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Faro
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787 Laminar Flow Nacelles

Fri Dec 11, 2009 12:07 pm

In this presentation, under the following link (click: Visionary Design --> Aerodynamics --> Next --> Next = "Laminar Flow Reduces Drag"):

http://www.newairplane.com/787/design_highlights/

Boeing talks of laminar flow being maintained over a greater proportion of the 787 nacelle than in existing installations (ie, mainly on the nacelle leading edges and a little behind). How is this achieved, any suction integrated in the set-up? Surely it's only a question of materials so that laminar flow nacelles leading edges should theoretically be retrofit-able to existing airliners too. When can we expect to see this?

Also, why are the nose-end of the 787 and the wing/empannage leading edges not similarly laminar-flowed?

Faro

[Edited 2009-12-11 04:11:22]
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oly720man
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RE: 787 Laminar Flow Nacelles

Fri Dec 11, 2009 5:40 pm



Quoting Faro (Thread starter):
How is this achieved, any suction integrated in the set-up?

Most likely the shape.

Very many moons ago RR did a lot of work at the lab I worked in on natural laminar flow and suction to improve engine nacelles. From what I remember the leading edge was a hyperellipse, giving a flatter front face than the usual squared relation for an ellipse would give.

Suction does work, but there are plenty of associated issues, not least the possibility of the holes getting blocked by dirt, dead insects, water, etc, and the pump and pipework losses associated with the suction system. Because the surface area of a nacelle is quite small in relation to the total surface area of the aircraft, savings using suction are quite marginal. If suction is going to be used it's most likely to appear on flying wings where its contribution will be more significant. But there will be certain operational issues that'll need to be overcome like what happens if it stops or there's a gradual deterioration in performance.

The historical problem with using suction was getting the suction distributed. The technique was tried in the late 40's with sintered porous materials, but not too successfully. It was only with the development of laser drilling where very small holes (0.2mm diam or less) could be produced in a surface quickly and easily, that suction techniques were successful. One of the problems with suction was the hole size itself. In earlier attempts with manually drilled holes, the diameter was such that suction caused local "plug hole" effects where the flow being sucked through the surface formed a jet structure that generated a wake and turbulence. With much smaller holes, the suction is felt very close to the surface and it's only the boundary layer immediately next to the surface that's removed.
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jetmech
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RE: 787 Laminar Flow Nacelles

Sat Dec 12, 2009 9:39 am



Quoting Faro (Thread starter):



Quoting Oly720man (Reply 1):
Most likely the shape.

I suspect that shape would have a lot to do with it. One way to promote laminar boundary layers is to produce a beneficial static pressure gradient along the surface in question. This pressure gradient would be one where the static pressure decreases as one moves along the surface.

On wings for example, such a pressure gradient generally exists from the leading edge to some region near the thickest point of the airfoil section. If one moves the thickest part of the airfoil aft, it is possible to extend the advantageous pressure gradient along a greater portion of the surface, and thus, provide conditions more conducive to a laminar boundary layer.

IIRC, this is what NACA did to produce the 6 series of airfoil sections, where the main advantage was a greater extent of laminar flow. I suspect Boeing may have done a similar thing with the 787 nacelle by moving the thickest part aft. Also, I seem to remember that Boeing uses a special paint on the nacelle to assist in maintaining the laminar boundary layer.

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Faro
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RE: 787 Laminar Flow Nacelles

Mon Dec 14, 2009 10:50 am



Quoting Oly720man (Reply 1):
Quoting Faro (Thread starter):
How is this achieved, any suction integrated in the set-up?

Most likely the shape.



Quoting Jetmech (Reply 2):
I suspect that shape would have a lot to do with it.

In that case, A, B and other manufacturers should be working on laminar flow nacelle mods for existing types, should they not? Seems an easy, elegant way to improve fuel burn. Is this the case?

Faro
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tdscanuck
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RE: 787 Laminar Flow Nacelles

Tue Dec 15, 2009 3:27 am



Quoting Faro (Reply 3):
In that case, A, B and other manufacturers should be working on laminar flow nacelle mods for existing types, should they not? Seems an easy, elegant way to improve fuel burn. Is this the case?

Recertifying a nacelle is not trivial...I'm not sure the savings would be enough to offset the cost of development (clean sheet designs don't have this problem because they have to pay the design and cert costs anyway).

Tom.
 
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RE: 787 Laminar Flow Nacelles

Tue Dec 15, 2009 7:43 am



Quoting Tdscanuck (Reply 4):
Recertifying a nacelle is not trivial...I'm not sure the savings would be enough to offset the cost of development (clean sheet designs don't have this problem because they have to pay the design and cert costs anyway).

Even if you are only re-designing the nacelle leading edges to hyper-elliptical shape? I'm no expert but would have thought that this would be a minor design/certification effort. After all, we're only talking about laminar flow over perhaps one seventh of the nacelle lengthwise.

