Blackbird
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BWB Wing Optimization Question

Tue Jan 01, 2008 8:41 pm

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?


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tdscanuck
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RE: BWB Wing Optimization Question

Wed Jan 02, 2008 6:06 am



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.
 
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RE: BWB Wing Optimization Question

Wed Jan 02, 2008 6:13 am

I still shudder at the thought of what it would cost to de-ice such a craft....

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RE: BWB Wing Optimization Question

Wed Jan 02, 2008 6:18 am

Why is the middle nacelle bigger?
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RE: BWB Wing Optimization Question

Wed Jan 02, 2008 8:17 am



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.
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RE: BWB Wing Optimization Question

Wed Jan 02, 2008 11:58 am

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.
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RE: BWB Wing Optimization Question

Thu Jan 03, 2008 12:35 am

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?


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RE: BWB Wing Optimization Question

Thu Jan 03, 2008 2:21 pm



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?
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RE: BWB Wing Optimization Question

Thu Jan 03, 2008 3:34 pm



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
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RE: BWB Wing Optimization Question

Thu Jan 03, 2008 5:34 pm



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.
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RE: BWB Wing Optimization Question

Thu Jan 03, 2008 5:57 pm



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:


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RE: BWB Wing Optimization Question

Thu Jan 03, 2008 6:06 pm

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?


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RE: BWB Wing Optimization Question

Thu Jan 03, 2008 8:47 pm



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.
 
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RE: BWB Wing Optimization Question

Thu Jan 03, 2008 11:02 pm

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

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RE: BWB Wing Optimization Question

Fri Jan 04, 2008 2:02 am



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.

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RE: BWB Wing Optimization Question

Fri Jan 04, 2008 2:06 am



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.
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RE: BWB Wing Optimization Question

Fri Jan 04, 2008 2:15 am



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 

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RE: BWB Wing Optimization Question

Fri Jan 04, 2008 2:57 am



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.)
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RE: BWB Wing Optimization Question

Fri Jan 04, 2008 5:07 am



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 
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Blackbird
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RE: BWB Wing Optimization Question

Fri Jan 04, 2008 5:30 am

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.


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RE: BWB Wing Optimization Question

Fri Jan 04, 2008 5:41 am

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.


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RE: BWB Wing Optimization Question

Fri Jan 04, 2008 7:13 am



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.
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RE: BWB Wing Optimization Question

Fri Jan 04, 2008 7:22 am



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.
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RE: BWB Wing Optimization Question

Fri Jan 04, 2008 5:49 pm



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.
 
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RE: BWB Wing Optimization Question

Fri Jan 04, 2008 7:15 pm



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.
 
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RE: BWB Wing Optimization Question

Fri Jan 04, 2008 8:18 pm

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 would be ring shaped like a bypass door or blow-in door) which would skim off the turbulent air before it reaches the rest of the engine, where'd you'd dump the turbulent flow I don't know) skim type design.

Could an electrical pump work as good/better than a turbocompressor pump? How would it fair reliability wise?


Tom,

If you were to put this in order, which is the most, and least efficient?

-Engine itself ingesting boundary layer to reduce drag
-Turbocompressor pump driven off engine to suck away boundary layer to reduce drag
-Venturi pump driven off engine used to suck away boundary layer to reduce drag
-Electrically driven pump driven off engine to suck away boundary layer to reduce drag

Efficiency wise, how does a venturi pump fair compared to a turbocompressor, or electrically-driven pump?


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RE: BWB Wing Optimization Question

Fri Jan 04, 2008 8:56 pm

I need to hang out in this forum more often. One really learns a great deal.  thumbsup 
 
tdscanuck
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RE: BWB Wing Optimization Question

Sat Jan 05, 2008 12:03 am



Quoting Blackbird (Reply 25):
Well the turbocomp pump would tax the engine less, especially if you skimmed the turbulence off properly...

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 drag reduction on the airframe, so you end up with a net benefit even though the engine is working a little harder. This is where focussing on individual system optimization can get you in trouble.

Quoting Blackbird (Reply 25):

If you were to put this in order, which is the most, and least efficient?

-Engine itself ingesting boundary layer to reduce drag
-Turbocompressor pump driven off engine to suck away boundary layer to reduce drag
-Venturi pump driven off engine used to suck away boundary layer to reduce drag
-Electrically driven pump driven off engine to suck away boundary layer to reduce drag

Efficiency wise, how does a venturi pump fair compared to a turbocompressor, or electrically-driven pump?

I'm assuming you mean efficiency to be minimum fuel burn. I'm not sure I can really answer this, since I'm not sure how much boundary layer we're talking about sucking away. If we're just talking about the inlet, I would expect the order to be:
1. Engine ingest the boundary layer
2. Electrically driven pump
3. Turbocompressor
4. Venturi

However, this ignored mechanical complexity, cost, etc., etc.

If we're talking about sucking the boundary layer off of larger portions of the airframe, the drag increment could be quite large and I really don't know how that trade would look.

