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airproxx
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"Clean Wing" Aircraft Future?

Sat Nov 10, 2012 12:33 am

I was just wondering, while chasing pics on a-net, what could be a short term future answer for improving commercial aircraft fuel burn. And I figured out that almost every modern aircraft for commercial aviation (except for executive aviation) is featured with pod mounted engines under the wings. The so-called pod must be downgrading a bit the Cx while, by opposition, a "clean wing" aircraft with rear-mounted engines should have a better wing aerodynamic efficiency....

I know, and I've read here that, for many reasons, manufacturers preferred so far a pod design for their next products.
But with nowadays oil prices sky-rocketing, we saw fuel burn improvements (such as blended winglets) allowing an efficiency improvement of about 2 to 5 %, be worth the deal....

My question is simple: for an aircraft the size of an A320/B737 to an aircraft the size of a 787, could such an efficiency be gained with a pure, clean wing, and rear mounted engines? Or is such a design completely obsolete now?

Thoughts?

I'd be glad to see your comments on this...
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HAL
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 12:57 am

There's more than just clean aerodynamics at work with wing-mounted engines. A lot of this is probably more suitable for the Tech/Ops forum, but in short, having the engines on the wing provides a stabilizing effect for wing flutter, so the wing as a whole can be made lighter and thinner than it would be without the engine. This saves considerable weight, and in many cases, outweighs the clean wing aerodynamic savings. That's just one example. Another would be the much heavier pylons required to carry the (very large) engines horizontally from the fuselage vs. hanging down from the wing. What you can be certain of is that the leagues of highly-paid aeronautical engineers always look closely at these questions when designing new aircraft, and what we have today are the best possible choices as expressed in the very similar designs coming from just about every manufacturer. When you look at designs ranging from the E-170 series all the way up to the 777 and A380, they're all so similar because those designs are what work best - and most economically - for today's aircraft, and today's construction methods.

HAL
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airproxx
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 1:28 am

Quoting HAL (Reply 1):

Understood! Thanks a lot for your very detailed reply... I was just hoping that we could see a 727X some day, but it won't be the case anytime soon if I got you well...
Always good to have that type of information though, I'm always curious!
Thanks again!

Cheers,

Dave
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Aesma
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 1:59 am

The engines weight on the wing actually help the wing structure since it's that much weight less coming from the fuselage, and even a counter-force. Rear mounted engines also mean a stronger fuselage is needed.

Airbus has proposed a clean wing design, not a 727 lookalike though :

http://www.blokeish.com/2010/07/the-...illustrates-air-transport-in-2050/

The last trijet in production is pretty close, the Falcon 7X.
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prebennorholm
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 2:01 am

When the 737 was launched almost fifty years ago the world was in chock. This was the very first twin jet airliner with underwing engines. How could Boeing come up with such an idea? After all they were very late to come up with a twin, and only after Caravelle, One-Eleven, DC-9, Tu-134 had shown the way with rear mounted engines. And after the successful 727. Boeing sure had to come up with a good explanation.

And they did!

Boeing claimed that by hanging the engines under the wing instead of 727 style on the fuselage they saved more than 1500 lbs structure in the fuselage and wing.

And the airlines loved to be able to fly an entire extra seat row of pax and baggage "for free".

Maybe they saved too much? Survivors of the Aloha "737 Convertible" accident may claim so.

But anyway, the rest is history. Since 737 EIS, how many new 100+ pax twin airliners have been designed with fuselage mounted engines? My count is zero. I'm not counting Boeing 717 and such. It's an updated and re-engined DC-9. Only small RJ's have been made with rear mounted engines because they were sitting too low to have underwing engines.

When talking RJ's, the VFW-614 was made with OVER-wing engines in order to avoid the structural disadvantage of rear engines, even if that had even more severe aerodynamic disadvantages than underwing engines.
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Ferret
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 2:20 am

Quoting HAL (Reply 1):
so the wing as a whole can be made lighter and thinner than it would be without the engine.

So here's my question to this point:
Not to question all the experts in the field, but I have often marveled at how thin the wing of the DC9/MD80s are. This is not scientific, but when you look at it compared to a comparable capacity 737 it sure seems like the 737 has to have a larger wing. I've always thought this because of the need to strengthen the wing, the profile must be larger and thus so too must the chord.
No?  

https://www.airliners.net/photo/Ameri...d=0c0db6e6ba57951c64aab0472c9c098a


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prebennorholm
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 2:25 am

Quoting Aesma (Reply 3):
Airbus has proposed a clean wing design, not a 727 lookalike though...

