Not-so-solid massive core wings: Lightening the core foam

[stanislavz]

Some pdf from my shelf, suitable for this thread.

 

[stanislavz]

And some more example, on how small amount of foam your need for you "general flying machine"

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www.flysquirrel.net

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www.flysquirrel.net

 

[BrianW]

V1, I think what you need to do is analyze a slice of the rib (say, an inch thick spanwise) as a beam to determine how large the lightening holes can be. First things first though. I'm assuming 1) a Rutan-style spar (UNI or graphite caps), 2) a similar rear spar that will be used to mount aileron hardware and transmit torsion back to a hard point on the fuselage, and 3) a composite skin that will carry torsion and contribute to resisting drag loads. Is that right?

So then you can draw a free body diagram of the nose block with the negative pressure all the way around the nose (remember it's negative all around, but more negative on top),
/snip/

No I do not remember that the pressure is negative all around the nose ~not at all!
Or else that stall sensor slot set low on the leading edge which squeals into action when the local pressure goes negative at high angle of attack - would not work.

 

[mcrae0104]

Oops, you're right, @BrianW. It depends on the angle of attack.

 

[REVAN]

I believe one of the BeLite ultralight models used this type of wing construction. I think they used a CNC router to cut what are basically 4 inch thick foam ribs with and internal truss structure remaining inside and two round hollows for the wing spars. One advantage to this is that the 'ribs' can have 3D features in each segment, allowing for more strength in relation to the weight that is removed (though I don't think BeLite did this). These 'ribs' were then just slid over the tube spars with no spacing between the foam rib segments (probably some adhesive between to hold them together). The structure was then overwrapped with fiberglass and hand finished LongEZ style. From the looks of it, it worked pretty well and made a light enough wing for a part 103 ultralight.

Using a smart application of CNC cut 3D features, a structure like this could likely be made strong enough to withstand mild vacuum pressures for bagging, possibly resulting in an even stronger and lighter wing. If going this extra distance, I would look to add channels to the top and bottom of the rib segments to lay in spar caps and let those carry the wing bending loads. I'd use smaller tubes that are there mainly for alignment of the wing during fabrication (could even be slid out and removed when done with fabrication). Why take on the expense of vacuum bagging the project to save a little weight on the wing skins while accepting the added weight of structurally inefficient tube style wing spars?

 

[BrianW]

I believe one of the BeLite ultralight models used this type of wing construction. I think they used a CNC router to cut what are basically 4 inch thick foam ribs with and internal truss structure remaining inside and two round hollows for the wing spars. One advantage to this is that the 'ribs' can have 3D features in each segment, allowing for more strength in relation to the weight that is removed (though I don't think BeLite did this). These 'ribs' were then just slid over the tube spars with no spacing between the foam rib segments (probably some adhesive between to hold them together). The structure was then overwrapped with fiberglass and hand finished LongEZ style. From the looks of it, it worked pretty well and made a light enough wing for a part 103 ultralight.
/snip/

In my fever dreams, over the tube and foam ribs, would go a hard shiny film, stronger than the Sripol film which left an orange peel finish; instead an optically smooth film like say cellulose sheet- but strong and light.

 

[TFF]

Peters finish would have been smooth if it was not directly on the foam.

 

[slociviccoupe]

Not same topic but since this process will apply im asking the question. Can laminated foam ribs (fiberglass or carbon each side) get lightening holes? Make a mold and do a wet layup on one side in the mold, add the foam rib to the mold, then another wet layup on top then vacuum bagged. Flangrs for wing skin could be made in the mold, foam will be positioned in the mold, and the skins on each side would contact eachother in the area where lightening holes were made. Use either cnc routered ribs or hotwire cut cores cut down into ribs.

 

[stanislavz]

Can laminated foam ribs (fiberglass or carbon each side) get lightening holes?

It may. As per Jim Marske book - you have one plie of 9oz fiberglass and corrugation in skin or holes and two plies. And kind of flange around holes.

Weight was similar.

 

[slociviccoupe]

Ive got the book. Dont understand much of the math. But would figure if your lamenating both sides anyways. If possible might as well reduce some foam if not necessary. And the induced angles and joining each side in middle of hole should add rigidity. At least my thouggts anyways. Ill have to make one and test compared to a standard plans built rib.

 

[BrianW]

Peters finish would have been smooth if it was not directly on the foam.

Are you thinking of the glass bead slurry/mud that some have used?

 

[stanislavz]

It may. As per Jim Marske book - you have one plie of 9oz fiberglass and corrugation in skin or holes and two plies. And kind of flange around holes.

Weight was similar.

And Jim is using one ply - he just cuts ribs from corrugated sheet using some metal shear.