Faro
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tdscanuck
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RE: 787 Laminar Flow Nacelles

Wed Dec 16, 2009 3:05 am



Quoting Faro (Reply 5):
Even if you are only re-designing the nacelle leading edges to hyper-elliptical shape? I'm no expert but would have thought that this would be a minor design/certification effort. After all, we're only talking about laminar flow over perhaps one seventh of the nacelle lengthwise.

Yeah, but you're talking about (potentially) altering the engine inlet flow. I don't see the regulators accepting that one by analysis. You'd also have to cover nacelle cooling and ice protection over again (and maybe fireextinguishing as well), although I can see all those maybe being done by analysis.

Tom.
 
oly720man
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RE: 787 Laminar Flow Nacelles

Wed Dec 16, 2009 10:28 am

Looking at other articles, it does seem that there are very tight profile tolerances on the nacelle inlet

http://www.flightglobal.com/articles...or-787s-in-bid-to-reduce-fuel.html

It will be interesting to see how it performs in service and whether abrasion will have any impact on performance.

I also see that it's Boeing rather than RR who have taken charge of this part of the design.


Laminar conditions are generally hard to achieve and maintain, and Boeing has put considerable effort into this design aspect to offset the drag of the 787’s extra large bypass engines at a time of rising fuel costs.


I suppose I assumed that it was the engine manufacturer who was responsible for the nacelle, with some input from the aircraft manufacturer. Or perhaps RR produced a nacelle with the potential for NLF and Boeing looked after the implementation of it in service.

I think the main problem with retrofitting new inlets is that they would to be longer than the inlets they'd be replacing, so there'd be no surface steps or gaps to trip the boundary layer. It wouldn't be a matter of unbolting one and fitting the other.

Comparing with, say, the A330


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Photo © Royal S King



I'm assuming that the part of the engine aft of the natural metal inlet on the A330, is a different piece, not the natural metal painted in company colours, and it's the small surface imperfections here that would trip a laminar boundary layer, if it was still laminar.
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RE: 787 Laminar Flow Nacelles

Wed Dec 16, 2009 11:30 am



Quoting Oly720man (Reply 7):
I think the main problem with retrofitting new inlets is that they would to be longer than the inlets they'd be replacing, so there'd be no surface steps or gaps to trip the boundary layer. It wouldn't be a matter of unbolting one and fitting the other.



Quoting Tdscanuck (Reply 6):
Yeah, but you're talking about (potentially) altering the engine inlet flow. I don't see the regulators accepting that one by analysis.

 checkmark 
Absolutely correct, yes. My impression that a easy retrofit was possible was triggered by the misperception of the 787's size and the size of the nacelles. It's sometimes hard to believe that it's practically as long as the A332, probably due to the 787's wider fuselage and bulbous nacelles. Proportion-wise, one's first impression is that it's 739-esque...

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RE: 787 Laminar Flow Nacelles

Thu Dec 17, 2009 4:47 am



Quoting Oly720man (Reply 7):

I also see that it's Boeing rather than RR who have taken charge of this part of the design.

That's normal. The outer mold lines of the whole aircraft is usually in the hands of the airframer, not the engine maker.

Quoting Oly720man (Reply 7):
I suppose I assumed that it was the engine manufacturer who was responsible for the nacelle, with some input from the aircraft manufacturer.

RR has a history of providing complete propulsion packages (engine/nacelle/thrust reverser), but they're the exception. Even in that case, the airframer will have huge say in the outer mold lines (i.e. the aerodynamics).

Tom.
 
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RE: 787 Laminar Flow Nacelles

Thu Dec 17, 2009 8:11 am

Not disputing that precise details of the lips might not make a difference but it raises (for me) the question of how much is lost with the "squashed" nacelles on the later 737s? While busy, anyone care to explain the squared off nacelles for many high speed jets - I don't suppose they have square fans and turbines!!  duck   duck 
 
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RE: 787 Laminar Flow Nacelles

Thu Dec 17, 2009 10:18 am



Quoting Baroque (Reply 10):
the squared off nacelles for many high speed jets

do you mean the rectangular inlets on combat aircraft?


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The short answer is shockwaves in supersonic flight. The intake is angled so that an oblique shockwave is generated from the upper lip and this helps create the pressure needed at the engine inlet, and a subsonic flow. You don't really want supersonic flows in inlets because they create shockwaves that generate high pressure variations. I think the only exception to this was Concorde that had the intake flow path profiled so that it deliberately created shocks to increase the engine pressure ratio. But Concorde was a lot longer and there was the space to do it. On combat aircraft there's not the room to do this.

Why rectangular? Well the F15 inlet, and others can be angled downwards


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and having a rectangle makes this easier.

The rectangular inlet on the F15 and others, serves the same shock generating purpose as the cone/half cone/quarter cone on other aircraft



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RE: 787 Laminar Flow Nacelles

Thu Dec 17, 2009 1:29 pm



Quoting Oly720man (Reply 11):
Quoting Baroque (Reply 10):
the squared off nacelles for many high speed jets

do you mean the rectangular inlets on combat aircraft?


Yes, and I know why they are rectangular, what I wondered was what does that do for fuel consumption subsonic which is where the ALL spend MOST of their time. Alvays ze difficult qvestuns!
 
