THE V-n on V-g DIAGRAM

The operating flight strength limitations of afl airplane are presented in the form of a V-n or V-g diagram. This chart usually is included in the aircraft flight handbook in the section dealing with operating limitations. A typical V-n diagram is shown in figure 5-3- The V-n diagram presented in figure 5-3 is intended to present the most important general features of such a diagram and does not neces­sarily represent the characteristics of any par­ticular airplane. Each airplane type has its own particular V-n diagram with specific V’s and »’s.

The flight operating strength of an airplane is presented on a graph whose horizontal scale is airspeed (V) and vertical scale is load factor (У). The presentation of the airplane strength is contingent on four factors being known:

(1) the aircraft gross weight, (2) the configura­tion of the aircraft (clean, external stores, flaps and landing gear position, etc.), (3) symmetry of loading (since a rolling pullout at high speed can reduce the structural limits to approxi­mately two-thirds of the symmetrical load limits) and (4) the applicable altitude. A change in any one of these four factors can cause important changes in operating limits.

For the airplane shown, the positive limit load factor is 7-5 and the positive ultimate load factor is 11.25 (7.5ХІ.5)- For negative lift flight conditions the negative limit load factor is 3.0 and the negative ultimate load factor is 4.5 (3-0x15). The limit airspeed is stated as 575 knots while the wing level stall speed is apparently 100 knots.

Figure 5.4 provides supplementary informa­tion to illustrate the significance of the V-n diagram of figure 5-3- The lines of maximum lift capability are the first points of importance on the V-n diagram. The subject aircraft is capable of developing no more than one posi­tive “g" at 100 knots, the wing level stall speed of the airplane. Since the maximum load factor varies with the square of the airspeed,

STRUCTURAL

the maximum positive lift capability of this airplane is 4 “g” at 200 knots, 9 g at 300 knots, 16 g at 400 knots, etc. Any load factor above this line is unavailable aerodynamically, i. e., the subject airplane cannot fly above the line of maximum lift capability. Essentially 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. Gen­erally, the negative CLmax is less than the posi­tive CLmax and the airplane may lack sufficient control power to maneuver in this direction.

If the subject airplane is flown at a positive load factor greater than the positive limit load factor of 7-5, structural damage will be possi­ble. When the airplane 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. If conditions of extreme emergency require load factors above the limit to prevent an immediate disaster, the airplane should be capable of withstanding the ultimate load factor without failure. The same situation exists in negative lift flight with the exception that the limit and ultimate load factors are of smaller magnitude and the negative limit load factor may not be the same value at all airspeeds. At speeds above the maximum level flight airspeed the negative limit load factor may be of smaller magnitude.

The limit airspeed (or redime speed) is a de­sign reference point for the airplane—the sub­ject airplane is limited to 575 knots. If flight is attempted beyond the limit airspeed struc- turahdamage or structural failure may result from a variety of phenomena. The airplane in flight above the limit airspeed may encounter: 00 critical gust (0 destructive flutter (0 aileron reversal 00 wing or surface divergence (0 critical compressibility effects such as stability and control problems, damaging buffet, etc

The occurrence of any one of these items could cause structural damage or failure of the pri­mary structure. A reasonable accounting of these items is required during the design of an airplane to prevent such occurrences in the re­quired operating regions. The limit airspeed of an airplane may be any value between termi­nal dive speed and 1.2 times the maximum level flight speed depending on the aircraft type and mission requirement. Whatever the resulting limit airspeed happens to be, it deserves due respect.

Thus, the airplane in flight is limited to a regime of airspeeds and g’s which do not exceed the limit (or redline) speed, do not exceed the limit load factor, and cannot exceed the maximum lift capability. The airplane must be operated within this “envelope” to prevent structural damage and ensure that the anticipated service life of the airplane is obtained. The pilot must appreciate the V-n diagram as describing the allowable combination of airspeeds and load factors for safe operation. Any maneuver, gust, or gust plus maneuver outside the structural envelope can cause structural damage and effectively shorten the service life of the airplane.

There are two points of great importance on the V-n diagram of figure 5-4. Point В is the intersection of the negative limit load factor and line of maximum negative lift capability. Any airspeed greater than point В provides a negative lift capability sufficient to damage the airplane; any airspeed less than point В does not provide negative lift capability sufficient to damage the airplane from excessive flight loads. Point A 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 point A provides a positive lift capability sufficient to damage the air­plane; any airspeed less than point A does not provide positive lift capability sufficient to

cause damage from excessive flight loads. The usual term given to the speed at point A is the “maneuver speed,” since consideration of subsonic aerodvnamics would predict mini­mum usable turn radius to occur at this con­dition. The maneuver speed is a valuable reference point since an airplane 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 airplane is below the maneuver speed.

The maneuver speed can be computed from the following equation:

VP= Va-Jn limit

where

VP= maneuver speed

Vs=stall speed

n limit = limit load factor

Of course, the stall speed and limit load factor must be appropriate for the airplane gross weight. One notable fact is that this speed, once properly computed, remains a constant value if no significant change takes place in the spanwise weight distribution. The ma­neuver speed of the subject aircraft of figure 5.4. would be

Vp=10QtJT5

= 274 knots