Hi guys,
I had a couple of questions in my AGK exam that I thought I had my head around, but according to my KDRs, I guess I don't.
Two questions were quite similar, and they related to reducing the rpm in a turbocharged aircraft with an automatic wastegate, and a CSU.
The first question said the aircraft was below it's critical altitude, and asked the effect on manifold pressure if the RPM was reduced, with no movement of the throttle.
According to the AGK book on page 2.10, when the RPM is decreased with no throttle movement, the manifold pressure gauge will show a slight increase.
My thinking was that when the RPM was reduced, there would be a slight rise in manifold pressure, however the absolute pressure controller would sense this, and the wastegate would open to reduce the speed of the turbine and compresser, thereby maintaining the manifold pressure. The aircraft is below it's critical altitude, so the turbocharger should be able to maintain upper deck pressure. So the answer I chose was that the manifold pressure would stay the same.
Looking at the books now, I believe this thinking was wrong because I didn't take into account that upper deck pressure remains constant, and the manifold pressure (between the throttle butterfly and inlet valve) is controlled by the throttle position. Does that mean that the turbocharger is irrelevant in this instance, and the MP would show a slight increase?
The second question was nearly identical, except in this case the aircraft was above it's critcal altitude. Would the MP still increase slightly with a decrease in RPM?
Thanks,
Craig
