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Naca airfoil performance in fabric wings

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Eduardo Fadul

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Hi all,

I have been looking for information/charts about fabric wings (CL,CD, CM), however I couldn't find any kind of paper/document. I am thinking in airfoils such as NACA 4415 or so, so nothing laminar just something pretty conventional.

How does they perform compared with conventional aluminum wings ??.

Any help will be really useful

Best regards
 

Topaz

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Fabric wings only hold a specific airfoil at each rib, and in the D-cell area ahead of the spar, if it's part of the design. In between the ribs, the fabric dips inwards a bit, creating a different airfoil. Also, 3D effects tend to dominate wing performance anyway, making airfoil selection absolutely critical only for things like sailplanes and high-performance sailplanes and top-performance cruising powered aircraft, neither of which is likely to use a fabric covered wing in the first place.

If your aircraft is of such critical performance that airfoil selection is also critical, a fabric-covered wing is probably a poor choice. If your aircraft is suitable for a fabric-covered wing, then don't obsess about airfoil choice. Choose a suitable family of airfoils by their overall, gross, characteristics, be conservative in your design process (allow margin for a slightly underperforming wing) and know that the ultimate numbers for your airplane will fall somewhere in that range.
 

clanon

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I was under the general impression of old type sailplanes with about 30% (D cell and beyond) rigid stuff (plywood) under the fabric with acceptable performance...(and some laminar flow):ponder:

PS: Designfoil and others could change from different surface finishes.To see the variations in Cl ; Cd ; and Pressure...
 

Topaz

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Didn't say, "unacceptable performance". The OP strongly suggests that he's looking for highly "predictable performance", based on a given airfoil section. That's going to be difficult in a real-world fabric wing. The different airfoil sections intra-rib will make that extremely difficult.

Also, the difference between old fabric-covered sailplanes and a modern high-performance example isn't much different than the change from a Bleriot and a 747. Not just "night and day". Totally separate universes.
 

Norman

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I have been looking for information/charts about fabric wings (CL,CD, CM), however I couldn't find any kind of paper/document. I am thinking in airfoils such as NACA 4415 or so, so nothing laminar just something pretty conventional.

How does they perform compared with conventional aluminum wings ??.
If you're building a perfectly smooth wing it makes a fairly large difference in drag but lift and pitching moment are hardly affected. I don't have wind tunnel data but here's an Xfoil analysis. I only put in some sag on the top surface because it's rather tedious moving all those points around but it shows the general trend. I think that the problem is not so much that the fabric deviates from the nominal shape as that there is a huge spike in the pressure distribution at the edge between the fabric and the D-tube. Putting this edge as far down stream of the minimum pressure point at possible would get it into an area where the turbulent boundary layer could cross it with minimum disturbance.
 

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Norman

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What about stall break ?
Beats me. My guess is that it just creates a separation bubble, after all there are thousands of planes flying reasonably safely with fabric skins. Xfoil isn't very reliable once the cl/alpha curve starts to round off near the top so even though it shows a slightly higher CLmax with the fabric sag I'd consider that to be in the noise. Within the linear range it's fairly accurate. The spike I mentioned does look hideous but I assumed that the edge of the D-tube would be at 25%c which just happens to be where the normal pressure minimum is. If it were 5 or 10% farther aft it might not have been so huge.
 

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Hot Wings

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Xfoil isn't very reliable once the cl/alpha curve starts to round off near the top so even though it shows a slightly higher CLmax with the fabric sag I
I haven't built or flown a model in decades, or even payed much attention when watching one fly to notice the fabric. Full size airplanes billow between the rib stitching in flight. In some cases the billow between the stitching chord wise is even noticeable. Is the tension is model plastic film covering high enough to maintain the sag in flight?

n66671_bonham_stubbington_2012_003.jpg
 

Norman

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I haven't built or flown a model in decades
As I recall from 30 years ago the colored mylar was quite a bit more elastic than fabrics or paper so I'm sure it would billow more under the same dynamic pressure but the pressure on a model is a small fraction of that on a full size airplane. I preferred a product called "Black Baron Micafilm" although sometimes I used saran wrap which seemed to work fine but dry grass could poke holes in it. Dr Hepperle's page that NoStealth linked to explains the problem better than I could have. There's just not much detailed data available on fabric wings. There have been some wind tunnel tests with practical construction wings but as I recall they were mostly with metal wings. There's some material in TOWS about that.
Full size airplanes billow between the rib stitching in flight. In some cases the billow between the stitching chord wise is even noticeable.
The part of the airfoil aft of the minimum pressure is just a fairing to bring the upper and lower boundary layers together smoothly (although the trailing edge angle is the chief determiner of pitching moment). The exact shape is not too critical as long as it's not bumpy in the chord-wise direction. Of course that line at the edge of the D-tube is a fairly substantial bump but the rest of the fabric should form a smooth surface. Billowing should be far less disruptive to the flow than sagging.
Is the tension in model plastic film covering high enough to maintain the sag in flight?
Yes... probably :ermm:

BTW the fiber orientation makes a huge difference in the apparent stiffness of fabric covering.
 

