VH Speed

VH speed is the maximum speed in level flight with maximum continuous power. VH speed is used to benchmark the value of VC. VC must not exceed certain maximum speed limitations, such as the maximum speed in level flight at maximum continuous power (VH) or a specified fraction of VH at sea level, as specified by regulations.

VA and VB Speeds

By definition VA represents the maximum speed at which the aircraft can be safely subjected to full and abrupt control movements without exceeding its structural limits. Learning just about VA isn’t sufficient as VA doesn’t consider external loads (by turbulence) which can be added to existing load created by control inputs.

VB, or turbulence penetration speed, is a critical airspeed limitation for transport-category aircraft, designed to ensure safe operation in rough air conditions. Unlike VA (Design Maneuvering Speed), which focuses on structural integrity during aggressive maneuvers, VB is specifically aimed at minimizing stress on the aircraft caused by gusts or turbulence. Again, VB isn’t perfect but introduces additional margin.

Read more: Importance of VB Speed

VC Speed

Design Cruise Speed (VC) is a crucial airspeed selected by aircraft designers to evaluate the structural strength requirements of the aircraft during normal cruising flight conditions. VC must provide adequate spacing from the design maneuvering speed (VB) and the design dive speed (VD) to allow for speed upsets during flight. Regulatory requirements typically mandate that VC be at least a certain value above VB (e.g., at least 43 knots above VB) and not greater than a certain fraction of VD (e.g., not greater than 0.8 VD). This ensures that the aircraft can safely maneuver and recover from unexpected speed changes during cruise. VC must not exceed certain maximum speed limitations, such as the maximum speed in level flight at maximum continuous power (VH) or a specified fraction of VH at sea level, as specified by regulations.

VD Speed

Design Dive Speed (VD) is a critical airspeed limitation defined by aircraft designers to ensure the structural integrity of the aircraft during high-speed flight conditions, particularly during dives. VD is based on the principle that an upset may occur when the aircraft is flying at its design cruising speed (VC), resulting in a shallow dive. As the aircraft enters the dive, its speed increases rapidly until recovery is initiated. VD is the maximum speed reached during this dive, taking into account the aerodynamic forces and structural loads experienced by the aircraft.

Aircraft designers calculate VD by assessing the structural strength of the airframe and determining the maximum loads it can withstand without experiencing permanent deformation or failure. VD serves as a crucial reference point for evaluating the aircraft’s strength and ensuring it can safely withstand the forces encountered during high-speed flight conditions. During the dive, if the resulting speed at VD is deemed unsuitable due to factors such as buffet or other high-speed effects, a demonstrated speed known as VDF (Flight Demonstrated Design Dive Speed) may be used instead. VDF is a speed that has been flight-tested and demonstrated to be safe for the aircraft’s structural integrity under high-speed conditions.