Builder Mods: Bob Axsom
Home > Builder Mods > Bob Axsom
Jump to newest entry
Wing Tip Mod
I removed the standard RV-6A wing tip which is 12" wide and made an airfoil template to start as the base for developing a much shorter wingtip. I taped file folders together and taped them to the outboard end of my tip tanks then traced around the tank on the pattern material (taped together file folders). The tip tank configuration is a 9 inch extension of each wing (18" total extension) using the exact same airfoil shape. Then I cut out the pattern very carefully to produce male and female patterns. The female pattern was taped in place on the opposite tip tank and this verified that the two wing ends are exactly the same shape as well as assuring that the airfoil pattern accurately represents the shape of both wings. Two photos are attached.
The objective of the development is to produce two new wing lengths for racing in an attempt to increase the airplane speed. One configuration will include the existing tip tanks and the other will attach directly to the basic wing. This will reduce the span by 3 feet and 1.5 feet respectively. With the tip tanks the existing wingspan is 1.5 feet greater than a stock RV-6A.
I expect to take quite a bit of time building two wood male molds (left and right) of the new, basically teardrop shaped, tips for hand lay up of the four fiberglass tips. The interface with the wing will be the same as the existing tips. I have to decide how much to set back the edge of the pattern to form a good clean transition from wing to tip. Right now I'm thinking roughly 1/8".
By Sunday morning I had made no development progress because of things I had to do around the house. I did buy some wood and clamps at Lowe’s Hardware and some epoxy resin & hardener (E-Z Poxy) from Aircraft Spruce via the internet. Perhaps more importantly, I continued to think about the mold shape and best way to implement it. I find when I think about something enough I can eliminate some errors before the works is done and come up with a better implementation concept. The time was not wasted. Today was focused on the wing tip mold development.
I had to buy a band saw to cut out the wood components of the wing tip mold and they are so big that I had to build a table to support the material overhanging the saw's work table. I did that today and then I was finally able to saw out the base plates for the male molds. Both of them were completed this afternoon and I was able to clamp them together and sand the edges before dinner. Now I've got to work on the mold parts that provide the tip shape outward from the base.
I have one of the rough cut molds glued up and clamped and the other one has one board glued and clamped to the base. It takes a lot of time because the band saw is a low powered but functional tool. I have three more segments to saw and that part of the process will be complete. It is looking well so far.
Thank God for slow drying wood glue. As I was going to bed I realized that I had started my second mold buildup exactly like the first one. Two left wing tip molds would not do me any good. I was able to drive a screwdriver under the single board glued to the second base and start building it up correctly on the other side.
9/7 - 9/30 (with 45 pictures)
I continue piddling with this but later today I will start "cutting wood" for the mold base. Two photos are attached. One shows the master pattern and the 3/32" set back mold base template. On the master pattern I drew the chord line from the leading edge of the nose to the trailing edge tip and 116 perpendicular rib station lines every 1/2" starting at the nose. Some observations were made as a result.The airfoil extends back 1/4" from the last drawn rib station line so the chord is 58.25". The maximum thickness of the airfoil is 7.75" and it is fairly constant over a fairly long section of the chord. Using the naked eye I estimate the maximum thickness occurs at 20 inches aft of the leading edge. At this point the airfoil above the chord line is 4.5" and airfoil below the chord line is 3.25"Since the airfoil is not perfectly symmetrical the tip will not be a perfectly symmetrical streamlined shape. I do not want the downward swooping surface on my short tips so the maximum extension of the tip when viewed from the from will have to move up if it is to follow the mid-point of the airfoil thickness with increasing rib stations. I will probably flatten the tip slightly to eliminate the appearance of concavity.The tip set used for short races with the 9" wide tip tanks removed will extend unsupported aft of the rear spar 15" and the configuration will be different to provide for that.
I mounted the glued up left wing tip mold block on a table that I built for the band saw work. Then I used a planer to roughly shape the block into the form of a wing tip. The lower surface of the airfoil is the shape I used for the plan (top) view of the wing tip. As a reference I established a line from the leading edge to the trailing edge which is exactly where two 2X4s are joined. Then I drew a line 2" from the upper surface where it is above the straight reference chord line. I used those two lines as boundaries for limiting the vertical shaping. Finally I very roughly sanded the surface of this left tip mold. Two photos were taken after each task and they are attached.
