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Cafe Racer Mk II (swappable)

Tench745

Tench745

Master member
Some time back I posted my first thread to these forums about my attempt to duplicate a fictitious airplane known as a Cafe Racer. Cafe Air Racer.jpg
More pictures here if you're interested in similar concepts.
The original was under powered and didn't really look much like the aircraft I was trying to duplicate. This past month I tried again and here is the result.
DSCF2286.JPG DSCF2287.JPG DSCF2288.JPG DSCF2289.JPG
The wheels are scratch built from EVA foam floor tiles. Landing gear is coat hanger, piano wire, and carbon-fiber tubing.

Maiden awaiting warm weather.

Flying weight: 652g (23oz)
Length Overall: 36"
Wingspan: 40"
Wing area: 260 square inches
Wing loading: 12.75oz/sqft
Cubic wing loading: 9.5


Motor: Turnigy 2830/11 1000kv motor
Servos: HXT 900
Prop: 8x6 slowfly
Battery: Turnigy 2200mah 3s
 
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SP0NZ

SP0NZ

FT CAD Gremlin
Staff member
Admin
Moderator
Mentor
That looks really cool. Make sure you get some video of the maiden.
 
Tench745

Tench745

Master member
Paint is on the way. Gotta figure out what I want to use.
I'm also debating throwing some balsa on the upper side of the cowl to mimic the framing of the concept.
I wish I could've done the engine in full detail too but there isn't room for that and an ft power pod.
 
saiga556

saiga556

Full of...
Your not gonna have much excess thrust with an 8x6 prop. I'd try a 9x6sf. I ran your setup through ecalc and you would get a much better thrust to weight with a 9x6.

I think hk suggest a 9x6-10x4.7 for that motor.
 
Tench745

Tench745

Master member
I've got a 9x4.7sf I ended up putting on my Jenny. If the 8x6 doesn't perform like I want I'll swap out to the larger one.
 
Tench745

Tench745

Master member
So, brief update on the Cafe Air Racer. I decided the 8x6 wouldn't be enough prop so I threw the 9x4.7 on it for when I finally maiden. It's the middle of winter here of course, gusting like crazy and another snow storm is on it's way today. So I've spent some of my free time detailing. DSCF2290.JPG DSCF2292.JPG
 
RacerX

RacerX

Member
Looks very nice. Impressive!

I wonder if you could print out a picture of an engine and glue that behind the cowl to simulate the concept art? You could do four separate printouts easily on any color printer. Just a thought.
 
Tench745

Tench745

Master member
Thanks, I've spent a lot of time staring at that picture. I've thought about doing the full engine, but with the way the powerpod fits inside etc I just don't see a good way. Printed graphics on the cowl would be okay, but not quite something I'd be happy with I think.
 
jamboree1

jamboree1

Active member
Tench745 said:

Maiden!
Possibly tail heavy. Pretty windy.
Click to expand...

Yup, looks a tad tail heavy to me but not so much that battery size or some weight can't overcome easily, sure looks like a neat and unique plane though
 
Tench745

Tench745

Master member
So, I love my cafe air racer. She's currently hanging from the ceiling in my room, the only plane I have ever hung up just to look at.
DSCF2403.JPG
When I first flew it, the air racer was a much faster plane than anything I'd flown previously. Now, after flying my versa wing (with a 1400kv motor running 3cells and a 7x5 prop) flying this can actually be somewhat sedate by comparison. (Not that the versa can't go slow too.)
Looking the plane over and applying some math from a model aircraft design book it looks like the H-stab should be about 1.5" wider than it is which would help correct for its pitch sensitivity. I did end up adding nose weight and dialing down the throws but even so it's still a bit pitchy.

The original gear was cf tube with clothes hanger wire epoxied in wherever it bent. The gear was raked aft due to a mis-calculation in wire lengths when I first bent it all up. This arrangement was lightweight and stiff as I'd hoped it would be. In practice, however, this turned out to be a negative; the clothes hangar bent too easily and the cf didn't flex enough to absorb the shocks of landing. Even good landings broke something, so the whole assembly was replaced.
The new gear was 7/64" music wire with a much thinner wire soldered in place to hold the main wire in a forward-raked position.
DSCF2402.JPG
This improved things a lot. The longer lengths of wire allowed for more spring in the gear. Even still, if you came down in anything less than a 3-point attitude the support wires would bend. This was rectified by securing a bamboo skewer to each of the support wires with 3, 1" long wraps of thread superglued in place. A particularly hard landing will break a skewer, allowing the gear to bend and save the fuselage but it's strong enough I can do touch-and-gos to my heart's content, even on grass.
 
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Tench745

Tench745

Master member
Even with the improvements of MK.II I find myself wishing I had gotten a little closer to the concept art this plane was based on.
Cafe Air Racer.jpg
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.
image.jpeg
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.
DSCF2398.JPG
DSCF2399.JPG
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.
 
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