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Aerodynamics: 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 it’s own Vg diagram which is valid at a certain weight and altitude.

Typical Vg diagram

Typical Vg diagram

The curved lines representing maximum lift capability are the first items of importance on the chart. The aircraft in the figure above is capable of developing no more than +1 G at 62mph, the wing level stall speed of the aircraft. since the maximum load factor varies with the square of the airspeed, the maximum positive lift capability of this aircraft is 2 G at 92 mph, 3 G at 112 mph, 4.4 G at 137 mph, and so forth. Any load factor above this line is unavailable aerodynamically (i.e., the aircraft will stall above that line). The same situation exists for negative lift flight with the exception that the speed necessary to produce a given negative load factor is higher than that to produce the same positive load factor.

If this aircraft is flow at a positive load factor greater than the positive limit load factor of 4.4, structural damage is possible.

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. The usual term given to this speed is maneuvering speed.

The other point of importance 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.

The limit airspeed (or redline) is a design reference point to the aircraft—the aircraft above is limited to 225 mph. If flight is attempted beyond the limit airspeed, structural damage may result.

The aircraft must be operated within this “envelope” to prevent structural damage. The pilot must appreciate the Vg diagram as describing the allowable combination of airspeeds and load factors for safe operation. Any maneuver, gust, or gust plus maneuver outside of the structural envelope can cause damage and shorten the service life of the aircraft.

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4 Comments

  1. John
    Posted January 19, 2016 at 9:02 am | Permalink

    This is a very helpful explanation, however it remains unclear just how, computationally, the statement “since the maximum load factor varies with the square of the airspeed, the maximum positive lift capability of this aircraft is 2 G at 92 mph, 3 G at 112 mph, 4.4 G at 137 mph, and so forth.” was determined. It would be very helpful for a simplified, stepwise discussion to be included about how to determined these important numbers. It would also be helpful to include a discussion of how the Vg diagram might appear differently if presented as CAS rather than IAS, and why negative G Vs etc. have substantially lower generated load factors (LF) for a given IAS than postive LF. Overall, this is a very helpful discussion that should be understood by all pilots. More than once I’ve observed pilots barrel into the ‘yellow’ even when encountering light to moderate chop. Not a really good idea, IMHO.

  2. John
    Posted January 19, 2016 at 9:09 am | Permalink

    An additional question that comes to mind with the Vg diagram, an for which I’ve seen no discussion is: How can owners and A&P update the Vg diagram for aircraft where lift enhancing devices (VG, STOL, etc.) are installed on the stock factory airfoils? It is fairly common for individual or multiple STC’s to reduce the IAS of Vs or Vso by 5-10%. A discussion of what this might do to positive LF Va and negative LF numbers would be extremely pertinent, as would a means of developing an updated depiction of the Vg performance envelope.

  3. cw
    Posted January 19, 2016 at 10:01 am | Permalink

    Thanks, I’ve seen this diagram, of course, but never fully understood it. This explanation is short, easy to follow, and very helpful.m

  4. Posted January 21, 2016 at 9:11 am | Permalink

    Hi John,
    Great points you made, we will definitely work on another post that might help simplify the Vg Diagram. Thanks again for the suggestions.

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