The Barn Find Build

[Eric Page]

Well, I tried tweaking my door frames this afternoon and much to my surprise, it worked! It took what the youngsters would call "getting medieval," but I was able to get a nice fit on both sides without breaking anything. Thanks again for the suggestions.

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Since I've been jumping around in the manual so much, I thought it would be a good idea to go back through the fuselage and wing chapters page by page to be sure I hadn't missed anything important. It turned out I had: the large cotter pins that serve as cable keepers under the four small pulleys just behind the seats. They were a bit of a chore to install, but getting them in made the review worthwhile.

I also found some things in the wing chapter that have me a bit worried about the rigging done by the first builder. I have no actual evidence that anything was done wrong, but it's keeping me up at night. I have a string of days off in mid-April, so I'm going to make a point of putting the plane back on some wheels, rolling it to a neighbor's hangar, re-fitting the wings and checking the measurements.

I've been getting frustrated lately with not being able to find things when I need them, so I spent the rest of the afternoon emptying all of the boxes I got from the first builder, deciding what was salvageable, what I would be selling or giving away, and what was trash. I whittled eight boxes down to two and regained access to an exterior door that had been blocked since the plane came home last July.

I have a couple more days of training for work coming up, but I should be back on the project on Saturday.

[Eric Page]

...And, two weeks later, here we are with very little done on the plane. I finally found some time this afternoon to get back to the flaperons. There was a little bit of Super-Fil to sand out, then I installed and squeezed all of the trailing edge rivets. It was definitely worth it to buy a rivet squeezer for this job, and I'm very happy with the appearance of the solid rivets vs pop rivets.

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Back to work tomorrow for four days, then a nice stretch of time off. Hope to make some good progress again...

[Eric Page]

Two more weeks and not much to show for it. I'm beginning to feel like I'm letting down the team here!

I did take the plane across the street to a neighbor's hangar a few days ago so that I could install the wings and check their rigging. That turned out to be a bit of a mixed bag.

I started by installing the gear, wheeling the fuselage over, then removing the gear and setting the plane up on jack stands. I had to shim the right side a bit, but fairly quickly got it level left-to-right.

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I found a scissor lift cart and tried using that to hold up the tail, but it very slowly leaked down, so within about 15 minutes the tail would be down about half a degree. I then commandeered a card table and some scrap wood and used them to level the fuselage fore-and-aft.

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Then a couple of neighbors helped me get the wings and struts installed. It turned out the first builder had primed the ends of the spar tubes after drilling the holes, so they needed reaming again before the bolts and pins would go through. My first fear was that the holes wouldn't be aligned vertically, but it took only a few gentle taps with a tack hammer to get the fasteners in. With that done, I set up the string line across the leading edge.

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This immediately revealed a problem: the wings are noticeably swept back. If you'll forgive my use of fuzzy jute twine, here are the left side measurements...

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...and here are the right side measurements:

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Please check my math: I used 5/16" (0.3125) for the left inboard measurement and 3/8" (0.375) for the right. Subtracting those from the 1" outboard measurement gives the height of the triangles (0.6875 and 0.625, respectively). Plugging those numbers into an online triangle solver (gimme a break; geometry class was a long time ago!) along with the 120" dimension along the spar and a 90 degree corner gives 0.328 degree sweep on the left and 0.298 degree sweep on the right.

Questions:

1. Are my wings scrap?

2. If not, then making this work will require moving some weight (e.g. avionics LRUs) aft, correct?

3. Looking at this excerpt from the Final Assembly chapter of the build manual...

CG Limits.jpg

...can I extrapolate that my aft CG limit would be farther aft than the zero-sweep limit? For example: 16" (zero sweep limit) - 14.75" (1 degree forward sweep limit) = 1.25" of aft limit change per degree of sweep. 1.25" x 0.3 degree sweep = 0.375". 16" + 0.375" = 16.375" adjusted aft limit.

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With my blood pressure well and truly spiked, I moved on to measuring dihedral and washout. Dihedral was within spec and equal on both sides, but washout was a mess. Fortunately, a bit of adjustment on the aft strut bearings put that right.

