ppl_05_e2

THE PRINCIPAL FLIGHT FORCES.

INTRODUCTION.

You have now studied in some detail the forces acting on an aircraft in fight: lift, drag, weight and thrust. In this chapter, we will look at the distribution of and interrelationship between the four fight forces, and how a pilot may control those forces to maintain the forces in balance (equilibrium), in steady, straight fight. We will also examine the role of the principal fight forces in turning fight.

The detailed study of the aircraft’s performance during take-off, climb, cruise, descent and landing will be covered in the Aircraft Performance section, which you will fnd later in this book.

We will begin by reminding ourselves that the fight forces act in different directions.

Lift acts at 90° to the relative airfow, and is considered as acting though the centre of pressure. If angle of attack is increased, but remains below the stalling angle, the centre of pressure will move forwards along the wing chord. With decreasing angle of attack, the centre of pressure will move rearwards.

Weight always acts vertically downwards through the aircraft’s centre of gravity.

During fight, the aircraft consumes fuel, and so the weight of the aircraft constantly changes. As individual fuel tanks empty, the position of the aircraft’s centre of gravity, may change. Its position must, however, remain within prescribed limits. When the aircraft manoeuvres in pitch, roll and yaw, the aircraft rotates about its centre of gravity.

Thrust may be considered to act along the line of the propeller shaft.

Drag may be considered to act parallel to the relative airfow so as to resist the motion of the aircraft. The actual line of action of the total drag is diffcult to determine except by experiment, and will vary with changing angle of attack.

As you can deduce from the above descriptions of the lines of action of the fight forces, and from what you have learnt so far, the four fight forces do not act through one point. So, if the designer and pilot require the aircraft to fy in steady straight fight, the interrelationship between the lines of action of the fight forces must be understood.

STRAIGHT FLIGHT AT CONSTANT SPEED.

Equilibrium.

In steady, straight fight, at constant speed, the four forces will be in equilibrium.

With the four forces all in equilibrium, the forces balance each other exactly, either one against one, or two together against the other two together. With the forces in equilibrium, the aircraft continues fying in its steady state without any change in attitude or speed. This state of equilibrium may be achieved in straight and level fight, a straight climb or a straight descent.

When manoeuvring, an aircraft rotates about its centre of gravity.

In steady,

straight flight, whether

maintaining altitude, climbing or

descending, all the flight forces acting on the aircraft are in equilibrium.

For a light aircraft in

steady, straight flight,

the lift and weight forces will typically be in the order of 10 times the magnitude of thrust and drag forces.

Figure 9.1 An idealistic state of equilibrium of the four main flight forces in straight and level flight.

You should note that, in reality, there is a considerable difference in the magnitude of the two pairs of forces: lift-weight, and thrust-drag. For a light aircraft the lift will typically be in the order of 10 times as great as the drag, giving a lift-drag ratio of 10. You have already learnt about the importance of lift-drag ratio.

Of course, if the throttle is closed, so that the engine is at idle, the thrust developed by the propeller is negligible and can be ignored. In this throttled-back situation, with the aircraft in a steady straight glide, there are only three forces acting on the aircraft. In a straight glide, it is these three forces, lift, weight and drag, which must be in equilibrium, as depicted in Figure 9.2. We briefy introduced the distribution of forces in the glide, in the chapter dealing with lift-drag ratio, and you will meet the glide again, in the En-Route section of Aircraft Performance, later in this volume. Figure 9.2 depicts an idealistic case of equilibrium for a straight glide, with the aircraft’s weight being balanced by the resultant (i.e. the total reaction) of the lift and drag forces.

Figure 9.2 An aircraft in a straight glide with three forces in equilibrium and no out-of-balance turning moment.

On an aircraft of conventional design, such as the PA28 Warrior that we depict here, whatever out-of-balance turning moment is generated by the principal fight forces, the required correcting moment is provided by the aircraft’s, tailplane or, in the case of the Warrior, the stabilator, (or all-fying tailplane).

