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Old 1st Jan 2011, 20:49
  #56 (permalink)  
Join Date: Oct 2005
Location: USA
Posts: 3,218
I'm somewhat surprised that none of my textbooks make any reference to "Inlet Drag" as defined by thrust reversal systems.

I might have to revise my thinking!
This is the typical reaction of most pilots when discussing the effects of ram drag or inlet drag with respect to reverse thrust. Most haven't been taught this, and react with distain; if it's not in my textbook, then it can't be true.

Then again, ask how many have been taught the basic thrust formula; gross thrust minus ram drag equals net thrust. Nearly none. The basic formula is all one needs to knkow to understand the effects of taking away the thrust; one is left with drag.

The common argument is that airplanes back up on reverse, therefore reverse airflow must be the primary mechanism of the retarding force during landing. This is a nonsensical argument, but it's commonly used. Thus, the "I've never heard of it" camp backs themselves up with "I can back up."

Certainly where redirection of exhuast or fan gasses can be used as a contributing factor without causing re-ingestion problems or aerodynamic problems, a manufacturer will do so. After all, there's no reason not to use every available element of force to retard the airplane during landing. That the re-directed airflow isn't the primary mechanism for slowing the airplane, however, is misunderstood; largely misunderstood because many seem to believe that if they don't know it or haven't heard it, then it can't be true.

Manufacturers generally don't go into an explanation of ram drag or inlet drag any more than they present the basic thrust formulas. Operationally, the pilot is more interested in when thrust must be applied and when it should be discontinued during the landing. Re-ingestion and FOD issues are much more compelling operationally than the base force which exists with or without pilot input. The drag is always there; removing the rear thrust component (or most of it, in the case of re-directing or blocking fan thrust, because jet thrust remains) does nothing more than allow the same force that was already present, to still be present.

Increasing the power setting increases the drag. It would increase thrust, but thrust has been blocked or divered; what we get is a drag rise.

By utilizing various cascade vanes, blocker doors, and other means, some component of the re-directed gas or airflow can also be used to slow the airplane on landing. The term "reverse thrust" misleads one to believe that this is the primary mechanism for slowing, when it is not. In fact, many bucket systems and reverser systems don't direct the fan or exhaust gas path forward at all, or very much forward at all. The retarding force we feel during reverse activation is much more simple than what we tend to think; we've done little more than remove the net thrust component of the equation, allowing what was already present, to remain. Our biggest concern as we slow is to protect the engine and to prevent FOD damage and watch for a compressor stall.
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