ppl_05_e2
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CHAPTER 13: THE STALL AND SPIN
14.If the Angle of Attack is increased beyond the Critical Angle of Attack, the wing will no longer produce suffcient lift to support the weight of the aircraft:
a.Unless the airspeed is greater than the normal stall speed
b.Regardless of airspeed or pitch attitude
c.Unless the pitch attitude is on or below the natural horizon
d.In which case, the control column should be pulled-back immediately
15.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
16.The reason for washout being designed into an aircraft wing is to:-
a.Increase the effectiveness of the faps
b.Cause the outboard section of the wing to stall frst
c.Decrease the effectiveness of the ailerons
d.Cause the inboard section of the wing to stall frst
17.When the aircraft is in a spin, the direction of spin is most reliably found by reference to which of the following indications?
a.Artifcial horizon
b.Slip indicator
c.Direction indicator
d.Turn needle
18.At the stall, the Centre of Pressure moving backwards will cause the nose to
_____ , and the decreased lift will cause the aircraft to _____.
a.Yaw, reduce speed
b.Drop, lose height
c.Rise, sink
d.Drop, reduce speed
19.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
20.On a given type of aircraft, the C of G moves forward as fuel is used; therefore:
a.The stalling speed will increase
b.The stalling speed will decrease
c.The stalling speed will remain exactly the same
d.Any of the above could be true
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CHAPTER 13: THE STALL AND SPIN
21.If a wing drops at the stall:
a.The downgoing wing becomes more stalled while the upgoing wing becomes less stalled
b.The wings should be immediately levelled by use of aileron
c.The secondary effect of yaw should be utilised
d.The upgoing wing becomes more stalled than the downgoing wing
22.Following a stall from straight fight, which of the symptoms below best describes the fully developed stall?
a.The airspeed begins to decrease
b.The nose pitches down and the aircraft loses height
c.The nose pitches up and the aircraft controls become sloppy
d.A wing tends to drop
23.When is the coeffcient of lift at a maximum?
a.At or just before the stall
b.With the elevator fully defected upwards
c.When the lift/drag ratio is at its most favourable
d.At about 4° angle of attack
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The answers to these questions can be found at the end of this book.
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CHAPTER 14
FLIGHT AND GROUND
LIMITATIONS
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CHAPTER 14: FLIGHT AND GROUND LIMITATIONS
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CHAPTER 14: FLIGHT AND GROUND LIMITATIONS
AIRCRAFT STRUCTURAL INTEGRITY.
An aircraft’s structure must be able to withstand the loads to which it is subjected during normal operations, in fight and on the ground.
In order to obtain type approval, the design of a light aircraft must conform to certain standards, among which is the requirement that the aircraft structure should have adequate strength and stiffness to withstand, without suffering structural damage, infight manoeuvring loads, plus induced, transient loads caused by turbulence, so that the aircraft may operate within its prescribed category; for instance, normal, utility, or aerobatic. Aircraft categories are defned in the ‘Aeroplanes’ volume of this series.
Figure 14.1 An aircraft’s structure must be able to withstand the flight loads it will encounter in its operational role.
The safety and integrity of the aircraft also demand that the aircraft’s structure be able to withstand loads which exceed the design loads by a prescribed safety margin. The factor which is allowed by the aircraft designer, in terms of the strength of the aircraft’s structure, for fight beyond normal operating limits is called the safety factor.
The designer cannot afford, however, to make the safety factor too great, or else the aircraft would become too heavy and could not operate effciently. For a light aircraft, the safety factor is normally in the order of 1.5. That means that the aircraft’s structure will support, without catastrophic damage, a load 1.5 times the greatest load envisaged by the designer. Loads greater than those allowed for by the safety factor will almost certainly cause serious structural damage to the airframe.
In order that an aircraft is not fown in a manner or confguration which would impose loads on the aircraft’s structure that it is not designed to withstand, the aircraft manufacturer specifes certain limitations on the operation of the aircraft. These limitations are generally expressed in terms of mass (or weight), airspeed, load factor, position of centre of gravity and wind speed.
General limitations are published in the Pilot’s Operating Handbook for the aircraft, and in the aircraft’s Check List, so that the pilot may know that he is to operate within these limitations in order to ensure the safety of his aircraft. A number of limitations, expressed in terms of selected limiting speed ranges, are also displayed to the pilot on the Airspeed Indicator (ASI), in the form of coloured arcs.
