The V-g Diagram
 
Article
The V-g Diagram
Date Updated
March 28, 2008
Category
Aerodynamics

Several of the factors that define an airplane's performance envelope can be combined in graphic formats. The V-g diagram is on example, relating velocity (V) to load factor (G). Each V-g diagram applies to one airplane type and the information is valid only for a specific weight, configuration, and altitude. V-g diagrams show the maximum amount of positive or negative lift the airplane is capable of generating at a given speed. They also show the safe load factor limitis and the load factor, or number of Gs, the airplane can sustain at various speeds. Flying within the boundaries depicted by the diagram (figure 1) minimizes the risk of stalls, spins, and structural damage.

Figure 1 - Airplane V-g diagram

Load Factor

Major points of the V-g diagram include the curved lines representing positive and negative maximum lift capability. These lines portray the maximum amount of lift the airplane can generate at the specified speed. The intersectionof these lines with the vertical speed lines indicates the maximum G-load capability at that speed. If you exceed the G-load limit at that speed, the airplane will stall. For example, at a speed of about 97 knots, this airplane will stall with a load factor of two Gs under the conditions represented. The horizontal positive and negative load factor limits represent the structural limitations of the airplane. Exceeding these values may cause structural damage.

Another important line on the V-g diagram is the normal stall speed, Vs. Note that at this speed, the airplane stalls at one positive G. This is the same speed shown by the lower limit of the green arc on the airspeed indicator. Above the one-G load factor, stalling speed increases. Any stall which occurs above the straight-and-level load of one G is an accelerated stall.

As discussed earlier, maneuvering speed (Va) is the maximum speed at which you can use full and abrupt control movement without casuing structural damage. Va occurs at the point where the curved line representing maximum positive lift capability intersects the maximum positive load factor limit. At this speed, a load in excess of 3.8 Gs will result in a stall. Above this speed, G loads may cause structural damage before a stall occurs.

The vertical line at a speed of 160 is the maximum structural cruising speed (Vno). It should not be exceeded in rough air. The speed range from Vs to Vno is the normal operating range and corresponds to the green arc on the airspeed indicator. The vertical line at a speed of about 195 represents the never-exceed speed, Vne. This speed corresponds to the red line. If you fly faster than Vne, there is a possibility of control surface flutter, airframe structural damage, or failure. The range from Vno to Vne is the caution range represented by the yellow arc on the airspeed indicator. Because turbulence and gusts are more likely to cause high load factors at higher speeds, this speed range should be used only in smooth air.

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