Because the tailplane or stabilator is situated at a considerable distance from the centre of gravity, about which the pitching motion of the aircraft takes place, the force generated by the tailplane or stabilator need be only small. In the case illustrated by Figure 9.6, the stabilator force of the PA 28 would be a downwards force, producing a correcting, nose-up pitching moment about the centre of gravity, to balance the turning moments generated by the main fight forces.

Figure 9.6 With this arrangement of the principal flight forces, equilibrium is achieved because of the balancing moment generated by the stabilator. All forces are in equilibrium, and there is no out-of-balance moment.

The out-of-

balance turning moments

caused by a

typical disposition of the thrust/ drag and lift/weight couples are balanced by the tailplane or stabilator force.

The basic function of the tailplane or stabilator, then, is to stabilise the aircraft in pitch. It does this by supplying the turning moment necessary to counter any out- of-balance moment in the pitching plane arising from the lift-weight and thrust-drag couples.

With the centre-

of-pressure behind the

centre of gravity

the balancing tailplane/ stabilator force, typically, acts downwards.

If the pilot increases power and

maintains a constant pitch attitude, the aircraft will begin to climb.

The balancing moment produced by a fxed tailplane, horizontal stabiliser, or stabilator can counter the resultant pitching moment produced by the principal fight forces for only one condition of level fight. However, in practical piloting, a pilot’s control over the elevators or stabilator allows him to modify that surface’s balancing moment in order to counter the resultant pitching moment of the main fight forces, for all conditions of fight. (See Figure 9.7, previous page) The student pilot is taught this control principle in his very frst fying lesson: fore and aft movements of the control column control the aircraft in pitch through movement of the elevators or stabilator.

More About the Relationship Between the Main Flight Forces - Increasing Power.

We have now seen, then, how the forces acting on an aircraft can be maintained in equilibrium by the balancing moment of the tailplane, horizontal stabiliser, or stabilator which is under the control of the pilot through his control of the elevators or stabilator

You have also learnt that a necessary condition for straight and level fight, at constant speed, is that all fight forces should be in equilibrium. But in order to maintain straight and level fight at a selection of airspeeds you must understand a little more of how the principal fight forces are interrelated.

Most importantly, you should understand that a change in any one of the forces of thrust, lift and drag will generate a change in the other two.

Look again at Figure 9.6, and let us imagine that, from this straight and level cruising situation, the pilot increases power by opening the throttle a little further. When the pilot increases power, thrust is increased. Thrust will now be greater than drag and the aircraft will accelerate. As the lift formula, Lift = CL ½ ρv2 S, tells us, the increase in speed will cause an increase in lift, which, because it is now greater than weight, will cause the aircraft to begin to climb. Furthermore, an increase in power will almost certainly cause a change in pitch attitude. In most cases, the nose will pitch up because of increased tailplane downforce from the greater slipstream, unless the pilot prevents the attitude change

If the nose does pitch up, there will be an increase in angle of attack, and, thus, in CL , leading to an increase in lift occurring before, and delaying, that due to the increase in speed. Drag will increase, too, both because of the rise in CD owing to the increasing angle of attack, and also because of the eventual extra speed (Drag = CD ½ ρV2 S). Drag will go on rising until thrust and drag balance each other again, and the aircraft stops accelerating. If the pilot does not choose to hold his pitch attitude constant, the increase in the angle of attack induced by the frst application of power will reach an angle where CD and drag have increased to such a level that the speed is now lower than when the pilot frst increased power. This decrease in speed will, of course, cause the lift force to reduce again despite the increase in CL. The decrease in speed will also decrease the drag, and so on and so forth until, eventually, a new equilibrium of the principal fight forces is established, which leaves the aircraft in a steady climb at a constant speed which is a little lower than the initial cruising speed, despite the climb having been initiated by an opening of the throttle.

The new state of equilibrium of the four principal fight forces is shown in Figure 9.8.

Note that in Figure 9.8, for simplicity, we have assumed that all the principal forces are acting through the same point, so that there are no turning moments.

As we do not have to consider turning moments, we can ignore the force produced by the tailplane or stabilator.

