Cindy the DragonFox (Highly Modified Kitfox model 1)

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Will Aldridge

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A few years ago I got caught up in the Trent Palmer/Kitfox craze and just had to have one. About a year and a half ago the stars aligned and I was able to buy one.81520-c613f3a95498f19513eef774f1521eb3.jpg I went in unprepared and although the seller assured me it was airworthy it wasn't. Don't get me wrong it was a solid airframe but all told it was worth $5k less than I paid for it. The rotax 532 needed a full overhaul, it also needed a new prop and the wing fuel tank was unusable.

Why "Cindy the DragonFox"? The first thing I learned about the early model Kitfoxes was that the really narrow bungee gear made the plane a handful on the ground and having already been bit by a tail dragger wanted as much help as I could get. Also the idea of breaking both legs if I dropped it in too hard was unappealing. So I bought a set of cub style cabane gear from Stace Schrader of Rocky Mountain Wings. They are 7 ft wide, a full 2 ft wider than stock and taller a well.
Next I didn't like the Kitfox round cowl so decided to build a new one. 20181028_220924.jpg
About this point I was thinking of a name for it and I thought that it was an older long legged fox with a nose job. That sounds like a super model. Cindy Crawford was the first Super model I can remember being aware of so it became Cindy.

Dragonfox is an homage to Mike Patey and Draco.
I'm hanging a BMW R1200 engine on the front so it's going to have a lot more hp, the wings and tail surfaces are all being modified, some carbon fiber for the cowling and wingtips will be used, and I'm currently working on a fairly radical new suspension system. There's a lot more going into it which I will detail later.
The reason for waiting so long to post this was that I was determined to do what I wanted to do and didn't feel like listening to differing opinions on it. I'm far enough along now that it should be obvious I'm committed. Little details like drag reduction I'm willing to solicit opinions on but the big ones are non negotiable. So far Ive got the new motor mount fabricated and the new Bush gear mounted. 20191001_103342.jpg20191001_103330.jpg
 

Will Aldridge

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One of the hazards of not being forthcoming about what you're attempting is that everyone has to guess and usually they guess wrong re the enlarging the tail surfaces. That's one of the reasons for starting this thread.

Re Stace Schrader I've heard he's been unreachable lately so that's how I took VB's comment.

Re structure for the bigger more powerful engine. 2 additional mounting points were added to the firewall that fortunately have direct load paths to primary structure. My other area of concern are the tail surfaces. I'm going to add 8 inches to the height and 4 inches of cord to the rudder, also 4 inches to the elevator chord as well. The vertical tail itself is fairly floppy. I'm going to be taking a leaf out of Mike Pateys book and skinning the horizontal and vertical stab in cf and bonding them both together so they support each other. Reason being the kitfox doesn't have any bracing above the horizontal stab to allow the wings to fold. Since I need the wings to fold to suit my mission, adding that bracing is impractical. Additionally I'm going to need at last 15 lbs of ballast in the tail, I'd rather any weight I have to add was doing something useful. So full airfoil tail surfaces, and additionally a just highlander style tail suspension that has 6" of travel vs the 3" the original tail leaf spring had to absorb a harder impact from more weight in the tail.

Note to the Mods: I started this in the member projects forum thinking only I would be able to post. Not sure if that's a perk for paying member's or not. If I simply didn't select the option could we split the discussion off into its own thread? I'm happy to answer questions.
 

Will Aldridge

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BMW R1200 DETAILS.
As we have heard perhaps thousands of times on this forum, auto conversions are fraught with peril. The 2 biggest issues seem to be the PSRU and if it's a more modern engine, the associated wiring harness and computer. Obviously a custom motor mount has to be built and every last fwf detail has to be custom designed. In addition other things are affected like weight and balance.

I chose the BMW because I wanted something different and I wanted a 100hp 4 stroke. Different excluded the rotax 912. It also had to be something I was confident of successfully converting, meaning there had to be a lot of them already flying. The R1200 is somewhat popular in Europe and there are at least 2 readily available options for the PSRU.

