Birth of a “Lion Cub!”
Well, it’s been a little while since my tease at kit bashing a Freewing Mirage into an IAI Kfir (“Kfir” is Hebrew for “Lion Cub”). We started with an assembly & flight review for the Freewing Mirage 2000 which out of the box flies awesome. However, the Kfir is such an awesome looking airplane and with canards and a little extra wing area we’ll add in the bashing process, I can only imagine that the airplane will fly even better! So, in this article, we’re covering the transformation process of turning this airplane into a Kfir and we’re using 3D printed parts as a part of that as well as employing some traditional building methods. Through this whole process we will be employing the foam refinishing method I covered in our How to Refinish a Foam Warbird series. I don’t plan to get into much detail about the foam prep work itself in this series as I want to focus on the kit bashing aspect to compliment the refinishing we did previously and use the next couple articles to go into more detail on painting, simple panel lines and weathering.
Now, one of the reasons that it’s been a little while is, in addition to of course a few distractions, is that I’ve been working out the 3D printed parts with a friend of mine. CAD modeling takes time and there were a number of parts that we ended up making. These include printing a new nose, the exhaust shroud and turkey feathers, the dorsal inlet, external wing tanks, lower ventral tank, and the afterburner cooling scoops and inlets on the fuselage. As a whole, we printed a total of 23 individual pieces for the conversion (many of the parts required multiple pieces to be printed).
THE KIT BASHING PROCESS
Before getting too into this, it’s important to talk a little bit about the transformation process. To start, the airplane had to be stripped down. This was done exactly the same as was done on our Lady Alice P-51 re-finish. While we were at it, I chose to fill in all the panel lines (they’re oversized and represent a Mirage 2000 anyhow) again like what we did with “Lady Alice” (the filler simply HobbyLite Balsa Filler). In terms of materials I’m simply using pink insulation foam (though, maybe it’s purple?) you can get at Home Depot along with balsa wood to craft the parts we’re not 3d printing. Depron is a good option too if you have it. Also, it really helps to have a decent drawing of the airplane you’re trying to model. I always start my projects with a good drawing and will usually search around in books and online to find one that I’m happy with. I will also look for a good plastic model too. This way, we have a tangible visualization of what the end result should look like. We’re turning one thing into something else so it’ll never be exact, but we can get it to look close. In this case, I made an overlay in photoshop of the Kfir and Mirage 2000 just to see where the primary variances were. Obviously, they’re different airplanes and we’ll have to make some concessions for scale, but we can get this Mirage 2000 to look like a very convincing representation of the Kfir without too much extra effort because the configurations are close to begin with. Having the drawing is helpful also in that it can be used to directly craft parts from. I like to enlarge the components needing to be crafted to the model scale so that it takes away most of the guess work on the planform shapes. This makes it a simple process of using the enlarged drawing to cut out the necessary parts and then shape them.
As mentioned, we used 3D printing for a number of parts, but I want to point out that most of the items printed can also be crafted out of foam or any other medium. To give you an idea, for the nose, it’s a matter of blocking out the basic side and top nose profiles and then sanding the nose to shape with a sanding block. For the aft exhaust shroud, that can be cut out of the existing fuselage by cutting the profile out and then clearing away a bit of foam to get the appropriate thicknesses at the back end, etc. The beauty of a kit bash like this is that we can make it whatever we want in whatever fashion! So I encourage you to experiment and have fun and don’t be afraid to fail because that’s how we learn and improve at this. The truth of the matter is too, when it comes to building, there are very few things that can’t be fixed. To give you an idea, here’s a similar conversion done by my friend where he carved the components from foam.
WINGS, CANARDS, AND DORSAL…OH MY!
That said, with the model stripped down and the panel lines all filled in, it’s time to get cracking on the conversion. In order to properly make this a Kfir, we’ve got to add all of the right features which are really what set the airplane apart from just your base mirage design. To explain it technically, we have to add pointy things and extra stuff…
Modifying the Wings
The wings were tackled first. In looking at the Kfir compared to the Mirage 2000 (and even the Mirage III), the kfir has a very distinct leading edge sawtooth at about the 2/3 span location. So to tackle that, it was a matter of blocking out that sawtooth shape with some excess so it can be embedded it into the kit wing from our Home Depot foam (using a blown up drawing of the Kfir wing as a guide) and gluing it into the kit wing. An appropriate portion of the kit wing leading edge was removed (I cut back to the first panel line on the top of the wing) and the new Kfir LE foam block was glued in its place with some 5 min epoxy. From there, a sanding block was used to sand the sawtooth LE contour to match the kit wing contour. IT’s important to use a sanding block and sand the LE as a whole so that we get a nice straight and smooth transition. Trying to hand sand will result in a wavy surface.
