370 FORMS OF ENERGY Fig. 160. The path of a baseball is curved by the spin given the ball by the pitcher. tion of the balls piles up air on one side and reduces the pressure on the other side, as shown in Fig. 161. The path of the ball is therefore deflected toward the direction of least pressure. A ping-pong ball will be supported in an air stream or a jet of water, as shown in Fig. 162, and is prevented from falling off the jet. Figure 163 shows how a card with a pin projecting up into the hole in a spool is held near the spool when air is blown through the spool, because the velocity of . Direction of Spinning Bail Direction of Air Relative to Ball Fig. 161. Bernoulli's principle applied to a "curved" baseball. Increased Pressure (Decreased Velocity) Decreased Pressure (Increased Velocity) the air is increased and therefore the pressure is decreased above the card to a value lower than the atmospheric pressure below it. The same principle is applied in the automobile carburetor, where a stream of air flows through a narrow passage and thus sucks in gasoline from the jets and mi.xes with it. The steam-injector, used in forcing water into boilers at high pressures, is based on this same principle. Two ships at anchor near each other in a river current or tide or two ships moving through the water side by side may be drawn together in the same way that the tennis balls hung near each other by long threads will be drawn together when a jet of air is blown between them. The Lift Opposes the Force of Gravity. The force which lifts the airplane from the earth and sustains it while in flight is called the lift. High Velocity Low Pressure A Fig. 162. A ball supported on a blast of air. — An illustration of Bernoulli's principle.
AERODYNAMICS 371 An airfoil is any surface which when moved relative to the air gives a useful dynamic action. The wings, ailerons, vertical tail fin, rudder, stabilizer, and elevators of an airplane are all airfoils because they give a useful dynamic action. Air CardT -Reduced Pressure Under and Near Opening Fig. 163. Why a card is held against a spool when one blows through the spool against the card. The pin keeps the card from slipping sidewise. The main airfoils which produce the lift are the wings. The Hft of an airfoil depends upon four factors: (1) the angle of attack, i.e., the angle at which the wing meets the air; (2) the density of the air; (3) the speed of the plane relative to the air; and (4) the design or shape of the airfoil. Lift increases as the angle of attack increases up to a certain point. Lift also increases with an increase in the density of the air and as the square of the speed of the airplane. the speed is doubled, the lift is increased four times. In taking off from the ground, the airplane has to attain a certain speed before the lift is sufficient to overcome the force of gravity. If the angle of attack exceeds what is called the critical angle, or hurhle point, the lift will decrease, and the airplane will stall. For an airplane of a given weight, the greater the wing area, the slower the airplane can fly without stalling. Wings are given a camber (curvature) to increase the lift, but in so doing the drag is increased and the speed is decreased. The factors affecting the lift are best seen in the formula: weight equals the product of a constant ^ for the type of airfoil, the density of the air, the area of the wing, and the velocity squared. If For an airplane of given weight any one of these four factors may be varied, provided Fig. that one other factor is varied to maintain the same total lift. * These constants for every type of airfoil have been obtained by wind-tunnel tests. Lift 164. The forces acting on an airplane in flight.