I completed the left tip mold through three coats of polyurethane, then waxed it and layer up the first fiberglass. I'm a little delayed on the right tip mold as my planer drive belt wore out. new one on order.
I got through a tricky part in the process, pulling the first layer off the mold. It came off fairly well using an ice cream stick and a bent coat hangar. Then I scraped the mold to remove epoxy resin deposits in various locations, sanded it lightly with 320 grit sandpaper and waxed it again. I put the first layer back on the waxed mold and applied another layer of fiberglass - ply #2. It is 24 hour cure resin but I intend to work with the skirts to pull them in tight before the final cure.
The 3rd layer is on. It took 50 minutes, from 11:20 to 12:10. It looks good. I am going to add a band of fiberglass to form a shoulder where the wing ends and the exposed wing tip starts. This will be the last full layer.
I trimmed the skirt in mid afternoon and this evening just before 8pm I removed it from the mold. It was as least as difficult as when I removed the first layer. I used the same basic approach but I had to get a heavy wire coat hanger to use as a separation tool this time. Finally, at 7:50 pm it snapped free and it looks very good in a raw state. It is ready for the transition to the integration phase.
I’ve been thinking about this a lot to make sure it comes out just right for appearance and performance. If I were going to mount platenuts on the wing tip fiberglass I would have to build it up a lot more that three layers. However, if I back the portion of the fiberglass that extends behind the wing skin with a thin aluminum strip the three plies should be adequate. The important thing is to do everything in a specific sequence to assure a precise fit that creates no installation loads except the compression of all elements between the flat screw head and the platenut and longitudinal tension of screws.
Here is how I intend to proceed. I will have to remove the left tip tank and the stock wing tip and save all of the painted screws in a marked container. All of the wires will have to be marked twice at this interface and cut in the middle of the slack. Then I will crimp knife splices on the two wire ends. The foil shielded strobe cable presents a special problem but I will have to work that out as I get in there. The electrical interface has to be configured so that so that it can be connected for normal flying with the tip tanks and stock tips in place and disconnected for racing with the new short tips. I plan to rely on knife splices, vinyl sleeving and wire bundle lacing tape as the primary materials.
When the open end of the wing is clear I will work the tip in place. Then I will drill 1/8” holes in tip mounting flange using the existing dimpled holes as a guide. Each hole will be clecoed to the wing after it is drilled. There is a 15” long recess needed in the rear of the tip to accommodate the aileron and I may need to take care of this before drilling the mounting holes and I may have to insert stiff foam in the tip opening to prevent its collapsing when I drill the holes. I’ll do what has to be done as the need becomes apparent.
When all of the holes are drilled I will remove the tip and clamp a 5/8” wide strip of 0.016” 2024T3 aluminum to the inside of the wing tip mounting flange and drill the mounting holes through the aluminum using the holes previously drilled in the tip as guides. Each hole will be clecoed after it is drilled with 1/8” clecoes. After the mounting holes are in the tips and the aluminum backing I will cleco a platenut on the outside of the fiberglass tip skin and drill the fiberglass and the aluminum strip through the platenut mounting holes with a 3/32” drill clecoing the first hole before I drill the second to assure alignment. Then the platenut will be removed and the center mounting hole will be enlarged as required to accommodate the skin dimple and the tip mounting screw. Then I will countersink the platenut mounting holes for 3/32’ flathead rivets. Every thing is set up for #8 hardware but the platenuts can be fixed dimpled platenuts or floating platenuts. Floating platenuts give more flexibility on position accuracy but they cost about 76 cents each while the dimpled ones are 32 cents. I expect the requirement to be around 100 for four tips so the difference is around $44. I’m thinking precision is a good thing.
The spacing on some of the mounting holes is large enough that I may add some rivets between mounting locations to secure the aluminum backing. Another unknown is the perfection of the fit between the tip and the end of the wing. There may be a need to build up of sand down isolated areas to get an acceptable fit.
