August 24, 2018, ©. Leeham News: In the last Corner we discussed how supersonic wave drag can be higher than any other drag for a blunt fuselage trying to fly at supersonic speeds. If we try to fly supersonically with a Dreamliner fuselage, the volume wave drag is four times as large as the whole Dreamliner drag when flying at its normal cruise speed.
Besides wave drag based on bodies volumes we also have wave drag when we angle lift surfaces like wings against air to create lift.
Like for the fuselage, the wave drag due to lift from the wings is an added drag on top of the normal drag due to lift, induced drag.
The drag is created when the wing redirects the air molecules downward to create lift. The shock waves which are created by the wing causes lift wave drag.
We are once again in the domain of “Bounce aerodynamics” as the air molecules have no idea a wing leading edge is coming. The pressure wave, which could make them move smoothly out-of-the-way around the wing, can’t give any pre-warning.
The pressure is traveling slower than the wing and therefore the leading edge is cutting like a knife-edge through the air, bouncing the air molecules to the side and downwards.
To create lift with as little bouncing as possible, the wings shall be thin and have a wide cord. Wingspan is not so important, as the speed is so high induced drag will be low even for a wing with a low aspect ratio.
A supersonic wing shall be thin. It shall have a low thickness compared with its cord length, like the Concorde wing in Figure 1.
Instead of the classical aspect ratio for a wing, which is the wingspan squared divided with the wing’s area, for supersonic wings we talk about the wing’s length aspect ratio which is the cord of the wing squared divided by the wing area.
If we halve the cord of the Concorde wing and keep the area the lift wave drag increases by 250% and passes the induced drag in size. With the normal Concorde wing, the lift wave drag is less than 25% of the induced drag during cruise.
Next Corner we will discuss why there is a drag hump when passing through Mach 1.