This very nontraditional Airbus proposal is optimized for low noise footprint rather than structural and aerodynamic efficiency. Both horizontal stabilizer and the twin vertical fins act as noise shields.

Such a design will see light of day only when the very last fraction of a decibel takes priority over fuel efficiency.

Even then it will be modified to take nosewheel related engine FOD into account.
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DocLightning
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 2:41 am

Quoting ferret (Reply 5):
So here's my question to this point:
Not to question all the experts in the field, but I have often marveled at how thin the wing of the DC9/MD80s are. This is not scientific, but when you look at it compared to a comparable capacity 737 it sure seems like the 737 has to have a larger wing. I've always thought this because of the need to strengthen the wing, the profile must be larger and thus so too must the chord.

You are thinking of the aircraft sitting on the ground and the forces involved.

When a clean wing is sitting on the ground, it is a cantilevered plank (with some fancy shaping) sticking out of the side of the aircraft. It only needs to hold up its own weight. When a wing with an engine is sitting on the ground, it is a cantilevered plank with a big engine sticking out of it, so it has to be stronger to support the engine. Right?

Wrong.

A wing is under its greatest stress not when the aircraft is on the ground, but when the aircraft is in flight. In flight, the entire force dynamic on the wing is reversed from at rest on the ground. The wing generates an upward force and the entire fuselage is literally resting on the wings. If the wing root were to suddenly turn to jelly, the wing would fly up and away from the aircraft (at least at first), not crash down to the ground, so the force on the wingtip is upward and the fuselage is trying to push the wing root downward. This means that there is an enormous bending force on the wing root as it resists the tendency of the wing to want to fold upward.

By mounting the engine on the wing you introduce a downward force outboard of the wing root and decrease that bending moment. This actually allows you to build a weaker wing because some of the unbalanced forces are evened out.
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tdscanuck
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 4:22 am

Quoting ferret (Reply 5):
This is not scientific, but when you look at it compared to a comparable capacity 737 it sure seems like the 737 has to have a larger wing. I've always thought this because of the need to strengthen the wing, the profile must be larger and thus so too must the chord.

In addition to what DocLightning said, you may be confusing size with strength. A bigger wing is not the same thing as a stronger wing, or as a heavier wing. Since wings are hollow, increasing the size of the profile while thinning down the skins can result in a wing that weights the same, has basically the same strength, and is considerably stiffer.

This is, for example, why I-beams exist...to get that amount of strength and stiffness with a solid bar would be smaller but also considerably heavier.

Tom.
 
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DocLightning
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 6:55 am

Quoting tdscanuck (Reply 8):
In addition to what DocLightning said, you may be confusing size with strength. A bigger wing is not the same thing as a stronger wing, or as a heavier wing. Since wings are hollow, increasing the size of the profile while thinning down the skins can result in a wing that weights the same, has basically the same strength, and is considerably stiffer.

And furthermore, longer wings make less drag but generate more lift, so they need heavier, stronger structures.

And then there's that Whitcomb Area Rule that I keep ticking tdscanuck off with.  
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rwessel
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 7:07 am

Quoting DocLightning (Reply 9):
And furthermore, longer wings make less drag but generate more lift, so they need heavier, stronger structures.

Not really. If it generated more lift than the airplane weighed, the aircraft would climb.

Induced drag, per unit lift, increases as either (or both) aspect ratio and wing area decreases.

Of course long thin structures have worse stress problems than short fat ones, hence high aspect ratio wings tend to be heavier for a given wing area than low aspect ratio wings. But it's not because they generate more lift - in level flight any wing will be generating exactly the amount of lift needed to hold up the airplane.
 
BMI727
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 7:10 am

Quoting ferret (Reply 5):
Not to question all the experts in the field, but I have often marveled at how thin the wing of the DC9/MD80s are. This is not scientific, but when you look at it compared to a comparable capacity 737 it sure seems like the 737 has to have a larger wing. I've always thought this because of the need to strengthen the wing, the profile must be larger and thus so too must the chord.

I haven't looked at the measurements, but I'd guess that the nondimensional thickness is about the same. DC-9s seem to have a pretty short chord.

Quoting DocLightning (Reply 9):
And then there's that Whitcomb Area Rule that I keep ticking tdscanuck off with.

