Well, first, Autoreply and I have commented on how classic massive cores are lighter than classic hollow (sandwich skin) at the modest chords we see in homebuilt airplanes. Autoreply has cited a doc where someone found the break even spot was around 2m, IIRC. Above that, you save weight by being hollow. Now, I emphasize that this is for skins with laminates on both sides, ribs, flanges, glues lines etc.
What is being proposed by Vigilant1 is to start with a massive core and then remove some of the core... Well, we know some can be removed safely. Rutan canards have historically removed a cylinder of foam spanwise for wing tip wiring and the rudder cable. It seems to survive a deletion of a 1" cylinder just fine, but that is what, a quarter pound in a Long Ez.
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...
I must improve the thinking behind the analysis method. First, let's box in the thinking - we are dealing with a foam core and fiber-resin skins and spars. There are four main loads on the wing skins in flight plus one for construction:
- Inflation - This one seems to be forgotten. In a 40 knot airplane it hardly matters, and in an airplane over 300 knots it pretty well defines skin structures. The air inside the cavities are stationary and thus the same as the atmosphere outside the plane. Outside the wings, the air is moving at some multiple of the airspeed of the airplane. TOWS Appendix I has this plotted for many symmetric forms at Cl=0 and at a one higher Cl for both surfaces. These velocity and thus pressure distributions apply when you add camber. You can estimate for any Cl. Using Bernoulli's Law, you can calculate the pressure outside and thus the pressure difference between inside and outside. Inflation effect is highest near the leading edge and diminishes as you go aft, and it goes with local V^2. Usually we then estimate stresses and deflections in constant thickness panels using Roark's Chapter 11, table 11.4. You have to do some interesting machinations of our asymmetric composite plate to use table 11.4, but it works. Doing the elliptical cutouts, well, that cries out for FEA. We can talk more about this if we want;
- Lift - Once you have done the forces trying to pull the skins away from the wing, you can also get the lift. It is just the difference between the load pulling the top one up and the load pulling the bottom one down. The lift is transmitted through the sandwich of foam and laminate to the spars;
- Bending - The combination of skin set and spar set make up the bending stiffness of the wing, and the wing curves upward under positive g. Near the root, the spar makes up most of the stiffness, with the skin pretty much going along for the ride. Out near the tip, a properly tailored spar's stiffness diminishes and wing skin usually dominates. The wing curves with the compressive strain (span wise) on one side and tensile strain (spanwise) on the other. Why emphasize spanwise? Well, the skins are in what ME's call plane stress. When we strain a plate in in one direction, at 90 degrees to that strain we get a significant strain of the opposite sense. So, the top skin gets compression spanwise and tension chordwise, while the bottom skin gets tension spanwise and compression chordwise;
- Torsion - The combination of skin set and spar set make up the torsional stiffness of the wing, and the wing usually twists leading edge down under positive g. The skin pretty much defines the torsional stiffness with the spar more or less along for the ride. The wing twists with the shear strain on the skins.
- Vacuum Bagging - This is like in flight inflation, except that you must carry the bag loads without the skin laminate helping and that the foam inside is in tension instead of compression. Oh, and this load is really big. 10 psi is 1440 lb/ft^2 which is about the dynamic pressure for 1100 fps. Hmm. Unless you are flying up into the transonic region, your skins won't be making this level of lead in flight.
Since we are probably talking the low speed end of the flight range, vacuum bag forces will probably be the BIG stressor on this structure.
Will the foam with cutouts stand the vacuum bagging? We EAA members can get SolidWorks for free, model up a wing profile with holes in it, extrude it to 10 feet, define the material with the specifics listed in the OP, apply an outside pressure of 10 psi and see what stresses and deflections it makes. If the foam's strength is exceeded, maybe you can investigate lower vacuum, or maybe vacuum bagging is simply out. But if the stresses in the foam are substantially below its strength, how much does it deflect into the cavities. You will probably have to fill and fair anyway, but that usually averages 0.015 to 0.030" thick or less than 0.09 lb/ft^2. If you have a bunch of filling to get the skin fair, you might be adding more weight by bagging than by doing open layups. All things you can check out with a SolidWorks model and a few simple calcs.
On to flight loads. The air is pulling out on the skins, and straining the panels to bulge outward. The wing is bending with axial strains both spanwise and chordwise in the panels. The wing is twisting with shear strains in the panels. And it does all of this at the same time. So all of these plane strain conditions must be superimposed and checked to see if they exceed the failure criteria for the asymmetric sandwich you are proposing.
You must stand the first three together and the last one by itself. We can do in plane loads and moments using composite plate theory.
What about me? Listen, I retired five years ago so I could do what I want, not what somebody else wants... I am building my airplane, trying to keep my wife happy, and other stuff that is important to me. If I spent my days doing your what-if's for you - and there are a bunch out there just begging for effort - my wife would be unhappy and my bird would never fly. You guys think this is so cool, do some analysis or some testing, or both.
It sounds like a great idea, and at small wing loadings and low speeds it can work. Peter Sripol is flying one. First, the solid core. It has proven to work just fine because even at the q's SpaceShipOne flew at, the forces pulling the skins away are less than the strength of the foam and the elastic deformations are tiny. Make a sandwich panel skin, with laminate on both sides of a core, and the neutral axis is somewhere near the middle of the sandwich, stiffnesses and strengths in bending go up by orders of magnitude. Bond the two skins together around the edges and it is way strong. Omit one laminate facing, and you can put in lots of foam before the neutral axis moves enough to start raising bending stiffness. And that is the problem - you lost a bunch of stiffness and strength... At the low end of the speed and wing loading regimes, this can work. But your skins have to stand your V-n envelope all the way to your Vdive. That means somebody better do the engineering and math. Unless...
An alternative of the "try it and see" school does present itself. If you have or can build a vacuum chamber and can connect outside pressure to inside tubes, you can perform a test series with test sections. There are tubes for applying atmospheric pressure inside vacuum bagged parts for fabrication. Run these inside the cutouts, so you can control inside pressure (run to outside air with a valve and gage) while the outside has vacuum that you also control. Do the designed experiment thing with large and small radii (cutouts) and large and small wall thickness (between cutout and laminate), throw in one case of mid wall and radius. Five permutations to build and test. Do each one twice, and you have some idea how much variability you will have as well as a good idea how much wall you will need and how small or large a radius you can stand for your flight envelope. Oh, I would put a scale in the back ground and another on the window so you can record your deflections as you ramp vacuum - deflection may stop you long before fractures do.
So what is holding your guys up? Come on, let's see some analysis and testing!
Billski
