Airframe structural Design

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AE 440Structures Discipline

Lecture 6

Eric Loth

For AE 440 A/C Lecture

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Structures Responsibilities

a. Determine V-n relationships and establish a V-n diagram

b. Establish and maintain load paths

c. Determine component materials (weights and dimensions to configuration) and consider environmental impact of acquisition, production and operations

d. Determine landing gear configuration

e. Carry out a detailed structural design of the wing box and wing attachment and structurally design landing gear (weights & dimensions to configuration)

f. Employ FEM Analysis

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Three Primary Structural Loads

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Typical Aircraft Loads

Limit Load Factors

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V-n Diagram

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/MainFrame?OpenFrameSet

Load Paths

• How loads are transferred from one structural member to another

• Wing skin—torsion and shear stress

• Wing ribs—maintain shape and rigidity, transfer loads from skin to spars

• Wing spars—bending moments

• Fuselage—bulkheads, framers, stringers

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Lift Distribution

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Spanwise Load Distribution

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Typical Wing Box (AE 441)

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Major Fuselage Loads

• Empennage loads due to trim, maneuvering, turbulence and gusts

• Pressure loads due to cabin pressurization

• Landing gear loads due to landing impact, taxiing and ground maneuvering

• Loads induced by the propulsion installation when the latter is attached to the fuselage

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Fuselage Structure

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Structural Components

• Spars• Ribs• Skin• Beam shear• Beam bending• Torsion

Cessna Citation II

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Materials

• Selection criteria– Strength– Stiffness– Density– Cost

• Typical materials– Aluminum– Composites – Steel– Titanium

• Mini-UAV materials– Plastic

– Balsa

– Fiber (cloth)

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Stress vs. Strain= Stiffness

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Landing Gear Loads• Vertical loads primarily caused by non-zero

touchdown rates and taxiing over rough surfaces

• Longitudinal loads primarily caused by braking loads and rolling friction loads

• Lateral loads primarily caused by ‘crabbed landings,’ cross-wind taxiing and ground turning

Common Landing Gear Configurations

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Samples of Structural Results from

Previous CDRs

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V-n Diagram

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Spanwise Lift Distribution

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Spanwise Load Distribution

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Moment Distribution

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Structural Weight and Deflection

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Structural Configuration (441 level)

I-beam gives good resistance to shear (web) and bending (caps)

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Structural Details (441 level)