That can be a bit of a wrench in the works. It probably won't drive many decisions (business jets do okay) but I imagine it can affect some dimensions. Long aircraft with tail engines would probably do poorly, since the rear mounted engines should pick up area from the wings and then pass it to the tail section. Just looking at picks, I think the MD-80 is not well area ruled compared to it's faster business jet counterparts.
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XT6Wagon
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 8:54 am

Quoting Aesma (Reply 3):
Airbus has proposed a clean wing design, not a 727 lookalike though :

While engines need far less MX than the days of the Comet, buried engines are an expensive nightmare compared to hung pods where you have nearly 360 degree access.

Also uncontained failures are far less dangerous. Again not as big of a problem today as it was even a couple decades ago, wrapping the engine deep in a wing root or fuselage is asking for extra issues.
 
sweair
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 9:16 am

The BWB will most certainly have its engines on the fuselage in pods. And this puppy will beat the tube and wing on efficiency by a lot. Once people get over the chock of non traditional design I think it will be the model how to build jet airliners.

Its not a new idea but now we have cfrp for the strange shapes and computers for control and also the thrust they need with just 2 or 3 engines. Almost all new UAVs are in that shape, it just is very good at lift vs drag a plus would be its radar cross section too, but that is hardly anything we care about for civilian airliners  

I hope I live long enough to fly on a BWB airliner, ok no window seat would be possible but if one has flown on a 2-5-2 DC10 and was seated in the middle seat you know its not the end of the earth, with current technology we could replicate windows in a very good way.
 
HAL
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 9:26 am

Quoting sweair (Reply 13):
I hope I live long enough to fly on a BWB airliner, ok no window seat would be possible but if one has flown on a 2-5-2 DC10 and was seated in the middle seat you know its not the end of the earth, with current technology we could replicate windows in a very good way.

Maybe a slight deviation from topic here, but you might get a more 'exciting' ride than you expected on a BWB. Consider for a moment how airplanes fly. When they turn, they roll in order to balance the aerodynamics of the maneuver. This roll moves around the axis of the fuselage, so that when the plane rolls, you (as a passenger) are no more than a dozen feet from the axis. Now picture yourself on a BWB, sitting out at the outermost seats. You're about 50 feet or more from the axis, so that every time the plane rolls for a turn you will move up and down a whole lot more than in a conventional tube fuselage. The G-forces felt by the passengers won't be that strong, but they will be very different from what they feel today. That motion is going to be one of the biggest problems to overcome in the design of the BWB because any time the plane starts rolling (i.e. turbulence, as well as maneuvering) you're going to have a planeload of nauseated passengers to deal with.

I'm not saying the BWB won't happen, but there's a lot more to consider in the design than just fuel economy.

HAL
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BMI727
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 9:36 am

Quoting HAL (Reply 14):
I'm not saying the BWB won't happen, but there's a lot more to consider in the design than just fuel economy.

I think the first ones we'll see are more likely to be hybrid BWB designs. Think something like the B-2.
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sweair
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 9:39 am

Quoting HAL (Reply 14):

We have had plenty of threads about that and it can be solved.

A BWB would be a clean wing , that was really what I should have written.
 
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antoniemey
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 9:50 am

Quoting prebennorholm (Reply 4):
Only small RJ's have been made with rear mounted engines because they were sitting too low to have underwing engines.

Some of the longer-range EMB-145 variants could have gotten away with wing-mounting if the engineers had wanted to, but by that point the design was kind of set...
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lightsaber
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 3:41 pm

Quoting airproxx (Thread starter):
what could be a short term future answer for improving commercial aircraft fuel burn.
Quoting airproxx (Thread starter):
a "clean wing" aircraft with rear-mounted engines should have a better wing aerodynamic efficiency....

As others have noted, the weight penalty is high. The wing is the main drag and the engines are the thrust. Any metal to move the thrust to the wing is heavy. Also, wing mounted engines are in clean air. Tail or pod mounted engines suffer a fuel burn penalty due to poor inlet conditions. Also, designing engine inlet for avoiding drag from the wing is easy if they are under the wing but tough for the engines.

Its funny, airframe designed concepts by aerodynamic engineers (which I am one) put the engines in the back. As soon as the propulsion or structural people come on board, the engines are back on the wings. The better BWB concepts I've seen put the engines back on the wing.

