3rd turbine in the tail?
 

This is probably the first time I've posted on this forum, but had a curiousity that I wanted to lay to rest. On some aircraft there is a 3rd turbine in the tail section of the airplane.. I've heard (from where, I can't remember) that this is primarily used to generate power for the aircraft, and is not necessarily used for thrust. Is this just myth or is it actually used to power the aircraft while in flight, etc? Thanks in advance!
Joel

Hello mandenbar,

It is what we call a Auxillery Power Unit (APU).
It's primary use is to provide the aircraft of electrical power and air to run the airconditioning packs and to start the engines.

The unit is mostly used on the ground but can also be used in flight as a back-up when one of the engine generators fails.

hope this helps.

regards Vincent

Well there's always the L-1011 or B727 but the "extra turbine" on those is a bit easier to spot ;)

We spell it "Auxiliary". I've also not heard of any that provide thrust that is used to power an aircraft. However, that does not mean to say that no thrust is generated - I'm sure it is, it's just that it's so relatively small it is of no consequence.

I believe its like a car's exhaust, any "propulsion" coming from the APU is just exhaust, the inlet of air is important to gain the power from the movement of the turbine.

yup the 3B used a single Rolls-Royce RB.162-86 engine for take off boost, it had just less than half or each of the main 3 engine's power ( 5,250lbs) .
visible here:
http://upload.wikimedia.org/wikipedi...dent_three.jpg

They certainly did. The engine was a bit binary though: On & "Full Welly" or Off. Apparenty, following a rejected takeoff, this thing was still going full tilt. It was somebody's job to remember to turn it off after such an event.

And the noise was impressive too

Yes , all the compressor blades were glass fibre. It was amazingley light weight as well, Rolls designed it as a VTOL engine. Those were the days, did my apprenticeship on the Trident.

Thanks for the info guys, my question has been answered :)

-Mandenbar

Another aircraft which uses the APU for forward thrust is the BAe 145, in fact it uses all Five of them!! :E

I know this thread has been a bit on the humorous (humourous??) side, but my recollection is that the 747-200 actually took a bit of credit for TO performance on the APU, and if APU were not operating there was a slight (250kg???) penalty.

Not sure if that applied to other 747's.