Not-so-solid massive core wings: Lightening the core foam
- Thread starter Vigilant1
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Thanks. I was wrong--about being wrong. I thought I'd read, in TOWS, that stagnation pressures can exceed q. However, I can't find that when I re-read the book today. More definitively, when I look at experimental data (NACA Report 563), I don't see any pressures around the airfoil that exceed the pitot pressure (except for minor instances that likely resulted from experimental error).
Maybe the source of my confusion regarding the possibility of pressures at the stagnation point exceeding q. was this: I (inadvisedly) spent some time reading about >compressible< flow and pressure patterns yesterday. With compressible flow, fluid density at the stagnation point can be higher than at the static point, which might change things a bit? I didn't want to risk re-reading that material to find a citation because it has nothing to do with my sub-150 knot world and I should never have entered that forest to begin with.
Thanks again for catching this--that's one less wrong thing I "know"!
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Seems like one could also make a burrito wing with a firmer foam on the outside, and a lighter foam on the inside.
REVAN
Well-Known Member
'q' (i.e. - 1/2*rho*V^2) is the stagnation pressure. By definition, they are one and the same.
Yes, true for incompressible fluids (which is what we care about in our sub-300 knot world). But, consideration of compressible fluids requires that we account for increases in rho (density) at the stagnation point. When that happens (using the same formula), stagnation pressure will be higher than dynamic pressure.
But you are correct that stagnation pressure = dynamic pressure at the airspeeds of concern to us.
Maybe on wings, but not on control surfaces. Many of the control surfaces on WWII fighters were fabric covered, including Mustang elevator and rudder, most of the Grumman fighters' surfaces, the Corsair. The US was not alone here, as German and Japanese fighters also had a lot of fabric covered control surfaces.
Billski
Can I ask your opinion on extending control surfaces chord for lighter weight of whole wing ? Example taken from BoKu carbon max and seeing how Cora ul is build with solid D nose and fabric rear part of the wing.
30% of D nose, 45% of fabric, 25% of controls, versus 65% of solid cored wing and 35% of control surface covered with fabric. For 10m2 / 107 sq ft we are wasting in ideal case 15-20 lb / 7-10 kg of weight if we change fabric to adequate solid skin. On latter one - half of it. Or nil, because transition from solid skin to fabric needs some filler, or you are cowering D cell with fabric too as done for most wooden wing with plywood D cell..
Dillpickle
Well-Known Member
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What is being proposed by Vigilant1 is to start with a massive core and then remove some of the core...
So, why is no one actually running some numbers? All this "theory" is pretty weak until you check on how much weight is removed and if the remaining structure will carry your flight and build loads. are you guys afraid of analysis? You can play "what if" and answer your own questions...
On to flight loads...
What about me? Listen, I retired five years ago so I could do what I want, not what somebody else wants...
So what is holding your guys up? Come on, let's see some analysis and testing!
Billski
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Man...this is some free college education stuff. My best professors showed us the tools and pointed the way, not spoon fed the answers...
What is being proposed by Vigilant1 is to start with a massive core and then remove some of the core...
So, why is no one actually running some numbers? All this "theory" is pretty weak until you check on how much weight is removed and if the remaining structure will carry your flight and build loads. are you guys afraid of analysis? You can play "what if" and answer your own questions...
On to flight loads...
What about me? Listen, I retired five years ago so I could do what I want, not what somebody else wants...
So what is holding your guys up? Come on, let's see some analysis and testing!
Billski
[/QUOTE]
Man...this is some free college education stuff. My best professors showed us the tools and pointed the way, not spoon fed the answers...
That's quite a find, thanks.
So, in the modeled case, the Reynolds number is 1,000,000 and the AoA is 15 deg. The airspeed field ("Mach") is listed as zero, I don't quite know why that is. Maybe it doesn't matter because Cp is a ratio between freestream pressure ("0") and the dynamic pressure ("1"). So, you can use the depiction to find the actual pressures simply by providing your own value of q for the airspeed and air density of interest (?)
If this model represented our 48" chord wing, we can assume the area inside the wing (black in the depiction) is at freestream static pressure. At 150 mph (dynamic pressure "q" = 0.4 psi (16.6 kiloPascals) at sea level) , then the maximum negative pressure we see in the depiction is the dark blue "Cp = -5.0" region on the nose. At this airspeed, q x (-5.0) = - 1.95 psi (or -83 kP). That is, on each square inch of the outside wing surface touching the dark blue, the wing skin (and the foam under it) is being pulled outward with a force of about 2 pounds (or, each cm2 is being pulled outward with 0.14 kgf).
