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JoeCanuck
Topic Author
Posts: 4704
Joined: Mon Dec 19, 2005 3:30 am

Bypass; Bigger Ain't Always Better: Nasa

Tue Dec 14, 2010 10:23 am

In a thread in Civ Av, there was much made of the benefits to big fan v. smaller fan on BPR and engine efficiency. One of the posters dug up a very interesting, (well...interesting to me that is), NASA study on theoretical re-engine options for the 737 and A320. For their very extensive study, they used the 737-800 as the benchmark for validating their experiments.

As one might expect, the study indicates that there are tradeoffs for each design. They even plug Ultra High Bypass engines into the formula, as well as GTF, variable vane nozzles and a number of other experimental variables to test the extremes of turbo fan concepts.

Perhaps this is old news and has been discussed already. In case it is new news, here is the complete study;

http://mdao.grc.nasa.gov/publications/NASA-TM-2009-215784.pdf


If that's a bit much, here are the conclusions;

Quote:
8.0 Conclusions

UHB engines are feasible fora 737/A320-class vehicle. In general, the larger diameter
associated with UHB engines can be accommodated on this class of vehicle with relatively
simple measures such as increased landing gear length or changes to wing dihedral (changes
to wing dihedral were not explored in this study).

Optimum engine fan pressure ratio depends on the metric of interest, as well as the ground
rules, basic engine architectures, and assumptions used in the analysis. With the ground rules,
architectures, and assumptions used in this study: empty and ramp weight (often surrogate
indicators of cost) are minimized with high fan pressure ratio; block fuel consumption is
minimized with a fan pressure ratio of ^1.6; block NOx emissions are minimized with high
fan pressure ratio; and LTO NOx and certification noise are minimized with fan pressure ratio
as low as possible. These fan pressure ratio trends do not change with variation in engine
overall pressure ratio or design Mach number.

The primary benefit of the geared fan approach is to enable viable propulsion systems at lower
fan pressure ratios than possible with a direct drive fan. The geared fan approach is preferred
for fan pressure ratios (top-of-climb) below 1.5 (roughly BPR >13). At a fan pressure ratio of
1.5, a low work LPC, direct drive engine can provide outcomes similar to a geared engine.
Above a fan pressure ratio of 1.5, a low work, direct drive engine provides a better overall
aircraft system (for the metrics tracked in this study) than the geared engines do.


• If the design goal is to minimize ramp weight, block fuel, or block NOx, with the design
ground rules and technology assumptions of this study geared fan engine technology is not
necessary (since the minimums for these metrics occur at fan pressure ratios greater than 1.5.).


• If the design goal is to minimize airport area environmental impacts (i.e., aircraft noise and
LTO NOx), a geared system would be the preferred approach because it enables a practical
low fan pressure ratio engine design.

• Among the cases analyzed, the best balanced designs, performing well across all the metrics
of interest (ramp weight, fuel consumption, emissions, and noise), are fan pressure ratio 1.5
designs; either the high work LPC with a geared fan, or the low work LPC with either a geared
or direct drive fan.

• Relative to 1998 EIS technology, the advanced 2015 EIS configurations have the potential for
significant benefits: up to 29%reduction in fuel consumption and 25 EPNdB cumulative noise
reduction. These benefits do not occur with the same engine design, however. The minimum
fuel consumption designs have ^12 EPNdB (cumulative) higher noise than the minimum noise
designs and the minimum noise designs have up to 6% higher fuel consumption than the
minimum fuel consumption designs.



Enjoy...!

[Edited 2010-12-14 02:49:04]
What the...?
 
wingscrubber
Posts: 823
Joined: Fri Sep 07, 2001 1:38 am

RE: Bypass; Bigger Ain't Always Better: Nasa

Tue Dec 14, 2010 5:04 pm

Joe - this is interesting to me too, thanks for sharing. I'll have to find that Civ Av post...

Its funny that there's no particular slam-dunk advantage of GTF over direct drive or vice versa, just that all future-gen engines will be quieter and more efficient than previous gen engines.

Also even though it does acknowledge that UHB engines are the direction development is headed, it doesn't give any concrete reasons why it's better, just that it's preferrable to maintain a lower fan pressure ratio for reduced noise and fuel efficiency and that the method of achieving that is with higher bypass in an isolated engine case, but not necessarily for the aircraft as a whole. (Page 78)

One point of note is at 6.2.1 Aircraft modifications, page 64 shows the increased drag/reduced mach of the reduced wing-sweep configuration(spiral 3), and on page 65 the M*(L/D) vs mach shows the trade in range which falls off at the high-mach number. This seems to dispell the trend of wanting to 'unsweep' wings for efficiency, when all it does is make you burn the same amount of fuel over a longer period, arriving later having spent the same gas. Going fast is good!! Right?

http://www.wired.com/autopia/2010/06...n-slows-down-to-match-jet-engines/

So maybe then, MIT needs to go back to the drawing board on this 'super efficient airliner' concept...?
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411A
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Joined: Mon Nov 12, 2001 10:34 am

RE: Bypass; Bigger Ain't Always Better: Nasa

Tue Dec 14, 2010 6:37 pm

Quoting Wingscrubber (Reply 1):
So maybe then, MIT needs to go back to the drawing board on this 'super efficient airliner' concept...?

