7 Stalling
Super Stall (Deep Stall)
A swept-back wing tends to stall first near the tips. Since the tips are situated well aft of the CG, the loss of lift at the tips causes the pitch attitude to increase rapidly and further increase the angle of attack. Figure 7.19.
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PITCH - UP
Stalling
TIP STALL
Figure 7.19 Pitch-up
This “automatic” increase in angle of attack, caused by pitch-up, stalls more of the wing. Drag will increase rapidly, lift will reduce and the aeroplane will start to sink at a constant, nose high, pitch attitude. This results in a rapid additional increase in angle of attack, Figure 7.20.
DOWNWARD INCLINED FLIGHT PATH |
TAILPLANE IMMERSED |
IN SEPARATED AIRFLOW |
FROM STALLED WING |
Figure 7.20 Super stall
Separated airflow from the stalled wing will immerse a high-set tailplane in low energy turbulent air, Figure 7.20. Elevator effectiveness is greatly reduced making it impossible for the pilot to decrease the angle of attack. The aeroplane will become stabilized in what is known as the “super stall” or “deep stall” condition.
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Clearly, the combination of a swept-back wing and a high mounted tailplane (‘T’ - Tail) are the factors involved in the “super or deep stall”. Of the two:
THE SWEPT-BACK WING IS THE MAJOR CONTRIBUTORY FACTOR.
It has been shown that the tendency for a swept-back wing to pitch-up can be reduced by design modifications (wing fences, vortilons and saw tooth leading edges) which minimize the root-to-tip spanwise flow of the boundary layer. These devices delay tip stall. Vortex generators are also frequently used on a swept wing to delay tip stall and improve the stall characteristics.
The wing root can also be encouraged to stall first. This can be done by modifying the aerofoil section at the root, fitting stall strips and by fitting less efficient leading edge flaps (Kruger flaps) to the inboard section of the wing.
Aircraft such as the DC-9, MD-80, Boeing 727, Fokker 28 and others, have swept-back wings and high mounted tailplanes (‘T’ - Tail). They also have rear, fuselage mounted engines. The only contribution rear mounted engines make is that they are the reason the designer placed the tailplane on top of the fin in the first place. In and of itself, mounting the engines on the rear fuselage does not contribute to super stall.
Super Stall Prevention - Stick Pusher
An aircraft design which exhibits super stall characteristics must be fitted with a device to prevent it from ever stalling. This device is a stick pusher. Once such an aircraft begins to stall it is too late; the progression to super stall is too fast for a human to respond, and the aircraft cannot then be un-stalled.
A stick pusher is a device, attached to the elevator control system, which physically pushes the control column forward, reducing the angle of attack before super stall can occur.
The force of the push is typically about 80 lb. This is regarded as being high enough to be effective but not too high to hold in a runaway situation. Provision is made to “dump” the stick pusher system in the event of a malfunction. Once dumped, the pusher cannot normally be reset in flight.
Once actuated, the stick pusher will automatically disengage once the angle of attack reduces below a suitable value.
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7 Stalling
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Factors that Affect Stall Speed
Page 148 details the CAS at which an aircraft stalls (VSR). We know that stalling is caused by exceeding the critical angle of attack. Stalling has nothing to do with the speed of the aircraft; the critical angle of attack can be exceeded at any aircraft speed. However, it has been shown that if an aircraft is flown in straight and level flight and speed reduced at a rate not exceeding 1 knot per second, the CAS at which it stalls can be identified. It is upon this reference stall speed (VSR) that the recommended take-off, manoeuvre, approach and landing speeds are based, to give an adequate margin from the stall during normal operations (1.05VSR, 1.1VSR, 1.2VSR, 1.3VSR etc).
Factors which can affect VSR are:
•Changes in weight.
•Manoeuvring the aircraft (increasing the load factor).
•Configuration changes (changes in CLMAX and pitching moment).
•CG position.
•Engine thrust and propeller slipstream.
•Mach number.
•Wing contamination.
•Heavy rain.
1g Stall Speed
In straight and level flight the weight of the aircraft is balanced by the lift.
Load Factor (n) or ‘g’ = |
Lift |
Weight |
While (n) is the correct symbol for load factor, the relationship between lift and weight has for years been popularly known as ‘g’. (1g corresponds to the force acting on us in every day life). If more lift is generated than weight, the load factor or ‘g’ will be greater than one; the force acting on the aircraft and everything in it, including the pilot, will be greater.
If Lift = Weight, the load factor will be one and from the lift formula:
L = ½ ρ V2 CL S
it can be seen that lift will change whenever any of the other factors in the formula change. We consider density (ρ) and wing area (S) constant for this example. If the engine is throttled back, drag will reduce speed (V) and, from the formula, it can be seen that lift would decrease. To keep lift constant and maintain 1g flight at a reduced speed, CL must be increased by increasing the angle of attack.
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Any further reduction in speed would need a further increase in angle of attack, each succeeding lower CAS corresponding to a greater angle of attack. Eventually, at a certain CAS, the wing reaches its stalling angle (CLMAX), beyond which any further increase in angle of attack, in an attempt to maintain lift, will precipitate a stall. We can transpose the lift formula to show this relationship:-
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Density altitude does not |
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VS1g |
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L |
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affect indicated stall speed |
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ρ |
CLMAX S |
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Effect of Weight Change on Stall Speed
At CLMAX for 1g flight, a change in weight requires a change in lift and it can be seen from the VS1g formula that, for instance, an increase in weight (lift) will increase VS1g
The relationship between basic stalling speeds at two different weights can be obtained from the following formula:
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VS1g new = VS1g old √ |
new weight |
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old weight |
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The angle of attack at which stall occurs will NOT be affected by the weight. (Provided that the appropriate value of CLMAX is not affected by speed - as it will be at speeds greater than M0.4, see page 177). To maintain a given angle of attack in level flight, it is necessary to change the dynamic pressure (CAS) if the weight is changed.
As an example: at a weight of 588 600 N an aircraft stalls at 150 kt CAS. What is the VS1g stall speed at a weight of 470 880 N?
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VS1g new = 150 √ |
470880 |
Weight does not |
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588600 |
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=134 knots CAS
It should be noted that a 20% reduction in weight has resulted in an approximate 10% reduction in stall speed. (As a “rule of thumb”, this relationship can be used to save calculator batteries, and time in the exam!). The change in stall speed due to an increase in weight can be calculated in the same way.
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