Aeroplane Construction and Operation

by John B. Rathbun

Eiffel 37 is essentially a high-speed wing having a high L/D ratio and a small lift co-efficient. The maximum lift-drag ratio of 20.4 is attained at a negative angle -08°. The value of Ky at this point is 0.00086, an extremely low figure. The maximum Ky is 0.00288 at 14.0°, the L/D ratio being 4.0 at this angle. Structurally it is the worst wing that we have yet discussed, being almost "paper thin" for a considerable distance near the trailing edge. The under surface is deeply cambered, with the maximum under camber about one-third from the trailing edge. It is impossible to use this wing without a very long overhang in the rear of the section, and like the Eiffel 32, the front spar must be very far forward. For those desiring flexible trailing edges, this is an ideal section. This wing is best adapted for speed scouts and racing machines because of its great L/D, but as its lift is small and the center of pressure movement rapid at the point of maximum lift-drag, it would be necessary to fly at a small range of angles and land at an extremely high speed. Any slight change in the angle of incidence causes the lift-drag ratio to drop at a rapid rate, and hence the wing could only be manipulated at its most efficient angle by an experienced pilot. Again, the angle of maximum L/D is only a few degrees from the angle of no lift.

U.S.A. Wing Sections. These wing sections were developed by the Aviation Section of the Signal Corps, United States Army, and are decided improvements on any wing sections yet published. The six U.S.A. wings cover a wide range of application, varying as they do, from the high speed sections to the heavy lift wings used on large machines. The data was first published by Captains Edgar S. Gorrell and H. S. Martin, U.S.A., by permission of Professor C. H. Peabody, Massachusetts Institute of Technology. An abstract of the paper by Alexander Klemin and T. H. Huff was afterwards printed in "Aviation and Aeronautical Engineering." While several of the curves are modifications of the R.A.F. sections already described, they are aerodynamically and structurally superior to the originals, and especial attention is called to the marked structural advantages.

U.S.A.-1 and U.S.A.-6 are essentially high speed sections with a very high lift-drag ratio, these wings being suitable for speed scouts or pursuit machines. The difference between the wings is very slight, U.S.A.-1 with K-000318 giving a better landing speed, while U.S.A.-6 is slightly more efficient at low angles and high speeds.

Fig. 14. U.S.A. Wing Sections Nos. 1-2-3-4-5-6, Showing the Ordinates at the Various Štations Expressed as Decimals of the Chord. U.S.A.-4 is a Heavy Lift Section, While U.S.A.-1 and U.S.A.-6.are High Speed Wings. For Any Particular Duty, the Above Wings Are Very Deep and Permit of Large Structural Members. The Center of Pressure Movement Is Comparatively Slight.

With 0° incidence, the ratio of U.S.A-1=11.0 while the lift-drag of U.S.A.-6 at 0° incidence is 13.0. The maximum lift of U.S.A.-1 is superior to that of Eiffel 32, and the maximum lift-drag ratio at equal speeds is far superior, being 17.8 against 14.50 of the Eiffel 32. Compared with the Eiffel 32 it will be seen that the U.S.A. sections are far better from a structural point of view, especially in the case of U.S.A.-1. The depth in the region of the rear spar is exceptionally great, about the same as that of the R.A.F.-6. While neither of the U.S.A. wings are as stable as the Eiffel 32, the motion of the C. P. is not sudden nor extensive at ordinary flight angles.

Probably one of the most remarkable of the United States Army wings is the U.S.A.-4 which has a higher maximum lift co-efficient (Ky) than even the R.A.F.-3. The maximum Ky of the U.S.A.-4 is 0.00364 compared with the R.A.F.-3 in which Ky (Maximum)=0.003481. Above 4° incidence, the lift-drag ratio of the U.S.A.-4 is generally better than that of the R.A.F.-3, the maximum L/D at 4° being considerably better. This is a most excellent wing for a heavy seaplane or bomber. The U.S.A.-2 has an upper surface similar to that of the R.A.F.-3, but the wing has been thickened for structural reasons, thus causing a modification in the lower surface. This results in no particular aerodynamic loss and it is much better at points near the rear edge for the reception of a deep and efficient rear spar.

U.S.A.-3 is a modification of U.S.A.-2, and like U.S.A.-2 would fall under the head of "All around wings," a type similar, but superior to R.A.F.-6. These wings are a compromise between the high speed and heavy lift types—suitable for training schools or exhibition flyers. Both have a fairly good L/D ratio and a corresponding value for Ky.

U.S.A.-5 has a very good maximum lift-drag ratio (16.21) and a good lift-drag ratio at the maximum Ky. Its maximum Ky is superior to all sections with the exception of U.S.A.-2 and 4. Structurally it is very good, being deep both fore and aft.

In review of the U.S.A. sections, it may be said that they are all remarkable in having a very heavy camber on both the upper and lower surfaces, and at the same time are efficient and structurally excellent. This rather contradicts the usual belief that a heavy camber will produce a low lift-drag ratio, a belief that is also proven false by the excellent performance of the Eiffel 37 section. The maximum Ky is also well sustained at and above 0.003. There is no sharp drop of lift at the "Stalling angle" and the working range of incidence is large.