Once install the tip with the #8 flathead screws which have been stored from the earlier tank removal I will draw a line showing the exact location of the end of the wing skin on the overlapped tip flange. I will remove that and install as 2” or 4” fiberglass strip up against the “end of wing skin” line to give a more uniform skin line and finished looking piece of work. After the epoxy resin cures I will blend in the outboard edge of the strip with sandpaper. Something else will probably come up but this is my going in plan.
Replacement drive belts received from Sears for my power planer at around 4pm today so I was able to tackle the rough cut and glued up 2X4 and plywood block. It is now roughly shaped and sanded. I need to make some templates off of the left tip mold to use in assuring the shape of the two tips are essentially mirror images of each other. I may use something simple like coat hanger wire for this task. Even in its rough shape it looks very similar to the left tip - at least to the unaided eye.
Friday September 21, 2007. I got up in the middle of the night and played with the coat hanger wire idea for profile checking. It doesn’t look promising at all. It seems to be too springy for a good check. I have to think about the some more as we walk this morning and when we go shopping for a ring for Jeanine. During our vacation last month she lost her wedding ring of almost 40 years and we would like to buy a new one before out anniversary on October 7.
I asked Jeanine if she had any idea of how I could compare the two molds and make sure they are the same. She suggested taking a tape measure and checking the distance around the surface at several common points. With the fact that I have a straight line boundary from the leading edge to the trailing edge, I think that is an excellent idea. I’m going to get in a little hand sanding with coarse sandpaper while Jeanine finishes breakfast and gets ready to go walking.
I got in an hour of sanding and hand planing the right mold. I also used spackle to fill the gaps between the plywood base and the 2x4s. Later today I will sand that flush. Then I plan to remove both molds from the supports and mark ˝” rib stations on the inboard surfaces of the molds. I can use the already marked master pattern to make this easy. These lines will be used to implement Jeanine’s idea of using the circumference measurements to determine conformity between the left and right molds.
From 4:05 pm until 5:08 I sanded the spackle and shaped the mold with more sanding and planing. Just before stopping to get ready to got to a dinner theater with friends, I spackled a few natural voids in the wood. This event tonight was a total surprise that I wish weren’t happening but we received a call from Pat Lyle saying that she and Budd had two extra tickets and would we like to go? They are very nice people so I sacrifice an evening of work for friendship. At my age friends don’t come easy but the wood will wait patiently for me to get back to it. If time permits I will sand the spackle tonight then the molds will be ready to remove for marking and measurement.
Saturday morning, September 22, 2007. I got up at 2:30 am and sanded the spackle smooth. It is now 9:26 am and I just got up. I have to eat breakfast, get cleaned up and take our daily 1.5 mile walk in the park before I can get back to work on the wing tips. The right wing tip mold looks right but I need to make sure it is the same size and shape as the left one before applying three coats of polyurethane to the mold surface.
I worked all afternoon and well into the evening on the mold for the right wing tip. First I needed to make sure the right and left molds were the mirror images of each other. I dismounted them both from their galvanized iron stands. I sat the left mold on the master pattern that I had marked off with ˝” rib stations at the beginning of this development. Since the molds are based on a pattern that is set back 3/32” from the perimeter of the master pattern the ends of the rib station marks were visible all around the mold. I used a blue fine point felt tipped pen to transfer the marks to the edge of the left mold. Beginning with the front of the mold I wrote the number of every fourth mark from 4 through 116 near the trailing edge.
Then turned the left mold over so the round surface was resting on the workbench top. I sat the right mold flat surface on the upturned flat surface of the left mold. They were a perfect match as I expected. The first conformance check was good. I transferred the rib station marks from the left mold directly to the right mold and numbered them as I had the ones on the left mold.
With both molds marked and numbered I separated them and placed them on the bench with the flat surface down, airfoil bottoms touching and squared up with the bench. I placed a level across the two molds and made progressive rough checks of the span wise conformance of the two molds from the leading edge to the trailing edge. The bubble was essentially centered for all checks except the rear five inches. The indication was that the new right mold was higher (longer in span) than the left mold at the rear end.
