Chapter

7

Stalling

Introduction					145
Cause of the Stall . . . . . . . . . .	. . . . . . . . . . . . . .	. . .	.		. 145
The Lift Curve . . . . . . . . . . . .	. . . . . . . . . . . . . .	. .	.	.	146
Stall Recovery . . . . . . . . . . . .	. . . . . . . . . . . . . .	. .	.	.	146
Aircraft Behaviour Close to the Stall					147
Use of Flight Controls Close to the Stall . .	. . . . . . . . . . . . . .	. . .	.		. 147
Stall Recognition . . . . . . . . . .	. . . . . . . . . . . . . .	. . .	.		. 148
Stall Speed					148
Stall Warning . . . . . . . . . . . .	. . . . . . . . . . . . . .	. .	.	.	150
Artificial Stall Warning Devices					151
Basic Stall Requirements (EASA and FAR)					154
Wing Design Characteristics . . . . . .	. . . . . . . . . . . . . .	. . .	.		. 154
The Effect of Aerofoil Section . . . . . .	. . . . . . . . . . . . . .	. .	.	.	154
The Effect of Wing Planform					156
Key Facts 1					162
Super Stall (Deep Stall)					166
Super Stall Prevention - Stick Pusher					167
Factors That Affect Stall Speed					168
1g Stall Speed . . . . . . . . . . . .	. . . . . . . . . . . . . .	. .	.	.	168
Effect of Weight Change on Stall Speed					169
Composition and Resolution of Forces . .	. . . . . . . . . . . . . .	. . .	.		. 170
Using Trigonometry to Resolve Forces					170
Lift Increase in a Level Turn					171
Effect of Load Factor on Stall Speed					172
Effect of High Lift Devices on Stall Speed . .	. . . . . . . . . . . . . .	. .	.	.	173
Effect of CG Position on Stall Speed . . . .	. . . . . . . . . . . . . .	. .	.	.	174
Effect of Landing Gear on the Stall Speed .	. . . . . . . . . . . . . .	. . .		.	175
Effect of Engine Power on Stall Speed					175

Continued Overleaf

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7 Stalling

Stalling 7

Effect of Mach Number (Compressibility) on Stall Speed . .	. .	. .	. . . . . .		. 177
Effect of Wing Contamination on Stall Speed					179
Warning to the Pilot of Icing-induced Stalls					181
Stabilizer Stall Due to Ice . . . . . . . . . . . . . .	. .	. .	. . . . . .		. 182
Effect of Heavy Rain on Stall Speed . . . . . . . . . .	. .	. .	. . . . . .		. 182
Stall and Recovery Characteristics of Canards					182
Spinning . . . . . . . . . . . . . . . . . . . .	. .	. .	. . . . . .		. 183
Primary Causes of a Spin . . . . . . . . . . . . . .	. .	. .	. . . . . .		. 183
Phases of a Spin					184
The Effect of Mass and Balance on Spins . . . . . . . .	. .	. .	. . . . . .		. 185
Spin Recovery . . . . . . . . . . . . . . . . . .	. .	. .	. . . . . .		. 185
Special Phenomena of Stall					187
High Speed Buffet (Shock Stall) . . . . . . . . . . . .	.	. .	. . . . . .	.	189
Answers to Questions on Page 173 . . . . . . . . . .	. .	. .	. . . . . .		. 191
Key Facts 2					192
Questions . . . . . . . . . . . . . . . . . . . .	.	. .	. . . . . .	.	195
Key Facts 1 (Completed) . . . . . . . . . . . . . .	. .	. .	. . . . . .		. 201
Key Facts 2 (Completed) . . . . . . . . . . . . . .	. .	. .	. . . . . .		. 204
Answers . . . . . . . . . . . . . . . . . . . .	. .	. .	. . . . . .		. 206

Note: Throughout this chapter reference will be made to EASA Certification Specifications (CS23, CS25) stall requirements etc, but it must be emphasised that these references are for training purposes only and are not subject to amendment action.

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Stalling 7

Introduction

Stalling is a potentially hazardous manoeuvre involving loss of height and loss of control. A pilot must be able to clearly and unmistakably identify an impending stall so that it can be prevented. Different types of aircraft exhibit various stall characteristics, some less desirable than others. Airworthiness authorities specify minimum stall qualities that an aircraft must possess.

Cause of the Stall

The CL of an aerofoil increases with angle of attack up to a maximum (CLMAX ). Any further increase above this stalling angle, or critical angle of attack, will make it impossible for the

airflow to smoothly follow the upper wing contour, and the flow will separate from the surface, causing CL to decrease and drag to increase rapidly. Since the CLMAX of an aerofoil corresponds to the minimum steady flight speed (the 1g stall speed), it is an important point of reference.

A stall is caused by airflow separation. Separation can occur when either the boundary layer has insufficient kinetic energy or the adverse pressure gradient becomes too great.

Figure 7.1 shows that at low angles of attack virtually no flow separation occurs before the trailing edge, the flow being attached over the rear part of the surface in the form of a turbulent boundary layer.

As angle of attack increases, the adverse pressure gradient increases, reducing the kinetic energy, and the boundary layer will begin to separate from the surface at the trailing edge.

Further increase in angle of attack makes the separation point move forward and the wing area that generates a pressure differential becomes smaller. At angles of attack higher than approximately 16°, the extremely steep adverse pressure gradient will have caused so much separation that insufficient lift is generated to balance the aircraft weight.

Stalling 7

Figure 7.1

It is important to remember that the angle of attack is the angle between the chord line and the relative airflow. Therefore, if the angle of attack is increased up to or beyond the critical angle, an aeroplane can be stalled at any airspeed or flight attitude.

An aeroplane can be stalled at any airspeed or attitude

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7 Stalling

The Lift Curve

	CLMAX
CL
7
Stalling				Stall
0	4	8	12	16
	Angle of Attack in Degrees (α)

Figure 7.2

Figure 7.2 shows that as the angle of attack increases from the zero lift value, the curve is linear over a considerable range. As the effects of separation begin to be felt, the slope of the curve begins to fall off. Eventually, lift reaches a maximum and begins to decrease. The angle at which it does so is called the stalling angle or critical angle of attack, and the corresponding value of lift coefficient is CLMAX. A typical stalling angle is about 16°.

Stall Recovery

To recover from a stall or prevent a full stall, the angle of attack must be decreased to reduce the adverse pressure gradient. This may consist of merely releasing back pressure, or it may be necessary to smoothly move the pitch control forward, depending on the aircraft design and severity of the stall. (Excessive forward movement of the pitch control, however, may impose a negative load on the wing and delay recovery). For most modern jet transport aircraft it is usually sufficient to lower the nose to the horizon or just below while applying maximum authorized power to minimize height loss.

On straight wing aircraft the rudder should be used to prevent wing drop during stall and recovery. On swept wing aircraft it is recommended that the ailerons be used to prevent wing drop, with a small amount of smoothly applied co-ordinated rudder. (The rudder on modern high speed jet transport aircraft is very powerful, and careless use can give too much roll, leading to pilot induced oscillation - PIO).

Allow airspeed to increase and recover lost altitude with moderate back pressure on the pitch control. Pulling too hard could trigger a secondary stall, or worse, could exceed the limit load factor and damage the aircraft structure. As angle of attack reduces below the critical angle, the adverse pressure gradient will decrease, airflow will re-attach, and lift and drag will return to their normal values.

146