Tom.
 
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RE: BWB Wing Optimization Question

Sat Jan 05, 2008 12:46 am

Just out of curiousity,

How is the engine ingesting the boundary layer more efficient than having an electric or turbocompressor pump? The turbocompressor pump would only degrade a tiny amount of engine power, while the whole engine would take a bigger hit if they ingested it directly right?

What am I missing?


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RE: BWB Wing Optimization Question

Sat Jan 05, 2008 1:31 am



Quoting Blackbird (Reply 28):
How is the engine ingesting the boundary layer more efficient than having an electric or turbocompressor pump? The turbocompressor pump would only degrade a tiny amount of engine power, while the whole engine would take a bigger hit if they ingested it directly right?

What am I missing?

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 enter the inlet. If you use a pump to suck away the boundary layer, the nacelle is in the freestream flow, which has higher nacelle drag than if the nacelle is partly in the boundary layer.

With boundary layer ingestion, if I'm reading the AIAA paper right, the engine runs a little less efficiently because you have less pressure recovery in the inlet but the total airframe drag goes down so you don't need as much thrust. Done right, the overall effect is a net decrease in fuel burn because the drag reduction overshadows the engine efficiency loss.

Tom.
 
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RE: BWB Wing Optimization Question

Sat Jan 05, 2008 1:44 am

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?


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RE: BWB Wing Optimization Question

Mon Jan 07, 2008 5:38 pm



Quoting Blackbird (Reply 30):
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?

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 energy to pressure), and slowing the air down is the direct cause of the nacelle drag. If you suck away the slower moving boundary layer, you improve pressure recover but increase nacelle drag for the same reason. Since they're two sides of the same phenomenon, separating them would probably be very difficult.

A variable geometry inlet should be able to achieve the best of both worlds by matching the current conditions to get the best balance of drag reduction and pressure recover, but that's a heavy and complex piece of equipment that probably couldn't buy its way onto a commercial jet.

Tom.
 
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RE: BWB Wing Optimization Question

Mon Jan 07, 2008 6:20 pm

Tdscanuck,

Wait... I thought the ingestion of the boundary layer was to improve airflow over the fuselage, reduce drag, not improve engine performance? Engine performance would be reduced a bit and drag from the fuselage would be improved. And I thought the drag-reduction wasn't the nacelle pods itself but the area on the aft of the wings in front of the engine area?

Couldn't you use a splitter plate to skim off the turbulent air heading right into the engine, and allow the turbulent parts of the flow to go around the pods which would be slower and would reduce the nacelle pod drag if that is a factor?


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RE: BWB Wing Optimization Question

Tue Jan 08, 2008 5:39 pm

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 splitter plates like the pleague, but the B-2 is a long range plane in need of fuel-efficient engines, so it really can't do without splitter plates.
 
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RE: BWB Wing Optimization Question

Tue Jan 08, 2008 6:18 pm

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;



...or are there effects, (not least of which might be the moment arm of the thust), which would prevent it in full scale?
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RE: BWB Wing Optimization Question

Tue Jan 08, 2008 8:04 pm



Quoting Rheinbote (Reply 33):
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 splitter plates like the pleague, but the B-2 is a long range plane in need of fuel-efficient engines, so it really can't do without splitter plates.

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 the engine in an S-duct, which pretty much rules out boundary layer ingestion.

Tom.
 
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RE: BWB Wing Optimization Question

Wed Jan 09, 2008 6:42 pm

Tom,

Can you answer my question on reply 32?


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RE: BWB Wing Optimization Question

Wed Jan 09, 2008 11:00 pm



Quoting Blackbird (Reply 32):
Wait... I thought the ingestion of the boundary layer was to improve airflow over the fuselage, reduce drag, not improve engine performance? Engine performance would be reduced a bit and drag from the fuselage would be improved. And I thought the drag-reduction wasn't the nacelle pods itself but the area on the aft of the wings in front of the engine area?

Couldn't you use a splitter plate to skim off the turbulent air heading right into the engine, and allow the turbulent parts of the flow to go around the pods which would be slower and would reduce the nacelle pod drag if that is a factor?

Ingestion of the boundary layer is, mostly, to decrease nacelle drag. The trade is worse engine performance. You probably get some drag reduction on the fuselage ahead of the inlet, but the majority (as I understand it) comes from lower ram drag in the inlet.

If you use a splitter plate to skim off the boundary layer the ram drag is going to go up.

The basic physics are something like this: the inlet is capable of ingesting more air than the engine needs. This is why you get a pressure rise at the fan face. It also means some of the air that is heading for the inlet can't actually go in, it has to go around. That means the air has to slow down (relative to the airframe), which is a momentum change. Any momentum change is reacted against the aircraft, which is the source of ram drag. If the air has already been slowed down by skin friction (the boundary layer) then it doesn't have to slow down as much approaching the inlet. That means less momentum change, which means less drag.

