The planes are usually made with a slight curve in them, so that they may the better catch the air and "scoop" it downwards, so to speak. They usually consist of fabric specially made for the purpose, stretched upon a light wooden framework. The whole framework is usually of wood with metal fittings frequently made of aluminium for the sake of lightness.
The engines have been mentioned in another chapter. The propeller which is almost invariably fixed directly upon the shaft of the engine has two blades only and not three as is usual with those of ships. Precisely why this should be so is not clear, but experience shows that two-bladed propellers are preferable for this work. They are made of wood, several layers being glued together under pressure, the resulting log being then carved out to the required shape. This makes a stronger thing than it would be if cut out of a single piece of wood.
All parts, engine, elevator, rudder and balancing
arrangement, are controlled by very simple means from the pilot's seat.
In monoplanes there is but one main plane, resembling a pair of bird's wings. Or if we care to look upon it as two planes, one each side of the "body," then we must call it a pair. Since the name "mono" indicates one it is best to think of it as one plane although it may be in two parts. The biplane has, as its name implies, two planes, but in that case there can be no doubt, since they are placed one above the other. Machines have been made with three planes and even with as many as five, but monoplanes and biplanes appear to hold the field.
It is not possible for an aeroplane to be in any sense armoured for protection against bullets: for defence the pilot has to depend upon his own cunning man[oe]uvres combined with the fast speed at which he can move. For offensive purposes he usually has a machine gun mounted right in front of him with which he can pour a stream of bullets into an opponent or even, by flying low, he can attack a body of infantry. It is recorded that one German prisoner during the war, speaking of the daring of the British pilots in thus attacking men on foot, exclaimed, "They will pull the caps off our heads next."
Some of the aeroplanes have their propeller behind the pilot and some have it in front. The latter, to distinguish them, are called "Tractor" machines, since in their case the propeller pulls them along. Now it is easy to see that a difficulty arises in such cases through the best position for the gun being
such that it throws its bullets right on to the propeller. But that has been overcome in a most simple yet ingenious way. The gun is itself operated by the engine with the result that a bullet can only be shot forth during those intervals when neither blade of the propeller is in the way. The propeller is moving so fast that it cannot be seen and the bullets are flying out in a continuous rattle, yet every bullet passes between the blades and not one ever touches.
It is easy to see that when an aeroplane is manned by a single man, as is often the case, he must have his hands very full indeed, what with the machine itself and the gun as well. In fact, he often has to leave the machine for a short time to look after itself while he busies himself with the gun.
Now there we see a sign of the wonderful work which has been done in the course of but a few years in the perfecting of the aeroplane, the result of a series of improvements in detail which make but a dreary story if related but which make all the difference between the risky, uncertain machine of a few years ago and the safe, reliable machine of to-day. Modern machines are inherently stable. The older ones had the elements of stability in them but they were so crudely proportioned that these inherent qualities did not have a chance to come into play.