Steve Pierce
BENEFACTOR
Graham, TX
I just finished installing the wing tip bows on my Clipper wings last night and both bows on a covered Super Cub just before Christmas. Someone on the Short Wing Piper Club website asked for any tips on installing wing tip bows. With it fresh on my mind I typed this up and thought someone here might can use it.
I have had to replace several tip bows on covered airplanes. I just finished replacing both tips on a Super Cub. It all depends on what finish and how easy you can get the tape off but here is what I did.
Get under the tape on the outboard aileron rib and pull it off. This serves two purposes. It gives you a good clean area to glue your new fabric to and gives a straight line. Trying to peel the finish off without creating big craters is a pain. After I pull the tape I cut the fabric out board of the rib several inches. Be sure and leave the rib stitching on the outboard aileron rib. This will keep the fabric tight. Also be careful not to cut the inter rib bracing tape until you get to the bow. Check the STC for your fabric system but most want a 2" glue seam for repairs. Remove the fabric. Peel the fabric back off of the outboard tip skin. You will have to remove the screws and slide the outboard tip skin out from behind the 3/4 wrap outboard leading edge skin. Remove the tip bow. Check for damaged ribs and reinforcement channels. If you only have two reinforcements between the front and rear spar and the tip bow get two more. J3s, PA11s, PA16s and probaly early PA20s only had two of these reinforcements and the tip bow tended to curl up as a result. Also the reinforcements tend to crack in the corners where they are bent.
The tip bows come from Univair in a round tube due to the oversize shipping if it was in it's true shape. I have a fixture that over bends the bow a little. I put it in the fixture and wet it. When it dries it comes out and is pretty much the shape you want. The problem I have had with trying to fit the bow to the wing right out of the tube is that the bow tends to pull and break the little tabs off of the ribs while you try to fit it. I use a round rasp and cut a half circle in the leading edge of the forward tip of the bow. Insert in behind the outboard aileron rib and pull the tip around to the front spar fitting and hold with a small C clamp. Pull the bow around and clamp it to the rear spar fitting the same way. After that I drill the holes through the tip bow into the 4 spar fittings. An angle drill works nice but you can eye ball it and drill it from the front also. I use draw type clecoes to hold it in place. I drill the spar fittings with a #21 since it takes an 8/32 screw and the hole that attaches to the ribs with a #30 for the 6/32. Pull the tip of the bow around and clamp the outboard aileron rib and the 3/4 rib. Be sure and put the reinforcement channel between the rear spar and the trailing edge tip of the 3/4 rib. This will place the rib in the correct place and should align your rib so it is straight. I align the outboard aileron rib with the aileron leaving an even gap between the rib and the aileron. It really sucks to have your aileron interfere with the rib. Don't ask how I know about this. Drill the holes and countersink them. To keep from having to drill more holes in the nose ribs I take two strips of aluminum and back drill the holes from the tip skin into the strips. I then screw these strips to the nose ribs in place of the tip skin. This lines the ribs up and also keeps from making swiss cheese of the ribs or the tip skin when you go to put the tip skin back on. Now I drill the hole in the tip bow for the nose ribs to attach to. These are #30 holes for the 6/32 screws and a small pilot hole for the wood screw. The drawing calls for a countersunk 3/4" long screw but I use a 1/2". 3/4" is too long. I then remove the bow and apply three coats of Polyfiber epoxy varnish per instructions. After it dries I install it. I apply varnish to each screw and also to the two nails in the trailing edge of the outboard aileron rib and the tip of the tip skin. I cut the trailing edge end of the tip bow off after completely installed flush with the outboard aileron rib and then varnish the cut. They come about 4" long. Take your time and you can get it all lined up and looking nice. Get in a hurry and you will be able to tell. I just finished installing the tip bows on my Clipper wings last night.
Install inter rib bracing tape to keep the ribs straight. As for the fabric on a repair I glue to the line where I pulled the rib tape off and stretch around the new tip bow and then do the same on the bottom. Shrink per your STC etc. I then stitch the overlap on the outboard aileron rib (it already has the original stitches under your glued seam) and the 3/4 rib, tape and finish.
