No, I did not say that "everything aft of the turbine is a net drag force". If you look carefully you will see that the RR figures include the statement that, "Exhaust unit (imediately aft of the turbines) 2419 lbs forward force". This is produced by the fact that pressure immediately behind the turbines is greater than ambient.
I we look only at these figures we may well ask, "so why don't we just get rid of the propelling nozzle?" Part of the answer is that if we removed the jet pipe and convergent propelling nozzle, the pressure behind the turbine would be lost as the air axpanded in all directions. This would cause us to lose the thrust that was being produced by the exhaust unit.
The jet pipe and propelling nozzle direct this expansion aft, and in this way it uses the excess pressure in the jet pipe to accelerate the air rearwards. This rearward acceleration produces thrust. In the RR figures this appears as the thrust on the exhaust unit.
If we look carefully at these nozzles in supersonic flight, the gas pressure on the convergent section tends to push the nozle aft, but that on the divergent section tends to push it forward.
But the excess pressure in the jet pipe is acting on the forward face of the propelling, nozzle so if the nozzle were to be suddenly released it would fly out of the back of the engine. This shows that the force on the propelling nozzle is acting reawards as stated in the RR figures.
But that is only part of the answer. The greatest benefit of the propelling nozzle becomes evident only as the speed of the aeroplane increases.
According to Newton's Second Law Thrust = Mass x Acceleration.
This means that to create thrust, the velocity of the air flowing out of the exhaust must be greater than that flowing in at the air inlet. In effect this means that the amount of thrust produced is proportional to the difference between the exhaust velocity and the TAS of the aeroplane. This in turn means that when TAS is equal to exhaust velocity there will be no thrust.
By accelerating the exhaust gas rearwards, the convergent propelling nozzle increases the thrust and also increases the TAS at which the thrust falls to zero. This permits us to reach higher airspeeds.
But convergent propelling nozzles cause acceleration only in subsonic airflows. So for very high supersonic speeds we need to replace the convergent propelling nozzle with one that is convergent-divergent. In these, the convergent part accelerates the air up to sonic speed, then the divergent part continues the acceleration at supersonic speeds.
The statement that "Concorde intakes suck the aircraft forward in supersonic flight" is incorrect.
For this idea to be true it would require the pressure inside the intake to be lower than ambient pressure. This would cause air to be sucked into the intake. But in high speed flight the air is rammed into forward facing intakes and the pressure inside the intake is greater than ambient.
A supersonic air intake is made up of a convegent duct, followed by a divergent duct. As supersonic air is rammed into the convergent section, its velocity decreases and its pressure increases. This produces an aft facing force (drag) on the convergent surfaces.
The convergent section is followed by a divergent section leading to the engine. By the time the air reaches the divergent section, its velocity is subsonic. So the air then continues to decelerate subsonically through the divergent section. This deceleration increases the pressure further.
Inside the divergent section the high pressure tends to push the divergent surfaces forward. It is this which produces the extra thrust. The thrust produced by the complete intake is equal to the forward force on the divergent surfaces minus the rearward force acting on the convergent surface. But there is no "sucking" involved in this process.
It might however be argued that at the start of the take-off run all jet engines suck the aircraft forward to some extent, because pressure in the inlet really is lower than ambient.
Last edited by Keith.Williams.; 10th Feb 2008 at 09:23.