aircraft structures: rigid and elastic

29
1 Aerospace Engineering Dr. John Valasek AIRCRAFT STRUCTURES: RIGID AND ELASTIC Dr. John Valasek Aerospace Engineering Texas A&M University AERO 401 November 1999

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AIRCRAFT STRUCTURES: RIGID AND ELASTIC. Dr. John Valasek Aerospace Engineering Texas A&M University AERO 401 November 1999. INTRODUCTION. early motivations. - PowerPoint PPT Presentation

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Page 1: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

1 Aerospace EngineeringDr. John Valasek

AIRCRAFT STRUCTURES:RIGID AND ELASTIC

Dr. John ValasekAerospace EngineeringTexas A&M University

AERO 401November 1999

Page 2: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

2 Aerospace EngineeringDr. John Valasek

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

Page 3: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

3 Aerospace EngineeringDr. John Valasek

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

Page 4: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

4 Aerospace EngineeringDr. John Valasek

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-37

F-106A

J-35D

Meteor Hurricane IIBf -110A

P-26

PupGauntlet

Gladiator

F-84G

F-84D

Fw 190A

MetalMonoplanes

Deltas

ThickSkinned

Jets

WoodBiplanes

Page 5: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

5 Aerospace EngineeringDr. John Valasek

WING CONSTRUCTION

Fabric covering wooden spars. Load carried by internal structure

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

positioned near aerodynamic

surfaces 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 failure

due to wood rot. Buckling of wings in flight

called a “striptease” in the

vernacular of the period.

the early years (1900 - 1918)

Moraine-Saulnier Type N “Bullet”

Page 6: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

6 Aerospace EngineeringDr. John Valasek

STATIC LOADS TESTING1920

Military Wing Sopwith D.1 No. 243 Squadron

Determining ultimate flight loads by testing to destruction

Page 7: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

7 Aerospace EngineeringDr. John Valasek

WING CONSTRUCTION

Built up steel spars with wood

reinforcement, 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

Page 8: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

8 Aerospace EngineeringDr. John Valasek

WING CONSTRUCTION

A major conceptual breakthrough:

most of the structural load is carried

by 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

Page 9: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

9 Aerospace EngineeringDr. John Valasek

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

Page 10: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

10 Aerospace EngineeringDr. John Valasek

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

Page 11: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

11 Aerospace EngineeringDr. John Valasek

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-101A

Javelin FAW.9

F-16XL

MiG-15F-86F

Hunter F.6

J35D

Mirage 2000F-5E

F-104G

F-18C

F-105D

EF2000

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

Page 12: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

12 Aerospace EngineeringDr. John Valasek

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 and

aileron reversal

contemporary (1976 - )

Page 13: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

13 Aerospace EngineeringDr. John Valasek

STATIC LOADS TESTING1998

Non-destructive testing including accurate measurement of deflections

Saab JAS 39 Gripen

Page 14: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

14 Aerospace EngineeringDr. John Valasek

TOTAL GROSS WEIGHT REDUCTION

projected

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 (%)

Structures

Aerodynamics

Propulsion

Systems

Source: Aerospace America, November 1997

Page 15: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

15 Aerospace EngineeringDr. John Valasek

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 1949

48 pax

490 MPH

3540 nm

Page 16: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

16 Aerospace EngineeringDr. John Valasek

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.

Page 17: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

17 Aerospace EngineeringDr. John Valasek

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

Page 18: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

18 Aerospace EngineeringDr. John Valasek

THE COMET STORY (4)America responds

Boeing 367-80 1954

118 pax*

582 MPH

3530 nm

Douglas DC-8 1958

132 pax

600 MPH

3550 nm

Page 19: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

19 Aerospace EngineeringDr. John Valasek

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

Page 20: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

20 Aerospace EngineeringDr. John Valasek

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.

Page 21: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

21 Aerospace EngineeringDr. John Valasek

THE ELASTIC AIRPLANEfact or fiction?

Page 22: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

22 Aerospace EngineeringDr. John Valasek

AEROELASTICITYwhen flexible structure meets dynamic

pressure

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

Page 23: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

23 Aerospace EngineeringDr. John Valasek

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

Page 24: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

24 Aerospace EngineeringDr. John Valasek

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

Boeing Model 707-320B

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

Page 25: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

25 Aerospace EngineeringDr. John Valasek

AEROELASTIC EFFECTS (2)

Example: elevator effectiveness degradation due to fuselage flexure.

aft fuselage bending

Boeing Model 707-320B

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 aerodynamic

load is: L C i KL qSh L w h e e hh

b g

Note 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, CC i l

C KqS cm

L w h e e h

L

h

h

h

b ge j1

LC i qS

C KqSh

L w h e e

L

h

h

b ge j1

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

Page 26: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

26 Aerospace EngineeringDr. John Valasek

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 s

CSc

x A xm i

T

ij iq

22 l q l q

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

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

Page 27: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

27 Aerospace EngineeringDr. John Valasek

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 Automatic

Flight Controls, Part II by J. Roskam

Page 28: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

28 Aerospace EngineeringDr. John Valasek

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.

Page 29: AIRCRAFT STRUCTURES: RIGID AND ELASTIC

29 Aerospace EngineeringDr. John Valasek

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.

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