I chose the SPG-4 from Airtrikes in Canada for 2 reasons, it's a little cheaper than the TakeOff version and reportedly a few lbs lighter. It does require a centrifugal clutch. I have encountered 2 small issues thus far. There are a couple bumps on the engine that interfere with the bell housing fitting tight against the mating surface and the other issue is the complete lack of any documentation on how to mount it. The only direction I got from the owner of Airtrikes was to do what seems logical. It is fairly straight forward if you look at it but you have to grind off 3 rivets that attach the clutch plate to the flywheel in order to bolt the new centrifugal clutch to the crank shaft. I figured it out but would have like some printed instructions.

The first doubt I had about installing the R1200 came when I received the wiring harness. It's HUGE!!20191026_133808.jpg
When I asked before purchase if there was an option to buy a shorter one the owner of Takeoff told me it's only a couple kg extra. That didn't prepare me for just how big the dumb thing is and absolutely indispensable for this project as Takeoff is the only vendor I know of that produces a harness for the R1200. An extra 4-5 lbs of wire on pretty massive for such a light plane. Ive been very conscious of every last bit of weight and this hurt.
 

Will Aldridge

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20200315_184609.jpg
SPG-4 gearbox with oil level window below prop flange
20200315_184502.jpg
Close up of engine, the throttle bodies are mounted upside down compared to the motorcycle mounting. Among other things it makes throttle cable routing easier. 20200315_184513.jpg
Ground away a little of the bell housing. Thought about cutting that nub off, but decided I might want to use it to mount something.
 

Will Aldridge

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I've been busy on this project, but mostly design work. I haven't made any parts in the real world for awhile. I'm completely revamping the control system and here are the first new parts20200523_124321.jpg
This is the torque tube and flap handle. I'm going to mill some slots in the handle and have a spring loaded push button that will engage the detents.
 

Will Aldridge

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I'm having a Mike Patey moment right now:
(I'm so excited) I feel almost like a little kid that's so excited that he can't sit still and is almost in danger of peeing his pants.

I'm no Mike Patey but I'm also doing a somewhat extreme new design suspension system for my plane and I've been working on parts for the last 6 months and the major assembly just came together for the first time today. Obviously Mike Patey showed me up before I ever got out of the gate, but hey we can't all be him.

The genesis of this comes from the fact that just about every high time pilot of the early foxes over on AvidFoxflyers.com seems to have had a hard enough landing to require significant repairs to the fuselage. There are a couple well known weaknesses in the early Fox/Avid fuselage. The first is the seat truss. If you have the original style bungee gear where the bungees wrap around the seat truss and you drop it in really hard (easy to do because the airfoil doesn't give any warning of impending stall it just lets go) the bungees squeeze the truss and bend it (and has been known to break pilot and passengers legs). If you have retrofitted a cub style cabane gear a hard landing can pull the lower longerons together. And the sides of the fuselage are weak as well and the longerons can be bent upward as well. The fix for pulling the longerons together is to have a tube across the top of the cabane. The fix for the weak fuselage sides is to add gussets in the framepost-36-13595151980036.jpg

I decided that I wanted something more for my plane. Of course monster shocks and Acme Aero get a lot advertising from the flying cowboys, and the idea of having a suspension that won't bounce me back into the air is appealing but I wanted to do something that no one had seen before.

Last summer my truck broke down and the shop I took it to also builds custom rock crawlers from scratch. Obviously they know a thing or 2 about suspension. So I showed them my plane and asked them what they would do if it were theirs. I had a cad model of the lower fuselage and hear legs, so I gave them that and they created a little animation, nothing fancy just the basic geometry and I fleshed it out from there.

So here is the basics:
20191122_203553.jpg

The green tubing is the seat truss (under the pilots knees) the purple tubes are the new cabane. As you can see it ties into the seat truss, making it twice as deep and therefore a lot stronger.