Quoting HAL (Reply 1):
having the engines on the wing provides a stabilizing effect for wing flutter, so the wing as a whole can be made lighter and thinner than it would be without the engine. This saves considerable weight, and in many cases, outweighs the clean wing aerodynamic savings.

   But do not neglect the thrust having to be moved to the wing where the drag is, in particular on takeoff.

Quoting prebennorholm (Reply 4):
Boeing claimed that by hanging the engines under the wing instead of 727 style on the fuselage they saved more than 1500 lbs structure in the fuselage and wing.

For a new 737 twin, the weight savings is more than double that as the understandings of what needs to be done has improved. The only reason left to do tail mounted engines is when the landing gear may be short and weight and other advantages of short landing gear outweigh the weight *and* volume penalty of tail mounted engines. Yes, tail mounted engines mean an un-used volume that has a (tiny) drag associated with it.

I expect the Hondajet will start the end of the era for tail mounted engines. In particular once we start seeing business jets with CFRP wings and aluminum bodies (which is what I think the future trend will be). Now, that will be a slow trend, but give it a few decades and that is what we'll see.

Quoting DocLightning (Reply 7):
A wing is under its greatest stress not when the aircraft is on the ground, but when the aircraft is in flight.

A wing is under its greatest stress when the aircraft hits a pothole on the ground.    There the cantilever effect of the engines outside of the gear is critical in reducing weight. I've worked on more than one project where the final engine placement was determined by pothole stress and not the aerodynamics.

Quoting HAL (Reply 14):
You're about 50 feet or more from the axis, so that every time the plane rolls for a turn you will move up and down a whole lot more than in a conventional tube fuselage.

I've seen BWB concepts for commercial service. No one is more than 50 feet from the axis. As others have noted, they are hybrid concepts like the B2 which has a fusalage. And the FAA has approved exit strategies about two years ago in discussions with the manufacturers to restart interest in the BWB.

Quoting rwessel (Reply 10):
Not really. If it generated more lift than the airplane weighed, the aircraft would climb.

Well, I'm for big wings as it is now possible to design for higher cruising altitudes.    That will have far more impact than engine placement on the wing. I expect with CFRP wings to see future airlines cruise higher (relative to their similarly sized prior peers). Altitude=less dense air=less drag.   

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D L X
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 4:01 pm

Quoting lightsaber (Reply 18):
No one is more than 50 feet from the axis.

50 feet from the axis is a pretty long way, don't you think? In a 30 degree bank, the window passengers will have gone 25 feet up from wings-level. That's quite a ride.
 
peterjohns
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 4:25 pm

Hi Airproxx-

Please take two things into consideration with your question-
1) The engines at the back also produce drag. The overall drag is not minimized by putting the engines somewhere else.

2) The engine size/ diameter has increased over the years due to larger fans which has proved to be the design of today.
Imagine hanging a large engine on the back... Look at the Citation X...
 
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ADent
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 4:54 pm

Quoting ferret (Reply 5):
So here's my question to this point:
Not to question all the experts in the field, but I have often marveled at how thin the wing of the DC9/MD80s are. This is not scientific, but when you look at it compared to a comparable capacity 737 it sure seems like the 737 has to have a larger wing.

I think Boeing likes bigger wings. You need to compare the 727 and DC9. Both are clean wings.


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sweair
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 5:47 pm

Quoting D L X (Reply 19):
50 feet from the axis is a pretty long way, don't you think? In a 30 degree bank, the window passengers will have gone 25 feet up from wings-level. That's quite a ride.

I don't think there will be any window seats, sure it will be a wider cross section than the A380 but 50 feet?!

Maybe controls will get even more limits under normal operation, we have had numerous threads about this already.
 
VC10er
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 6:40 pm

Reading this is very interesting to me given my name! VC10! Back when I took my first flight at the age of 6, I flew the VC10 from JFK to Glasgow. The engineers back in 1960 must have been so confident in placing all 4 engines on the rear of a 707 sized aircraft. We all know that the VC10 was not very successful and very noisy for those on the ground. But she is still flying for the RAF (in small numbers) so there must be some virtue to it's design..no?
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Fabo
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 11:50 pm

Quoting DocLightning (Reply 7):
By mounting the engine on the wing you introduce a downward force outboard of the wing root and decrease that bending moment. This actually allows you to build a weaker wing because some of the unbalanced forces are evened out.

