User:Eas4200c.f08.gator.edwards/Chapter One Notes

Emphasis of the course includes: the understanding of mechanics, the ability to formulate problems, to be able to judge correctness of solutions, and to avoid the old ad hoc methods of structural analysis.

There are two basic aircraft design goals: we want the aircraft to be both light and strong.

The desired aircraft material properties include: high stiffness, high strength, and lightweight.

Stiffness is the Young's Modulus $\displaystyle E$ (slope) in linear relation between stress $\displaystyle \sigma$ and strain $\displaystyle \epsilon$ , i.e., $\displaystyle \sigma$ = $\displaystyle E$ $\displaystyle \epsilon$

Strength includes yield stess $\displaystyle \sigma$ Y, and ultimate stress (rupture stress) $\displaystyle \sigma$ u.

Toughness is the material's ability to resist fracture and damage. This is also called fracture toughness.

Some high stiffness and high strength materials include: steel alloys, titanium alloys (which have lower stiffness and strength than steel alloys), and aluminum alloys (which have lower stiffness and strength than titanium alloys).

A high stiffness and low toughness material is glass.

Some low stiffness and high toughness materials include: plastic and nylon. An example of a material with good fracture toughness is aluminum (which has lower stiffness, strength, and fracture toughness than steel alloys) and it is used in aircraft skin.

Some high stiffness and high toughness materials include: composite materials.

composite materials- fiber reinforced composites where the material used as the base material (called the matrix) is reinforced with many fibers (which increases strength and stiffness).

There are two aspects of an aircraft structure we will consider: its geometry and the material that it is made out of. Two styles of the geometry of the aircraft is monocoque and semi-monocoque.

Monocoque -team gator

Semi-monocoque design of structural skin and internal stiffening on a Boeing 747

Monocoque, also known as structural skin, is a way to use an object's outer skin as a structural support. This is opposed to using an internal support system with a skin that is not used to bear loads. Airplanes used to be made of wood and then were covered with a cloth. In the 1920's aluminum prices fell and metal began to be used both for inner structures and for the skin. Monocoque construction has advanced as a result of the use of composites in aircraft. Wing structures have taken advantage of this and can have stiffness and strength in one direction without wasting material in another by layering the materials in certain ways. Flexibility can also be increased without being any heavier or stiffer than needed (such as helicopter blades being able to just be twisted instead of rotated). Combining the use of structural skin as well as internal stiffening provides strength against buckling in compression. This combination is called semi-monocoque.

Some other considerations that need to be made for aircraft construction include the fact that the geometry is limited by aerodynamic considerations such as lift and drag of the airfoil. Also the fact that we want the aircraft to be used to its full potential and for there not to be any wasted weight. Optimizing the aircraft materials used is also important. This can be done by replacing aluminum and titanium with fiber reinforced composites, which saves up to 30 to 40 percent in weight.

Composite Materials in Aircraft -team gator

An example of which parts of an airplane are made out of what kinds of materials can be found in the F18 Hornet. Aluminum is used in the skin. Steel is used to make the landing gears. Titanium is used to make the engine encasing structure and other structural parts (stabilator, wing, cockpit cover). Carbon Epoxy is used in the fuselage, aileron (control surface along trailing edge of wing), stabilator, and the vertical stabilizer.

If you would like to see locations of where toughened graphite, graphite, hybrid and fiberglass are located in a Boeing 777 you can click here. For an example of an aircraft showing sections of glass, aramid, and carbon fiber reinforced plastic, you can try this site.

Boeing 787

The Boeing 787 is a new aircraft from Boeing that rolled out on July 8, 2007. In the 787, 50% by weight is composite (80% by volume) which means that it is much lighter and stronger than previous planes made with traditional materials. The plane is made of about 35 tons of carbon-fiber reinforced plastic. The composite constructed parts include the interioe, doors, tail, wings and the fuselage.

Fiber Metal Composite - team gator

Fiber-reinforced composites are stonger and lighter than traditional materials. Their strength to weight ratio makes them ideal materials to use in such things as aircraft parts. These composites are made by layering sheets of the fiber cloth into a mold the shape of the desired product. Then the mold is filled with an epoxy and either air cured or heated. The final product is stiff, strong, and corrosion resistant.The Airbus A350 as well as the previously mentioned Boeing 787 are largely composed of composite materials. This will make the aircraft much lighter than the traditional aluminum fuselage as well as having a superior fatigue resistance which will need less maintenance.

A metal matrix composite is made up of at least two parts: a metal and a different metal or another material (often a ceramic or organic compound). These are made by beginning with a metal matrix and dispersing a reinforcing material into it. Other purposes for using this method besides stength is to improve thermal conductivity, friction coefficients, or to improve wear resistance. An example of use of metal matrix composites in aircraft can be found in the F-16 Fighting Falcon. A structural component of this jet is made of monofilament silicon carbide fibers in a titanium matrix. The advantages over polymer matrix composites include moisture and fire resistance, better electrical and thermal conductivity and resistance to radiation. The disadvantages include being more costly and is more difficult to manufacture.

References

Boeing 787 in Wikipedia

Metal Matrix Composite

Carbon Fiber Reinforced Plastic

Monocoque