Is it me? I'm never sure if Peter really means what he says.

pj747 said: The reversing thrust on the C-17 goes back through the engine... ???

pj747 then said: Its a statement, unrealted to the picture, other than the fact the topic in indeed about the C-17 Globemaster III

Hummmmmm???  :-/

Peter,

The air from the reverse thrust does not go through the engine. If it did, in any great quantity, it could cause the engine to compressor stall. Most modern large-fan jet engines only reverse (deflect) the fan air, the bulk of the thrust. Smaller engines and older large fan's reversed the fan and the turbine air. This caused too many problems with stalls and unbalanced air flow through the engine. Pilots are instructed to be at idle thrust by 80 knots, so the deflected air is not brought back into the inlet, along with other stuff like dirt or stones that could get sucked into the inlet - not to mention baggage bins!  :P

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boeing247 asked: Lou-- did many propliners have reverse thrust?

All of the turboprops alter the blades pitch to control the thrust of the "disk." There is a position called beta where the thrust is about zero. In flight beta can produce a lot of drag since the diameter of the propeller (disk) produces drag equivalent to a solid disk. You can hear the sound produced by the spinning disk as it goes through this phase. The C-130, DHC-6 and most turboprops can use this ability to rapidly alter the blade angle for fine control of the thrust including reverse to back up the plane. Older prop planes also had the ability to change the blade angle and produce reverse thrust. Planes like the Boeing 337 with the P&W-4360 had to be treated with kid gloves when using a lot of reverse so as not to fry the engine or foul the plugs.

Here is a video of a C-17 backing up after landing at an airshow. The C-17 reverse is deflected up and to the side so as to avoid sucking-up stuff off the runway.  In the airline industry doing something like this would cause the CEO to have a heart attack since the chance of engine damage doing this is very high. Wait a minute, CEO don't have a heart so that could not happen!  ;D

C-17 backing up after landing...
http://www.youtube.com/watch?v=T9rdzqOgNrA

A few related videos...
C-17 landing in Afghanistan on a dirt runway. What do you think the descent rate was on touchdown?
http://www.youtube.com/watch?v=diX3x6ffvSA&feature=related
Boeing 737 taxing in snow. You can see the area in front of the engine where stuff is sucked into the inlet.
http://www.youtube.com/watch?v=L1DkUOOfDFk&feature=related
Short video showing the fan blocker doors in operation on a large fan engine.
http://www.youtube.com/watch?v=92ixq3iHFxQ&feature=related

Lou