With dihedral and washout set, I locked down the strut bearing jam nuts and started test fitting the jury struts. I discovered that I'm missing one of the four "rings" that hold the jury struts to the wing strut tubes, so I could only check one side at a time.

The first builder had wrapped the wing strut tubes with masking tape at the ring locations and left it in place for 20 years, so it had turned into superglued paper. Removing that from one wing strut took over an hour, so I packed it in for today and came home to seek advice from the Kitfox hive mind.

[CoClimber]

I'm going to take a stab at this. The total CG range is 6.5 inches. Your mean sweep is ~.35" which is ~5% of your range. An aft CG is dangerous so if you use the factory numbers, you automatically build in an additional safety margin. Aerodynamically, that sweep back shouldn't be detectable in flight. The slightest side slip would be far greater than that angle.

Basically, I think you are good to go.

[efwd]

Hi Eric. When setting up for the sweep, the blocks of wood or whatever are adhered to the spar itself. The leading edge plastic is not installed. I would recommend that you factor into the math that added material. In spite of the fact that Jigs are utilized to establish the correct placement of the leading edge, there is a fair amount of opportunity to get that placement a little off. Be certain you are measuring those spots from the locations that the manual suggests.

[jiott]

Agree with Eddie, make sure the plastic leading edge sticks out beyond the spar the same amount inboard and outboard; if not adjust your measurements.

Assuming your approx. 1/3 degree sweep-back is correct, I think you are fine. It will help compensate for a heavy tail. Most of the SS7's tend to be somewhat tail heavy. On mine, with full load of fuel, baggage and passenger, I always reach rear CG limit before GVW. I have never reached fwd cg limit.

[alexM]

With that 0.3 degree aft sweep I think you're going to want to go with turbine power (kidding).

I will counter that an Aft CG condition is NOT dangerous, it is in fact desirable. It's being beyond Aft CG which is dangerous. At the aft limit you can expect slower stall, slower landing with more elevator authority (already an issue with the Kitfox), lighter control feel. My old Cardinal RG with its stabilator loved a good load in the back. At the aft limit it cruised 10 knots faster than at the forward limit.

I wouldn't spin a plane at the aft limit as a habit, but I have no fear of being loaded towards the rear.

And damn cool seeing your plane with the wings on. Looks familiar.

Oh, and I have round tube jury strut brackets which are surplus to my needs. I recently found that I own both round tube and aero jury struts.

[Eric Page]

...if you use the factory numbers, you automatically build in an additional safety margin. Aerodynamically, that sweep back shouldn't be detectable in flight. The slightest side slip would be far greater than that angle. Basically, I think you are good to go.

I like safety margin, and 3/8" additional range isn't useful anyway, so I'll do that. I had a feeling that 0.3 deg wouldn't much affect the flight characteristics of such a benign airplane.

Hi Eric. When setting up for the sweep, the blocks of wood or whatever are adhered to the spar itself. The leading edge plastic is not installed. I would recommend that you factor into the math that added material. In spite of the fact that Jigs are utilized to establish the correct placement of the leading edge, there is a fair amount of opportunity to get that placement a little off. Be certain you are measuring those spots from the locations that the manual suggests.

I noticed that, but the PVC leading edge is already Hysol'ed in place so I was stuck with it. I did verify that it's the same thickness from the spar tube at the root and at the 120" point, so it didn't make any difference to the measurements.

I've actually got three build manuals (Series 5 in PDF form, early Series 7 in printed form and current Series 7 downloaded from Kitfox) and I read all three to be sure I was doing the steps the right way. There's actually one item that's confused between manuals. In the section on checking dihedral, both the Series 5 and early Series 7 manuals say to use the front spar in the text, but the rear spar in the diagram. The current Series 7 manual says to use the front spar in both places, so that's what I did.

Agree with Eddie, make sure the plastic leading edge sticks out beyond the spar the same amount inboard and outboard; if not adjust your measurements.

Assuming your approx. 1/3 degree sweep-back is correct, I think you are fine. It will help compensate for a heavy tail. Most of the SS7's tend to be somewhat tail heavy. On mine, with full load of fuel, baggage and passenger, I always reach rear CG limit before GVW. I have never reached fwd cg limit.

