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Mimark06
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Aerodynamics...

Fri Apr 21, 2017 10:36 pm

With the introduction of the 787 and then the A350 we saw a 'new' nose, new wing shapes etc. Although some of these refinements in aerodynamics seem to bring back memories of the Nimrod and the Comet etc, my question is… is there a perfect shape for a given airliner and why hasn't it been found yet. There is talk of special winglets on the A380 which would decrease fuel consumption etc, but why wasn't that known when they had the models flying in the wind tunnel? The nose of the 737 hasn't changed in decades and the latest iterations haven't changed the nose at all, is that the perfect shape? Obviously I'm just an enthusiast here but hopefully someone in the know could explain to the laymen among us the complexities involved in designing an airliner. I realize the shape of the thing is just one consideration among thousands, or is the shape just the end result of those many other considerations? Thanks in advance…
 
IPFreely
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Re: Aerodynamics...

Fri Apr 21, 2017 11:37 pm

If you're talking about optimizing fuel economy only, the "perfect shape" is a "flying wing" a la the B2 bomber.
 
WIederling
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Re: Aerodynamics...

Sun Apr 23, 2017 2:41 pm

IPFreely wrote:
If you're talking about optimizing fuel economy only, the "perfect shape" is a "flying wing" a la the B2 bomber.


Stealth craft tend to not shine in the "perfect aerodynamic shape" domain. :-)
B2 takes MTOW * .5 for 10,000nm range ( no payload, unrefueled from Janes via WP:DE:B2 )
A330-200F takes MTOW * .45 to for 9300nm ( MTOW * .5 fuel would move that to ~10,100nm
( A330 for the comparable max fuel fraction.)
Murphy is an optimist
 
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PITingres
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Re: Aerodynamics...

Sun Apr 23, 2017 3:06 pm

There's no perfect shape because there are multiple competing metrics to measure against. You might have a shape that's aerodynamically fantastic but can't be built with any known material, or won't hold up the rest of the plane, or would cost $1B per wingset, or would have no fuel tankage space, or would be great in cruise but not so much for takeoff/landing, or the reverse. And on and on. Some things change because new designs, materials or construction techniques become available. Some things don't change because they would affect the rest of the plane systems too much, or cause a re-certification ($$$$$$), or because the improvement wouldn't be worth the trouble.

The A380 wing fences were chosen to keep the wingspan within the code F box, or so I understand it, and Airbus wanted the current wingspan for future expansion and probably a bunch of other reasons. The 737 nose hasn't changed because even though it's sub-optimal, nose changes would seem to affect a ton of things from cockpit layout to front structure and systems, and it probably wouldn't be worth the cause which I guess would be large.

Edited to add: another reason airplanes go out the door designed as they are, and this is an important one, is to get the airplane out the door. I don't doubt that Airbus could have come up with an A380 winglet that beat the fence while maintaining the desired aerodynamics, lift, and tankage requirements. But it might have taken months? years? of fiddling. One of the hardest engineering skills to learn is when something is good enough, stop optimizing, ship it, and move on.
Fly, you fools! Fly!
 
benbeny
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Re: Aerodynamics...

Sun Apr 23, 2017 4:01 pm

And you need to design it so you can further refine it in the future, when the demand is there. It's a compromise. Likewise, 737 cockpit is a head-banger, but you need to keep it for commonality purpose. Behind those panels may come so many structural things that you cannot change, or too expensive to change, so you're stuck with that dimension and that layout. I think perfection doesn't come from any particular design, but comes from so many imperfect design parts, that when you combine all of them, it becomes so beautifully matched till they function so flawlessly in their intended purposes.
 
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SomebodyInTLS
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Re: Aerodynamics...

Mon Apr 24, 2017 12:16 pm

Not only are you compromising aero against cost/engineering/materials etc. etc., you're also compromising aero for one mission against aero for another against aero for high altitude against aero for low altitude against aero for high speed against aero for low speed etc.

There just is no single "perfect shape".
"As with most things related to aircraft design, it's all about the trade-offs and much more nuanced than A.net likes to make out."
 
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Starlionblue
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Re: Aerodynamics...

Mon Apr 24, 2017 12:46 pm

Mimark06 wrote:
With the introduction of the 787 and then the A350 we saw a 'new' nose, new wing shapes etc. Although some of these refinements in aerodynamics seem to bring back memories of the Nimrod and the Comet etc, my question is… is there a perfect shape for a given airliner and why hasn't it been found yet. There is talk of special winglets on the A380 which would decrease fuel consumption etc, but why wasn't that known when they had the models flying in the wind tunnel? The nose of the 737 hasn't changed in decades and the latest iterations haven't changed the nose at all, is that the perfect shape? Obviously I'm just an enthusiast here but hopefully someone in the know could explain to the laymen among us the complexities involved in designing an airliner. I realize the shape of the thing is just one consideration among thousands, or is the shape just the end result of those many other considerations? Thanks in advance…


Looking at the 787 and 350 vs earlier generations, there is no more "notch" under the windshield. Newer radars and other pointy end equipment is smaller, meaning the nose can be more rounded, with the pilots sitting further forward, which maintains the required visibility.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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lightsaber
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Re: Aerodynamics...

