136 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1908. 



The total weight of the machine, with aviator, is 650 pounds. 

 It has a load of about If pounds per square foot of supporting sur- 

 face. Two pneumatic-tired bicycle wheels are attached to the lower 

 part of the frame. 



With this machine, Mr. G. H. Curtiss, on July 4, 1908, won the 

 Scientific American trophy by covering the distance of over a mile in 

 one minute and forty-two and two-fifths seconds at a speed of about 

 39 miles per hour. 



SOME GENERAL CONSIDERATIONS WHICH GOVERN THE DESIGN OE AN 



AEROPLANE. 



The design of an aeroplane may be considered under the heads of 

 support, resistance and propulsion, stability, and control. 



In this class of flying machines, since the buoyancy is practically 

 insignificant, support must be obtained from the dynamic reaction 

 of the atmosphere itself. In its simplest form, an aeroplane may 

 be considered as a single plane surface moving through the air. 

 The law of pressure on such a surface has been determined and may 

 be expressed as follows: 



P = 2A'(rAV- sin a (1) 



in which P is the normal pressure upon the plane, ^• is a constant 

 of figure, o- the density of the air, A is the area of the plane, V the 

 relative velocity of translation of the plane through the air, and a 

 the angle of flight. 



This is the form taken by Duchemin's formula for small angles 

 of flight such as are usually employed in practice. The equation 

 shows that the upward pressure on the plane varies directly with the 

 area of the plane, with the sine of the angle of flight, with the density 

 of the air, and also with the square of the velocity of translation. 



It is evident that the total upward pressure developed must be at 

 least equal to the weight of the plane and its load, in order to support 

 the system. If P is greater than the weight, the machine will ascend ; 

 if less, it will descend. 



The constant k depends only upon the shape and aspect of the plane, 

 and should be determined by experiment. For example, with a plane 

 1 foot square ka = 0.00167, as determined by Langley, when P is 

 expressed in pounds per square foot, and V in feet per second. 



Equation (1) may be written 



P 



AV^ = : 



2k(T sm a 



If P and a are kept constant then the equation has the form 



AV- = constant. (Q) 



