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Old Jan 11, 2005, 12:47 PM
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Falcon, Colorado
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Kevlar hinge delam

Hey guys.. Im having a problem with live hinges on a DLG wing.. I am bagging a blue foam core that has been sanded smooth and cleaned. I use 1.7 oz kevlar that is 1" wide, cut on a 45 degree bias and stretched lengthwise to about a 70 degree bias for my hinge. This results in a hinge that is about 1/2 inch wide. Problem is that when I cut the flaperon and flex it over the top of the wing to break the hinge, I get a delam on parts of the flaperon or the back of the wing. It usually extends about 1/8" back from the actual hinge line. It is easy to fix with a little epoxy, but it is kindof a pain to do. This only happens on wings that I make without facing the surfaces in the layup. Is there a way to make the kevlar adhere to the foam better? Also, is there any other known way to bag the facing into the layup other then what is on the CRRC site? Cutting and facing a foam triangle to press the faces of the surfaces is a little bit tedious, and seems to produce an inaccurate facing surface.
Thanks!
Jim
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Old Jan 11, 2005, 02:04 PM
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The hinge will delaminate if some combination of hinge stiffness and a weak bond between the kevlar skin and foam is present. To avoid having the hinge delaminate when it is first bent over you want a very flexible hinge line and a good bond to the foam.

To make the hinge line flexible you must remove as much stuff as possible from the hingeline that is not Kevlar before attempting to fold the hinge for the first time. The stuff to be removed might be fiberglass or epoxy which is over/in the hinge line. Use a small tringle file to file through any glass over the hinge line and also file through any epoxy all the way down to the kevlar skin. Keep filing until you are hitting raw Kevlar. Do your layup such that the Kevlar is the first fabric up against the core and any glass is outside the Kevlar. The only non Kevlar stuff in the hinge line will then be some epoxy in the Kevlar fabric or under it. The epoxy bonds in the Kevlar fabric break as the hinge is bent over for the first time. The epoxy under the Kevlar skin is sanded off gently after the hinge is bent over. This loosens the hinge even more, making the hinge as loose as a tape hinge.

The other part of the equation is getting a good bond between the foam and the Kevlar skin. The prefacing routine you talk about will take care of this. If you don't do the prefacing routine then all it takes is rolling a narrow stripe of extra epoxy along the hinge line area of the layup prior to bagging. The super dry layups that are frequently done on DLG wings just do not allow enough epoxy for the firm bond that is needed at the hinge line.

You can also achieve much nicer and stronger trailing edges if you roll some extra epoxy into the trailing edge area of the layup.

Phil
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Old Jan 12, 2005, 12:21 PM
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threcixty's Avatar
Falcon, Colorado
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I think the problem was that I was soaking too much epoxy out of the hinge area before bagging. I do add some epoxy, usually with a little cabosil added, to the trailing edge before bagging. Ill have to give that a try (without cabosil) on the hinge. I thought about cutting a couple of grooves on the edge of the hinge as to add carbon rod or cabosilled epoxy to sort of "face" the surface. It wouldnt work as well as facing with carbon cloth or glass, but it would save a lot of work.
Thanks for the reply, Phil!
Laterz,
Jim
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Old Jan 12, 2005, 03:11 PM
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Draw the hinge line with wax crayon before epoxy. Draw the light line under the inside of the Kevlar side so the wax doen't penetrate all the way through. No scrape off epoxy along the line but blot the epoxy.
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Last edited by Ollie; Jan 12, 2005 at 03:16 PM.
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Old Jan 12, 2005, 08:56 PM
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Falcon, Colorado
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Crayon, huh?? I like it! Never thought about doing it that way. What about the top surface of the wing, as in the finished side of the hinge? I usually use fiberglass ontop of the kevlar for its finishing qualities. Do I need to do anything with that?? I just started having problems with the hinges when I started to blot up excess epoxy, but these ideas you guys have are great. Thanks!
Jim
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Old Jan 12, 2005, 10:31 PM
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I must say that I don't understand Ollie's post at all. To draw the hinge line on the mylar you should draw a line on the back side of the mylar with a black sharpie marker. This works for unpainted wings, you can see the line through the mylar and the layup. If the wing is painted, you can draw on the dried paint with a red sharpie marker. The red is less likely to show through the paint on the finished wing.

