Fig. 8. Pressure Distribution for Thin Circular Section. Fig. 9. Shows the Effect of Increasing the Camber. (Eiffel)
By taking the sum of the pressures at the various parts of the surface, it was found that the total corresponded to the lift of the entire aerofoil, thus proving the correctness of the investigation. The sum of the drag forces measured at the different openings gave a lower total than the total drag measured by the balance, and this at once suggests that the difference was due to the skin friction effect that of course gave no pressure indication. The truth of this deduction is still further proved by the fact that the drag values were more nearly equal at large angles where the turbulence formed a greater percentage of the total drag.
Figs. 8, 9, 10, 11, 12 are pressure distribution curves taken along the section of several aerofoil surfaces. These are due to Eiffel. In Fig. 8 is the pressure curve for a thin circular aerofoil section, the depth of the curve measured from the chordal line being 1/13.5 of the chord. The vacuum distribution of the upper surface is indicated by the upper dotted curve, while the pressure on the bottom surface is given by the solid curve under the aerofoil. The pressures are given by the vertical column of figures at the right and are in terms of inches of water, that is, the pressure required for the support of a water column of the specified height. Figures lying above 0 and marked (-), refer to a vacuum or negative pressures, while the figures below zero are positive pressures above the atmospheric. The entering edge is at the right, and the angle of incidence in all cases is 6°.
It will be seen that the vacuum jumps up very suddenly to a maximum at the leading edge, and again drops as suddenly to about one-half the maximum. From this point it again gradually increases near the center, and then declines toward the trailing edge. It will also be seen that the pressure on the lower surface, given by the solid curve, is far less than the pressure due to the upper surface. Since the lower pressure curve crosses up, and over the zero line at a point near the trailing edge, it is evident that the supper surface near the trailing edge is under a positive pressure, or a pressure that acts down and against the lift. The pressures in any case are very minute, the maximum suction being 0.3546 inch of water, while the maximum pressure on the under surface is only 0.085 inch.
Fig. 9 shows the effect of increasing the curvature or camber, the aerofoil in this case having a depth equal to 1/7 the chord, or nearly double the camber of the first. The sharp peak at the entering edge of the pressure curve is slightly reduced, but the remaining suction pressures over the rest of the surface are much increased, indicating a marked increase in the total pressure. The pressure at the center is now nearly equal to the front peak, and the pressure is generally more evenly distributed. There is a vacuum over the entire upper surface and a positive pressure over the lower. The general increase in pressure due to the increased camber is the result of the greater downward deviation of the air stream, and the corresponding greater change in the momentum of the air. The speed at which the tests were made was 10 meters per second, or 22.4 miles per hour. The curves are only true at the center of the aerofoil length and for an aspect ratio of 6.
The average pressure over the entire surface in Fig. 8 is 1.202 pounds per square foot, and that of Fig. 9 is 1.440 pounds, a difference of 0.238 pound per square foot due to the doubling camber (16.5 per cent). Another aerofoil with a camber of only 1/27 gave an average pressure of 0.853 pound per square foot under the same conditions. A flat plane gave 0.546. Tabulation of these values will show the results more clearly.
Camber of Surface | Av. Pres. Per Sq. Ft. | Inc. in Pres. in Lbs./Sq. Ft. | Efficiency | |
|---|---|---|---|---|
Top. | Bottom. | |||
Flat Plane. | 0.546 | 0.000 | 0.89 | 0.11 |
1/27 | 0.853 | 0.307 | 0.72 | 0.28 |
1/13.5 | 1.202 | 0.349 | 0.71 | 0.29 |
1/7 | 1.440 | 0.238 | 0.59 | 0.41 |
In this table, the "Efficiencies" are the relative lift efficiencies of the top and bottom surfaces. For example, in the case of the 1/7 camber the top surface lifts 59 per cent, and the bottom 41 per cent of the total lift.
Fig. 10 is a thin aerofoil of parabolic form, while Fig. 11 is an approximation to the comparatively thick wing of a bird. In both these sections it will be noted that the front peak is not much greater than the secondary peak, and that the latter is nearer the leading edge than with the circular aerofoils. Also that the drop between the peaks is small or entirely lacking. The lower surface of the trailing edge is subjected to a greater down pressure in the case of the thin parabola, and there is also a considerable down pressure on the upper leading edge. The pressure in Fig. 10 is 1.00 pound per square foot, and that of No. 11 is 1.205, while the efficiency of the top surfaces is respectively 72 and 74 per cent.
