Flight Corridor The speed-altitude band where flight sustained by aerodynamic forces is technically possible is called the flight corridor. The subsonic Boeing 747 and supersonic Concorde have flight corridors within the conventional boundary (indicated in cyan). The high-altitude solar powered Centurion is able to operate beyond conventional boundaries. Structural design is often concerned with flight vehicles within conventional boundaries
Centurion Remotely piloted, solar-powered airplane developed under NASA for surveillance purposes. The airplane was believed to be the first aircraft designed to achieve sustained horizontal flight at altitudes of 90,000 to 100,000 feet. The Centurion has 206-foot-long wings and used batteries to supply power to the craft's 14 electric motors and electronic systems.
Aircraft missions and stages Applied loads depend on the mission of the aircraft, e.g. transport, fighter, aerobatic mission, etc. The stages during any aircraft mission can be roughly divided into: (a) taxi and takeoff, (b) cruising, (c) maneuver, and (d) landing Design loads must be carefully established for every stage of the aircraft mission The objective of structural design is to maintain the shape and integrity of the aircraft during each part of the mission and stage.
Weight & load factors Control of weight important in aircraft design Limit load maximum load in normal operation Proof load limit load x proof factor (1.0-1.25) Ultimate load limit load x ultimate factor (usually 1.5)
Load Limits Structure must withstand proof load without detrimental distortion Structure must not fail until ultimate load is achieved Must be matched to the flight envelope (boundary depicting the limits of speed that the aircraft cannot safely exceed)
Flight Envelope (V-n diagram) Basic strength and flight performance limits for any aircraft is contained in the flight envelope. During flight, it is possible to apply positive and negative limit loads n 1 to n 3 without stalling the aircraft. A particular flight envelope is applicable to only one altitude as the maximum lift coefficient reduces with increased altitude
Human perception of load factor +1, all occupants of the aircraft feel that their weight is normal. Greater than +1 all occupants feel heavier than usual. Zero, all occupants feel weightless. Negative, all occupants feel they are upside down (the aeroplane is flown upside down)
Load Factor Ranges Type of aircraft commercial transport airplanes light airplanes aerobatic airplanes helicopters Load factor range -1 to +2.5-1.5 to +3.8-3 to +6-1 to 3.5
Airfoil basics An airfoil is any surface designed to produce lift or thrust when air passes over it The angle of attack α changes the amount of lift and drag on the aerofoil The chord is the distance between the leading to trailing edge along the chord line Aerofoils are generally classified as unsymmetrical or symmetrical
Pressure on airfoil Changing the angle of attack alters the pressure distribution on airfoils The centre of pressure is the point on a body where the total sum of a pressure field acts, causing a force and no moment about that point On an unsymmetrical airfoil, the centre of pressure moves forward along the airfoil surface as the angle of attack increases On a symmetrical airfoil, centre of pressure movement with angle of attack change is very limited
Airfoil aerodynamic centre The aerodynamic centre is the location along the chord line where moment is invariant with angle of attack Unless otherwise stated, the pitching moments of airfoils are always taken about the aerodynamic centre AC lies approximately at the quarter-chord position at subsonic speeds and at approximately the half-chord position at supersonic speeds
Types of Airframe Loads Inertia Loads Maneuver Loads Gust Loads
Inertia Loads Appear when aircraft undergoes acceleration and deceleration They affect the airframe during landings, take-off, maneuvers, gust Important parameters to note Force Mass moment of inertia Torque 04-01-AircraftCarrierTrapping 04-02-AircraftCarrierCatapult
Inertia Loads (1) For a rigid body undergoing constant angular velocity (8.1) (8.2)
Inertia Loads (2) For a rigid body undergoing angular acceleration (8.3) (8.4)
Inertia Loads (3) Torque about the axis of rotation produced by inertia force is (8.5) If I CG is the moment of inertia through the CG (8.4) 04-03-JetBlueNoseGear
Symmetric Maneuver Loads There are infinite number of flight conditions within flight envelope Corners A, C, D1, D2, E & F in flight envelope are critical points for investigation In symmetric maneuver, motion of aircraft initiated by movement of control surfaces in plane of symmetry
Level Flight (1) For vertical equilibrium For horizontal equilibrium assumes that n (load factor) = 1 for commercial aircraft on level flight (8.8) (8.7) ρ density of air V aircraft speed S wing area c mean chord C M,CG coefficient of moment Taking moments about CG (8.9) pitching moment of the aircraft about the CG = 1 ρ 2 2 V ScC M, CG
Level Flight (2) Lift (where C L is the coefficient of lift) As first approximation, take P = 0 so that (8.10) As second approximation, P is substituted to obtain a more accurate value of L and the procedure is repeated. Assuming P, D & T are small and taking L=W (8.11) 04-04-Levelflight.wmv
Pull-Out From Dive For vertical equilibrium (8.12) For horizontal equilibrium (8.13) Taking moments about CG (8.14) 04-05-F14Flyby
Steady Pull-Out For equilibrium along flight normal Taking L = nw (8.15) At lowest point θ = 0 (8.16) Smaller radius (more severe pullout) n is larger Could lead to - Increased load on structure - Possibility of stalling 04-06-RaptorStallTest 04-07-Fighter_SU37
Correctly Banked Turn For horizontal equilibrium For vertical equilibrium (8.17) (8.18) For L=nW (8.20) Greater bank angle higher load factor (8.21) For tighter turn higher bank angle 04-08-C19_BankStallCrash.wmv
Gust Loads Movements of air in turbulence are generally known as gusts They cause changes in wing incidence and subject the aircraft to sudden or gradual change in lift In high speed aircraft, this may cause higher loads than control initiated maneouvers 04-09-CrossWindLandings
Single or discrete gust A distribution of vertical gust velocity over a given finite length or period of time. Sharp-edged gust: Aerodynamic forces determined by instantaneous incidence of the particular lifting surface. Generally leads to overestimation of gust loads. Graded gust: Gust velocity increases linearly to a maximum over a gust gradient distance H. 1 cosine gust: Gust velocity is given by u(t) = U/2[1 cos(πt/t)].
Continuous gusts Has freedom from arbitrary assumptions of gust shapes and sizes Assumes that gust velocity is a random variable comprising a large number of sinusoidal components Power spectral analysis is a common method of evaluating continuous gusts Requires a large amount of experimental data for analysis
Sine wave summation The addition of sine functions (of the right amplitude and phase) can be used to create a sawtooth or rectangular function. This illustrates that all functions can be decomposed to a series of sine waves of different frequencies
Sharp-edged gust (1) For aircraft flying with speed V with wing incidence α 0 entering a gust of upward velocity u. Changes in lift and load factor are: 04-10-FedExGustCrash
Sharp-edged gust (2) Changes occurring at the tail are: V E equivalent airspeed, u E equivalent gust velocity, S T tailplane area
Vertical Gust Suppression System The Dreamliner has sensors embedded in the composite skin that will detect tiny changes in pressure caused by wind gusts. The flight-control system automatically makes adjustments to smooth out the ride before the plane gets bounced around.