What about machining the honeycomb pattern, maybe a little larger, into your core? So basically the spots that you have cut out for the honeycomb. After that is all cut out, wrap the core with veneer top and bottom to keep epoxy out. A core with hollow spots, more or less.
Make sense?
One of my custom skwals has a really funky core layup that is wrapped in veneer and carbon...i gotta take some pics...really funky layup compared to what is the 'norm'.
Twizz 2012/2013
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That would require some kind of airvent, like the gore-tex ones windsurfers use, to prevent blowout/blow-in of the hollow cambers with different airpressures, eg on top of the very mountain, or at the base, sun or shade etc.
I wonder whether the honeycomb would be able to cope with the difference in airpressure with no chance of equallizing the pressure.
greets, kerr
I wonder whether the honeycomb would be able to cope with the difference in airpressure with no chance of equallizing the pressure.
greets, kerr
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twizzstyle
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I never thought about the pressure change, but it wont' be that much.
If I layup the skis on a standard day (my house is near enough to sea level that I don't care), it'll have 14.7psi inside the ski.
The top of my mountain is something like 6,000ft? 7,000ft? Let's just say I go up to 10,000ft (in my dreams). Pressure up there (standard day) is about 9.5psi. ~5psi delta, I would think it could handle that? Certainly if it can maintain a bond under bending loads, it can take 5psi of pressure.
Busy today today skibuilding with KevyWevy. We got his new cores profiled and basically everything ready for layup. I cut out all the pieces for a new powder mold on the CNC. Lots of camber in the middle section, with pretty tasty early-rise.
Unfortunately I leave for a work trip end of this week (to hawaii... it could be worse
) So the layups for all of these skis (two pairs for me, two pairs for Kevin) will have to wait a couple of weeks.
If I layup the skis on a standard day (my house is near enough to sea level that I don't care), it'll have 14.7psi inside the ski.
The top of my mountain is something like 6,000ft? 7,000ft? Let's just say I go up to 10,000ft (in my dreams). Pressure up there (standard day) is about 9.5psi. ~5psi delta, I would think it could handle that? Certainly if it can maintain a bond under bending loads, it can take 5psi of pressure.
Busy today today skibuilding with KevyWevy. We got his new cores profiled and basically everything ready for layup. I cut out all the pieces for a new powder mold on the CNC. Lots of camber in the middle section, with pretty tasty early-rise.
Unfortunately I leave for a work trip end of this week (to hawaii... it could be worse
) So the layups for all of these skis (two pairs for me, two pairs for Kevin) will have to wait a couple of weeks.
What's the CTE of air? I would have thought implosion (or at least some dimpling or "suck") as you took it out of the press at sea level would be a bigger concern than overpressure after everything is all nicely cured. leave it in the press hot a while? a cursory google suggests that you'd lose about 20% density of air between room and pressing temps. less PSI than the mountain climb, but it is immediately after pressing.
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twizzstyle
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There's no such thing as CTE with a gas, you use.... someone's law. Can't remember the name, but it's PV=nRT
Pressure and volume vary proportionally with temperature (absolute temperature).
So that's even better - as sea level a heated cure, would cause it to contract after cooling. Then when I went up to the mountain, it would expand again. Solved
Pressure and volume vary proportionally with temperature (absolute temperature).
So that's even better - as sea level a heated cure, would cause it to contract after cooling. Then when I went up to the mountain, it would expand again. Solved
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troublemaker
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Think about why they started putting gas in bags of potato chips in the 1950's. When they would ship them over the rockies the chips would crush. So in theroy you need some kind of pressure in the camber, or when you go to a higher elevation the the camber would/could collapse. Maybe causing a concave base or top sheet, probably not, but we are spliting hairs here. Also think about canning fruit, you can when the fruit is hot and as it cools it pulls the lid down for a good seal.
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twizzstyle
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I think the bigger issue would be a positive pressure inside the ski. I'm not worried about crushing, this honeycomb can take a LOT of force. I could park a car on top and it wouldn't do anything. But a positive pressure inside (pushing the laminate out) could overcome the already weak bond.
