Speed stability is the behavior of the speed after a disturbance at a fixed power setting. The behavior of an aircraft’s speed after it has been disturbed is a consequence of the drag values experienced by the aircraft frame. Speed is said to be stable if after it has been disturbed from its trimmed state it returns naturally to its original speed. For example:
1. An increase in speeds leads to an increase in drag, thus causing a return to the original speed.
2. A decrease in speed leads to a decrease in drag, thus causing a return to the original speed.
Speed is said to be unstable if after it has been disturbed from its trimmed state the speed divergence continues, resulting in a negative speed stability. For example:
1. A decrease in speed leads to an increase in drag, which causes a further decrease in speed, thus causing a negative speed divergence.
2. An increase in speed leads to a decrease in drag, which causes a further increase in speed, thus causing a positive speed divergence.
A speed margin is the difference between the aircraft’s normal maximum permitted operating speed and its higher certified testing speed.
For a piston-engined propeller aircraft:
VNO is the normal operating maximum permitted speed. VNE is the higher, never exceeded operating speed. VDF is the maximum demonstrated flight diving speed, established during design certification flight trials.
The piston-engined propeller aircraft enjoys a relatively large margin between VNO and VDF and has very little overspeed tendencies. Therefore, the speed margin for a piston-engined propeller aircraft is not very significant.
For the jet aircraft:
VMO/MMO is the maximum indicated operating speed in knots or Mach number. This is the normal maximum operating speed, which ensures an aircraft’s structural integrity and adequate handling qualities. VDF/MDF is the maximum demonstrated flight diving speed in knots or Mach number established during the design certification flight trials. This flight diving speed incurs reduced aircraft structural integrity and often a lower level of handling qualities. The jet aircraft’s margin between VMO/MMO and VDF/MDF is relatively small, and because of its low cruise drag and the enormous power available from its jet engines, especially at low altitudes, the jet aircraft has a distinct overspeed tendency. Therefore, the speed margin on a jet aircraft is very significant.