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Old Feb 18, 2012, 04:56 PM
Entropy is happening!
Jim.Thompson's Avatar
Australia, NSW, Bellingen
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Kevlar hinges

I am building my first vac bagged wing with kevlar hinges. So far, I am less than impressed with the method!
Facing the surfaces is a real fuss, with them in situ (or course). There must be a trick to it. So far, its taken me much longer than it would to cut the surfaces off the wing, face, sand and fit them accurately, and then silicone hinge them. The last bit takes only minutes.
I must be missing something.
Any hints fellow builders?

Jim.
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Old Feb 18, 2012, 05:43 PM
speedfreek
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Newcastle, Australia
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Jim,

This is what I do....

Fold back the surface and use some tape to hold it back in place so the surface is laying down vertical with the area to be faced horizontal. Use a piping bag, carefully put your splooge along the back of the surface so it doesnt cross the hinge. You can put some tape along the hinge to protect it as added insurance if need be. The splooge will level off on its own and leave a meniscus which doesnt require any sanding.

And youre done.


Waz
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Old Feb 18, 2012, 05:46 PM
Entropy is happening!
Jim.Thompson's Avatar
Australia, NSW, Bellingen
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Quote:
Originally Posted by alkoo View Post
Jim,

This is what I do....

Fold back the surface and use some tape to hold it back in place so the surface is laying down vertical with the area to be faced horizontal. Use a piping bag, carefully put your splooge along the back of the surface so it doesnt cross the hinge. You can put some tape along the hinge to protect it as added insurance if need be. The splooge will level off on its own and leave meniscus which doesnt require any sanding.

And youre done.


Waz
That does sound neat Waz. And similar for the ends and the control surfaces I guess?

Jim.
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Old Feb 18, 2012, 05:56 PM
speedfreek
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Jim,

I never bother with the ends. No need.
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Old Feb 18, 2012, 05:59 PM
AvB
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Jim, what sort of plane is it? You might not need to get stressed about it at all. If it's a light air flyer it mightn't need anything. Or just a light painting with some epoxy or thin splooge. My Aeromod Miraj only has a bit of splooge painted over the foam ... the surfaces are not stiff, but it flies incredibly well. My normal vacbags (SC1, 2, etc) have not had any facing at all ... just raw cut blue foam. Don't worry about trying to make it look good.

If you want them to be stiff however, it's better to put some carbon tow or rolled-up light cloth into the gap. Splooge does help, but doesn't give anything like the stiffness of joining the top and bottom surfaces with glass/ carbon. Tape the surfaces back as Waz described and dig out the foam. A slow dremel with carbide tip etc does a great job but be careful. Use an Xacto knife to scrap the underside of the skins free of foam. Then stand the wing between blocks on the bench, cuts upwards, and work your wetted up material into the gap. Wipe everything clean. Lay some sort of plastic or film or Glad Bake over the cut and push round rods, straws or something into the gap to press the material into it. Tape it down and walk away. You will be amazed how stiff they become.

It's much more important to get the control surfaces stiff than to solidly join the cut surfaces of the wing (dragspar area). But if you need lots of stiffness and torsional strength, you'll need to do the dragspar area too.
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Old Feb 18, 2012, 06:05 PM
speedfreek
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AvB,

Depends what sort of splooge you use. I use a mixture of 50/50 micro balloons and colloidal silica/cotton flox and it goes hard like concrete. The result is a VERY stiff surface with no need for carbon tow.
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Old Feb 18, 2012, 10:59 PM
Entropy is happening!
Jim.Thompson's Avatar
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Thanks for those tips Andrew and Waz.
Andrew,
I certainly want the top and bottom skins tied together and would not consider the job finished if they weren't. The plane is a 2mtr. sloper, a rebuild of my earlier "Endorphin", you may have seen pics of it posted formerly. In this case it is a very light layup and want sufficient strength for basic front side aerobatics.

Waz,
The cotton flock/silica (West 403 or equivalent) mix is a good idea. I have only used "Q" cells up to date.

