IMPROVEMENT OF PERFORMANCE - AEROPLANE or AIRPLANE.
A loss of between one quarter and one third of the power of the engine in the transmission assembly would hardly be tolerated in road or rail transport. Transport by water, however, is burdened by a loss identical in character and similar in amount. It arises from the fact that the propulsive force is obtained by driving a fluid backwards. This condition cannot be entirely overcome, but propeller efficiency can be and is being greatly increased. Commercial aeroplanes have an altitude operating range from sealevel to about 15,000 feet. At 15,000 feet the air density is slightly more than half the air density at sea level. It is obvious that a constant pitch propeller cannot have high efficiency throughout that marked range of air density. If it is designed for high efficiency at the surface, where maximum power is desired for take-off, it will have low efficiency at normal cruising altitudes. To meet the changing densities with changing altitudes, resort has been made to adjustable pitch, controllable pitch, and automatic propellers. The first is adjustable on the surface with motors dead. The second is capable of control by the pilot while in flight. The last is automatic. All these types are at present in use as standard equipment. (See AIRSCREW.) Resistance of the Body.—The majority of aeroplanes have in the nose of the body an air-cooled engine, or a water-cooled engine with a radiator. With this arrangement the resistance of the body cannot be separated from that which is incurred in cooling the engine. This latter is equivalent to a reduction in the power of the engine of from 15% to 25% at the cruising speed of a normal commercial aeroplane. At higher speeds the loss is greater.
It is natural to try to recover some of this by placing as much as possible of the load carried (fuel, crew, passengers) behind the radiating surfaces. But a large part of the loss remains, be cause such parts as these could, at least in principle, be placed in a stream-line body of very low resistance. Such bodies would, how ever, be heavy, and the conventional tractor aeroplane is by no means a bad compromise, especially when an air-cooled engine is used.
When a very high speed is required, as for example in racing aeroplanes, the cooling resistance can be eliminated by using a water-cooled engine in combination with a radiator whose cooling surface is part of the smooth surface of the wings. Such a radia tor has been used in the Schneider Cup and Pulitzer Trophy races. The consensus of the opinion of designers appears to be that it is as yet unsuitable for employment in commercial aeroplanes.
The undercarriage of an aeroplane has a high resistance, mainly due to the wheels. This condition has been met in two ways. The wheels and landing gear may be covered with cowling in a streamline shape or the landing gear may be retractable. In the latter case the landing gear is hinged in such a manner as to permit it to fold into the wings, fuselage, or engine nacelles. The retraction is usually activated by hydraulic or electric systems. Both methods of resistance re duction are in current use. The cowling (commonly called "breeches") is more common for small aeroplanes and the retract able gear for large ones.
A few years ago the aeroplane was a mass of external struts and bracing with cables between wings, from wings to fuselage and engine bed, and be tween the various fixed and movable controls. The tendency at present is toward "clean" design. Modern construction methods and materials permit all bracing for monoplanes to be internal. Whereas this requires increase in weight, it is largely balanced by increase in aerodynamic efficiency. Most large, high speed trans ports and some military aeroplanes are at present aerodynamically "clean" and highly streamlined, with an ever increasing tendency in that direction.
Of recent years much has been done in the development of superchargers. They were originally designed to avoid power loss with increases in operating altitude. As an unsupercharged engine is taken to high altitudes with consequent decreases in atmospheric pressure, power decreases rapidly. The supercharger increases the manifold pres sure and prevents undue power losses. Many modern engines operate at the surface with a manifold pressure well in excess of atmospheric pressure with a resulting improvement in sealevel performance.