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CHAPTER 14: FLIGHT AND GROUND LIMITATIONS
Figure 14.2 The Pilot’s Operating |
Figure 14.3 The ASI has coloured arcs |
Handbook. |
showing certain limiting speed-ranges. |
LIMITATIONS.
The designed structural strength of an aircraft is determined by the magnitude and direction of the loads that the designer expects to be imposed on the aircraft, in fight and on the ground. The word load is just another name for force. You have already learnt a considerable amount about the main loads acting on an aircraft: lift, weight, thrust and drag.
Stress and Strain.
All loads induce stresses in an aircraft’s structure which must be limited to ensure the aircraft’s continuing structural integrity. Stress is defned simply as load per unit area of the structure supporting the load.
Load
Stress =
Area
The greater the load, the greater the stress. The greater the surface area, the lower the stress.
Any amount of stress produces a defection or deformation in the component under stress. Such defection and deformation is called strain. Normal operating stress is not a problem for an aircraft. All airframe components will experience acceptable levels of strain under normal fight loads and stresses, (for instance, the wings may defect markedly under certain load factors, as they are designed to do) but will return to normal dimensions and positions when the loads are removed. If, however, stresses on the airframe become excessive, due to the application of excessive fight loads, strain in a component may become permanent. Any permanent deformation must be regarded as damage to the airframe. If an airframe is loaded beyond its design limit, the associated stresses will result in such excessive strain that one or more components may break. The stress imposed on an aircraft by thrust forces are low in a light aircraft.
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CHAPTER 14: FLIGHT AND GROUND LIMITATIONS
Stresses arising from lift and weight loads can be very high, however; so it is lift and weight which are the principal loads which must be limited in fight and on the ground. The lift forces acting on an airframe are dependent, practically, on speed, load factor (manoeuvres) and angle of attack (Lift = CL ½ ρV2 S). Consequently, limit load considerations are closely related to speed limitations. Drag forces are also dependent on speed.
In this Chapter we shall be looking, primarily, at how aircraft manoeuvres and speeds need to be subject to limitations in order to ensure the aircraft’s safety. But frst let us look, briefy, again at some of the Principles of Flight concepts that we have already met; this time from the point of view of their importance in setting ground and fight limitations, designed to ensure the safety of an aircraft. We will reconsider briefy the concepts of lift, weight, Centre of Gravity position, airspeed, load factor, and windspeed.
Lift.
We have considered lift as acting through the Centre of Pressure on the wing. This is a sound assumption for predicting and measuring the resultant effect of lift on the aircraft. But, in reality, lift is actually developed over the whole surface area of the aircraft, and is a spread load, though not equally spread.
In fight, the various degrees of magnitude of the lift force, necessary to support the load factors of different manoeuvres, cause bending moments to be applied to the wing spars. The bending moments themselves vary in magnitude along the wing. The stress caused by bending is greatest at the point at which the wings are attached to the fuselage; that is, at the wing root. The bending moment will be zero at the wing-tip and maximum at the root. If a limit is not set for the lift force that an aircraft generates, bending moments in the wing may become dangerously large, causing stresses and strains that the wing structure has not been designed to deal with.
Weight.
Even though the weight of an aircraft may be considered as acting as a point load through the aircraft’s Centre of Gravity, weight, like lift, may be viewed as a spread load. Each individual component part of an aircraft possesses its own weight.
Where the phenomenon of futter on a control surface is concerned, for instance, it is the weight and Centre of Gravity of the control surface, alone, which must be considered.
The aircraft’s all-up weight is of considerable importance, and must have prescribed limits because of the effect on the aircraft’s structural integrity, both in fight and on the ground. All-up weight will also affect the various limiting speeds, including stalling speed, both straight and level and under different load factors. The combined weight of individual components, and their position in the aircraft will determine the position of the aircraft’s Centre of Gravity. In this respect, certain weights, such as crew weight, will have minimum as well as maximum limits, or else there is a danger that the position of the aircraft’s Centre of Gravity could lie outside its prescribed limits.
During fight, the aircraft consumes fuel, so the weight of the aircraft constantly changes. As fuel tanks empty, the distribution of the weight throughout the aircraft changes; consequently, the position of the aircraft’s Centre of Gravity changes, too. For reasons that you learnt earlier in this volume, it is important that the aircraft’s
Centre of Gravity should remain within limits for the duration of the fight.
