Spitfire Sloper

Check out Russ Thompson's beautiful, scratch-built, fiberglass Spitfire slope plane. Russ created this plane for the 5th annual PSS Festival at the Cajon Pass, and it took 4th place in the "Prop" class.

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Introduction

I did not have any slopers in flying order when I heard that the 5th Annual PSS Festival would be taking place up on the Cajon Pass over Memorial Day weekend. I had been flying powered planes and had not built any new slopers since I balled up the last one I had a couple of years before. I really wanted to enter one of my Spitfires in the Festival, but it was only four weeks away. I knew that if I were going to have a plane ready, I would have to do something extreme. I convinced my boss to let me have the following week off in order to build one. In this article, I will cover the building of that plane from start to finish.
I have divided the construction into the following sections.
  • Building the Fuselage
  • Cutting the Wing Cores
  • Building the Wing
  • Building the Horizontal Stabilizer
  • Putting it all Together
  • Painting

Construction

Building the Fuselage
First thing to do in this project was to lay-up the fuselage. I generally use three layers of six-ounce cloth in my fuselages, because Point Fermin has very good lift and the planes fly better at 90 to 100 ounces for a wing loading of about 30 to 35 ounces per square-foot. When I mix the resin for the first layer, I pour off a little into a separate cup and add some Cabosil to thicken it. I fill the exhaust pipes and any sharp corners where bubbles could possibly form with the thickened resin. I then lay down the first layer of cloth and wet it.
As soon as I am done laying up the last layer and the resin has kicked, I trim the excess glass. It is very easy to do at this stage, while the resin is still "green".

To keep the vertical stabilizer from "oil canning", I install a balsa block inside. The balsa block is glued in with resin. I install it with the grain running side-to-side, so that the forces pull endwise on the grain rather than trying to split it lengthwise. The block is sanded down until the other half of the fuselage just meets at the seam around the vertical. I also install a bulkhead in the aft section of the fuselage to help stiffen that area and add a little more strength for the horizontal stabilizer. I normally install a nyrod for the elevator in this bulkhead, but I thought I might be able to save some time this time around by using a carbon fiber pushrod. You will see later that this actually made it harder to mount the servos, and I have since gone back to using a nyrod here. Installation of the bulkhead and the nyrod must be done before the fuselage halves are seamed together.
When it comes time to put the fuselage halves together, I pull some fibers from some heavy boat cloth, and then wet them out with resin. I lay them around the seam of the vertical stabilizer and apply some resin thickened with Cabosil to the wood block and the rear bulkhead. The other half of the fuselage is set in place and weighted down with bags of lead shot.

After the tail sets up, the fuselage halves are taped together with masking tape to keep them aligned. The access holes are cut out in the hatch area and under the wing saddle with a jigsaw. The fuselage is then seamed with two-inch seam tape.

Cutting the Wing Cores
Because the wing is elliptical, the planform of the wing is wider in the middle than it would normally be with a straight tapered wing. Thus, it follows that the airfoil must also be thicker at the mid point. To cut this kind of wing, all you need to do is bow the foam a little before you cut the airfoil. It is actually quite simple with the exception of two minor caveats. First, it is very important that you create a nice gradual, even bow in the foam. Second, the foam must be bowed up in the middle when cutting the bottom of the airfoil, and down in the middle when cutting the top of the airfoil. The amount of bow will vary depending on the planform you are trying to build. This is something that will require a little trial and error to get right. For this wing, I found that bowing the foam about 3/32" in either direction gives me just about the right amount of extra thickness in the middle to accommodate the planform.
I prefer to cut the bottom of the airfoil first. The reason is that as the wire passes through, it will melt away around .02 of .03 of foam (a little less than 1/32"). The foam will drop down by that amount. When it comes time to cut the top of the airfoil, the wire will not burn away the edges of the leading and trailing edges. It is a little difficult to visualize, but if you try cutting the top of the airfoil first, you will see what I mean.
The first thing to do is to get setup to cut the bottom of the airfoil. The foam must be bowed up in the middle for this cut, so I place 3/32" balsa shims at the ends and 3/16" shims in the middle section.

Next, lay a piece of 3/4" particleboard over the shims and screw it down to the cutting board on the ends. The stiffness of the particleboard creates an even bow for the foam to sit on.

The foam is then weighted down on top of the particleboard to hold it in place during cutting.

I position and clamp down the airfoil template to the cutting board so it will not move during cutting.

I make the first cut.

The foam is flipped over and the airfoil template is turned around. The bottom of the other wing is then cut.

Now it's time to change the setup. To cut the top of the airfoil, the foam must be bowed down in the middle. The 3/16" shims in the middle are removed, while the 3/32" shims on the ends remain. The particleboard is then screwed down to the cutting board in the middle. The foam is again weighted down, and the top airfoil template is positioned and clamped down.