This is how I have done it and it has worked for me. Hope it helps.
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I have had to replace several tip bows on covered airplanes. I just finished replacing both tips on a Super Cub. It all depends on what finish and how easy you can get the tape off but here is what I did.
Get under the tape on the outboard aileron rib and pull it off. This serves two purposes. It gives you a good clean area to glue your new fabric to and gives a straight line. Trying to peel the finish off without creating big craters is a pain. After I pull the tape I cut the fabric out board of the rib several inches. Be sure and leave the rib stitching on the outboard aileron rib. This will keep the fabric tight. Also be careful not to cut the inter rib bracing tape until you get to the bow. Check the STC for your fabric system but most want a 2" glue seam for repairs. Remove the fabric. Peel the fabric back off of the outboard tip skin. You will have to remove the screws and slide the outboard tip skin out from behind the 3/4 wrap outboard leading edge skin. Remove the tip bow. Check for damaged ribs and reinforcement channels. If you only have two reinforcements between the front and rear spar and the tip bow get two more. J3s, PA11s, PA16s and probaly early PA20s only had two of these reinforcements and the tip bow tended to curl up as a result. Also the reinforcements tend to crack in the corners where they are bent.
The tip bows come from Univair in a round tube due to the oversize shipping if it was in it's true shape. I have a fixture that over bends the bow a little. I put it in the fixture and wet it. When it dries it comes out and is pretty much the shape you want. The problem I have had with trying to fit the bow to the wing right out of the tube is that the bow tends to pull and break the little tabs off of the ribs while you try to fit it. I use a round rasp and cut a half circle in the leading edge of the forward tip of the bow. Insert in behind the outboard aileron rib and pull the tip around to the front spar fitting and hold with a small C clamp. Pull the bow around and clamp it to the rear spar fitting the same way. After that I drill the holes through the tip bow into the 4 spar fittings. An angle drill works nice but you can eye ball it and drill it from the front also. I use draw type clecoes to hold it in place. I drill the spar fittings with a #21 since it takes an 8/32 screw and the hole that attaches to the ribs with a #30 for the 6/32. Pull the tip of the bow around and clamp the outboard aileron rib and the 3/4 rib. Be sure and put the reinforcement channel between the rear spar and the trailing edge tip of the 3/4 rib. This will place the rib in the correct place and should align your rib so it is straight. I align the outboard aileron rib with the aileron leaving an even gap between the rib and the aileron. It really sucks to have your aileron interfere with the rib. Don't ask how I know about this. Drill the holes and countersink them. To keep from having to drill more holes in the nose ribs I take two strips of aluminum and back drill the holes from the tip skin into the strips. I then screw these strips to the nose ribs in place of the tip skin. This lines the ribs up and also keeps from making swiss cheese of the ribs or the tip skin when you go to put the tip skin back on. Now I drill the hole in the tip bow for the nose ribs to attach to. These are #30 holes for the 6/32 screws and a small pilot hole for the wood screw. The drawing calls for a countersunk 3/4" long screw but I use a 1/2". 3/4" is too long. I then remove the bow and apply three coats of Polyfiber epoxy varnish per instructions. After it dries I install it. I apply varnish to each screw and also to the two nails in the trailing edge of the outboard aileron rib and the tip of the tip skin. I cut the trailing edge end of the tip bow off after completely installed flush with the outboard aileron rib and then varnish the cut. They come about 4" long. Take your time and you can get it all lined up and looking nice. Get in a hurry and you will be able to tell. I just finished installing the tip bows on my Clipper wings last night.
Install inter rib bracing tape to keep the ribs straight. As for the fabric on a repair I glue to the line where I pulled the rib tape off and stretch around the new tip bow and then do the same on the bottom. Shrink per your STC etc. I then stitch the overlap on the outboard aileron rib (it already has the original stitches under your glued seam) and the 3/4 rib, tape and finish.
This is how I have done it and it has worked for me. Hope it helps.
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