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BWB Wing Optimization Question  
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Posted (6 years 7 months 3 days 4 hours ago) and read 5331 times:

I was shown awhile back a graphic of a BWB which reflected the design they were currently using and an optimized design.

This was basically the non-optimized design



This was the optimized design (the original design was smaller, I printed out, used a pen to smooth out the pixellatedness, then scanned it back in at a larger size)
http://i161.photobucket.com/albums/t240/AVKent882/BWB-Optim001.jpg


The question is, if the optimized shape is the best shape... why does Boeing's X-48B look more like the original BWB design even though the other one would be better?


Andrea Kent

53 replies: All unread, showing first 25:
 
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 1, posted (6 years 7 months 2 days 18 hours ago) and read 5299 times:



Quoting Blackbird (Thread starter):
The question is, if the optimized shape is the best shape... why does Boeing's X-48B look more like the original BWB design even though the other one would be better?

The key question is probably "optimized for what?". The graphics you have appear to be for commercial passenger transports. I'm pretty sure the X-48B's are primarily geared towards research for military missions. I strongly suspect the optimum configuration isn't the same for both cases.

Tom.


User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 2, posted (6 years 7 months 2 days 18 hours ago) and read 5295 times:
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I still shudder at the thought of what it would cost to de-ice such a craft....

2H4



Intentionally Left Blank
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17002 posts, RR: 67
Reply 3, posted (6 years 7 months 2 days 18 hours ago) and read 5291 times:

Why is the middle nacelle bigger?


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 4, posted (6 years 7 months 2 days 16 hours ago) and read 5281 times:



Quoting Starlionblue (Reply 3):

Probably because it is in the middle?  Wink

Think of the DC-10/MD-11, look at the distance in front of the engines, there will be a more turbulent layer before the middle engine's intake than either side. So to maintain the same fan-face air pressure (to result in the same thrust), the nacelle would have to be longer.



The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offlineKeta From Germany, joined Mar 2005, 448 posts, RR: 0
Reply 5, posted (6 years 7 months 2 days 12 hours ago) and read 5261 times:

When did you see the optimized design? Maybe Boeing worked on the design of the X-48 before the optimized shape was made. Or it might be what Tdscanuck says.


Where there's a will, there's a way
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 6, posted (6 years 7 months 2 days ago) and read 5211 times:

Tdscanuck,

I actually don't know. I would guess cruise, but you sound like you have WAY more aerodynamic engineering expertise than I do. Do you have any guesses?


2H4,

LOL, probably a bloody fortune


Keta,

To the best of my knowledge, the optimized design shape was made BEFORE the X-48 was developed.


And this one is kind of a question for everybody...

Why did BWB's plan to use the fan itself to skim the turbulent layer away, which would obviously reduce engine power? Couldn't they develop a means of getting rid of the turbulent layer by using a turbocompressor driven device mounted under the engine pods to skim off the turbulent air -- it would get the job done, burn less fuel and provide better compressor efficiency. Am I missing something?


Andrea Kent


User currently offline3201 From , joined Dec 1969, posts, RR:
Reply 7, posted (6 years 7 months 1 day 10 hours ago) and read 5189 times:



Quoting Tdscanuck (Reply 1):
The key question is probably "optimized for what?".

 checkmark 
Depending on the timeframe, and what the objective and constraints were... there is no such thing as a single "optimized" design. The A380 and the A350 are both optimized, by the same people, but not identical looking. Something as simple as a change in the projected average fuel price can change a design visibly.

Quoting Blackbird (Reply 6):
but you sound like you have WAY more aerodynamic engineering expertise than I do

For one thing, optimizing just for aerodynamics does not give you the most optimal design. If you look through aircraft design optimization literature, there are a lot of studies that optimize partial objectives without all constraints -- they're useful, because they develop or validate specific physical or optimization models, but they don't necessarily result in a "best" or "good" design. As an example, it's pretty widely known that you need to put in a landing gear placement constraint to get good results when optimizing traditional wing/tail combinations.

