268 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
to cruise for long distances, compressibility will for some time be 
relatively unimportant and laminar-flow forms offer outstanding 
prospects. In round figures range and economical cruising speed are 
inversely proportional to the square root of Cp. If we can halve 
_@po, both range and cruising speed will rise by 40 percent. 
We must not dismiss too lightly the possibility of cruising economi- 
cally, at great height, at very high speeds—speeds at which com- 
WING LOADING LB/SQ FT 
HEIGHT, THOUSANDS OF FEET. 
Figure §8.—Critical wing loading vs. 
height. True level speed 450 m. p. h.; 
flow laminar up to 60 percent of chord; 
airfoil thickness 15 to 16 percent. 
pressibility may well have a dominating influence on design. With a 
laminar flow extending over the majority of the surface of the air- 
plane we may reasonably expect to be able to cruise at 450 m.p.h.— 
a Mach number of about 0.7. Considering the airfoil alone, because 
of necessary thickness and camber, sonic speed will occur at a point 
near the surface when the lift coefficient reaches a certain value. 
Hence, the wing loading must not exceed a figure dependent on the 
