[Home] [Up] [Core Cutting] [Core Preparation] [Fabric - Mylar Preparation] [Lay-up First Steps] [Lay-up Next Steps] [Trimming and Finishing] [Hinging the Surfaces]

   Site Map:      Mail Me:      Payments/Conditions:   Feedback/Order Form:    
      

Here is an abridged version of Phil's Kevlar Skin Hinging 
(see SALGlider files section for a full copy)

There are three major sections (and I hope to add some pictures soon)
Basic Philosophy
Tools required
Hinging Process

First, the basic philosophy: 

For skin hinges, you must arrange for there to be Kevlar over the hinge line. When you do the hinging, you must remove everything from the hinge line that is not Kevlar. These other things which must be removed would include; fibreglass, carbon, epoxy or anything else which might be in the lay-up at the hinge line location. This is the key to getting Kevlar skin hinges to be as loose as tape hinges. If you have tried Kevlar skin hinges before and had trouble with them being tight, it is because you did not remove every non Kevlar bit from the hinge line. The most common thing to be left in the hinge line is epoxy, especially from the back side of the hinge line. A Kevlar skin hinge works because of Kevlar's toughness. An impact to a Kevlar skin may break the epoxy bonds which hold the Kevlar fibres together, but will usually not break the actual Kevlar fibres. When we make a skin hinge, what we do is break all of the epoxy bonds along the hinge line and leave the Kevlar fibres intact. The bits of Kevlar fibre that cross the hinge line are then just little short bits of Kevlar "rope" which are free to flex like any other bit of rope or string. It is Kevlar's toughness that allows the epoxy bonds to be broken without tearing the actual fibres. For the hinge to be loose, you must break all of the epoxy bonds along the hinge line, both above and below the Kevlar.

There are many different ways to "arrange to have Kevlar over the hinge line".  Wings like HL wings with just Kevlar and no glass are OK since there is no paint and the raised bits of Kevlar fuzz don't show as they do when the surface is painted. Kevlar hand launch wings have no special requirements prior to bagging. There is only the single layer of Kevlar, no glass or carbon near the hinge line. If you have Kevlar doublers on the Flaperons or near the hinge line, you should make sure that the edge of the doubler is at least 1/8" away from the hinge line. If it is too close you may have trouble because the first time you try to break the hinge loose, it may want to crease at the edge of the doubler rather than the desired hinge line.

Special tools:

The first tool is a control surface template. This is just a template made from a piece of plastic laminate material. This is the stuff used for laminating countertops and is frequently called by it's brand name of Formica. It is just a piece of the laminate material the same size and shape as the control surface. You can draw out the desired shape, cut close to the line with a band saw and finish sand it down to the line with a disc sander and finally a long sanding block. You just need to be sure and get a nice straight , clean edge especially at the hinge line edge.

The second tool will be used to file the actual hinge line. Get a small triangle file about 9" long. You don't want the handle for it, just use the file alone, without the handle. You will need to cut the tip of the file off since the teeth do not go all the way to the end. Also, as the file gets dull, you can "freshen it up" by cutting about 1/4" off the end of the file. I keep using the same file this way until it gets so short as to be hard to hold. Use an abrasive cut-off disc to cut the file. I have a little cross cut mitre saw that I got from Micro Mark, but I suppose that a Dremel with a cut-off disc would work also. Another tool that I have used for filing the hinge line is a Dremel cut-off wheel. I would use one of the thicker ones. I would use it as a scraper, held by hand, not mounted in the power tool. It would simply be used as a scraper, pulled along the edge of the template by hand. I don't understand why, but I used to think that the cut-off wheel scraper worked better than the file. Now I have gone back to the file.

You will need one of those "snap off" knives. These are the cheap little knives with blades consisting of about 10 blade points that you snap off when they get dull. You will also need some single edge razor blades.

You will need a tool to create the little gap at the ends of the surfaces. This is made by simply gluing two pieces of 220 grit wet or dry sand paper back to back with 3M77. This sanding tool is pulled through the gap between the surfaces and cleans up the cut line and gives it just the right gap.

You will be using the plastic laminate templates to cut the surfaces. These templates will be attached temporarily to the wing with a fine mist of 3M77. If this is done just right, then the template will stay put for the cutting/filing operation but will come right off when you want it to and will only leave a little residue behind which can be easily brushed off. I stick the template to the back of my 3M77 spray booth ( a six foot long cardboard box, standing on end) and give it one quick spray. What you want is a fine mist of adhesive, no big drops or globs. To achieve this, you should have a nearly full can which is not too cold. You should be careful to hold the button down all the way and not have you finger blocking part of the spray. Also this technique works best on templates that have been used a few times before. Brand new templates, without a residue of old 3M77 on them don't work quite as well in terms of not leaving a residue on the wing. In other words, never clean the old residue off of your templates. So just one quick spray and immediately place the template on the wing, flush to trailing edge. I use a small spacer to get the correct distance from the wing root for the flap, usually 1" from the root. After the cutting or filing operation, just pull the template off and brush/rub the 3M77 residue off with your fingers. If your technique was not quite right and there is some residue remaining, you can carefully wash it off with a little mineral spirits on a paper towel. You must be careful not to get any solvent on the bare foam, and only a damp towel, not enough to soak through the skin.

Now we are finally ready to create a hinge. 