Without seeing the source for the optimized design you show, it's hard to tell what's going on. Do you have a reference?

Quoting Blackbird (Reply 6):
Couldn't they develop a means of getting rid of the turbulent layer by using a turbocompressor driven device mounted under the engine pods to skim off the turbulent air -- it would get the job done, burn less fuel and provide better compressor efficiency.

Even if it improved overall aircraft performance, with the added mechanical losses and weight, I'd be worried about reliability. Would you be able to fly if it was inop, or would the performance hit ground you? What if it failed during flight, would you have to do flight planning scenarios to ensure you could safely divert if it failed?


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17002 posts, RR: 67
Reply 8, posted (6 years 7 months 1 day 9 hours ago) and read 5177 times:



Quoting 3201 (Reply 7):
For one thing, optimizing just for aerodynamics does not give you the most optimal design.

If nothing else, you have to decide on aerodynamic optimization for a certain speed range. As soon as you have variable speeds...

I guess some of the most aerodynamically optimized designs are by Rutan. But Proteus and Voyager are very far from ideal shapes for carrying self loading cargo. Big grin



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineSEPilot From United States of America, joined Dec 2006, 6836 posts, RR: 46
Reply 9, posted (6 years 7 months 1 day 7 hours ago) and read 5165 times:



Quoting Tdscanuck (Reply 1):

The key question is probably "optimized for what?"

 checkmark   checkmark 
You also have to be specific as to what you mean by "optimized". For commercial transports that usually means lowest cost per unit carried, but for military applications that is a minor consideration. I envision an eventual BWB airliner as being somewhat of a cross between the "tube with wings" and a true BWB; or perhaps a cross between a tube with wings and a delta wing. The point is that optimization for aerodynamics alone will often impose constraints that limit utilization, and hence render it uneconomic. An example: high wings are inherently more efficient than low wings, due to the necessity of having dihedral (which wastes lift) on the low wings to provide stability (gravity providing the same effect for high wings-hence the absence of low winged birds). But the difficulty of providing acceptable landing gear on high winged planes has made the breed extremely scarce in transport aircraft; it works out that the cost and weight of the landing gear outweighs the benefits of greater aerodynamic efficiency. There are many possibilities for an eventual BWB transport, but much work remains to be done, and the end result will be a compromise between an aerodynamically optimized design and practical considerations.



The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 10, posted (6 years 7 months 1 day 6 hours ago) and read 5162 times:



Quoting Blackbird (Reply 6):
Why did BWB's plan to use the fan itself to skim the turbulent layer away, which would obviously reduce engine power? Couldn't they develop a means of getting rid of the turbulent layer by using a turbocompressor driven device mounted under the engine pods to skim off the turbulent air -- it would get the job done, burn less fuel and provide better compressor efficiency. Am I missing something?

I wasn't aware that the BWB designs were ingesting the boundary layer into the engines. I can't tell from the photos in this thread since you can't see the lower edge of the inlet. I'd be surprised if they don't have a splitter plate or some offset in there though. Keep in mind that the boundary layer is on the order of inches to a few feet thick at the trailing edge...on an aircraft the size of a notional commercial BWB, that much offset would look very small on a drawing.

If it really does have flush intakes, I'd expect something like the B-2's solution with splitter plates:


Tom.


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 11, posted (6 years 7 months 1 day 6 hours ago) and read 5159 times:

Tom,

I don't know the exact particulars, but I remember them talking about using the engines to ingest the boundary layer. Splitter plates do sound like a better idea. But if extra suction was required, you could put a turbocompressor driven pump or some device to suck the layer out which would require less engine power loss right?


Andrea Kent


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 12, posted (6 years 7 months 1 day 3 hours ago) and read 5151 times:



Quoting Blackbird (Reply 11):
But if extra suction was required, you could put a turbocompressor driven pump or some device to suck the layer out which would require less engine power loss right?

