ppl_05_e2

ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 10: LIFT AUGMENTATION

But as the angle of attack increases, the wing’s centre of pressure (CP) moves forward, increasing the pressure differential between the wing’s upper and lower surfaces. The resultant aerodynamic force pulls the slat open. See Figure 10.33.

Figure 10.33 At high angles of attack, pressure differential pulls the slat open to create a slot.

In comparison to the pitch change caused by operation of the trailing edge faps, operation of the slats causes only a small change to the pitch moment of the wing. This is because although the centre of pressure moves forward to operate the slats, it does not move so far forward that the pressure distribution pattern over the wing is modifed signifcantly.

Figure 10.34 When the slat deploys, the change in pitch moment is small, because the position of the Centre of Pressure alters very little.

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Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 10: LIFT AUGMENTATION QUESTIONS

Representative PPL - type questions to test your theoretical knowledge of Lift Augmentation.

1.The leading-edge slot allows fight at higher angles of attack:

a.providing an extra lifting surface and hence increase the lift available

b.changing the shape and hence the lift characteristics of the wing

c.re-energising the airfow over the top of the wing, and delaying separation of the boundary layer

d.decreasing lift and hence induced drag

2.The maximum gliding distance from 6000 feet, in still air, for an aircraft in clean confguration, with a lift/drag ratio of 8:1, is approximately 8 nautical miles. If faps are deployed:

a.the maximum gliding distance will increase

b.the maximum gliding distance will be less

c.Lift / Drag ratio will be unaffected but will be achieved at a lower airspeed

d.the maximum gliding distance will be unaffected

3.The maximum speed at which the aircraft can be fown with faps extended is called:

a.VYSE

b VFE

c.VNE

d.VNO

4.In which of the following approach scenarios would you normally select full fap?

a.When commencing the fnal approach

b.On going around

c.When landing into a strong headwind

d.In the latter stages of the approach, when satisfed that you can safely touch down in the designated landing area

5.Which of the following four options describes the consequence of taking off with the manufacturer’s recommended take off fap setting selected?

a.An increase in the length of the take off run compared to a non-fap take off

b.A decrease in the length of the take off run compared to a non-fap take off

c.A greater angle of climb

d.Easier avoidance of obstacles at the end of a runway

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ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 10: LIFT AUGMENTATION QUESTIONS

6.With the faps lowered, the stalling speed will:

a.increase

b.decrease

c.increase, but occur at a higher angle of attack

d.remain the same

7.When faps are lowered the stalling angle of attack of the wing:

a.remains the same, but CLMAX increases

b.increases and CLMAX increases

c.decreases, but CLMAX increases

d.decreases, but CLMAX remains the same

8.A pilot lowers the faps while keeping the airspeed constant. In order to maintain level fight, the nose of the aircraft:

a.must be lowered

b.must be raised

c.must be held at the same attitude but power must be increased

d.must be held at the same attitude and power required will be constant

9.If a landing is to be made without faps the landing speed will be:

a.reduced

b.increased

c.the same as for a landing with faps

d.the same as for a landing with faps but with a steeper approach

10.Lowering the faps during a landing approach:

a.permits approaches at a higher indicated airspeed

b.decreases the angle of descent without increasing power

c.eliminates foating

d.increases the angle of descent without increasing the airspeed

Question	1	2	3	4	5	6	7	8	9	10
Answer

The answers to these questions can be found at the end of this book.

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Customer: Oleg Ostapenko E-mail: [e-mail] Customer: Oleg Ostapenko E-mail: [e-mail]

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ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail] Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 11

STABILITY

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Order: 6026

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 11: STABILITY

Figure 11.1 Positive Static Stability.

Figure 11.2 Neutral Static Stability.

Figure 11.3 Negative Static Stability.

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Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 11: STABILITY

AIRCRAFT STABILITY.

INTRODUCTION.

The dictionary defnes stability as a state of being “frmly fxed”, “established”, “not to be moved”, “not changeable”. Applied to an aircraft, the word stability means that if the aircraft is fying at a certain attitude, it will tend to maintain that attitude.

However, if a stable aircraft is temporarily disturbed from a steady state of fight, it will tend to return to its original attitude without any control movements by the pilot. Stability is inherent in the aircraft by design and varies from one aircraft to another, depending on type and role. An aerobatic aircraft, for instance, would normally be less stable than an aircraft designed specifcally for touring.

Displacement of an aircraft from its state of steady fight may occur because of turbulence or a sudden gust of wind. Any force which displaces the aircraft in this way is usually of a temporary nature.

An aircraft possesses two types of stability: static and dynamic.

A stable aircraft

will tend to return to its

original attitude

after it has been displaced.

Static Stability.

Static stability refers to the initial response of an aircraft when disturbed from a given attitude or fight path. Static stability can be further subdivided into three different types.

•Positive static stability. (See Figure 11.1).

•Neutral static stability. (See Figure 11.2).

•Negative static stability. (See Figure 11.3).

If an aircraft tends to return to its original state or attitude after it has been displaced, it is said to possess positive static stability; put more simply, the aircraft is said to be stable. However, if the aircraft tends to remain in the state or attitude that it acquires after a disturbance, it is said to be neutrally, statically stable. Negative static stability is the tendency for the aircraft, once disturbed, to continue to depart further from its original attitude or fight path. Such an aircraft is said to be unstable.

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Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 11: STABILITY

Dynamic Stability.

