AERO 401: Aircraft Structures and Materials - Aerospace...

1 Aerospace EngineeringDr. John Valasek

AIRCRAFT STRUCTURES:RIGID AND ELASTIC

Dr. John ValasekAerospace EngineeringTexas A&M University

AERO 401November 1999


INTRODUCTION

The main factor that governs the choice of materials and structural form is the ratio of the load on the structure to its dimensions. mission type and speed

Very early aircraft operated at low speeds, and therefore loads were low in relation to aircraft size. Wing loadings were typically 5 - 10 psf. best option was to concentrate compression loads into a few small rod-like members

and diffuse tensions into fabric and wires Low power engines of the time made structural lightness an expedient

wood and fabric were best choice, and simple to obtain aircraft of similar dimension were less than the weight of comparable modern ones metals were entirely out of the question

Biplanes were prevalent because early monoplanes suffered from catastrophic structural failures (probably caused by aeroelastic effects which were unknown at the time). WWI dogfight load factors could be as high as 4g

early motivations


INTRODUCTION

For high speed flight, the main factor that governs the choice of materials and structural form is the high temperature environment caused by kinetic heating in sustained supersonic flight.

Except for one or two exceptions, the top speed of fighter aircraft have traditionally been limited not by aerodynamics or propulsion but by the choice of materials. without advances in structural efficiency the performance improvements due to

advances in aerodynamics and propulsion would not have been realized Existing fighter aircraft as a rule do not have long supersonic endurance, and so

have metalic leading edges (for reasons of rain and birdstrikes). The proposed U.S. High Speed Civil Transport (HSCT) is critically dependent

on advanced structures and materials technology.

modern motivations


WING LOADING

Wing loadings based on maximum takeoff weight.

The great rise in wing loading occurred during the 1930’s and 40’s.

The generation of fighters with thick skins lessened the trend slightly.

Note the difference in F-16 takeoff wing loadings: F-16A air superiority F-16C multi-role

fighter aircraft trends (1910 - 2000)

0

20

40

60

80

100

120

140

160

1910 1930 1950 1970 1990 2010Year

Win

g Lo

adin

g (p

sf)

Thick Skinned Jets DeltasMetal Monoplanes Wooden Biplanes

Thick Skinned

Jets

Deltas

WoodenBiplanes Metal

Monoplanes

F-16C

MiG-31F-104A

F-15EF-14A

F-15CF-4EF-4B

MiG-29A

Kf ir Rafale

EF2000Mirage 2000

F-16AYF-12A

J-37F-106A

J-35D

Meteor Hurricane IIBf -110A

P-26

PupGauntlet

Gladiator

F-84GF-84D

Fw 190A

MetalMonoplanes

Deltas

ThickSkinned

Jets

WoodBiplanes


WING CONSTRUCTION

Fabric covering wooden spars. Load carried by internal structure

plus bracing wires. Typical of WWI aircraft. Load bearing members are

positioned near aerodynamicsurfaces where the stresses are highest.

Upper surface in compression, lower surface in tension. Stresses near the neutral axis are low and lightening holes can be used.

Susceptibility to structural failuredue to wood rot.

Buckling of wings in flightcalled a “striptease” in the vernacular of the period.

the early years (1900 - 1918)

Moraine-Saulnier Type N “Bullet”


STATIC LOADS TESTING1920

Military Wing Sopwith D.1 No. 243 Squadron

Determining ultimate flight loads by testing to destruction


WING CONSTRUCTION

Built up steel spars with woodreinforcement, covered with fabric.

Warren type truss. Load carried by internal

structure plus bracing wires. Intended to be the “best of both

worlds” in terms of greater structural strength due to inclusion of steel, and lower cost, ease of manufacture, and ease of maintenance due to fabric covering.

Ended up being “worst of both worlds” mix of steel and wood not as strong as steel alone fabric unable to withstand higher speeds

permitted by stronger structure

the inter-war years (1919 - 1938)

Hawker Hurricane Mk. XII


WING CONSTRUCTION

A major conceptual breakthrough:most of the structural load is carriedby the external structure.

Semi-Monocoque construction the thin skin can easily handle tension to handle compression without

buckling, the skin is attached to the spars and stringers Stressing the skin results in an even higher load carrying capability.

total result is a structure very stiff in bending. requires mechanical fasteners (rivets). permits higher speeds / lower drag.

Discovered in 1925 by Dr. H. Wagner, termed the ‘Wagner Theory of the Diagonal-Tension Field Beam,’

Standard construction type today.

WWII to Korea and after (1939 - 1955)

Messerschmitt Me 262 Sturmvogel


WING WEIGHTfighter aircraft trends (1930 - 1980)

Normalized to P-51 baseline span (accounting for planform, section, materials).

Modern jet wings are much lighter than 1940’s prop wings P-51 14.5% WTO

F-15A 3% WTO

If modern wings had to be built using 1940’s technology, they would virtually be solid aluminum alloys or steel.

Structural efficiency has improved greatly with time.

