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Кировоградская Лётная Академия Национального Авиационного Университета

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ID: 3658

Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com

CHAPTER 13: THE STALL AND SPIN

THE POINT OF STALL AND RECOVERY FROM THE STALL.

During normal flying operations, you should never stall the aircraft except, in the case of a tail-wheel aircraft, at the moment of touchdown, when the aircraft wheels are only a few inches above the ground, with the aircraft in the correct attitude for a three-point landing.

The most important lesson you learn from the stalling exercise, then, will most certainly be how to recognise the signs of the approaching stall, so that you can make sure that no stall occurs.

However, in your flying training, your instructor will demonstrate the fully-developed stall to you and teach you how to recover from it using the standard stall recovery procedure.

Figure 13.23 The point of stall. Lift has decreased below the value of weight, and the Centre of Pressure has moved rearwards.

First of all, let us remind ourselves that when a fully-developed stall occurs the wing has reached the angle of attack for maximum coefficient of lift, CL MAX, around 16° for most wing aerofoil sections on training aircraft. Any further increase of angle of attack will cause the lift to decrease abruptly, while drag increases rapidly. Figure 13.23 depicts the lift-weight couple as the aircraft is at the stall. Lift has decreased below the value of weight, and the centre of pressure has moved rearwards. Notice

Figure 13.24 When the aircraft stalls, the nose drops and the aircraft accelerates, the CP moving forward as lift is regained.

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CHAPTER 13: THE STALL AND SPIN

Several

hundred feet can be lost

during stall recovery, so a stall near the ground could be catastrophic.

To recover

from a fullydeveloped

stall, always use the standard stall recovery procedure.

that this sequence of events sets up a nose-down couple which will actually cause the nose to pitch downwards and so aid in the recovery from the stall. Conventional aircraft wings are designed with lift characteristics which aid in stall recovery in this way.

When the nose pitches down at the stall, the aircraft loses height, by several hundred feet generally speaking, and accelerates. The angle of attack is then reduced such that the wing becomes unstalled. (See Figure 13.24).

However, unless the pilot carries out the standard stall recovery procedure, this “design feature” recovery will be unstable. The lift force would increase, the centre of pressure would move forward again, causing the nose to rise sufficiently for the aircraft to enter a second stall. The standard stall recovery should, therefore, always be carried out, with the controls being operated in a smooth and progressive manner. Power may or may not be used in stall recovery.

Your flying instructor will teach you all about the details of the standard stall recovery procedure; here, we consider only the Principles of Flight aspects of stall recovery.

These are fairly simple.

You have learnt that a stall occurs because the angle of attack between the relative airflow and the aircraft’s wing has reached the stalling angle of attack. An approaching stall which is developing from an initial situation of level flight, or a glide, will be indicated to the pilot by a higher than normal nose attitude and low and decreasing speed. To recover from a stall from level flight or a glide, therefore, the pilot must select a lower angle of attack and allow the speed to increase to normal flying speed. The initial action, then, will be for the pilot to move the control column centrally forward, to select a suitable stall recovery attitude. It is vitally important that the control column is not displaced laterally as the pilot selects the recovery attitude. We discuss this issue in greater detail below. Your instructor will teach you the rest of the stall recovery procedure.

Always be aware that, with a load factor of greater than 1 acting on the airframe, for instance in a turn or when recovering from a high-speed dive, the aircraft can stall from any attitude. Nevertheless, you would still recover from the stall, or prevent the stall occurring, by decreasing the angle of attack, sometimes referred to, in these circumstances, as “unloading” the aircraft.

Figure 13.25 When a stall occurs, one wing may stall before the other.

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ID: 3658

Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com

CHAPTER 13: THE STALL AND SPIN

Wing Drop at the Stall.

You have learnt that, for several reasons, for instance, out-of-balance flight, wing contamination or even small differences in the manufacture of the wings, when a stall occurs, one wing may stall before the other.

If one wing stalls before the other, that wing will drop and the aircraft first rolls and then yaws towards the more stalled wing; if the pilot does not intervene, this situation may lead to autorotation. Wing drop at the stall is, therefore, an unstable and potentially hazardous situation. You will learn about autorotation and the spin in the next section of this chapter. Here, we will look at the Principles of Flight considerations of why wing drop at the stall is an unstable condition, and how it can be corrected.

