Multishaft Turboprop Engine

Multishaft Turboprop Engine

It is obvious that the multishaft concepts can be extended to a turboprop
engine with the propeller turbine replacing the role of the fan and most of
the role of the thrust nozzle. The off-design behavior would be similar to
that discussed in the previous section on the two-shaft jet engine.

Afterburning

A photograph of a single-shaft turbojet with afterburning was presented
in Fig. 1.15. Two designs of two-shaft turbofan engines with afterburners
are shown in Fig. 1.29. Notice that the gas stream from the engine is mixed
with that of the fan ahead of the afterburner. The static pressures of both
streams must be the same where they merge, e.g., at point A in the upper
figure. This requirement affects the off-design performance of both the fan
and gas generator, but the general principles previously noted still apply.
Multistage fans and afterburners typify configurations needed by supersonic
aircraft. Except for their proportions, the rotating structures resemble
those of Fig. 1.27. The function of the mixers just ahead of the afterburners
is explained in Chapter 2 of Ref. 2.
Igniting an afterburner poses a special off-design and control problem
that is treated in Chapter 2 of Ref. 2. In brief, the effective Mach number
of the flow in the nozzle throat is unity during normal operation. When the

gas temperature is suddenly raised, the nozzle throat area must rapidly
increase to pass the mass flow without an excessive pressure rise. The rate
of change of the fuel flow must be accurately synchronized with the rate of
change of the throat area. If the area increases too rapidly, thrust is
momentarily lost just when an increase is demanded. If it is opened too
slowly, the fan and possibly the compressor are pushed into surge. The
magnitude of the control problem varies inversely with the permissible
surge margins of the compressors.