aircraft structural considerations j.b.rogers/structures j. byron rogers, p.e

Aircraft Structural Aircraft Structural ConsiderationsConsiderations

J.B.Rogers/Structures

J. Byron Rogers, P.E.J. Byron Rogers, P.E.

Structural Considerations

• The structure will not fail!

- Not statically under any static design ultimate load case

• Ultimate Load is typically 1.5 * Limit Load

• Covers part tolerances, statistical allowables, load exceedance

- Not after repeated loads within the lifetime of the vehicle

• The structure will not deflect such that something does not work anymore!

- Doors will open when they are supposed to

- Nothing will yield

- Control surfaces will move through expected range

- No unexpected shock waves will form

• Structure will meet specified durability/ damage tolerance/ fail safety requirements.

- No failures with specified damage within allowed inspection intervals

In Simple Terms

What Do You Need to Consider?

• Are the external loads accurate and complete?

• Are good internal load paths provided?

• Are the internal loads balanced for each component and part? (Are free body diagrams provided?)

• Do the material allowables meet the criteria/requirements? (Static strength, DDT, thermal, manufacturing/processing considerations)

• Does the certification basis demonstrate compliance with criteria/requirements– Detail analysis– Tests– Reports

J. B. Rogers/Structures

Imagine yourself as the Certifying or Procuring Agency designated representative. You are responsible for assuring that the vehicle complies with all structural criteria and requirements. What would it take to convince you that the design was safe and should be certified?

• 400 passengers

• 40 year service life

• All weather

• Maintainable

• Reliable

• Damage Tolerant

• RPV

• Long Range

• Loiter XX Hours w/o refueling

Different Objectives - Different Configurations - Similar Process

Aircraft Structural Considerations

• Military Fighter/Attack

• Carrier Suitable

• Mach 2

• nz = 7.5g Internal LoadsLoad Paths

AnalysisSizing

CriteriaRequirementsObjectives• FAR’s• MIL Specs• SOW/PDS

Configuration

External LoadsEnvironments•Pressures•Inertia•Thermal•Acoustic

MethodsAllowables

Certification•Tests•Reports

Aircraft Loads, Conditions & Requirements

Flight Loads:• Maneuver• Gust• Control Deflection• Buffet• Inertia• Vibration

Ground Loads:• Vertical Load Factor• Braking• Bumps• Turns• Catapult• Arrested Landing• Aborted Takeoff• Spin-Up• Spring Back• One Wheel/Two Wheel• Towing• Ground Winds• Break Away

Other Loads & Conditions:• Jacking• Pressurization• Crash• Actuation• Bird Strike• Lightning Strike• Hail• Power Plant• Thermal• Fatigue• Damage Tolerance• Fail Safety• Acoustics• Ground Handling

Specific Conditions are defined per:• FAR Vol III (23 and 25)…………………….Commercial• Mil-A-8860-8870 and SD-24L……………. Military

Requirements Have Evolved With Experience/Lessons Learned

Commercial Transport

• Wings/Body

The airplane must be capable of successfully completing a flight during which likely structural damage occurs as a result of - Impact with a 4-pound bird when the velocity of the airplane relative to the bird along the airplane's flight path is equal to Vc at sea level or 0.85Vc at 8,000 feet, whichever is more critical;

• Empennage

The empennage structure must be designed to assure capability of continued safe flight and landing of the airplane after impact with an 8-pound bird when the velocity of the airplane (relative to the bird along the airplane's flight path) is equal to VC at sea level

Military

Specifications typically require that catastrophic structural failure or loss of control of aircraft be prevented after a defined limit of structural damage has occurred as a result of in-flight bird strike.

Requirement: Bird Strike

No penetration of cockpit• Danger to crewNo penetration of fuel tanks:• In-flight fire hazard• Fuel lossNo damage to control surface actuation/controls

This sounds like a nicesafeguard, but is it really necessary?


Every Requirement and Condition is There for a Reason!