If you really did need extra suction, you could do it with a turbocompressor pump. I'd go for a jet (Venturi) pump myself, just for mechanical simplicity, although you'd give up some efficiency that way.

Tom.


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 13, posted (6 years 7 months 1 day 1 hour ago) and read 5139 times:

What's the difference between a venturi pump and a turbocompressor pump?

Andrea Kent


User currently offline3201 From , joined Dec 1969, posts, RR:
Reply 14, posted (6 years 7 months 22 hours ago) and read 5120 times:



Quoting Starlionblue (Reply 8):
If nothing else, you have to decide on aerodynamic optimization for a certain speed range.

Still not good enough. If nothing else, need to consider weight as well, and for aerodynamics of the entire aircraft (not just the wing) need to include trim drag, and...

Quoting Starlionblue (Reply 8):
I guess some of the most aerodynamically optimized designs are by Rutan.

Rutan's designs are definitely not aerodynamically optimized -- he always uses canards, what cannot be aerodynamically optimal. I know you won't believe me, but maybe you' ll believe OldAeroGuy in this thread:

Quoting OldAeroGuy (Reply 30):

Canards are less efficient aerodynamically than conventional configurations in two primary areas:

1) Induced drag due to the high fraction of total lift that must be carried by a stable canard. Since the span of the canard is less than that of the wing, higher induced drag is the result.

2) Higher wetted area drag due to the inability of a stable canard to stall the wing. Since the wing cannot develop its full lift potential on a canard configuration, the total lifting surface area on a canard (canard wing) is greater than that of a conventional configuration (wing h. tail) for the same takeoff/landing operating speeds.

For these reasons, stable canards are less efficient than conventional configurations for all phases of flight. The number of canards airplanes built vs the number of conventional airplanes tell the tale.



User currently offline3201 From , joined Dec 1969, posts, RR:
Reply 15, posted (6 years 7 months 22 hours ago) and read 5119 times:



Quoting Tdscanuck (Reply 10):
I wasn't aware that the BWB designs were ingesting the boundary layer into the engines.

This is another one that has been discussed here at least twice -- here is an AIAA paper that talks about it, and here is a thread where it was discussed, and here is another.


User currently offline2H4 From United States of America, joined Oct 2004, 8955 posts, RR: 60
Reply 16, posted (6 years 7 months 22 hours ago) and read 5117 times:
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Quoting OldAeroGuy:
Induced drag due to the high fraction of total lift that must be carried by a stable canard. Since the span of the canard is less than that of the wing, higher induced drag is the result.

But when examining the total induced drag, one must take the aspect ratio into account, no? If we consider only the span of the canard, we're not looking at the whole picture.

That said, considering the vast discrepancy between OldAeroGuy's level of knowledge, and 2H4's level of knowledge, I'm sure I'm missing something.  biggrin 

2H4



Intentionally Left Blank
User currently offline3201 From , joined Dec 1969, posts, RR:
Reply 17, posted (6 years 7 months 21 hours ago) and read 5111 times:



Quoting 2H4 (Reply 16):
But when examining the total induced drag, one must take the aspect ratio into account, no? If we consider only the span

No -- induced drag is based on lift per unit span. Aspect ratio gets involved when you do lift and drag coefficients, but if you look at total lift and total drag, not the coefficients, there's no AR in the equation, it's span.

(But at lower AR and the same total lift and the same span, you have way more S, so a lower wing loading, more parasite drag, a lower lift coefficient, etc. This is why usually you talk about going to higher AR.)


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17002 posts, RR: 67
Reply 18, posted (6 years 7 months 19 hours ago) and read 5101 times:



Quoting 3201 (Reply 14):

Rutan's designs are definitely not aerodynamically optimized -- he always uses canards, what cannot be aerodynamically optimal. I know you won't believe me, but maybe you' ll believe OldAeroGuy in this thread:

Why won't I believe you?  scratchchin 



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 19, posted (6 years 7 months 19 hours ago) and read 5097 times:

The statement of canards not being efficient doesn't make sense to me. A canard produces lift, a tail produces downforce which takes away some lift from the wing. A canard sounds like a better choice even though it slightly reduces the wing's lift which rides in the wake.

