Extreme Points of a V-n Diagram

The corner points of the flight envelope (see Figure 5.2) is of interest for stress engi­neers. Enhancing structures would establish aircraft weight that must be predicted at the conceptual design phase.

Figure 5.3 shows various attitudes in pitch-plane maneuvers associated with the V-n diagram, each of which is explained herein. The maneuver is a transient situa­tion, and the various positions shown in Figure 5.3 can occur under more than one scenario. Only the attitudes associated with the predominant cases in pitch-plane maneuvers are addressed below. Negative g is when the maneuver force is directed in the opposite direction toward the pilot’s head, irrespective of his or her orienta­tion relative to the Earth.

Positive Loads

This is when an aircraft (and its occupants) experiences a force more than its normal weight. An aircraft stalls at a maneuver reaching amax; following are the various scenarios. In level flight at 1 g, the aircraft angle of attack, a, increases with slowing down of speed and reaches its maximum value, amax, at which the aircraft would stall at a speed VS.

1. Positive High Angle of Attack (+PHA). This occurs during a pull-up maneuver that raises the aircraft nose in a high pulling g-force, reaching the limit. The aircraft could stall if it is pulled harder. At the limit load of n, the aircraft reaches +PHA at aircraft speeds of VA.

2. Positive Intermediate Angle of Attack (+PIA). This occurs at a high-speed level flight when control is actuated to set the wing incidence at an angle of attack. The aircraft has a maximum operating speed limit of VC when +PIA reaches the maximum limit load of n, in maneuver; it is now in transition.

3. Positive Low Angle of Attack (+PLA). This occurs when an aircraft gains the maximum allowable speed, sometimes in a shallow dive (dive speed, VD). Then, at a very small elevator pull (i. e., low angle of attack), the aircraft would hit the maximum limit load of n. Some high-powered military aircraft can reach VD during level flight. The higher the speed, the lower is the angle of attack, a, to reach the limit load – at the highest speed, it would be +PLA.

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