CHAPTER V 



ON SUSTAINING SURFACES 

 The following general considerations may conveniently precede the particu- 

 lar description of the balancing of the aerodrome. 



In " Experiments in Aerodynamics," I have given the result of trials, show- 

 ing that the pressure (or total resistance) of a wind on a surface 1 foot square, 

 moving normally at the velocity of 1 foot per second, is 0.00166 pounds, and that 

 this pressure increases directly as the surface of the plane, and (within our ex- 

 perimental condition) as the square of the velocity, 1 results in general accord- 

 ance with those of earlier observers. 



I have further shown by independent investigations that while the shape of 

 the plane is of secondary importance if its movement be normal, the shape and 

 " aspect " greatly affect the resultant pressure when the plane is inclined at a 

 small angle, and propelled by such a force that its flight is horizontal, that is, 

 under the actual conditions of soaring flight. 



I have given on page 60 of " Aerodynamics," the primary equations, 

 P=P 90 F(a)=kAV 2 F(<x), . 

 W=P a cos a=hAV*F(a) cos a, 

 R = P a sm a=kAV z F(a) sin a, 



where TT r is the weight of the plane under examination (sometimes called the 

 "lift") ; jR the horizontal component of pressure (sometimes called the "drift") ; 

 k is the constant already given ; A the area in square feet ; V, the velocity in feet 

 per second; F a function of a (to be determined by experiment) ; a the angle 

 which, under these conditions, gives horizontal flight. 



I have also given on page 66 of the same work the following table showing 

 the actual values obtained by experiment on a plane, 30x4.8 inches ( = 1 sq.ft.), 

 weighing 500 grammes (1.1 pounds) : 



' This pressure per unit of area varies with the area itself, but in a degree which is negligible 

 for our immediate purpose. 



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