CHAPTER V
ON SUSTAINING SURFACES
The following general considerations may conveniently precede the particular description of the balancing of the aerodrome.
In “Experiments in Aerodynamics,” I have given the result of trials, showing 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 experimental condition) as the square of the velocity,[20] results in general accordance 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,
Pα = P90F(α) = kAV2F(α),
W = Pα cos α = kAV2F(α) cos α,
R = Pα sin α = kAV2F(α) sin α,
where W is the weight of the plane under examination (sometimes called the “lift”); R 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 α (to be determined by experiment); α 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, 30 × 4.8 inches (= 1 sq. ft.), weighing 500 grammes (1.1 pounds):