Turboprops

[DanTheMan]

We have been studying aircraft propulsion systems in my thermodynamics class the past two weeks, and i just had a quick question about what my professor said about turboprops.

In a turbofan engine the turbine is designed to provide just enough power to run the compressor, the fan, and any auxillary equipment. The rest of the combusted gases exit out the nozzle as thrust. I understand that no problem. Then my professor says that a turbo prop is designed the same way, only there is no cowling over the fan. Meaning that it is still powered by thurst and the prop on the front just gives it a very high bypass ratio. I had always thought that in a turboprop, the turbine was designed to convert all of the energy from the combusted gases to turn the prop through a gear reduction and the thurst is negligeble.

any turboprop pilots care to help? thanks

[FL270]

It's been a while since Turbine Engines class (or my last trip to Simuflite for that matter) but if memory serves the bulk of the thrust generated by a turboprop engine comes from the prop. A small amount (I won't quote a percentage since I can't remember the exact number) is created by the exhaust. In a turboprop engine, you have the same intake, compression, and combustion as you do in a turbofan. When the combusted air passes through the turbine section, it's turning a propeller as well as the compressor before being exhausted. In a way it's similar to the turbofan, in that the combusted air flowing through the turbine section is turning both the "fan" section and the "power" section of the turbofan engine. Hope this makes sense ... like I said, it's been a while since I've been in the books on it!

FL270

[DanTheMan]

[ QUOTE ]
if memory serves the bulk of the thrust generated by a turboprop engine comes from the prop. A small amount (I won't quote a percentage since I can't remember the exact number) is created by the exhaust.

[/ QUOTE ]

that's what i figured. I'll have to correct my professor, that will be fun because the guy is an ass. Thanks.

[lilrkt]

Another way to look at this is that the exhaust gases on a turboprop exit from an exhaust pipe not usually pointed rearward.

[FL270]

What turboprops would those be? The PT6's on my King Air exhaust rearward, as do the engines on most turboprop commuter aircraft I know of.

In the case of the PT6 the engine is "reverse flow" ... meaning the movement through the engine is "back to front" ... but the air still enters and exits in the conventional direction ... the inlet duct is under the prop spinner ... the inlet is ducted back and makes a 180 degree turn to enter the engine from the rear, move forward, then make another 180 degree turn to exit through the exhaust stacks.

FL270

[Mr_Creepy]

They're all pointed rearward from what I've seen, even in bass ackwards mountings like the PT6.

Your instructor is sort of right though, think of a Turboprop as a high-bypass Turbofan, just with no cowling over the fan (the prop).

Turbofans are fancy turboprops. In the CRJ 80% of the air goes through and 90% of the thrust thrust comes from the fan. The fan is more efficient for thrust than straight exhaust, especially at speeds in the mach .70 to .84 range.

Straight turbojets do better at higher speeds, or so I am told - never flew one!

[FlyChicaga]

Yup, very little of the thrust is generated from the turbine exhaust. I can't remember how much, but in the 5-10% range.

[ananoman]

For turbofan engines you can usually get a rough estimate of the thrust generated by the fan vs. the engine core by looking at the bypass ratio. It is not exact, but if you have a bypass ratio of 5:1 then you can count on about 80+% of the thrust coming from the fan.

With turboprops there is still some thrust generated by the exhaust. This is why you will often see the term 'equivalent horsepower' or 'ehp' for short. If shaft horsepower is used to rate a turboprop engine it may not give a true indication of the engine's power. Ehp takes into account the shaft horsepower and the thrust from the exhaust. Remember that nothing is perfect and it would not be feasible to recover all the engines power and channel it to the prop. Even in recip. engines there can be alot of wasted power in the exhaust. The last of the great recips were the 'turbo-compound' radials like the Wright R3350 and P&W R4360. They used power recovery turbines to recover in excess of 500 hp from the exhaust and feed it back into the prop.

In a straight turbojet 100% of the engines thrust comes from the core. In these engines there is alot of 'extra' air that is accelerated through the core and this can be tapped off as bleed air and used for pressurization and deicing, with little loss of thrust. With turboprops and high bypass turbofans taking bleed air from the core can cause a noticable loss of power. This is why it may be necessary to do a 'no packs' take off. It is also common on smaller jets to not use bleed air for deicing the wings. Hawkers use TKS and some Citations use a combination of heat (not sure if it is bleed air or electric) and pneumatic boots (heat is used inboard, so ice chunks are not thrown into the engines).

[DanTheMan]

Now when you guys all say that the fan is producing 80%-90%, this is at cruise flight right? Is it different at lower altitudes like takeoff for example?

[aloft]

[ QUOTE ]
I had always thought that in a turboprop, the turbine was designed to convert all of the energy from the combusted gases to turn the prop through a gear reduction and the thurst is negligeble.

[/ QUOTE ]Half-right. Thrust is negligible, but not all turboprops put "all" the energy to work. Look up the difference between direct-drive and free-turbine designs sometime.

[ QUOTE ]
I'll have to correct my professor, that will be fun because the guy is an ass.

[/ QUOTE ] That's a bright idea....not.

[raysalmon]

[ QUOTE ]
Now when you guys all say that the fan is producing 80%-90%, this is at cruise flight right? Is it different at lower altitudes like takeoff for example?

[/ QUOTE ]

The fan produces a proportionately higher percentage of the total thrust at low altitudes. As air density thins with altitude, the "exhaust" thrust percentage (of total) increases....although I highly doubt it does above 50%.

Ray

[jergar999]

When I flew Metros the figure was 300lbs. of thrust on the TPE-331's.

Now I fly Kingairs (850hp at the prop, 903 ESHP, thus 53 lbs of thrust from the PT-6.

[DanTheMan]

[ QUOTE ]
That's a bright idea....not.

[/ QUOTE ]

Relax Aloft, I was joking.

[ QUOTE ]
Now I fly Kingairs (850hp at the prop, 903 ESHP, thus 53 lbs of thrust from the PT-6.

[/ QUOTE ]

That is exactly the answer I was looking for. Thanks a lot!