747/767/777 Daily Highlight Report03 April 2001

Multiple Bird Strike - One Bird Entered Flight Deck 767 (L/N 447)American Airlines reported that on April 2nd during climb from Paris,at 12,000 feet, the reference airplane struck multiple birds impactingvarious locations on the aircraft. One bird entered the flight deckvia the P1-1 panel on the captain's left side. All flight controls andsystems functioned normally. The crew elected to return to Paris wherean uneventful landing was made. The airplane is currently AOG inParis.


• USAF - 34,856 bird strikes reported between Jan 1985 and Feb 1998– 33,262 non-damaging (less than $10,000)– 7,358 struck wings– 764 were Horned Larks– 348 were Turkey Vultures (+98 Black Vultures)

• Over 25,000 reported strikes to Civil aircraft between 1988 and 1992

• CAA estimates that UK registered A/C of over 12,000 lbs strike a bird about once every 1,000 flights

Aircraft Loads, Conditions & RequirementsRequirement: Bird Strike

Lightening Striking All Nippon Airlines, Osaka, Japan

Lightning Strike: Same Story


In-Flight Hail: Same Story

S/N 1019 took off from Lyon, France and climbed to approximately 3000 ft when large hail/ice was encountered. The slats were stowed as the aircraft took damage for approximately 3-5 second. Aircraft landed back at Lyon successfully. No other aircraft in the area incurred similar damage.

The fan blades were inspected and were said to be in pristine condition.

The aircraft will be ferried from Lyon to Marshalls of Cambridge to be repaired.


Yaw Maneuver andLateral Gust

Buffet

Positive CheckedManeuver

Negative CheckedManeuver

Negative Maneuver

Lateral Maneuver

Negative Gust

Engine Blade Out

Taxi

Negative Maneuverand Braking

Gust

Cabin Pressure

Aileron Roll

Positive DynamicGust

Positive Maneuverand Static Gust


Different Load Conditionsare Critical for Different Areas

Typical Commercial Transport Critical Static Load Conditions

Auxiliary Front Spar

LeadingEdge

RearSpar

LogoLights

FixedTrailing Edge

Surge TankVent Scoop

Rib 5

Rib 8

Rib 10

Rib 12

Rib 14

Wet Area

Jack Screw Ftg Rib 2

Rib 1

Center Box

Fuel VentTube/ReturnLine

Surge Tank

• External loads (pressures/inertia)• Durability/Damage Tolerance• Crash• Failed Refueling Valve• Hail and bird strike• Lightning strike• Material utilization

Structural ConsiderationsBird Strike

Stall Buffet Failed RefuelValve

Abrupt Up Elevator

Lightning Strikeminimum skin thickness

Upper Surface Skin/StringersDurability/Damage TolerancePositive Maneuver

Lower Surface Skin/StringersNegative Maneuver

Abrupt Elevator

Negative Maneuver

Balanced Maneuver

Negative Gust

Stall Buffet9G Crash

3G Side LoadCrash

Typical Commercial Transport Critical Load Conditions


Internal Loads/Load Paths• Aircraft structure is designed to be light weight

=> Typically very thin gage

• Members are arranged to carry loads efficiently (in-plane)

• shear webs

• axial members

• Out-of-plane loads are carried to redistribution members where the loads are converted to in-plane components

Stiffened Skin Panel

Built-Up Spar

Body Panel

• Consider all load conditions and requirements

• Develop a static load balance for each critical condition

– Apply loads realistically

– Determine where they are going to be balanced

• Cut sections to determine local internal loads

• Provide a path for the loads to follow

(Load will follow stiffest path!)

Note: Most members serve more than one function

So how do we get internal members to carry loads efficiently?

Do this for local loads as well as for general vehicle loads

Internal Loads/Load Paths

Lift

ThrustBalance

LoadMoment

Weight

CGDrag

• Primary Structural Components are fuselage, wing, and tail (horizontal and vertical stabilizers)

• Fuselage consists of skins, longerons, and frames

• Wing and Stabilizers consist of covers, spars, and ribs

Spars

Ribs

Frames

Longerons

What do these members do?