20191122_203833.jpg

This image shows the air shocks in place. One is forward and one aft of the cabane.20191122_210757.jpg

And here is the full setup (one side only for clarity). As the gear leg deflects upward it pulls on the bottom of the blue tube which compresses the air shock. It should be noted that the bolt that the blue tube pivots on also holds the end of the shock on the front side of the cabane.

20191122_204431.jpg

The way I have the geometry set up I get about 14.75" inches of vertical travel at the wheels (the shock have 4" of travel). My guess is that the bungee setup would have had in the neighborhood of 8" of travel, so I've got nearly double the travel to soak up a real belly whopper of a landing. It should be noted that I'm a rookie pilot that hasn't put any time in my log book in 6 years. So the ability to take a hard hit and not need to be repaired is high on my list of needs.

And the big reveal!!!! What has me so excited!!!
20200523_173540.jpg

The main assembly is all together for the first time!!! And it all fits together amazingly well. Obviously just hardware store bolts at the moment but man this has been a long time coming and I'm excited. And of course Fox Shocks for a Kitfox is certainly apropos.

A couple more items of note:
1. This setup allows the gear to hang down 4" more in the unloaded condition than the bungee setup. With the bungees and no load the axles are parallel with the ground. You can see in the screen grab that the tires are not vertical.

2. This system changes the direction of the forces. As mentioned one of the failure modes pulled the longerons together. Now the forces are pushing the longerons apart, and structures are usually stronger in tension than compression.

3. I imagine that a hard enough landing will probably bend something, but my suspiscion is that with this system it will be more likely to be the gear legs or axle than the fuselage. I'd much rather replace things that can be unbolted than tear into the fuselage to cut out and weld in new tubes.
 

Will Aldridge

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Came across this French site that sells R1200 conversions:


And at the bottom of the page there is this walk around video. Of particular interest to me was the cooling plenums.


I tried emailing them and asked if they'd be willing to sell them to me. So far, no answer. Not sure if France is still on lockdown due to Corona Virus or for whatever reason they don't want to sell to me. In any case I figured I should start making my own.

I honestly have no idea how this will turn out but had to start somewhere. So using some plywood scraps and hot glue, this is where I end the day. 20200525_215041.jpg

I did clean my disaster of a garage today, and I can tell already that will increase my productivity.
 

Will Aldridge

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Bought a 3d printer recently for this project. My first ever print was a new housing for the turn coordinator.

I have a Lift Reserve Indicator that will go on top of the glare shield. I made this housing to hold the glass and ball of the turn coordinator, which will be glued to the bottom of the LRI. Current version doesn't quite fit the glass very well and it's a little flimsy. But I think the idea is sound. Pretty good for the very first part I ever 3d printed. 20200622_224903.jpg20200622_225017.jpg20200622_224914.jpg
 
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Will Aldridge

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I was wondering if PLA (the filament I used for the above part) would last in my plane. I accidentally left the back plate of the new housing next to the cooling fan exhaust port of my laptop:
20200624_175919.jpg
That's a definite no.
I'm going to be using carbon fiber for the top of the cabin instead of the normal polycarbonate for 2 reasons;
1. Weight, I think I can save at least 1 pound, maybe even a pound and a half which is huge.
2. I don't like heat and anything that's going add heat to the cockpit can be omitted as far a I'm concerned.

75% of the time in my logbook is in 172's and I never really missed having the skylight and never had one in any vehicle I've owned and never missed it there either.
 

Will Aldridge

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This latest 3d print is obviously something that could have been accomplished in other ways but I'm learning the ins and outs and it is the first larger item I have printed. I've tried a couple other larger prints but they failed mostly due to inadequate bed preparation and incorrect settings.

This is a template to correctly position the new pvc leading edge:20200627_112613.jpg

As you can see it's translucent because I printed it at almost the thinnest setting with minimum infill. It's quite flimsy and i damaged it simply trying to detach it from the print bed. But if I'm careful it'll do the job.