Here you can see that after the cores are cut, the airfoil is thicker in the middle section.

Building the Wing
Unfortunately, I forgot to take pictures during the first stages of building the wing. Basically, before the cores are sheeted the leading and trailing edges are sliced off to make way for the leading and trailing edge material. The cores must also be sanded lightly with some fine grit sandpaper (220 or finer) to remove the little hairs left on the foam from cutting. The cores are sheeted with 1/32" plywood. I like to use Southern's Sorghum to bond the wing skins because it doesn't delaminate in the heat of the sun. The leading and sub-trailing edges are applied, and the planform is traced from the wing template and roughed out with a jigsaw. I used Poplar for the leading edges and tips this time, because that's what I happened to have lying around. Any wood that is harder than balsa will work. The harder wood helps to prevent dings in the leading edge during landings.

The leading edges and tips are roughed down to shape with a planer. I used a power planer, but a hand planer will work as well. You want to get the majority of the excess wood off so that you are close to the finished airfoil. The next step is to finish shaping the airfoil with a sanding block.

Now the aileron stock is tacked in place and cut to the shape of the planform. The next step is what separates the men from the boys, shaping the airfoil. There isn't really any easy way to do this; it's just a matter of a little elbow grease. The truer the airfoil the better the plane will fly. Remember, much of the curve on the bottom of the airfoil is removed when the leading edge of the core is sliced off. It is important to reshape this in the wood, or you will end up with something closer to a flat bottom airfoil that will fly much more slowly.

Now that the airfoil is done, the wing is almost ready for glassing. First, the ailerons must be cut off, and the dihedral set by propping up the wing tips with a couple of 2x4" pieces of wood. The wing halves can be tacked together at the root to keep them aligned. An extra 4" wide strip of cloth is cut and laid under the main layer of cloth at the wing root for reinforcement in that area.
When the wing is glassed, the excess resin should be squeegeed off. A playing card works well for this. You should leave enough resin so that there are no bubbles in the cloth, but you should not have any puddles. More resin does not mean more strength. In fact, the resin will tend to crack rather than flexing if it is too thick. The glass should look like wet cloth with the texture of the weave visible on the surface. Once the resin gels, it is a good idea to trim the excess cloth around the edges with a razor blade. It is much easier to do this while the resin is still soft. Next, glass the bottom of the wing.

One thing I think many builders do not take seriously enough is control surface flutter. There is no question about it; these planes go fast. If an aileron flutters in a dive, the plane will cork screw into the hill in about a second-and-a-half. Therefore, rather than using the mild steel control rods that are available, I fabricate my own from 1/8" piano wire and brass tubing with silver soldered joints.

The aileron control horns are now installed. It is important to make note of the approximate location of the fuselage sides and locate the control horns so that there will not be any interference once the wing is glassed onto the fuselage. The wing must be slotted. I use some 80-grit sandpaper wrapped around a piece of brass tubing to clean up the slot so that the control horns fit nicely. I also use a Dremel to cut out some reliefs at the center ends so that the control horns will be able to move. The bottom of the wing is also the bottom of the airplane, so it is important not to grind all the way through. Before installing the control horns, I apply a small amount of oil at each end of the brass tubing. If any resin happens to get on that area, the oil will keep it from sticking and locking up the aileron linkage. I set the control horns in place and use some resin, thickened with Cabosil, to fill in the slot around the brass tubing. Next, I lay a strip of cloth over the area to reinforce it.

Now the wing can be sand coated. I brush on a thin, even coat of sanding resin over the entire wing. This can later be sanded to a smooth finish that is ready for painting.

While the wing is drying, the ailerons must also be glassed and sand coated.

After the sand coat is dry, the wing is sanded. A good tool for this is a random orbital sander with 100-grit paper. A random orbital sander will yield a better finish than trying to sand the wing by hand, and the job can be done in about 20 minutes. Now you only have to finish cleaning up the edges with a sanding block.

Building the Horizontal Stabilizer
Building the horizontal stabilizer is a fairly simple process. I like to use two pieces of 3/32" balsa with 1/64" plywood in the middle. This is not so much for strength, but rather for convenience. The 1/64" ply can be slotted out where the hinges will go so that when the elevators are cut off, the hinge slots are already there. This also makes for a perfectly straight hinge line.
First, I cut the 1/64" plywood to the shape of the horizontal stabilizer. I then cut out the slots for the hinges.

I cut out two pieces of 3/32" balsa sheeting and use medium CA glue to sandwich the 1/64" plywood between them. I shape the edges down with a sanding block, glass, and sand coat.

