GATION ; AVIATION, CIVIL.) Number of Planes.—The appearance of aeroplanes in front and side elevation is more varied than that in plan, owing mainly to the use of one or more planes. As many as four planes have been used on aeroplanes, but the great majority of existing ma chines have either one or two planes.
An analysis of the aeroplanes described in Jane's All the World's Aircraft for 1934 shows that, out of a total of nearly IIOO types, 52% were monoplanes and 48% biplanes, with no triplanes or quadruplanes. A similar analysis for 1927 gives, for about 600 types,3i% monoplanes and 69% biplanes. In 1922 there were 22% monoplane types to 78% biplanes with 8 triplanes and one quad ruplane. This trend is due partly to the development of wing sections of great depth in relation to their chord, and partly to a desire to eliminate external wing bracing, but principally to the fact that the structural strength of the biplane is no longer a pri mary consideration in view of recent developments in stressed skin wing and fuselage construction.
In fig. 8, separate analyses are shown diagrammatically for aeroplanes produced in Great Britain, France, Germany, the United States and other countries. It should be noted that the figures given above refer to types and include both commercial and war aeroplanes and seaplanes. Some of these naturally exist only in small numbers, and some may be confined even to one experimental machine, but there is evidence that all have actually been made and flown. These figures show that the biplane is hold ing its preponderance only in Great Britain. The monoplane has taken the lead in the United States, France and Germany.
Confining attention to monoplanes and biplanes, the chief variations in front elevation are shown in fig. 9. The high wing monoplane (a) is used mainly where it is desired to have some of the advantages of external bracing. The low wing monoplane (b) has rather less resistance and is less liable to damage by wind when on the ground, an important advantage. The latter type is the more numerous.
The biplane with wings of equal span (c) is the most com mon form of aeroplane, but wings of unequal span (d) (some times called sesquiplane, i.e., I-1 plane) have many advantages. The monoplane is rapidly taking the leading role. Fig. 9 (e) shows a large biplane with two or more engines. The right and left hand wings are generally not in line, but form a very flat upward Vee. This dihedral angle (which is generally given as the angle between either wing and the horizontal, and varies from o° to 5°) is introduced in order to improve the lateral stability of the machine.
The corresponding side elevations of these aeroplanes are given in fig. 1o. In a biplane the relative position of the planes as seen in this view varies as shown in (c), (d) and (e). In (c) and (d) the planes are staggered, i.e., the line joining the leading edge of the upper and lower planes is not perpendicular to their chords. The angle of stagger is generally between o° and 3o° and a varia tion over this range has little if any effect on the efficiency of the wings. In (d) the chord of the lower wing is smaller than that of the upper wing, a common variant, especially when the spans are also unequal (fig. 9). In (e) the wings have equal chords and no stagger. The tail plane is here shown as a biplane, in which form its weight can be made low for a considerable area.
They perform practically the same function as the rudder of a boat and can be seen in more detail in fig. 9 of article AERO NAUTICS. The fixed fin is usually made adjustable for modern multiengine aeroplanes. The range of adjustment is sufficiently great to permit a setting such that the aeroplane will fly straight, when one of the wing engines is not operating, without undue exertion on the part of the pilot. This is particularly important on multiengined aeroplanes since many accidents have resulted from the inability of the pilot to hold a straight course after one engine has failed at low altitude. The operating engine has a tendency to lift the wing on its side and pull the aeroplane into a power spin.
A comparatively recent and highly efficient method of "trim ming" the aeroplane for flight is the Flettner Control. This is a hinged section of the trailing edge of the rudder which is adjust able from the pilot's seat. As the Flettner control is moved to the right, the airflow on its surface forces the rudder to the left. A sufficient movement of the control is permitted to compensate for the loss of one engine's power as far as directional stability is concerned. When the Flettner Controls are used the fixed surfaces need not be adjustable.
As aeroplanes increase in size the amount of pressure which must be applied to the controls by the pilot increases to such an extent that his strength may be overtaxed in sharp manoeuvres. To reduce the effort required auxiliary control surfaces have been in certain instances introduced. In appearance they are simi lar to the Flettner controls but, instead of being operated by the pilot, are automatic. As the right foot pedal is pressed by the pilot and the rudder moves to the right, the auxiliary control auto matically moves to the left. The airflow thus assists the pilot in making his turn by relieving pressure on the foot pedal.
Both the Flettner controls and the automatic auxiliary controls are used on elevators (movable horizontal control surfaces) as well as rudders and could be used on ailerons if size of the aero plane warrants.
The automatic pilot has many advantages. It produces more steady flight than is otherwise possible and permits the pilot to attend his radio and navigational duties with greater freedom. Al though it is not a normal practice, it may be set for takeoff or landing.