P-51B

P-36A

P-26A

F-100C

F-106A

F-111F-14A

F-15A

F-16A

F-4CF-104A

F-84F

F-86A

0

0.2

0.4

0.6

0.8

1

1.2

1930 1940 1950 1960 1970 1980 1990

Year of Service Entry

Nor

mal

ized

Win

g W

eigh

t


WING CONSTRUCTION High transonic and supersonic flight speeds mandated wings with

very low thickness ratios large bending strength sweepback torsion thicker skins

and therefore more structural material. Solid wings were one answer (F-104). A better solution is integral wings

skin and stringers are machined from a single large piece of material eliminates mechanical fasteners good surface finish (low drag) “Wet Wing”; no bladders, but integral:

fuel tank torque box skin

significant increase in fuel volume structural synergism

supersonic to post Vietnam (1955 - 1975)

McDonnell F-101A Voodoo


INTERNAL FUEL LOAD

Comparison of integral tanks and bladder tanks.

For the same area, integral tanks offer greater capacity.

Notable aircraft: F-101A fuselage fuel F-15E conformal tanks Su-27 overload condition

fighter aircraft trends (1945 - 2000)

0

2000

4000

6000

8000

10000

12000

0 200 400 600 800 1000

Wing Area (m2)

Inte

rnal

Fue

l Vol

ume

(kg)

YF-22

YF-23F-15E

Su-27(o)

Mirage 4000Su-27

F-14A

F-101AJavelin FAW.9

F-16XL

MiG-15F-86F

Hunter F.6

J35D

Mirage 2000F-5E

F-104G

F-18C

F-105DEF2000

Rafale DF-100C

J-8

F-15C

F-86H

F-8A

J37 F-15A

F-18E

F-16C

Lavi

F-84F

Bladder TanksPre-1955

Deltas

Integral Tanks

Deltas

Integral Tanks

Bladder TanksPre - 1955

Sukhoi Su-27 Flanker


WING CONSTRUCTION The quest to save weight while still retaining good mechanical properties. Concept: reduce structural mass by reducing material density, instead of increasing

mechanical properties like strength stiffness toughness

For most materials: 10% strength increase, 3% weight reduction 10% density reduction, 10% weight reduction

Execution is usually in the form of various types of alloys and composites. Drawbacks include

cost difficulty in manufacturing undesirable aeroelastic effects

such as reduced roll rates andaileron reversal

contemporary (1976 - )


STATIC LOADS TESTING1998

Non-destructive testing including accurate measurement of deflections

Saab JAS 39 Gripen


TOTAL GROSS WEIGHT REDUCTIONprojected

11

8

8

24

19

22

16

9

16

13

13

18

5

12

11

5

5

2

23

12

12

13

13

12

15

8

10

14

2

15

13

18

9

3

7

6

7

5

85

0 10 20 30 40 50 60

SUPER Long Haul

SUPER Premium

SUPER Business

LONG HAUL Conv.

LONG HAUL Blended

GLOBAL CARGO Long

GLOBAL CARGO Short

STOL Medium Range

STOL Short Range

TILTROTOR

Gross Weight Reduction (%)

StructuresAerodynamicsPropulsionSystems

Source: Aerospace America, November 1997


THE COMET STORY (1)1949 A New Era Begins

The DeHavilland D.H. - 106 ushers in the jet age in

commercial air passenger transport

DeHavilland D.H.-106 Comet 194948 pax

490 MPH3540 nm


THE COMET STORY (2)

Five aircraft are lost two due to stall at takeoff three inflight, due to “unknown” causes

1953 - 1954 Tragedies

BOAC Comet Yoke-Peter, serial G-ALYP, (the first Comet I in scheduled service) crashes off the island of Elba in the Mediterranean Sea, 10 January 1954. 35 pax plus crew are lost.

South African Airways Comet crashes off the island of Stromboli in the Mediterranean Sea, 8 April 1954. 14 pax plus crew are lost.

Deep sea salvage using sonar and underwater television cameras is used for the first time to locate aircraft wreckage.


THE COMET STORY (3)

The Particulars pressurized cabin multiple pressurizations / depressurizations square windows

The Mechanism crack propagation

The Result structural failure resulting from repeated loading/unloading cycles

The Phenomena Cyclic Fatigue

1955 The Cause Revealed


THE COMET STORY (4)America responds

Boeing 367-80 1954118 pax*582 MPH3530 nm

Douglas DC-8 1958132 pax

600 MPH3550 nm


The improved “safe version” Comet 3 (1955) and improved range (transatlantic) Comet 4 (1958) are offered.

In 1958 the Comet 4 begins the very first regularly scheduled transatlantic jet service. westbound flights still had to refuel at Gander, Newfoundland

One year later, the DC-8 and B707 firmly captured the market due to higher speed and significantly larger passenger capacity. Comet 4: 76 pax at 500 MPH B707: 176 pax at 600 MPH

Comets are eventually sold to the Royal Navy as Nimrod AEW aircraft.