Also looking at the Kfir planform to the Mirage, the elevons on the Kfir have a slightly larger chord. So, to simulate this, I glued a 3/4″ wide by 1/4″ thick balsa wood strip to the trailing edge and sanded it to shape. The nice benefit of this is that it not only increases the wing area (not a bad thing for this model), but it also allows us to sand the TE down to a thinner point vs. the 1/4″ thick TE you get out of the box. One thing to note is that the full size airplane uses conical camber for vortex enhanced lift and so actually has a fairly drooped leading edge at the tip that results in a down turned trailing edge. So, while sanding the TE, I did a little treatment with filler and the balsa trailing edge to get the rounded trailing edge at the tip. I didn’t fully characterize the drooped LE based on the stock wing shape, but I wasn’t too concerned about that.
Building the Canards
The literal translation of the word “canard” is “duck” which originates in French. How it became associated with a small flying surface forward of the main wing on an airplane, I’ll never know, but the Kfir has “ducks” and we need to build them! The canard design on the Kfir is very distinctive and is one of the reasons that I like the airplane so much. It is a fixed (non-moving) lifting canard which helps the turning capability of the airplane. In fact, they realized a pretty significant turn radius improvement compared to similar airframes without them.
To build canards, I went back to my roots and made them from balsa wood. They’re each made from two laminations of 1/16″ balsa cut to the correct planform (using again a blown up printout from our drawing). I allowed excess at the root to allow for what would be required when gluing the canards into the fuselage, no more than 1/4″ or so. Between the laminations were glued some 1/64″ plywood ribbing. The 1/64″ ply helps reinforce the structure to make it more resilient to bumping…because I guarantee, these canards will get bumped! Also, with the ply in there, you can get a really strong and sharp trailing edge. So, once laminated, the whole thing was sanded with a symmetric airfoil cross section root to tip. For the airfoil, it’s purely arbitrary in shape to blend the laminations and get a nice transition to the trailing edge. Truthfully, the cross section isn’t that critical. The important part is to sand them symmetrically left to right and avoid any strange camber. The surfaces are quite thin, but that is as intended and with the ply in there makes it a non-issue.
Making the Dorsal
For the dorsal, I went back and forth about whether to build it up from balsa, or just carve it from foam and I ultimately decided to just go with the latter. For a one off part it was the path of least resistance for me. Also, by making it from foam, it allowed me to easily custom fit the base to the fuselage and the forward area for the 3D printed dorsal inlet as I went vs having to figure it out up front. To carve the dorsal from foam, I first off blocked out the planform (gotta have that drawing!) from the foam sheet and then sanded it down into a symmetric airfoil root and tip. It’s actually not a difficult process, but it helps to have a nice coarse file for the majority of the heavy lifting and then a good quality sanding block for the final shaping. Oh, and expect to make LOTS of foam dust! In the process, it’s important to mark the center chord line at the root and tip so you have a good reference for the symmetry. Also, be sure to sand the surface with a sanding block as a whole so as to ensure you have even spanwise thickness. If you focus on the root or tip too much, you’ll end up having nice contours root and tip, but the whole center section will have a bulge to it when you look at it from the front. Placing a straight edge over the surface spanwise will give you an idea if this is an issue. To finish out the dorsal, it needs some structural reinforcement. To do that, I replaced the trailing edge with a 3/4″ wide strip of balsa wood (similar to what we did on the wing). From there I added a vertical grain shear web through the center using 3/16″ thick balsa. I then tied that to some longitudinal 1/8″ thick balsa cap spars that were embedded into the foam and glued to the back of the shear web. This was necessary for the bending and torsional strength of the whole surface.
Lastly, it was necessary to work out the trailing edge wing root fairings. The Mirage 2000 has an extended fairing which the Kfir doesn’t have, plus we’ve added a nice and sharp trailing edge, so we definitely needed an update. To make it, I simply cut the kit fairing back to where the wood trailing edge mates to the fuselage and glued a piece of foam back that was in the appropriate planform. From there, it was sanded in cross-section to create the transition and then glued into the slot in the fuselage. It was then finished off with some filler so that everything blended well and looked right.
PUTTING IT ALL TOGETHER
With all of the components built, it’s time to put it all together to finish the transformation. For the large parts (dorsal and canards), epoxy is the glue of choice. When time is needed to set the parts, then 30 minute epoxy is a necessity. The last thing you want is to have the epoxy start to set before the parts are even ready!
3D Printed Nose, Exhaust Shroud and Other Details
To transform the fuselage into that of a Kfir, the first things done were to glue the 3D printed nose and exhaust shroud on. The nose was a super easy installation since it was designed to simply replace the stock nose. So, it was glued on and then filled around to flare the contour (I used my trusty balsa filler) so we get a nice transition. Also, to finish out the chin fairing on the underside, I added a small foam block to the back of the lower chin and sanded it to shape to complete out the nose side profile.
For the exhaust shroud and turkey feathers, it was simply a matter of cutting the back part of the fuselage off at the last primary panel line and then gluing the 3D printed shroud on in its place (again, this was the design intent). The fuselage as a whole is shorter on the Kfir, so this accomplishes that. Also, when gluing, it was important to ensure it was glued on straight with the drag chute at top dead center.
