Properties of the Eiffel Sections (32-36-37). Three of the Eiffel sections are shown by Figs. 10, 11 and 12, these Sections being selected out of an enormous number tested in the Eiffel laboratories. They differ widely, both aerodynamically and structurally, from the R.A.F. aerocurves just illustrated.
Fig. 10-11-12 Ordinates for Three Eiffel Wing Sections
Eiffel 32 is a very stable wing, as has already been pointed out, but the value of the maximum L/D ratio is in doubt as this quantity is very susceptible to changes in the wind velocity—much more than in the average wing. Since Eiffel's tests were carried out at much higher velocity than at the M. I. T., his lift-drift values at the higher speeds were naturally much better than those obtained by the American Laboratory. When tested at 67.2 miles per hour the lift-drift ratio for the Eiffel 32 was 184 while at 22.4 miles per hour, the ratio dropped to 13.4. This test alone will give an idea as to the variation possible with changes in scale and wind velocity. The following table gives the results of tests carried out at the Massachusetts laboratory, reported by Alexander Klemin and G. M. Denkinger in "Aviation and Aeronautical Engineering." Wind speed, 30 miles per hour.
The C. P. Travel in the Eiffel wing is very small, as will be seen from Table 3. At -2° the C. P. is 0.33 of the chord from the leading edge and only moves back to 0.378 at an angle of 20°, the intermediate changes being very gradual, reaching a minimum of 0.304 at 6° incidence. The maximum Ky of Eiffel 32 is 0.002908, while for the R.A.F.-6 wing, Ky = 0.003045 maximum, both co-efficients being a maximum at 16° incidence, but the lift-drag at maximum Ky is much better for the R.A.F.-6.
Structurally, the Eiffel 32 is at a disadvantage when compared with the R.A.F. sections since it is very narrow at points near the trailing edge. This would necessitate moving the rear spar well up toward the center with the front spar located very near the leading edge. This is the type of wing used in a large number of German machines. It will also be noted that there is a very pronounced reverse curve or "Reflex" in the rear portion, the trailing edge actually curving up from the chord line.
Eiffel 36 is a much thicker wing than either of the other Eiffel curves shown, and is deficient in most aerodynamical respects. It has a low value for Ky and a poor lift-drag ratio. It has, however, been used on several American training machines, probably for the reason that it permits of sturdy construction.
Fig. 13 Characteristic Curves for Eiffel Wings Sections