In the new state of equilibrium, then, the fight path has changed from straight and level to a climb. As we have said, the speed will have reduced, and the lift, you may be surprised to learn, is also very slightly reduced. But thrust and the drag have both increased. However, the important point is that the resultant of the thrust and lift forces is equal and opposite to the resultant of the drag and weight forces, so equilibrium does, indeed, exist and the aircraft is moving at constant velocity again, in accordance with Newton’s Second Law.

Figure 9.8 Here, the aircraft is climbing with all four principal flight forces in equilibrium. (NB: the assumption here, for simplicity, is that all forces are acting through the same point so that there are no turning moments.)

So the aircraft is in a steady climb, with the lift force a little less than when the aircraft was fying level. In fact, lift is even a little less than weight. On frst consideration, this statement seems as if it cannot be true. But it is, indeed, a fact. In this steady climb, both the thrust and the lift are contributing to the resultant force which is causing the climb, while the opposing force, which is necessary for equilibrium and, thus, constant velocity, is the resultant of the drag and the weight.

The lift has, indeed, reduced slightly, but it is important to understand that the pilot could not have initiated the climb unless the lift had, at frst, increased as the pilot increased power and speed, or alternatively increased the angle of attack by raising the nose

So, in order that an aircraft may begin to climb, the lift must always initially increase in order to change the fight path of the aircraft (Newton’s 1st Law); but once the aircraft has settled down into a steady climb at constant airspeed, the lift reduces and is less than the weight. You should know, though, that with the relatively low angles of climb achievable by light aircraft, the reduction in lift below the value for weight is very small indeed. (This is not the case, however, for high performance fghter aircraft, and, of course, the Harrier can dispense with wing-generated lift force altogether.)

The sequence of force changes in establishing the aircraft in the climb may sound a little chaotic, but, in reality, the situation is readily controlled by the pilot. The pilot adjusts his power setting (RPM) and pitch attitude to achieve the climbing speed and

An aircraft’s

weight always acts vertically

downwards

towards the centre of the Earth.

In a steady climb, lift is a little less than weight.

To increase speed in level

flight, the pilot increases

power and decreases CL by pitching the nose downwards by an appropriate amount.

rate of climb that he desires. A light aircraft will invariably climb with full power, and the pilot will select the pitch attitude to give the speed for either the best rate of climb or the best angle of climb. Cruise climbs, though, are also an option for en-route fying. Your fying instructor will teach you all about climbing.

More About the Relationship Between the Main Flight Forces - Increasing Speed in Level Flight.

When a pilot increases power from level fight, it is not always because he wishes to initiate a climb. He will very often want to increase speed in straight and level fight, and to maintain his altitude while simply fying faster.

If a pilot opens the throttle and does not modify his pitch attitude, we know now that the aircraft will climb. But if a pilot does not want to climb but to increase speed, provided he has remembered the lift formula, Lift = CL ½ ρV2 S, he will know exactly what to do. As he increases power, he will, at the same time, move the control column forward slightly to adjust his pitch attitude and so reduce the value of CL by reducing the angle of attack of the wing, thus preventing the increase in airspeed from increasing the lift. The pilot continues to reduce the angle of attack as the airspeed increases, until the airspeed ceases to rise.

The increasing speed will, of course, increase drag (Drag = CD ½ ρV2 S) and the airspeed will settle down at its new higher value as the drag force approaches the value of the newly increased force of thrust, and when the fight forces again reach equilibrium, with greater values for thrust and drag, and slightly different lines of action for all fight forces. This new equilibrium situation may be easier to appreciate if we consider the simplifed diagram at Figure 9.9, showing only the principal fight forces, and with the aircraft at a typical attitude for a light aeroplane fying at a high cruise speed. Again, for simplicity, we have assumed that all the principal forces are acting through the same point, so that there is no turning moment and no need for a tailplane force.

Figure 9.9 Here, the aircraft is flying straight and level in a high cruise-speed attitude, with all four principal flight forces in equilibrium.