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1/6 scale Cafe Air Racer Mk III

Tench745

Well-known member
#1
*This is a reposting of the build I began to cover in the Cafe Racer Mk II thread, but I have decided to move it to it's own. All further building and testing of the new Mk III design will be covered here.*

Even with the improvements of the MK.II I find myself wishing I had gotten a little closer to the concept art the plane was based on.
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I've been curious about using paperless DTFB and Keith Sparks' foam building methods to construct a lighter, and much more finished looking aircraft. As such, I've begun re-drawing my Cafe Racer for building in raw foam, possibly to be glassed. When I first started obsessing over this fictional airplane I took the time to draw a top and side view from the original artwork, focusing strongly on the relative sizes, shapes, and dimensions of each part.
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It was this 2-view drawing I began by taking measurements off of. Happily, the original artwork has a figure. Assuming a figure height of 6 feet I estimated that the chord at the wing root was about 4 feet. On my sketches the wing root was 2". Thus, my sketch was 1/24 of the fictional full-scale prototype.
Interestingly, this means the full-scale would have an 18 foot wingspan, almost identical to that of Art Chester's Goon.
Now that I had a scale of the drawings I had to decide on a scale for the model. Since the wings are the most important thing on an airplane I used the same root-chord as a start for scaling the new model. MK.II had a 9" root chord. For ease of maths I chose an 8" root chord for the new one, making it a 1/6 model of the full scale and allowing me to simply quadruple any measurement I took off my drawings.
All of this gave me the following:
Wingspan: 36"
Length: 36"
Wing area: 185 square inches

Satisfied that this would be flyable if kept to the same weight as the MK.II version I proceeded with drawing the basic shapes in CAD. In order to experiment with the building techniques and get an idea of the final size I decided to start with the focal point of the whole shebang, the exposed engine.
Below is the prototype engine. I have yet to attach the cylinders and construct the cooling fins, but so far I'm pretty happy with it.
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White gorilla glue is super easy to work with and the DTFB cuts, sands, and glues well once the paper is removed.
I was happy to see the crankcase is large enough to contain a motor, esc, and possibly even the battery.

Got looking at my crankcase yesterday, decided it was too wide, so a few minutes and a long razor blade later I had this. Neat to see the framing from the inside like this. Good too, since the case was almost sanded through in the front on top.
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I cut 1/8" off each side to slim the whole thing down. It looks a lot better now but the camshaft/lifter area needs to be reworked still.
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I'm testing using foam plates for the cooling fin/cylinder arrangement. They work pretty well and can be sanded to shape. I'm not sure how durable they might be. We'll see.
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Side project, I decided to try and duplicate the little toolbox on the wing. Something innocuous, but so much detail nonetheless. Since the wrenches on the wing are too big to fit into the drawers I made the assumption that there must be a tray in the top of the box as well. I think it turned out pretty good. It's much more distressed than the one in the artwork, but it IS a mechanic's toolbox, is it not?
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Just a pair of pictures showing all 6 cylinder blanks glued up and drying.
Also, this is the narrowed case. I've moved one of the pushrod locations from the front to the rear of the engine so I can offset the cylinders from one side to the other.
 

Attachments

Tench745

Well-known member
#2
Inspired by the design-off and spurred by the availability of some free time, I've started working on this project again.
Looking at my wings, I decided it would be impractical to make foam full-span ailerons so I've substituted 3/8 x 1" aileron stock.
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I was also unhappy with the shape and finish on the elevators, so those were replaced with some larger aileron stock I had laying around. Sanding will be necessary to get these down to the correct thickness. I'm sacrificing some of the shape of the original, but I can live with that for now. All balsa will probably be glassed, or at least sealed with WBPU.
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Initially I had a foam former for the forward fuselage bulkhead. On rethinking things I realized this would be the frame member that took all loads from the engine mount and landing gear, so it was replaced with 1/8" ply. The lower portion is laminated three layers thick to receive the landing gear legs through a slot in the fuselage skin. (Old gear legs used for visual, not a final piece.
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And, last but not least, here is my pilot. She needs her face painted, hair done-up, and a new set of clothes. She's probably too heavy to actually fly in the plane, but she's in the picture, so she gets built. ;)
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Nerobro

A Severe Lack of Sense
#3
You need a reply just for encouragement. Where did the concept art come from? And.. I can't wait to see the end result.
 