If you experience a wing drop at the stall it must be dealt with immediately. On first consideration, the wing drop indicates a roll and your instinctive reaction may be to correct the roll by applying aileron in the opposite direction. However, this would be an incorrect and dangerous action.

As we have already mentioned, the basic reason why the wing-drop may occur at the stall is because one wing stalls before the other. As the wing drops, its angle of attack increases still further, and so the dropping wing becomes even more stalled. Conversely, the angle of attack of the up-going wing decreases; this wing, therefore, is less stalled and may even have become unstalled. The increased drag of the stalled, down-going wing causes the aircraft to yaw towards it. This yaw, in turn, tends to speed up the up-going wing and, thus add to its lift. The rolling and yawing will continue until the pilot intervenes. If he does not intervene, the aircraft will enter a spin.

How, then, should a pilot react to wing-drop at the stall? Firstly and most importantly, he must not try to prevent the wing-drop with aileron. That action, by deflecting the aileron on the dropping wing downwards would increase the angle of attack of the dropping wing still further. As the dropping wing has already exceeded its stalling angle of attack, using aileron to try to raise the wing would make the wing even more stalled and so make the wing drop even more severe. (See Figure 13.26).

At the stall, never attempt to lift a

dropping wing with aileron.

Figure 13.26 The dropping wing has already exceeded its stalling angle of attack. Using aileron to prevent wing-drop would aggravate the stall and so make the wing-drop even more severe.

Increasing the severity of the wing-drop would, of course, reinforce the roll. Increased roll would lead to yaw which would induce more roll, and so on, until autorotation set in, putting the aircraft into a spin. Therefore, on no account should aileron be used to prevent or correct wing drop at the stall.

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CHAPTER 13: THE STALL AND SPIN

As you have learnt, the extra drag of the down-going wing causes yaw in the direction of the wing drop which, because it increases the speed and therefore the lift of the up-going wing, may give rise to aerodynamic conditions which may lead to further roll and further yaw and, then, to autorotation and the spin.

But, if the pilot prevents any further yaw, the above sequence of events will be broken.

Rudder is, therefore, the answer to wing drop at the stall. Rudder is used to stop the yaw and minimise the roll. The pilot should not try to level the wings with rudder; he should apply sufficient rudder to prevent further yaw while, at the same time, unstalling the wings using the elevator. As soon as the wings are unstalled, the pilot can centralise the rudder and roll the wings level using coordinated aileron and rudder.

The flying aspects of dealing with wing drop at the stall will be covered by your flying instructor.

Stall Warning.

Warning of the approaching stall may be aerodynamic in nature, as in the case of the pre-stall buffet, decreasing speed, ineffective controls or high nose attitude, or the stall warning may be electro-mechanical as in the case with audio warning devices.

THE SPIN.

When an aircraft spins, it is in a state of stalled flight, and is losing height rapidly in a steep helical descent, yawing, rolling and pitching, at the same time. Both wings are stalled, and the aircraft is auto-rotating under the influence of yawing and rolling moments.

Figure 13.27 The spin is a state of stalled flight with the aircraft in a steep descent, yawing, rolling and pitching at the same time.

Unless an aircraft is being spun intentionally, the spin will almost certainly have arisen because its controls were mishandled during a stall. We examined above how a wing drop at the stall can lead to a spin if the pilot does not react correctly. (See Figure 13.28).

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CHAPTER 13: THE STALL AND SPIN

Figure 13.28 Wing drop at the stall may lead to a spin.

Now we can make a further fundamental statement about the spin. For a spin to develop, yaw must be present at the point of the stall. This yaw is very often caused by one wing stalling first and dropping.

In order better to understand how yaw may be present at the stall, we need to look at the drag situation if wing drop does occur at the stall.

At low speeds and high angles of attack, total drag consists almost exclusively of induced drag. When both wings are stalled, both wings experience high levels of induced drag. But if a wing drops at the stall, the up-going wing is less stalled than the down-going wing (See Figure 13.28). The drag on the up-going wing is, therefore, less than that on the down-going wing, while its lift is greater. This state of affairs is depicted in Figure 13.29 where CL and CD curves are plotted against angle of attack.