This is, btw, also why airplanes first use fuel that is stored in fuselage, and only then start using fuel stored in wings. Fuel stored in wings helps to distribute weight along the wing, counteracting what is in the fuselage.
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ZaphodB
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RE: "Clean Wing" Aircraft Future?

Sat Nov 10, 2012 11:52 pm

Quoting VC10er (Reply 23):

Reading this is very interesting to me given my name! VC10! Back when I took my first flight at the age of 6, I flew the VC10 from JFK to Glasgow. The engineers back in 1960 must have been so confident in placing all 4 engines on the rear of a 707 sized aircraft. We all know that the VC10 was not very successful and very noisy for those on the ground. But she is still flying for the RAF (in small numbers) so there must be some virtue to it's design..no?

From a passenger's perspective: yes, absolutely - then 'ten' remains the best thing since sliced bread. Breezes through rough air that would be spilling the G&Ts on any A or B. And then there's the greasers (or are RAF pilots just that much better?).
From a bean counter's perspective: no, unless you can get away with charging higher fares.

I'll take a rear engine, clean wing bird any day in preference to A or B but the economics seem to be pointing in the other direction.
 
strangr
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RE: "Clean Wing" Aircraft Future?

Sun Nov 11, 2012 12:11 am

So if clean wing is such an idea for the future, has Aviation evolution gone full circle.
 
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lightsaber
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RE: "Clean Wing" Aircraft Future?

Sun Nov 11, 2012 1:38 am

Quoting D L X (Reply 19):
Quoting lightsaber (Reply 18):
No one is more than 50 feet from the axis.

50 feet from the axis is a pretty long way, don't you think? In a 30 degree bank, the window passengers will have gone 25 feet up from wings-level. That's quite a ride.

50 feet from the axis implies a 100' cabin width. I've seen modern concepts to 40 foot cabin widths, but not 100 feet. The only 100 foot cabin widths I can recall are from 1950s Popular mechanics magazines...

Quoting sweair (Reply 22):
I don't think there will be any window seats, sure it will be a wider cross section than the A380 but 50 feet?!

I haven't seen any serious proposals since Northrop's early concepts without a passenger fuselage (or 'hybrid BWB').

Quoting strangr (Reply 26):
So if clean wing is such an idea for the future

  

Look at the Nasa studies, they want the aircraft to block engine noise. That is with engines on top of a BWB (such as the X-48C, not the X-48B, they have a major engine configuration change) or Northrop's proposed PSC BWBs.
This article has images of both BWBs.
http://www.aviationweek.com/Blogs.as...f5d94b-36ca-4da3-b16f-c773e76268d1

Now, NASA has publicized the Boeing design more as Northrop has refused to compromise performance for a noise goal that airlines would never pay for (e.g., vertical stabilizers). That means Boeing will have more research subsidized and then a last minute configuration change.  

We might see a 'clean wing' on a propfan, but that would be it. Long haul is not going to adopt the concept.

Quoting ZaphodB (Reply 25):
I'll take a rear engine, clean wing bird any day in preference to A or B but the economics seem to be pointing in the other direction.

For passenger duty, the economics is what matters. Engines in the rear, as noted before, add weight, subtract a little cabin volume (due to structure) and detract from engine efficiency (due to body boundary layer effects). They are a fine compromise on smaller planes or applications where short landing gear is a preference (business jets), but not where there are jetways.


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DocLightning
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RE: "Clean Wing" Aircraft Future?

Sun Nov 11, 2012 1:41 am

Quoting rwessel (Reply 10):
Not really. If it generated more lift than the airplane weighed, the aircraft would climb.

I worded that poorly. A long, thin wing will move the center of lift outboard and make a larger bending moment. And then there is your point about long, thin wings having worse stress problems.

Quoting lightsaber (Reply 18):
A wing is under its greatest stress when the aircraft hits a pothole on the ground.

I did not know that. Is that because all the force is concentrated straight up the gear strut? Is this worse than a hard landing?
-Doc Lightning-

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prebennorholm
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RE: "Clean Wing" Aircraft Future?

Sun Nov 11, 2012 2:03 am

Quoting lightsaber (Reply 18):
Quoting prebennorholm (Reply 4):
Boeing claimed that by hanging the engines under the wing instead of 727 style on the fuselage they saved more than 1500 lbs structure in the fuselage and wing.

For a new 737 twin, the weight savings is more than double that...

Sure yes. But 1500 lbs was what Boeing claimed 47-48 years ago when the the 731 was on the drawing board.

