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.
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.