Airplane Flaps

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Within limits, greater camber produces greater lift. However, a less cambered wing allows for higher airspeeds at cruise. With the invention of retractable flaps we can have both, to a point. Adding flaps increases the camber of the airfoil. As a result, the airplane can fly at slower airspeeds without stalling. Then with the flaps retracted the camber is reduced and the airplane can achieve higher airspeeds at cruise.

Different designs have a wide range of effects on stall speeds. Some planes may only get two- to five-knot decreases in stall speed when flaps are used, while others allow them to fly fifteen to twenty knots slower. Specialized slow-flight aircraft can seemingly hang suspended motionless when their flaps are deployed and land in less than a few hundred feet of runway.

There are many different flap designs, but we will focus on three: the plain flap, the split flap, and the fowler flap. The plain flap is hinged at the rear of the wing and basically pivots down around its hinge point.

The second flap design, split flaps, also increases the camber of the underside of the airfoil, but is located on the aft, bottom portion of the wing. Unlike the plain and fowler flaps, it does not change the camber of the upper surface of the wing.

The fowler flaps are the most efficient of the three shown in Figure. In addition to moving down, they also move back from the trailing edge of the wing. This not only increases the camber of the airfoil on the upper and lower surface, but also increases wing area. Additionally, depending on how they are designed, slots between the wing and sections of flap can improve the flow of air over the wing at high angles of attack, further reducing the speed at which turbulence becomes strong enough to stall the wing. The next time you ride the airlines, notice how the flaps extend down and away from the trailing edge of the wing during takeoff or landing. This is part of the system that allows a jet that cruises at Mach .8 to land at relatively slow airspeeds.

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