Aircraft Structural Loadings PDF

Lect. 8: Aircraft Structural Loadings MM2514 AIRCRAFT STRUCTURES & REPAIR  Contents 1. Basic Aircraft Loads Types of Aircraft Loadings Main Forces Acting on an Aircraft Loads on Major Aircraft Structu...

Lect. 8: Aircraft Structural Loadings MM2514 AIRCRAFT STRUCTURES & REPAIR  Contents 1. Basic Aircraft Loads Types of Aircraft Loadings Main Forces Acting on an Aircraft Loads on Major Aircraft Structures 2. Equilibrium of Aircraft Loadings 3. Load Factor & Effect on Aircraft Structures MM2514 AIRCRAFT STRUCTURES & REPAIR  Basic Aircraft Loads Types of Aircraft Loadings An aircraft must be able to withstand the most severe loadings that it could possibly experience throughout its service life. Not only must it be capable of sustaining external loads e.g. ground and flight loads, it must also be able to withstand a certain limit of foreign body impact damage (FOD) during its flight. In addition, the aircraft structures must also be able to tolerate accumulated and undetected damages until the next maintenance opportunity, without undergoing catastrophic failure. MM2514 AIRCRAFT STRUCTURES & REPAIR  Basic Aircraft Loads Examples of aircraft loadings: Ground loads – Load induced during landing, taxiing etc Flight loads – Wing and tail loads, manoeuvring loads, gust loads etc FOD – Bird strike, impact due to tyre burst, ground incidents etc Undetected damages – Fatigue cracks on pylon struts etc MM2514 AIRCRAFT STRUCTURES & REPAIR  Basic Aircraft Loads Main Forces Acting on an Aircraft An aircraft in flight is under the influence of four main forces: Lift (L) Force acting vertically upwards on the wings Weight (W) Weight of the aircraft (deadweight and payloads) acting vertically downwards through the Centre of Gravity (CG) Thrust (T) Force generated from the engine that pushes the aircraft in the forward direction horizontally Drag (D) Force due to air resistance and acting horizontally backwards MM2514 AIRCRAFT STRUCTURES & REPAIR  Basic Aircraft Loads L T D CG TL W Flight Forces Legend T Thrust L Lift W Weight D Drag TL Tail Load CG Centre of Gravity of Aircraft MM2514 AIRCRAFT STRUCTURES & REPAIR  Basic Aircraft Loads Loads on Major Aircraft Structures The external loads (e.g. flight loads) and dead weight/payloads that the aircraft is subjected to are carried and transferred by the various aircraft structures in the form of internal loads. Loads e.g. axial, shear, bending etc are applied to major areas of airframe assembly, e.g. body and wing skin panels, frames, floor beams, wing ribs etc. In aircraft structural design, the actual loads applied to the aircraft structure for specified flight and ground conditions are known as “limit design loads”. Aircraft structures must be capable of withstanding these loads without any permanent deformation. MM2514 AIRCRAFT STRUCTURES & REPAIR  Basic Aircraft Loads Loads on Major Aircraft Structures The ultimate design loads are the limit loads multiplied by a safety factor. Aircraft structures are designed to endure ultimate design loads without failure. The inclusion of a safety factor means that aircraft structures possess reserve strength above that is required to sustain the expected limit design loads. Per the requirement of FAR (Federal Aviation Regulations) Part 25: Ultimate Design Load = Limit Design Load x Factor of Safety (1.5) MM2514 AIRCRAFT STRUCTURES & REPAIR  Basic Aircraft Loads Wing Loads Basic external loads on the cantilevered wings are:  Lift  Thrust loads from wing-mounted engines  Drag  As the centre of lift is not located at midway between the front and rear spars, a torsional load will be generated. Additional torsional loads are also created are:  Thrust loads (engines are mounted below wing level)  Other loads acting on the leading/trailing edge devices MM2514 AIRCRAFT STRUCTURES & REPAIR  Basic Aircraft Loads Fuselage Loads Flight loads acting on fuselage are:  Lift, thrust and other loads from the wing  Balancing tail load  Drag  Small amount of lift on fuselage itself Other loads acting on the fuselage  Weight of structures  Pressure loads due to pressurization of aircraft (pressure vessel)  Secondary loads such as payloads, galleys etc MM2514 AIRCRAFT STRUCTURES & REPAIR  Basic Aircraft Loads Fuselage Loads Fuselage skin is the main load carrying Cabin Pressure structure. All loads from the frames, bulkheads and floor beams must be transferred to the skin Payload Majority of the hoop tension loads due to the cabin pressurization are carried by the skin, while the frames take about 15% of these loads The stringers and skin bear the primary flight loads Payloads and dead weight loads are applied to the floor beams and frames and transferred to the skin via the shear ties MM2514 AIRCRAFT STRUCTURES & REPAIR  Basic Aircraft Loads 400 lb 300 lb 200 lb 300 lb 80 lb 60 lb 40 lb 40 lb 100 lb a a a 600 lb 1 2 3 4 5 6 7 8 9 d b 10 11 600 lb 12 13 17 16 15 14 18 b b c c a 400 lb R1 R2 a = 20” b = 30” c = 25” d = 15” MM2514 AIRCRAFT STRUCTURES & REPAIR  Equilibrium of Aircraft Loadings For aircraft undergoing