Jim.

who has already resolved to go back to silicone hinges next time. So much time and fuss involved with this!
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Old Feb 18, 2012, 11:42 PM
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Hey Jim, I would persist with the kevlar hinges. They are sooooo much better- less sloppy, and less likely to rip in a bad landing etc,and also less springy so you dont stress your servos at full flap deflection.. I bet on your next one you'll be like, hey, that wasn't too bad after all!
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Old Feb 19, 2012, 12:19 AM
ERS....Energy Retention System
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Quote:
Originally Posted by josh18 View Post
Hey Jim, I would persist with the kevlar hinges. They are sooooo much better- less sloppy, and less likely to rip in a bad landing etc,and also less springy so you dont stress your servos at full flap deflection.. I bet on your next one you'll be like, hey, that wasn't too bad after all!
I agree..Kevlar are much easier and stronger. , ive v-bagged lot of wings , and never done anything...Don't read into it to much.. For a light front sider not much is needed. For the speed you are going to be flying at you wouldn't notice if they where done or not.

I'm with WAZ/ avb...just cut a little out and sploog it. Dryish mix , so you can do drag and controls at the same time....
Or just CA some aluminum along both....that's the easiest and would take about 1/2 hour....

Tim
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Old Feb 19, 2012, 12:26 AM
Entropy is happening!
Jim.Thompson's Avatar
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[QUOTE=josh18;20789581].................................................. ........................................
less springy so you dont stress your servos at full flap deflection.. ...................QUOTE]

Interesting that you mention that Josh. These (kevlar) ones are incredibly stiff and the springiest hinges I have ever made! So much so, that I'm not sure how to free them more as they are certainly not functional as they are. I've scraped and scraped, filed, sanded with various implements at various times. They are very slightly improved, but still far, far too stiff for my satisfaction.
My silicone hinges almost drop with their own weight in comparison!

Puzzled??

Jim.

edit: I used 200 gsm kevlar. Perhaps that is why they are so stiff?
edit2: I've just had another session at trying to free the hinges up. Failed! I've decided to walk away from them for a while, I'm at the point of cutting them off with the dremel and then siliconing them after facing etc. I know the kevlar will not sand smooth for this, but I don't see much option. The wing is too good just to scrap!
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Old Feb 19, 2012, 01:43 AM
ERS....Energy Retention System
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Hi Jim
To free up all Kevlar hinges , all you need to do is , HEAT along the hinge line with heat gun( be carful) or a normal hair dryer ( safer)... While moving the surface up and down.
This warms the epoxy in the weave and free's it up.....we do it to all planes we build. You can get them to floppy and they will hang down by them selves..

We also use 200gram kevlar..

Hope that helps.

Tim
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Old Feb 19, 2012, 01:44 AM
ERS....Energy Retention System
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Hinges

Tim
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Old Feb 19, 2012, 02:05 AM
Entropy is happening!
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Thanks Tim. I will give that a go next session.

edit: I have applied some heat and it has certainly improved the situation. They are almost free enough to be operational, but not quite. I know that I have removed enough glass off the kevlar on the outer skin side of each one, because the kevlar fibres are just starting to look furry indicating some have been severed. However, in the inside I don't think I have filed/sanded the glass down enough. The flaps will open to crow deployed position sufficiently freely, but not flex in the up direction very well. Similar for the ailerons, they flex up ok, but not down. This suggests I have to remove more material from the inside. I'll just keep it aside and have a go every now and then to free them up. Otherwise, I will break out the dremel and cut them all off in frustration.

Jim.
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Old Feb 19, 2012, 04:12 AM
by ZIPPER
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East Kurrajong Sydney Australia
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Ive tried kevlar hinges a number of times with light cloth and heavy cloth, pulled tight on the bias and left at 90deg and found them to be a total PIA. They always sore off with heavy landings where the silicon would hold together.
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Old Feb 19, 2012, 04:13 AM
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Hey jim, what are you using to scrape the hinge line? I have found a broken dremel cut off wheel is best. I reckon this would be better than say, a needle file, cause the cut off wheel makes about a 1mm wide flat scraped area where a needle file might make a smaller scrape. The larger scrape seems to help stop the opposing sides of the hinge binding on deflection.The thicker the skin, the wider scrape seems to bee needed. Keep it up Jim, when you get it right it is way quicker and to me, heaps more satisfying than silicon hinges.
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