Airframe Loads

Flight CorridorThe 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

Airframe Loads

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.

Airframe Loads

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.

Airframe Loads

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)

Airframe Loads

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)

Airframe Loads

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 n1 to n3 without stalling the aircraft.

A particular flight envelope is applicable to only one altitudeas the maximum lift coefficient reduces with increased altitude

Airframe Loads

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)

Airframe Loads

Load Factor Ranges

-1 to 3.5helicopters

-3 to +6aerobatic airplanes

-1.5 to +3.8light airplanes

-1 to +2.5commercial transport airplanes

Load factor rangeType of aircraft

Airframe Loads

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

Airframe Loads

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

Airframe Loads

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

Airframe Loads

Types of Airframe Loads

• Inertia Loads• Maneuver Loads• Gust Loads

Airframe 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-AircraftCarrierTrapping04-02-AircraftCarrierCatapult

Airframe Loads

Inertia Loads (1)

(8.1)

(8.2)

For a rigid body undergoing constant angular velocity

Airframe Loads

Inertia Loads (2)

(8.3)

(8.4)

For a rigid body undergoing angular acceleration

Airframe Loads

Inertia Loads (3)

(8.5)

(8.4)

Torque about the axis of rotation produced by inertia force is

If ICG is the moment of inertia through the CG

04-03-JetBlueNoseGear

Airframe Loads

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

Airframe Loads

Level Flight (1)

(8.7)

(8.8)

For vertical equilibrium

For horizontal equilibrium

Taking moments about CG (8.9)

CGMScCV ,2

21 ρpitching moment of the aircraft about the CG =

ρ – density of airV – aircraft speedS – wing areac – mean chordCM,CG – coefficient of moment

assumes that n (load factor) = 1 for commercial aircraft on level flight

Airframe Loads

Level Flight (2)

(8.10)As first approximation, take P = 0 so that

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)

Lift (where CL is the coefficient of lift)

04-04-Levelflight.wmv

Airframe Loads

Pull-Out From Dive

(8.12)

(8.13)

For vertical equilibrium

For horizontal equilibrium

Taking moments about CG (8.14)

04-05-F14Flyby

Airframe Loads

Steady Pull-Out

(8.15)

For equilibrium along flight normal

At lowest point θ = 0

Smaller radius (more severe pullout) – n is larger

(8.16)

Taking L = nW

Could lead to- Increased load on structure- Possibility of stalling

04-06-RaptorStallTest04-07-Fighter_SU37

Airframe Loads

Correctly Banked Turn

(8.17)

(8.18)For vertical equilibrium

For horizontal equilibrium

For L=nW (8.20)

Greater bank angle – higher load factor

(8.21)

For tighter turn – higher bank angle

04-08-C19_BankStallCrash.wmv

Airframe Loads

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

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Airframe Loads

Single or discrete gustA 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)].

Airframe Loads

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

Airframe Loads

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

Airframe Loads

Sharp-edged gust (1)For aircraft flying with speed V with wing incidence α0 entering a gust of upward velocity u. Changes in lift and load factor are:

04-10-FedExGustCrash

Airframe Loads

Sharp-edged gust (2)Changes occurring at the tail are:

VE – equivalent airspeed, uE – equivalent gust velocity, ST – tailplane area

Airframe Loads

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.