The practical application of these rules is greatly limited by the uncertainty that attaches to the actual position of the center of pressure, and this fact and also the numerical values involved may be illustrated by examples.
CONDITION OF AERODROME NO. 6, NOVEMBER 28, 1896
The weight was 12.5 kilos. On November 28, the steam pressure was less than 100 pounds, and the thrust may be taken at 4.5 kilos. The distance bd was 25 cm.
Hence
12.5 × ab = 4.5 x 25 cm.
ab = 9 cm.
This appears to give the position of CP1, but CP1 is a resultant of the pressure on both wings, and its position is determined by the empirical rule just cited. We [p050] cannot tell in fact, then, with exactness how to adjust the wings so that CG1−CP1 may be 9 cm., and equilibrium was in fact obtained in flight when (the empirically determined) CG1−CP1 = 3 cm.
Again, let it be supposed that CP1 was really over CG1 . . . . The distance of the center of gravity below the center of pressure is 43 cm. = R.
Then
α = sin−14.5 × 2512.5 × 43 = 12° nearly.