STABILITY AND CONTROL - AEROPLANE or AIRPLANE.
The general principles of stability and control are dealt with in the article AERODYNAMICS, and the stabilizing and controlling The fore and aft control required can generally be obtained area of elevator if the ratio area of fixed tail plane • is about two-thirds.
Lateral stability depends mainly on the provision of an ade quate fin and dihedral angle. While desirable for comfort, lateral stability is of minor importance compared with lateral control.
Ailerons of an area between
and k of that of the wings are usually provided, and give adequate control at all normal flying speeds. At low speeds, however (more ac curately, at angles of attack of the wings in the neighbourhood of stalling), not only are normal ailerons ineffective, but they may actually become a source of danger, their use aggravating an inherent tendency of the stalled wings to force the aeroplane into a spiral dive or spin. (See AILERON.) Rudder Control.—An effective rudder is as essential for safety at low speeds as effective ailerons. The efficiency of any given rudder depends on its being arranged so that the body does not shield it, but it is found that when the product of the ratios surfaces have been referred to above. The necessary disposition and dimensions of these surfaces are decided in practice chiefly by direct comparison with previous designs. Since, however, aero planes are not merely scale copies of one type, some rational means of using the accumulated experience of previous work is essential, and it is this that research into the fundamentals of stability and control is able to supply. Needless to say, even these methods break down when the aeroplane in question is not broadly similar to those from which the data are derived, and then nothing short of a thorough investigation, assisted by model ex periments, will suffice. Very often, even with the best that can be done in this way, uncertainties remain, and the designer is forced, as in other branches of engineering design, to rely upon his ability to draw conclusions from insufficient premises.
Stability, the property of returning to a steady state of motion when deliberately or accidentally disturbed, im plies as a necessary preliminary of the existence of equilibrium, or "trim," in that state. An aeroplane is trimmed for a steady con dition of flight by an adjustment of its tail plane, or adjustment of the Flettner controls, the elevators being left free. The degree of stability generally considered desirable for a commercial aero plane can be attained by arranging the size of tail plane and the fore and aft position of the centre of gravity within certain limits. The further back the centre of gravity, the larger is the necessary tail area. The fore and aft position of the centre of gravity in relation to the wing is defined by its distance from a plane pass ing through the leading edge of the wing perpendicular to the chord, for a monoplane ; for a biplane an equivalent monoplane wing is substituted. In normal aeroplanes this distance is usually about one-third the chord, though it has varied between one quarter and one-half. The height of the centre of gravity of an aeroplane in relation to the wings has a secondary effect on its stability. In particular a low centre of gravity is not necessary for stability. With the centre of gravity in the first-mentioned position, stability will be secured providing the product of the ratios area of tail plane and elevator and area of wings distance of tail plane from centre of gravity chord of wings is greater than about 0.35. The latter ratio is usually about 3, and the former one-eighth.
is above 1/2o, a satisfactory control is obtained. The latter ratio is usually about I, so that the former should not be less than one-twentieth. The area of the rudder is generally about 4 that of the rudder and fin combined.
The efficiency and sensitivity of control surfaces have been ma terially increased by placing tabs on the trailing edges. The tabs are quite small pieces of comparatively thin metal so located on the trailing edges as will furnish the greatest effect. Tabs smooth out the effect of the controls and increase control stability. They are used on ailerons, rudder and elevators.
