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Slow-Speed Flight

in Aerodynamic Factors

Anytime an aircraft is flying near the stalling speed or the region of reversed command, such as in final approach for a normal landing, the initial part of a go around, or maneuvering in slow flight, it is operating in what is called slow-speed flight. If the aircraft weighs 4,000 pounds, the lift produced by the aircraft must be 4,000 pounds. When lift is less than 4,000 pounds, the aircraft is no longer able to sustain level flight, and consequently descends. During intentional descents, this is an important factor and is used in the total control of the aircraft.


However, because lift is required during low speed flight and is characterized by high angles of attack, flaps or other high lift devices are needed to either change the camber of the airfoil, or delay the boundary level separation. Plain and split flaps [Figure 2-11] are most commonly used to change the camber of an airfoil. It should be noted that with the application of flaps, the aircraft will stall at a lower angle of attack. The basic wing stalls at 18° without flaps but with the application of the flaps extended (to CL-MAX position) the new angle of attack at which point the aircraft will stall is 15°. However, the value of lift (flaps extended to the CL-MAX position) produces more lift than lift at 18° on the basic wing.

Figure 2-11. Various Types of Flaps.

Figure 2-11. Various Types of Flaps.

Delaying the boundary layer separation is another way to increase CL-MAX. Several methods are employed (such as suction and use of a blowing boundary layer control), but the most common device used on general aviation light aircraft is the vortex generator. Small strips of metal placed along the wing (usually in front of the control surfaces) create turbulence. The turbulence in turn mixes high energy air from outside the boundary layer with boundary layer air. The effect is similar to other boundary layer devices. [Figure 2-12]

Figure 2-12. Vortex Generators.

Figure 2-12. Vortex Generators.

 

 

 

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