## Instrument Flying Handbook Aerodynamic Factors Slow Speed Flight

Instrument Flying
Handbook

Preface

 Figure 2-10. Region of Speed Stability. An aircraft flying in steady, level flight at point C is in equilibrium. [Figure 2-10] If the speed were increased or decreased slightly, the aircraft would tend to remain at that speed. This is because the curve is relatively flat and a slight change in speed does not produce any significant excess or deficiency in power. It has the characteristic of neutral stability, i.e., the aircraft's tendency is to remain at the new speed. Reversed Command 'Ike characteristics of flight in the region of reversed command are illustrated at point B on the curve in Figure 2-10. If the aircraft is established in steady, level flight at point B, lilt is equal to weight. and the power available is set equal to the power required. When the airspeed is increased greater than point B: an excess of power exists. This causes the aircraft to accelerate to an even higher speed. When the aircraft is slowed to some airspeed lower than point B, a deficiency of power exists. The natural tendency of the aircraft is to continue to slow to an even lower airspeed. This tendency toward instability happens because the variation of excess power to either side of point B magnifies the original change in speed. Although the static longitudinal stability of the aircraft tries to maintain the original trimmed condition, this instability is more of an influence because of the increased induced drag due to the higher angles of attack in slow-speed flight. Trim The term trim refers to employing adjustable aerodynamic devices on the aircraft to adjust forces so the pilot does not have to manually hold pressure on the controls. One means is to employ trim tabs, A trim tab is a small, adjustable hinged surface, located on the trailing edge of the elevator, aileron, or rudder control surfaces. (Some aircraft use adjustable stabilizers instead of trim tabs for pitch trim.) Trimming is accomplished by deflecting the tab in the direction opposite to that in which the primary control surface must he held. The force of the airflow striking the tab causes the main control surface to he deflected to a position that contacts the unbalanced condition of the aircraft. Because the trim tabs use airflow to function, trim is a function of speed. Any change in speed results in the need to re-trim the aircraft. An aircraft properly trimmed in pitch seeks to return to the original speed before the change. It is very important for instrument pilots to keep the aircraft in constant trim. This reduces the pilot's workload significantly, allowing attention to other duties without compromising aircraft control. Slow-Speed Flight 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 lilt 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, 01' delay the boundary level separation. Plain and split flaps [Figure 2-!!] are most commonly used to change the camber of an airfoil. It should be noted that with the application of Raps, 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 C1 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 CLmax position) produces more lift than lift at 18° on the basic wing.

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