Page:Aerial Flight - Volume 1 - Aerodynamics - Frederick Lanchester - 1906.djvu/411

plates J riveted to a shank K which fits into a socket in the balance arm A (Fig. 158) being secured by a set screw L.

In making any determination the area of the normal plane is adjusted until the beam is in equilibrium. The coefficient of skin-friction $\xi$ is then calculated from the relation of the areas of the normal and friction planes multiplied by their respective distances from the pivot axis.

Determination, June 19th, 1907, Cobley Hill, Alvechurch.

Wind velocity (estimated) 20 to 40 miles per hour.^[1]

Friction plane No. 1, cedar shellac varnished and roughly polished. In pterygoid aspect.

	Length.		Breadth.		Leverage.
Normal plane	2.5″	$\times$	.95″	$\times$	3.25	= 7.7
Friction plane	16.25″	$\times$	5″	$\times$	10.12	= 823

$\xi ={\frac {7.7}{823}}=.00936$

Determination, June 23rd, 1907.

High wind.

Friction plane No. 2, cedar filled and water gilt and burnished. In pterygoid aspect.

	Length.		Breadth.		Leverage.
Normal plane	2.25″	$\times$	.9″	$\times$	3.25	= 7.3
Friction plane	16.25″	$\times$	5″	$\times$	10.12	= 823

$\xi ={\frac {7.3}{823}}=.00888$

Friction plane No. 1 (polished cedar), substituted for No. 2 as above, showed no appreciable change of balance.

With width of normal plane increased to 1 in., both planes, Nos. 1 and 2, gave insufficient reaction to balance pressure on normal plane. It is therefore to be concluded that for a well varnished surface or for polished metal, under the conditions of

1. Determinations made with aerodynamic balance are approximately independent of velocity of wind; a rough estimate is sufficient for the purposes of record.

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