Tom.
 
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RE: BWB Wing Optimization Question

Wed Jan 09, 2008 11:19 pm

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?
 
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RE: BWB Wing Optimization Question

Thu Jan 10, 2008 12:31 am



Quoting Blackbird (Reply 38):
Is the ram-drag reduction really worth the loss of engine performance?

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 dependent on how well it's done. In other words, it's probably really easy to make the overall equation worse but there are small pockets where it's better. This is kind of similar to the case of the HondaJet nacelles...effective, but only within a very narrow design space.

Quoting Blackbird (Reply 38):
Why does the X-48 not use such a boundary layer ingestion?

No idea. If I had to guess, I'd say it's because the X-48 is primarily for researching flight controls (I think) and they aren't going for minimum fuel burn. It's a research aircraft and something like boundary layer ingestion is probably better studied in a wind tunnel than in the field at this stage of the game.

Tom.
 
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RE: BWB Wing Optimization Question

Thu Jan 10, 2008 12:41 am

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 stalling or surging during maneuvers in flight and such?

BTW: The nacelle design on the BWB drawing (the first picture in post one) is that the design for boundary layer ingestion or not?


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RE: BWB Wing Optimization Question

Thu Jan 10, 2008 1:20 am



Quoting Blackbird (Reply 40):
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 stalling or surging during maneuvers in flight and such?

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 goes to low. The risk is that, done wrong, the drag decrease from eating the boundary layer won't be big enough to overcome the reduced engine efficiency due to lower inlet pressure. You're trading two sensitive and highly coupled design parameters against one another...if you don't trade carefully, the system-level gain is negative.

Quoting Blackbird (Reply 40):
BTW: The nacelle design on the BWB drawing (the first picture in post one) is that the design for boundary layer ingestion or not?

It sort of looks like it, but I don't think you can tell for sure because you can't see the bottom edge of the inlet. The top and front views are no help and, in the side view, it looks like the view of the inlet base is blocked by the outer upper wing surface. It could be a boundary layer ingestion inlet or there could be a scoop out of the center aft fuselage so the inlet is proud of the upper wing surface.

Tom.
 
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RE: BWB Wing Optimization Question

Thu Jan 10, 2008 1:26 am



Quoting Blackbird (Reply 38):

Its a model, turbulence and Reynolds number are a direct relation to size, you cannot scale down such things. A radio-controlled airplane, literally, need not worry about such things and they wouldn't move that fast to investigate long term effects and such.

The X-48 model was the demonstrate flight characteristics of blended wing body aircraft, not sucking boundry layers into the engine; well, as I understood it.
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Blackbird
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RE: BWB Wing Optimization Question

Thu Jan 10, 2008 3:08 am

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 drag of the engine pods and wouldn't reduce ram-drag per-se, but wouldn't you still reduce drag by trimming away some of that turbulent flow in front of the pods?

If so, how significant would just that be?


Andrea Kent
 
tdscanuck
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RE: BWB Wing Optimization Question

Thu Jan 10, 2008 7:42 pm



Quoting Blackbird (Reply 43):

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 drag of the engine pods and wouldn't reduce ram-drag per-se, but wouldn't you still reduce drag by trimming away some of that turbulent flow in front of the pods?

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 point of sucking away turbulent boundary layers).

Quoting Blackbird (Reply 43):
If so, how significant would just that be?

I have no idea. It should scale with the amount of area that you're affecting. Laminar boundary layers still have some drag so, at least to first order, if you're sucking in the boundary layer over 5% of the skin you'd get less than 5% skin friction drag reduction (no real effect on form or induced drag).

Tom.
 
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RE: BWB Wing Optimization Question

Thu Jan 10, 2008 9:06 pm

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 range of 6,000 nm, 7,500 nm, or 8,500 nm @ a cruise speed of mach 0.85, mach 0.88, and mach 0.90?


Andrea Kent
 
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RE: BWB Wing Optimization Question

Thu Jan 10, 2008 9:24 pm



Quoting Blackbird (Reply 45):
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 range of 6,000 nm, 7,500 nm, or 8,500 nm @ a cruise speed of mach 0.85, mach 0.88, and mach 0.90?

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. You'd have to run an optimizer to figure it out.

Tom.
 
Blackbird
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RE: BWB Wing Optimization Question

Fri Jan 11, 2008 2:11 am

Tom,

Do you think such a system would work as good as they say it would?


Andrea Kent
 
tdscanuck
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RE: BWB Wing Optimization Question

Sat Jan 12, 2008 3:02 am



Quoting Blackbird (Reply 47):

Do you think such a system would work as good as they say it would?

I can safely say I have no idea.

Tom.
 
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RE: BWB Wing Optimization Question

Thu Jan 17, 2008 12:05 am



Quoting 2H4 (Reply 2):
I still shudder at the thought of what it would cost to de-ice such a craft....

2H4

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 would have concieved of to de-ice the aircraft more efficiently.
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