doldi1989web.de Posted November 28, 2018 Posted November 28, 2018 There Is a system which prevents Thrust reduction at M 1.23 and higher. Why there Is such a system? Couleur someone explain this to me?
Eddie Posted November 28, 2018 Posted November 28, 2018 It’s to prevent engine damage/compressor stalls and surges due to high speed/high airflow. Pretty much all modern fast jets have such a feature in the engine control system.
doldi1989web.de Posted November 28, 2018 Author Posted November 28, 2018 Could I imagine this as a preventiom from Airflow-Damage because the Thrust that Is given stabilizes the RPM that the Engines can handle with the massive airflow? If there won‘t be such a system, the compressor-blades are breaking?
Eddie Posted November 28, 2018 Posted November 28, 2018 The thrust isn't overly relevant, its the engine RPM/airflow in to the engine that is important. The engine control systems control the engine RPM through fuel flow, the. Thrust produced is a effect of that. The compressor blades would be highly unlikely to break, what is more likely are things such as a flameout or compressor stall due to unstable airflow.
doldi1989web.de Posted November 28, 2018 Author Posted November 28, 2018 Ah ok! Cause of reduced rpm of the blades there Is a Compressor stall in effect the high flow in Contrast to the lower Rpm?
Banzaiib Posted November 28, 2018 Posted November 28, 2018 (edited) Airflow relative to the face of the compressor must be subsonic. Supersonic air is compressible, which means the comrpessor would stall and could damage the engine. The F-18 does not have variable geometry inlets to adjust the shock waves to modulate the intake airflow like the F-15 and F-14 have. This is why there's a cap of Mach 1.8 at altitude, and why there is a thrust reduction at lower altitudes when approaching mach 1.23. Even when the SR-71 is moving at mach 3, the airflow to the compressor face is sub-sonic. This is achieved by adjusting the position of the iconic cones that stick out from the front of the engines to change the shape of the shockwaves. Edited November 28, 2018 by Banzaiib
Banzaiib Posted November 28, 2018 Posted November 28, 2018 (edited) The reasoning behind this was, if I recall correctly, to reduce maintenance, increase engine lifespan, reduce cost and reduce weight. What's really the utility of going above 1.8 in 99% of the use of the jet? The faster you go the more fuel you burn and the Legacy bug doesn't have big tanks to sustain that anyway. Add payload and the drag penalty goes up, the fuel burn goes up and the whole thing just compounds itself. Yep, I recall reading about the inlet geometry being quite a hassle for some F-14 crews... they'd get stuck closed, potentially affecting takeoff performance... the crews would mess with the circuit breakers... etc. etc... F-18 was supposed to be easier and cheaper to maintain... makes sense to me... then again, so was the space shuttle :) Edited November 28, 2018 by Banzaiib
DirtySanchez Posted November 30, 2018 Posted November 30, 2018 This is why there's a cap of Mach 1.8 at altitude, and why there is a thrust reduction at lower altitudes when approaching mach 1.23. There's no thrust reduction approaching Mach 1.23. The FADEC/DEEC/EEC/whatever it's called in the Hornet prevents the engine from operating below MIL when faster than Mach 1.23. Big difference. As Eddie already explained, this protects the engine from compressor stalls, surges, and flameouts that would likely occur if the engine were allowed to decelerate under those conditions. The jet will still decelerate at MIL, and once slower than the threshold the engine will respond to throttle positions below MIL.
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