![]() ![]() The chief assumption when modelling fluid flow as incompressible is that the density of an infinitesimally small parcel of that fluid is constant in time. At low speeds (generally below Mach 0.5) air is essentially incompressible which allows scientists and engineers to make a number of simplifying assumptions when modelling the motion of the aircraft through the air. To understand how and why shockwaves form as an aircraft approaches sonic speeds you need to appreciate that air is a fluid and that it is compressible. The effect of sweeping a wing on the normal velocity component Shockwave Formation This is a simplification of the actual flow distribution as the discontinuity at the wingtip and other effects like interference between the wing root and fuselage are neglected however, the essential flow dynamics and central idea behind wing sweep are well captured by this explanation. Since the velocity normal to the leading edge is responsible for the magnitude of the pressure distribution over the top and bottom surface of the wing, it follows that the reduced velocity \(U_ \) will result in a smaller pressure difference between the two surfaces and consequently a reduction in the lift produced by the swept wing all other things being equal. The swept wing shown in the image below decreases the velocity normal to the leading edge by almost 30 % of the freestream velocity at a sweep angle of 45°. Thus by sweeping the wing back, one could reduce the velocity component over the wing which would therefore delay the formation of shockwaves at high speed as the wing would be seeing an effective velocity less than the actual freestream velocity. He described how the aerodynamic properties of the wing are dominated by the component of air flow normal to the leading edge of the wing section and not the freestream velocity. Adolf Busemann who presented his findings at the Volta Conference in Italy in 1935. The idea of sweeping a wing to reduce the onset of compressibility effects and to delay the formation of shock waves came from a German scientist Dr. Miles M.52 (L) and Bell X-1 (R) unswept aircraft. In fact, the British equivalent to the Bell X-1, named the Miles M52 was cancelled once the benefits of swept-wing technology came to light. While the first aircraft to break the sound barrier, the Bell X-1, made use of a straight wing, it required a rocket engine to produce sufficient thrust to overcome the large drag rise and suffered from controllability issues at high speed. The ability to delay the formation of the shock waves has a dramatic positive effect on the total drag produced by the aircraft as it approaches Mach 1.Įarly attempts to fly at sonic speeds in straight-winged aircraft were characterised by a noticeable drag increase and violent shuddering of the airframe as these high speeds were approached. The sweep has the effect of delaying the formation of shock waves on the surface of the wing caused by the compressibility of air at high speeds. Wing sweep is primarily used on aircraft that fly in the transonic and supersonic regions. The sweep angle of a wing is the angle at which the wing is translated backwards (or occasionally forwards) relative to the root chord of the wing. ![]() The sweep angle of the Boeing 747-400 wing is approximately 37.5 degrees This is most easily seen when viewing the wing in planform as shown on the Boeing 747-400 below. We'll now spend some time looking at wing sweep and discuss how correctly applying a sweep angle to a wing is a necessity if you are designing an aircraft that operates anywhere near the transonic or supersonic region.Ī wing is said to be swept when a straight line between two corresponding chord locations (given as percentage of chord) on the root and the tip are angled relative to the lateral coordinate of the aircraft. The concept of wing sweep was introduced in the previous post which discussed the concepts of Wing Area and Aspect Ratio and their importance in the design of a new aircraft. ![]()
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