Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/403

Whence, $V=36\ {\mbox{ft./sec.}}\tan \gamma =.246$ or $\gamma =13^{\circ }-50'$ or $\sin \gamma =.239.$

Resistance in line of flight
	$=W\sin \gamma =.102\times .239=$		$.0244$
Aerodynamic resistance
		$=W\beta ={\frac {102}{20}}=$	$.0050$
∴ Skin resistance (lbs.)		=	——— $.0194$

or in poundals per sq. ft.

$={\frac {.0194\times 32.2}{.504}}=1.24$

But normal plane reaction at 36 ft./sec. (poundals),

$P=.7\times .078\times 36\times 36=71$

If we make an allowance in respect of the aeroplane in accordance with §§ 182, 183, and 184, deducting half the area, we have, 72.65 – 22 = 50.65 sq. in. = .351 sq. ft. in lieu of .504 as above. Or skin resistance in poundals per sq. ft.

$={\frac {.0194\times 32.2}{.351}}=1.78$

∴ $\quad \xi ={\frac {1.78}{71}}=.025$

In the case of an aerodone having a natural velocity as high as 36 ft./sec, it is impossible to be sure, in so short a flight as 65 feet, that the true natural velocity and gliding angle are recorded; in a short flight the launching velocity and angle have a serious influence on the flight path.

If the altitude of discharge could be increased to 50 feet or thereabouts, with a flight path of some 200 feet length, this difficulty would be overcome, or at least its importance would be reduced to a negligible quantity.^[1]

1. The author had intended repeating these experiments under more favourable circumstances, but the difficulty of hitting the right weather conditions, at an appointed place, away from home, at a time that is otherwise convenient, has hitherto proved insuperable.

381