Man's physical universe

xanabras

368 FORMS OF ENERGY

The drag increases with increases in the angle of attack, until at the

burble point (the angle of attack at which the air no longer flows

smoothly over the airfoil) the airplane stalls, i.e., it ceases to maintain

altitude because the lift has been destroyed.

Fig. 156. National Advisory Committee for Aeronautics full-scale wind

tunnel, Langley Field, Va. The Brewster XF2A-1 airplane mounted for test

in the full-scale tunnel, the largest wind tunnel in the world. The airplane is

supported in the jet on struts which transmit the forces to balances in the house

below. Streamline fairings around the struts shield them from the air stream

to eliminate extraneous forces that would not act on the airplane in flight. The

air stream in this tunnel is 60 feet wide and 30 feet deep. Two 353-2 foot propellers,

operated by 4000-horsepower electric motors, produce a wind of 118 miles per

hour past the airplane. (Courtesy of the National Advisory Committee for

Aeronautics.)

The greater the camber (i.e., curvature), the greater will be the drag.

Inasmuch as the lifting power is also increased by increasing the

camber, the airplane-designer is forced to compromise depending on

whether he wishes high speed or high lift characteristics.

The Lift of an Airplane Is an Application of Bernoulli's Principle.

A Venturi tube is a short tube of small diameter with large openings

in the front and the rear. The fiow of air through the Venturi tube

causes the pressure to drop in the tube in proportion to the velocity

of the flow of air.

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