Originally Posted by
bvcu
BVCU and system can cause lack of power as i've already posted .You can have correct rpm and jpt but low thrust if bleeds open. I suspect it was discounted as the power was adequate for takeoff which i would have thought proved things with a relatively short runway , tailwind and warm day.
Re the above, if the bleed valves are open, that will produce a higher JPT for a given rpm. Thrust (mass flow rate) is directly related to the compressor delivery pressure and the fuel flow (given fixed compressor and turbine geometry). Open compressor bleeds reduce the compressor delivery pressure (CDP) for any given selected throttle position, this results in the accel control unit scheduling an increase in fuel flow, through the servo fuel system to produce a CDP increase to restore the mass flow rate. An engine producing a given thrust with bleeds open will have a higher rpm and jet than the same engine at the same thrust with the bleeds closed. I've seen this on testbeds on the Avon & Spey, and a number of other engines