514 ANI^UAL REPORT SMITHSONIAN INSTITUTION, 193 9 



This may or may not be the order of relative importance, accord- 

 ing to the viewpoint from which "importance" is judged. Thus the 

 order of importance for planes in commercial service and in military 

 or naval service would not, in general, be the same. It will, moreover, 

 be convenient to discuss the present trend with regard to these various 

 characteristics in a somewhat different order, and I shall take first 

 No. 2, carrying capacity. 



The laws of fluid mechanics t«ll us immediately that the lift of 

 an airplane wing increases directly with the area, with the square 

 of the speed, and, over most of the practical working range, with 

 the angle of attack of the air on the face of the wing. This angle 

 may be represented by the angle between the direction of motion of 

 the plane and a fore and aft line touching the under face of the wing. 

 It represents what may be called the attitude of the wing relative 

 to its direction of motion. 



It would appear then, that in order to increase the lift of an air- 

 plane wing, we have only to select some combination of larger area, 

 higher speed, and greater angle of attack. It is not, however, quite 

 so simple. What we are ultimately concerned with is not so much 

 the gross lift of the plane as the useful part of this lift — that is, 

 the balance of the total lift over and above what is necessary to sus- 

 tain the weight of the plane as a whole, including engines and pro- 

 pellers. In thus approaching the economics of airplane operation, 

 we recognize three subdivisions of the total lift : 



1. The weight of the plane, including power plant (engines and propellers). 



2. The useful load subdivided into — 



a. Fuel and oil, operating personnel, supplies. 



b. Pay load — i)aBsengers, mail, express matter, etc. 



Again, the weight of the plane together with its power plant may 

 be viewed as a dead load which must be sustained in the air simply in 

 order to realize the purposes of flight ; whereas the useful load repre- 

 sents the part of the total lift disposable as may be desired, between 

 fuel, supplies, and personnel on the one hand, and pay load on the 

 other. Thus with a small fuel load, a relatively large pay load may 

 be carried for a short distance; with a large fuel load, a relatively 

 small pay load may be carried for a long distance. 



Now to return to the results following an increase in wing area, or, 

 in general, an increase in the size of the plane. As we have seen, 

 other things being equal, the lift will increase with the area. But 

 the weight of the structure will increase likewise, and the results on 

 the useful load are, therefore, by no means assured. In order to follow 

 this point more closely, we must now discriminate between the weight 

 of the plane simply as a structure and the weight of the power plant 

 which it carries. I have referred to the sum of these two weights as 



