RESEARCH FOR AERONAUTICS—-FARREN 267 
the range of operation of power plants so that propulsive power is 
independent of height. Taking an airplane with the characteristics 
of the Spitfire (table 1), and assuming that Cp)=0.022 under all con- 
ditions, the curve of speed against height is shown in figure 7 labeled 
A. The line of sonic speed, Mach number=1, is crossed at 65,000 feet. 
In practice the effect of the compressibility of air begins to be felt at 
about M=0.65 at 33,000 feet at a speed of about 430 m.p.h., and the 
rapid rise of Cpo with U/ brings the curve for greater heights down 
to about the level of curve A,. The loss of speed is very large. 
HEIGHT, THOUSANOS OF FEET. 
Figure 7.—True level speed vs. height, showing influ- 
ence of reduction of Cpo and of compressibility. 
Propulsive power, 4.5 T. H. P./sq. ft. wing surface ; 
wing loading, 28 lbs./sq. ft. ; aspect ratio, 5.6. 
If, by devising forms that will ensure some measure of laminar 
flow, we can halve Cp, and at the same time avoid compressibility 
effects, we get curve B. But if compressibility has the same kind of 
effect as on the original airplane, the result will be to depress the 
speed to curve B,. Similarly, reducing Cp. to one quarter of the 
original value, we get curves C and C. 
If we are to reach really high speeds economically, it is clear that 
we must devote at least as much effort to avoiding or reducing the 
effect of compressibility as to reducing the “low speed” value of Cpo. 
On the other hand, at speeds at which it is likely to be economical 
