Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/397

Now pressure per sq. ft. in pounds at 11.5 ft./sec. is given by expression

$P={\frac {.7\ \rho \ V^{2}}{g}}=$ ${\frac {.7\times .078\times 11.5\times 11.5}{32.2}}=.224$

which in grams becomes

$.224\times 453.6=101.5\ {\mbox{grams}}.$

∴ $\quad \xi ={\frac {2.175}{101.5}}=.0214$

The above example is one of several determinations made by this method. Generally speaking, the flight measurements showed greater variation than in the example given; the day of these experiments was exceptionally calm, and the aerodone used (No. 1) made a long series of good straight glides without mishap; a performance which it is not always easy to obtain. Flights of circular or otherwise curved path need to be rejected.

The results of different series of experiments were found to give values of $\xi$ varying from a trifle over .012 to nearly .030 as a maximum.

Using the value above determined ($\xi =$ .0214) we may calculate the total skin resistance of the model employed.

Total area (without added surface)

= 3.77 sq. in. = .0262 sq. ft. or

resistance = .0262 $\times$ 101.5 $\times$ .021 = .0558 grams.

But total resistance = .1172 grams, hence we may audit the resistance account for this model as follows:—

Frictional	=	.0558
Aerodynamic	=	.0614
		———
Total	=	.1172
		———

A result which appears to be quite consistent, as showing the model to be approximately designed for the conditions of least resistance within the limits of experimental error.^[1] (§ 164.)

1. We may compare the pressure relation of this model with the values laid down in Table IX., § 185.
   Area (effective) of aerofoil on ${\frac {2\ L^{2}}{3\ {\mbox{n}}}}$ basis is

375