Figure 13.29 Wing-drop at the stall - CL and CD curves for the up-going and down-going wings.

In this situation, then, the aircraft first rolls and then yaws towards the more stalled wing. The angle of attack on the up-going wing is then reduced, increasing lift and

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CHAPTER 13: THE STALL AND SPIN

reducing drag on that wing. The dropping wing, consequently, drops even more sharply, increasing angle of attack, aggravating the stall, decreasing its lift, and increasing induced drag, and so on. The unstable rolling and yawing movements towards the lower wing would reinforce each other, leading to autorotation; use of aileron would worsen the situation further. The aircraft would then begin to spin, losing height rapidly in a steep, helical, auto-rotating descent, yawing, rolling and pitching, at the same time.

The aircraft eventually settles down at a steady rate of autorotation. In the spin, although the rate of descent is high, airspeed is not excessive because total drag also remains very high. Because of the relatively low airspeed, load factors are low posing little danger of structural damage to the aircraft.

The situation of the spin, then, is entirely different to that of the spiral dive where speed increases rapidly and structural loads are high.

Determining Direction of the Spin.

There are basically two ways to determine the direction of spin. In good visual flight conditions the direction of spin can be established by looking along the nose of the aircraft to the ground.

Figure 13.30 The turn needle will always indicate the direction of spin.

Alternatively, the Pilot may refer to the Turn and Slip Indicator. The turn needle of the Turn and Slip Indicator will be fully deflected in the direction of the spin. The Turn and Slip Indicator in Figure 13.30 is indicating a spin to the right.

Spinning is not taught in the Private Pilot’s Licence flying syllabus. But it is essential that you understand what makes an aircraft spin so that you can avoid any mishandling of the controls that may lead to a spin. If you undertake the necessary training in an aircraft approved for spinning, you may have an opportunity to try spinning for yourself. However, in your PPL training emphasis will be on spin avoidance, so that you never enter a spin unintentionally.

Spin Recovery.

As a PPL student you will not be required to practise spins; neither is this book a manual of flying instruction. But the general Principles of Flight considerations for spin recovery may be of interest to you. Note that, because both wings are stalled in

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ID: 3658

Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com

CHAPTER 13: THE STALL AND SPIN

a spin, the ailerons are ineffective and cannot be used to recover from a spin. Use of the ailerons will merely deepen the stall on the respective wing.

Spin recovery is generally effected in the following manner:

•Rudder is applied against the direction of spin. This action has the effect of reducing the yaw and slowing down rotation.

•The control column (held central so that there is no aileron deflection) is moved progressively forward to reduce angle of attack and unstall the wings. The forward movement of the control column is continued until the spin stops.

•The rudder is centralised as soon as the spin stops.

•There should then be no delay in levelling out from the dive, or else the aircraft will accelerate rapidly and could exceed VNE. Unnecessary loss of height must also be avoided. The recovery from the dive should be smooth in order to avoid subjecting the aircraft to high load factors which might lead to a further stall.

NB: Spin recovery technique is aircraft type specific and is laid down in the POH.

Spin Avoidance.

Your PPL training in terms of the spin, will most probably be confined to spin avoidance training.

You have learnt that the spin can only occur when the aircraft is stalled, and when the aircraft is moving in the yawing plane at the moment of the stall, either because the pilot is misusing the rudder or because a wing drops at the stall, inducing a yaw in the direction of wing drop.

Consequently, the first lesson to learn about spin avoidance, is that you should avoid the stall. If you never allow your aircraft to stall, it cannot spin.

A second very important aspect of spin avoidance is that you should understand how a spin may develop from a stall through mishandling of the controls. With this understanding, you are unlikely ever to spin inadvertently.

Three very important considerations about correct use of the controls at the stall are:

•If you are practising stalling and stall recovery, make sure that you are applying no yaw at the moment of stall; otherwise the stall may lead to a spin.

