Pilots are using several graphs and charts in their flight operations. Four important envelopes are as follows: 1.Diagram of variations of aircraft lift coefficient versus Mach number (CL –M)
2.Diagram of variations of airspeed versus altitude (V –h) 3.Diagram of variations of center of gravity versus aircraft weight (Xcg –W) 4.Diagram of variations of airspeed versus load factor (V –n) One of the most important diagrams is referred to as flight envelope. This envelope demonstrates the variations of airspeed versus load factor (V –n). In another word, it depicts the aircraft limit load factor as a function of airspeed. One of the primary reasons that this diagram is highly important is that, the maximum load factor; that is extracted from this graph; is a reference number in aircraft structural design. If the maximum load factor is under-calculated, the aircraft cannot withstand flight load safely. For this reason, it is recommended to structural engineers to recalculate the V-n diagram on their own as a safety factor. In this section, details of the technique to plot the V- n diagram in introduced. Figure 9.10 shows a typical V-n diagram for a GA aircraft. This diagram is, in fact, a combination of two diagrams: 1. The V-n diagram without consideration of gust, 2. The V-n diagram on the effect of gust. In this section, we first have another look on the load factor and then present new concepts on load factor. Then the phenomena of gust and gust load are described. At the end of this section, the technique to plot V –n diagram is completely described.
LOAD FACTOR: In determining the structural loads, the main wing, horizontal and vertical stabilizers, and fuselage are considered separately. The loads on these are due to due to a combination of static and dynamic weights, and aerodynamic forces. It is defined as the ratio of lift to the weight, n=L/W. In level flight, the lift produced by the wings equals to the weight, so that n=1. However, during maneuvers such as climb to altitude, acceleration to high speed, sustained or instantaneous turn rate associated with the combat, significantly larger load factors may occur. Since they set the limit on the internal structure, it is very important that the maximum load factor be determined. The load factor limits for different aircraft is given in table (from thamos corck) S.No Type Load Factor n Max Min 1. General aviation (normal) 3.1 -1.25 2. General aviation (utility) +4.4 -1.8 3. homebuilt +6 -3 4 General aviation (acrobatic) 6.0 -3.0 5 Commercial transport 3.5 -1.5 6 Fighter 7.75 -4.5 For the fighter aircraft Nmax=7.75 Nmin=-4.5
Gust envelops: Gust loads are unsteady aerodynamic loads that are produced by atmospheric turbulence. They represend load factor that is added to the aerodynamic loads, wich presented in the previous section.