Fuselage, Wings and Stabilizing Surfaces — Page 11, Lesson 13

Let’s start with the forces acting on the aircraft structure, because everything else in this chapter builds on that. An aircraft is subject to various forces that act on the structure both on the ground and in flight. During flight, the wings produce lift, which tends to bend the wing upwards. As a result, there will be compression on the upper surface and tension on the lower surface. So the top of the wing gets squeezed together, and the bottom gets stretched apart. Lift also causes a torsional force, which twists the wing. Drag will also act on components such as the landing gear, bending them backwards, whilst the mass of the aircraft will pull it downwards. Now, an aircraft flying straight and level at a constant speed will be subject to 1g. That’s the normal acceleration due to gravity. Any change in attitude will change the g, which in turn alters the weight of the structure and the loads. So if you pull up or bank, the g-loading changes, and that changes how much the structure is loaded. It should be noted that the loads on an aircraft that experiences engine failure will change considerably. On a twin, the remaining engine would still be producing thrust on one side of the aircraft. In addition to changes in the loads on the wings, the asymmetric thrust would produce a yawing moment, which in turn would need to be corrected by the use of opposite rudder. There would be increased loading on the fin and the fuselage structure. So the fin and fuselage have to be designed to handle that extra load. Now let’s look at the design loads. The Design Limit Load, or DLL, is the maximum load that the designer would expect the airframe or component to experience in service. The standard DLLs are: for Transport Aircraft, +2.5 and -1.0. For Utility Aircraft, 4.4. And for Aerobatic Aircraft, 6. These values are based on ‘g’-forces and derived from failure values determined experimentally at the design stage. So the DLL is the load you expect in normal service, expressed in g. Then we have the Design Ultimate Load, or DUL. The DUL is the DLL multiplied by the safety factor. The minimum safety factor specified in design requirements is 1.5. The structure must withstand DUL without collapse. So the ultimate load is the limit load times the safety factor, and the structure has to survive that without failing. The safety factor is the ratio of the ultimate load to the limit load. So the formula is SF = DUL divided by DLL. That’s the safety factor equals the ultimate load over the limit load. Now, the design philosophies. The aircraft manufacturer will attempt to design an aircraft to take into account all the loads that it may experience in flight. There are various guidelines, formulae, and experience to guide them in the design of a good fail-safe or damage tolerant structure. First, Safe Life. The safe life of an aircraft structure is defined as the minimum life during which it is known that no catastrophic damage should occur. Life-counts for components of assemblies may be recorded as a number of flying hours, cycles of landing, pressurization events, accelerations, or even on a calendar basis. After the elapsed life-count or fatigue cycle—typically pressurisations or landings—has been reached, the item is replaced or overhauled. In the interim, the operational life of the aircraft, and to minimize the chances of failure due to fatigue, aircraft designers apply the principle of fail-safe construction or damage tolerance. So the safe life philosophy says: we know how long this part will last before catastrophic damage could occur, and we replace or overhaul it before that point. The life-count can be in flying hours, landing cycles, pressurization events, accelerations, or calendar time. And to minimize fatigue failure during that operational life, we also apply fail-safe construction or damage tolerance. Let me make sure you’ve got the key numbers: Transport Aircraft DLL is +2.5 and -1.0. Utility is 4.4. Aerobatic is 6. The minimum safety factor is 1.5. And the safety factor formula is SF = DUL divided by DLL. Now, the figures I have here show the detection of faults reliant upon a planned inspection programme, and the damage tolerant structure and fatigue concepts. Let me bring those up for you. So the key idea is: the DLL is what you expect in service, the DUL is what the structure must survive, and the safety factor bridges them. The safe life philosophy tells you when to replace parts, and fail-safe or damage tolerance design minimizes the chance of fatigue failure during the operational life.