Best selling variant today, the 738, is an entirely different bird. Twice as long cabin, longer wings, twice as heavy plane, twice as powerful and twice as heavy engines.

If we imagine tail mounted engines on both 731 and 738, then not only are the 738 engines twice as heavy, but in a pothole they have a much longer moment arm related to the MLG, so the fuselage structure reinforcement needed would be a lot more than double on the 738 compared to the 731.

If we assume that Boeing was fairly right when calculating 1500 lbs weight penalty for tail engines on the 731, then it is probably fairly correct to assume the weight penalty on the 738 to be at least some 4,000 to 5,000 lbs.

Quoting ZaphodB (Reply 25):
From a passenger's perspective: yes, absolutely - then 'ten' remains the best thing since sliced bread. Breezes through rough air that would be spilling the G&Ts on any A or B. And then there's the greasers (or are RAF pilots just that much better?).

Some forty years ago I saw in the aviation press some B&W pictures of VC10's which had landed hard. The fuselage had wrinkles all over. The accompanying text told that the VC10 should always meet terra firma gently - more so than most planes, or it gets wrinkles like a Ju-52 because of the four heavy engines on the tail.

On this forum some 4-5-6 years ago someone gave us a link to a video clip of a DC-9 hard landing test where the entire tail broke off as soon as the MLG touched the runway. I wonder if it is still out there. Does somebody have a link?
Always keep your number of landings equal to your number of take-offs
 
VC10er
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RE: "Clean Wing" Aircraft Future?

Sun Nov 11, 2012 2:17 am

Why does the military aircraft (unfortunately named the "warthog") have 2 giant turbo fans mounted on top-rear? The advantage being?
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lightsaber
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RE: "Clean Wing" Aircraft Future?

Sun Nov 11, 2012 2:25 am

Quoting DocLightning (Reply 28):
I did not know that. Is that because all the force is concentrated straight up the gear strut? Is this worse than a hard landing?

I'm an 'aero weannie,' so if I start talking all the stress paths, I'll get something off.    But it is a worse case stress condition than a hard landing due to the rate of landing gear contraction that also has a very abrupt rearward component (breaks are 'squeezed on and off') and already compressed landing gear. When I used to do engine design, we would have the engines moved out to increase the engine cantilever which would ironically reduce wing weight due to reduced stresses due to the pothole event. Now some of this is certainly due to the hard landing being a primary design consideration and thus the pothole creating the most stress as it isn't the primary design criteria, but rather a later optimization. You know how engineering is, the art of compromise (with lots of fun math).  
Quoting prebennorholm (Reply 29):
twice as heavy engines.

   Please understand I'm trying to make my posts more concise.   

Quoting prebennorholm (Reply 29):
then it is probably fairly correct to assume the weight penalty on the 738 to be at least some 4,000 to 5,000 lbs.

I would concur with that estimate.

Quoting prebennorholm (Reply 29):
Some forty years ago I saw in the aviation press some B&W pictures of VC10's which had landed hard. The fuselage had wrinkles all over. The accompanying text told that the VC10 should always meet terra firma gently - more so than most planes, or it gets wrinkles like a Ju-52 because of the four heavy engines on the tail.

One cannot always choose a soft landing. In particular if it is an airline such as DL which hires a large number of Navy reserve pilots. When I was at Pratt they had habits that were accelerating engine wear. Our project to cut the wear was halted by one photo of a taxiway incursion in front of a Pratt powered 752 that our changes would have prevented the landing aircraft from taking off as quickly and missing the 'incursion aircraft.' It was a difference of maybe 8 feet, but that would have changed a near miss to an impact. What matters with the VC10 is US Navy pilots like to 'stick it' onto the runway. That means the aircraft must be designed for thousands of hard landings. For today's aircraft, that means more weight.

Quoting prebennorholm (Reply 29):
On this forum some 4-5-6 years ago someone gave us a link to a video clip of a DC-9 hard landing test where the entire tail broke off

It was the related MD-80 (perhaps tested when it was the DC-9 Super 80?):
http://www.youtube.com/watch?v=x9deMKE-iek


Ok, I wanted to correct you it was only the tail cone, until I found the above. While I couldn't find a link, those have dropped off during hard landings due to the emergency release mechanism failing (but the plane could still fly)

A video of a test of the tail cone slide:
http://www.youtube.com/watch?v=oLFJcA3Hlgo

Test of the tail cone release (no slide):
http://www.youtube.com/watch?v=3wW03Ax1FiI

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zippyjet
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RE: "Clean Wing" Aircraft Future?

