The Aeroplane

THE AEROPLANE Fig. 9 on p. 247 gives a diagram of an aeroplane indicating its principal parts and their names.

The main purpose of the various parts is clear from the figure which represents a biplane; the wings support the machine; to secure steady motion in a vertical plane a tail plane is necessary, and in order that the motion in that plane may be controllable part of the tail is hinged and can be raised and lowered thus form ing the elevator.

Steady motion in the horizontal direction calls for a vertical fin at some distance behind the main planes and, to control the direction of that motion, a rudder is added to guide the machine.

The tail plane and fin are connected to the wings by the fuse lage, in the fore part of which is the cockpit containing the pilot's seat and the controls. A portion of the rear extremity of each wing is hinged about an axis parallel to the length of the wing, thus forming an aileron. Lowering an aileron increases the angle of incidence and therefore the lift of that portion of the wing; raising it has the opposite effect. The pilot is thus able to control any rocking of the aeroplane about a longitudinal axis.

Control wires pass from these movable parts to the pilot's seat, those from the rudder going to the rudder bar worked by his feet, while those from the elevator are connected to the control column or joy stick, a rod fixed in front of the pilot by a universal joint at its lower end. The connections are such that the longitudinal motion of the column raises or lowers the elevator while a trans verse motion affects the aileron.

In a tractor machine, as shown in the drawing, engine and air screw are in front of the pilot ; in a pusher machine they would be placed at his back. The planes are connected by struts, while rigid ity is secured to the structure by a series of wires arranged to keep the two planes in fixed relative position. The longerons are the lower main members of the body structure which are prolonged forward so as to support the engine; the machine is carried on the landing chassis, an under carriage on wheels fitted with springs so as to absorb shock on landing. At the rear is the tail skid, which rests on the ground when taxying.

The distance between the wings is the gap. In many machines the upper wing is not set exactly above the lower but either rather in front when the machine is set to be positively staggered or behind when the stagger is negative.

In a monoplane there is only one pair of wings.

Details of Structure.

We turn now to the actual design and structure of aircraft, remembering that a fuller account appears in the article AEROPLANE.

For most purposes the design must be such that the aeroplane will fly safely with a minimum expenditure of power. Since the total weight carried is fixed, the structure must be light in order that the useful load, that part of the load not made up of the weight of the machine including the engine, petrol, oil and neces sary equipment, may be as large as possible ; the machine must be controllable ; the stability and manoeuvrability desirable will depend on the purposes for which it is required.

Landing is made more easy if the speed is slow, hence the stalling speed, the minimum speed of flight, should be kept as low as other requirements permit ; precaution against fire should be taken, and, for a civil machine, the comfort of the passengers is of great importance ; steps have recently been taken to reduce the noise, often most trying to passengers, to about the same as that met with in an express train.

In the earlier machines the main structure was of wood and the wings were covered with linen cloth painted or doped with a kind of varnish to secure stiffness and protect the material.

In

more modern machines much of the main structure is of metal, while in a few, particularly sea-planes and flying boats, metal in the form of a thin sheet is used for the covering of the wings. The need to combine lightness and strength in aircraft has led to much metallurgical and engineering research, and by the use of new alloys many desirable changes in the structure have become possible.

The functions of the various parts of the machine are discussed in full detail in the article AEROPLANE and their use in controlling the flight at the will of the pilot described.

Prevention of Stalling.

By these means, so long as the ma chine has sufficient speed to produce the necessary lift, it is under control: but, if from any cause stalling incidence is passed, the lift is suddenly reduced, controls cease to be fully effective and the machine falls rapidly. If this takes place at a sufficient altitude the pilot may be able to prevent the accident by diving the machine, increasing the speed and bringing the controls into action again or, on the other hand, the machine may develop a spin, which it is difficult to stop, and be wrecked, before the pilot can regain control.

Now we have seen that the stalling speed is inversely propor tional to the maximum lift coefficient. By increasing this, we re duce the stalling speed, i.e., we enable the machine to be flown at a speed at which it would otherwise be uncontrollable. Various attempts have been made to put it into the power of the pilot to increase the lift in case of emergency; the Handley Page slot is one of these. This consists of a slot or narrow aperture cut in each wing, parallel to its length; the slot is ordinarily closed but the pilot has the means of opening it and thus the lift is increased. In a more recent form, also due to Handley Page, the slot opens automatically when stalling incidence is passed.

The use of the slot alone has certain concurrent disadvantages. These, however, can be overcome by a suitable adjustment of the ailerons, and mechanism has been devised combining the two, so that when the slot is opened the ailerons simultaneously take up the correct position.

Fig. to (p. 248) shows a wing fitted with a Handley Page slot.

Loading: Load Factor.

In a modern engineering structure the stresses to which each component member will be subjected can usually be calculated and the necessary strength provided. In an aeroplane the stresses depend on the velocity and accelera tion of the machine, its position in the air, the gusts of wind to which it may be subjected, etc. While it is possible to cal culate them for steady horizontal flight and for certain other definite conditions of motion, the calculation, which for modern machines is specially complicated, cannot embrace all conditions.

To allow for this a load factor determined as the result of experience is assigned to each member of the structure; the calculated stress, including as usual a factor of safety, is multi plied by the load factor and the result is the stress which the member is designed to bear.