How does reverse thrust ingest FOD at low speed?

[AirBadger]

I understand that on jets you disengage reverse thrust at a certain speed like 60 knots to prevent fod ingestion, but how exactly does fod get in the engine with the reverseres deployed? On most turbofans bypass air is deflected outward, away from the engine so you would think that fod would get blown away?

 

[Amish RakeFight]

Thrust reversers need to be stowed below a certain speed so that FOD isn't ingested at the turbine inlet. This is due to the deceleration of the aircraft and the speed at which the TR's are moving the thrust forwards.

When you activate the TR's, some of the thrust ends up moving forward. Once the plane has decelerated to a slow enough speed, the reverse thrust ends up moving forward and ahead of the aircraft causing any debris in front of the engines to be stirred up. This is where the FOD issue comes into play.

 

[DrProc]

Watch airplanes land on a wet runway and you will understand. The water on the pavement acts like a piece of metal, rock, etc would act when the engines are in reverse.

 

[Fedora]

The air thrown ahead of the aircraft engine will throw debis along with it. The description of the wet runway will help you visualise it very well.
At higher speeds, the aircraft "overflys" if you will, this debris. (going too fast to suck it up). At slower speeds, the debis is thrown right out in front of the intake where the engine can move into the debris cloud and pick up a lot of possible damaging items. High speed taxiways are particularly bad spots for this because a lot of debris is naturally thrown off to the edges of the runway by departing and landing aircraft.
Reverse idle is often the limit below 60 kts and while taxiing.
Jet engine reverse is typically most effective at higher speeds anyway.

 

[avbug]

I understand that on jets you disengage reverse thrust at a certain speed like 60 knots to prevent fod ingestion, but how exactly does fod get in the engine with the reverseres deployed? On most turbofans bypass air is deflected outward, away from the engine so you would think that fod would get blown away?

Airbadger,

FOD is Foriegn Object Damage, and fod material doesn't come from the engine. It's stirred up or picked up by the engine. Some older airplanes such as 200 series 737's used bleed air ahead of the engine to blow dust and gravel aside during taxi, takeoff, and landing. You still occasionally see these in locations where the aircraft operate from rough condition fields.

Operation of reversers at lower speeds can also result in ingesting exhaust gasses, which can lead to rough engine operation or even a flame-out in unusual circumstancs.

The exhaust stream diverted during reverser operation isn't really there to provide braking action. Accordingly, it isn't reversed to provide thrust forward. Generally some degree of forward component of thrust does exist, though not always, and the diverted flow isn't intended for that purpose. It's diverted to prevent it from driving the aircraft; it's removing forward thrust.

Useable thrust is what's coming out the back of the engine, the nozzle or tailpipe jet effluence thrust, minus intake drag. An engine that produces five thousand pounds of thrust but has two thousand pounds of intake drag loss...produces three thousand pounds. Block the thrust coming out the back and all you have left is the two thousnd pounds of intake drag...and that's what is used to slow the aircraft...not the diverted airflow. It's a combination of taking away the thrust by diverting it, and then allowing intake drag to have it's effect.

Diversion of the airflow, rather than merely blocking it, is necessary because the exhaust gasses must go somewhere, and because it prevents a compressor stall.