The word dynamic refers to movement, and the expression dynamic stability refers to the response of an aircraft tending to correct any displacement from its fight path or attitude over a period of time. Dynamic stability is subdivided into:

•Positive dynamic stability. (See Figure 11.4a).

•Neutral dynamic stability. (See Figure 11.4b).

•Negative dynamic stability. (See Figure 11.4c).

Let us assume that an aircraft has been disturbed in such a way that its nose pitches up, as depicted in Figure 11.4, at Points (a), (b) and (c). In order to return to its original attitude, the aircraft’s initial response must be to pitch nose down again. But, of course, as the aircraft attempts to regain its original attitude it is continuing along its fight path.

Figure 11.4 Degrees of Dynamic Stability.

Static and dynamic

stability can be positive,

neutral or negative, but an aircraft must be statically stable to be dynamically stable.

If the aircraft possesses positive dynamic stability the amplitude of its displacement from its fight path will decrease with passing time (see Figure 11.4a) and the aircraft will soon regain its original fight path.

If the aircraft has neutral dynamic stability the amplitude of its oscillations about its original fight path remains constant (see Figure 11.4b).

Negative dynamic stability is present when the amplitude of the aircraft’s oscillations increases over time, with the aircraft moving further and further away from its original fight path (see Figure 11.4c.).

It should be noted that in all the above situations the aircraft tended to try to regain its original fight path and displayed positive static stability, even when it was not dynamically stable. An aircraft that has positive dynamic stability must possess positive static stability.

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ID: 3658

Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 11: STABILITY

THE AIRCRAFT’S THREE AXES.

An aircraft has three reference axes about which it rotates in fight, when manoeuvred.

They are: the longitudinal axis, the lateral axis and the normal axis. All three axes pass through the Centre of Gravity (C of G) of the aircraft as shown in Figure 11.5.

Figure 11.5 The three axes about which an aircraft rotates.

The longitudinal axis runs from nose to tail, passing through the C of G. Movement about the longitudinal axis is called roll. Whenever you consider stability, think of the motion about the axis concerned. An aircraft that is laterally stable, i.e. stable in roll, is stable about its longitudinal axis.

The lateral axis runs parallel to a line from wing tip to wing tip and passes through the C of G. Movement about the lateral axis is called pitch. An aircraft which is stable in pitch is said to be longitudinally stable.

The normal axis passes vertically through the C of G at 90º to the longitudinal axis. Movement about the normal axis is called yaw. Stability in yaw about the normal axis is termed directional stability.

An aircraft may not be equally stable about all three axes. Generally, an aircraft will be designed with pronounced directional stability, reduced, but still positive, longitudinal stability, and weak to neutral lateral stability.

During development and before certifcation for general use, an aircraft should be able to demonstrate adequate stability in order to maintain steady-state fight throughout its speed range, whilst still allowing proper response to the fight controls. In other words, the aircraft must be stable, but also manoeuvrable. This requirement leads to a “stability compromise” whose extent depends on the aircraft’s role.

A very stable aircraft would show slow response to the pilot’s control movements, which would reduce the aircraft’s controllability and manoeuvrability; the opposite would apply if the aircraft were neutrally stable or unstable, and the aircraft would be more manoeuvrable but more diffcult to fy smoothly and steadily.

The longitudinal

axis runs from nose to tail,

passing through

the Centre of Gravity.

The lateral axis

runs parallel to a line from

wing tip to wing

tip and passes through the C of G.

The normal

axis passes vertically

through the

C of G at 90º to the longitudinal axis.

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Customer: Oleg Ostapenko E-mail: [e-mail]

Customer: Oleg Ostapenko E-mail: [e-mail]

CHAPTER 11: STABILITY

For an aircraft to be stable, aerodynamic forces must produce a corrective turning moment to any disturbance which the aircraft may encounter. We will now consider an aircraft’s stability about each of the three axes.

Longitudinal stability is

stability about the lateral

axis, in the pitching plane.

The centre of pressure of a

wing moves with changing

angle of attack.

LONGITUDINAL STABILITY.

Longitudinal stability (in the pitching plane), is stability about the lateral axis. An aircraft which is longitudinally stable will, following a disturbance in the pitching plane, tend to return to its original pitch attitude.

As you have already learnt, the magnitudes and lines of action of the 4 principal fight forces (lift, weight, thrust and drag), are such that, considered on their own, any disturbance of the aircraft from equilibrium would produce an unstable and uncontrollable pitching moment. For this reason, a tailplane, or horizontal stabiliser is required to generate a balancing moment and give the aircraft longitudinal stability.

Figure 11.6 The tailplane gives an aircraft longitudinal stability.

A conventional aircraft, then, possesses positive longitudinal stability by virtue of its tailplane, or horizontal stabiliser. Let us now examine longitudinal stability a little more closely.

If you think back to what you have learnt about couples and turning moments and examine Figure 11.6, you will see that the degree of longitudinal stability that an aircraft possesses depends on, amongst other things, the position of the Centre of Gravity (through which the weight acts) relative to the position of the Centre of Pressure (through which the lift force acts), and on the distance of the tailplane from the Centre of Gravity.

It should be clear that, when considered in isolation, a wing is dynamically unstable longitudinally. Look at Figure 11.7, overleaf. As the wing begins to pitch up (that is, as angle of attack increases), the Centre of Pressure moves forward and the lift force increases in magnitude. The wing would have a certain amount of static stability if its Centre of Gravity were in front of the initial position of the Centre of Pressure, but the wing is highly unstable dynamically and can not return to equilibrium unless a balancing turning moment is applied to it.

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