Brief announcements

• I reviewed due dates on e-learning
  • Moved the final to its appropriate time 12/17 12:30PM
  • Moved the project back two days to 12/10 2:30PM
• Grader has completed the homework
• Last homework assignment will be optional (practice only, solutions immediately available). Fair game on the exam.
• Don’t forget about the limited number of licenses for software!

• Please fill out the course evaluations!
• Please let me know how the online version of the course went… better, worse, both?

Aero structures history

Early flight: The Wright Brothers

First flight

• Dec 17, 1903
• 120 ft in 12 seconds
• Principal features
  • Wood (spruce and bamboo)
  • Fabric skins on ribbed wings
  • Piano wire, rods, metal fittings

• Enabling technologies
  • wind tunnel testing for improved airfoils and propellers
    • rudimentary understanding of aerodynamics
  • improvements in flight controls
    • use of wing warping and canard
    • tested on kites and gliders
  • improvements in internal combustion engines
    • significant advantage in power to weight ratio over steam engines and other alternatives
• Note: our remaining discussion and this course will largely ignore improvements in aerodynamics

Truss structures in early airframes

• Allowed high stiffness to weight ratio
  • Were based on civil engineering bridge structures
  • This characteristic was necessary given the engine specific power available
  • Structurally an advantage over the predecessors which modeled bird and bat-like structures

• Biplane configuration allowed for additional lifting surface area
  • Thus, a reduction of induced drag – dominant drag at higher lift coefficients needed for low speeds
• Allowed control-ability through varying tension in the bracing wires
• Useful for wings (biplane) and for fuselage
• Wing coverings were muslin (lightweight cotton cloth in a plain weave)
  • Fabric was on the bias (45 degrees to the spar) and helped carry drag and inertial loads
• Primary structures were primarily of wood

This arrangement became the “standard” for several years

Early monoplane

• Development of the fuselage was one of the next big advancements.
  • Pilot and accessories no longer need sit right on the wing
  • Low winged monoplane allowed energy absorption to occur below the pilot and passengers

Early monoplane

• Airfoils were still thin…
• Spars were not designed to carry lifting loads on their own
• Wires above and below the fuselage aided in carrying landing loads

Early monoplane

• Is there a downside to wire bracing? Yes!
  • Compression and buckling failure due to the load component that is parallel to the wing
  • Drag
• Internal wire bracing (inside the wing) was eventually employed
  • Strength and durability were principal concerns with internal bracing

Cantilevered wings

• Cantilevered spars eliminated drag, no bracing (internal or external)
• Earliest practical version was the Fokker Dr-1 1912
• Spar box (plywood web with spruce stringer)

• Structural skins were used on wing and fuselage
• Planes were still multi-wing (triplane) – increased surface area and decreased bending moments

Metallic aircraft

Metallic aircraft represent a maturing technology

• First practical all metal aircraft was the Junkers J-1 monoplane, 1915
• 1930s – Metallic skins began to be used. Corrugated skin added durability, however did not carry much load

• Arguments in favor of metal:
  • Manufacturing synergies with automobiles, etc
  • “Fire resistance”
  • Consistent/predictable material properties
  • Limited supply of spruce after WWI

Monocoque Structures

Modern transport aircraft

• Pressurized fuselage allowed for a more comfortable 8000ft cabin pressure
• Circular fuselage is essentially a pressure vessel for efficient maintenance of cabin pressure
• Cutouts such as windows and doors caused stress concentrations and fatigue cracks. Rounded corners were employed.
  • De Havilland Comet was the first major tragedy associated with low cycle fatigue cracking
• Pressurized cabins and metallic load bearing structures ushered in an era where fatigue was a dominant concern of the structural designer

Challenges in modern aerostructures

Sandwich structures and composite materials

• High stiffness with low weight material configuration
• Core carries the shear load which the face sheets carry the tensile and compressive loads associated with bending
• Early example of sandwich structure was De Havilland Mosquito which had balsa core sandwiched between spruce face sheets

• Carbon and boron fibers are commonly used face sheets due to high specific stiffness and specific strength
• Core is often constructed of foam with high compressive strength
• Core and face can be adhesively bonded or molded in place
• Other cores are corrugated metals or metallic foams
• Early composite aircraft was the Raytheon/Beach Starship 2000 (1986)

Tailored stiffness–composite materials

• Aeroelastic tailoring: use of directional differences in properties to carry load and cause intentional deformations that have aeroelastic advantages (stiffer in one mode of deformation than another)

• Grumman X-29A–Forward swept wing with aeroelastic tailoring.
  • The advantage is that the wing tips stall last instead of first, allowing greater control
  • Downside is that the aeroelastic deformation causes positive feedback on wing loading (wing loading drives higher angles of attack, drives higher wing loading)–Divergence
• Bend/twist coupling allows minimization of divergence effects

Wing warping

Challenges

• Maintain structural integrity during large changes in configuration
• Control

Other safety innovations in modern aircraft

• Fail-safe design – redundant load paths in the event of failure

• Safe-life structures – cracks cannot be prevented but the structures is designed to eliminate catastrophic crack propagation over the life of the airframe

• System optimization – loosely, to optimize the aerospace vehicle system instead of individual components

Material innovations

• Improved alloys (aluminum, titanium, magnesium, steel) allow for high strength or fatigue resistance
• Welding, adhesive bonding, etc allows elimination of rivets (holes) that cause stress concentrations
• Composite materials
  • Naturally have a high fatigue resistance
  • Fatigue prediction much less refined than in metals
  • More difficult to inspect
  • New techniques for repair
  • New methods for lightning strike mitigation
• Material resistance to temperature becomes an issue for supersonic and hypersonic airframes
  • 85% Titanium in the SR71-Blackbird

Temperature resistance

Environmental impact

• Fuel economy, greenhouse gas emissions
• Aircraft noise, airport–residential conflicts
• Recyclability, clean manufacturing, life-cycle impact

3D Printing

World first printed plane

• LA Times

International politics

• Trade subsidies
• Technology transfer
• Gross domestic product

“In the year 2054, the entire defense budget will purchase just one aircraft. This aircraft will have to be shared by the Air Force and Navy 3 days each per week except for leap year, when it will be made available to the Marines for the extra day.” – Norm Augustine 1986