It has been my professional experience that some MIT graduates have what can be best described as pie-in-the-sky concepts where civil aircraft design (and operation) are concerned.
IE: looks good on paper, not so practical in reality.
 
TSS
Posts: 3738
Joined: Fri Dec 29, 2006 3:52 pm

RE: Bypass; Bigger Ain't Always Better: Nasa

Tue Dec 14, 2010 6:56 pm

Quoting 411A (Reply 2):
It has been my professional experience that some MIT graduates have what can be best described as pie-in-the-sky concepts where civil aircraft design (and operation) are concerned.
IE: looks good on paper, not so practical in reality.

That has been my non-professional observation as well. The critical design considerations that any civil aircraft must be:
A. built on an assembly line,
B. sold at a competitive price, and
C. serviced in the field,
often seem to be ignored in favor of the "Wow!" factor.
Able to kill active threads stone dead with a single post!
 
pilotpip
Posts: 2844
Joined: Fri Sep 19, 2003 3:26 pm

RE: Bypass; Bigger Ain't Always Better: Nasa

Tue Dec 14, 2010 8:35 pm

Quoting TSS (Reply 3):

That has been my non-professional observation as well. The critical design considerations that any civil aircraft must be:
A. built on an assembly line,
B. sold at a competitive price, and
C. serviced in the field,
often seem to be ignored in favor of the "Wow!" factor.

And completely thrown out the window when you throw government red tape in the equation which is why Cessna is still producing the same piston single it did 50 years ago with only minor changes.
DMI
 
wingscrubber
Posts: 823
Joined: Fri Sep 07, 2001 1:38 am

RE: Bypass; Bigger Ain't Always Better: Nasa

Tue Dec 14, 2010 9:13 pm

I volunteered to grade some papers for the AIAA design build fly contest once - I graded one from MIT and must admit there is some terrific talent coming from that school, but at the same time there's plenty of naive 'pie in the sky' too.
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pilotpip
Posts: 2844
Joined: Fri Sep 19, 2003 3:26 pm

RE: Bypass; Bigger Ain't Always Better: Nasa

Tue Dec 14, 2010 11:54 pm

Keep in mind though, if we didn't have people with that "Pie in the sky" attitude, we would still be stuck on the ground. All the items we consider to be essential, or improving our quality of life were at one time nothing more than a crazy idea.
DMI
 
JoeCanuck
Topic Author
Posts: 4704
Joined: Mon Dec 19, 2005 3:30 am

RE: Bypass; Bigger Ain't Always Better: Nasa

Wed Dec 15, 2010 6:54 am

I thought this paper was interesting for a number of reasons.

It based its formulas on, and tested them against, real world performance results achieved by the 737-800, adjusting their prediction models to match actual results, then using the modified models to make projections.

In using the -800, they use a model which has a good chance of being subjected to the very concepts they were discussing, again making it a real world case.

They used a wide variety of variable, which they tested one at a time and in combination to get a very wide variety of possible results.

It seemed to me to be more realistic and practical than your average pie in the sky study.

Regardless, it gave me whole minutes of fascinating reading.
What the...?
 
411A
Posts: 1788
Joined: Mon Nov 12, 2001 10:34 am

RE: Bypass; Bigger Ain't Always Better: Nasa

Wed Dec 15, 2010 7:42 am

Quoting TSS (Reply 3):
That has been my non-professional observation as well. The critical design considerations that any civil aircraft must be:
A. built on an assembly line,
B. sold at a competitive price, and
C. serviced in the field,
often seem to be ignored in favor of the "Wow!" factor.

Not only that.
Consider the problems that might be found on re-engining a known design.
Disregarding for a moment such things as bleed air pneumatic systems (etc) that need to be redesigned, there is the slight problem of accomodating the engine on the wing.
One MIT guy might say...'well, just extend the landing gear just a bit to accomodate the larger engine'.
However, the aircraft manufacturers structures guy might then say...'wait a minute, that longer landing gear won't fit in the gear bay any longer, and, oh by the way, we can't make the gear bay any bigger without redesigning the wing box.'
Many variables involed...very expensive ones.

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