Glass over the hinge line is OK. You just want to do the layup such that the glass is on the outside (Kevlar is closest to the foam). It is no problem to file the hinge line when there is glass over the Kevlar. In fact a little glass can make the hinge line a little cleaner looking. When filing the hinge line you just need to expect that it will take a little more filing to get through the glass. Be sure that you file all the way down to raw Kevlar before folding the hinge for the first time.

From reading your last post it is clear that your delam problem is caused by the dry layup. The problem will go away again if you just roll a narrow stripe of epoxy along the hinge line area of the layup.

You could also simplify things by just using bias 1.7oz Kevlar or even 0-90 Kevlar for the hinge. The shearing ("stretched lengthwise to 70 dehgrees") of the Kevlar that you describe is not required.

By the way, There is a rather long, ponderous document about skin hinging in the SALglider YAHOO group files section;

http://groups.yahoo.com/group/SALglider/

Look in "Phil's vac bag writeups"

Phil
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Old Jan 13, 2005, 11:35 AM
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Quote:
Originally Posted by Phil Barnes
I must say that I don't understand Ollie's post at all. To draw the hinge line on the mylar you should draw a line on the back side of the mylar with a black sharpie marker. This works for unpainted wings, you can see the line through the mylar and the layup. If the wing is painted, you can draw on the dried paint with a red sharpie marker. The red is less likely to show through the paint on the finished wing.


Phil
Basically he's just using the crayon as a wax to keep the resin out of the hingline. He draws on the kevlar itself to get the wax on just the hingeline. I haven't tried it myself but it should make it easy to free the hinge later on, less filing/scraping.
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Old Jan 13, 2005, 12:37 PM
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Falcon, Colorado
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Phil: Shearing the kevlar to about 70 degrees is easy... 45 deg. bias kevlar, 1.5 inches wide pulled on either side as far as it will go, will make 70 degree hinge cloth so it will have better "leverage" in the hinge, and the hinge will flex easier. I will try adding more epoxy in the next layup so that it will stick better and let you know how it comes out.

Ollie: I like the crayon idea, but how do you get the accuracy of the crayon line on the kevlar? It seems that it would be difficult to get everything to end up in the exact place as it would have to for this idea to work.

Any ideas on how to press a fiberglass flaperon facing into the core WITHOUT using the foam triangle that was cut from the channel?? It is such a pain to get a good looking facing using this foam, and I was thinking more along the lines of two small mylar strips glued onto the mylar to form a triangle that the entire layup goes over. Ideas??

Laterz,
Jim
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Old Jan 13, 2005, 12:56 PM
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Adding wax to your fabrics cannot possibly help your delam condition, and you will not get a score line hinge once there is dry fabric where the wax prevented absorption.

If you are trying to face the flaperons during bagging, the Ib J. method is best. Otherwise you're on your own to develop a better method. Once I ruined at foam triangle and made a new one out of very light balsa rather than scrap the core, but that was much more work to get it to fit perfectly. For production work (>20 models) I would have teflon triangle machined which could be reused.
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Old Jan 13, 2005, 01:19 PM
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Falcon, Colorado
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[QUOTE=Badger]Adding wax to your fabrics cannot possibly help your delam condition, and you will not get a score line hinge once there is dry fabric where the wax prevented absorption.

I think that the crayon only prevents epoxy from sticking to the surface of the kevlar, i.e. inside of the flaperon channel, so that there is no need to score the hinge before you flex it.


If you are trying to face the flaperons during bagging, the Ib J. method is best. Otherwise you're on your own to develop a better method.

What is the Ib J. method?? I'm guessing that it is the foam triangle faced with tape, but not sure.

Also, any ideas on where to get teflon in the dimensions that would work for this?? I haven't looked so I dont know.
Thanks!
Jim
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Old Jan 13, 2005, 01:34 PM
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Quote:
Originally Posted by threcixty
I think that the crayon only prevents epoxy from sticking to the surface of the kevlar, i.e. inside of the flaperon channel, so that there is no need to score the hinge before you flex it.

