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P20 powered Crusader 3B, old school building, modern fab

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P20 powered Crusader 3B, old school building, modern fab

Old 09-27-2023, 08:44 AM
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S_Ellzey
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Default P20 powered Crusader 3B, old school building, modern fab

I built the original Crusader 3 with the P20 several years ago. Its construction was fairly simple: balsa sheeted foam wings with a generally simple balsa fuselage and tails. It flew very nicely, and I enjoyed it for many years. But, over time a structural issue forced me to retire it. I decided its replacement would not have the same limitations that I gave myself on the original. This time the shape would be unconstrained, much of the design philosophy included the concept of “slinky”. The construction would use the technique I learn from the Modeling Master Sam Snyder. But this approach would use modern design and fabrication.




This machine is 5% larger than the original, since it was always tight inside. It uses electric retracts, which gets rid the air tank and valve servo, but otherwise reuses all the original equipment.
The original was drawn on paper, and did not have much detail. The 3B was designed in 3D CAD (Pro-E), and all the internal structure was to be laser cut.
The following images show the Pro-E model.




The model includes all the parts that go into the model. The screw holes for the servos, landing gear and engine were included in the design. Once all the parts were defined each one had to be exported to the laser cutting software, Xtool Creative Space, which would drive my Xtool D1 laser cutter. Once in that software the parts are arranged to make the best use of a sheet of plywood. Roughly half of the parts were 1/16” ply and the rest 1/8”. All told it would take 4 sheets of 35”x17” plywood to cut all the parts, but I had some very useable areas left on two of them.


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Old 09-27-2023, 09:13 AM
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The first part that actually needed to be built was the inlet duct, it would have to be inserted as the frames where put in place. A plug was 3D printed, sanded smooth, primed, sanded some more, primed some more and sanded some more, then waxed with Partall and coated with PVA. A coat of primer was applied and once that was getting firm two layers of 3.2 oz cloth was applied. The plugs where made 1/4’” longer on each side so that the glass would lay flat across the area it would be cut.


With the all of this I had a pile of parts read to go.


Now began the task of putting together the most complex 3D puzzle ever created by man.



At this point nothing has been glued, but the structure is fairly stiff. There are little legs on most of the frames that keep everything strait when it sets on a flat surface.

Last edited by S_Ellzey; 09-27-2023 at 12:46 PM. Reason: Correct spelling
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Old 09-27-2023, 09:31 AM
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Kick ass lil bird. Fast AF and supremely stable for such a lil bird
Old 09-27-2023, 09:42 AM
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Next the structure was weighted down on to the building table and a steel 1-2-3 block was used to keep the frames and ribs square while CA was applied to the joints. Once all the structure was CA’d together a thin epoxy was used to reinforce the joints. Balsa tri-stock was used on the highly loaded joints, hinge blocks, spar caps and shear webs, and leading edge structure was added.



If you look carefully you will see a number of places where two pieces of ply are running parallel, or nearly so. This is where the hatches and control surfaces will be parted off later.

At its point the entire upper surface was block sanded so that the edges of the plywood would match the slope of the skin that would be applied.

Next the upper wing skins were glued on, I did not take a picture of this step for some reason. The following photo shows the bottom of the model after the top wing sheeting was added and the building legs were removed. At this point the structure was so stiff that I decided it was no longer necessary so use building legs. In this photo you can see the additional features added to the inside of the wing, like wheel wells, and additional shear webs to stiffen the skin.


The nose was a bit flexible, so a couple of balsa sticks were tacked on the top and bottom to stabilize it better (not shown). Next the lower wing sheeting was added a long with the Leading Edge Extension (LEX) skins.


There is still a leading edge cap and a LEX cap to be added, so those edges shown now will be extended.
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Old 09-27-2023, 10:23 AM
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And now the real fun starts - planking the fuselage!

For some odd reason I decided to start with the hardest part first: the aft upper fuselage that includes the inlet cowl. This was much tricker that I thought it would be. Turns out they you have to twist the plank about 180 degrees to make part of it work. I broke several planks before I got it worked out, but soaking part of them in alcohol made them soft enough that it would be done. The planking process involves holding the plank in place and then use thin CA to glue it to the frames and to the plank next to it. I have a simple balsa stripper that uses a #11 blade and will cut any width you like. While the planking might look rough, it is actually fairly smooth at this point, it is the CA that makes it look rough. They are hard to see here, but there are several small holes in the skin that show where the engine cover will be parted off. There is a former that is taped to the trailing edge that the planks go inside of. This allows the very aft end to be formed to the correct shape. This area is glassed on the inside for stiffness and then the former and the planks that pass through are cut off.


The upper forward part of the fuselage proved to be a much easier job, maybe in part to what I learned on the back. I started by putting in several of the upper centerline planks and then stared to build up to them.


From this point it was pretty quick and easy to finish this area. Again there are small holes that define the corners of the hatches.


The bottom forward fuselage was very simple to sheet, not planked. The bottom aft fuselage was planked just like the top front.

