AeronautMitch
New member
Hi, first thread so here goes! I took an FT Mini Sportster and incorporated several modifications to make it better resemble a genuine Gee Bee, specifically the Model X Sportster. My goal here is to share my process and get some feedback, because I'm learning as I go and if there's a better way to do something I'd like to hear it!
Eventually I'd like to share my modified plans, not just to share the aesthetics , but also some practical features I incorporated, like a detachable wing and hidden strip-aileron linkages.
I absolutely love any 30's racing aircraft, especially the Gee Bee planes built by the Granville Brothers. I always wanted to build one but I've been out of the hobby for 10 years so I needed an easy build to get my skills back up to speed. Why the Model X Sportster and not a Gee Bee Z or R1 Supersportster? Because I wasn't ready to learn how to do a built up design with lots of curves quite yet. The model X has an inline engine rather than a radial so the standard box fuselage, as you can imagine, is naturally an easier build. So the FT Mini Sportster was the easiest place to start from.
If I'm going to be honest, I'm kind of a snob about the aesthetics of any plane that claims to be an air racing "Sportster", so naturally I have some things I'd change about Josh's design. It looks like a frankenplane - nose of a crop duster, engine of a Piper Cub, wings of a gee bee, tail feathers of a Curtiss early 40's fighter, and gangly landing gear of a rubber band powered stick of balsa. No offense, just nothing felt right. Those features looked great on the Old Fogey and Old Speedster, but the FT Sportster just doesn't have that signature "Built-in Tailwind" aesthetic that Bob Hall and the Granvilles originally came up with back in 1930 with their Model X Sportster - the first of their prize-winning line of low-wing racing aircraft. It's something primarily about the shape of the tail feathers and fuselage profile.
I would've chosen the larger FT Sportster so it'd be easier to fly, but I wanted to fly it on our Independence Day vacation to the in-law's farmhouse, so I had to make it and my gear fit in the back of a loaded down Toyota Matrix. Thusly, I was forced into the mini version and had to make the wing detachable.
Here's the Model X, the first of the Sportsters. Notice the inverted inline four barrel engine and rectangular nose profile. Also the tail that fairs into the top of the turtle deck. I have to give credit to Tom Allen - on his website his plans of an 18" balsa version was helpful to get the tail just right. Check it out here if you're interested.
A screenshot of my working modifications. A lot of the changes haven't made it into my Illustrator file yet, in particular the wing attachment method, and modified wing slot.
Here are the wings after applying the modified paint template. The only template provided was for the full sized Sportster so I had to scale them down and of course mirror it to better match the real scalloped shape for wing bottom. Spray painting didn't work for me, so I went with light weight colored tape and cut it with a razor blade to shape. Maybe heat activated covering, like Monokote would be easier?
A key modification I made was to remove all the paper from all the internal surfaces of the foamboard, including the fuselage. I realized that when the wing and fuselage are folded and glued, the outer layer of paper is put under tension, giving it enough strength as to make the inside layer of paper unnecessary. Like the skin of a monocoque fuselage. It was a gamble, but was proven in several decent crashes (naturally lol).
Note for anyone building an FT Sportster or Mini: If you chose to remove the paper from the inside of the fuselage, you can skip the fuselage crease step.
Without paper on the inside surface of the wing, the airfoil took a more symmetrical shape. I figured it'd do this but I wasn't certain of the exact shape, so I made this template to trim away foam from the original slot shape. A balsa spar was used in place of a foamboard one.
I kept the paper on the under-cambered tips.
The elevator flexes way too much without any reinforcement. I tried a couple of solutions but here's what I came up with, with what I had on hand in a pinch. It helps but it's just balsa so still flexes at that thin. I pull up hard and it'll nose right. It's a plane of quirks! Next time I'll do something better, like carbon fiber.
