft. Examination of the lift coefficient curves for each type of aircraft will help explain some of these differences. [The figure] shows the curves for a straight wing, propeller driven aircraft and a swept wing, jet aircraft. Note: the type of propulsion doesn't matter to the curves, it is the wing sweep.
Three differences can be seen from these curves:
- The straight wing aircraft has a higher value of CL. Recall the basic lift equation:
- The swept wing aircraft must fly at a higher AOA to achieve maximum lift.
- There is a sudden reduction of CL for the straight wing aircraft at stall, but not for the swept wing aircraft.
It can be seen that if the lift equals the aircraft weight, stall speed will be a minimum when the values of the lift coefficient is a maximum (CL(MAX)), and that the higher the value of CL(MAX), the lower the stall speed will be. This, of course, means that the straight wing aircraft has a lower VS for the same weight and wing area.
The straight wing aircraft will reach its maximum lift at an earlier angle of attack, but the amount of lift produced by the wing drops sharply with any further increases in AOA.
The swept wing aircraft requires a higher angle of attack to reach its maximum lift and further increases in AOA only result in smaller decreases in lift production.
How Wing Shape Affects Stall Warning and Roll Control
Figure: Stall progression, from Eddie's notes.
[Dole, pg. 126] Stall of a wing will occur where the ratio of the local lift coefficient Cl to the wing lift coefficient CL is highest. . . . The straight wing is usually rectangular, or it has a slight taper. The wing area near the wing tip is large compared to the lift in that area, and the wing loading is, therefore, low. . . . The maximum wing loading is at the wing root. Stall will, therefore, start at the wing root and then spread outward and forward as shown in [the figure].
Swept wings have more taper, with the extreme being the delta wing with a taper ratio of zero. The delta wing's area at the tip is very small, and the wing loading is very high. Stall starts where the wing loading is highest, so the stall starts from the wing tip and progresses inboard and forward.
Straight wing aircraft, with stall starting at the roots, seem to have advantages over swept wing aircraft, which stall at the tips. First, there is more adequate stall warning, caused by separated air buffeting the fuselage. Second, the ailerons of the straight wing aircraft are not enveloped in the stalled air until the stall has progressed outward from the wing roots, compared to the relatively early envelopment from tip stalling.
A straight wing airplane gives you an early warning of a stall with buffeting near the fuselage; you have full use of your ailerons until the stall has progressed well beyond the earliest warning.
A swept wing airplane gives only the slightest warning at the earliest stages of the stall, the buffet will be far from the fuselage. As the stall progresses, the ailerons quickly become impacted by the turbulent air at the wing tips.
How Tail Design Affects Pitch Control and Stall
Figure: B-747 vertical forces, from Eddie's notes.
Low Tailed Aircraft
[Dole, pg. 258] The forces in the pitching plane are shown [in the figure]. Assume the aircraft is trimmed in straight and level flight and that the aircraft's CG is forward of the AC (as shown). A downward balancing force on the tail is required. In case of an aft CG location, an upward force on the tail is required.
Figure: B-747 Prestall, from Eddie's notes.
If the speed is reduced, a gentle up deflection of the elevator is required to maintain altitude, and the downward load on the tail is increased. The static stability of the aircraft causes a nose-down pitching tendency, which has to be resisted by further up elevator to keep the nose from dropping. The low set tail soon becomes engulfed in the turbulent, low energy air from the wing wake. This reduces the efficiency of the tail.
The tail becomes less effective as the aircraft begins to stall.
Figure: B-747 stalled, from Eddie's notes.
At the stall, two distinct things happen. The airplane responds to the traditional nose-down pitching tendency at the stall, and the whole airplane responds with a nose-down pitch. Second, at the moment of stall, the wing wake passes straight aft and goes above the low set tail. This leaves the tail in undisturbed, high energy air, and it now is at a high positive AOA, causing an upward lift on the tail. This lift increases the nose-down pitching tendency.
The decrease in the wing's lift causes a nose-down pitching moment. As the air above the wing goes above the horizontal stabilizer, the tail down force is decreased which also causes a nose-down moment. The aircraft is said to stall "conventionally."
T-Tailed Swept-Wing Aircraft
Figure: G450 vertical forces, from Eddie's notes.
[Dole, pg. 258] Again assume that an aircraft is trimmed in straight and level flight.
Figure: G450 prestall, from Eddie's notes.
As the aircraft is slowed toward the stall, the handling characteristics are much the same as the low tailed aircraft, except that the high tail remains clear of the wing wake and retains its effectiveness. Continued speed reduction is, therefore, more efficient.
The tail does not lose its effectiveness as the angle of attack is first increased.
Figure: G450 stalled, from Eddie's notes.
At the stall two distinct things happen. The swept wing, high tailed airplane tends to suffer a marked nose-up pitch after the stall (this is explained in detail later), and the wing wake, which has now become low energy turbulent air, passes straight aft and immerses the T tail and makes it incapable of combating the nose-up pitch and so the aircraft continues to pitch up. The great reduction in lift and increase in drag cause a rapidly increasing descent path. Thus the AOA is increased, and the pitch-up problem is worsened. The airplane is thus well on its way to extreme angles of attack and deep stall.
The effect of wing sweep. In practice the whole wing does not stall at the same instant. Swept and tapered wings will tend to stall at the tips first because of the high wing loading at the tips. The boundary layer outflow also resulting from wing sweep slows the airflow and reduces the lift near the tips and further worsens the situation. This loss of lift outboard (and therefore aft) causes the center of pressure to move forward, which augments the pitch-up tendency.
