Title is 30 Pt Helvetica Bold

1
Aircraft Structural Considerations
J. Byron Rogers, P.E.
J.B.Rogers/Structures
2
Structural Considerations
In Simple Terms

• 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

3
What Do You Need to Consider?
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?

• 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
4
Aircraft Structural Considerations
Different Objectives - Different Configurations -
Similar Process

• 400 passengers
• 40 year service life
• All weather
• Maintainable
• Reliable
• Damage Tolerant

• Military Fighter/Attack
• Carrier Suitable
• Mach 2
• nz 7.5g

• RPV
• Long Range
• Loiter XX Hours w/o refueling

5
Aircraft Loads, Conditions Requirements
Requirements Have Evolved With Experience/Lessons
Learned

• 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

6
Aircraft Loads, Conditions Requirements
Requirement Bird Strike

• 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.

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

This sounds like a nice safeguard, but is it
really necessary?
7
Aircraft Loads, Conditions Requirements
Every Requirement and Condition is There for a
Reason!
8
Aircraft Loads, Conditions Requirements
Requirement Bird Strike

• 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

9
Aircraft Loads, Conditions Requirements
Lightning Strike Same Story
Lightening Striking All Nippon Airlines, Osaka,
Japan
10
Aircraft Loads, Conditions Requirements
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.
11
Aircraft Loads, Conditions Requirements
Typical Commercial Transport Critical Static Load
Conditions
Positive Dynamic Gust
Positive Maneuver and Static Gust
Aileron Roll
Yaw Maneuver and Lateral Gust
Negative Maneuver
Negative Maneuver and Braking
Buffet
Positive Checked Maneuver
Negative Checked Maneuver
Gust
Lateral Maneuver
Taxi
Negative Gust
Cabin Pressure
Engine Blade Out
Different Load Conditions are Critical for
Different Areas
12
Aircraft Loads, Conditions Requirements
Typical Commercial Transport Critical Load
Conditions
Structural Considerations

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

13
Internal Loads/Load Paths

• Aircraft structure is designed to be light weight
  gt 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
14
Internal Loads/Load Paths
So how do we get internal members to carry loads
efficiently?

• 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

Do this for local loads as well as for general
vehicle loads
15
Internal Loads/Load Paths

• 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

What do these members do?
16
Internal Loads/Load Paths - Fuselage
Consider fuselage to act as a beam
For a downward tail load, body will carry a shear
and a bending moment
Bending moment is carried based on Mc/I
distribution
Crown longerons and skin carry tension loads due
to bending moment
Skins carry shear load in-plane with VQ/I
distribution
Lower longerons (with effective skin) carry
compression axial loads due to bending moment
Keel Beam added to restore load path on lower
surface (wing carry through and wheel well areas)
17
Internal Loads/Load Paths - Fuselage
Crown Panel
Longerons (stringers) carry axial loads
Skins carry shear, torsion and tension
Frames provided to reduce longeron column length
Frames also support cargo floor and passenger
floor beams (react end loads into skins as shear)
Floor beams tied to frames (react vertical load)
and to a longitudinal beam to react forward
loads (landing and crash)
Seat rails run fore-aft and are supported by
floor beams
18
Internal Loads/Load Paths - Fuselage
Body skins also carry external and
compartment pressures as a membrane.
For duel-lobe configurations, longitudinal
beam (crease beam) and floor beams react
out-of-plane load component at lobe intersection
19
Internal Loads/Load Paths - Wing/Stabilizer
Internal structure consists primarily of Covers,
Spars, and Ribs
20
Internal Loads/Load Paths - Wing/Stabilizer
Wing acts like cantilevered beam under
distributed pressure loading. Shear, Moment, and
Torsion (about elastic axis) are beamed to
fuselage and balance tail load, inertia, and
other side wing load.
V
T
M
T
V
21
Internal Loads/Load Paths - Wing/Stabilizer
Main Types of Wing Primary Structure
Thin Skin ( many stringers and ribs)
Thick Skin ( many spars, few ribs)

• 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
Section Shear Flow
Spar Webs Carry Shear (V) Shell Carries Torque (T)
22
Internal Loads/Load Paths - Wing/Stabilizer
Effective Area for Pressure Loads
Pressure Inertia Loads
Built-In Curvature Loads
Crushing Loads on a Rib
23
Internal Loads/Load Paths - Wing/Stabilizer
Ribs redistribute pressure and inertia loads into
cellular box structure.
Internal External Pressure, Inertia,
Curvature, and Crushing Loads
24
Internal Loads/Load Paths - Wing/Stabilizer

• 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

Shear Tied Rib
Intermediate Rib
25
Internal Loads/Load Paths - Wing/Stabilizer
Emergency Landing (Crashworthy) Fuel Loads
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.
Fuel Loading - Roll Rate
26
Internal Loads/Load Paths - Wing/Stabilizer
Ribs redistribute concentrated loads into
cellular box structure.

