22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



The occurrence of a critical speed for a given body in a given atti- 

 tude is paralleled by the practically much more important phenome- 

 non of the occurrence of a critical attitude at a given speed. Just as 

 the nature of the fluid motion and the law of resistance of a given 

 body change rapidly at a certain critical range of speed, so there are 

 similar rapid changes in the motion and the forces at the critical angle 

 of attack for a given aerofoil at a given speed. 



Remarks on the Resistance of Flat Plates Normal to 



THE Wind 



From equation (8) it would appear that when R is proportional to 

 S^ it must also be proportional to D^, and yet this does not seem to 

 be true for flat plates normal to the wind. On the contrary, while 



-fizai ^^ nearly independent of S, the results of various observers 



indicate that it increases somewhat as the diameter of the plate 

 increases from a few inches to a few feet. Leaving aside the improb- 

 able supposition that this efifect is only apparent and due to observa- 

 tional errors, the most obvious explanation is that compressibility 

 may not be entirely negligible. If that is the explanation, equation 

 (9) and not equation (8) is the one to be used, and it is quite con- 

 ceivable that (f)( — ^ > ) might have such a form as to be independ- 

 ent of 5* without being entirely independent of D. Computations of 

 the amount of compression to be expected at the speeds in question * 

 seem to show that the discrepancies are too large to be accounted 

 for in this way. But it may be remarked that in some of the details 

 of the turbulence much higher speeds may occur than the speed of the 

 wind as a whole. Hence compression might occur locally, in some 

 parts of the field about the body, to such an extent as to modify the 

 flow and so afifect the resistance, even though computations based on 

 the average speed of the wind might indicate that the effects of com- 

 pression could not possibly be appreciable under the given experi- 

 mental conditions. 



Mr. Hunsaker's observations on circular disks suggest, however, 

 that there may be another interpretation of the efifect in question 

 which does not oblige us to have recourse to the unlikely supposition 

 that compressibility is of importance. If, -as appears from these ex- 

 periments, there is a critical range of speed determined by the form of 



^ See Bairstow and Booth : Rep. British Adv. Committee for Aeronautics, 

 1910-11, p. 21. 



