Originally Posted by billt460
Originally Posted by Astro14
Bill - just landed from Manchester, so just saw this...The reverser on that engine type operates by placing several small blocker doors in the fan exhaust when the sleeve slides back. That fan flow is directed forward through the grating that you see in the picture. Core exhaust is still directed rearward.
I wondered about that, but wasn't sure. So, at full reverse thrust, the fan portion of the thrust output is required to "fight" the non redirected core jet thrust, which continues rearward. On a large turbofan engine like the 747 uses, about what percentage of the core jet thrust accounts for the total thrust of the engine?
Looking at those big turbofans it doesn't look like there is any way they could incorporate any type of clamshell to redirect it.
Yeah, a clamshell only works on a narrow engine, like a turbojet or low bypass turbofan.
I know the bypass ratio on the P&W 4056 (which our 747-400 were powered by) is about 5:1, so the airflow through the fan is 5 times that of the core, but I don't know if that means the ratio of fan/core airflow, or ratio of fan/core thrust, is anywhere near that during reverse.
I suspect the ratio of thrust far lower, as the core is still a straight path, while the fan is directed at a 45 degree angle around the engine, and I don't know the effect of the reverse blockers on N1 vs. N2 speed. An engineer who works on these could tell you.
Still, in all, it's a net reverse thrust. But it's nowhere near as effective as forward thrust.
Compressor stalls can cause damage. Fundamentally, a compressor stall happens when the smooth airflow through the engine is disrupted. Each of those blades, be they big, in the fan, or smaller in the later stages of the compressor, is a wing. Turbulent airflow causes those little wings to experience a stall, they don't create the pressure that they did, and the air stops moving in the direction it was moving (and supposed to be moving).
When the airflow is disturbed, you get all sorts of negative things happening. The turbine is subjected to uneven heating, and parts of it can greatly exceed the limit temp, damaging it. The compressor and fan experience shockwaves as the flow reverses, and that shock can damage them.
Most compressor stalls are caused by the ingestion of turbulent air. During reverse thrust. In a heavy crosswind at low speed. Very high, or very low AOA. During missile or gun firing in a fighter. Anything that causes the normally smooth flow through the inlet to the face of the engine to be disrupted. Turbine engines like smooth air at the compressor face. It has to be subsonic as well, which is why you see complex inlets on high speed airplanes.
Compressor stalls can be caused by mechanical problems in the engine itself. Variable stair vanes that fail to respond to changing airflow, the wiring amount of fuel for the operating conditions, that sort of thing.
Often, in a modern, FADEC engine, normal engine operation can be restored by reducing thrust to idle. Reduced fuel flow greatly reduces the pressure, and the amount of airflow, inside the engine, and those blades come out of stall.
Sometimes, though, you have to shut the engine down. Removing combustion allows the air to simply move through the engine (assuming in flight), cooling it and re-establishing a normal rotation relationship between N1 and N2 and a normal flow through the engine.
So, from a pilot perspective, if you have a compressor stall, you start by moving the throttle to idle. If it works, you slowly advance the throttle towards desired power. If it doesn't work, you shut the engine down, and let it cool/stabilize. Then you restart and see if it runs.
If there was no damage, it should run normally. If there was damage, it will let you know, and you either operate it at reduced power or shut it off completely.
![[Linked Image]](https://upload.wikimedia.org/wikipedia/commons/5/52/GE90_blades_dsc04669.jpg "[Linked Image]")
