Gust Envelope

Encountering unpredictable atmospheric disturbance is unavoidable. Weather warnings are helpful but full avoidance is not possible. A gust can hit an aircraft from any angle and the gust envelope is shown in a separate set of diagrams. The most serious type is a vertical gust (see Figure 5.1), which affects load factor n. The vertical gust increases the angle of attack, a, developing AL. Regulatory agencies have specified vertical gust rates that must be superimposed on the V-n diagrams to describe the operation limits. It is common practice to combine the maneuver and gust envelope in one diagram, as shown in Figure 5.4. The FAR provides a detailed description of required gust loads. To stay within the ultimate load, the limits of vertical gust speeds are reduced with increases in aircraft speed. Pilots should fly at a lower speed if high turbulence is encountered. The gust envelope crosses the limit load and its boundary varies with increases in speed. Equation 5.5 shows that

Table 5.3. FAR-specified gust velocity

Altitudes 20,000 ft

Altitudes 50,000 ft

and below

and above

VB (rough air gust)

66 ft/s

38 ft/s

VC (gust at max design speed)

50 ft/s

25 ft/s

VD (gust at max dive speed)

25 ft/s

12.5 ft/s

aircraft with low wing-loading (W/SW) and flying at high speed are affected more by gust load.

VB is the design speed for maximum gust intensity. This definition assumes that the aircraft is in steady-level flight at speed VB when it enters an idealized upward gust of air, which instantaneously increases the aircraft angle of attack and, hence, the load factor. The increase in the angle of attack must not stall the aircraft – that is, take it beyond the positive or negative stall boundaries.

From statistical observations, the regulatory agencies have established the max­imum gust load at 66 ft/s. Except for extreme weather conditions, this gust limit is essentially all-weather flying. In a gust, the aircraft load may cross the limit load but it must not exceed the ultimate load, as shown in Figure 5.4. If an aircraft crossed the limit load, then an appropriate action through inspection is taken.

Table 5.3 outlines the construction of a V-n diagram superimposed with a gust load. Flight speed, VB, is determined by the gust loads and can be summarized as shown in the table.

Linear interpolation is used to obtain appropriate velocities between 20,000 and 50,000 ft. The construction of V-n diagrams is relatively easy using aircraft specifi­cations, in which the corner points of V-n diagrams are specified. Computations to superimpose gust lines are more complex, for which FAR has provided the semi­empirical relations.

Vertical-gust velocity, Ug, on forward velocity, V, would result in an increase of the angle of attack, A a = Ug/ V, that would generate an increase in load fac­tor An = (Cbx Ug/V)/(W/S). Airspeed V is varied to obtain An versus speed. DATCOM and ESDU provide the expressions needed to obtain CLa. A typical V-n diagram with gust speeds intersecting the lines is illustrated in Figures 5.2 and 5.4.

VC is the design cruise speed. For transport aircraft, the VC must not be less than VB + 43 knots. The JARs contain more precise definitions as well as definitions for several other speeds.

In civil aviation, the maximum maneuver load factor is typically + 2.5 for air­craft weighing less than 50,000 lbs. The appropriate expression to calculate the load factor is as follows:

n = 2.1 + 24,000/(W + 10,000) up to a maximum of 3.8 (5.6)

This is the required maneuver-load factor at all speeds up to VC, unless the maxi­mum achievable load factor is limited by a stall.

Within the limit load, the negative value of n is -1.0 at speeds up to VC, decreas­ing linearly to 0 at VD. The maximum elevator deflection at VA and pitch rates from VA to VD also must be considered.

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