Mine's a 5, but I'm using a 7 elevator, two servos behind the seat and a comm antenna in the vertical stab, so it'll be a little heavier aft than it otherwise would be. I'm inclined to press ahead with avionics forward, and if I have forward CG limit problems I can always move a couple of boxes aft later. I also have a larger spare tailwheel assembly that I can swap in for additional far-aft weight.

With that 0.3 degree aft sweep I think you're going to want to go with turbine power (kidding).

I wanted that anyway! Hard to believe no one has put a small t-prop on a Kitfox yet. Solar Turbines should sponsor one of the YouTube guys. I'd watch that video.

I will counter that an Aft CG condition is NOT dangerous, it is in fact desirable. It's being beyond Aft CG which is dangerous. At the aft limit you can expect slower stall, slower landing with more elevator authority (already an issue with the Kitfox), lighter control feel.

All true! I'll just introduce my plane at airshows as "The Swept-Wing Kitfox." It'll never win a Lindy, so a little fighter-like notoriety will have to do.

Oh, and I have round tube jury strut brackets which are surplus to my needs. I recently found that I own both round tube and aero jury struts.

Excellent; SOLD! Speaking of which, have you had a minute to sort through that box of goodies in your hangar?

[alexM]

I've actually got three build manuals (Series 5 in PDF form, early Series 7 in printed form and current Series 7 downloaded from Kitfox) and I read all three to be sure I was doing the steps the right way. There's actually one item that's confused between manuals. In the section on checking dihedral, both the Series 5 and early Series 7 manuals say to use the front spar in the text, but the rear spar in the diagram. The current Series 7 manual says to use the front spar in both places, so that's what I did.

[and]

Excellent; SOLD! Speaking of which, have you had a minute to sort through that box of goodies in your hangar?

No but I've tripped over it enough times that I need to get that done. I'll try to get that done tonight.

I got myself confused with the series 5 and 7 manuals on this step too, which is probably why John discourages people from referencing both manuals. The text of my series 5 manual matches the drawing, and has you use the aft spar for setting dihedral and then moving the forward lift strut attach fitting to check/set washout. My confusion was because I read the words but projected my imagination into what they meant. Once I got real careful I saw what was going on.

Series 7 has you set dihedral using front spar, setting washout with the lift strut fitting on the aft spar - which makes WAY more sense to this model airplane guy. After I worked it out I came to the conclusion that it would result in the exact same setup from two different directions. Upon further reflection I realized that's not true. The series 7 will end up with 0.2 degrees more dihedral.

Reason? With both configurations you get your 1 degree dihedral. So far, so good. Then with the series 5 you remove 0.5" dihedral on the other spar, while on the 7 you add 0.5" dihedral to the other spar.

You say potato, I say potato. Hey that doesn't work when you type it out.

And believe me, I've considered a Solar T-62 in something. I would like to design a scaled down OV-10 Bronco and use a pair of them.

[Eric Page]

No but I've tripped over it enough times that I need to get that done. I'll try to get that done tonight.

Why do you think I put the box right in front of your fuselage?! Just kidding -- thank you.

Series 7 has you set dihedral using front spar, setting washout with the lift strut fitting on the aft spar - which makes WAY more sense to this model airplane guy. After I worked it out I came to the conclusion that it would result in the exact same setup from two different directions. Upon further reflection I realized that's not true. The series 7 will end up with 0.2 degrees more dihedral.

Reason? With both configurations you get your 1 degree dihedral. So far, so good. Then with the series 5 you remove 0.5" dihedral on the other spar, while on the 7 you add 0.5" dihedral to the other spar.

I'm afraid I didn't think it through that far, but that makes sense. I used the procedure in the current Series 7 manual, figuring that since the other two contradicted themselves and that one didn't, it must have been corrected.

And believe me, I've considered a Solar T-62 in something. I would like to design a scaled down OV-10 Bronco and use a pair of them.

Oh boy, that would be a really cool airplane! You're going to need tip tanks, drop tanks and a belly tank to run two T-62s.

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I disassembled the plane last evening (again, with the help of two neighbors) and brought it back to my garage. Now I'm off to work for four days...