•If you experience wing drop at the stall, respond to this in the correct manner. Never attempt to use aileron to lift a dropping wing at the stall. Apply rudder in the opposite direction to the wing drop to stop further yaw and minimise the roll. Do not try to level the wings with rudder. At the same time, move the control column centrally forward to unstall the wings. As soon as the wings are unstalled and normal flying speed in indicated on the ASI, centralise the rudder and roll the wings level using coordinated aileron and rudder.

•Ease out of the dive, smoothly.

The spin is a condition of stalled flight.

Never allow your aircraft to stall, and it will not spin.

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Order: 6026

Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com

CHAPTER 13: THE STALL AND SPIN

Learn the

signs of the approaching

stall thoroughly and always take prompt action to prevent the stall from occurring.

Distractions when you are flying at high angles of attack may lead to mishandling of the controls. System malfunctions, preoccupation with equipment, navigation, radio calls and avoidance of other aircraft are just some of the everyday distractions that you might encounter and which might lead to uncoordinated control movements at a critical stage of flight.

Conclusion.

Finally, to end this chapter, the importance of stall recognition and prompt application of correct recovery action cannot be over-emphasised. Remember, too, that on takeoff and landing, when the aircraft is close to the ground, the airspeed is low and the controls sluggish, with the possible presence of turbulence, and all the other distractions that take your attention at these critical stages of fight, there may be insuffcient height to complete a recovery if the aircraft should stall and spin.

Always remain vigilant, therefore, and never get into a situation where your aircraft can stall without your having intended it.

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CHAPTER 13: THE STALL AND SPIN

Representative PPL - type questions to test your theoretical knowledge of Stalling and Spinning.

1.An aeroplane will stall at the same:

a.angle of attack and attitude with relation to the horizon

b.airspeed regardless of the attitude with relation to the horizon

c.angle of attack regardless of the attitude with relation to the horizon

d.indicated airspeed regardless of altitude, bank angle and load factor

2.A typical stalling angle of attack for a wing without sweepback is:

a.4°

b.16°

c.30°

d.45°

3.If the aircraft weight is increased, the stalling angle of attack will:

a.remain the same

b.decrease

c.increase

d.the position of the C of G does not affect the stall speed

4.If the angle of attack is increased above the stalling angle:

a.lift and drag will both decrease

b.lift will decrease and drag will increase

c.lift will increase and drag will decrease

d.lift and drag will both increase

5.The angle of attack at which an aeroplane stalls:

a.will occur at smaller angles of attack fying downwind than when fying upwind

b.is dependent upon the speed of the airfow over the wing

c.is a function of speed and density altitude

d.will remain constant regardless of gross weight

6.In a steady, level turn, at 60° angle of bank, the stalling speed of an aircraft which has a straight fight stalling speed of 60 knots IAS, would be:

a.43 kt

b.60 kt

c.84 kt

d.120 kt

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Order: 6026

Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com

CHAPTER 13: THE STALL AND SPIN

7.The stalling speed of an aircraft in a steady turn would be:

a.the same as in level fight

b.lower than in level fight

c.higher than in level fight, and a lower angle of attack

d.higher than in level fight and at the same angle of attack

8.A wing with washout would have:

a.the tip chord less than the root chord

b.the tip angle of incidence less than the root angle of incidence

c.the tip angle of incidence greater than the root angle of incidence

d.the tip camber less than the root camber

9.If aircraft weight is increased, stalling speed will:

a.remain the same

b.decrease

c.increase

d.weight does not affect the stalling speed

10.Stalling may be delayed until a higher angle of attack is reached by:

a.increasing the adverse pressure gradient

b.increasing the surface roughness of the wing top surface

c.distortion of the leading edge by ice build-up

d.increasing the kinetic energy of the boundary layer

11.Slots increase the stalling angle of attack by:

a.Increasing leading edge camber

b.Retarding the onset of turbulence and delaying separation of the boundary layer

c.Reducing the effective angle of attack

d.Reducing span-wise fow

12.An aeroplane wing stalls when:

a.The indicated airspeed is too low

b.The critical angle of attack is exceeded

c.The laminar airfow becomes turbulent

d.It is subjected to unusually high ‘G’ forces

13.The stalling speed of an aircraft is a function of the:

a.Inverse of the Load Factor

b.Indicated airspeed

c.Square of the weight

d.Square root of the Load Factor

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