The Theory and Practice of Model Aeroplaning

by V. E. Johnson

Fig. 2.—Design for an Aeroplane Model (Power Driven).
This design is attributed to Professor Langley.

Built up fabric-covered aeroplanes[7] gain in lightness, but lose in resistance. In the case of curved surfaces this difference is considerably more; one reason, undoubtedly, is that in a built up model surface there is nearly always a tendency to make this curvature excessive, and much more than it should be. Having called attention to this under the head of resistance, we will leave it now to recur to it later when considering the aerofoil proper.

Fig. 3.—Horizontal Section of Vertical Strut (enlarged.)

§ 7. Allusion has been made in this chapter to skin friction, but no value given for its coefficient.[8] Lanchester's value for planes from ½ to l½ sq. ft. in area, moving about 20 to 30 ft. per second, is

0·009 to 0·015.

Professor Zahm (Washington) gives 0·0026 lb. per sq. ft. at 25 ft. per second, and at 37 ft. per second, 0·005, and the formula

f = 0·00000778l ^·93v^1·85

f being the average friction in lb. per sq. in., l the length in feet, and v the velocity in ft. per second. He also experimented with various kinds of surfaces, some rough, some smooth, etc.