What is "turbo-prop"???

[UnAnswerd]

Someone basically described it as a jet engine operating a conventional propeller. Can anyone be more specific about the internal workings of this design??? Also, were there ever any conventional piston-engine aircraft that utilized pressurized cabins and were capable of operating above say 25,000' MSL??? Just interested...

 

[FL000]

That is basically correct. Here's a simple diagram:

http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Oliver_Fleshman/3-36.jpg

 

[idleclamp]

That's pretty much the jist of it. There are two primary designs, Garrett and Pratt & Whitney. Garrett uses a gearbox to to slow down the revolutions of the turbine to get them to optimum prop speed. Example: A TFE-331 spin about 41,270 rpm (depending on series) at 100% power, but the prop is spinning at 2000 rpm at 100%. They're what you call "direct drive", which basically means if you spin the prop, you spin the turbine. Consequently though, the gearbox weighs more than the engine does. As for the Pratts, they have a free spinning prop. In layman's terms, the prop is not connected to the engine. Instead the engine (on all models that I can think of right now, i.e. King Airs, except the B100, which has Garretts) is put in backwards, and the exhaust pressure is used to spin a free wheel turbine connected to the prop. In theory, you could start a Pratt, hold the prop during start, and keep it from spinning. Not so with a Garrett.


As for piston airplanes above FL250, yes they do exist. But they all have turbo-chargers, similiar to a car (in principle). A couple of examples would be Cessna 414A, and 421. They are cabin-class piston twins with pressurized cabins, that can go at least to FL250, if not above. Hope this helps.

 

[banned username 2]

This might help too, its a little crude, but may clear things up a bit...

http://www.grc.nasa.gov/WWW/K-12/airplane/aturbp.html

http://wings.avkids.com/Book/Propulsion/Images/trboprp.jpg

http://www.aircraftenginedesign.com/abe_right4.html

http://www.aerofiles.com/turbos.html

Hope this helps...http://www.grc.nasa.gov/WWW/K-12/airplane/aturbp.html

 

[minitour]

So whats the point?

Why not take off the prop, vent the exhaust out the back side and make it a jet?

Isn't that the idea? You compress, add fuel, ignite the air/fuel mix, and the exhaust has nowhere to go out the back and gives you thrust? Or is that rockets....

-mini

 

[Rally]

Props are more efficient at lower altitudes then regular jet/turbofans. Thats why you see alot of regional's with short flights flying turboprops, no time/distance to climb a take a advantage of pure jet/turbofan engine likewise jets are really inefficient at low altitudes. Someone can probably post some fuel flows, you'd be surprised what a jet burns just taxing vs in cruise at altitude. In a nutshell props low altitude and jets high altitude. Think distance of the overall flight, is it worth it to climb? For instance a flight from say Key West to Fort Myers is say 100 miles. By the time a Jet climbed a efficient altitude it would already be time to descent, a t-prop could just climb to its "low altitude".

 

[Rally]

Its late and I'm tired excuse grammar. Hope the grammar police are not up.

 

[banned username 2]

Why not take off the prop, vent the exhaust out the back side and make it a jet?

Turbo-Props also use a smaller jet core (plus an efficient prop) than what would be required to derive the same thrust if used as a pure jet. By using a smaller core, they are burning less fuel and therefore much more efficient.

 

[enigma]

As for the Pratts, they have a free spinning prop. In layman's terms, the prop is not connected to the engine. Instead the engine (on all models that I can think of right now, i.e. King Airs, except the B100, which has Garretts) is put in backwards, and the exhaust pressure is used to spin a free wheel turbine connected to the prop. In theory, you could start a Pratt, hold the prop during start, and keep it from spinning. Not so with a Garrett.

Idleclamp, I'm sure that you know what you meant to say, but I think that you might have lost something when you put it in laymens terms. Please let me try and make a small clarification.

The prop is most definitely connected to the engine. If memory serves, (it has been 12 years since I flew a PT-6), the first stage turbine is connected to the compressor section, and the remaining power extraction turbines are connected to the prop-reduction unit. This is much the same as a twin spool jet where the first stage turbines (N2)are connected to the high pressure compressors. They are connected with a hollow tube which is the outer of a concentric shaft arrangement. The inner concentric shaft connects the low pressure turbines (N1)with the low pressure compressors. The two sections are not physically connected.

regards,
enigma

 

[idleclamp]

Idleclamp, I'm sure that you know what you meant to say, but I think that you might have lost something when you put it in laymens terms. Please let me try and make a small clarification.

The prop is most definitely connected to the engine. If memory serves, (it has been 12 years since I flew a PT-6), the first stage turbine is connected to the compressor section, and the remaining power extraction turbines are connected to the prop-reduction unit. This is much the same as a twin spool jet where the first stage turbines (N2)are connected to the high pressure compressors. They are connected with a hollow tube which is the outer of a concentric shaft arrangement. The inner concentric shaft connects the low pressure turbines (N1)with the low pressure compressors. The two sections are not physically connected.

regards,
enigma

Well, I've never flown a Pratt turboprop before, so I'll take your word for it. I was just relaying how it was explained to me years ago. Thanks for the lesson. But if you're going for NOISE, Garrett is definitely the way to go. I always loved watching people put their fingers in their ears when I taxied onto a ramp. That's what we called the "Garrett Salute".