IFH 4-9

Uncontrolled Turbulence Controlled Vortices Figure 4-13. Vortex generators. Plain Split Fowler Slotted Figure 4-12. Plain and split flaps. accept minor speed changes knowing that when the pitch is returned to the initial setting, the speed returns to the original setting. This reduces the pilot’s workload. Aircraft are usually slowed to a normal landing speed when on the final approach just prior to landing. When slowed to 65 knots, (1.3 VSO), the airplane will be close to point C. [Figure 4-14] At this point, precise control of the pitch and power becomes more crucial for maintaining the correct speed. Pitch and power coordination is necessary because the speed stability is relatively neutral since the speed tends to remain at the new value and not return to the original setting. In addition to the need for more precise airspeed control, the pilot normally changes the aircraft’s configuration by extending landing flaps. This configuration change means the pilot must be alert to unwanted pitch changes at a low altitude. However, because lift is required during low speed flight and is characterized by high AOA, 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 4-12] 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 AOA. For example, if the basic wing stalls at 18° without flaps, then with the addition of flaps to the CL-MAX position, the new AOA that the wing 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. 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 4-13] Small Airplanes Most small airplanes maintain a speed well in excess of 1.3 times VSO on an instrument approach. An airplane with a stall speed of 50 knots (VSO) has a normal approach speed of 65 knots. However, this same airplane may maintain 90 knots (1.8 VSO) while on the final segment of an instrument approach. The landing gear will most likely be extended at the beginning of the descent to the minimum descent altitude, or upon intercepting the glideslope of the instrument landing system. The pilot may also select an intermediate flap setting for this phase of the approach. The airplane at this speed has good positive speed stability, as represented by point A on Figure 4-11. Flying in this regime permits the pilot to make slight pitch changes without changing power settings, and