Supersonic Wing Designs
Text Contents ] Chapter Contents ] History of Supersonic Flight ] Definitions - Pressure Waves ] Critical Mach Number ] Increasing Mcr ] Shockwave Formation ] Energy Loss at a Shockwave ] [ Supersonic Wing Designs ] Supersonic Flight Effect on Stability ] Supersonic Engine Inlets ]

 

Time is money, as the saying goes. If time is worth enough money then supersonic flight makes sense.

Supersonic flight will always be more expensive than subsonic flight. We have already seen that the CDp is higher in supersonic flight than in subsonic flight. Therefore, more fuel will always be required for supersonic flight than for subsonic flight.

However, a well designed supersonic aircraft has commercial potential. Several are currently on the drawing boards, along with supersonic business jets. Whether the future holds many or a few supersonic airliners depends entirely on the demands of the traveling public. It is certainly well within the ability of aeronautical engineers to build a safe and reasonably efficient supersonic aircraft.

In this section we will consider some of the challenges and known solutions to sustained flight in the speed range from about Mach 2 to Mach 5.

Supersonic Airfoils

The normal shock wave which formed in Transonic flight moves back to the trailing edge of the wing as the aircraft reaches the speed of sound. In this position it posses relatively little problem. Once the aircraft accelerates beyond the speed of sound the shock wave will will remain attached to the trailing edge of the wing, but will sweep back becoming a weaker Oblique Shock Wave. We will explore the Oblique shock wave further below.

The Oblique shock wave has the characteristic of slowing the airflow which passes through it, but not necessarily to subsonic speed. IE, the airflow behind a normal shock wave is always subsonic, but the airflow behind an Oblique Shock Wave may still be supersonic.

The Oblique shock wave is not really much of a problem for supersonic design. It does represent a certain amount of drag, since energy goes into it's formation, but very little can be done about that.

The primary drag problem in supersonic flight comes from the bow wave which forms ahead of the wing. The bow wave causes an area of very high pressure to form just in front of the wing. This causes a large increase in pressure drag. 

There are two ways for the designer to deal with the Bow wave:

  1. Sweep the Wings
  2. Or, use a "Supersonic Airfoil"

Ideal Supersonic Airfoils

One solution to the drag caused by the bow wave is to make the leading edge of the wing very sharp. This design feature will allow the bow wave to attach to the leading edge thus eliminating the area of high pressure ahead of the wing.

The problem with sharp leading edges is that they will be very poor in subsonic flight. Therefore, such aircraft will have very high stall speeds and are generally unsuited to use as modern airliners because they would require very long runways. However the sharp leading edge wing is OK for a missile.

Someday sharp leading edges may be used on supersonic airliners if vectored thrust is used to provide low speed lift for either vertical takeoff or reduced ground run takeoffs. (more on sharp edges in supersonic flow.)

Swept Wings in Supersonic Flight

Swept wing in supersonic flight

As mentioned above, swept wings are the second option an aircraft designer has to minimize the drag caused by the bow wave.

Earlier we learned that swept wings raise the critical mach number thus making modern airliners more efficient at high subsonic speeds. Now we will see that even more sweep will reduce the effect of the bow wave in supersonic flight.

Every part of the airplane which strikes the airflow and slows it to subsonic speeds will produce a shock wave (the bow wave.) This shock wave will sweep back at an angle known as the mach angle (The mach angle is simply 1/Sin(M) where M is the Mach number of the aircraft. IE, an aircraft flying at mach 2 will produce shock waves which trail back at a 30 degree angle.) These are of course Oblique shock waves.

Earlier we learned that the airflow behind an Oblique shock wave is still supersonic. although it is slowed down. However, the component of the airflow at flight angles to the Oblique shock wave will always be subsonic. This concept is shown in the diagram to the left.

If a wing is placed behind the shock wave as shown above, then the air flowing at right angles over that wing will be subsonic, even though the aircraft is flying faster than the speed of sound. Therefore, a subsonic airfoil, with round leading edges can be used without creating a bow wave.

Required Sweep Angle For Supersonic Flight

In order for the above procedure to work the wings must be swept back more than the mach angle. IE, to fly at mach 2 the wings should be swept at least 30 degrees, to fly even faster will require more sweep. However, as long as the designer can accommodate this requirement the wing will react as though it is in subsonic flight. Therefore, the designer will be able to use a conventional airfoil with a round leading edge. The air will flow smoothly around this wing and no bow wave will form. This is the design strategy of choice for most modern supersonic designs.

The primary advantage of using the swept wing, with a round leading edge airfoil is that low speed characteristics, including stall, will be good and therefore takeoff and landing should be less of a problem.

The disadvantages of the highly swept wing include a very large stalling angle of attack which will require unusually nose high attitudes for landing and also excessive lateral stability making control difficult.

Summary

It is important to note that the supersonic airfoil designs, with very sharp leading edges are generally only applicable as an alternative to swept wings. In other words both sweep and sharp leading edge airfoils would not normally be applied to the same aircraft.

Supersonic airfoils are used extensively on devices such as missiles, because subsonic performance is not important for such devices. However, for passenger carrying aircraft subsonic takeoff and landing performance makes conventional airfoils, with round noses more suitable. This could possibly change in the future if very high speed aircraft designers are combined with vectored thrust VTOL capabilities. Generally aircraft designers concede that straight wings with supersonic airfoils are superior above Mach 2. However, they have inherently high stall speeds and therefore require very long runways.

 

Text Contents ] Chapter Contents ] History of Supersonic Flight ] Definitions - Pressure Waves ] Critical Mach Number ] Increasing Mcr ] Shockwave Formation ] Energy Loss at a Shockwave ] [ Supersonic Wing Designs ] Supersonic Flight Effect on Stability ] Supersonic Engine Inlets ]