PPRuNe Forums - View Single Post - reversers and a/c speed
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Old 1st Jan 2011, 16:28
  #50 (permalink)  
PBL
 
Join Date: Sep 2000
Location: Bielefeld, Germany
Posts: 955
Intuitively the mechanics of reverse thrust are simple.

If we are talking braking action, we are talking forces along (roughly) the direction of the length of the fuselage, the x-axis in airplane-based coordinates. Braking means generating some force in the negative-x direction.

Let's consider how this is done by a jet or turbofan engine in the "reverse thrust" regime. You are sucking a lot of air in through the front of the duct, in the negative-x direction, and you are reducing its velocity in the x direction effectively to zero (you are chucking it back out perpendicular) or slightly negative (you are chucking it back out a few degrees forward). That momentum-reduction produces a sizeable force in the negative-x direction, which is experienced as braking force.

If you suck air in faster, there is more momentum to reduce to zero, and you get correspondingly more force if you so reduce. There are two ways in which you can suck air in faster. One is by having a faster free airstream velocity and absorbing it; the other is by revving your engine faster. The first explains why TR is more effective at higher ground speed; the second why you increase power on the engine to achieve higher braking.

It seems to me to be mostly definitional what you call this momentum-stopping force. If Guppy wants to call it "inlet drag", then I guess I'll call it "Annelise Merriweather" and invite him to explain why his term is more apposite than mine.

This business of a "disc of air" generating some kind of resistance seems to me to be poppycock. As usual, being a bit familiar with basic physics helps enormously.

(Of course, pedants could calculate how big a flat circular plate would have to be to generate the equivalent force simply by pure drag as the engine in "reverse-thrust" regime generates, and some PPRuNe wit could say "see, *that's* the circular disc of air generated by reverse thrust!" But that's just the fun of PPRuNe.)

PBL
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