When it comes to covering a model airplane’s wings, you have to make sure the fabric covering material is adhered properly to the wing structure. The wing is the part that produces lift so the model can fly and more than the fuselage and the tail surfaces, it is subjected a lot of stress. To get the most strength from the wing structure, the covering has to be taught and wrinkle free. Here’s how I did it with my 1/3-scale Fokker Triplane Project. To get the basics of the covering process using Stits Lite fabric, check out my post at: http://www.modelairplanenews.com/blog/2013/06/10/covering-like-the-real-thing-7-tips-for-applying-stits-lite-fabric/
Covering the wings
As with all covering jobs, the secret to a smooth professional finish is preparation of the surfaces being covered. Sand the wood smooth and make sure you have even seams and edges that do not project up. Everything needs to be smooth and clean. A good wiping with a Tack Cloth will do wonders!
With all wings, I cover them with two pieces of fabric, one on the bottom (applied first,) and one on the top surface. You can also the covering in 4 pieces ( two top and two Bottom) with a seam in the center of the wing.
The Fokker triplane has undercambered bottom surfaces on it wings so for this technique I start off by applying glue to the bottom of the wing. Poly Tak adhesive is heat activated so be sure to apply the adhesive to all the edges of the wing. This includes the wingtips, the LE and the TE.
Because of the concave shape of the rib bottoms, I apply a second coat to the capstrips. If you heat the fabric up while shrinking the fabric, it can pop off the ribs. Two coats of adhesive work well to prevent this.
Here you can see the glue applied to the ribs and the LE of the wing. I also apply adhesive along the spar running from tip to tip as well as between the spar and the leading edge.
Once the glue has dried, I drape the fabric over the wing to cut it to length. Be sure to leave a couple inches of extra material all around the wing so you can have something to grab and pull to get the slack out of the fabric.
Clean off your workbench and place the wing over the the fabric. I now apply some glue on the fabric all around the wing. This will help prevent snags while you trim the fabric to size.
Flip the wing bottom side up and center the fabric over the structure. Lightly tack the fabric in a couple of places to hold it from slipping off.
Now, starting in the center of the wing, apply heat with your iron to tack the fabric down along the spar. Pull the fabric taught as you work your way to the tips. Work in small 3 to 4 inch sections. Let the glue cool before moving to the next section. Don’t worry about any creases in the fabric now. When it is shrunk tight, they will vanish!
Now, before you start sealing around the wing’s outer edges, tack each of the ribs to the cloth. Apply heat only the the tops of the ribs. Let them cool before moving on to the next rib and work your way from the center of the wing out to the wing tip. Don’t worry about removing wrinkles from the open bay areas.
Apply heat to the center edge of the covering while pulling it tight. This shows the top of the wing, but it is the same on the underside.
At the center wing cutout area, split the covering as shown (use a sharp knife blade,) and trim it to size (for about a 1/2 inch overlap,) and glue the fabric down to seal to the edges. Here you see some of the Poly Brush sealer applied to check for areas where the fabric might have lifted from the wood. You can do this for each covering panel you apply, or after all the fabric has been attached. I does not affect the Poly Tak adhesive.
With Stits Lite fabric, wingtips become very easy to do. Simply grab the extra material and apply heat with the iron along the edges while pulling the fabric tight. Remove the heat but hold pressure until the glue has cooled. Start in the center and work outward along the edge to the LE and the TE. The photo (above) shows the bottom of the wing tip area.
Now go along the LE and the TE and check that everything has been sealed down properly. Again, you can use a little of the Poly Brush to make sure the fabric is tight against the wood. If not, apply heat and lift the fabric up off the wing, apply a little more glue, let dry and re-tack to seal it down.
Here’s the center cutout area. As you can see the material has been trimmed backed and sealed to the inner edges. The Poly Brush sealer shows the fabric is all tight against the wood.
This close up shows the scalloped TE. There are no wrinkles or slack spots in the cloth. The edges overlaps about 3/16 inch onto the opposite side for a tight seam. I heat it to tack the cloth the TE and let cool. I then trim the fabric and seal it down.
Tech Tip: When it comes time to add the Poly Brush Sealer, do not apply it to the entire length of the rib capstrip all in one go. This will soften the glue bond and the fabric can pop free of the undercambered (concave) rib shape. Instead, apply the sealer to the back third of the rib, allow the sealer to dry, then apply the sealer to the front third and let it dry. After the sealer has dried, apply more to the center third of the capstrip and let it dry. Using this technique, the fabric will remain tightly bonded to the ribs.
(Above) Here you see all the ribs with the first coat of sealer applied. Sealing with Poly Brush is the last step in covering and the first step in preparing the fabric for paint and finish.
Now use the same process to attach the rest of the fabric to the wing. Do the bottom surfaces first and then apply the top covering. Get it all smoothed down nice and wrinkle free, then get all the edges sealed down before applying heat for the final fabric shrinking.
For areas where there something protrudes from the wing, like this inter-plane strut attachment (above,) you have to cut an opening that fits neatly around the part, then glue the fabric down around it. You do this before you start sealing down all the outer edges.
Here you see that I have also pulled the fabric tight and tacked it to the wing tip rib. Now you can use the same technique mentioned earlier to seal the center edge seam followed by the leading and trailing edges.
Here’s the finished top wing surface at the wing tip. There are no wrinkles or sags in the fabric. Now is the time to apply heat to the fabric to shrink everything to a smooth finish. Start in the center of each wing panel and work your way to the outer edges. Do not apply a lot of heat to any of the seams or they can come loose causing the fabric to slacken. Once all the fabric is shrunk and cooled off, you can apply two coats of the Poly Brush sealer to seal the fabric weave.
That’s it! Your wing is now ready to finish and paint! Stay tuned for more. I will be back soon to show how to paint and finish your Stits Lite covered model.