Sun Nov 11, 2012 2:32 am

Quoting airproxx (Reply 2):

A crazy question but, could we ever see a combination bird such as the late great L1011 and DC-10/MD11's? Here's why;
if we ever create or perfect hypersonic commercial travel how about a plane with wing pod engines but maybe 2 aft mounted engines for say the thrust cycle, getting the plane to hypersonic speeds. I've read even somewhere on A-Net that today to create a commercial hypersonic bird with just wing mounted engines with all the noise reduction and other environmental controls you'd have to use multiple coach bus sized engines which would create issues in itself. Why not split the engines if needed?
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RE: "Clean Wing" Aircraft Future?

Sun Nov 11, 2012 2:39 am

Quoting VC10er (Reply 30):

Why does the military aircraft (unfortunately named the "warthog") have 2 giant turbo fans mounted on top-rear? The advantage being?

Its to protect the engines from FOD on unimproved runways. It was desgined to operated from bombed out airports, roads, grass fields, etc. So there was no certainty it would be a clean surface like a well kept airport in peace time.

Also protects them from AA fire as the wing blocks much of the angles for gunfire from ground level.

Weight was also not as bad as a civilian plane as they are mounted between the tail and wing instead of with the tail. Fuselage also doesn't suffer volume constraints from having the engines there like a civilian plane. Another good thing with the tail behind is it makes the A10 much harder to lockup with IR missiles. A re-engine to a more modern engine with higher bypass would really help this too, but good luck with that.
 
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RE: "Clean Wing" Aircraft Future?

Sun Nov 11, 2012 3:39 am

Quoting VC10er (Reply 30):
Why does the military aircraft (unfortunately named the "warthog") have 2 giant turbo fans mounted on top-rear? The advantage being?

They're really not that big...I have a fan blade from one on my desk. As for why...what XT6Wagon said.

Quoting zippyjet (Reply 32):
if we ever create or perfect hypersonic commercial travel how about a plane with wing pod engines but maybe 2 aft mounted engines for say the thrust cycle, getting the plane to hypersonic speeds.

I'll be shocked if any hypersonic vehicle has podded engines. Hanging those suckers out in the free stream to generate shockwaves that will come back to hit the wing or fuselage just seems like a terrible idea.

Tom.
 
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RE: "Clean Wing" Aircraft Future?

Sun Nov 11, 2012 3:52 am

Quoting tdscanuck (Reply 34):
I'll be shocked if any hypersonic vehicle has podded engines. Hanging those suckers out in the free stream to generate shockwaves that will come back to hit the wing or fuselage just seems like a terrible idea.

Hypersonic aircraft would probably continue to have belly mounted intakes to take advantage of extra compression from the nose shockwave.
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RE: "Clean Wing" Aircraft Future?

Sun Nov 11, 2012 3:53 am

Quoting VC10er (Reply 30):
Why does the military aircraft (unfortunately named the "warthog") have 2 giant turbo fans mounted on top-rear? The advantage being?

Add to what XT6Wagon said

1. The engines blow over the horizontal stabilizer which blocks the IR signature making it tougher to shoot the plane down (as he noted) with the world's first 'high bypass' engines. (More cold air mixing with the core flow with mixing time granted by the horizontal stabilizer blocking direct view of the engine exhaust.) (Minor enhancement of his comment.)

Delete, had torque balance incorrect... (late edit).

Quoting XT6Wagon (Reply 33):
A re-engine to a more modern engine with higher bypass would really help this too, but good luck with that.

The main reason for a re-engine is more thrust at altitude. This improves cruise range and battle capabilities in high altitude locations in addition to range improvement due to more modern engines. Newer engines with better fan/core mixing would help the IR signature, but that is not the drive behind a re-engine.

Lightsaber

[Edited 2012-11-10 20:04:07]
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RE: "Clean Wing" Aircraft Future?

Sun Nov 11, 2012 4:05 am

Quoting Aesma (Reply 3):
Airbus has proposed a clean wing design, not a 727 lookalike though :

One part of that concept that bugs me is the engines will suffer a large efficiency hit as their 'pods' have so much area it will create drag upon the engine thrust resulting in reduced engine efficiency. That is a nice looking low noise design, but there are high costs to achieve that low noise.

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