I sand the horizontal stabilizer smooth, and then cut the elevators off with a straight edge. I bevel the leading edges of the elevators slightly, just enough so that they will be able to move without binding. I mark the hole locations and drill holes into the elevators where the control horn will be inserted. I glue the control horn into the elevators with 5-minute epoxy, making sure that the elevators stay straight at the hinge line and remain flat until the glue dries. A good way to do this is to set the fixed part of the horizontal stabilizer and the elevators on sheets of balsa with a gap in between so that the control horn can hang down between them.

Putting it all Together
All the major components are now done, but there is still quite a bit of tedious work to do before the wing can be glassed.

On this particular plane, the hatch only provides access to the nose area. There isn't any way to get to the aileron linkage after the wing is glassed on, so the linkage has to be made up first. One way to make life a little easier is to mount the aileron servo in the wing. The linkage can be made and connected on the bench before setting the fuselage in place. However, I like to have access to the aileron servo, so I prefer to mount both servos up in the nose area on servo rails. This makes the aileron pushrods longer, so I make them out of brass tubing so that they will be nice and stiff. The tricky part is figuring out how long the pushrods should be. I mount the servos in the fuselage first, set the fuselage on the wing, and take the measurements. Once I have the length, I silver solder the threaded ends into the tubing and screw on the clevises. Here, I have used ball links to connect the aileron pushrods to the control horns. I put Locktite on the threads of the ball link mounts to make sure they will not come off easily. Once the wing is glassed on, the aileron pushrods will be ready.

The best way I have found to get the incidence right is to use the bench as a reference. I block the wing up so that it is level, side-to-side and front-to-back. At the wing root, I make sure the height of the trailing edge is the same as the height to the center of the leading edge. I set the fuselage on the wing and block it up at the proper angle. Once the height of the tail is set, I stack up some nice flat pieces of wood to get to the height that I need to mark the slot where the horizontal stabilizer will go.

There is the tricky little matter of connecting the elevator pushrod to the control horn. It's not an exact science. When using a nyrod, I fish the rod out through the slot in the tail and install the threaded end and clevis. Before sliding the horizontal stabilizer in, I slide one-half of the elevator through the slot and connect the clevis to the horn. It is just a matter of monkeying around with the whole unruly thing until I can get it all back through the slot so it is inside the fuselage. Once that is done, I slide the horizontal stabilizer in place.
In the case of this plane, I clipped the clevis onto the elevator control horn and slid the elevator in place. With the horn inside the fuselage, I pointed the clevis forward and carefully put the pushrod in from the front. With a little finesse, I was able to screw the threaded end of the pushrod into the clevis with it already inside the fuselage.
It's always a little tricky no matter what you do. However, this is a small price to pay to have a clean exterior with no control horns showing.
Since we are on the subject, you will recall that I previously mentioned that I usually mount a nyrod in the aft bulkhead before the fuselage halves are joined. For this particular plane, I decided to try using a carbon fiber pushrod for the elevator. I was hoping to save a little time by eliminating the need to mount the nyrod. However, this presented a challenge with respect to how to mount the servos. The problem is that the carbon fiber rod had to be run down the center of the fuselage, but the aileron servo also needs to be mounted in the center in order to have equal throw on the ailerons. What I ended up doing was to mount the elevator servo up on blocks so that the elevator pushrod would clear the aileron linkage. It turned out to be a really ugly installation, and I have since gone back to using a nyrod. The nice thing about using a nyrod is that it can be routed neatly along the port side of the fuselage where it is out of the way of everything.

Now that all the linkages are done, it's time to glass the wing on. Here I have everything blocked up and ready to go. Notice the blocks under the horizontal stabilizer as well. In preparation for glassing, I first sand the fuselage in the area that I will be glassing to rough it up a bit and to remove any wax that was left on the surface from the mold. I pull some fibers from some heavy boat cloth, enough to make a 1/4" strand for each side once I wet them out with resin. I also cut a strip of cloth to lay over the fillets after the fibers are in place.

I wet out the fibers and lay them along the edge of the wing fillets.

I lay the strip of cloth over the fibers, overlapping down onto the wing and up onto the fuselage. This will help the fibers form a smooth fillet and adds strength. Once the fillets are dry, just a thin coat of some lightweight body filler is all that is needed to smooth and blend them in nicely.

The horizontal stabilizer fillets are done the same way, but not with as many fibers.

Now it's time for a little bodywork. I wrap a piece of sandpaper around a chunk of PVC pipe, and I have the best filleting tool known to man.

A couple of hour's worth of filling and sanding (and filling and sanding and filling and sanding...) and she's ready for paint.

For the majority of the nose weight, I like to use a mix of lead shot and resin. It will conform to any shape and once dry, it won't move. All that is needed is a small piece or two to fine-tune the CG.