The kitfox model 1 was designed to be the absolute in simplicity of construction and used the aluminum tube spar as the leading edge, unfortunately its not very good from an aerodynamic perspective. The same airfoil was used as well for the models 2 and 3. Harry Riblett designed a new airfoil for the model 4 that used a pvc leading edge to give a proper profile20200627_113809.jpg
The red profile is the original kitfox airfoil and the black is the Riblett design. The original is obviously very highly cambered and does quite well getting the plane off the ground (or water) quickly but that comes at the expense of cruise and top end speed.

In any case multiple individuals have found that there are significant performance gains to be had by adding the pvc leading edge to the earlier airfoil. A 2-3 mph increase in top speed, a similar decrease in stall speed, a 100-200 fpm increase in climb rate have been reported. Also the original airfoil doesn't give much warning of an impending stall, it just stops flying. Incidentally the Just Aircraft Highlander uses the same(or very similar) airfoil as the early foxes and at least one that I know of has used the Kitfox pvc leading edge

So with the wings uncovered I wasn't going to pass up the opportunity for that kind of improvement in performance. 20200627_112628.jpg

20200627_112711.jpg

Not to the point of epoxying them to the wing yet but now that I've made the template I'm one step closer
 

Will Aldridge

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Got a present in the mail today:
20200629_201958.jpg20200629_202009.jpg

It's a hydrostatic pressure fuel sending unit. Basically it measures fuel level by measuring the weight of the fuel above it. My fuel system is slightly unorthodox. The aircraft came with a non useable 6 gallon aluminum tank in the wing and the 9 gallon roto molded header tank that was mounted just aft of the firewall. I got rid of that 9 gallon tank because of cg reasons heavier engine etc. The wing tank was unusable because the aluminum tanks had a habit of cracking welds due to flexing of the wings. The initial solution to that problem was to slosh the tank. The sloshing compound unfortunately didn't adhere well and that left the very real risk of clogging the finger strainers. I've cut the bottom out of the tank and started cleaning all the sloshing compound out, then I'll seal all the seams of the tank and rivet a new plate of aluminum on. I'm doing this because i don't really trust any of the fiberglass or plastic tanks not to go soft from ethanol(which I'll try to avoid but that might not always be possible).

Anyway I wanted more than 6 gallons of fuel capacity and wanted the most lightweight reliable tank I could get so I bought a 6 gallon tank from bam manufacturing.
20200629_201637.jpg
The tank is shown in roughly the position it will occupy on the aircraft.

The fuel will drain from the right wing tank into this tank(highest fitting is for venting and fitting on very end of the tank is inlet from the wing tank)20200629_201702.jpg and there will be a fuel pump at the bottom outlet.20200629_201657.jpg
This obviously doesn't allow for a traditional fuel sending unit and I've been scratching my head wondering what to do when someone on teamkitfox.com posted a link to this, problem solved. Not sure if it will read the combined pressure of the wing tank and the header tank but there is a sight gauge on the wing tank. Fortunately it is very lightweight:20200629_201811.jpg
And hopefully it can tolerate a few g's.

It's supposed to work with just about any electrical fuel gauge.
 

Will Aldridge

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Reading Sport Aviation can have unintended consequences.

This article:20200704_194548.jpg
In the June edition led to a complete revision of my plans for the tail surfaces of my plane.
20200704_194555.jpg
That stabilator got me thinking about how I would go about putting something similar on my kitfox. First thought was the Evans Volksplane VP-1, but it's tail surfaces didn't meet all the specs, so I looked at the VP-2 and decided that they did meet my needs. Then I decided I needed a sanity check, so I asked the resident volksplane expert Fritz what he thought of putting vp-2 tailfeathers on a Kitfox and he said he thought it was a great idea. Not only did he think it was a great idea he had both the vertical and horizontal tail feathers and told me i could have them if I came and got them. The holiday weekend was the perfect time and was even better because Fritz was going to visit family in Albuquerque which knocked about 7 hours off the round trip. 20200704_190524.jpg20200704_191513.jpg
Just got back with my prizes. Considering material and shipping costs and time to build I came out ahead by going down to get them.

It's going to be a little interesting getting it all to work but I've got it about 75% designed.
 

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