Sonic Cruiser and transsonic aerodynamics
 

Just curious: Boeings Sonic Cruiser concept seems to be intented for high subsonic/just supersonic speeds. That looks like long range cruising in the very least attractive speed-environment to me. Could anybody please give some info on what really happens in terms of aerodynamics with an aircraft at that speed? Heard of shockwaves, local supersonic airflows, dramatic changes of cg, vibrations, engine problems and so on. And how fast will it go in real life (if ever)? Thanks.

'Local supersonic flow' says it all really.

Because air is accelerated over the upper surface of the wing, there are regions where the flow becomes supersonic over a conventional aerofoil even at Mach numbers as low as about 0.4.

Transonic (or 'compressible' flow) flow is generally bad, because energy has to be put in to compress the air, and this wastes a lot of energy. (The air is heated as it is compressed, then some of the heat is dissipated, so that energy used to compress it is 'lost'.

Local supersonic flow is more serious, because shockwaves develop, and this produces 'wave drag', which is very significant, so most civil aircraft are designed to minimise these effects.

The two traditional approaches to this are:

Sweeping the wings - This effectively makes the air move diagonally over the wing, so that the component normal to the wing is slower than the free stream velocity. The component parallel to the wing essentially has no effect. This clearly incurs a penalty in that a swept wing is not as effective per unit wing area as an unswept wing.

Supercritical wing sections - More recently, wing sections have been developed which reduce the 'peak' velocity over the wing and distribute it more evenly, thus delaying the transonic drag rise. These sections are less efficient than conventional sections, but sweep angle can be reduced, yielding both structural and aerodynamic benefits. (Compare the sweep on a 747 with that of a more modern aircraft!)

As for the sonic cruiser, I can't believe that there will be no local supersonic flow, so I am keen to find out what Boeing's solution will be. It all sounds like pie in the sky at the moment though. (Mmm.... Pie!)

Pie

Thanks Pie

I am curious as to why you believe Supercritical Wings are less efficient than conventional wings.

Surely for a given thickness to chord ratio, Supercritical wings produce greater lift than conventional wings. This is primarily due to the camber on the lower surface towards the trailing edge.

Granted this creates structural problems, as so the wing might be heavier. But I don't know whether this cancels out the increased in lift.

Anyone care to shed light on this matter ?

Thanks

Supercritical sections, like everything in engineering, are a compromise. As I have stated, their purpose is to allow lower sweep angles on aircraft which fly in the high subsonic range, which is a very worthwhile purpose in itself. This is what they are optimised for, and as such, they deviate from what would be the optimised design for a lower speed aerofoil. If they worked better than a low speed aerofoil, even at low speed, then everybody would be using them, from gliders upwards.

I'm not an expert in aerodynamics, and I know very little about the aerofoils themselves, but I do know that when selecting an aerofoil, like anything else, you can't have everything.