The copy of the Rolls Royce book "The Jet Engine" that I have here at home is a 1973 version. I examined it for an ISBN number but it (my old copy) does not have one. You can certainly get it from POOLEYS or TRANSAIR and probably PILOTWAREHOUSE through their websites. It does not go into the theory to any really great depth (no atom splitting or gene splicing equations), but it gives a good overall description of how jet engines work.
The air velocity at the face of the fan will vary with RPM. But should ideally be constant at any given RPM, regardless of TAS. To achieve this the engine sucks in the air when the aircraft TAS is very low. So before brake release in the take-off run, although the aircraft TAS is zero, the velocity at the face of the fan may be about 200 or 300 knots. But the pressure and air density in the intake are very low.
As aircraft TAS increases, there is less of a need to suck in the air, so pressure in the intake gradually rises towards ambient. At higher aircraft speeds the air is forced into the intake at a speed greater than that required by the fan. So the air inlet is designed to decelerate the air, converting as much of the excess velocity (kinetic energy) as possible into static pressure. But unless the divergence of the intake duct can be varied in flight, the airspeed at the fan face will vary to some extent with TAS.
For low speed flight (up to typical airliner cruise speeds) the intakes are of the slightly divergent PITOT type. When the TAS is greater than the required compressor inlet speed the divergent intake slows the air thereby increasing its static pressure and density.
For higher speeds (transonic and supersonic) a PITOT intake would create too much drag and be very inefficient. This is because the individual shock waves that form around the lip combine to form a strong shockwave right across the throat of the intake. The air passing through this shockwave would be decelerated very abruptly. Although this would increase its static pressure and density, it would also cause a large increase in temperature. This increased inlet temperature would reduce the thermal efficiency of the engine. In addition to this, excess air would spill around the lips of the intake, causing lots of turbulence and drag.
To overcome these problems, supersonic aircraft use either multi-shock spike-type intakes (as in the MIG21 and SR71), or variable area moving ramp intakes (as in CONCORDE, and most modern supersonic combat aircraft). These intakes use a series of oblique shockwaves to gradually decelerate and compress the air. Also, by moving the internal ramp, these intakes change their frontal area and convergence/diverenge ratios to match the incoming airspeed. The more gradual deceleration produced by these methods causes much lower increases in temperature. The overall effect is a very large increase in pressure before the air enters the compressor.
The exhaust systems of high speed aircraft are also designed to achieve very high exhaust gas velocities. In a simple convergent duct type propelling nozzle, the maximum possible velocity at the throat of the nozzle is the local speed of sound. This is 661 knots at 15 degrees C, but is equal to 39.84 x the square root of the absolute temperature.
At ISA msl this gives 38.94 x the square root of (15 + 273) = 661 Knots. But the temperature in a jet pipe is much higher than ambient. If we assume a 400 degree C jet pipe temperature, this will give a local speed of sound of 1010 Knots. This is why aircraft with simple convergent propelling nozzles can achieve supersonic speeds. The TAS of the aircraft may be more than 661 Knots, but the exhaust velocity is much greater, so the engine is still producing thrust.
But if we just keep opening the throttle we will eventually reach a point where the velocity at the throat of the nozzle is equal to the local speed of sound. If we now open the throttle even more, the jet pipe pressure will continue to increase, but the exhaust gas velocity at the throat of the nozzle will remain constant. Instead of going faster, the air simply leaves the nozzle with some excess static pressure. This pressure is then wasted as the air expands outside of the aircraft. This situation is called a CHOKED NOZZLE condition.
It is in this choked nozzle condition that we get a small amount of pressure thrust due to the pressure difference across the nozzle. Pressure thrust is equal to the nozzle area multiplied by the difference between the ambient pressure and that in the jetpipe. But this is a very inefficient method of creating thrust, so it is best avoided if possible.
If we want to go really fast we need to get even greater exhaust velocities. This can be done in two ways. Firstly if we use reheat by burning extra fuel in the jetpipe we will increase the exhaust gas temperature. This will increase the local speed of sound at the nozzle thereby enabling our simple convergent nozzle to achieve higher exhaust velocities. But the exhaust gas velocity at the throat of the nozzle is still only equal to the (now rather higher) local speed of sound.
To get truly supersonic exhaust velocities we need to use a more complex propelling nozzle in the form of a convergent duct followed by a divergent duct. The convergent duct accelerates the gas to local speed of sound. This sonic gas then flows into the divergent duct, which continues to accelerate it to supersonic velocities. The explantion of this effect is not really very complicated, but works best with diagrams, so I won't try it here. But to describe the resilts we need simply reverse the effects of ducts on low speed flow. At subsonic speeds a convergent duct increases velocity, decreases temperature and decreases static pressure. At supesonic speeds these effects are produced by a divergent duct.
If you look at any modern supersonic aircraft, the propulsion system will usually include variable area intakes, reheat and variable area convergent-divergent propelling nozzles.
Last edited by john_tullamarine; 2nd Jan 2004 at 07:42.