Why are thrust reversers ineffective at low speed?

[Mukka]

If I may make an analogy - what would you expect to have the biggest effect on speed - opening a car door at 5 mph or 70 mph?

No equations necessary!

[Checkboard]

Mukka - there is a difference between drag and thrust.

[Mukka]

Checkboard,
I think I've fallen into the trap of thinking that reverse thrust pushes against the surrounding airflow, the push being more effective at higher speed.

I thought the propulsive efficiency of the jet engine is greatest when there is a small difference in speed between the jet exhaust and the surrounding airflow, because the "push" is most effective then.
This doesn't make sense any more though!

Are you saying that thrust decreases with speed because the massflow through the engine decreases?

[Snakum]

A question about activating reverse thrust in large aircraft ...

It appears one would go to idle thrust at touchdown (mains), and following nose-wheel touchdown (I'm assuming a squat switch) the levers are pulled up and back to activate thrust reversers with the engines at idle.

Is this the correct GENERAL sequence?

Is it common to spool the engines back up while in reverse thrust (other than using RT for pushback) for short runway, for contaminated runway, or other possible over-run situations?

Just curious ...

Minh

[Checkboard]

It appears one would go to idle thrust at touchdown (mains),

Well, usually at some height above touchdown, like 30 feet or so...

and following nose-wheel touchdown (I'm assuming a squat switch)

Actually it varies, on the 733 it is at the earlier of 10 feet on either radalt, or air/ground logic (i.e. squat switch) activation, and the thrust levers at idle (the reverse levers won't rise unless the thrust levers are at idle.

the levers are pulled up and back to activate thrust reversers with the engines at idle.

Yep.

Is it common to spool the engines back up while in reverse thrust

This depends on company procedures. Using reverse thrust puts a little stress on various aeroplane bits, so some companies ask for idle reverse, unless the situation demands othersie (like a wet runway). An interlock will hold the piggyback levers at the first detent until the reversers are at least 60% deployed, after tat you can pull the levers back to increase reverse thrust. The amount of reverse thrust permitted to you varies with the aircraft and engine combination.