Pilots are using several graphs and charts in their flight operations. Four important envelopesare 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 envelopedemonstrates the variations of airspeed versus load factor (V – n). In another word, it depictsthe aircraft limit load factor as a function of airspeed. One of the primary reasons that thisdiagram is highly important is that, the maximum load factor; that is extracted from thisgraph; is a reference number in aircraft structural design. If the maximum load factor isunder-calculated, the aircraft cannot withstand flight load safely. For this reason, it isrecommended to structural engineers to recalculate the V-n diagram on their own as a safetyfactor.In this section, details of the technique to plot the V- n diagram in introduced. Figure 9.10shows a typical V-n diagram for a GA aircraft. This diagram is, in fact, a combination of twodiagrams: 1. The V-n diagram without consideration of gust, 2. The V-n diagram on theeffect of gust. In this section, we first have another look on the load factor and then presentnew concepts on load factor. Then the phenomena of gust and gust load are described. At theend 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, andfuselage are considered separately. The loads on these are due to due to a combination ofstatic 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 thewings equals to the weight, so that n=1. However, during maneuvers such as climb toaltitude, acceleration to high speed, sustained or instantaneous turn rate associated with thecombat, significantly larger load factors may occur. Since they set the limit on the internalstructure, 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.NoTypeLoad Factor nMaxMin1.General aviation (normal)3.1-1.252.General aviation (utility)+4.4-1.83.homebuilt+6-34General aviation (acrobatic)6.0-3.05Commercial transport3.5-1.56Fighter7.75-4.5For the fighter aircraftNmax=7.75Nmin=-4.5
M 2 1Upward gust load∆n =Gust envelops:Gust loads are unsteady aerodynamic loads that are produced byatmospheric turbulence. They represend load factor that is added to the aerodynamic loads,wich presented in the previous section.