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CHAPTER 14: FLIGHT AND GROUND LIMITATIONS
All aircraft have published maximum weights designed to protect their structure, and which the pilot must ensure are not exceeded. Among these are: maximum gross weight, maximum take-off weight and maximum landing weight. If weight is increased, lift will have to increase; so, as the lift formula teaches us, in any given phase of fight, a heavy aircraft will either have to fy at a greater angle of attack (i.e. higher CL) than a lighter aircraft, or with the same CL and at greater speed.
You will often fnd the word mass substituted for weight, and, when manoeuvring, it is, scientifcally speaking, mass which is the true consideration. In your practical fying, though, do not worry about the difference between mass and weight. As you have learnt, the Earth’s gravitational acceleration is as good as constant in the realms in which aircraft fy, so weight and mass can be considered as giving us the same type of information. However, as a student of Principles of Flight, you now know that 1 kg mass is equal to 9.81 Newtons weight. This issue has been dealt with fully in the chapter on Weight & Mass.
Performance considerations, as well as structural considerations, lead to the imposition of maximum weights. Performance considerations are related to temperature, density, runway conditions, etc, and are dealt with later in this book, in the section on Aircraft Performance.
You will learn more about maximum weights and masses in Volume 6 of this series which contains the topic Mass and Balance.
The Line of Action of a Force or Load.
The four principal fight forces also give rise to secondary forces which impose different types of load and stresses on the aircraft. We have already seen that lift can cause considerable defections and oscillations of the control surfaces (futter) at high speeds. If the aircraft, when fying fast, is also subjected to turbulence, the transient loads caused by the turbulence can be very high. If the pilot manoeuvres his aircraft in this situation, displacement of the control surfaces will impose even higher, twisting loads on the wings, tailplane and fuselage. The lowering of faps and undercarriage would exacerbate the situation when transient loads are high.
This latter point is important. It is not only the magnitude of a load acting on the aircraft’s structure which determines whether the load remains within limits, but also the line of action of the applied load. The fight forces developed by the ailerons when they are defected, for instance, apply a twisting moment to the control surface and wing. It is only the degree of stiffness of the aileron (that is, its resistance to strain) which allows the aerodynamic force generated by the aileron to cause the aircraft to roll. The wing is subject to twisting forces whose line of action changes because the centre of pressure of the wing moves fore and aft with changing angle of attack. The same phenomenon occurs at the tailplane, which in turn can apply twisting and bending moments to the fuselage.
All these considerations have to borne in mind by the aircraft designer when he is deciding on how strong an aircraft structure needs to be, and when establishing fight limitations.
Airspeed.
As the lift and drag equations (Lift = CL ½ ρ v2 S and Drag = CD ½ ρ v2 S) tell us, the aerodynamically generated lift and drag loads applied to an aircraft’s structure are directly proportional to the square of the airspeed. Of these two loads, it is lift which
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CHAPTER 14: FLIGHT AND GROUND LIMITATIONS
is the greater, and which gives rise to the loads which can cause structural damage to the aircraft.
In the lift equation, v is the true airspeed. You have already learnt, though, that indicated airspeed (the speed given by the airspeed indicator (ASI)) is a measure of that part of the lift equation designated by the term ½ ρ v2, otherwise known as dynamic pressure. Therefore, as dynamic pressure is, at the same time, a factor in the lift equation which determines the magnitude of the lift, and also a factor which can be read directly by the pilot from the ASI as indicated airspeed, all speed limitations published by the manufacturer are indicated airspeeds.
You may remember from Chapter 4 that indicated airspeed, vi, is related to true airspeed, v, by a factor, known as relative density, which represents the ratio between the actual density of the airfow being measured, ρ, and the density at sea-level in the ICAO Standard Atmosphere (ISA), ρisa , which is 1.225 kg/m3. The formula relating vi to v is as follows:
ρ
vi = sv
ρisa
So, as lift is directly proportional to v2, and as vi, is directly proportional to v, lift is also directly proportional to vi2 . This square relationship between lift and airspeed means that if a limiting airspeed is exceeded by even a small amount, the effect on lift can be great. For instance, doubling the airspeed will increase the lift force by 4. Great attention must, therefore, be paid to your airspeed if you are fying near the upper airspeed limitations.