If you are trying to face the flaperons during bagging, the Ib J. method is best. Otherwise you're on your own to develop a better method.

What is the Ib J. method??

Also, any ideas on where to get teflon in the dimensions that would work for this?? I haven't looked so I dont know.
Thanks!
Jim
You will still have to score the kevlar hinge on the outside of the wing to remove epoxy, there will always be a layer of epoxy next to the mylar.

The Ib J method is the foam/tape triangle, though I don't know if he uses tape or just scrapes out the foam.

Go to a machine shop and get a quote on the teflon strip. Any industiral plastic supplier will sell teflon sheet. You will not be able to make the teflon triangle strip with home tools.

Many people have used the standard method to get good results so my suggestion is to practice and be careful rather than trying to develop new procedures and tools.
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Old Jan 14, 2005, 10:38 AM
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Falcon, Colorado
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Badger:

Not really trying to find new methods. I just want to see if anyone has used an alternative that works better then the foam strip. I don't like the quality of the facing in that it is similar to a leading edge when you cut the mylar short and allow the bag to face the l.e. I works, but it isn't "precision" by any means, and it doesn't seem to be related to the craftsmanship involved. I am trying to figure out methods that could be used for production wings, and would have a very high quality finish while requiring less time and hassle then the foam channel. You guys have given me some great ideas! Keep 'em coming if you can.

Jim

p.s. I could just make 'em polyhedral, but that isn't any fun!
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Old Jan 14, 2005, 11:38 PM
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Quote:
Originally Posted by Phil Barnes
By the way, There is a rather long, ponderous document about skin hinging in the SALglider YAHOO group files section;

http://groups.yahoo.com/group/SALglider/

Look in "Phil's vac bag writeups"
Phil's vacuum bagging write ups are great. I employ many of Phil's techniques with great success.

A couple additional ideas that have worked well for me with slope wings:

1) Install 1/16 to 1/8-inch CF or FG weblets on either side of the hinge line. This stiffens both surfaces and improves the skin to core bond close to the hinge line.

2) Make 1/16 to 1/8-inch slits into the foam cores on either side of the hinge line. It does not help as much as the weblets, but the the skin to foam bond is improved with little effort.

Checkout the following links to see what weblets look like and how they are installed:

http://www.rcgroups.com/forums/showt...=282362&page=2

http://www.rchomepage.com/~jdecker/gallery/wing01
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Old Jan 15, 2005, 08:48 AM
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Falcon, Colorado
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Quote:
Originally Posted by Sled Driver
Phil's vacuum bagging write ups are great. I employ many of Phil's techniques with great success.

A couple additional ideas that have worked well for me with slope wings:

1) Install 1/16 to 1/8-inch CF or FG weblets on either side of the hinge line. This stiffens both surfaces and improves the skin to core bond close to the hinge line.

2) Make 1/16 to 1/8-inch slits into the foam cores on either side of the hinge line. It does not help as much as the weblets, but the the skin to foam bond is improved with little effort.

Checkout the following links to see what weblets look like and how they are installed:

http://www.rcgroups.com/forums/showt...=282362&page=2


http://www.rchomepage.com/~jdecker/gallery/wing01
Good stuff... I was thinking about the "weblet" method before. One could cut slots the same way as cutting the triangle channel, but without going as deep or seperating the foam, and then install cf weblets instead of facing the entire surface. This probably wouldn't make as strong a surface as wrapping the face with fiberglass, but it doesn't need to be that strong on a DLG anyways. Heck, I never had flutter on non faced flaperons, and I throw the plane as high as anyone out there. So, I guess it just comes down to the trick factor. Too bad you couldn't just skin the wing without any work in the flap channel, i.e. like a polyhedral wing, and then cut a slot in the middle of the channel and then press a triangle shaped mandrel into the wing to crush the foam (and fiberglass facings) into a channel shape. Thanks for the help guys!
Laterz
Jim
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