A,18 • TRANSITION ON AIRPLANE CONFIGURATIONS 



chord as a function of /c/S*. Varying techniques were used for measuring 

 the transition Reynolds number, principally that of rise in total drag 

 coefficient and surface tubes. The detector tubes were located in various 

 positions and correction to a fixed position does not seem practicable. 

 The effect of shape of the roughness elements is obvious as is the large 

 influence of air stream turbulence. 



Loftin's results [62] refer to three-dimensional roughness elements in 

 the Langley two-dimensional low turbulence wind tunnel and were ana- 

 lyzed by him in terms of the Reynolds number of the roughness element 

 Ukk/v. The observed value was a function of the ratio of the diameter d 

 of the cylindrical projections to the height k, varying from a value of 

 about 1000 for d/k = 0.5 to 200 for d/k = 7.0. For d/k = 1.0, the value 

 of UTck/v was 750. This compares with Klebanoff, Schubauer, and Tid- 

 strom's value of 300 for spherical elements [23]. 



Von Doenhoff [63] studied the effect of sand grain roughness elements 

 on airfoils in considerable detail. His results, also obtained in a low turbu- 

 lence wind tunnel, give a critical Reynolds number of 250 based on nomi- 

 nal particle size, or 600 based on maximum particle size. The available 

 data suggest that the critical Reynolds number of a roughness element is 

 affected as much or more by the turbulence of the air stream in which the 

 measurements are made as by the shape of the element (see Art. 5). 



In the absence of sufficient data one can only conclude that the tran- 

 sition Reynolds number on airfoils is a function of air stream turbulence, 

 pressure gradient, and surface roughness and that all variables have im- 

 portant effects. 



The flight data on transition on airfoils presents as confusing a picture 

 as the wind tunnel data. While the effects of air stream turbulence are 

 presumably absent, the angle of attack changes with speed so that the 

 Reynolds number cannot be systematically varied for a fixed angle of 

 attack. The largest influence, however, appears to be that of surface wavi- 

 ness, for increased care to secure smooth and fair surfaces has given higher 

 and higher values. In the last eighteen years the values of {xJc)Re have 

 increased from 3.5 X 10^ for conventional airfoil sections in 1938 to 

 11.4 X 10® and 17.0 X 10^ for low drag sections in recent years. Some 

 of the more recent work is still classified but the highest observed values 

 are those given in [64\- It seems impractical to secure the required free- 

 dom from surface waviness and roughness in normal construction and 

 operation of aircraft to realize these high values. 



A, 18. Transition on Airplane Configurations and on Airplanes 

 in Flight. Additional variables influence transition on three-dimensional 

 airplane configurations in wind tunnels and on airplanes in flight. The 

 pressure distribution at wing-body junctures and nacelle- wing fairings is 

 often such as to bring transition close to the leading edge of the wing. 



<45) 



