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Old 20th Apr 2013, 17:33
  #145 (permalink)  
Join Date: Apr 2013
Location: Richmond, Virginia
Age: 65
Posts: 4
Many thanks for replying to my post, I will, when I have time look at the materials you mention. No disagreement on the inlet being a diffuser. I think the mail area of confusion lies in the statements that point to the inlet static pressure being much above the outside static pressure, no argument there. The aircraft is not standing still, the inlet or the entire engine is not "thrusting against static pressuer outside the aircraaft". It is, rather thrusting against the "ram pressure", whic is MANY times geater than static pressure. It is the energy in ram pressure that the inlet can recover and convert to pressure rise, this, is one measure of inlet performance. As the (mach 3.1 or so) air is slowed thru a series of shocks, (in the case of the SR-71) it is terminated in the inlet section. The diffusion process builds pressure but at the expense of DRAG. Inlets produce drag, not thrust. You are not thrusting against outide pressure (which at 50,000 feet ain't much), but against the RAM pressure. If you want to say the the air compressed by the Sr-71 that is bypassed and fed to the afterburner inlet produces the majority of thrust, so be it, I am sure that is correct. I have "done the math", seems like everyone posting is not familiar with PRESSURE THRUST, which is covered in elementary text on any type of jet or rocket propulsion. To go even further and state that the engine is just processing airflow, while the inlet is doing the lion's share of the work is a false statement. It was stated by Kelly Johnson himslef, but later he was corrected by engineering and he, among friends admitted he was initailly wrong. Yes, I have done the math, and adding up and summing drag and positive pressures inside the engine is not necessary or even correct, unless you want ot calculate Pressure Thrust. In the end it is momentum exchange in the exhaust gases that creates thrust, and yeah, some state the "exhaust system " produes X amount of thrust. All propulsion components work together to provide thrust. I will study the book you mentioned, and try to see what you are saying. Again, thank you for the response, I know you for sure have done the math, and I will continue to educate myslef in this area. I will get back with you when I have done some more studying. I still find this an extremely intersting subject!
Tomtech is offline