a steady (constant speed) level flight, the primary external loads are balanced, i.e. the aircraft flies in a precise balance of these forces :- - Lift equals weight and tail load - Thrust equals drag L T D CG TL W MM2514 AIRCRAFT STRUCTURES & REPAIR  Equilibrium of Aircraft Loadings Based on the principles of mechanics, for conditions of equilibrium:  Summation of forces equals zero, i.e. ∑Fy = L(↑) + W(↓) + TL = 0 ∑Fx = T(←) + D(→) = 0  Summation of moments (about any point along fuselage) equals zero, i.e. ∑M = 0 If the forces are unbalanced, there would be a net force generated in a particular direction, resulting in acceleration in that direction For steady level flight to take place, the wing must produce exactly enough lift to balance the weight and tail load, and engines must generate the exact thrust to balance the drag MM2514 AIRCRAFT STRUCTURES & REPAIR  Load Factor & Effect on Aircraft Structures The loads imposed on the wings during flight are stated in terms of “Load Factor” Load Factor – ratio of total load supported by the aircraft wings to actual weight of aircraft (includes payload) Lift Usually expressed in terms of gravity pull Centrifugal Load Factor in a Turn Force Gravity Load Factor Additional load is generated when the flight path changes direction at high speeds with abrupt control movements The aircraft structures must be able to sustain such additional loads generated due to flight conditions MM2514 AIRCRAFT STRUCTURES & REPAIR Safety Factor in Engineering on Aircraft Structures Any parts of an aircraft structure can be made to fatigue or fail, subjected it to loadings that is more than its maximum material strength. To ensure safety, there are several points that engineers need to consider and these includes, 1. Ensure adequate safety margin 2. The Need to consider the effects of uncertainties in loading 3. The Need to consider all potential modes of failure (Note: FAR25.303 Factor of safety. Unless otherwise specified, a factor of safety of 1.5 must be applied to the prescribed limit load which are considered external loads on the structure. When a loading condition is prescribed in terms of ultimate loads, a factor of safety need not be applied unless otherwise specified ) MM2514 AIRCRAFT STRUCTURES & REPAIR Other Formula (please copy them down) Ultimate Factor of Safety (U.F.S.) = Ultimate Load / Limit Load Design is usually conservative and an additional “Margin of Safety” (M.O.S.) is used/ Limit MOS = (Tested Limit X - Limit X) / Limit X Ultimate MOS = (Tested Ultimate X - Ultimate X) / Ultimate X Design Ultimate = Design Limit x Factor of Safety Design Limit = nlimit x weight MM2514 AIRCRAFT STRUCTURES & REPAIR  V-n Diagram MM2514 AIRCRAFT STRUCTURES & REPAIR  What is a V-n diagram in general? The V-n diagram is simply a plot between the load factor and the velocity of the aircraft. The V-n diagrams plays an important role in design of Aircraft. These diagrams are use primarily in the determination of combination of flight conditions and load factors to which the aircraft structure must be designed. Every aircraft has its own V-n Diagram. MM2514 AIRCRAFT STRUCTURES & REPAIR  V-n diagram = Acft load factor vs Acft velocity Load factor is defined as the ratio of the aerodynamic load to the weight of the aircraft. Aircraft has to perform different loading conditions at different speeds, controls and high loads due to stormy weather. These variation of loads are impossible to investigate at the same time. There are structural limitations on the maximum load factor allowed for a given airplane. MM2514 AIRCRAFT STRUCTURES & REPAIR There are two categories of structural limitations in airplane design: 1 Limit Load Factor: This is the boundary associated with permanent structural deformation of one or more parts of the airplane. If n is less than the limit load factor, the structure may deflect during maneuver, but it will return to its original state when n = 1. If n is greater than the limit load factor, then the airplane structure will experience a permanent deformation, i.e., it will incur structural damage. 2 Ultimate Load Factor: This is the boundary associated with outright structural failure. If n is greater than the ultimate load factor, parts of the airplane will break. MM2514 AIRCRAFT STRUCTURES & REPAIR  For airplane design, the limit load factor depends on the type of the aircraft. Some typical values for limit load factors are given below: Aircraft type npositive nnegative Normal general aviation 2.5 to 3.8 -1.5 to -1 Aerobatic aviation 6 -3 Civil transport 3 to 4 -2 to -1 Fighter 6.5 to 9 -6 to -3 MM2514 AIRCRAFT STRUCTURES & REPAIR MM2514 AIRCRAFT STRUCTURES & REPAIR MM2514 AIRCRAFT STRUCTURES & REPAIR MM2514 AIRCRAFT STRUCTURES & REPAIR Load Calculation to Note: 1 Design Ultimate = Design Limit x Factor of Safety 2 Design Limit = nlimit x weight 3. M.O.S. = Tested Ultimate - Ultimate Ultimate MM2514 AIRCRAFT STRUCTURES & REPAIR MM2514 AIRCRAFT STRUCTURES & REPAIR MM2514 AIRCRAFT STRUCTURES & REPAIR  Any Questions??? MM2514 AIRCRAFT STRUCTURES & REPAIR