The only issue is that it doesn't ride the downwash like a tailplane would and big-flaps are harder to pull off with canards.


Andrea Kent


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 20, posted (6 years 7 months 19 hours ago) and read 5094 times:

3201,

Still, wouldn't it be better to use a turbocompressor driven device to injest the boundary layer which only takes a tiny bit out of the engine than have the engine do it directly which would take more out of the engine? I know it reduces drag, but turbulence is not good for a jet engine efficiency wise.


Andrea Kent


User currently offline3201 From , joined Dec 1969, posts, RR:
Reply 21, posted (6 years 7 months 17 hours ago) and read 5086 times:



Quoting Blackbird (Reply 19):
The statement of canards not being efficient doesn't make sense to me. A canard produces lift, a tail produces downforce which takes away some lift from the wing. A canard sounds like a better choice even though it slightly reduces the wing's lift which rides in the wake.

The only issue is that it doesn't ride the downwash like a tailplane would and big-flaps are harder to pull off with canards.

If you correctly include even a subset of the relevant factors, and optimize the size and position of the canard and main wing, they both move very far back, and the canard becomes the bigger of the two. I don't have too many references handy, but here's one.


User currently offline3201 From , joined Dec 1969, posts, RR:
Reply 22, posted (6 years 7 months 17 hours ago) and read 5085 times:



Quoting Blackbird (Reply 20):

Still, wouldn't it be better to use a turbocompressor driven device to injest the boundary layer which only takes a tiny bit out of the engine than have the engine do it directly which would take more out of the engine? I know it reduces drag, but turbulence is not good for a jet engine efficiency wise.

I don't know -- I've never seen any results of such a study, and don't have a good feel for it off the top of my head. In general, though, I hate to see another part/system introduced that's critical to performance and reduces the worth of your asset (as an aircraft operator) when it's inop, plus probably increases the cost of periodic maintenance as well, and increases production costs so either increases acquisition cost or decreases airframe manufacturer profit. Can you make it as reliable as a winglet? Would it offset some of the factors I list by making the engines themselves more reliable, cheaper to maintain, cheaper to install initially? The great paradox is that the more benefit it provides, the more screwed you are if it's ever broken and you want to fly.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 23, posted (6 years 7 months 6 hours ago) and read 5054 times:



Quoting Blackbird (Reply 13):
What's the difference between a venturi pump and a turbocompressor pump?

A turbocompressor uses a two rotors to move fluid around. The turbine takes high pressure air and converts it to mechanical work. That work is used to drive another rotor which acts as a pump/compressor.

A venturi pump blows a high pressure stream through a venturi, which drops the pressure down. You put a tap just below the venturi throat and use this to suck fluid from wherever you want to get it. It's not tremendously efficient, but its small and light, has zero moving parts, and basically infinite service life when used with non-abrasive fluids.

Quoting Blackbird (Reply 19):
The statement of canards not being efficient doesn't make sense to me. A canard produces lift, a tail produces downforce which takes away some lift from the wing. A canard sounds like a better choice even though it slightly reduces the wing's lift which rides in the wake.

Long and semi-exhaustive discussion of this very topic. In a nutshell, a canard can do better than a horizontal stabilizer under some circumstances but you have to give up on other things:
(Dis-) Advantages Of Canard And V-Tail Designs (by Flexo Sep 21 2007 in Tech Ops)

Tom.


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 24, posted (6 years 7 months 5 hours ago) and read 5049 times:



Quoting 3201 (Reply 15):

This is another one that has been discussed here at least twice -- here is an AIAA paper that talks about it, and here is a thread where it was discussed, and here is another.

Great references. Thank you 3201! The AIAA paper is especially interesting...far to readably written for typical academia.

Tom.


