Aircraft Design of Structures and Wings

Aircraft Design of Structures and Wings

For aircraft flying at or near supersonic speeds, the air flow over the aircraft is very different from that in subsonic flight, and the designer has a new set of problems to face. An aircraft flying through the air generates pressure waves, which move at the speed of sound. At speeds below the local speed of sound these pressure waves warn the oncoming air that the aircraft is approaching. As aircraft speeds approach the speed of sound, these pressure waves no longer travel significantly faster than the aircraft, and a shock wave will begin to form on parts of the aircraft. Air pressure, speed and temperature change very suddenly across this shock wave, and the flow behind it is quite turbulent. The effect on the aircraft is a reduction in lift, an increase in drag, changes in trim and possibly buffeting of the controls. Designers have reduced the effects of these problems with better designs, particularly swept wings. To understand how this is achieved, consider the airflow to be made up of two components - one at right angles to the wing leading edge and one parallel to the leading edge. By sweeping the wings back (or forwards), the component at right angles to the leading edge is reduced as the cosine of the sweep angle. The larger the sweepback angle the higher the speed at which this component will remain subsonic. Highly swept aircraft wings give other problems, particularly structural, and are more difficult and expensive to build than simple straight wings.

Aerodynamic problems associated with swept wings include an increased likelihood of tip stalling, which can lead to spinning, and a reduced lift-to-drag ratio.

Once flying above the speed of sound, the airflow is changed to become steady again, although quite different to subsonic conditions. Shock waves form whenever there is a change in airflow direction, and curved shapes no longer produce the lowest drag - sharp edges create a single shock wave, and are generally more efficient than curves. The plan form of wings becomes very important, and a low aspect ratio and highly swept wings are most suitable. The reduced aspect ratio of these wings makes life easier for the aircraft structural design, but the loads are very high, and the structure must be strong and stiff.

The main disadvantage of swept wings is that, at a given angle of attack, they produce less lift than an unswept wing of the same general dimensions. This is because the geometry of the sweep angle acts to reduce the effective angle of attack. So when the aircraft is flying slowly, for instance during take-off, a larger angle of attack is required to provide sufficient lift. Sweepback changing in flight has been tried on some aircraft. Delta wings have high sweepback and are strong with a large chord at the wing root.