Because the swept wing stalls first at the tips, there is more lift inboard which is forward, causing a pitch up. The tail becomes immersed in the turbulent air and has less "grip" and is less effective combating this tendency, making the pitch up tendency worse.
Proper Stall Recovery Procedure
Figure: Jet versus piston drag, from Eddie's notes.
[Davies, pg. 92] In the [figure shown] the total drag curves have been plotted against speed for both a piston-engined and a jet aircraft. The two speed scales at the bottom show the relative changes, as a function of stall speed. . . . Notice the VIMD [the point of minimum drag] of 1.2 VS for the piston, this is typical. On the jet, however, VIMD is much higher at around 1.4 VS; this is typical of the latest jets, although earlier models were nearer 1.3 VS.
Flight below VIMD on a piston-engined aeroplane is quite well identified by the steepness of the drag curve; in flight, as well as on paper! But the relative flatness of the drag curve around VIMD for the jet does not produce any noticeable changes in flying qualities (other than a vague and irritating lack of speed stability), and all this is a bit of a trap.
All this means that when, for example, holding in the clean configuration in a jet transport at high altitude, with a speed in the neighborhood of 1.5 to 1.6 VS, care is necessary to avoid any decrease in airspeed; if this should occur the aeroplane will quietly slide up the back end of the drag curve. Although at higher lift coefficients associated with lower speeds some extra lift is produced, the drag increases faster than the lift increases, the L/D ratio deteriorates and the net result is a steepening of the flight path. So the aeroplane begins to sink. This sinking tendency can be salvaged in two ways:
- The nose can be put down to reduce incidence and allow the aeroplane to accelerate to a speed above VIMD, when steady flight conditions can be again established, but as this always involves a height loss it can rarely be used.
- The thrust can be increased to accelerate the aeroplane to a value above VIMD to establish steady flight conditions again. it is important to emphasize that the amount of thrust to accelerate the aeroplane is quite large.
So, if you suspect that this is what is occurring, don't play around with small increments of thrust hoping to sort things out without making it obvious that something has gone wrong — give it a handful. Fly the aeroplane with some determination and get it re-established in a steady flight condition as soon as possible.
The text from D. P. Davies is a bit dated and I think you can substitute "straight wing" for "Piston-engined aircraft" and "swept wing" for "jet aircraft." But his bottom line is a good one: don't play around, give it all the power you can. Now adding what we know from the thirty years of history since this text was written, also give it a handful of pitch, you need to break the stall.
Figure: Proper stall recovery technique, from Eddie's notes.
Your prime directive here is to get the wing comfortably flying again and that means you have to decrease the angle of attack. The best way to do that during a stall, or even an approach to a stall, is to decrease pitch. That is going to take a conscious push forward on a swept wing, T-tailed aircraft.
If you don't decrease pitch enough and the stick shaker becomes a pusher, do not override the pusher! Let it pitch the nose over and then try again to manually recover.
What if you are low to the ground you say? Well how much altitude is needed for the recovery? If you practice this realistically in the simulator it rarely takes more than 300'. If you are any lower than that you are no longer managing a stall, you are managing a very hard landing.
Update: AC 120-109
In late 2012, the FAA released AC 120-109 Stall and Stick Pusher Training which acknowledges "Reduction of AOA is the most important response when confronted with a stall event." It further outlines the following stall recover template:
- Autopilot and autothrottle ... Disconnect
- Nose down pitch control ... Apply until stall warning is eliminated
Nose down pitch trim ... As Needed
- Bank ... Wings Level
- Thrust ... As Needed
- Speed brakes/Spoilers ... Retract
- Return to the desired flight path.
While maintaining the attitude of the airplane, disconnect the autopilot and autothrottle. Ensure the pitch attitude does not increase when disconnecting the autopilot. This may be very important in out-of-trim situations. Manual control is essential to recovery in all situations. Leaving the autopilot or autothrottle connected may result in inadvertent changes or adjustments that may not be easily recognized or appropriate, especially during high workload situations.
Reducing the angle of attack is crucial for recovery. This will also address autopilot-induced excessive nose up trim. If the control column does not provide sufficient response, pitch trim may be necessary. However, excessive use of pitch trim may aggravate the condition, or may result in loss of control or high structural loads.
This orients the lift vector for recovery.
During a stall recovery, maximum thrust is not always needed. A stall can occur at high thrust or at idle thrust. Therefore, the thrust is to be adjusted accordingly during the recovery. For airplanes with engines installed below the wing, applying maximum thrust may create a strong nose-up pitching moment if airspeed is low. For airplanes with engines mounted above the wings, thrust application creates a helpful pitch-down tendency. For propeller-driven airplanes, thrust application increases the airflow around the wing, assisting in stall recovery.
This will improve lift and stall margin.
Apply gentle action for recovery to avoid secondary stalls then return to desired flight path.
AC 120-109 Stall and Stick Pusher Training, 8/6/12, U.S. Department of Transportation
Davies, D. P., Handling the Big Jets, Civil Aviation Authority, Kingsway, London, 1985.
Dole, Charles E., Flight Theory and Aerodynamics, 1981, John Wiley & Sons, Inc, New York, NY, 1981.
Gulfstream G450 Aircraft Operating Manual (Historical), Revision 24, September 18, 2009.