 

[wsimpso1]

Not same topic but since this process will apply im asking the question. Can laminated foam ribs (fiberglass or carbon each side) get lightening holes? Make a mold and do a wet layup on one side in the mold, add the foam rib to the mold, then another wet layup on top then vacuum bagged. Flangrs for wing skin could be made in the mold, foam will be positioned in the mold, and the skins on each side would contact eachother in the area where lightening holes were made. Use either cnc routered ribs or hotwire cut cores cut down into ribs.

That scheme works, the issues are usually just one of what can you execute most lightly. A lot of whether you mold a composite rib or saw one from laminated plate and tab it in and use flange transfer technique also comes down to if the tabbing etc and one thin glue line is lighter than the molded flange and two glue lines.

 

[BBerson]

I saw a RC model that was 3D Printed out of "foam". It can be printed in hollow shapes impossible to Hotwire.

 

[Hephaestus]

I saw a RC model that was 3D Printed out of "foam". It can be printed in hollow shapes impossible to Hotwire.

LW-ASA and PLA.

It's not printed out of foam it's printed with a material that foams at specific temperatures. They're nice but weak, if someone added carbon strand within it...

 

[Vigilant1]

Quick notes:

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.

Thanks for the lead to the TOWS Appendix 1 charts. These, plus the associated text in Ch3 and Ch4, will be a good starting point for understanding the pressures around the wing section. I don't think I would have remembered this info was so close at hand. Luckily, I won't need to define every case, just the "worst cases" from a wing-core standpoint.

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.

For all these reasons, I think it is best to just plan to not subject the "Swiss cheese" core to vacuum bagging pressures. Why do it? I can just leave the cut cores in place, or fill the hot-wire kerf with layer or two of tape on the outside of the cutout and slide them back in. Or, do it pneumatically, if that proves best. The cores will never experience those forces (in magnitude or direction) once in service. I suppose it's even possible that putting them under pressure while the skins cure could "lock in" stresses that might best be avoided.

... and the dynamic pressure would also be pretty small (e.g. "q" at sea level and 150 knots is just 0.39 psi, so we might see that at the airfoil nose/stagnation point).

I was wrong about this. Pressures at the nose of the airfoil can exceed "q".

Thanks again for the input!

 

[mcrae0104]

Pressures at the nose of the airfoil can exceed "q".

Do you have a reference for this? It's not readily apparent that stagnation pressure can exceed q, but I would be happy to learn something new.

 

[wsimpso1]

Quick notes:

Thanks for the lead to the TOWS Appendix 1 charts. These, plus the associated text in Ch3 and Ch4, will be a good starting point for understanding the pressures around the wing section. I don't think I would have remembered this info was so close at hand. Luckily, I won't need to define every case, just the "worst cases" from a wing-core standpoint.

For all these reasons, I think it is best to just plan to not subject the "Swiss cheese" core to vacuum bagging pressures. Why do it? I can just leave the cut cores in place, or fill the hot-wire kerf with layer or two of tape on the outside of the cutout and slide them back in. Or, do it pneumatically, if that proves best. The cores will never experience those forces (in magnitude or direction) once in service. I suppose it's even possible that putting them under pressure while the skins cure could "lock in" stresses that might best be avoided.


I was wrong about this. Pressures at the nose of the airfoil can exceed "q".

Thanks again for the input!

I agree with the processing approaches for molding these parts. I am still concerned that the skin panels where foam has been removed could have either excess stresses or excess deflections or both.

Note that plots in TOWS App I are for Cl= 0 and some low Cl. That is fine as far as it goes, but at max g as seen at Va, Vd, and between, Cl are much higher. The (V/v)^2 plot for higher Cl may be estimated by extrapolating. Much higher local V^2 can occur over the upper surface then, particularly forward of the spar.

Billski

 

[Sockmonkey]

I believe one of the BeLite ultralight models used this type of wing construction. I think they used a CNC router to cut what are basically 4 inch thick foam ribs with and internal truss structure remaining inside and two round hollows for the wing spars. One advantage to this is that the 'ribs' can have 3D features in each segment, allowing for more strength in relation to the weight that is removed (though I don't think BeLite did this). These 'ribs' were then just slid over the tube spars with no spacing between the foam rib segments (probably some adhesive between to hold them together). The structure was then overwrapped with fiberglass and hand finished LongEZ style. From the looks of it, it worked pretty well and made a light enough wing for a part 103 ultralight.

Building like that it would be easy to increase the size and/or number of cutouts as you go from root to tip for optimal mass/strength distribution.

 

[BBerson]

Inflation - This one seems to be forgotten.

I think what you are saying here is that those areas that have cores removed (post 11, the thin remaining foam areas) will be sucked outward and fail. Makes sense.
Fabric would have more tensile strength than foam and would bulge out some but not fail. But fabric must be stitched to a certain maximum panel size or would tear loose.
I think I read that the Vickers Wellington was at the upper limit of fabric at around 300 mph.