Aeroplane Construction and Operation

by John B. Rathbun

Small speed scouts, rarely if ever, have any overhang since the object of these machines is to make them as small and compact as possible.

CHAPTER VIII. EFFECTS OF PLAN FORM. (TANDEM AEROPLANES.)

General Notes. Up to the present we have considered only two wing outlines, the rectangular and the wing with raked tips. In addition, we have considered only the effect taking place on monoplane surfaces. For the purpose of obtaining longitudinal stability, or for distributing the lift upon two or more following surfaces, the plan view in some aeroplanes has been somewhat modified as in the Dunne, Langley and Ago. Undoubtedly the simple rectangular wing, or the wing with raked tips, have proved the most efficient from an aerodynamic standpoint, but as the layman is usually interested in distorted wing shapes, or odd-looking outlines, I will describe the effect of such stabilizing forms upon the lift and drag.

Figs. 1-9 show the usual range of wing forms, at least those that have been used on well known machines, and all of them have flown with varying results. In any case, the variations in the values of the lift and lift-drag are not excessive, the extreme cases varying possibly not more than 20 per cent on either side of the values for a plain rectangular wing. While almost perfect longitudinal stability can be obtained in other ways, by less loss than by changing the plan form, certain manufacturers still adhere to one or more deviations from the more usual rectangular form.

Fig. 1 shows a plan view of a machine with a rectangular wing, and No. 2 shows the machine provided with raked tips. Fig. 3 is a wing with an inclined entering edge, as used on the English Mann biplane. Fig. 4 is the German Ago with a diamond form surface, the evident purpose of which is to simplify the wing spar bracing as shown by the dotted lines. By bringing the wing spars 'together at the tips, the spars themselves form a triangle to resist the drag stresses. Fig. 5 is the common form of "sweep back" or "retreated wing" as used in the Standard H-3 training biplane, and several other modern biplanes. While this arrangement undoubtedly assists longitudinal stability, it causes certain losses that will be described later. The inherently stable Dunne is shown by Fig. 6 in which the sweep back is increased to almost 90 degrees. In fact the retreat is so great that no tail or stabilizing surfaces are used at all, the elevator functions being performed by the ailerons. It should be noted that the swept back tips really act as stabilizers since they trail back far behind the center of gravity and center of pressure. The ailerons (a) are not needed for lateral balance, hence ascent and descent—and also turning in a horizontal plane—are performed by the ailerons in setting them in different relative positions. Being far to the rear they are very effective elevators, although their action and the extreme retreat of the wings, causes a considerable drag. This is somewhat offset by the absence of tail resistance. An Austrian or German "Taube" is shown by Fig. 7 with the negative trailing wing tips (a), that greatly assist longitudinal stability, but which are decidedly inefficient. This is evidenced by the fact that neither the Germans nor Austrians build this machine at present—at least for active service. The tips (a) are bent up at the rear and thus form a "negative" angle of incidence with the main lifting surface. This was the original invention of Igo Etrich, an Austrian, and as with everything else, the idea was promptly grabbed by the Germans at the beginning of the war and claimed as their own idea. Etrich was one of the pioneers in aviation, a science that did not prosper in Germany until the impracticability of the Zeppelin for universal service was an accepted fact.

Figs. 1-9. Plan Views of Different Wing Arrangements and Wing Outlines.

Fig. 8 is the wing outline of the Bleriot monoplane, a representative wing used on the earlier monoplanes. This is really reversed rake, and hence does not stand for efficiency in lift. A tandem aeroplane is shown by Fig. 9 in which the leading surface is (m) and the trailing wing is (n). The tail (t) may, or may not be included. This is a type that has been neglected in its practical development although it has been repeatedly proposed. The Langley machine, the Montgomery glider, and the Richardson are of this type.

In all the figures the wing ribs are indicated by the thin full lines passing across the width of the wing, with the tail at (t). The arrow represents the line of flight. It will be noted that with any but rectangular wing outlines the rib lengths are different, throughout the wing. This makes this wing a bad manufacturing proposition, and a difficult and expensive wing to repair. To provide against emergencies the aviator must keep a complete extra wing in reserve for repair parts, while the manufacturer is put to the expense of a great number of rib molds, and must also keep a large number of ribs in stock. There is a constant difficulty due to the fact that mistakes are often made in ordering the ribs for repair, and altogether, anything but a rectangular wing is a decided nuisance.

Sweep Back or Retreat. With swept back wings, as shown in Fig. 5, the center of pressure movement is peculiar. The C. P. moves forward when reducing the incidence at small angles, and thus tends to reduce head diving, but at very large angles the C. P. again moves forward, tending to increase the angles further and thus stall the machine. This reversal of C. P. movement takes place at about 10° to 12°, and the movement is sharper and further with each increase in the sweep back. At ordinary angles of flight, say at from 0° to 6° the forward C. P. movement is satisfactory, but at low speeds and high angles stability is only partially secured, and hence for the total performance sweep back is not to be desired. The wing section used in the above investigation was an R.A.F.-6 with an aspect ratio of 6.