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. Transonic Acceleration
A supersonic jet is to cruise at supersonic speeds. But to reach supersonic speeds, it has to accelerate from take-off to the supersonic cruise speed But in between there is a speed regime with maximum wave drag and minimum engine performance which may determine engine size. This critical speed is the transonic acceleration point at low supersonic speed (about Mach 1.1).
Because wave drag is at its maximum, it dominates drag and (for rough estimations) the other pans may be omitted. So cqs. (49) or (50) become:
CD • 128 V2/(kSI*) ♦ KL • 02SCJ/(2ji/2) (59)
or in physical units
D = Kv2SV2/{nSI40)q + KLV2W2/{2nql2) (60)
where the first term for volume wave drag is dominant.
At Mach 2 the Mach-angle is 30*. at Mach 1.2 about 60’ and at Mach l just 90*. This
means, tliat at Mach 1 all disturbances produced by the aircraft arc radiated in a plane normal to the flight path and so stay in the relative position to the plane, at least when neglecting the local Mach number variations. (But these local Mach number variations arc the reason, that stationary or very slowly accelerated flight at Mach 1 is possible).
To minimize transonic wave drag, the configuration must be slender w ith a smooth variation of the total aircraft’s cross section area distribution These cross sections must include the variations of the engine s stream tubes For the large Mach angles of about 90*. changes of cross sectional area must be balanced in a very short streamwisc distance; but due to the nearly stationary propagation, the area distribution within each cross section is not so important. This may lead to so called "Coke-bottle" fuselages which balance (strong) variations of wing, tailplanc, engine areas by fuselage area changes.
To enable low wave drag at transonic and supersonic cruise speed, smooth area variations of all aircraft parts arc recommended.
It is easy to design a perfect supersonic or hypersonic aircraft which will not be able to accelerate to supersonic speeds. This was demonstrated several times in the past! And Concorde needs its fuel gulling afterburners to overcome this drag during a very slow ly accelerated part of the flight. If Concorde docs not sucecd in the first attempt to accelerate to supersonic speeds for a transatlantic flight, it has to return for refuelling!
