Table 1: Direct Access to the Sub-Disciplines of Aerospace Engineering

Aerodynamics; Fluids
 Structures; 
Solids
Materials
 Propulsion
 Astronautics
  Flight Mechanics 
Controls; Avionics
 Design;
Manufacturing

DESIGN-CENTERED INTRODUCTION TO AEROSPACE ENGINEERING

12. STRUCTURES & MATERIALS

Structural Nomenclature.
Loads:
As shown below, there are different kinds of loads. These are combinations of 5 basic types of loads: compression, tension, shear, bending and torsion.
The deflections resulting from such loads are called by corresponding names:
"axial deflection", "shear deflection", "bending deflection", "torsional deflection", etc. Below are shown some extreme examples of these deflections. In practice, the deflections are quite small, but since aircraft are large, we can indeed see the wings bending on a large aircraft. The wing tips of a large aircraft, for example, might move up and down through more than six feet relative to the fuselage when the aircraft goes from the ground, where the wings only have to support their own weight, to flight, where they support the weight of the entire aircraft.
The engine mount  takes the weight of the engine, which causes tension, and the shear load as the engine pulls the aircraft along. Of course the resulting deflections are hardly noticeable (unlike the exaggerated situation sketched above!)
.          The wings produce lift, so that they are under a distributed load. The engine mounts and the fuslage are like point-loads, hanging down from the wings. Under these loads, the wings bend a little.
The wings are also under torsional load. The wing tips, for example, may tend to twist with their front sides pointing up, under the influence of the aerodynamic load (the pitching moment). The wing roots are attached to the fuselage, so they can't go anywhere. This causes the wings to twist a little. The wings have to be designed to take this torsion as well.

Consider the situation existing on an aircraft moving at high speed. At the very front of the aircraft, the air is getting pushed by the nose of the aircraft. In other words, the flow is brought to a stop relative to the aircraft, at the nose. The pressure becomes the stagnation pressure. On an aircraft flying at very high speed, this pressure may be 100 times the surrounding atmospheric pressure. At the same time, lets say the engines are mounted in the fuselage (to reduce drag), and are causing enough thrust to push the aircraft along, overcoming all the drag. The fuselage is getting compressed from both ends. It may tend to "buckle" as shown. This is not as far-fetched as the above figure might indicate. The author remembers talking to a classmate who went to work during a summer. His job was to analyze the strength of the "pitot tube", (the long  tube  that sticks out at the very front of test aircraft). Apparently these tough metal tubes had been buckling!
 

Members
Column : carries compression
Cable ("tensile member") carries tension
Panel and Web carry Shear
Beam and Spar carry Bending

 
Plate may carry compression/tension.
Shell (its alwas curved) carries compression-tension and Shear.
The part of a beam that carries tension-compression is called "flange" or "cap".
Moment of Inertia is proportional to (Mass)(distance)2. So, for maximum strength against deformation about an axis, place the maximum mass at the maximum distance from that axis.

 

I-beam and C-channel for bending.

Box for torsion strength

Skin for shear


 

Ribs for tension

Struts to prevent buckling in compression.

The g-factor in aircraft structural design: Acceleration effect on structural loads.
Consider an engine of mass Me hanging from a wing. It pulls down at point A with a force of Meg   under steady conditions. Now consider the same configuration in a hard landing. The aircraft experiences a deceleration of 6gs: (6 time 9.8 m/s2). Now the force at point A is 6Meg. If you drop down in free fall 20 feet, and the landing gear compresses by 1 foot incoming to rest, the deceleration is 20.3 g's! This is why pilots have to be careful in landing and designers have to be careful in designing. Typically, commercial aircraft structures are designed for 3g loads. It becomes prohibitive to design them for much higher g-factors: the structure weight goes up too much!  The landing gear is provided with enough compression length and damping to reduce the landing deceleration to well below this limit. This is also why such aircraft should not try fancy maneuvers like fighter aircraft. Fighters are designed to well over 9g's. The limit here is not so much the structure breaking as the pilot not being able to function, or "blacking out" altogether. Missiles, or Uninhabited Aerial Vehicles (UAVs) can be designed to higher g-factors, because the guidance system may be able to withstand higher g-loads than human pilots.
Boeing 747-400 under construction. Boeing 777-200 Wing . Below: A V-22 lifts a Humvee: a dicery problem in stability and structural design. Boeing Joint Strike Fighter parts being built.
Joint Strike Fighter wing construction. Joint Strike Fighter engine parts.F-16 assembly line. Another structures problem: designing pressurized space suits which will let an astronaut explore alien worlds. Or ride in "comfort".  Thrust-vectoring nozzle: the nozzle components must be able to rotate a flow against enormous forces and moments, while surviving temperatures of perhaps 2000K, and still be as light as possible.

Go to the next section: High Speed Flight

Go to the previous section: Stability

Go to the Course Outline
 
 

Aerodynamics; Fluids
 Structures; 
Solids
Materials
 Propulsion
 Astronautics
  Flight Mechanics 
Controls; Avionics
 Design;
Manufacturing