Internal Loads/Load Paths

For a downward tail load, body will carry a shear and a bending moment

Skins carry shear load in-plane with VQ/I distribution

Lower longerons (with effective skin)carry compression axial loads due tobending moment

Crown longerons and skin carry tension loads due to bending moment

Bending moment is carried based on Mc/I distribution

Consider fuselage to act as a beam

Keel Beam added to restore load path on lower surface (wing carry through and wheel well areas)

z

y

0.31.82

16.57

71.32

66.62

57.22

-31.82-16.57

-57.22

-66.62

-71.32

-66.62

-57.22-31.82

-16.57 0. 16.5731.82

57.22

66.62

Bruhn Figure A21.62

12

3

4

5

6

7

8

9

1011 11'

10'

9'

8'

7'

6'

5'

4'

3'

2'1'

zy

2000 200011.5" 11.5"

Bruhn Section 21.12 (Fig A21.62)

Internal Loads/Load Paths - Fuselage

Longerons (stringers) carry axial loads

Frames provided to reducelongeron column length

Skins carry shear, torsionand tension

Frames also support cargo floor and passenger floor beams (react end loads into skins as shear)

Seat rails run fore-aft and are supported by floor beams

Floor beams tied to frames (react vertical load) and to a longitudinal beam to react forward loads (landing and crash)

d

h

Stringer Systemd > h

d

h

Longeron Systemd < h

Internal Loads/Load Paths - FuselageCrown Panel

Body skins also carry external and compartmentpressures as a membrane.

For duel-lobe configurations, longitudinal beam(crease beam) and floor beams react out-of-planeload component at lobe intersection

Internal Loads/Load Paths - Fuselage

Internal Loads/Load Paths - Wing/Stabilizer

Internal structure consists primarily of Covers, Spars, and Ribs

Elastic Axis

Wing acts like cantilevered beamunder distributed pressure loading.Shear, Moment, and Torsion (about elastic axis) are beamed to fuselage and balance tail load, inertia, and other side wing load.

Typical VMT for Horizontal Stabilizer

-50

0

50

100

150

200

250

0 0.2 0.4 0.6 0.8 1 1.2

Percent Semispan

Sh

ea

r (1

0^

3 lb

s),

Mo

me

nt

(10

^5

in-

lbs

), T

ors

ion

(1

0^

5 in

-lb

s)

Shear (V)

Moment (M)

Torsion (T)


VT

M

VT

Transports & Bombers• Deep Sections• Skin Supported by

Stringers Carries Bending Moments

Fighters• Thin Sections• Unstiffened Skins• Skin and Spar

Chords Carry Bending Moment

Section Bending Moments

Main Types of Wing Primary Structure

Thin Skin ( many stringers and ribs) Thick Skin ( many spars, few ribs)

Stringers would not be efficient

Section Shear Flow

Vz = 1000 lbs

23.3

17.410.07.728.3

51.9

28.3 7.7 10.0 17.4

Vz = 1000 lbs

7.74

1.845.534.716.44

18.5118.8111.23 18.92

6.44 4.71 5.53 1.84


Spar Webs Carry Shear (V)Shell Carries Torque (T)

2

1

Pseg i

Pseg i

Prib i

Prib = Pseg i * (sin 2 - sin 1) i

Mid-spanbetween ribs

Mid-spanbetween ribs

Rib

Rib

1, 2 are the “as built” angles

Pseg i is load at ribi

Segment

Built-In Curvature Loads

bs

S

StringerEffective Area

Rib

Effective Area for Pressure Loads

PP

P P

L1 L2Q

Q

Q = PM (L1 + L2)

EI 2

Crushing Loads on a Rib

Pressure + Inertia Loads

Ribs Covers

External + Internal Pressures + Inertia


.1 .3 .6 .9 .9 .6 .6 .3 .1

.7 1.0 1.3 1.6 1.7 1.6 1.2 1.0 .8 .6 .41.41.5

10"15"

5"

6.67"

21"18"

60"

.2 .6 1.1 .9 .8 .6 .2

1.2 2.6 3.3 2.4 1.6.7

3.0

S.C.

