With the aid of diagrams we will consider the arrangement of loads on the whole aircraft in four cases, cruising straight and level, turning, stalling and landing.
The loading situation in the straight and level cruise condition is as shown in Fig. 4.8.
In Section 4.2 we discussed inertia factors (load factors) which are written as ng. In the cruise the inertia factor is 1 g and applies to the weights of all the elements of structure and equipment which constitute the whole aircraft. Sometimes the inertia factor is referred to as an accel-eration factor or accelaccel-eration coefficient. There is nothing wrong with such
Fig. 4.8 Cruising flight.
a descriptive name except that in the straight and level condition we are left to explain the statement ‘. . . the acceleration factor is 1 g’, when clearly the aircraft is not being accelerated in any direction. Remember that whether it is called ‘inertia’, ‘load’ or ‘acceleration’ it is always a factor and as such is a device used by engineers to bring into calculation the apparent increase in weight of objects which are being accelerated. As a multiplying factor it represents an unaltered situation when it is 1.0.
When an aircraft is turning it suffers an apparent weight increase due to the centrifugal force which is trying to make it go straight on instead of round the curved path of the turn. In a correctly executed manoeuvre the loads balance as shown in Fig. 4.9.
Stalling occurs when the lift surfaces of the aircraft are no longer able to provide enough lift to balance the weight, and the speed of the aircraft at which this occurs is called the stalling speed. Even this is not simple, because we can have one stalling speed with a ‘clean’ wing and another lower speed when flaps, slats and any other of a selection of lift-improving devices are added. We also have the situation described above, where during a turn the apparent weight of the aircraft is increased, so that the ability of the wing to provide enough lift will disappear at a rather higher speed in a turn than when the aircraft is flying straight and level.
The aerodynamic action of stalling is a breakdown of the airflow over the top surface of the wing, as shown in Fig. 4.1 and, except for some high-speed situations, occurs at an angle of attack of about 15°.
The structural load situation is that immediately after the stalling of the wing the aircraft starts to pitch nose down and, more or less, free fall; that Fig. 4.9 Loads in a turn.
is all of the individual parts of the aircraft appear weightless. In Section 4.1.2 we spoke of the ‘free body’ concept with all the loads in balance, so we might expect that when the lift disappears the opposing and balancing weight will also disappear. In fact the ideal situation is not quite realised as, although the wing has stalled, it is still producing a small amount of lift but at a point further aft than prior to the stall. The weight now signifi-cantly exceeds the available lift and is also acting forward of this lift thus tending to pitch the aircraft nose downwards and, with the aid of the down force on the tailplane, producing an overall acceleration earthwards. In the stall case the unbalanced tailplane load causes a nose down pitching rotation which aerodynamically allows the aircraft speed to build up and stable flying conditions to be restored (see Fig. 4.10). Structurally the loads involved on the main structure in the stall are not usually of any signifi-cance but the loads on control surfaces and flaps may be important.
Landing loads are a major consideration in the design of the structure and are involved with the designed cushioning characteristics of the undercarriage. The stiffer the springs of the undercarriage the more rapid the vertical deceleration of the landing aircraft and the higher the loads involved on the structure. The Airworthiness Requirements are very descriptive and demanding on this subject. Helicopters and, even more obviously, deck-landing aircraft generate high landing loads.
If we look back at Figs. 4.7–4.9 we can see that the general loading on any major part of the aircraft structure is a bending or a twisting load or both. This general loading pattern is shown in Fig. 4.11 and will be dis-cussed in the next chapter.
4.7 Review of the key points
The principal forces acting upon an aircraft are lift, weight, thrust and drag. In straight and level flight lift equals weight and thrust equals drag.
It can be useful to resolve a force into two other forces at 90° to each other. For example the force produced by a wing moving through the air is resolved into:
Fig. 4.10 Loads out of balance in a stall.
•
lift acting at 90° to the direction of travel; and•
drag acting parallel to the direction of travel.A force acting at a point displaced from a support will produce a turning moment in addition to the direct force. Moment is force multiplied by the distance from the support (see Fig. 4.3).
When an aircraft (or any other object for that matter) is in a steady state, i.e. not accelerating in any direction, although it might be moving at a steady speed, all forces and all moments acting on the aircraft are in balance. For an aircraft to change direction (manoeuvre) some of the loads acting upon the aircraft must be increased and this will cause the aircraft to accelerate. The increased loads are often quoted as a multiple of the normal force due to gravity, e.g. an aircraft might perform a 3 g turn where the loads acting upon the aircraft are three times those experienced in straight and level flight. Airworthiness codes usually specify what factors (multiples of nominal gravity) to apply when designing the structure.
4.8 References
Carpenter, C. Flightwise. Shrewsbury: Airlife Publishing.
Thurston, D.B. Design for Flying. New York: McGraw-Hill.
(See also references for Chapter 13.)
Tye, W. Handbook of Aeronautics, No. 1, Structural Principles and Data, Part 1 ‘Structural airworthiness’, 4th edn. London: Pitman.
Fig. 4.11 Loads on the wing.