The fins were skinned in the normal method. The leading edge cap was added. The Lex cap is a bit interesting, I sandwich a piece of 1/32” ply between 3/32” balsa leaving a little of the ply extending inward. That edge is then slipped between the skins when the cap is glued on. When sanded to shape the edge is the plywood edge which holds up way better than balsa would. My RCM article on the Crusader 2 used the same technique. I also add two layers of 0.75 oz cloth between the trailing edge sheeting to provide a stronger trailing edge.

The following photo shows the model getting is weigh in (2.87 lbs) with all the wood in place except for the inlet lip, it had to go on pretty late in the process.


Old 09-27-2023, 10:59 AM
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At this point the effort becomes the normal wood finishing technique: lots of block sanding, puttying joints to create fillets and more sanding. Once everything looked smooth a layer of 0.56 oz cloth was applied with Zap finishing resin. This was followed with more sanding and a little puttying. The fins have not been attached yet to make it easier to sand everything. Have I mentioned that there is a lot of sanding involved in this? Finally the first coat of primer was applied, this is the time that you learn how many flaws you have to fix.


At this point the putty, sand, prime, sand cycle is repeated.

Once the finish looked good it was time to part off the hatches. For this an Exact saw blade with the spine removed is work in between the ply parts that define the hatch edges and the hatch is cut free. Before I started sheet/planking the holes for the alignment pins had been put in place. This was also the first time that I was able to check fit the engine: the mounting holes where dead on. If you look carefully at the area the fins will attach to you can make out a layer of masking tape that matches the shape of the base of the fin. That was added before glassing and was removed right before the fins are glued on.


The control surfaces were cut off like the hatches. They had to have balsa block added to their leading edge which of course had to be sanded, puttied, primed and sanded some more. Holes for Robert hinge point where included in the design, so all of those were in place when it came time to hinge.

The 1/16” gap between the hatches and the fuselage was filled with either 1/16” balsa or putty. This is also when the inlet lip was added.



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Old 09-27-2023, 11:43 AM
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Finally it’s time to paint. I had decided that I wanted to go with an original F-16 three tone gray air superiority scheme (current F-16s use a two tone scheme).

I have never used Klasscoat paint before, but having seen the results some friends have achieved with I decided to go with it. I managed to find the FS paint numbers for the colors I needed and sure enough Klasscoat had them. So I ordered them with the satin hardener.

The entire bottom and some areas on the top got the lightest shade of gray.


The canopy was painted with some PPG DBC I had on the shelf and cleared with PPG 2021.

The upper nose and the fins where painted with the medium gray with a cardboard circle setting about 1/4” off the surface covering the tail splash area.

Stencils were made using a Cricut vinyl cutter and Orical stencil vinyl. The stencils on the top protected the light gray that was applied first. The ones on the bottom were sprayed through to apply that graphic.


More stencils were used to add the tail number and the text for the tail splash.


The final graphic is the Crusader Knight, which is a story unto itself. I am not much of a graphics artist. When I looked for a good bit of art to use I could not find anything that I liked. At the same time there was quite a bit of discussion online about AI art sites, so I went to one and put in some thoughts about what I wanted, and the very first image returned had an amazing vibe. I looked for more to see if I could generate anything better, but I kept going back to the first image. I took that and cleaned it up for my use, but I was not totally happy with the helmet. I sent the image to a friend who is a great artist. Without me saying what I was not happy with he said there was something wrong with the helmet, it was not in the right place. He went in and created a new helmet, and if you look in the eye slit you can see the eye brow, eye lid and the top of the cheek. The only way that I could maintain that level of detail was to use a water slide decal. So that was laser printed, and applied. A light coat of satin clear was sprayed over it.


Then all of the equipment was stuffed in.



Overall I am pleased with how it turned out. The Klasscoat paint work good, I’d use it again.




Old 09-27-2023, 12:40 PM
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The first flight went well, the model lift off much like the original, but with a shorter takeoff roll. The model trimmed up nicely and after a few laps was flying straight and level hands off. I turned the gyro on and started to set the gains. When it came time to land it slowed down nicely, and touched down smoothly. The second flight went pretty much the same.

Now moving on to the attempt of the third flight. As I went through the servo checks one of the elevons cycled a couple times and then died. I checked wires and connectors to no avail, that JR servo was dead. Had it died a few minutes later I would have lost the model. Given the availability of JR servos I found a Spektrum servo that was close, it had a bit more torque and was a bit faster. It was the same width, in fact the mounting holes had the same spacing. It was also thinner, but it was taller. I got four of them in and then took a dremel after my nice laser cut servo mounts. Oh well, they work well and the model flys great with them.

One difference I could feel was that this version was much faster. I put a better inlet duct on it, so the engine should be running better, and the back of the model is cleaner, so even though it 5% larger it appears to have less drag. I am also getting longer flight times on the same fuel tank. The original model used a 5 minute timer, and I was not thinking of extending it. With this one I am at 6 minutes and still have 30 - 40% of the fuel remaining.

When you look at the technology available to us today; CAD design software, 3D printers and laser wood cutters; there has never been a better time to be a scratch builder.

Steven
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Old 09-28-2023, 03:50 PM
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Plus, it flies great! :-)
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