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RE: 787 Laminar Flow Nacelles

Thu Dec 17, 2009 2:58 pm

Ah, OK.

I'm not sure that anyone has gone to the trouble of investigating the effects of different intake shapes on engine fuel consumption, though there will be effects on the overall aerodynamics of the aircraft. There aren't any planes I can think of that have different intake shapes apart from the F/A-18 which went from oval on early versions to square with the Super Hornet. I once saw an early wind tunnel model of the Tornado that had side intakes like a Mirage and, presumably, studies showed that the rectangular ones were better.

As long as the intake provides the correct pressure distribution and flow rate to the front face of the engine then the engine's happy and the intake, to some extent, is irrelevant. After that any effects are external aerodynamics/drag which will have an impact on fuel consumption
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RE: 787 Laminar Flow Nacelles

Fri Dec 18, 2009 4:20 am



Quoting Faro (Reply 8):
easy retrofit

I suspect that one would need to pay attention to a fair portion of the nacelle aft of the lip to achieve a greater extent of laminar flow.

If the Reynolds number is low enough, the boundary layer may possibly remain as a laminar one for the entire distance over which it interacts with the surface of the object. As one increases the Reynolds number, the boundary layer will likely transition from laminar to turbulent at some point on the surface, with this transition point moving further forward as the Reynolds number continues to increase.

At the high Reynolds numbers encountered in actual flight, the region of the surface with a laminar boundary layer would most likely be very small. Thus, to extend the surface region subjected to a laminar boundary layer at high Reynolds numbers requires much careful design to achieve, most probably with careful shaping and surface quality.

Any slight disturbance such as steps or irregularities in the surface, dust, insects, dirt and even vibration can be enough to initiate transition and cause the boundary layer to "prematurely" turn from laminar to turbulent at elevated Reynolds numbers.

Apparently, laminar boundary layers can remain so even in the presence of large disturbances if the Reynolds number is low enough. Laminar boundary layers can also remain so at very high Reynolds numbers, but one then needs to take extreme measures to ensure the homogeneity of the fluid, a disturbance free environment as well as a very smooth and carefully shaped object

http://en.wikipedia.org/wiki/Reynolds_number

Regards, JetMech
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RE: 787 Laminar Flow Nacelles

Fri Dec 18, 2009 9:00 am



Quoting JetMech (Reply 14):
smooth and carefully shaped object

Thanx JetMech for the detailed feedback re Reynolds. One thing that just stuck me though: those 787 nacelles leading edges are not bright and shiny metal. Does this mean that matte surfaces can somehow also be hyper-smooth? Some new technology involved here, funky coatings perhaps?

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RE: 787 Laminar Flow Nacelles

Fri Dec 18, 2009 3:41 pm



Quoting Faro (Reply 3):
In that case, A, B and other manufacturers should be working on laminar flow nacelle mods for existing types, should they not?

CFM has developed an elongated nacelle that will house the CFM56-7B Evolution engine when it starts testing. The nacelle has been flown on a 737-900ER, but it looks like all the benefits will come from the engine itself, not the nacelle.
 
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RE: 787 Laminar Flow Nacelles

Fri Dec 18, 2009 7:30 pm

Slightly off topic but the engine pylons appear to be yawed inboard at the leading edge.

Am I seeing something that isn't there or has Boeing canted them to streamline the pylons with the airflow around the fuselage?
 
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RE: 787 Laminar Flow Nacelles

Sat Dec 19, 2009 4:57 am



Quoting Faro (Reply 15):
those 787 nacelles leading edges are not bright and shiny metal. Does this mean that matte surfaces can somehow also be hyper-smooth?

I think they're titanium, which is almost never shiny unless you really work at it. Provided the scale of the roughness is smaller than the boundary layer thickness, which isn't particularly hard, the surface looks "smooth" to the airflow.

Quoting Aerodog (Reply 17):
Slightly off topic but the engine pylons appear to be yawed inboard at the leading edge.

Am I seeing something that isn't there or has Boeing canted them to streamline the pylons with the airflow around the fuselage?

It appears to be real. You can see the tilt and cant on the scaled drawings:
http://www.boeing.com/commercial/airports/acaps/787sec9.pdf

I can't think of any reason you wouldn't align them with the local flow, and the flow should be slightly outboard due to both displacement by the forward fuselage and induced flow from the wings.

Tom.
 
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RE: 787 Laminar Flow Nacelles

Sat Dec 19, 2009 10:16 am



Quoting Tdscanuck (Reply 18):
Quoting Faro (Reply 15):
those 787 nacelles leading edges are not bright and shiny metal. Does this mean that matte surfaces can somehow also be hyper-smooth?

I think they're titanium, which is almost never shiny unless you really work at it. Provided the scale of the roughness is smaller than the boundary layer thickness, which isn't particularly hard, the surface looks "smooth" to the airflow.

They are in fact aluminium (refer above link in thread opener: click on Visionary Design --> Composites). This Composites page has a materials-composition diagram for the 787 which shows that the only titanium part is the tail cone.

My money is on a funky coating on the aluminium nacelle leading edge surface...

Faro
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