Turbo-prop Vs. Turbo-Jet

[no1pilot2000]

What is the difference between how a turbo prop and turbo jet creates its thrust? Also, how does the reverse thrust mechanism work on each aircraft?

 

[Weasel Keeper]

A turbojet throws the thrust out the exhaust, where a turboprop gets it's thrust from the prop. TJets are much bigger engines so they can use the thrust from the exhaust. Turboprop engines (also jet engines) are generally smaller, but are geared through the Prop Gear Box to create good prop speed.

Reverse on turboprops...the prop blades adjust the pitch opposite by the control levers through a pitch control unit...creating foward thrust which acts as a brake (or reverse if sitting still). Turbojet reverse..."clam shells" actuate behind the exhaust to direct the engine thrust foward and stop the foward thrust...acting as a brake or reverse.

 

[NoPax]

http://www.aerospaceweb.org/question/propulsion/q0033.shtml

Has information and diagrams on turbo-jets, turbo-fans, turbo-props, ram-jets and turbo-shafts.

In a typical turbo-prop, 85% of it's thrust is produced by the exhaust-gas driven propeller, and the remaining 15% of the thrust is produced by the jetcore. The gearbox that Weasel Keeper was referring to is to prevent the prop from turning at a very high RPM.

How a turbo-jet works -

The key to making a jet engine work is the compression of the incoming air. If uncompressed, the air-fuel mixture won't burn and the engine can't generate any thrust. Most members of the jet family employ a section of compressors, consisting of rotating blades, that slow the incoming air to create a high pressure. This compressed air is then forced into a combustion section where it is mixed with fuel and burned. As the high-pressure gases are exhausted, they are passed through a turbine section consisting of more rotating blades. In this region, the exhausting gases turn the turbine blades which are connected by a shaft to the compressor blades at the front of the engine. Thus, the exhaust turns the turbines which turn the compressors to bring in more air and keep the engine going. The combustion gases then continue to expand out through the nozzle creating a forward thrust

 

[avbug]

As a student pilot, your time and effort studying the aircraft you'll be flying might be more profitable. Regardless, if you begin to study aircraft and have a question about something that isn't fully explained or covered in your books, that's one thing. If you begin asking basic questions that you could research and find the answer to yourself...that's another.

Do some work, put in some effort, and then ask the question.

 

[Weasel Keeper]

NoPax said

The gearbox that Weasel Keeper was referring to is to prevent the prop from turning at a very high RPM.

Wouldn't that be the over speed governor (ummm...shaaaah!)? The PGB transfers engine power to the prop. Why would a gearbox prevent a prop from turning at high RPMs? That would be like your car's transmission only preventing high RPMs and providing no foward gearing motion. A PGB is the transmission from a jet engine to your prop thrust. Have you seen a turboprop PGB? It's awfully big just to limit RPMs...

Some times an A&P is a good thing to have...

 

[Mr Wu]

NoPax said

Wouldn't that be the over speed governor (ummm...shaaaah!)? The PGB transfers engine power to the prop. Why would a gearbox prevent a prop from turning at high RPMs? That would be like your car's transmission only preventing high RPMs and providing no foward gearing motion. A PGB is the transmission from a jet engine to your prop thrust. Have you seen a turboprop PGB? It's awfully big just to limit RPMs...

Some times an A&P is a good thing to have...

The PGB is also a reduction gearbox, preventing the prop from turning at 30000 rpm. When I was in A&P school, I learned that 30,000 rpm is a very high rpm. Yes, an A&P is a good thing to have but so is reading comprehension. The reason that the PGB is there is to reduce the RPM of the prop and to increase the available torque. You could mount the prop directly to the output shaft of the engine but it would come apart due to the high centrifugal forces caused by the rotation speed. If it didn't come apart, no torque would be produced.

The governor limits the speed of the prop but in a lower range, usually lower than 10% of the engine speed. If the governor fails there is no way that the prop will reach the same speed as the engine due to the reduction in the gearbox.

 

[avbug]

Typical operating speed for a turboprop gas generator section is around 30,000 rpm. Typical propeller speed is approximately 2,000 rpm. The reduction is not to prevent the propeller from flying apart at high RPM's. The reduction is purely for the purposes of propeller efficiency. The propeller is an airfoil, just like the wing, and is subject to aerodynamic limitations. Among those are speed limitations. Speed is greatest at the propeller tips, and it is the tip limiting speed that is partially responsible for establishing the propeller RPM limitations.

Blade strength limitations are part of that, but the primary reason that the propeller is limited is a combination of blade harmonics and aerodynamic efficiency...both of which are closely tied together.

 

[NoPax]

The PGB is also a reduction gearbox, preventing the prop from turning at 30000 rpm. When I was in A&P school, I learned that 30,000 rpm is a very high rpm. Yes, an A&P is a good thing to have but so is reading comprehension. The reason that the PGB is there is to reduce the RPM of the prop and to increase the available torque. You could mount the prop directly to the output shaft of the engine but it would come apart due to the high centrifugal forces caused by the rotation speed. If it didn't come apart, no torque would be produced.

The governor limits the speed of the prop but in a lower range, usually lower than 10% of the engine speed. If the governor fails there is no way that the prop will reach the same speed as the engine due to the reduction in the gearbox.

Thank-you.

30000 RPM to 2000 RPM, is a reduction of RPM of 15:1, for all the reasons Avbug stated...did I miss anything...if I did, sorry.

Now to the original poster...In the airplane you are learning, is the engine RPM faster, slower or the same as the propeller RPM? Why?

 

[pgcfii2002]

Turbojet...100% of the exhaust is shot out the back and creates....thrust.
Turboprop....the exhaust drives the turbine blades which drive the propeller (?)

 

[Weasel Keeper]

I stand corrected...and would definately not want to stand next to a prop turning at 30,000RPM.