On the underside of the wing, where the Cp is moderately positive (e.g. +0.4), we'd have some pushing in on the wing skin. Where the Cp is +0.4, every sq inch would see 0.4 x 0.4psi = 0.16 pound of "inward" pressure against the skin.
If the depiction is for our wing at a lower airspeed or ambient pressure, then the forces would be lower, proportionate with q.
I think a simple physical model could be constructed. It would be fiddly and look like an exploded harpsicord, but it would be cheap and able to (roughly) test various skin/foam core configurations. More later . . .
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Most of the force is forward of the main spar. Some wings could leave that forward part of the wing core solid.
The aft parts could be cored. Forward loading on the wing doesn't hurt for flutter either.
The aft parts could be cored. Forward loading on the wing doesn't hurt for flutter either.
Could explain this more ?centre of mass of the wing ?
While we know that the quarter cord point is where the pressures balance ( regardless of Cl), the depiction posted by Stanislav (and other depictions I've seen) look like it is more forward than that. The extended lever arm aft probably explains how the forces balance at 25% chord.
The foam in the front is definitely under more concentrated external (negative and positive) pressure from the skin. On an absolute basis, relative to the specified strength of the foams typically used, the forces are still very small (in this depiction and values of q that we're talking about). Estimated tensile load of 2 psi at that nose section, rated foam tensile strength of maybe 85 psi. At the tensile modulus listed for similar XPS, we should expect a 6" long chunk to stretch less than .002" under this tension. So, even forward of the spar, it looks like there might be some opportunity to reduce the foam, if desired.
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Most WW2 vintage airplanes had fabric-covered control surfaces for flutter control. First, fabric-covering is very light-weight. That means that you only need small lead or tungsten balance weights forward of the hinge-line.
You are speaking about C.P or C.A of an air foil? I am mixed with them.. Now digging this info for my Not-right V strutted composite wing.
Again from java foil :
And that we do need is only its position at highest CL - high speed pull-up in real world. On any other Cl - it will be smaller at any given point. Again from Java foil :
And then we open any aircraft design book, and we have simplified load per chord in triangle shape :
And it was really fun to feel it live
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Why not build a carbon fiber/Kevlar shell that attaches to the spars (crude black lines), then foam around it and vacuum bag a carbon fiber/kevlar shell over it?
The reason I ask about kevlar in the matrix is that is is now a popular mix many are marketing it and carbon fiber alone is so "kinetic" it basically "explodes" under "impact" and the kevlar has a chance of keeping things more orderly in an accident.
- "Foam around it" with what?
- Drawing: The spars need to bond to the skin, can't pass serious loads through foam.
- How would we cut the final foam wing profile?
- I don't see any reason to put Kevlar in a wing skin. It isn't particularly stiff and it absorbs water.
But maybe I'm missing something.
A solid regular core works just fine, I just want to make that core lighter with judicious removal of some XPS foam.
Just a quick and dirty idea, but basically attache the internal structure to the spars yes. ... that doesn't mean that the outer skin cannot or does not have structural carry through to the spars especially if it is necessary.
Yes, wing center mass at 25% chord would be ideal for flutter. Rotor blades are balanced at 25%. Most wings are not, but why not have more foam density at front?
Well, sort of. Composite facing and then foam, with no composite facing on the other side will be way softer and way weaker than with a facing on the other side. It is not the end of the world, but done properly (thick enough foam, narrow enough space with foam missing) with a slow enough airplane (all of these terms are relative), it can live long. But if you are willing to go to these lengths to make it light, why are you building in fiberglass?
Fiberglass is great for folks who are going low drag first, and are willing to take a small penalty on weight. Even if you can make a holey cored fiberglass wing stay in one piece, bulging skins first trip the boundary layer from laminar to turbulent for an increment of drag, then as deflection gets bigger, it adds some more drag. So now you are a little heavy and not low drag. If you can stand that, knock yourself out... analytically first. I talked above about how to do all of this.
If you are really after low weight first and low drag second, you can go Tailwind/Falco/GP style. Wood spars, stick built ribs, plywood skins, and then a really light fiberglass skin. Generally lighter than fiberglass, nearly as low drag if profiled correctly, can be painted any color you like, and if you go to the light end of the wing loading spectrum, skip the plywood between the D-tube and drag spar, like the Buttercup.
If you are not concerned with cost and want low drag and low weight, you can build in graphite fiber. Maybe even lighter than the wooden wing and can be made as laminar as any other airplane ever built.
But it all comes back to "what are you really trying to do, and why?"
Billski