V=19,200 #

a. Applied Rib Loads (Load in 103 lbs)

b. Loads Resolved to Stiffeners and Reacted at Shear Center

T=47,000 in-#

qt

qv

V=19,200 #

T=47,000 in-#

Loads at Shear Center Balanced by Shear Flows

2.448"

19,200 #

q=-515 #/in

q=-435 #/in

q=-423 #/in

q=395 #/in

q=383 #/in

q=475 #/inQ=9275#

Q=8554#

Calculated Shear Flow Balance - Stiffened Skin

Internal + External Pressure, Inertia, Curvature, and Crushing Loads

Internal Loads/Load Paths - Wing/StabilizerRibs redistribute pressure and inertia loads into cellular box structure.

Ribs

• React and distribute air/fuel pressure loads

• React panel crushing loads

• React curvature loads

• Maintain wing/stabilizer chordwise contour

• Limit skin or skin/stringer column length

• React Local Concentrated Loads• Landing gear• Power plant• Fuselage attachments• Ailerons• Flaps• Lift devices

• May Act as Fuel Boundaries

Rear Spar Front SparStringer TieSpar Tie

AFT FITTING FORWARD FITTING MACHINED RIB

FLOOR FITTINGFLOOR FITTING

FLOOR FITTING FLOOR FITTING

Up

Aft

Shear Tied Rib

Intermediate Rib


Rib 2

Rib 8

136.3" 83.0"

219.3"

If the time ‘T’ for fuel to flow from the upstream side of the barrier to fill a volume of air defined in the 1g flight condition is greater that 0.5 second, the internal baffle can be considered to be a solid pressure barrier.

Conversely, an internal baffle may not be considered as a pressure boundary if the volume of air in the fuel cell downstream of the barrier is not adequate to meet the above criteria. In such cases, the pressures due to the hydrostatic fuel head must be calculated without consideration of this internal baffle.

P = 0.34 * K * L (6.5 pound/gallon fuel density)

Where: P = design pressure at location ‘a’; L = reference distance, feet, between the point of pressure and the farthest tank boundary in the direction of loading; K is defined in the table.

Emergency Landing (Crashworthy) Fuel Loads

Fuel Loading - Roll Rate Loading Condition KForward 9

Aft 1.5Inboard 1.5

Outboard 1.5Downward 6

Upward 3


F = Mr2

F = Mr

= angular acceleration = angular velocity

r

Concentrated Loads

• Landing Gear

• Power Plant

• Fuselage Attachments

• Ailerons

• Flaps

• Lift devices

Engine Pylon

Wing Elastic Axis

PP C.G.

Front Spar

Ribs redistribute concentrated loads into cellular box structure.


Chord

Upright or Rib Post

Web

ChordStrut

Rib Post

• Carry Wing Shear Loads

• With Covers, Carry Torsion

• React Local Concentrated Loads

• May Also Act as Fuel Boundaries

Spars are Primarily Shear Beams3 Basic Types of Spars

Stiffened Web

Sinewave

Truss Beam

Exception toin-plane shear loading

Fuel Pressures

Fuel LoadsBird StrikeCost

Access

Thin SectionFighter Wing

bs1

L wmax = (bs1/2 + bs2/2) * p

bs1/2 + bs2/2


5.0" (5 PL)

20.0"

Chords:

Area =0.6 in2

Iy =.20 in4

Struts/Posts:

Area =0.45 in2

Iy =.06 in4

Web:t =0.1 in

q1 q2 q3

q1 = 200 lbs/in

q2 = 200 lbs/in

q3 = 500 lbs/in

0

500

500700700700900

0

200200

9500 6000 250017500 13000

900 q = q =q =q =q = q =

25009500 60001300017500

q(applied) =q(applied) =q(applied) =

Web Type Spar

Example Geometry and Applied Loads

Most Common Type(Usually Diagonal Tension)Light Weight/Low CostSimple Internal LoadsPoor AccessModerate to High Assembly Cost

For a shear beam, q = V/h (web shear flow)P = M/h (chord load)h = Distance between chord centroids

