September 7, 2018, ©. Leeham News: In the last Corner we looked at the drag hump which is created by several supersonic effects when an SST (SuperSonic Transport) passes Mach 1.
Now we will look at other aerodynamic problems facing an SST.
We have seen passing the Sound Barrier is one of the challenges for an SST. The drag hump requires slender aerodynamics and strong engines.
Once past the Sound Barrier, another problem appears. A subsonic aircraft has the center of lift for the wing at about one-third of the wing cord. Past Mach 1 the center of lift passes well aft of this point, the pressure distribution at supersonic flight is different.
To keep level flight, one must trim the aircraft with more up-elevator. This causes increased trim drag. For a tailed design like the Aerion AS2 in Figure 1, the tail, which is placed at a distance to the wing, can create the nose up moment without excessive elevator angle and therefore drag.
For a tailless Delta like the Concorde or Boom SST (Figure 3), the tail arm is short and the wing elevons need a higher angle to counter the nose down pitch moment. Trim drag is high.
To counter this the Concorde had a center of gravity which could be moved back at supersonic flight, Figure 2, and then forward again for flight below Mach 1 and landing.
This reduced the elevon trim needed and therefore the trim drag. The Boom SST has an engine in the tail. It remains to be seen how it will counter trim drag.
Another problem with SSTs is the landing. The wings need a higher angle of attack to generate the lift needed at the slow speeds of a landing.
A tailed design like the AS2 can have powerful flaps which create lift. But these also lower the angle of attack when this lift is developed. A wing with deployed flaps generates a strong nose down pitch moment. Once again, the tail arm of the AS2 is handy to control the pitch of the aircraft during landing.
A tailless delta has no flaps and the lift is generated at high angles of attack. I flew the SAAB Draken delta and it developed the landing lift at 11° angle of attack. The Concorde and Boom SST planforms are similar; so the angle of attack will be in this bracket, Figure 4.
At 11 degrees nose up angle for the aircraft, the pilot can no longer see the runway once at landing speed. The Concorde and Tu-144 solved the problem with a droop nose. How the Boom SST solves the problem is not clear.
There are more aerodynamic problem areas with an SST. A major one is the heating of the aircraft skin when flying at high supersonic speeds. This will be the subject for our next Corner.