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readytotaxi
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A Fuel Question, Jet V Prop

Mon Apr 21, 2008 4:14 pm

I have no idea so decided to ask the experts.
A 4 engine jet v a 4 engine prop traveling at same speed over 1,000 miles, which would eat the most fuel?

I kinda think its the jet but not sure, reason for the question?
A friend said "with the price of fuel rising why don't airlines use props and take longer to get there and save money."
Is there any logic to her question?
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Flighty
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RE: A Fuel Question, Jet V Prop

Mon Apr 21, 2008 4:52 pm

I can answer this.

The substance of your question is: comparing a Q400 propliner at 350 knots with a slowed-down E-170 at 350 knots, which burns more fuel? The propliner would burn a lot less fuel.

One problem with your friend's point is, there is no "4 engine prop" for a 1,000 mile journey that compares with a "4 engine jet" other than perhaps an Avro RJ85, which is a fuel guzzler. Other than a Dash-7, which is outdated.

Generally speaking, the Q400 saves around 30-40% compared to its nearest competitor, ironically Bombardier's own CRJ-705 by the same manufacturer. So your friend is right, in theory journeys up to 1,000 miles should be flown by prop to save fuel. The problem is, props are a bit slow. At 1,000 miles, you will spend 1 extra hour in-flight compared to a jet. A jet is most fuel efficient at cruise speeds (550mph or 475 knots). So, slowing them down won't help anything.

Anything over 1,000 miles should be considered impractical for a prop in airline service.

Given today's available new aircraft, we could say that 300-400 new Q400 aircraft, flying missions up to 750 miles, would improve our national fuel efficiency compared to the smaller jets we use. As for bigger jets, there is no moden large prop to provide comparison.
 
futurecaptain
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RE: A Fuel Question, Jet V Prop

Mon Apr 21, 2008 4:55 pm



Quoting Readytotaxi (Thread starter):
A 4 engine jet v a 4 engine prop traveling at same speed over 1,000 miles, which would eat the most fuel?

Are we assuming both planes are the same size too? Props just don't like to be scaled up too big so a jet will generally carry several times what a prop will on a given route. I know you only asked about fuel burn, but payload in return for the fuel burned is a consideration too.
In general though a jet will burn more fuel on a given route. In return, in general, a jet is faster and the engine is more reliable than a prop.

Quoting Readytotaxi (Thread starter):
"with the price of fuel rising why don't airlines use props and take longer to get there and save money."

American (ATR)
Continental (Q400)
Frontier/Lynx (Q400)
Horizon(Q400)
Island Air (Q400)
ASA (ATR)
Fed Ex (ATR)

And that is just the tip of the iceberg. Props are in service all over the world.
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readytotaxi
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RE: A Fuel Question, Jet V Prop

Mon Apr 21, 2008 5:28 pm



Quoting Flighty (Reply 1):

Thank you, excellent reply.
you don't get a second chance to make a first impression!
 
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readytotaxi
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RE: A Fuel Question, Jet V Prop

Mon Apr 21, 2008 5:30 pm



Quoting Futurecaptain (Reply 2):

Thank you, I don't think she had considered payload, just bums on seats
you don't get a second chance to make a first impression!
 
9VSIO
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RE: A Fuel Question, Jet V Prop

Tue Apr 22, 2008 4:40 am

What happens if we talk piston prop instead of turbo-prop?
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KELPkid
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RE: A Fuel Question, Jet V Prop

Tue Apr 22, 2008 5:34 am



Quoting 9VSIO (Reply 5):
What happens if we talk piston prop instead of turbo-prop?

For a GA-sized plane, piston powerplants are definitely more fuel effecient. I think as you go up in size, that advantage disappears...also, piston powerplants usually have much reduced power output at typical turbine cruise altitudes.
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thegeek
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RE: A Fuel Question, Jet V Prop

Tue Apr 22, 2008 7:26 am



Quoting Readytotaxi (Thread starter):
I have no idea so decided to ask the experts.
A 4 engine jet v a 4 engine prop traveling at same speed over 1,000 miles, which would eat the most fuel?

The jet will eat the most fuel, but the prop will not fly as fast. With more expensive fuel, I expect turboprops to sell better after just about becoming extinct in the late 1990s. Both ATR and Bombardier are looking at stretching their ATR-72 and Q400 respectively. 1000mi is a long way for a prop to fly though. You are looking at paying more than a 30min flight time penalty over that sort of sector length though. Props would likely be confined to sectors less than about 400mi (644km)

Of course, they will be twins, not quads.
 
rwessel
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RE: A Fuel Question, Jet V Prop

Tue Apr 22, 2008 10:26 pm



Quoting KELPkid (Reply 6):
piston powerplants usually have much reduced power output at typical turbine cruise altitudes.

As, of course, do turbines. And you can't even turbo charge a turbine to compensate.

Large piston internal combustion engines are not necessarily much less efficient that turbines of the same output, but become massively impractical from a complexity, reliability, and/or mass standpoint. For example, there are 90,000shp marine diesels that are 52% efficient in common use, and the best Otto cycle engine have made it to about 60% in the lab. Large gas turbines generally run in the mid-to-high 60s, and some of the largest, when coupled with devices to extract and reuse the excess heat energy in their exhaust streams (for example, you can heat a boiler and run a steam turbine with the exhaust), can make it up into the eighties, but of course that's at least theoretically possible for piston ICEs as well. But don't make too much of the apparent difference - the large piston ICEs are built with more of an emphasis on cost and reliability. Of course large output turbines are miniscule compared to their piston equivalents, and tend to run much smoother.

Small turbines, OTOH, have significant issues trying to maintain a high compression ratio, but it can be done. Very small turbines often run in the low-to-mid 20s, while the best small(er) piston ICEs are in the upper 20s to mid 30s. Basically it’s easier to seal a small combustion chamber with a piston ring than it is to control the leakage past the turbine.
 
KELPkid
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RE: A Fuel Question, Jet V Prop

Tue Apr 22, 2008 10:51 pm



Quoting Rwessel (Reply 8):
As, of course, do turbines. And you can't even turbo charge a turbine to compensate.

Large piston internal combustion engines are not necessarily much less efficient that turbines of the same output, but become massively impractical from a complexity, reliability, and/or mass standpoint.

It would be nice if Pratt and Whitney still engineered behemoth radials, and if Curtiss-Wright were still around to try and make the case for "corncob" radials in this day and age  Wink I'd love to see what could become of a modern large aviation piston engineering effort, like about propliner-sized engines...Wonder what kind of weight savings we'd see with modern technology advances? Wouldn't it be cool, too, if someone tried a modern aviation turbodiesel (large, of course, not like the Thielert or the SMA engines), you could even run it off of Jet-A fuel  Smile
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
thegeek
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RE: A Fuel Question, Jet V Prop

Wed Apr 23, 2008 9:05 am



Quoting Rwessel (Reply 8):

I think your efficiency numbers are largely optimistic, except the the 52% for a turbo-compound diesel. Going from memory a theoretical Otto cycle needs to get over 10:1 compression to reach 60% efficiency, and that assumes instantaneous combustion and no pumping losses. A gas turbine backed by a steam plant can crack 60% efficiency in practice. It could only get into 80% if the waste heat is productively used.

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