Painting
I typically use Model Master Paints for the camouflage. Although, lately the acrylic enamels are getting hard to come by in California because they don't conform to a bill that was recently passed. I haven't tried the water-based paints yet, but I hope that they are made available in all the same colors and more importantly, that they spray and cover as nicely as the enamels do.
In preparation for painting, I use a heavy scratch filler primer. This will smooth over any minor scratches left from sanding. Another benefit is that once the plane is all one color, pinholes and other small imperfections can be easily seen and fixed.
I usually use light grey primer, but this time around, all I could find was this dark red. The problem with a dark primer is that colors like yellow and white do not cover it well.

After the primer dries, I wet sand the entire plane with 600-grit paper. This knocks down the over spray and leaves a nice, clean, smooth surface for the paint. Although the entire plane gets wet, I try not to run too much water on the hinge lines, because there is bare balsa in that area.

For spraying the colors, I use a Pasche airbrush. An airbrush works well for a plane this size. For all of the initial painting, I use a #5 tip, which can be opened up to spray about the same width as a spray can, but will spray much more evenly. A #5 tip works well for painting large areas and for blocking out the majority of the camouflage.
I like to paint the invasion strips first. It is much easier to mask off the stripes than it is to mask off the rest of the plane in order to paint the stripes. First, I block out a big area in Flat White. I then mask off where the white strips will be and spray the Flat Black. As I mentioned earlier, it is difficult to get yellow to cover well over such a dark primer, so I also sprayed the leading edges white to serve as an undercoating for the yellow.

After the stripes have been painted, I mask over the entire area, and then spray the leading edges with Insignia Yellow. Here I have also painted the unit band and the spinner Duck Egg Blue.

The leading edges, spinner, and unit band are masked off before I spray the bottom of the plane with Light Ghost Grey.

Now I mask off the bottom of the entire plane, including the bottom of the horizontal stabilizer, and I am ready to block out the green portion of the scheme with Dark Green.

After the green, I block out the grey portion using Medium Grey. Once I have both colors of the main scheme blocked out, I switch to a #1 tip. This tip can be dialed down for much finer detail. After that, I go back and clean up the edges between the grey and green colors with the finer tip to sharpen up the pattern a bit. The trick here is to work close to the surface and always try to spray away from the opposite color. When spraying grey, spray toward the middle of the grey, and when spraying green, spray toward the middle of the green.

Now, this is the moment that we've all been waiting for, the unveiling. The main scheme is done. It is now just a matter of finishing a few details such as the canopy, exhaust pipes, and unit markings.

Here I have just finished the canopy. I like to use Ford and GM Engine Blue as the base color, and then mix a small amount with some Gloss White to create a slightly lighter shade. I use the lighter shade to add some highlights. I have found that the effect is better if the contrast between the two shades is fairly small. What I am aiming for is to create the illusion of the sky reflecting off the glass. If you look at the sky on a clear day, you will notice that it is a lighter blue near the horizon and darker overhead. I use the lighter shade to add some highlights around the bottom edges and try to blend it upward.

Painting the rest of the markings was tedious to say the least. Here I have the side of the plane masked off with Frisket film in order to spray the yellow band of the roundel. This had to be done for every color and the unit letters as well.

Finally, I could finish the plane. For the exhaust pipes, I mixed Flat Black with rust colored paint to darken it a little, because the rust is a little too orange. For the exhaust on the side of the plane, I mixed jet exhaust, rust, and Flat Black to make a sludgy brownish grey, which I misted on the side of the plane. I also used this color for the gunfire blow back on the wings. The colors used for the roundels were Insignia Blue, Insignia Red, Insignia Yellow, and Flat White. (Insignia White is glossy.)

Test flight

I flew the plane for the first time at Bluff Cove in Palos Verdes, CA. I like to test my planes there because the cliff is about 450' high and there is plenty of room to dive the plane and get it flying if I have some sort of problem on the first launch. The wind was blowing around 12 to 15 knots out of the southwest. This is really about the minimum for this plane, but at least I was able to get a test flight done. The plane actually flew quite well for the first flight. It only required a couple of clicks of down trim to get it flying level and the ailerons were right on. The CG seems fine where it was at 25%, although I was not able to gain enough altitude to do any dive testing. Now I am just waiting patiently for a good stiff 20 to 25 knot day.

Conclusion

In conclusion, I would say that this plane took me about twice as long to build as I had originally estimated. Actual building time was in the neighborhood of 70 to 80 hours, but it was all a labor of love. I would not trade the satisfaction I get out of this hobby for anything, and I am lucky to have a wife who understands that it is worth every minute to me to spend a week of my vacation time doing what I love to do. Now it's time to start on my next project, a new Me-109.
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Feb 01, 2017, 05:20 PM
Registered User
that plane is beautiful great job!
I don't fly gliders but man that is nice, now to just find an arf like it!
Feb 24, 2017, 10:46 PM
taste the high speed dirt
grrttmyers's Avatar
Cool.....love it


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