Mon Apr 24, 2017 1:43 pm

Current designs, starting with the A380, are partial lifting body. Far more advanced than old concepts. This is a major disadvantage of the MAX and NEO. Cest la vie. Time to market and the parts chain are important.

WIederling wrote:
IPFreely wrote:
If you're talking about optimizing fuel economy only, the "perfect shape" is a "flying wing" a la the B2 bomber.


Stealth craft tend to not shine in the "perfect aerodynamic shape" domain. :-)
B2 takes MTOW * .5 for 10,000nm range ( no payload, unrefueled from Janes via WP:DE:B2 )
A330-200F takes MTOW * .45 to for 9300nm ( MTOW * .5 fuel would move that to ~10,100nm
( A330 for the comparable max fuel fraction.)

Stealth does degrade aerodynamics. But the flying wing or BWB is far more aerodynamic than a cigar with wings.

The goal is to minimize wet area to Wing area. For more wing area allows for higher cruise which lowers the fuel burn.

With folding wingtips, I think we'll see 'fat bodies' transitioning to the wing.

The issue with BWBs is they do have huge optimal wingspans.

The other issue is drag rudders and a loss of engine thrust. This prevents a twin. So Boeing prefers a trijet and Northrop (probably with Airbus), a quad. The preferences are based on engine mounting schemes.

Lightsaber
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IPFreely
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Re: Aerodynamics...

Mon Apr 24, 2017 2:37 pm

lightsaber wrote:
But the flying wing or BWB is far more aerodynamic than a cigar with wings.


Exactly. Current military incarnations of the flying wing are a compromise between fuel efficiency, the need for stealth, and the ability to carry large payloads. A basic flying wing with no stealth requirements will always be more efficient than any traditional airliner regardless of tweaks to the shape of the nose, etc.
 
WIederling
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Re: Aerodynamics...

Tue Apr 25, 2017 4:31 pm

IPFreely wrote:
lightsaber wrote:
But the flying wing or BWB is far more aerodynamic than a cigar with wings.


Exactly. Current military incarnations of the flying wing are a compromise between fuel efficiency, the need for stealth, and the ability to carry large payloads. A basic flying wing with no stealth requirements will always be more efficient than any traditional airliner regardless of tweaks to the shape of the nose, etc.


A330 and the B2 seem to be on par efficiency wise. i.e. with 50% of MTOW fuel load and no further payload they have similar range.
( WP::DE::B2 says 18,000km/~10,000 nm transfer range for 75t fuel and 152t MTOW. 110t for 240t MTOW bring an A330 out to ~~9,500nm)
Murphy is an optimist
 
benbeny
Posts: 250
Joined: Thu Nov 03, 2016 1:44 pm

Re: Aerodynamics...

Wed Apr 26, 2017 2:27 pm

IPFreely wrote:
lightsaber wrote:
But the flying wing or BWB is far more aerodynamic than a cigar with wings.


Exactly. Current military incarnations of the flying wing are a compromise between fuel efficiency, the need for stealth, and the ability to carry large payloads. A basic flying wing with no stealth requirements will always be more efficient than any traditional airliner regardless of tweaks to the shape of the nose, etc.

But doesn't the lift induce drag?
 
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SomebodyInTLS
Posts: 1933
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Re: Aerodynamics...

Wed Apr 26, 2017 3:58 pm

benbeny wrote:
IPFreely wrote:
lightsaber wrote:
But the flying wing or BWB is far more aerodynamic than a cigar with wings.


Exactly. Current military incarnations of the flying wing are a compromise between fuel efficiency, the need for stealth, and the ability to carry large payloads. A basic flying wing with no stealth requirements will always be more efficient than any traditional airliner regardless of tweaks to the shape of the nose, etc.

But doesn't the lift induce drag?


Yes, but with a flying wing you can create the amount of lift you need (Y) with a certain amount of drag (X).

If you add a tube you will still need at least that amount of lift (Y), so you get at least that amount of drag (X) - but then the tube itself creates extra drag (Z) for almost no extra lift.

In other words, for the same total weight the wing+tube version gets lift Y for drag X+Z.
"As with most things related to aircraft design, it's all about the trade-offs and much more nuanced than A.net likes to make out."

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