I remounted the molds on the iron stands so that I could measure the rounded surface dimension at selected rib stations. The measurement procedure was to hook the end of a tape measure on the edge of the mold base on one side at a particular rib station then wrap it around the tip mold to the edge of the mold base on the other side at the same rib station. I measured the left mold and recorded the measurement then made the same measurement on the right mold and recorded that measurement. They were very similar except at the rear, which was consistent with the level check. There were some excesses at other stations in the right mold stations that were not revealed by the level check. Since these stations passed the level check the excesses had to do with the side contours.
These are the recorded data (measurements in inches and fractions of inches converted mathematically to decimal with no precision adjustment beyond zero fill to two decimal places):
Station Left Mold Right Mold
4 5.875 5.875
8 8.125 8.125
12 9.6875 9.625
16 10.75 10.8125
20 11.5625 11.625
24 12.125 12.1875
28 12.5625 12.625
32 12.8125 13.00
36 13.0625 13.15625
40 13.0625 13.1875
44 12.9375 13.125
48 12.75 12.875
52 12.50 12.5625
56 12.1875 12.28125
60 11.8125 11.84375
64 11.375 11.34375
68 10.875 10.75
72 10.25 10.125
76 9.625 9.5625
80 9.00 8.8125
84 8.3125 8.25
88 7.625 7.625
92 6.9375 6.9375
96 6.3125 6.25
100 5.53125 5.53125
104 4.625 4.8125
108 3.75 4.03125
112 3.00 3.3125
I reviewed the recorded data and did nothing with differences on the order of 1/16”or if the right mold was smaller than the left mold. The rear of the right mold was the only part that really seemed to require work but I did work on some other areas as well. Wood was removed from the right tip mold with a plane and #60 grit sandpaper resulting in better conformity with the left tip mold. As the mold was reworked the dimensions in key areas were rechecked until I found them to be close enough to accept. The corrected dimensions for the right mold are:
˝ Rib Final
I painted the right mold at this point with Min-Wax clear glossy polyurethane. I finished at 8:47 pm. So I will have to lightly sand it and apply a second coat in three to four hours.
At 00:30 am I completed the second coat of polyurethane. In three to four hours I will do this again. After the third coat it needs to cure for 24 hours.
It’s Monday 9-24-2007 and everything is ready to fiberglass but I have to go to the dentist. This means that I will not get the first layer on until this afternoon. I will be anxious to see how the new mold release wax works on this tip. I opened the can last night for a look and it is a whitish looking wax the is hard. I think this will make the removal process much easier.
I finally got started on the first layer of fiberglass on the right wing tip mold at 1:21 pm. I cleaned off the front section of the workbench and then I taped down a layer of newspaper and two long overlapped sheets of waxed paper. This was for the impregnating the glass cloth with EZ-Poxy Resin before placing it on the mold. I laid the cloth on the mold and cut it a few inches oversize before moving it to the workbench.
I read the instructions on the mold release wax and ignored all of the preparation steps requiring more of the company’s products. I did apply, let dry and wipe off the wax five times. The wax is made by a company in El Monte, California. That city has a special place in my memory because it is where I landed and left my airplane every day for fifteen years as part of my commute to work.
I put on a pair of vinyl gloves to protect my hands and started to work with the resin. The wing tips really take a lot of resin per layer. I poured 8.5 oz. of resin in a large cup then added enough hardener to bring the weight up to 11.7 oz. I stirred it for three minutes then it was ready to apply. I cut a one-inch slice off of the end of a small kitchen sponge to do the glass impregnation with and started pounding the resin into the cloth with the end of the sponge. Around one hour later the cloth was saturated and I transferred it to the mold and carefully shaped it to conform the mold without wrinkles or air bubbles. I will work with it through the night to make sure it stays that way. It looks good right now.
Monday - I trimmed the skirt of the first fiberglass layer on the right tip at 7 pm. It looks great. On the left tip I removed the first layer from the mold after about 11 hours. It was not fully cured but I was concerned that I might not be able to get it off if it were fully cured. I used the new mold release wax but I don’t know how well it works. Right now I’m leaning toward building up the whole three layers before pulling the fiberglass from the mold. Yeah, I’m sweating.