Always work with the wing in it's foam beds. Start with the wing upside down in it's top bed. Attach the flap template with 3M77 as described above using an appropriate sized spacer to get the correct distance from the root end. File the hinge line with the triangle file. You want to remove the thin fibreglass layer and the epoxy which is over the hinge line. Run one flat side of the file against the template. Keep filing as long as you are able to raise dust. When you get close to the Kevlar, there will be less dust. Also the feel of the file changes. As you hit the Kevlar, the file will no longer glide along, but will start to bog down in the Kevlar. If you are really aggressive, the file will suddenly catch in the Kevlar fabric and stop. This will make a little tear in the Kevlar and is a final warning to stop filing. In general, the more you file, the looser the hinge gets but if you file too far you will begin to weaken the Kevlar. 

After filing the hinge line, use the snap off knife to cut the skin at the ends of the template. You need not cut more than the skin except the last 1/2" or so at the trailing edge. You want to cut all the way through at the trailing edge so as to leave a mark on the top skin which will be used as an alignment mark when the template is attached to the top. Leave the flap template in place and attach the aileron template to the bottom of the wing. Make it flush to the trailing edge and but it up to the flap template. Use the snap off knife to cut the hinge line in front of the aileron template and the skin at the tip of the aileron. Remember that the snap off blade is only to be used to cut the skin. You do not want to try to cut the foam as it will likely tear. I hold the knife in such a way that the tip of the blade only penetrates a short distance into the wing. 

After completing the hinge line cut, Pick up the aileron template and reattach it to the bottom of the wing a short distance back from the first cut. Do not re spray it with 3M77, it should stick again well enough. The second cut will be made far enough back to get the correct gap needed for down aileron deflection. I just eyeball this distance, it's really not a critical dimension. After this second cut, you may remove the two templates and brush off the 3M77 residue. I tend to leave the little strip of skin in the aileron hinge gap for now, although you could also peel it off. I do tend to peel it off at this stage if it is a carbon skin. 

Turn the wing over and place it in the top bed. Attach the flap template flush to the trailing edge and aligned spanwise with the little cut marks left from the previous cut. Use the snapoff knife to cut the flap hinge line and the two ends of the flap. Once again, just the skin, not the foam. Attach the aileron template. Flush to the trailing edge and butted to the flap template. File the aileron hinge line. It is usually easier to finish filing the top hinge after removing the template. You need to file enough of it to create a channel for the file to run in. After removing the template you can continue filing in this channel with the file fully upright rather than having one of it's faces flat to the template. This will make it easier to finish filing the hinge line. Before removing the template, don't forget to cut the skin at the tip of the aileron. 

If this wing needs to have flap reflex then a second cut is required at the flap hinge line. With an all Kevlar skin, the challenge is to make this cut as close as possible to the first cut. A very small gap is all that is needed to get the proper reflex. The hazard is that if you try to make the second cut too close to the first, then you may simply push the skin down into the foam with the knife rather than cutting it.. Once this happens you will have a typical Kevlar mess which will be very difficult to clean up. To avoid this problem, you should always start this cut with a new knife point. You should make the cut with a few light passes of the knife rather than pressing too hard. The closest gap I can do reliably is somewhere between 1/32" and 1/16", once again just eyeball distance. When you have finished this cut, you may remove the templates and brush off the 3M7 residue. 

The next step is to cut the foam at the ends of the control surfaces and create the proper gap between the surfaces. Use the snap off blade to make the initial cut. Run the blade out to it's full length and use a sawing motion to cut the foam. Of course, the skin is already cut, and this acts as a guide for the knife when cutting the foam. Now pull the gap sanding tool through the gap a few times to clean up and widen the gap. The only cutting left now is the foam at the hinge line. Use a single edge razor blade for this. I slide the wing partially off the bed far enough so that the control surface being cut free is overhanging into empty space. As I pull the razor blade through the foam, I use my other hand to push down on the surface. This will take pressure off the blade by pulling open the cut line. This will allow me to feel when the blade contacts the underlying Kevlar skin and allow me to tell how much pressure I am putting on the skin with the blade. I will make the cut with two or three passes. The first cut is made about 1/2 way through the foam and is carefully aimed so as to hit as close as possible to the hinge line on the opposite surface of the wing. The second cut is made with all the focus on the correct depth. The aim is taken care of with the first cut. The second cut should go all the way to the Kevlar on the opposite skin and lightly drag the tip of the blade along the Kevlar. The third pass is just used to ensure that nothing was missed on the second pass. 

You should now be able to fold the hinge line over. If done well you can bend the hinge over a full 180 degrees and lay the surface all the way back onto the main wing surface. Some surfaces will not quite go all the way to 180 degrees, the ones with thick skins will not go quite that far. 

After folding over the flap hinge, You will need to remove the little sliver of foam and skin that needs to be removed for clearance for flap reflex. Just use a new single edge razor blade to slice this off. With the flap held all the way back against the bottom of the wing, you can just run the blade along and slice off the sliver of foam. 

The last step for the flap hinge is to use a 100 grit sanding block to lightly sand the back of the Kevlar hinge. This is to remove any foam that is still extending past the hinge line and to remove epoxy residue from the back side of the Kevlar. Be sure to only sand lightly on the Kevlar as it is very vulnerable at this point and you can easily weaken or even sand all the way through the Kevlar if you are not careful.

The final step is to cut the aileron hinge line and sand the back of the hinge in the same manner as was used for the flap hinge.

Phil

<< Back to Trimming and Finishing                ^Back Up to XP3 Techniques home page