Yesterday I gave Kevin the go-ahead to try and break the test piece I did. It was actually pretty cool - while there was lots of "bubbling" with the veneer coming off the honeycomb, he was actually able to bend the core to nearly a 90 deg angle before it actually broke (and that took a LOT of effort to get to that point). So while the top sheet might end up looking like crap, and it might turn into a noodle - it would still be a skiable noodle! Still need to do more testing before I'm confident to put it in a ski though.
Yesterday I gave Kevin the go-ahead to try and break the test piece I did. It was actually pretty cool - while there was lots of "bubbling" with the veneer coming off the honeycomb, he was actually able to bend the core to nearly a 90 deg angle before it actually broke (and that took a LOT of effort to get to that point). So while the top sheet might end up looking like crap, and it might turn into a noodle - it would still be a skiable noodle! Still need to do more testing before I'm confident to put it in a ski though.
The reason I said you should think about pressure changes in a hollow camber ski, where you close off the cambers by a veneer of some kind, is some practical experience building wooden hollow core skimboards and surfboards.
These boards, although bigger in size and volume, would almost explode on me, when I laid them after a session in the sun with the valve shut firmly, forgetting I should open the valve immediately afterwards. The expanding gas caused them to seriously belly-out and heard a few alarming cracks. Unscrewing the valve I saw the tension release of the wood, and saw that it wouldn't be a good idea to let that happen all too often. Just by laying it in the sun for a couple of minutes!
Whether the ski's will hold the pressure you can calculate very easy, with the common gas law (pV=nRT) as described above, knowing the volume/area of the hollow cambers and F=m(gas,air)*A(camber,total)
Divide by the number of cambers if you want to see the value of F just for one camber. The mass of air = V*density@Temp
If you know the amount of force the walls of the camber will hold, if F exceeds this value you've got a problem, if not you're good to go with the honeycomb.
I don't expect when you're routing out cambers with thin walls out of wood and top if off with a wood veneer, it would hold up, based on my experience with hollow core skimboards. The gas expands and contracts and there is nothing that you can actually do about, except perhaps installing a vent/valve like the most windsurfboards have nowadays, a gore-tex vent, which keeps the water out, but leaves the watervapour/air free to go.
These boards, although bigger in size and volume, would almost explode on me, when I laid them after a session in the sun with the valve shut firmly, forgetting I should open the valve immediately afterwards. The expanding gas caused them to seriously belly-out and heard a few alarming cracks. Unscrewing the valve I saw the tension release of the wood, and saw that it wouldn't be a good idea to let that happen all too often. Just by laying it in the sun for a couple of minutes!
Whether the ski's will hold the pressure you can calculate very easy, with the common gas law (pV=nRT) as described above, knowing the volume/area of the hollow cambers and F=m(gas,air)*A(camber,total)
Divide by the number of cambers if you want to see the value of F just for one camber. The mass of air = V*density@Temp
If you know the amount of force the walls of the camber will hold, if F exceeds this value you've got a problem, if not you're good to go with the honeycomb.
I don't expect when you're routing out cambers with thin walls out of wood and top if off with a wood veneer, it would hold up, based on my experience with hollow core skimboards. The gas expands and contracts and there is nothing that you can actually do about, except perhaps installing a vent/valve like the most windsurfboards have nowadays, a gore-tex vent, which keeps the water out, but leaves the watervapour/air free to go.
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twizzstyle
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I just now actually read this last post... and the engineer inside me can't keep my mouth shut.
This equation you listed for the force inside a chamber?
F=m(gas,air)*A(chamber,total)
huh? Sorry, but no. Force does not equal a mass times an area. The F=ma you're thinking of is mass times acceleration, THAT is a force, but that's not applicable here.
In this case it is pressure times area, mass is irrelevant. Pressure is found using PV=nRT.
This equation you listed for the force inside a chamber?
F=m(gas,air)*A(chamber,total)
huh? Sorry, but no. Force does not equal a mass times an area. The F=ma you're thinking of is mass times acceleration, THAT is a force, but that's not applicable here.
In this case it is pressure times area, mass is irrelevant. Pressure is found using PV=nRT.