• Concentrated Loads
• Landing Gear
• Power Plant
• Fuselage Attachments
• Ailerons
• Flaps
• Lift devices

27
Internal Loads/Load Paths - Wing/Stabilizer
3 Basic Types of Spars
Spars are Primarily Shear Beams
Fuel Loads Bird Strike Cost

• Carry Wing Shear Loads
• With Covers, Carry Torsion
• React Local Concentrated Loads
• May Also Act as Fuel Boundaries

Stiffened Web
Exception to in-plane shear loading
Thin Section Fighter Wing
Fuel Pressures
Sinewave
Truss Beam
Access
28
Internal Loads/Load Paths - Wing/Stabilizer
Web Type Spar
Most Common Type (Usually Diagonal Tension) Light
Weight/Low Cost Simple Internal Loads Poor
Access Moderate to High Assembly Cost
Framed Out Access Hole
For a shear beam, q V/h (web shear flow) P
M/h (chord load) h Distance between chord
centroids
29
Internal Loads/Load Paths - Wing/Stabilizer
Simple Truss
Eccentricity Issues Less Simple Joint
Loads Simple Assembly Good Access
Line Up Loads!
Fixed End Truss
Complicated Internal Loads Complex Joint
Loads Low Assembly Cost Good Access
30
Internal Loads/Load Paths - Wing/Stabilizer
Stiffened Skin (many ribs)
Shear Tied Ribs _at_ Concentrated Load Locations
31
Internal Loads/Load Paths - Arrangement
This Slide Intentionally Left Blank See Class
Handout
32
Internal Loads/Load Paths - Arrangement
Longeron System (d lt h)
Wing Fold
Frames _at_ Direction Changes in Load Carrying
Members
Multi-Spar (unstiffened skins, few ribs)
Frames _at_ Concentrated Load Points
33
Internal Loads/Load Paths - Arrangement
Longeron System (d lt h)
Wing Fold
Stub Ribs
Dielectric material
Multi-Spar (unstiffened skins, few ribs)
Frames _at_ Concentrated Load Points
34
Internal Loads/Load Paths - Arrangement
No Fuselage, No Vertical Stabilizer
Ribs _at_ Concentrated Load Points
Deep Section Stiffened Skins (many ribs)
35
Internal Loads/Load Paths - Arrangement
Two Structural Boxes
Forward Structural Box
Aft Structural Box
Big Hole in the Middle
36
Aircraft Structural Considerations

• Now you have
• Developed a Configuration to Address the
  Requirements, Criteria, Objectives
• Provided Internal Load Paths
• Developed the Internal Loads
• Whats 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
37
Internal Load Balance
In-Plane Load Balance
Web shear flows and stiffener loads were
developed for each load condition
Fore-aft stiffener loads (lbs)
Shear flow (lbs/in)
Lateral stiffener loads (lbs)
38
Internal Load Balance

• Load Balance
• Normal Pressures
• In-Plane Components

p Pressure (psi)
88.4p lbs
V
Develop shear, moment, axial, and torsion diagrams
104.9p lbs
M 1665p in- lbs
M
39
Analysis Methods

• Most 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 _at_ Fastened Joints

Most Static Load Critical Structure is Stability
Driven
40
Analysis Methods

• Methods Generally Developed from
• NACA Tests and Reports
• IRAD and CRAD Tests/Studies
• Experience/Lessons Learned
• Each Company Has its own Methods Manuals

41
Preliminary Sizing - CT Horizontal Stabilizer
Main Box Cover Panel

• CONSTRAINTS
• C/S Depth
• Skin min gage (.08fuel 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) _at_ Cruise
• Preliminary D/DT Cutoff
• Flexure-Torsion Mode Stability
• Pure Torsion Mode Stability

42
Preliminary Sizing - CT Horizontal Stabilizer
Front Spar

• 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

43
Aircraft Structural Considerations

• Now 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

44
Preliminary Sizing
Considering 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

Aencl is enclosed area