From here, to avoid hangar rash during handling, my preference was to finish out everything on the underside first before gluing the dorsal on (I considered that the near last step). So, first of all, the 3D printed 3-piece ventral tank was glued on with thin CA. One nice thing (maybe the only nice thing 😉 ) with EPO is that regular CA’s don’t melt it. Also, in the process, I finished all of the additional filling needed on the underside (you’ll notice that I filled in the wing/fuselage intersection as it was a large gap and I have no intention of removing the wings).
From there, I worked out the 3D printed wing tank installation. I pulled the magnets from the tanks and pylons that come in the kit and then used them for the 3d printed Kfir tanks. It was all done with regular CA, nothing special really. The key was to just ensure that the magnets when both are together mate up flat and square to the top of the pylon so it sits right in the wing slot. So, when glueing them in, I glued the magnets in the tank pylons first, but with the two mating magnets together. This allowed me to set the magnets in the pylon as necessary so that the mated magnets were flat across the pylon.
With the underside work done, it was finally time to glue the dorsal. To do that, it was first necessary to fill in all of the excess space where the stock dorsal sat. That was done by tracing the base of the stock dorsal onto our Home Depot foam and then cutting out the center for the new Kfir dorsal. The contact surface is the base within the fuselage, so the additional foam around the dorsal helps give it a little more structural rigidity. Note too that I removed a portion of the dorsal trailing edge at the base for the drag chute fairing and also squared off the nose to mate up with the 3D printed dorsal inlet. It was important to ensure that the dorsal inlet and dorsal surfaces mated up well before physically gluing the dorsal on so lots of dry fitting and sanding was done until it all fit right. So, with the wings on the airplane and the airplane leveled on the bench, I used 30 minute epoxy to glue the dorsal in place. A triangle was used to align the dorsal trailing edge to make sure it was perfectly vertical.
From there, the drag chute fairing was completed using some spare foam and filler. I roughed out the contours in foam separate from the airplane with a sanding block and then did the final shape work with the parts glued on the airplane. There was a bit of filler needed to get a nice clean transition but it came out nice and looks about right. It took a couple passes of sanding and filling to get the final shape I was looking for.
For the canards, the position was determined simply by measuring 1/4″ up from the slot for the Mirage 2000 strakes and marking a line parallel to that slot. A Dremel with a 1/8″ cutter bit was used to cut a slot into the foam and then the canards were glued into the slot with about a 1/4″ material recessed into the foam. Again, this was all done with the airplane leveled on the bench so as to ensure everything was square. Also, since I was using 5 minute epoxy, I glued one canard at a time. There wouldn’t be enough time to do both at once. Once in, filler was used to clean it all up.
To finish out our Kfir transformation, we needed to ditch the Mirage 2000 dual nose tires. The Kfir has a single nose tire, so I picked up from MotionRC the nose strut that comes from the 80mm Super Scorpion and installed the lower fork to the stock Mirage 2000 strut. Also, the Kfir has a 3-piece nose door setup and so I decided to make that since I had the geometry already sorted from my big Mirage IIIRS. To do this, I first cut the stock nose door in half with an Xacto knife. You don’t have to cut through but rather score across the door a couple times and then you can bend it to break it free cleanly. Once cut, I used 0.032 diameter wire to bend a hinge that’s bent to offset the door (see the graphic to the left for the pattern or click here for full size pdf). The ends of the wire insert into some brass tubing that is glued into the nose gear well which allows the wire frame to pivot around. The key is the placement of the brass tube pivots as this sets the alignment and pivot point for the door. To glue the pivot tubes in, the door was taped in the closed position and balsa wood standoffs were used for the tubes to be glued to set the pivot height within the gear well. Also, on the front door a 1/64″ ply ramp was added which extends just past the bottom of the front door. This way when the doors open or close, the rear door slides under the front door on that ramp. Also, that little ply extension acts to pull and hold the rear door closed with the gear retracted. Lastly I used some balsa wood which were glued to the doors to create offsets so that the doors look correct in the down position.
To finish it all off I made a quick pitot tube out of concentric brass tubing and 1/16″ diameter wire. I soldered a small wood screw in one end which serves as the attachment into the 3D Printed nose chin. It’s a 1/8″ diameter tube with smaller sizes used to create the general shape. Bevels were sounded around the edges as needed to get the correct look and shape. A “looks about right” approach was taken, so nothing too crazy.
GIVE IT A TRY!
I know it was a lot to cover, but we now have a fully assembled Kfir! In terms of time, it really wasn’t that much, maybe 20 hours at the most for the physical conversion work (the article and video took longer to put together than the actual build work). Next time we’ll cover some quick and easy cockpit detailing including painting and installing a new, more detailed pilot, which really help improve the looks while also hide the unsightly texture that is so common in an EPO airplane cockpit. See you next time!