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Vg diagram explained | Load Factor and Accelerated Stalls

FAA-H-8083-25B Pilots Handbook of Aeronautical Knowledge

Vg Diagram
The flight operating strength of an aircraft is presented on a graph whose vertical scale is based on load factor. The diagram is called a Vg diagram—velocity versus G loads or load factor. Each aircraft has its own Vg diagram that is valid at a certain weight and altitude.

If the aircraft is flown at a positive load factor greater than the positive limit load factor, structural damage is possible. When the aircraft is operated in this region, objectionable permanent deformation of the primary structure may take place and a high rate of fatigue damage is incurred. Operation above the limit load factor must be avoided in normal operation.

There are two other points of importance on the Vg diagram. One point is the intersection of the positive limit load factor and the line of maximum positive lift capability. The airspeed at this point is the minimum airspeed at which the limit load can be developed aerodynamically. Any airspeed greater than this provides a positive lift capability sufficient to damage the aircraft. Conversely, any airspeed less than this does not provide positive lift capability sufficient to cause damage from excessive flight loads. The usual term given to this speed is “maneuvering speed,” since consideration of subsonic aerodynamics would predict minimum usable turn radius or maneuverability to occur at this condition. The maneuver speed is a valuable reference point, since an aircraft operating below this point cannot produce a damaging positive flight load. Any combination of maneuver and gust cannot create damage due to excess airload when the aircraft is below the maneuver speed.

The other point of importance on the Vg diagram is the intersection of the negative limit load factor and line of maximum negative lift capability. Any airspeed greater than this provides a negative lift capability sufficient to damage the aircraft; any airspeed less than this does not provide negative lift capability sufficient to damage the aircraft from excessive flight loads.

Maneuvering speed at gross weight can be found in the Type Certificate Data Sheet for your airplane. Newer airplanes also have it in the Airplane Flight Manual or Pilots Handbook. Maximum structural cruising speed and never exceed speed are found on your airspeed indicator.

The very best explanation of maneuvering speed that I have seen is in this video by Rod Machado.

And this one explains why maneuvering speed varies with weight.

Rule of thumb: For every 2% decrease in aircraft weight from max gross, decrease maneuvering speed by 10%.

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