Yes, that was indeed not the right formula, i cocked up typing and reading to correct mistakes before/after pressing post.
Still needs a force though to break a chamberwall, probably bad idea to type it down now afterwards, but i meant somewhere along the line pressure = Force/Area, and to calculate F use the mass of air and gravity (a) in a chamber. Wouldnt that be correct? The pv=nrt formula just gives the ambient pressure, you would want to know the force it puts onto the walls?
It would be nice to see some hollow core ski's being built, and see whether they can cope easily with the pressure changes without an equalizer of any kind, as you've planned for. My skimboards certainly didnt withstand pressure changes with ease, the vent - except when skimming ofcourse- has to be open all the time, otherwise it will crack and twist, even standing in the garage at sea level.
Perhaps, as you intented to do, filling the hole with small-chamber-honeycomb, will be way better than big chambers of air.
kerr
Still needs a force though to break a chamberwall, probably bad idea to type it down now afterwards, but i meant somewhere along the line pressure = Force/Area, and to calculate F use the mass of air and gravity (a) in a chamber. Wouldnt that be correct? The pv=nrt formula just gives the ambient pressure, you would want to know the force it puts onto the walls?
It would be nice to see some hollow core ski's being built, and see whether they can cope easily with the pressure changes without an equalizer of any kind, as you've planned for. My skimboards certainly didnt withstand pressure changes with ease, the vent - except when skimming ofcourse- has to be open all the time, otherwise it will crack and twist, even standing in the garage at sea level.
Perhaps, as you intented to do, filling the hole with small-chamber-honeycomb, will be way better than big chambers of air.
kerr
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twizzstyle
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You're getting even further off now!kerriser wrote: but i meant somewhere along the line pressure = Force/Area, and to calculate F use the mass of air and gravity (a) in a chamber. Wouldnt that be correct? The pv=nrt formula just gives the ambient pressure, you would want to know the force it puts onto the walls?
Pressure acts in all directions. If you have 14psi inside a chamber, it is pushing with 14psi on to top, bottom, and sides. Forget gravity.
14.7 pounds is how much a column of air weighs that is 1 square inch in cross section, from sea level, all the way to space. You're talking about calculating the weight of the air inside the chamber - which is going to be a TINY number.
The walls of a chamber don't have a single force level that it fails at. The amount of force for a failure will be different if its a point load (like a needle) or spread out (air pressure). What you care about here is at what pressure it will fail at.
Forget gravity, and forget force. The pressure inside the chamber relative to the ambient pressure (that's a key detail!) is what will or won't cause a failure. 14.7psi inside the chamber, and 14.7psi outside the chamber means they balance out. 14.7psi in the chamber, and say 10psi outside (up on a high mountain), means you've got 4.7psi of delta, pushing outwards on the chamber walls.
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FigmentOriginal
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twizzstyle
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If you can use your CNC to route out an interconnected honey comb structure in parts where it doesn't matter too much, eg where you can save a couple of grams, and connect them via some sort of channel front to back, so you can use a gore-tex airvent (just a strip of fabric) to cancel out pressure/temperature differences.
I know you weren't impressed by the numbers when you compared pressures at sea and summit level, but there is not only a difference in pressure, but in temperature as well. I can agree on the fact that it's perhaps not a big problem when skiing, but when standing in a garage when you're not skiing during summer when temperature is at its highest, well it would be anyone's guess. /
Thirty degrees (°C), 54°F warmer will cause a 10% increase in pressure, if I'm not mistaken, so perhaps if you ski's can cope with the difference between sea&summit, they can take this one as well.
kerr.
I know you weren't impressed by the numbers when you compared pressures at sea and summit level, but there is not only a difference in pressure, but in temperature as well. I can agree on the fact that it's perhaps not a big problem when skiing, but when standing in a garage when you're not skiing during summer when temperature is at its highest, well it would be anyone's guess. /
Thirty degrees (°C), 54°F warmer will cause a 10% increase in pressure, if I'm not mistaken, so perhaps if you ski's can cope with the difference between sea&summit, they can take this one as well.
kerr.
Last edited by kerriser on Mon Nov 05, 2012 11:26 am, edited 2 times in total.