516 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 3 9 



plane is, therefore, not subject to this law, even under the conditions 

 where the weight of the structure would be. This helps to reduce 

 the rate of increase of weight of plane with increase in size. 



Another factor in increase of lift is higher speed, and this has 

 been used with telling effect since lift increases as the square of the 

 speed. However, increased speed means larger engines and more 

 weight of power plant, as well as more lift per unit of area and, 

 therefore, more loading per square foot of wing area. The law of 

 compensation holds here; if 'we would obtain more lift we must 

 buy it at a price. 



A third factor in increase of lift is increased angle of attack 

 between the air and the wing. This is indeed effective, but it 

 demands too heavy a price. With the increase of lift comes also 

 an increase of drag or resistance to the motion of the plane. This 

 means more engine and more power plant weight for the same 

 speed, or a reduced speed for the same power plant weight. The 

 price is too high and can only be afforded during periods of 

 maneuver at low speed, as in taking off from the ground, or landing 

 thereon, where the decrease in lift due to decreased speed is com- 

 pensated by the increase due to greater angle of attack. 



Other contributing causes have helped along in the same direction 

 of evading the consequences of this square-cube law, and it would 

 be a brave man who would now attempt to place a limit to future 

 increase in size. 



Twenty yeare ago, during the Great War, there was no commer- 

 cial aviation. The total lift, that is, the total loaded weight of 

 military planes was, for the most part, from 3,000 to 5,000 pounds. 

 The earliest commercial transport planes had a total loaded weight 

 from 5,000 to 10,000 pounds. The planes in current transport 

 service weigh loaded 20,000 to 40,000 pounds; the China Clipper 

 has a loaded weight of 52,000 pounds. The new Boeing flying boat 

 will weigh loaded 82,000 pounds, the new Douglas 4, which has been 

 undergoing test flights, will have a loaded weight of 65,000 pounds. 

 Beyond these figures, new designs have been submitted by four 

 builders for trans- Atlantic service, the loaded weight of which will 

 be about 200,000 pounds. 



Some further particulars of these latest planes may be of interest. 

 The present standard Douglas 3, of 24,000 total lift, has a wing 

 spread of 95 feet, a wing area of 985 square feet, a wing loading of 

 24.3 — that is, each square foot of wing area supports in flight 24.3 

 pounds. It has engines developing 2,000 horsepower, a cruising 

 speed of 170 miles per hour, a range of 500 miles with a pay load 

 of 5,200 pounds including 21 passengers. Corresponding figures 

 for the present China Clippers are : weight 52,000, wing spread 130 feet, 