The airspeed that should never be exceeded is designated as VNE. VNE is indicated on airspeed indicators by a red line. This airspeed is calculated by the designer for each aircraft model. If an aircraft exceeds VNE, deformation of or structural damage to one or more airframe components may occur. An aircraft may be safely fown at VNE in smooth, calm conditions, but it is not safe to fy at or near VNE in turbulence.
Of course, fying at too low an airspeed can cause lift to reduce below the value necessary to support the weight of the aircraft, and lead to a stall. Consequently, there are lower airspeed limits, too. The principal airspeed limitations are: the stall speed from straight and level fight (i.e. where the load factor =1), VS, the neverexceed speed, VNE, the maximum normal operating airspeed, VNO, the maximum manoeuvring speed, VA, the maximum speed with faps extended, VFE , and maximum speeds related to the extension and retraction of retractable undercarriages, VLO and VLE.
The importance of airspeed as a measure of an aircraft’s structural limitations is dealt with in more detail later in this chapter, in the section on the Manoeuvring Envelope.
Load Factor.
The designed strength of the aircraft structure is not only determined by the basic aerodynamic loads imposed on it in fight, or by its weight alone when it is stationary on the ground. In fight, loads are increased whenever the aircraft manoeuvres.
The “inertial multiplication” of the lift and weight forces during manoeuvring subjects the aircraft to stresses which the designer must take into account when he is calculating the required strength of the aircraft’s structure. Two very similar aircraft
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CHAPTER 14: FLIGHT AND GROUND LIMITATIONS
may need to have different structural strengths depending on the role for which they are designed; for instance, whether they are aerobatic aircraft or touring aircraft. An aircraft whose structure is designed to be strong enough for touring fight and basic manoeuvres only (plus a safety factor), could suffer catastrophic damage if it were to carry out certain aerobatic manoeuvres for which it was not “stressed”. An aircraft must be cleared for any aerobatic manoeuvres that a pilot may wish to carry out. In a touring aircraft, manoeuvres which could cause damage to the airframe will be explicitly forbidden in the aircraft’s Type Certifcate, and in the Flight Manual or Pilot’s
Operating Handbook. These documents will also contain details of limiting speeds and load factors which may be different for two seemingly identical aircraft.
An aircraft manufacturer will specify a limit to the load factor both positive and negative, to which the aircraft may be subjected. Exceeding those limits can lead to structural damage or failure.
You have learnt that whereas lift is always equal to weight, in steady, straight fight, lift must be greater than weight during turns. When lift is greater than weight, the loads acting on an aircraft increase by a factor called the load factor. Load factor is expressed by the ratio of lift to weight, such that:
Load Factor = |
Lift |
|
Weight |
||
|
In steady straight fight, then, with lift equal to weight, load factor is 1, but in any form of accelerated fight, when the velocity of the aircraft is changing, the load factor will be greater or less than one 1. In positive accelerations, such as turns, the load factor will be greater than 1. An inverted loop would bring the load factor below one, and will actually cause it to be a negative factor. Remember that an aircraft is accelerating when either its speed or direction is changing.
A light aircraft’s linear acceleration, along its line of fight, is usually small enough to be neglected. Linear acceleration is greatest at take-off, or when bringing the aircraft to a rapid stop after landing (negative acceleration or deceleration), but, in neither case is the acceleration likely to be as great as that of a car. However, aircraft accelerations necessary for turning fight, pulling out of dives, or for other manoeuvres, are signifcant. Consequently, the load factors produced by those accelerations will be high, (or, less frequently, low, in negative g manoeuvres). For instance, a 60° banked level turn requires lift to be twice the magnitude of weight, generating a load factor of 2. Pulling out of a dive or fying a loop is likely to subject the aircraft to higher load factors.
We will return to load factor, later in this chapter.
Position of Centre of Gravity.
The effect of the position of the aircraft’s Centre of Gravity (C of G) on aircraft stability and control has been covered earlier in this book. The topic is also dealt with in some detail in the subject ‘Mass & Balance’, elsewhere in this series of text books. We will just emphasise, here, that the aircraft’s C of G must lie within the specifed forward and aft limits (see Figure 14.4), calculated by the aircraft designer, and must remain within those limits, throughout the fight. If the C of G is too far forward, comparatively larger control column movements would be necessary to manoeuvre
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