THE COMET STORY (5)the lead is lost for good


V-22 design life is 10,000 hours, or 20 years of flying ops. Airplane and helicopter induced loads will be encountered.

takeoffs landings airplane and helicopter maneuvers rough field and shipboard operations ground maneuvers (braking and taxiing)

FATIGUE TESTINGensuring long term structural integrity

Source: Aviation Week & Space Technology, 20 April 1998

Boeing V-22 Osprey

For acceptance, structural integrity of airframe is tested to multiple lifetimes. Two for low-cycle loadings (20,000 hrs),

three for high-cycle loadings (30,000 hrs) Minimum 7,000 hours in airplane mode,

3,000 hours in VTOL mode. No damage at 4g, 310 kts, and 2.8g, 345 kts. At end of first test lifetime, airframe is

disassembled and inspected.


THE ELASTIC AIRPLANEfact or fiction?


AEROELASTICITYwhen flexible structure meets dynamic pressure

Source: Air International, Vol. 52 No. 3, March 1997


ELASTIC AIRCRAFT All aircraft are elastic to some extent. The designed-in level of airframe elasticity is dictated by:

operational requirements and constraints aerodynamics materials economics safety, e.g “bend but don’t break”

Some aircraft types are significantly more elastic than others: Aircraft which are generally rigid

fighters F-15 Eagle general aviation Cessna 172 homebuilts made of conventional materials Thorpe T-18

Aircraft which are generally elastic supersonic cruise Concorde large and long range transports and bombers Boeing 777 homebuilts made of composite materials GlassAir

practical considerations

Rutan Voyager


AEROELASTIC EFFECTS (1)

Compared to the rigid aircraft: elastic weathercock stability has

essentially equal yet opposite slope for 0.1 M 0.9

elastic weathercock stability is reduced 85% at M = 0.9

Example: Boeing Model 707-320B Weathercock Stability Elastic stability derivatives are a

strong function of dynamic pressure and therefore speed and altitude.

steady-state stability derivatives

Boeing Model 707-320B


Source: Airplane Flight Dynamics and Automatic Flight Controls, Part II by J. Roskam


AEROELASTIC EFFECTS (2)

Example: elevator effectiveness degradation due to fuselage flexure.

aft fuselage bending


Model the horizontal tail as a flexible cantilever beam:

Under a vertical load Lh the fuselage will produce an elastically

induced angular deflection KLh. An up load produces a negative

change in horizontal tail angle-of-attack. The total aerodynamicload is: L C i KL qSh L w h e e hh

b gNote that Lh is a function of itself. Solving for this load:

At high dynamic pressure the loads decreases because the denominator grows large. Converting to a pitching moment coefficient and differentiating with respect to e, C

C i l

C KqS cm

L w h e e h

Lh

h

h

b ge j1

LC i qS

C KqSh

L w h e e

L

h

h

b ge j1



by .

MODELING AEROELASTICITY

Analytical derivatives are obtained by influence coefficient methods.

Aerodynamic [A] rigid body

perturbed-state stability derivatives

Each element aij is the aerodynamic

force induced on panel i as a result

of a unit change in angle-of-attack

on panel j. The column of aerodynamic

forces is related to [Aij] and

the airplane angle-of-attack distribution

F q AA ij JEi io t n s

FAEio t

Jin sC

Scx A xm i

Tij iq

2

2 l q l q


Converting to pitching moment coefficient and taking the derivative with respect to pitch rate, gives the rigid body pitch damping derivative.


JIG SHAPE (1)

It is assumed that: The aircraft is held in its elastic equilibrium shape by an elastic

equilibrium load distribution (gravity, aerodynamic, thrust). The aircraft is elastically deformed in the equilibrium state.

strain energy is “pent up” in the structure While under equilibrium loads, the center of gravity does not

correspond to a specific point on the structure of the airplane. When equilibrium loads are removed, the C.G. is a fixed point on the

structure of the aircraft in its undeformed or jig shape.

equilibrium states of elastic aircraft

Elastic Equilibrium State

Undeformed or Jig Shape

Source: Airplane Flight Dynamics and AutomaticFlight Controls, Part II by J. Roskam


JIG SHAPE (2)equilibrium states of elastic aircraft

Elements of a calculated “jig shape matrix” must be translated into “jigging points” for the assembly jigs.

Determination of the jig shape is usually performed by computer. Computer controlled laser-guided alignment is used during assembly.


ELASTIC AIRCRAFT Multiple and simultaneous aeroelastic behaviours are typically encountered:

aileron reversal wing divergence loss of longitudinal control power due to aft fuselage bending

Aeroelastic effects on stability and control derivatives are usually significant and always vary strongly with flight condition.

Steady-state and perturbed state stability and control derivatives are fundamentally different for elastic aircraft: inertial effects due to mass distribution invoke elastic deformations, altering the

aerodynamic loading Elastic aircraft must be designed, manufactured, and built to a jig shape to

achieve a specific desired cruise shape under flight loads. Many analytical modeling techniques exist of varying complexity and

accuracy.

summary

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