Thick skin Laminar airfoil.

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Monty

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In reading one of Orion's posts in another thread he mentioned using a thick (.035) skin with countersunk rivets (not dimpled) to achieve a smooth wing profile.

I thought this deserved more discussion, since aluminum and laminar profiles are usually considered to be mutually exclusive.

Lets say, you used thick skins and thinner internal structure, so that the internal structure would tend to conform to the skin rather than the other way around. Further, lets say you made a really good jig with form blocks to profile the wing skin properly. Something like CNC cut MDF.

Could you get a laminar section to perform? Is it worth the trouble? Is it worth the weight. On my little bird, the wing would weigh twice as much as it does now, to accomplish this. The drag reduction seems to be significant if one compares "standard roughness" to smooth finish laminar section.

However, when one compares smooth finish to smooth finish the results are different entirely. So the question is: Is "standard roughness" realistic for typical sheet metal construction?

I seem to recall reading that things aren't typically that bad. The rivets are in a row chord-wise, so the disturbance is not as bad as you would think.

The laminar sections have more pitching moment. Trim drag goes up. The laminar sections all perform better at high CL numbers, but my design has relatively low CL numbers in flight, because of the stall speed requirement.....Seems to me there is really not much to gain in this department. Is going off down this path chasing ghosts?

Cessna citation: various models use the 230XX and modified versions
Questair Venture: 230XX

So it seems speed and these old sections are compatible.

It would seem that the major gains for the more exotic sections happen at high CL ie. Sailplanes, or high altitude, not so much for sport planes.

Is this Heresy?
 

orion

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Well, your thinking is in the right direction but may need a bit of tweaking. First off, the thicker skin in this type of configuration acts as the actual spar cap so the only significant structural buildup you'd need is at the root for the wing attachment. The internal structure is now really just the webbing for and reinforcement of the skin membrane. As such, your wing should not need to be twice as heavy since the skin is replacing the spar stack-up. Yes, this needs proper design but it should not be too difficult.

And yes, with the skin properly formed and also having a smooth blend with the leading edge, this should work as well for laminar flow as any molded composite skin would.

And BTW, standard roughness is not used in any real-world design so to answer your question, no, it's not realistic. This is a roughness medium that was used in the original NACA airfoil work and has become the standard for roughness definition in many of those tests. The roughness is however extreme to say the least - about equivalent to 36 grit sand paper. You're never going to see this level of surface contamination short of flying through a swarm of locusts. Even protruding rivet heads are not going to cause this much boundary layer interference.

Regarding the laminar sections and pitching moments, if you're talking about sections like the NLF family, then yes, those coefficients are sizable. But keep in mind that those sections were developed more for larger type commuter aircraft more so than for the light general aviation plane. Those applications can deal with the larger coefficient since the wing loading is quite high and the high unflapped CLmax is put to good use. A small plane does not need these qualities. Even the older 64-xxx or 65-xxx series of sections (or the Riblett equivalent) will do a much better job of what you're most likely after. So no, this is not heresy - your just seeing the light.

In the overall scheme of things, the wing is only a small fraction of the drag number for the airframe. Choosing the right airfoil is a game of compromise and optimization for cruise and should be a fairly quick process. True, if you're designing a commercial craft then the choice becomes more critical but for the light airplane other details carry as much, if not more weight when dealing with the issues of efficient design.
 

Topaz

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Aluminum laminar flow isn't impossible. The example that comes to mind immediately is the Swearington SX300. Fairly thick skins (I don't recall *how* thick, offhand, and am posting this from my phone), stretch-formed over a mandrel, then riveted to the substructure by the builder. The substructure needs to be robust to keep the skins from deforming due to aerodynamic forces.

So yeah, it's possible. But it should be obvious that this isn't your average Van's RV-x, either. The SX300 was an expensive, complex kit, reportedly one of the most difficult to build that's been offered to the market. It's hard for most people to justify that cost and complexity against the 20kts or so gained from it.

Sailplanes need extensive laminar flow for high performance because we're sucking our energy out of whatever we can find in the local environment. A sportplane generally doesn't need that level of refinement. The SX300 was the airborne equivalent of a Ferrari: performance at any price.
 

orion

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The substructure needs to be robust to keep the skins from deforming due to aerodynamic forces.
Well, actually just the opposite. Proper structural design is of course important but said process must recognize the benefit of said thicker skin. The larger skin gauge is more capable of handling the loads and maintaining panel stability than a thinner skin. The thicker skin could actually eliminate any internal stringers and may not require as many ribs. One of the best examples of this construction is on the Aerostar.
 

DeepStall

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The way most of the studies I've encountered have ended up, weight is king. Laminar flow might buy you less drag, but a lighter wing with a lighter engine that burns less gas usually does about as well, for less $$. Realistically, unless you're scrupulous about keeping the bugs off the leading edge, the flow is probably going to transition to turbulent pretty early on anyway. So, my take would be that building for large areas of laminar flow isn't worth the trouble for most powered GA aircraft.

Also, don't think of "laminar" as a black and white condition; realize it's shades of gray. In parts of the wing operating in clean air, the flow is laminar at the leading edge. Somewhere along the upper surface, the flow trips, and it's almost always turbulent there by the trailing edge. "Laminar" airfoils just attempt to delay the upper surface transition to turbulence to relatively far aft.
 

autoreply

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Could you get a laminar section to perform? Is it worth the trouble? Is it worth the weight. On my little bird, the wing would weigh twice as much as it does now, to accomplish this. The drag reduction seems to be significant if one compares "standard roughness" to smooth finish laminar section.
The question is; how much.

You'll always get laminar flow. Even with normal (non countersunk) rivets you'll get 10%, maybe more. Gliders on the other hand get to 95% on the lower side and 75-80% on the top skin and are a universe apart from your typical Cresna profile drag. Yepz, even at a low lift coefficient of 0.2 or so, they're a couple times better, with virtually no Cm, because the flaps are reflexed. The question is just; how much are you willing to trade off for it....

I would have a good though about why you're building in aluminium. If I recall correctly you prefer the material which is of course perfectly fine. You might consider several options if you're looking for a decrease in profile drag. Here again, it's a trade-off between your preference and lower drag, and, as Orion has said, it's a limited part of your total drag.

The Rutan method (hotwired foam) comes to mind as cheap, fast and simple. Alternatively, a glued alu skin might be an interesting option, it's proven by Fokker and on the Cri Cri.

One other note, I think, though not based on quantitative data, that a major part of the non-laminar behavior of alu skins comes from the wrinkles in flight. Around your ribs, spar and so on, you get tiny wrinkles, tricking the flow into a turbulent one, since the wing does bend enough to deform the skin. A typical composite skin is at least 1/4" thick (and via the Rutan method it's as thick as the wing itself) and thus doesn't really wrinkle.

If I were n your position, with your preference for alu, I would give Rutan-like wings a good thought. IMHO, an aluminium aircraft with composite wings looks like a cheap, simple, fast method to build a fast aircraft.
 

Topaz

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Well, actually just the opposite. Proper structural design is of course important but said process must recognize the benefit of said thicker skin. The larger skin gauge is more capable of handling the loads and maintaining panel stability than a thinner skin. The thicker skin could actually eliminate any internal stringers and may not require as many ribs. One of the best examples of this construction is on the Aerostar.
Ah, structures. Apparently I still have a long way to go... :ermm:

Thanks, Orion. I'll get this down someday. :)
 
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