Peter...now you are mixing and matching accounting methods...
Yes...ram drag is what we subtract from the gross thrust...ram drag is freestream speed (aircraft speed) times mass flow...so that is why thrust = mass flow * (jetpipe exit speed – freestream speed)...
However...if we want to do a control volume analysis of the engine...then we do it as mentioned...btw we can also do the whole engine as a single control volume and we will get the thrust the exact same way as I did for the inlet duct...using the same math...
And again it helps to make a sketch...we see from such a sketch that we can again use the impulse method to calculate thrust...
Let's say again we have a mass flow of 100 kg/s...and an inlet area of 1 m^2...at the nozzle the area is 0.5 m^2 and the pressure is let's say 2 times the atmospheric pressure...the speed of the jepipe exit is 500 m/s...the aircraft is flying at 250 m/s...~M8 at FL 350...
The simple formula for thrust is mass flow times jetpipe velocity...minus ram drag which is mass flow times freestream V...so our ram drag is 25,000 N...and our “gross” thrust is 50,000 N...from which we subtract ram drag of 25,000 N and we have net thrust of 25 kN...
We can get the same result if we now do a control volume analysis and take into account the pressure times area at the front inlet and aft nozzle respectively...the atmospheric pressure is 22.5 kN/m^2 so the front pressure pushing the engine 22.5 kN/m^2 * 1 m^2 inlet area = 22.5 kN...
The nozzle pressure is double...but the area is smaller by half...so it is a wash...we then take our inlet momentum (ram drag) and our exit momentum and we find that we have 25 kN thrust...
We could do this control volume thing for each component and still will get the same net thrust at the end...
The important thing is not to mix and match ideas...pick one bookkeeping method and stick with it...
Regards,
Gordon.
PS: being an Ontarian myself...I'm curious if you are related to the TV newsman of the same name...?
