What is Ripstop Nylon  and where do I get it (is it expensive?

Ripstop Nylon is a tightly woven fabric that has a double weave.  The weave has a very tight thread pattern with a second weave that adds a tight "grid" into the weave.  The grid is about 1/4" square.  Commercial uses of ripstop include parachute canopies, sailboat spinnakers, tents, flags, windsocks and clothing. 

Ripstop applied to a foam combat wing greatly adds to the strength of the wing and makes it much more resistant to damage from mid-air strikes. It is hard to tear, but gives slightly to absorb impact.  It ties all the elements of the wing together and will help you survive mid-airs in a match.

If you go into your local fabric store (Hancock's Fabric or JoAnn's) and ask for ripstop nylon you will as likely be met with blank stares as by someone who knows what it is.  When you go into a fabric store ask for the "nylons" section.  That is where you will find ripstop.  At Hancock's ripstop is most likely to have the bolt number 128.  There are varying weights of ripstop.  The one carried in fabric stores is about 1.5 ounces per square foot.  There are lighter weights but they are hard to find.  You might get lucky and find some so keep on looking.

Ripstop found in most fabric stores is "water resistant".  It has been coated with a resin on one side.  After searching half the world for non-coated ripstop I have come to the conclusion that you just have to be lucky to find it.  The Hancock bolt #128 is coated but has more porosity than some of the other types of ripstop.  Fluids can flow through it but just barely.  In a moment we'll talk about how to handle getting the adhesive to flow through it.  For right now try to find a color of ripstop you like at either Hancocks or JoAnn fabric so we can get started.  You will pay about $5 for a linear yard 60" wide.

What do I use to apply ripstop?

The most forgiving adhesive for ripstop is water based polyurethane (wpb).  The WPB will not attack the foam, has a good potlife and is forgiving when applying.  I have used several brands with success and recommend ACE hardware brand wpb for its cost and characteristics.  Other folks have had great success with other wpbs and I encourage you to experiment to find the one that works best for you.

Because WPB cures by evaporation you should never apply it when it is raining.  I did some wings when it was pouring outside and wasn't a happy camper with the results.

I use a cheap plastic bristle brush that is two inches wide and cost under $3 to put on the wpb. 

The Secret Technique for Applying Ripstop Nylon to a foam wing

The first step is to cut your ripstop to size.  I made up a cardboard template that was oversize to cut out the panels.  I use four pieces of ripstop to cover a wing.  A bottom left and bottom right, a top left and top right.

Ripstop is tough stuff and will dull a conventional XActo blade quickly.  I went to a fabric store and bought a rotary cutter by Olfna and a cutting mat.  This has made cutting the ripstop a much easier proposition. The rotary cutter also works wonderfully when you are cutting glass cloth so you will get a lot of use from it.

Look at your ripstop.  The resin coating is only on one side.  The uncoated side should be the one that goes onto the foam.  When you cut your panels mark the LE and which side is the "inside".  You will make your life a lot easier if you go through this extra step.

Right now don't worry about your ailerons they will come later.

The first secret of applying ripstop is to treat it so that the wpb will flow through it.  The best way to do this is to go into the kitchen and grab one of your wife's big cooking pots and fill it about half full of water.  Put it on the stove and heat it up, not to boiling but close.  About halfway through heating it pour some wpb into the pot.  Mix it for a little while to make sure it is dissloved in the water then put two panels ( a left and right) of ripstop into the water.

You want to leave it in the hot water for at least thirty minutes.  The water and the heat will swell the fibers slightly and allow the ripstop to flow into it and gain a grip.  The ripstop you put into the water will penetrate the weave and help you get a good grip.

While the ripstop soup is on the stove take your wing and make sure it is smooth and ready for application of the rsn.  Blow the dust off or use a tack rag to get it clean.  Take your bristle brush and apply a generous coat of wpb to one side of the wings bottom.  We always do the bottom first so that the top will overlap and the seam will not show.  We are basically vain about our airplanes appearance.

Remove a panel of ripstop and lay it down on the wing that you have coated with wpb.  Stretch and tug it to get out the wrinkles and the bubbles, using your brush paint it span wise and chord wise till it lays smooth.  It takes about 5-10 minutes to get all the bubbles and wrinkles worked out and the LE and TE down smoothly.  Repeat the process for the other bottom wing panel.  Go into the house, pour two fingers of your favorite attitude adjustment elixir and go back to the shop to admire your work.  I find that if I check it every thirty minutes for the first two hours I can fix any imperfections that come up.  Don't put on more wpb instead set your brush in a glass of clean water and use the water to encourage the bumps and bubbles to lay down.

Let the wing sit overnight, and 24 hours is best.  I know it sounds slow, I usually do two or three wings at a time and this makes me feel like I am really building fast.  Now grab about three fresh XActo blades and trim off the excess.  You will be surprised at how fast the rsn dulls blades.  If any bubbles have come up, cut a slit in them with the Xacto, and squirt some foam safe CA into the bubble then rub it down with something that won't stick to the CA, I use a small piece of plastic film when I do this.

Do the top the same way, wait 24 hours, trim it and lets go on to the ailerons.  I use four pieces of rsn to do the ailerons.  I cut them about four inches wide by the length of the panel.  I bevel the ailerons for top hinging, meaning the top mates with the wing while the bottom is "open" due to the bevel.  I begin with the top.  I pin the aileron in place reflexed downwards.  Again I soak the rsn in hot water before application, coat the wing top where the rsn will be placed and the aileron and then apply the rsn over the wing TE and aileron.  When dry I do the bottom.  The aileron is reflexed upwards and the rsn is pushed into the bevel with a long straight edge.  Once dry they are trimmed out and now you are ready for the final coat for fuel proofing.

I believe wpb is fuel proof if you let cure for about a week.  I never have that much time before I need the plane so I fuel proof with epoxy.  I've tried several types and have settled on 30 minute Zpoxy.  I thin it 100% with drug store 91% isopropyl alcohol.  I mix about an ouce of epoxy and add an ounce of alcohol.  I paint this on with a brush and set the wing aside.  If the alcohol doesn't flow well enough I dip the brush in alcohol and paint the areas out so they are wetted with epoxy and alcohol.

On a 72" bat trick wing this entire process only added about an ounce and half to the final weight of the plane.  Good luck!