Aeroplane Construction and Operation

by John B. Rathbun

Fig. 2. (Upper). Shows a Slight "Reflex" or Upward Turn of the Trailing Edge. Fig. 3. (Lower) Shows an Excessive Reflex Which Greatly Reduces the C.P. Movement.

By putting a reverse curve in the trailing edge of a wing, as shown by Fig. 2, the stability of the wing may be increased to a surprising degree, but the lift and efficiency are correspondingly reduced with each increase in the amount of reverse curvature. In this way, stability is attained at the expense of efficiency and lifting power. With the rear edge raised about 0.037 of the chord, the N. P. L. found that the center of pressure could be held stationary, but the loss of lift was about 25 per cent and the loss of efficiency amounted practically 12 per cent. With very slight reverse curvatures it has been possible to maintain the lift and efficiency, and at the same time to keep the center of pressure movement down to a reasonable extent. The New U.S.A. sections and the Eiffel No. 32 section are examples of excellent sections in which a slight reverse or "reflex" curvature is used. The Eiffel 32 wing is efficient, and at the same the center of pressure movement between incident angles of 0° and 10° is practically negligible. This wing is thin in the neighborhood of the trailing edge, and it is very difficult to obtain a strong rear spar.

Wing Selection. No single wing section is adapted to all purposes. Some wings give a great lift but are inefficient at small angles and with light loading. There are others that give a low lift but are very efficient at the small angles used on high speed machines. As before explained, there are very stable sections that give but poor results when considered from the standpoint of lift and efficiency. The selection of any one wing section depends upon the type of machine upon which it is to be used, whether it is to be a small speed machine or a heavy flying boat or bombing plane.

There are a multitude of wing sections, each possessing certain admirable features and also certain faults. To list all of the wings that have been tried or proposed would require a book many times the size of this, and for this reason I have kept the list of wings confined to those that have been most commonly employed on prominent machines, or that have shown evidence of highly desirable and special qualities. This selection has been made with a view of including wings of widely varying characteristics so that the data can be applied to a wide range of aeroplane types. Wings suitable for both speed and weight carrying machines have been included.

The wings described are the U.S.A. Sections No. 1, 2, 3, 4, 5 and 6; the R.A.F. Sections Nos. 3 and 6, and the well known Eiffel Wings No. 32, 36 and 37. The data given for these wings is obtained from wind tunnel tests made at the Massachusetts Institute of Technology, the National Physical Laboratory (England), and the Eiffel Laboratory in Paris. For each of these sections the lift co-efficient (Ky), the lift-drift ratio (L/D), and the drag co-efficient (Kx) are given in terms of miles per hour and pounds per square foot. Since these are the results for model wings, there are certain corrections to be made when the full size wing is considered, these corrections being made necessary by the fact that the drag does not vary at the same rate as the lift. This "Size" or "Scale" correction is a function of the product of the wing span in feet by the velocity of the wind in feet per second. A large value of the product results in a better wing performance, or in other words, the large wing will always give better lift-drag ratios than would be indicated by the model tests. The lift co-efficient Ky is practically unaffected by variations in the product. If the model tests are taken without correction, the designer will always be on the safe side in calculating the power. The method of making the scale corrections will be taken up later.

Of all the sections described, the R.A.F.-6 is probably the best known. The data on this wing is most complete, and in reality it is a sort of standard by which the performance of other wings is compared. Data has been published which describes the performance of the R.A.F.-6 used in monoplane, biplane and triplane form; and with almost every conceivable degree of stagger, sweep back, and decalage. In addition to the laboratory data, the wing has also been used with great success on full size machines, principally of the "Primary trainer" class where an "All around" class of wing is particularly desirable. It is excellent from a structural standpoint since the section is comparatively deep in the vicinity of the trailing edge. The U.S.A. sections are of comparatively recent development and are decided improvements on the R.A.F. and Eiffel sections. The only objection is the limited amount of data that is available on these wings—limited at least when the R.A.F. data is considered—as we have only the figures for the monoplane arrangement.

WING SELECTION.

(1) Lift-Drag Ratio. The lift-drag ratio (L/D) of a wing is the measure of wing efficiency. Numerically, this is equal to the lift divided by the horizontal drag, both quantities being expressed in pounds. The greater the weight supported by a given horizontal drag, the less will be the power required for the propulsion of the aeroplane, hence a high value of L/D indicates a desirable wing section—at least from a power standpoint. In the expression L/D, L = lift in pounds, and D = horizontal drag in pounds. Unfortunately, this is not the only important factor, since a wing having a great lift-drag is usually deficient in lift or is sometimes structurally weak.

The lift-drag ratio varies with the angle of incidence (i), reaching a maximum at an angle of about 4° in the majority of wings. The angle of incidence at which the lift-drag is a maximum is generally taken as the angle of incidence for normal horizontal flight. At angles either greater or less, the L/D falls off, generally at a very rapid rate, and the power increases correspondingly. Very efficient wings may have a ratio higher than L/D=20 at an angle of about 4°, while at 16° incidence the value may be reduced to L/D = 4, or even less. The lift is generally greatest at about 16°. The amount of variation in the lift, and lift-drag, corresponding to changes in the incidence differs among the different types of wings and must be determined by actual test.

After finding a wing with a good value of L/D, the value of the lift co-efficient Ky should be determined at the angle of the maximum L/D. With two wings having the same lift-drag ratio, the wing having the greatest lift (Ky) at this point is the most desirable wing as the greater lift will require less area and will therefore result in less head resistance and less weight. Any increase in the area not only increases the weight of the wing surface proper, but also increases the wiring and weights of the structural members. With heavy machines, such as seaplanes or bomb droppers, a high value of Ky is necessary if the area is to be kept within practical limits. A small fast scouting plane requires the best possible lift-drag ratio at small angles, but requires only a small lift co-efficient. At speeds of over 100 miles per hour a small increase in the resistance will cause a great increase in the power.