Throttle position is not directly related to fuel flow in a jet either, if you leave your throttle position constant and climb, your engine speed will vary a little but your fuel burn will decrease. EPR could remain similar, but you have less air to pressurize the higher you go. It is simply a pressure reading not a thrust amount.think of it like a 2500psi. pressure washer , at the end of the day you only move as much water as your water spicket can produce. Now think fire hose....

 

Good friend of mine teaches jet engine operations to pilots for Pratt & Whitney, the textbooks are great

 

Eric

It really seems strange that you would be able to leave the throttles at what constitutes climb thrust all the way up. I would be curious to pick your friends brain on that one as it doesn't seem to make sense to me. But maybe my understanding of EPR is worng. My assumptions have always been that EPR has nothing to do with the throttle position and everything to do with differential pressure between inlet and outlet. Sure adding fuel through the throttle will cause the engine to turn faster which generates a a greater differential pressure.

 

So.....here is the question for someone much smater than I am. As you go up, inlet pressure decreases. If the engine speed remains constant, then the ratio should stay the same. However, as you increase altitude, though the ratio would stay the same, the thrust would decrease. This is based on my understanding of Boyles law which summed up in this situation basically boils down to volume in = volume out. Now as the engines reach lower ambient temps, they are able to run a higher EPR and thus compensate for the loss of thrust that would exist if a constant EPR was kept. So whithout outside influence of throttle increase how is this possible? Does the 727 have better fuel control requiring less manual inputs?

Your understanding of EPR isn't wrong, it's exactly as you describe. Nor is EPR physically related to thrust lever position. However, you've assumed thrust doesn't decrease with altitude, but it does exactly that. That isn't a problem for aircraft performance as drag decreases too. There's nothing particularly special about the JT8D control system, quite the reverse, but as it happens you don't need to adjust thrust lever setting much to maintain the rated climb EPR. The EPR the engine produces at that thrust lever position will change as the aircraft climbs, but so also will the climb EPR setting, and the two roughly keep in alignment at the same lever angle.

Throttle position is not directly related to fuel flow in a jet either, if you leave your throttle position constant and climb, your engine speed will vary a little but your fuel burn will decrease. EPR could remain similar, but you have less air to pressurize the higher you go. It is simply a pressure reading not a thrust amount.

 

No, EPR is actually not a pressure reading, it's a ratio of two pressures, and changes of air density at different altitudes affect both pressures (exhaust press in the numerator and compressor press in the denominator), so EPR remains a reliable metric to measure thrust relative to maximum thrust at that air density throughout the altitude envelope. All measures of thrust (RPM or EPR) for a turbojet/turbofan are relative and not direct measures of thrust, which decreases with air density.

 

Regards

right, ratio, not raw pressure reading, thanks!