I couldn’t stand it any more. I got up at midnight and pulled the first ply from the mold. It came off cleanly except near the rear few inches of the tip. A little strip, three or four inches long and 1/8” wide was stuck to the mold. If I had let it cure fully I might have had to cut it off. As it was it yielded finally and the tip looks good.
I scraped and sanded the problem area on the mold then applied five coats of mold release wax. Then I cut more cloth, mixed more resin and applied the second ply over the first on the right wing tip mold. I think this is going to work out well.
Tuesday Morning September 25, 2007. I spent 16 minutes just before 10 am cutting off the skirt of the second ply. It is looking very good. I found one place where there was a gap in a pleat like form at the base of the mold. Because it was not completely cured I was able to push it in to join the first ply after the skirt removal was done in that area. Once again I did the right thing by working with the fiberglass before it was completely cured. The hard bond between the first layer and the mold or the delaminated ripple at the base would have been significant problems in the completed wing tip. The third ply should be fairly easy without another intermediate removal of the fiberglass from the mold. We are going to be gone all day and I have to vacuum the floors today so that will not happen until late this evening.
We went to see “The Brave One” in Rogers and it was a very good movie staring Jodie Foster. We went shopping for a few things at Belk’s and rain made the driving hazardous so we got home later than planned. I never got to the floors but I did get the third ply of fiberglass on the mold at 6 pm. It is 8:40 pm and the resin seems more wet than usual at this point. I expect to trim the skirt by midnight but if I didn’t get the resin mix right it will be later.
Finishing this third layer of fiberglass on the right wing tip will be the last significant development work until we get back from our 40th anniversary trip to New York. We are going to fly the plane there in its normal cross country cruise configuration but when we get back on the 9th of October all efforts will be focused on integrating the new wing tips with the airplane.
Wednesday morning at fifteen minutes past midnight I got out of bed and went out to the garage to check the condition of the latest layer of fiberglass. It was starting to stiffen up – just right for trimming the skirt. It came off easily and neatly. I will wait until sometime tomorrow before I pull the new wing tip from the mold. I can tell by the way the fiberglass moved relative to the mold that there will be no trouble getting it off when the time is right.
I got up again at 4:21 am and decided to try to remove the right wing tip from the mold. It came off cleanly with no difficulty at all. It is a lighter shade of amber than the left wing tip at this point. I have lightly sanded the left wing tip and the right one is less than half cured so they may bee more similar in appearance later on. I also sat the two molds together and it is impossible to miss the degraded difference in appearance of the left one from the right. The special mold release wax definitely makes a difference. Regardless of the mold appearance I am very pleased with the two wing tips at this point in the development process.
It’s Sunday, the last day of September. Tuesday we takeoff on our 40th Anniversary trip to New York. I am looking forward to that of course but it is agonizing to have the fiberglass wing tips ready for integration but having to wait for the trip to be over before I can start. Every day I go out to the garage and take the tips off of the molds and study them. They look really good and I can’t image that they will not give an increase in speed. I am going to the airport in a few minutes to put the New York trip routing in the GPS receivers. While I am there I will tape the tips on the wing. That will give me some satisfaction.
It’s 6:28 pm now and I have gone to the hangar loaded the New York flight plan in the two GPS receivers. That was a chore but then I got to hold the wing tips up to the airplane and see how they will look, well kind of. The wings are so slick from wax that it is very difficult to hold the tips in position. They want to slide sideways and I had to be very careful; that they didn’t slide off of the back of the wing. I was able to get enough masking tape on the right one to hold it roughly in place but it was risky and I didn’t even try the left one. I think the short range configuration is going to be faster but I’m not sure about the long range race configuration. I am going to build the long range tips as well and test them. As always, if they are better they will be used and if not they go up in the attic with the other modifications that didn’t work. When I got back home I sanded the rough edge where the lay-up skirts were trimmed off. I can hardly wait until we get back from New York and I can complete the short range version.
I don't know if these photos will pass the bite limit but I will try. The first four photos are of the tip in the short range placement the last two have the tip in the long range placement.