Upper Chord

Lower Chord

Upper Sill

Lower Sill

L1

L2

Framed Out Access Hole

Web Type Spar


q =900 lbs/in q =900 lbs/in

q(applied) =500 lbs/in

9500 2,5002,500 2,50017,500 9500

10,00010,307

6,000

10,307

6,000

14,431

4,000

14,431

06,000

14,431

9500

14,43114,431

13,000

0

13,000

0

14,00014,431

13,000

17,500


q(applied) = 200 lbs/in

0

q =900 lbs/in q =900 lbs/in q(applied) =500 lbs/in

9514 2,4202,676 2,48917,500 9,514

8,936

9,151

5,996

12,974

5,996

13,657

9514

13,657

11,014

12,336

1,207

12,336

0

14,000

11,014

14,970

17,500


q(applied) = 200 lbs/in

2,6769514

5,99613,000

87

119

5,996

12,9742,793

155

1064

3558

1763 155

1064

1064155

1551339

1763

248987

1,985

1,294

3,871

119

1,081

9,151

1,361

187

1,844

1871,827

163

1,422

1791,775

163 609

179 1,908

1,294

2,600

640

1,727 640

1,473

212

2,210

212

2165

155

155

2,210

1872

163

205

163

6092,712

9,685

2,712

3,877

640

4,926640

1,727

Truss with Full Fixity at Ends of Struts

Simple Truss

Complicated Internal LoadsComplex Joint LoadsLow Assembly CostGood Access

Eccentricity IssuesLess Simple Joint LoadsSimple AssemblyGood Access

Chord Centroidal Axis

Strut Centroidal Axis

Line Up Loads!

Simple Truss

Fixed End Truss


Stiffened Skin(many ribs)

Shear Tied Ribs @ Concentrated Load Locations


Internal Loads/Load Paths - Arrangement

This Slide Intentionally Left BlankSee Class Handout

Longeron System(d < h)

Multi-Spar(unstiffenedskins, few ribs)

Frames @ Concentrated Load Points

Frames @ Direction Changes in Load Carrying Members

Wing Fold


Multi-Spar(unstiffenedskins, few ribs)

Longeron System(d < h)

Dielectric material

Stub Ribs

Frames @ Concentrated Load Points

Wing Fold


Deep SectionStiffened Skins(many ribs)

No Fuselage, No Vertical Stabilizer

Ribs @ Concentrated Load Points


Two Structural Boxes

ForwardStructuralBox

Aft Structural Box

Big Hole in the Middle


Aircraft Structural ConsiderationsNow you have • Developed a Configuration to Address the Requirements,

Criteria, & Objectives• Provided Internal Load Paths• Developed the Internal Loads

What’s Next?

Conduct Analysis & Sizing• Identify Internal Loads for Each Part• Balance Loads & Reactions (free body diagrams)• Develop Shear, Moment, and Axial Loads (and diagrams)• Conduct Analyses/Sizing using Appropriate Loads, Methods, and

Allowables

Certification• Tests• Reports

Note: “Aircraft Materials” and “Design Validation Testing” will be the subjects of future lectures

Web shear flows and stiffener loads were developed for each load condition

676

79 18 18 86

689

7.45" 60 41

439 440

48

451 452

-11

675 19421875

156 165

2482 582 583 893 895 909 596 597 2566

67

18 13860 41 48-11 67134

54 107115 38 3665 11591

372 22 832294 216 143 22397 302759

18 5460 41 48-11 6747 5447

-1031890 367 -861 -1767 -2085 -955 -245 760 2279

103

-420-259 -451 -441 -597 432 389 557

420

Internal Load Balance

In-Plane Load Balance

Shear flow (lbs/in)

Lateral stiffener loads (lbs)

Fore-aft stiffener loads (lbs)

88.4p lbs

Internal Load Balance

wmax = 7.475p lbs/in

88.4p lbs104.9p lbs

58.61 in

23.1p lbs23.1p lbs

V

M

M = 1665p in- lbs

104.9p lbs

p = Pressure (psi)

