Aeroplane Construction and Operation

by John B. Rathbun

(2) Maximum Lift (Ky). With a given wing area and weight, the maximum value of the lift co-efficient (Ky) determines the slow speed, or landing speed, of the aeroplane. The greater the value of Ky, the slower can be the landing speed. For safety, the landing speed should be as low as possible.

In the majority of wings, the maximum lift occurs at about 16° of incidence, and in several sections this maximum is fairly well sustained over a considerable range of angle. The angle of maximum lift is variously known as the "Stalling angle" or the "Burble point," since a change of angle in either direction reduces the lift and tends to stall the aeroplane. For safety, the angle range for maximum lift should be as great as possible, for if the lift falls off very rapidly with an increase in the angle of incidence, the pilot may easily increase the angle too far and drop the machine. In the R.A.F.-3 wing, the lift is little altered through an angle range of from 14° to 16.5°, the maximum occurring at 15.7°, while with the R.A.F.-4, the lift drops very suddenly on increasing the angle above 15°. The range of the stalling angle in any of the wings can be increased by suitable biplane or triplane arrangements. If large values of lift are accompanied by a fairly good L/D value at large angles, the wing section will be suitable for heavy machines.

(3) Center of Pressure Movement. The center of pressure movement with varying angles of incidence is of the greatest importance, since it not only determines the longitudinal stability but also has an important effect upon the loading of the wing spars and ribs. With the majority of wings a decrease in the angle of incidence causes the center of pressure to move back toward the trailing edge and hence tends to cause nose diving. When decreased beyond 0° the movement is very sharp and quick, the C. P. moving nearly half the chord width in the change from 0° to -1.5°. The smaller the angle, the more rapid will be the movement. Between 6° and 16°, the center of pressure lies near a point 0.3 of the chord from the entering edge in the majority of wing sections. Reducing the angle from 6° to 2° moves the C. P. back to approximately 0.4 of the chord from the entering edge.

There are wing sections, however, in which the C. P. movement is exceedingly small, the Eiffel 32 being a notable example of this type. This wing is exceedingly stable, as the C. P. remains at a trifle more than 0.30 of the chord through nearly the total range of flight angles. An aeroplane equipped with the Eiffel 32 wing could be provided with exceedingly small tail surfaces without a tendency to dive. Should the elevator become inoperative through accident, the machine could probably be landed without danger. This wing has certain objectionable features, however, that offset the advantages.

It will be noted that with the unstable wings the center of pressure movement always tends to aggravate the wing attitude. If the machine is diving, the decrease in angle causes the C. P. to move back and still further increase the diving tendency. If the angle is suddenly increased, the C. P. moves forward and increases the tendency toward stalling.

If the center of pressure could be held stationary at one point, the wing spars could be arranged so that each spar would take its proper proportion of the load. As it is, either spar may be called upon to carry anywhere from three-fourths of the load to entire load, since at extreme angles the C. P. is likely to lie directly on either of the spars. Since the rear spar is always shallow and inefficient, this is most undesirable. This condition alone to a certain extent counterbalances the structural disadvantage of the thin Eiffel 32 section. Although the spars in this wing must of necessity be shallow, they can be arranged so that each spar will take its proper share of the load and with the assurance that the loading will remain constant throughout the range of flight angles. The comparatively deep front spar could be moved back until it carried the greater part of the load, thus relieving the rear spar.

With a good lift-drag ratio, and a comparatively high value of Ky, the center of pressure movement should be an important consideration in the selection of a wing. It should be remembered in this regard that the stability effects of the C. P. movement can be offset to a considerable extent by suitable biplane arrangements.

(4) Structural Considerations. For large, heavy machines, the structural factor often ranks in importance with the lift-drag ratio and the lift co-efficient. It is also of extreme importance in speed scouts where the number of interplane struts are to be at a minimum and where the bending moment on the wing spars is likely to be great in consequence. A deep, thick wing section permits of deep strong wing spars. The strength of a spar increases with the square of its depth, but only in direct proportion to its width. Thus, doubling the depth of the spar increases the strength four times, while doubling the width only doubles the strength. The increase in weight would be the same in both cases.

While very deep wings are not usually efficient, when considered from the wing section tests alone, the total efficiency of the wing construction when mounted on the machine is greater than would be supposed. This is due to the lightness of the spars and to the reduction in head resistance made possible by a greater spacing of the interplane struts. Thus, the deep wing alone may have a low L/D in a model test, but its structural advantages give a high total efficiency for the machine assembled.

Summary. It will be seen from the foregoing matter that the selection of a wing consists in making a series of compromises and that no single wing section can be expected to fulfill all conditions. With the purpose of the proposed aeroplane thoroughly in mind, the various sections are taken up one by one, until a wing is found that most usefully compromises with all of the conditions. Reducing this investigation to its simplest elements we must follow the routine as described above: (1) Lift-drift ratio and value of Ky at this ratio. (2) Maximum value of Ky and L/D at this lift. (3) Center of pressure movement. (4) Depth of wing and structural characteristics.