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kenish
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Platinum
Re: Fuel efficiency a mandate in aircraft design
kenish   6/4/2012 11:25:48 PM
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@Bob Salter <I assume that one engine only could allow a plane to fly and land safely if one engine fails.>

Yes, twin-engine aircraft have to demonstrate they can fly for long periods on one engine to get certified.  The term is ETOPS; here's a Wiki: http://en.wikipedia.org/wiki/ETOPS  The Wiki is a bit weak explaining that ETOPS involves not only design, but also maintenance and crew training.  Many airlines will have ETOPS and non-ETOPS versions of the same aircraft type in their fleets.  Also ETOPS applies to isolated areas (arctic, mountains, etc) and not just over the water....and dozens of factors go into ETOPS route planning on a flight-by-flight basis.

Nice thing to know when you're flying from LA to Sydney in a 777

 

N. Christopher Perry
User Rank
Gold
Re: Fuel efficiency a mandate in aircraft design
N. Christopher Perry   5/29/2012 2:29:13 PM
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I think the ideal temperature depends on a number of factors.  In some cases the turbine inlet temperatures are getting way beyond 1340°C.  Mitsubishi Heavy Industries recently achieved in inlet temperature of 1600°C in a co-generation application.  The actual turbine inlet temperatures for higher performance aircraft engines is not as well published (I think they are something of a trade secret).

Bleed air cooling is still a staple in gas turbines.  There are various advances being made on that front as well.

One of the really big developments being made on this latest generation of aircraft is the elimination of other bleed air uses, which originally supported aircraft HVAC systems among other things.  Apparently, running those systems by electric means is providing significant overall efficiency improvements, both in the engines themselves and in the reduced aircraft weight/volume.  Similar to this is the switch from hydraulic to electrically driven systems, or electro-hydraulic systems.

Bob Salter
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Iron
Re: Fuel efficiency a mandate in aircraft design
Bob Salter   5/29/2012 1:38:07 PM
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I remember from a thermodynamics course that the formula for theoretical efficiency of any internal combustion engine was the compression ratio raised to a certain exponent. I believe this is why Diesel engines are more fuel efficient than gasoline engines. One has to realize that C/R is the ratio of the actual air/fuel pressure at TDC in a piston engine to atmosphere, not the mechanical ratio advertised. At legal cruising speeds, the throttle in a gas engline is barely cracked resulting a a fairly low cylinder pressure, while a  Diesel does not have a throttle. So, raising C.R. in a jet engine would have efficiency advantages. The temperature 1340C has always stuck in my memory as being the goal for stoichemetric temperature at turbine inlet. Shouldn't this have depended on compressor outlet temperature also?  The higher the compression, the higher this parameter would be. When I initially learned about gas turbine engines for a manufacturer, air would be bled from one of the latter compressor stages and directed to the high pressure turbine section as cooling air. I was amused that 525C air would be considered cooling air.

 

N. Christopher Perry
User Rank
Gold
Re: Fuel efficiency a mandate in aircraft design
N. Christopher Perry   5/29/2012 12:13:44 PM
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I believe the stoichemetric air-fuel ratio challenge has largely been addressed.  The real push has been to maximize the compression ratio.  This is currently limited by how hot the turbine inlet temperatures can get, which ends up being a thermal management/materials problem.  The push to single crystal super alloy turbine blades with epitaxial ceramic thermal barrier coatings have pushed these limits well beyond 1200°C, which is allows compression ratios of more than 40:1.

Bob Salter
User Rank
Iron
Re: Fuel efficiency a mandate in aircraft design
Bob Salter   5/29/2012 11:13:59 AM
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Many years ago I realized that the speed of commercial aircraft, excluding the now retired SST Concord, has remained fairly close to the same MPH as the old Boeing 707 introduced in the 1950s. 550 MPH appears to be the upper limit, and this has been dictated by the speed of sound that has stayed the same. At cruising altitude, a plane is operating around 85 % of Mach 1 which is trans sonic and no man's land.  So one facet  left for the airframe and jet engine designers to do is increase efficency. I've been out of aerospace for 40 years, but do remember that one goal at that time was to reach a stoichemetric air-fuel ratio. I'm sure this was accomplished years ago and would like to hear from anyone if and when this was done. The original 707 with 4 jet engines followed a decade later by the 747 with 4 huge turbofan engines were leaders for overseas transport. Now, the thrust and reliability of turbofans has grown so much that only two are needed to power large wide-body transports.  I assume that one engine only could allow a plane to fly and land safely if one engine fails.

N. Christopher Perry
User Rank
Gold
Re: Fuel efficiency a mandate in aircraft design
N. Christopher Perry   5/25/2012 11:29:47 AM
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@ Charles:  That's probably a redition of the MAX 9 varient, which is the next gen of the 737-900ER.  The -900ER are rated for 220 passengers, as apposed to the old pre -700 series planes that are on rated for <140 passengers.

Charles Murray
User Rank
Blogger
Re: Fuel efficiency a mandate in aircraft design
Charles Murray   5/23/2012 9:02:33 PM
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I agree that the time is well spent. I do wonder, though, if this aircraft will have the same stubby appearance that the 737 is known for. It appears to be sleeker here.

naperlou
User Rank
Blogger
Re: Fuel efficiency a mandate in aircraft design
naperlou   5/23/2012 10:03:50 AM
Beth, I expect they do a lot of simulation.  When I was with GE (Aerospace) the internal supercomputer center was at the Aircraft Engines plant. 

William, you have a good point there as well.  Since the planes and engines will be around for a long time, the extra development time is time well spent. 

williamlweaver
User Rank
Platinum
Re: Fuel efficiency a mandate in aircraft design
williamlweaver   5/23/2012 8:10:38 AM
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This is an impressive engine. It uses the second-generation Twin Annular Pre Swirl (TAPS II) combustor which is not designed for fuel efficiency, but for NOx reduction. It's great to learn that this low NOx engine can also be made fuel efficient. I'm guessing there is additional optimization and fuel efficiency to be found by running test engines on actual aircraft. There is only so much you can learn from a ground-based static test. Collecting data under actual conditions of drag, air temperature, density, and quality should lead to additional enhancements -- albeit taking a bit more development time.

Beth Stackpole
User Rank
Blogger
Fuel efficiency a mandate in aircraft design
Beth Stackpole   5/23/2012 7:15:09 AM
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Fuel efficiency has to be part of next-generation aircraft engine design as it is in car design given the huge pressures surging oil prices put on the airline companies' ability to squeeze out any kind of profit. I'm curious how much of that balance between weight, drag, and other aerodynamic factors is studied in wind tunnels or through high-performance CFD simulations. My guess is more and more of the latter.



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