25 Blackbird : 3201, Well the turbocomp pump would tax the engine less, especially if you skimmed the turbulence off properly... a splitter plate, or an annular (it
26 Post contains images Stitch : I need to hang out in this forum more often. One really learns a great deal.
27 Tdscanuck : The main message I got from the threads and AIAA paper on boundary layer ingestion is that the amount you tax the engine isn't as big as the overall
28 Blackbird : Just out of curiousity, How is the engine ingesting the boundary layer more efficient than having an electric or turbocompressor pump? The turbocompre
29 Tdscanuck : You're missing the fact that using a pump to suck away the boundary layer ahead of the inlet negates the benefit on nacelle drag of having slower air
30 Blackbird : Is there any way to figuratively have your cake and eat it too in regards to getting the high pressure recovery and skim off the turbulent flow? Andre
31 Tdscanuck : I'm not sure that's possible. The increased pressure recover in the inlet is a direct result of having to slow the air down more (converting kinetic
32 Blackbird : Tdscanuck, Wait... I thought the ingestion of the boundary layer was to improve airflow over the fuselage, reduce drag, not improve engine performance
33 Rheinbote : If making an axial turbofan digest boundary layer air was a good idea, the B-2 wouldn't have splitter plates. On a stealth plane, you want to avoid sp
34 Post contains images BAe146QT : Maybe they're on little pylons, like a normal engine inverted? I know that the X-48B is subscale, but is there any reason why they couldn't do this; .
35 Tdscanuck : But on a stealth airplane you also really want to avoid exposing the fan face, even more than you want to avoid splitter plates. That means burying t
36 Blackbird : Tom, Can you answer my question on reply 32? Andrea Kent
37 Tdscanuck : Ingestion of the boundary layer is, mostly, to decrease nacelle drag. The trade is worse engine performance. You probably get some drag reduction on
38 Blackbird : Tom, Is the ram-drag reduction really worth the loss of engine performance? Andrea Kent BTW:Why does the X-48 not use such a boundary layer ingestion?
39 Tdscanuck : According to the AIAA/Stanford study, yes, if you do it right. Nacelle/wing interaction is a tricky topic on a good day so I'm sure it's highly depen
40 Blackbird : Tom In regards to the parameters where the Boundary-Layer Ingestion that WOULDN'T work: Are you saying that the engines would be on the verge of stall
41 Tdscanuck : Not necessarily. Although boundary layer ingestion will lower your fan face pressure, that's not going to lead to a surge/stall unless the pressure g
42 Lehpron : Its a model, turbulence and Reynolds number are a direct relation to size, you cannot scale down such things. A radio-controlled airplane, literally,
43 Blackbird : Tom, Even if you used suction to draw away the turbulent flow from the fuselage with a pump and use a normal air intake... you wouldn't reduce the dra
44 Tdscanuck : I assume you'd reduce the skin friction drag in the area of the suction, assuming you could maintain a laminar boundary layer (which is the whole poi
45 Blackbird : If the whole boundary layer ingestion worked right -- how much of a performance boost could it provide? How much range would it add to a plane with a
46 Tdscanuck : For % changes for one case, take a look at the AIAA paper that 3201 referenced in Reply 15. For specific range/cruise speed cases, I have no idea. Yo
47 Blackbird : Tom, Do you think such a system would work as good as they say it would? Andrea Kent
48 Tdscanuck : I can safely say I have no idea. Tom.
49 CJAContinental : Well, I would account for advances in technology before stating something like that. On that note though, I would have to agree that a new technology
50 Flipdewaf : The BWB's wont likely be going anywhere near that speed though, the thickness causes them to suffer from the flow going supersonic doesn't it? That's
51 Tdscanuck : I'm not sure that it's the absolute thickness that would be an issue, it would be the fine-ness. If you pick the same thickness ratio and sweep, I'm
52 Flipdewaf : just going on what my aerodynamics teacher told me, he said a cruise up M0.7 is about as high as he thinks it could handle. Fred
53 3201 : Not sure why he thought that, but I'm guessing he hasn't done any BWB research himself and is just going on some basic rule of thumb about wings or a
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