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Eduardo Fadul

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Thank you very much for all your answer, really give a better understand.

The plane that i have in mind is something really basic where safety and economy are the main goals. It will be something like a far prettily Rans S12.

i will be sharing more info as soon as i get something more mature.
 

Eduardo Fadul

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I have been trying to size the wings to fulfill with the stall speed parameter of no flaps, however I always ends with some huge wings, regardless if they are composites/metal/fabric.

I knew that the requirements says

(4) A maximum stalling speed or minimum steady flight speed without the use of lift-enhancing devices (VS1) of not more than 45 knots CAS at the aircraft’s maximum certificated takeoff weight and most critical center of gravity.
What if the minimum flaps set is (aprox) 10° to fulfill with LSA requirements? I mean, when the flaps are fully retracted they are at 10°, never reach a 0°.

For that reason I would like to know your opinion about that. As this configuration will be the "normal" cruise configuration, Could this be acceptable??

The impact in CDo and CDi is few however the improve in lift is considerable.

regards
 

TFF

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It is going to fly like you have 10% flaps down. It will not fly clean and crisp. How much more wing does your calculations come out to?
 

Eduardo Fadul

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Comes to almost twice the size.

With a single slot I comes achieve (theoretical) up to 2.1, however with a clean wing I only could achieve something around 1.1


S= (2*Wto)/(ro*(Vstall^2)*CL)

S= (2*1320)/(0.0023769*(70^2)*1.1)=206 ft^2
S= (2*1320)/(0.0023769*(70^2)*1.9)=119 ft^2
S= (2*1320)/(0.0023769*(70^2)*2.1)=107 ft^2

That is why I am really thinking in use a wing with something like 10° of flaps all the time. It is not the most clever decision however I think that it is the only one practical :ermm: :ermm:
 

Topaz

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... That is why I am really thinking in use a wing with something like 10° of flaps all the time. It is not the most clever decision however I think that it is the only one practical :ermm: :ermm:
The increased trim drag from carrying that amount of pitching moment from the always-deflected flaps will almost certainly well outweigh the drag of simply going with the larger wing. If you're not span-constrained, I'd start looking for low-moment, high-lift airfoils and simply accept that you're going to need a larger wing than you'd like. If your design is constrained in span, time to consider a biplane or tandem-wing design.
 
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Eduardo Fadul

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I made a fast (and wrong) analysis and you are complete right.

The general numbers shows that the Cm of the wing comes to almost twice its original value.

002.jpg

I am a little worried about the increase of the wing's weight. However, that is something easy to handle.

However, I would like to know your opinion. Do you think that the FAA would accept that, or they will say that we are anyway utilizing a high lift device?

regards
 

Topaz

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You're allowed to use a high-lift device on an LSA. It's just that you have to meet the 45 knot stall requirement without that device being deployed. Having the "up-stop" on a flap permanently affixed at 10° down-flap is probably "legal", but I'm sure you'll get an odd look from the FAA. About the only way I think it'd pass their scrutiny is if the up-stop is a permanent part of the structure, and not adjustable in any way, at any time.

But the point is moot, really. You've already seen the huge penalty in Cm for using this arrangement. That's going to translate into a really big increase in trim drag, and probably more than the drag of a larger no-flap wing, even with the increased weight. There's no "absolute good" in making the wing the tiniest possible size, particularly in a design that's going to have a regulatory top-speed constraint, like LSA.

Take a look at similar aircraft and see what their wing areas are. Yours should be coming out in the same ballpark, for a similar mission and certification standard. Somehow they managed to do it. You can, too.
 

BJC

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Comes to almost twice the size.

With a single slot I comes achieve (theoretical) up to 2.1, however with a clean wing I only could achieve something around 1.1


S= (2*Wto)/(ro*(Vstall^2)*CL)

S= (2*1320)/(0.0023769*(70^2)*1.1)=206 ft^2
S= (2*1320)/(0.0023769*(70^2)*1.9)=119 ft^2
S= (2*1320)/(0.0023769*(70^2)*2.1)=107 ft^2

That is why I am really thinking in use a wing with something like 10° of flaps all the time. It is not the most clever decision however I think that it is the only one practical :ermm: :ermm:
Power Point presentation about stall speeds here, in post 12.
http://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/16647-wing-max-cl-compared-existing-homebuilts.html#post189711


BJC
 
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