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.
AERODYNAMICS 369 The Venturi tube is an application of Bernoulli's principle, which states that when the rate of flow of gases or liquids is increased the pressure decreases. Venturi tubes increase the rate of flow of a gas or liquid under a given pressure in the constricted portion of the tube. The gauges shown in the Venturi tube in Fig. 157 can be graduated so as to show rates of flow of liquids Fig. 157. Bernoulli's principle. The pressure is reduced at the constricted portion of the pipe. or gases through the Venturi tube. Meters can even show the amount of water used in any given time. Such meters are called Venturi meters. Figure 158 shows one type of Venturi "Pitot tube" used with the air-speed indicator of an airplane. The two open ends of the Venturi "Pitot tube" are mounted in the Fig. 158. Principle of the air-speed indicator, an application of Bernoulli's principle. airplane so as to face the direction to which the airplane is pointed. The difference in pressure on the two sides of the air-speed indicator varies with the air speed ; the instrument may be graduated to read in terms of miles per hour. Figure 159 shows how the streamlining of the wing causes the air above the wing to travel faster than the air below the wing. The increase in the velocity of the air above the wing thus decreases the pressure above the wing, and the decrease in the velocity of the air below the wing increases the pressure below the wing. This combination of low and high pressures produces a pull and push on the wing Fig. 159. The circulatory stream of air about the wing of an airplane, which combined with the sweep of the air from left to right yields a velocity of air above the wing which is greater than that below. which is called the lift. low pressure above the airfoil, From 65 to 100 per cent of the lift is due to the depending upon the angle of attack. Bernoulli's Principle Has Many Other Interesting and Practical Applications. A "cut" tennis ball, a "sliced" golf ball, or a "curved" baseball given a spin when pitched will follow a curved path because the rota-