AIRPLANE OR AEROPLANE - MINIMUM SPEED OF FLIGHT It is the impossibility of steady flight below the stalling speed that constitutes the chief danger of air travel. Many attempts have been made to avoid this source of danger by devising an aeroplane which would be capable, as a whole, of remaining sta tionary in the air. Such machines have generally employed either flapping or rotating wings, the so-called ORNITHOPTERS and HELICOPTERS. The former have shared the legendary fate of Icarus. The latter have been recently proven possible of construc tion and flight. However, their reliability and practicability are still to be proven. See GYROPLANE.
There is no fundamental obstacle to reducing the minimum speed of a normal aeroplane to some 25m.p.h., instead of the 4o to 75m.p.h. quoted above as typical of modern practice. In fact the earliest flying machine had minimum speeds of this order. But the general proportions of such a machine would be uneco nomical from every other point of view. In particular almost the whole of the available weight would be absorbed by the structure and power plant, leaving no margin for paying load.
The relation between the total weight (IV lb.), the area of the wing (S sq. ft.) and the aerodynamic coefficient (k,) which de termines their lift at any given speed (V ft./sec.) (see AERO DYNAMICS) iS IV = p being the density of the air.
This may be put, for average ground level density, in the form where w= W/S and is called the wing loading, in pounds per square foot. Fig. i shows the possible combinations of w and kr, which will give any selected speed. Fig. 2 shows the cross sec tion by a vertical plane parallel to the direction of flight of some modern wing sections. For these the maximum value of kr, (which in virtue of the above relation corresponds to the mini mum flying speed) is between 0•55 and 0•65, represented by the shaded band in fig. 1. Modern commercial aircraft using such wings have loadings between 9 and 20 lb. per square foot. A re duction in the stalling speed can be made only by reducing the wing loading or by using a shape of wing section which will give a higher maximum value of The former course leads to a re duction in the paying load. Much attention has therefore been devoted to the latter method.
A most effective high lift wing section is the "slotted" wing of F. Handley Page (fig. 3). With this the maximum lift coefficient can be increased some 5o% or more. The action of the device is, broadly speaking, to cause the flow characteristic of stalling (see AERODYNAMICS) to occur at a larger angle of incidence.' The in crease in lift is dependent on the relative position of the brain and auxiliary wings, i.e., on the size and shape of the slot between them, which can be varied in flight. With the slot closed the characteristics are nearly the same as those of a normal wing. In this respect form a (fig. 3) is superior to form b, but in prac tice there are, with both, constructional difficulties in the way of ensuring a smooth and unbroken surface when the slot is closed.
Another method which has been widely used, is to vary the curvature, or "camber," of the wing (fig. 4). In general, the greater the curvature of a wing section the higher is the maximum lift, but the lower the efficiency at high speeds. This device at tempts to secure both high lift and high efficiency by enabling the pilot to alter the curvature to suit the conditions of flight.
The increase of lift attainable in this way is much less than can be achieved with a slot, the max imum being about io%. An automatic form of this device has also been used, the trailing flap being held down by elastic cord and therefore rising as the air pressure increases, i.e., as the speed increases. By combining both these methods very high maximum lifts have been ob tained.
A variation of the system of changing the camber is the use of flaps. Their use is now standard on the majority of airliners, hav ing been necessitated by the greatly increased speeds. The in crease in high speed and in cruis ing speed has inevitably increased the stalling speed and thus the landing speed. The employment of flaps has kept the landing speed at a practical figure. The use of flaps on the take-off also increases the lift and makes shorter take-off possible at the expense of speed. They are con structed by hinging the under surface of the trailing edge of the wing in such a manner that the trailing edge may be opened by forcing down the under surface by a system of control from the pilot's seat for landings and take-offs. When in the air the flaps are raised (or closed) and the wing resumes normality. Flaps increase the lift at or near stalling speed by about 25%.