N2,
Does ram effect now provide the bulk of compressive efficiency at altitude and crz speed?
At usual operating thrust the engine exhaust achieves supersonic velocity, referred to as being "choked". Thrust levels above this increase the mass flow through the engine, but the exhaust speed is fixed. As in my original explaination this means that the faster the engine is going (in terms of TAS), the less thrust it is producing (but at a greater efficiency).
There is also a loss through
momentum drag - if you are an airmolecule hanging around in the sky, and are scooped up by an engine rushing past, then the engine has to accelerate you up to its speed as you pass into the engine, and this is known as momentum drag, which has a net effect of reducing the thrust.
On the upside is the "ram ratio effect". The higher dynamic pressure into the front of the engine increases the mass flow (Force =
mass x acceleration, so the more mass the engine processes, the more thrust it produces). The higher mass flow requires a higher fuel flow to meet it, the ram effect covers the losses from momentum drag, but doesn't cover the losses associated with the higher airspeed.
Elevation:
it has something to do with the exhaust gas. The cooler the outside air temp, the more the exhaust gas expands as it exits the chamber hence creating more forward "push".
The jet engine is a
reaction engine - it produces thrust by throwing mass out the back, what happens to the exhasut after it has passes the jet nozzle has no effect on thrust produced. See below for more on temperature, though.
This common misconception also has people thinking that the jet blast deflectors seen on carriers increase the takeoff thrust of the jets on a cat launch - they don't, they just stop people & equipment behind the jet being toasted
twistedenginestarter:
The specific fuel consumption of jet engines decreases as the outside air temperature decreases for constant revolutions per minute (RPM) and TAS.
The reason the statement refers to a constant RPM is that the RPM controls the pressure ratio of the engine, which is the dominating factor in engine efficiency.
Having said that, when the inlet air temperature is lowered, a given heat addition can provide greater changes in pressure or volume & greater efficiency. If the RPM was limited due to EGT considerations a higher RPM can be achieved as well.