Propelling nozzle
Propelling nozzles also act as downstream restrictors, the consequences of which constitute an important aspect of engine design.
Rocket engines — the extreme case — owe their distinctive shape to the very high area ratios of their nozzles.
At low airspeeds, such a setup causes the nozzle to act as if it had variable geometry by preventing it from choking and allowing it to accelerate and decelerate exhaust gas approaching the throat and divergent section, respectively.
The afterburners on combat aircraft require a bigger nozzle to prevent adversely affecting the operation of the engine.
The amount of this air varies significantly across the flight envelope and ejector nozzles are well suited to matching the airflow between the intake system and engine.
They both used tertiary blow-in doors (open at lower speeds) and free-floating overlapping flaps for a final nozzle.
[12] Turbofan installations which do not require a secondary airflow to be pumped by the engine exhaust use the variable geometry C-D nozzle.
[13] These engines don't require the external cooling air needed by turbojets (hot afterburner casing).
[14] The primary and secondary petals may be hinged together and actuated by the same mechanism to provide afterburner control and high nozzle pressure ratio expansion as on the EJ200 (Eurofighter).
Modern high by-pass turbofans have triangular serrations, called chevrons, which protrude slightly into the propelling jet.
The nozzle, by virtue of setting the back-pressure, acts as a downstream restrictor to the compressor, and thus determines what goes into the front of the engine.
At non-afterburner thrust settings the exit area was too big for the closed engine nozzle giving over-expansion.
[11] For complete expansion to ambient pressure, and hence maximum nozzle thrust or efficiency, the required area ratio increases with flight Mach number.
If the divergence is too short giving too small an exit area the exhaust will not expand to ambient pressure in the nozzle and there will be lost thrust potential[20] With increasing Mach number there may come a point where the nozzle exit area is as big as the engine nacelle diameter or aircraft afterbody diameter.
Nozzles are thus limited to the installation size and the loss in thrust incurred is a trade off with other considerations such as lower drag, less weight.
[25] Some very early jet engines that were not equipped with an afterburner, such as the BMW 003 and the Jumo 004 (which had a design ),[26] had a variable area nozzle formed by a translating plug known as a Zwiebel [wild onion] from its shape.
In some applications, such as the J79 installation in various aircraft, during fast throttle advances, the nozzle area may be prevented from closing beyond a certain point to allow a more rapid increase in RPM[29] and hence faster time to maximum thrust.
In the case of a 2-spool turbojet, such as the Olympus 593 in Concorde, the nozzle area may be varied to enable simultaneous achievement of maximum low-pressure compressor speed and maximum turbine entry temperature over the wide range of engine entry temperatures which occurs with flight speeds up to Mach 2.
To run a turbofan to give maximum airflow (thrust), the nozzle area may be controlled to keep the fan operating line in its optimum position.
If the nozzle did not open for some reason, and the pilot did not react by cancelling the afterburner selection, typical controls of that period[32] (e.g. the J47 in the F-86L), could cause the turbine blades to overheat and fail.
These exhausts converted some of the waste energy of the (internal combustion) engines exhaust-flow into a small amount of forward thrust by accelerating the hot gasses in a rearward direction to a speed greater than that of the aircraft.
[34] On the 1944 de Havilland Hornet's Rolls-Royce Merlin 130/131 engines the thrust from the multi-ejector exhausts were equivalent to an extra 70bhp per-engine at full-throttle height.
