Kitfox Series 7 Super Sport - Show Me Fox

[Jason Murphy]

Received a call from the DPE, CFI-I check ride will be rescheduled he accidentally double booked himself. I decided to take a break and work on the plane instead. Multi Engine training begins next Thursday.

Prior to starting I spoke with another builder who has the Hoerner Style wing tip seeking advice or lessons learned while they had fitted their wingtip. I was advised to fit the wingtips before covering. Clamp straight edges to the wing to assist in alignment. Trim only the top first. Drill some temporary holes and cleco in place. If needed these holes can be patched later. After getting the top just right clamp and mark the bottom. Trim up the bottom rinse and repeat. It will take several fitting sessions to get things right. Then turn your focus to the trailing edge. Using a straight edge mark and trim.

Dry fit wing tip onto right wing using clamps. Noted interference of the forward spar tube with the wing tip. Used an 8 ft level as a straight edge placed across the top of the wing ribs. It appears I will need to compensate for a slight droop.

Dressed up the end of the spar tube to eliminate interference. Dry fit wingtip using clamps again. I am starting to get the feeling this will be another exceptionally tedious task if I am going to make this look right.

Following the advice received earlier in the day I will need to trim the topside first. I am thinking I could layout a pattern on the aluminum strip and drill No 40 pilot holes. After I get things trimmed, sanded, and fit I can use my angle drill attachment to back drill / match drill the wingtip from the inside. This would ensure I have proper edge margin on the aluminum. I would then bring the holes up to full size.

I am also wondering if the wingtip would benefit from using a countersunk washer and countersunk flat head screw to hold it in place. I believe the pressure would be better distributed but this may require a slightly wider aluminum strip to serve as the wingtip attachment point. I am open to feedback and suggestions.

Pictures to follow or click the link

https://eaabuilderslog.org/?s=ShowMeFox

[Jason Murphy]

Tried emailing the pictures to myself and attaching them via web browser today. They are JPG images... No Joy

[Jason Murphy]

Used an 8? level across the cap strips of the ribs to help me determine the wingtip is flush with the wing.

Used a 4? level across the leading edge. Brought the wingtip flush with each straight edge. Clamped wingtip in place. Using the outboard edge of the No 10 cap strips I measured 5 mm from the cap strip which was actually the largest deviation from the cap strip. I then connected all the dots using a ruler and a reinforced cut off wheel made a rough cut and snuck up on my cut line with sandpaper. I now have a much straighter edge. I am going to repeat the process on the bottom edge and start making plans for final fitting.

I have the photo feature quasi working, it seems to only allow attachment of one picture. Regardless the photo that is attached is a faring on a PC12. I would like to know if anyone has used countersunk washers with flat head screws to attach the wingtip. I eyeballed the thickness of a washer and I wonder if it is thicker than the fiberglass. I also believe I would need to have wider attachment strips to allow adequate edge margins on the mounting strip and wingtip. The photo appears to be a No 8 screw and there is approximately 5/8 to 3/4? center to edge distance.

IMG_1433.jpg

[jrevens]

... I would like to know if anyone has used countersunk washers with flat head screws to attach the wingtip...

I did... #8 SS screws and SS washers. I epoxied the washers to the fiberglass.

[Jason Murphy]

Finished up my CFII rating and I have some time to work on the Kitfox before the Multi Engine Checkride. This weekend has been dedicated to getting back on track. A fair amount of repeating work which had been completed with the goal of better results.

Seems like I am still having trouble uploading photos. Attached is a link to my EAA builders log where photos have been posted.

Finishing the work in moving the no 9 rib. The reinforcement brackets are back on and new trailing edge has been fitted and bonded into place.

The wingtip has been fitted. This accounts for the vast majority of time spent. Using clamps the wing tip was mounted on the wing. The outboard edge of the No 10 rib was used as the standard which measurements were taken. The goal was to remove as little as possible from the wingtip to allow additional room to add a recognition light in each wingtip. I started by placing the wingtip on the wing. I measured the largest gap and then laid out a trim line. I sanded back to the trim line, mounted the wingtip and checked for a nice uniform fit to the no 10 rib. Rinse and repeat taking approximately 2mm of material at a time until the inboard edge of the wingtip was nice and uniform. I ended up removing about 7mm of material. I back drilled the wingtip and secured with a few clecos.

I then proceeded to ensure the wingtip was flush with the top of the wing. To do this I laid a 8 ft level across the top camber of the wing. Again using the outboard edge as my measurement standard. I removed 2 mm from the top of the wing which raised the outboard edge of the wingtip. I ended up about 1/8? to 1/16? high on the outboard edge. I am going to keep this to allow for any sag or settling which may occur.

The remaining holes were back drilled in their final position and cleco in place. The trailing edge was cut slightly proud of the trailing edge. It was sanded smooth. I am going to refer to the EAA composite books on the best way to close this out. Also the outboard edge is approximately 1/4? thick and the inboard edge is about 1/8? thick. I would like to make the length a uniform thickness.

Superfil applied to transition false ribs. I also applied superfil to the outboard segment of the leading edge to create a nice smooth transition between the wingtip and the wing.

Regarding edge margin for the nut plates. I consulted with Ted Myers an A&P in Fulton MO who offers builder assist services and has built and finished several kitfox. The question was do you need to provide the proper edge margin for the screw hole as well as the 3/32 rivets or simply need edge margin for the rivets because the nut plate will be handling the stresses of flight. He advised you should allow for the largest hole which would be the No 8 screw.

Allowing edge margin in the aluminum would require that you get very close to the edge of the fiberglass so it's quite a catch 22. The question was also posed to the factory and I was advised there really isn't enough material to get an ideal edge margin and you need to find a compromise. They have used flat head No 6 screws on their STI model and had no trouble.

I have decided to allow 0.200? of edge margin to accommodate the 3/32 rivets. What I may end up doing is drill the mounting strip for a No 8 screw then go ahead and open up the weak area with a file and create a series of tabs. I also created a nut plate jig using a nut plate and no 40 drill. I then marked the modified nut plate with nail polish. This allowed me to locate and drill my holes for mounting the nut plate.

[Jason Murphy]

I did... #8 SS screws and SS washers. I epoxied the washers to the fiberglass.

Thanks John, those look really good. I like the concept of using a countersunk washer to better distribute the stress and forces applied to the hardware and wingtip. Although a pan head screw smooshes the wingtip and holds it in place in my mind the setup you went with is a little more kind to everything involved. The only drawback I could see is it uses a little more hardware. Good thing Wicks Aircraft is not terribly far away and small orders are pretty cheap to ship from spruce.