Load Balance:• Normal Pressures• In-Plane Components

Develop shear, moment, axial, and torsion diagrams

Analysis MethodsMost Methods are Unique to Aerospace Industry and are Semi-empirical

• Diagonal Tension– Forced Crippling– Permanent Buckling– Gross Allowable Web Stress (Shear Rupture)– Secondary Bending Moments

• Lightening Holes/Flanged Holes• Beaded Shear Panels• Local Buckling• Crippling• Effective Width of Buckled Sheet• Sheet Wrinkling• Buckling in Bending

– Formed members with or w/o attached skin– Extruded members with or w/o attached skin

• Tension Fittings/Clips• Lugs• Joggles• Bearing/Bypass Interaction @ Fastened Joints• Effects of Defects

Most Static Load Critical Structure is Stability Driven

Analysis Methods

Methods Generally Developed from:• NACA Tests and Reports• IRAD and CRAD Tests/Studies• Experience/Lessons LearnedEach Company Has its own Methods

Manuals

CONSTRAINTS

• C/S Depth

• Skin min gage (.08”fuel areas or .05” other)

• Stringer attach flange width (e/d & clearance)

• Minimum stringer machining gage (.05”)

• Producibility

STATIC CHECKS ( Each Stringer; Each Rib Bay)

• Crippling

• Johnson-Euler Column (Axial Compression) Fixity Coefficient C = 1

• Johnson-Euler Column (Axial Compression) + Shear C = 1

• Flexure (pressure acting singularly, C = 4)

• Beam Column (C = 4)

• Skin Stability between stringers (compression + shear) @ Cruise

• Preliminary D/DT Cutoff

• Flexure-Torsion Mode Stability

• Pure Torsion Mode Stability

bF

bA

tF

tO tAH

bS

tF

tOpad

Preliminary Sizing - CT Horizontal Stabilizer Main Box Cover Panel

CONSTRAINTS

• Standard sheet thicknesses

• Minimum chord machining gage (.08”)

• Upright attach flange thickness

• Upright and chord attach flange widths

• Shear stability limits (80% DLL in wet areas)

STATIC CHECKS (Each Bay)

• Webs/Uprights

- Net shear

- Shear rupture

- Bearing

- Fastener shear

- Uprt/Chd net shear + tension

- Irequired

- Forced crippling

- Upright column

- Upright flexure

• Chords

- Crippling

- Maximum compression

- Column stability

- Net tension

- D/DT cutoff

t1

t3

t2 = tav g w3

w1 w2wb

wa

Preliminary Sizing - CT Horizontal StabilizerFront Spar

Aircraft Structural ConsiderationsNow you have • Developed a Configuration to Address the Requirements,

Criteria, & Objectives• Provided Internal Load Paths• Developed the Internal Loads

Conducted Analysis & Sizing• Identified Internal Loads for Each Part• Balanced Loads & Reactions (free body diagrams)• Developed Shear, Moment, and Axial Loads (and diagrams)• Conducted Analyses/Sizing using Appropriate Loads, Methods,

and Allowables

Cycle would be iterated 1 - 3 times.

Certification• Tests• Reports

Preliminary SizingConsidering How Little Time You Have, What Can You Do?

• Develop External Loads• Provide Good Internal Load Paths• Develop the Internal Loads at a Few Locations

• 2 Body Cuts• Mc/(Ad2)• Vq/(Ad2) or V/(h)• T/(2Aencl)

• 2 Wing Cuts• M/h Cover Axial Loads• Split V between spars

(balance about SC or centroid)• T/2Aencl Assume covers and outer

spars carry all torsion• Size to Cut-Off Ultimate Stress or Strain

• Aluminum 40 ksi (compression)40 ksi (tension)

• CEP .004 in/in (compression).0045 in/in (tension)

• Assume Shear Resistant for Shear and Torsion• Fighter Covers no Buckling at Ultimate

Aencl is enclosed area

Aencl h

S.C.T

V

a

L

Va/LV - Va/L

q = T/(2Aencl)

z

yh V/(2h)V/(2h)

z

y

aircraft structural considerations j.b.rogers/structures j. byron rogers, p.e

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