Tench745

Well-known member
#4
Concept art is from a Brazilian, I believe, artist. He's got a few 3D printed static models of his designs available through Facebook under "Imaquinaria." I appreciate the encouragement, not many people seem to care about designs that aren't based on an existing model or full scale plane. Of course, the fact that my builds always stall out probably doesn't help. :)
I need to work on planning better before cutting my first balsa and foam. I often leave the hard decisions until things are already built. This one could be flyable before winter, but I'm moving soon, so it's debatable.
 

Tench745

Well-known member
#6
I've found that every time I set a deadline, I just watch as life drags me past it. I'd love to get it done this summer, but next is more likely.

I bounce between this and my Searey project, and another design concept I started sketching recently. Besides these, I have a pair of static models to finish and an army of wargaming miniatures for a friend. I need fewer hobbies, or more time. ;)
 

Tench745

Well-known member
#7
Bunch of pictures this time. About 8 hours of work represented in these pictures.
I glassed both wing halves.
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I began to cogitate on how to best attach the v-stab and decided a balsa strip up the tailpost and a bbq skewer to reinforce the leading edge.
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To help take the stresses of the skewer I laid up a layer of glass inside the fuselage
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then laminated another piece with glass
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which then was glued in place at an angle so that it would be braced on all four sides.
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The fuselage then got a coat of primer while the glassing set-up and the filler I had applied to the glassed wings cured. The new elevator halves were cut an drilled for the crossover bar and the tailpost was slotted for two more ca hinges from the rudder.
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Here are the tailfeathers in place in their final configuration. I think paint will have to happen before they can be permanently installed.
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I also added 1/32" (I think) plywood end plates on the root-end of the wing panels and glassed them in place. These will get drilled for the wing joiner tube. Servo placement and pushrods will have to be finalized and I'll have to figure out how and where to add a battery hatch/tray before I install the upper skin of the fuselage. The power system is currently undecided. Until that choice is made I won't have any idea of how my cg sits. Ideally the motor will fit inside the dummy engine, but I don't know if that is even possible.
 

Tench745

Well-known member
#8
Short updade: Ordered a 3550, 1250kv motor, 60 amp esc (probably overkill but cheaper than a 40), and 9g servos. (I've used 5g for everything except the Cafe Air Racer Mk II)
 

Tench745

Well-known member
#9
So, quick update:
Wings, H-stab and V-stab have all been painted. Rear turtledeck is built. Motor has been mounted temporarily for a balance check. Landing gear has been built and temporarily attached. Wheels are stand-ins
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She's tail heavy with a 30A ESC, even with all the servos forward.
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A 60A ESC makes it just about right. Hopefully that will get me pretty close once the electronics are fully installed.
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Tench745

Well-known member
#10
I fabricated an aluminum motor mount, seems to work but am unhappy with it so it will probably change. Started working out servo placement and ran the Elevator and Rudder pushrods. Now I'm done in the aft section of the fuselage, and have glued on the turtledeck. This will need a rather large hatch cut in above the wing spar to allow for inserting the battery, mounting the engine mount, servicing servos, and securing the wings in place. Once that hatch is cut I will reinforce the area with balsa and glass the last of the fuselage. I'm getting close.

Current weight: 925g (32.6oz)
 
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Tench745

Well-known member
#12
In one of the many distractions that come up during any build, I took a moment off from my Ryan NYP build to touch on the Cafe Racer MK3 again. I was stuck for a long time trying to come up with a way to make the wings removable/interchangeable while still having the option of flaperons. Finally, I gave up and just buried some servos in the lower wing surface. Running numbers on this build, I do not think it will be flyable with the stock wings. I'm guessing 36oz+ flying weight on only 185sqin of wing. Not looking promising. Still, I'm going to keep at it and at least do some taxi tests with the small wings. Pictures to follow. Looking at maybe a 9x7 3 blade for 67oz of thrust and 60mph in level flight.
 
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Tench745

Well-known member
#14
Stole a page from Wilsonman's Bugatti build (sort of) for my tailwheel. The wheel housing isn't removable, but a pair of plywood brackets hold the rear wheel's axle firmly into the v-stab.
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This is how she sits now. V-stab and H-stab are permanently installed, elevator and rudder pushrods are connected, aileron servos are installed and connected, wing attach screws installed, and dummy engine is in progress.
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Tench745

Well-known member
#20
Such an awesome project - any chance we will get to see it in person at the FF17 east exhibition 'race'?
If I can get the finances together to actually be at FF2017 I will be bringing all my planes. The cafe racer MKII and III should be there, as will -fingers crossed- my Ryan NYP.