584 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 3 8 



pointed out in his 1936 James Forrest lecture, to make the flow remain 

 laminar over the whole surface would be to reduce the drag to one- 

 tenth of the values now normal. This I do not think we shall ever 

 attain, but whether we do or not, we can at least ensure that wing 

 surfaces shall be made exceedingly smooth, particularly near the 

 leading edge and very particularly on the upper surface of the wing; 

 and we can see that when flush riveting cannot be used, the shape, and 

 place, of the rivets is such that they do not project through the laminar 

 layer. Steps can also be taken, now that the technique is known, to 

 reduce greatly the engine cooling drag — even if it may be to convert 

 it most marvelously into a small thrust. 



Action on these lines applied to the designs of today soon brings 

 one close to the most formidable of all nature's fences; that is the 

 natural limit to the speed with which the air is able to get out of the 

 way of the advancing airplane. The speed at which air can move 

 when pushed is the same as the velocity of sound, and once the 

 airplane speed approaches this boundary, it becomes more and more 

 difficult to push away the air in front. And there is nothing we can 

 do to increase the velocity of sound. 



Perhaps we should study the mechanism more closely. When a 

 body moves it compresses the air just in front of it, and the resulting 

 pressure is communicated to the air farther ahead. This communi- 

 cation is, as I have said, carried out at the velocity of sound in the 

 medium. In air of normal sea-level pressure and temperature, this 

 velocity is 750 miles per hour. At greater altitudes it is less, not 

 because the pressure is less, but because the temperature is. If the 

 temperature were everywhere the same, the velocity of sound would 

 not change. Actually the square of the velocity is directly propor- 

 tional to the absolute temperature, so that in the stratosphere instead 

 of being 750 miles per hour it is only 650 miles per hour. 



What precisely happens when the speed reaches this limit? When 

 that happens, the air ahead cannot be "warned" of what is coming. 

 This leads to as many shocks and collisions as if an unlighted motor 

 car tried to get through a crowd of deaf people on a dark night. 

 When an airplane moves as fast as, or faster than, the velocity of 

 sound, collisions with the air particles are inevitable and there will 

 be an enormous loss of energy, through conversion into heat in the 

 resulting shock waves. Hence, the drag coefficient rises to a high 

 value as the velocity of sound is reached, and although it subsequently 

 recovers somewhat, it still remains very much higher than the sub- 

 sonic figure. Moreover, every roughness point on the airplane 

 produces its own shock wave and therefore its own particular loss of 

 energy. It is true that the lift coefficient also increases at this time, 

 but the ratio of the lift to drag is substantially reduced and a less 

 efficient airplane results. To some extent this might be thought to 



