In regard to the lift coefficient Ky, it was noted that this factor was decreased with every degree in the angle of sweep back. At the incidence angle of 4° for the maximum value of L/D, the value of Ky decreased from 0.00143 with a sweep back of 0° (Straight wings), to 0.00120 with a sweep back of 30°. At the same incidence, but with a sweep back of 10°, the lift became: Ky = 0.00130. With a retreat of 20°, Ky- 0.00129. The lift-drag also suffered with an increase in retreat, this being 17.00 with straight wings, at 16.5 at 10°, 16.2 at 20°, and 12.8 at 30°. Up to, and including a retreat of 20°, the loss in lift-drag is not so bad, but in the change from 20° to 30° there is a very great loss.
Fig. 10. (Upper) Various Angles Made by Wings During Experiments. (Below) The Center of Pressure Movement with Varying Angles.
The "angle of retreat" herein specified is such that each wing is moved back through an angle of one-half the total given retreat (r) angle in Fig. 5. That is, with a retreat of 30°, each wing section makes an angle of 15° with the entering edge of a pair of straight wings. It is more usual to specify the included angle between the leading edges as indicated by (S) in Fig. 5. In the upper portion of Fig. 10 is shown the various angles of the wings during the experiments, while below is the C. P. movement according to the different angles of retreat. The above is based on experiments made by H. E. Rossell and C. L. Brand, assistant Naval Constructors, U. S. Navy, and published in "Aerial Age."
The center of pressure referred to is that at the forward point of the middle longitudinal section of each wing, and with a given incidence the C. P. is thrown to the rear by about 0.2 of the chord by a sweep back of 10 degrees, and 0.4 of the chord for a sweep back of 20°. The center of gravity of the machine will thus have to be moved to the rear if sweep back is employed.
In making a turn a machine with sweep back has a natural tendency to bank up in the correct attitude, and even a retreat of 10° will add nearly 100 per cent to the banking tendency when compared to a pair of straight wings. It will be seen that with swept back wings, the leading edge is radial, consequently meets the air stream at more nearly a right angle. This gives more lift to the outer wing than would be the case with straight entering edges, while the inner end losses increase correspondingly in lift and hence tends further to depress the inner tip. The greatest value of sweep back is found in its resistance to side slip. If the machine should be "over banked," and tend to slip down toward the inner side of the turn, the backward angle of the inner wings will cause the entering edge to meet the side stream at more nearly right angles, and thus tend to reduce the bank and the inner slide slip. In this respect the retreat is an aid to lateral stability. On the other hand, the sweep back tends to keep the machine rolling in rough weather for side gusts then meet the inclined wing edges at an effective angle. This is particularly noticeable in landing.
Raked Tips. This subject was discussed under "Standard Wing Section," but it may be repeated here, that Eiffel in an experiment with the Coanda Wing (Eiffel 38), found that the L/D of the raked wing was 20 per cent higher than with the same wing in rectangular form. This value would not be safe to assume with all sections.
Fig. 11. Influence of "Wash Down" on the Rear Wings of a Tandem Pair.