I didn't like the protruding servos and linkages on the FT version - too easy to damage and visually breaks up the clean lines, so I made things ten times harder on myself and chose a solution that incorporated an internal servo. I came across what's called a "strip aileron" linkage method, seen above. The first image shows the 2nd wing I made to prototype the system to get the geometry and placement correct before cutting into anything. On the 2nd image you can see the final solution. I'm cheap and didn't want to spend the money to buy Dubro Strip Aileron hardware, but I had lots of Dubro EZ Links and extra pushrod, so I used them. Some of the supplied pushrod guide tube glued in place held the rods in place. When everything was glued down, I sneakily hid the rods with Avery label paper. I had never done this so it took a ton of tinkering! A Turnigy 9gram metal gear is what I went with.
If you're going to go this route, I suggest you use adjustable linkage stoppers on the servo arms.
The other significant change is the removable wing. I must give credit and thanks to Zetoyoc on youtube and his removable wing FT P-40 video, it was so helpful! I was originally going to go with nylon bolts but it may have been even more complicated and heavy.
Here it is, showing the barbecue skewer that holds it in place at the front, and wooden tabs at the back. I reinforced the front insert with a length of balsa, and small squares of plastic at the holes. Upon a cartwheel (oops!) the front insert took the brunt of the force, so this works surprisingly well as a shock absorber. The front wing insert is a separate piece from the wing making it easy to replace when it's damaged. You may notice that I covered over the nose. It's the perfect compartment for an RID module.
I removed the middle piece of foam from the top of the fuselage, and replaced it with a piece located mid fuselage that covers the aileron servo and acts as a receiver and ESC tray. The 4-channel micro Orange RX is perfectly snug in place when fitted on its side. In fact it's super snug with a 30A ESC and 1300mah 2cell.
For the hatch, I wanted a curved cross section, so made a 2nd former and hollowed it out to act as an air scoop. The hatch is secured in front with two short pins held down by a popsicle stick. I deleted the air escape on the bottom of the fuselage, so the air flows out the back of the tail above the horizontal stabilizer. It's hard to see, but I didn't seal the back end of turtle deck I made from card stock. You really can't tell that there's openings on either side of the rudder unless you look directly from behind. For my own shameless advertising, my logo and slogan are printed on Avery laser printer labels and cut out. Folks used to complain about the FAA requiring our registration number to be on our planes, but I think if you print it out with a nice font and put it on the tail it can look fairly legit.
It's nice that I was able to find an actual image of the Model X Sportster's instrument panel.
For choice of motor and prop I went with an Anoel D2822 1800Kv and 9x3.8 Slow Fly on a prop saver, powered by a Nanotech 1300mah 2 cell.
I figure it was the best for what I had on hand and actually flies nicely with that power setup.
When fully loaded it's 340 grams and flies around happily at half throttle. Acrobatics require a little more, probably 3/4 throttle. A 1300mah is the largest that'll fit, but from timing my flights and checking the battery, I'd say it'd safely do a sporty 10min flight.
One major problem I'm having stems from choosing a crap ESC. It has no brake, so landings have been frustrating. In fact I'm not going to fly it again till I get a better one. The plane floats at half or quarter power, but pull it all the way back and it drops like a rock. I'm certain it's that slow fly prop spinning free that sucks all the energy out of my landing approach. I'm thankful to have flown in a harvested wheat field, where all the stalks catch it very gingerly, even on the lead weight landings. One quirk I'm noticing is with the rudder - when I give it full rudder, right or left, it'll lower the nose. What do you think that's about? Servo cross talk, or strange aerodynamic effect? Tail servos are junky HXT900's.
Next is to finish the landing gear. Otherwise it's not a GeeBee. And to get a braking ESC, or proper landings aren't happening. I'm working on wing mounted and based on the shape of the GeeBee Z's. Ideally I'd like them to be detachable. Perhaps a slot-in method? It'll have the structural wire sandwiched between the struts, built out of two pieces of foam. Thoughts on this would be appreciated, and I'm ready for any jokes about the planform of the wheelpants. Just pair the jokes with constructive feedback please and I guarantee to laugh at them.
Next time I do this I'll start with the larger FT Sportster, and adjust the placement of the wing and underside shape of the fuselage. I'd like to have it fully low-wing and try the bolt on method because I've never done that before. Also a different coloring method. The tape is cool, but cutting it to shape introduces challenges.
That's about it for now. I'm not sure if this can handle video so I'll hold off for now.
Please share thoughts!
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