There is apt to be considerable confusion about the use of the term °length° when ap plied to aeroplanes, owing to the fact that wings fly through the air broadside-on. It is natural, when speaking of the wings alone, to refer to their longest dimension as their length, as one would do when speaking of any other object ; but, when the machine is in flight, one might equally remark how short is the length of the wings compared with that of the machine as a whole, meaning thereby the measurement of the wing from its leading edge to its trailing edge. For this reason it has be come customary to apply the terms °span° and °chord° in this connection, and haying an ob vious derivation they justify their existence as technical words and are deserving of general use. One speaks of the span of a bridge and chord of the arc of a circle; both expressions have an analogous significance in reference to the aeroplane.
Thus, the wings, which are the supporting members of the machine, form a kind of bridge that spans the air in order to hold aloft the weight; the wings themselves, as has already been mentioned, are cambered so that a string stretched between the leading edge and the trailing edge would occupy the position of the chord to the arc that is formed by the wing surface. This arc, by way, is not circular, but is of such contour as is found best by experiment on models that are tested in an experimental wind tunnel. The highest point of the curvature of any wing section is always nearer the front edge than it is to the trailing avoid confusion of terms; actually it is merely a pivoted plane just like a rudder, but arranged horizontally instead of vertically. In a mono plane, the rudder and the elevator form part of the tail of the machine. The term °tail° applies to a group of organs of which the two just mentioned are hinged and movable to per form the directional functions under the pilot's control. A third plane horizontally arranged like the elevator, but rigidly fixed, is commonly added in order to confer some degree of natural °longitudinal stability° in flight. In those aeroplanes that have long boat-like bodies ex tending the full length of the machine, this fixed tail plane is often a mere fin-like excres cence. On other types, however, the rigid por tion of the tail is a much larger. affair ; in either case, it commonly carries the elevator as an extension in the form of a hinged flap. Instead of flapping its wings, to do which would in volve constructional difficulties, the machine carries a propeller. This is usually a two-bladed unit built of timber, and it usually measures about eight feet or more in diameter. Owing to its high speed of rotation, which commonly is between 1,000 and 1,300 revolutions per minute, the propeller is invisible in some of the machines that are photographed in flight. It is common practice to put the propeller in front and to mount it -upon the engine crank shaft. A general study of the designs of the machines makes it very clear that the forward position—or "tractor* screw as the propeller in front is often called — is structurally con venient. Monoplanes were from the first designed with long girder-like bodies, which necessitates a single air screw being placed either in front of the wings or behind the tail.
When this form of body became more common on biplanes the tractor screw accompanied it. There exists a wide difference of expert opinion regarding the most effective plane area. Some of the most efficient aerofoils (consult Loening, 'Military Aeroplanes)) have been developed by the Frenchman Eiffel. Experi ments at the Royal Aircraft Factory in Eng land have also been productive of some inter esting wing surfaces and more recently tests made for the Signal Corps of the United States army at the Massachusetts Institute of Tech nology have developed some interesting facts concerning aerofoils. It is well recognized, however, that the matter of plane area is largely governed by the factor of velocity, and that slow speed machines require larger aero foils than those operating at higher velocities Since slow speed is not a desirable qualification of aircraft in general, and tends to influence the angle of incidence, thus increasing the air resistance and requiring the employment of ex cessively high driving power, the general con structive effort is being directed to the pro duction of high speed machines having small aerofoils and powerful engines, the velocities commonly attained ranging from 65 to 150 miles an hour, with a speed of 180 miles an hour an easy probability in the near future.
The solution of the problem of control naturally follows the successful accomplish ment of aeroplane sustentation, and in the successful execution of the acts of balancing, banking, and steering, depends the success of the machine as a practical commercial craft. In the design and construction of air craft there are 10 essentials to be borne in mind: (1) An aeroplane must have sufficient com bined speed and plane area to raise its intended load, together with its own weight. (2) The greater the speed the less the plane area, and the less the necessary angle of that plane for the same effect. (3) To counteract the resist ance set up by means of gaining momentum while on the ground, which is additional to the resistance the machine will have when once clear: a. extra power is required, or b. extra plane surface to meet the power we have, c. a better effect for the power we have, d. an outside agency that will assist. Extra power means more weight, extra plane means more resistance, better effect for the power we have means improvement in the engine or means of propulsion, an outside agency means a fixed starting point. (4) The planes must always be sufficient to permit of a safe landing. (5) Their exact shape depends upon the type of machine constructed, the means employed for obtaining lateral and longitudinal balance and stability, and varies so greatly that no particular rule can be laid down. (6) The planes should be constructed of materials as strong as the end in view permits, and should in themselves create as little useless resistance as possible. (7) The general arrangement should be as simple as the design allows. (8) The control should be simple and easy of manipulation. (9) The balance should be automatic. (10) Al though it is difficult to obtain, a means of keeping afloat until a safe landing is made without engine power perfects the aeroplane.