Turboprop and turbofan service ceiling

[ScottG]

I think I have a grasp on the efficiencies with each of the two in terms of speed, but why is it that a turbofan will fly so much higher than a turboprop. My understanding is that they are much the same engine, both using a turbine but one uses a prop and one uses a ducted fan. So again, why can one go higher than the other? It seems to me that low air density would have the same effect on a fan blade as it would a prop. Maybe the fact that the fan is in an air duct, it is able to spin much fast than a prop, which looses efficiency approaching mach1 at the tips? Just guesses........

[Dugie8]

In "general" terms, an airplane's service ceiling is usually more of a factor of maintaing cabin altitude around 8 to 10,000 feet. Be it either a limitation on the amount of bleed air the engine can supply to the cabin to maintain the desired "altitude" or a structual limitation on how much of a pressure differential the pressure vessel can tolerate

Another limitation could be O2 availability. To be able to make it from the highest altitude attainable to, and I may be wrong on the reg, 14,500 feet MSL there needs to be enough O2 on board to meet certain requirements.

There are other things like, service ceiling being where you can climbe at 100 fpm, and absolute ceiling being where you can climb at 50 fpm (I may have thos a little backwards). For the most part, when talking about service ceilings for turbo props and jets it is more a matter of "high dive" requirements, structural limits, etc than it is the engines ability to pull the airplane through the air.

[ScottG]

That was one of my thoughts (cabin press), could fuel efficiency be gained on a turboprop if it was certified to fly higher then?

[Firebird2XC]

Originally Posted by ScottG

That was one of my thoughts (cabin press), could fuel efficiency be gained on a turboprop if it was certified to fly higher then?

Yes. I was at FL 240 in a Beech 1900 in July. Our fuel flow was greatly reduced, just like in a jet.

The efficiency of altitude comes from the efficiency of a turbine engine.

Theoretically, a turboprop could fly as high as jet, but propeller efficiency decreases with altitude. Turbofans derive a large portion of their thrust generated from their fans, but not all of it.

I know alot of turboprops, because of design specs, aren't limited to lower altitudes by their powerplants. The Beech 1900, for example, is limited to FL 250 by the maximum cabin pressure differential of 5.2:1.

Hope this helps.

[ScottG]

Pretty much what I was looking for, thanks. So, if cabin pressure and non power plant design limitations were removed a turbo prop could fly almost as high as a high bypass jet, but the actual jet exhaust portion on the thrust enables a little higher flight level?

[Dugie8]

Originally Posted by ScottG

Pretty much what I was looking for, thanks. So, if cabin pressure and non power plant design limitations were removed a turbo prop could fly almost as high as a high bypass jet, but the actual jet exhaust portion on the thrust enables a little higher flight level?

Yes and no, yes being the simple straight forward answer, no being that a turbofan (high bypass) at altitude has almost a 50/50 split of power from the fan and "exhaust" where on the ground and lower altitudes is close to 70/30 or even 80/20. A turbofan looses just as much power as a turbo prop as they go up, remember for all intents and purposes these are still basically normally aspirated engines, ie no turbo charger type device feeding high pressure air into the intake.

It is true a turbo fan gives more power per lbs of weight of motor, BUT a turbo prop will always win in NM per LBS of fuel (true measure of efficiencey).

[ScottG]

Bingo, question answered and a well deserved thank you to all.

[NJA_Capt]

Originally Posted by Dugie8

....being that a turbofan (high bypass) at altitude has almost a 50/50 split of power from the fan and "exhaust" where on the ground and lower altitudes is close to 70/30 or even 80/20.

I have forgotten the actual numbers but the high altitude numbers are far from 50/50. Low altitude thrust comes from the fan and high altitude thrust comes from the turbine section. At FL510, there is almost no thrust coming from the fan section on the AE3007 (5:1). All practical thrust is from the turbine section. At sea level almost all thrust is from the fan section.

[Dugie8]

Originally Posted by NJA_Capt

I have forgotten the actual numbers but the high altitude numbers are far from 50/50. Low altitude thrust comes from the fan and high altitude thrust comes from the turbine section. At FL510, there is almost no thrust coming from the fan section on the AE3007 (5:1). All practical thrust is from the turbine section. At sea level almost all thrust is from the fan section.

Well, a CFM was close to 50/50 and the JTD15-B is about 40/60 (according to SimCom). Those are run in the high to mid 30s.