Akom's Tech Ruminations

Various tech outbursts - code and solutions to practical problems

Pitts S2A Bottlenose motor mount rebuild Toys

Posted by Akom • Monday, October 13. 2008 • Category: Toys

I didn't see this coming - hesitated in my landing approach, caught a gust of wind, clipped a cute little pine tree - and dove straight down 15 feet, carefully missing the soft lawn and hitting the packed dirt road. Front end demolished - engine cowl and engine mount are in pieces, fuselage cracked. The prop saver may have helped the motor survive this adventure.
New Engine Mount (mount cross is inside bottle)

I tried to find suppliers for these parts (again), but nothing turned up. Other than buying a whole new plane, I don't see a way of getting a new motor mount (that seems a little silly, no?). The fuselage is easy, Gorilla glue got that back together in an hour or two. (In fact, I've already glued the fuse with it once, and that weld did not break).

But the mount?

I tried making a new firewall motor mount out of foam. I am not that neat though... foam kept breaking, and it just didn't seem like a good design. And then, there it was... a perfectly good square Fiji water bottle, sitting in plain sight... I stared at it for a bit and the motor was just begging to take the place of the bottle cap! The square cross-section did seem like it might slide on to the plane in place of the engine cowl.

New motor mount

I cut the bottle in half and slide on it did not. It is about 1/4" too narrow. I tried using boiling water to stretch the material, but it only shrank instead :-)... and deformed the bottle a bit. I didn't have another, so I made do - I just slit the bottle at all 4 corners until I could stretch it over the plane. It fits! Now to mount the motor.

I had to run to the hardware store and get some 3mm x 0.50(pitch) x 20mm(length) screws and lock washers. Then the cross just went into the bottle neck. I had to cut off the bottle cap thread section from the mouth to make the screws reach the motor (just took a hot knife to it). The bottle is made of some sturdy plastic, it's really rather unfortunate that all that just goes to waste when not recycled, after what, an hour of use? Well this one is going to live on for a while (I hope).
Motor protector bumper
So the motor is mounted to the bottle (forgot to run the wires through the neck so I just made a hole and ran them through), now I just pull the bottle onto the plane and tape it in place (packing tape does a lot in this hobby).

Done!

Well, not exactly. Now that the motor is effectively firewall mounted, it's more exposed... and the plane likes to nose-in. So I found a block of foam that I taped under the nose - to keep the prop/motor from striking the ground. I'd have installed a wheel there if I had one handy :-)
Nose-In Bumper in action

Flight testing

I made the bottle just long enough so that the motor is as far forward as it was with stock mounts. I also am not using the weight I had in the old cowl. The sleek shape of the bottle (even as heat-mangled as mine) looks much more aerodynamic than the stock cowl, which basically is a brick right behind the prop!

Before long I was back at the park, and the plane flies again! It seems to be a little faster, but that's hard for me to measure. What I can measure is flight time, which should translate to an improvement in efficiency.

So with my 1800mAh 3E battery I take off at full throttle, climb to decent altitude and leisurely circle. First of all, it now cruises at about 60% throttle, without loss of altitude. It still certainly does not give me unlimited vertical, but it feels more confident when climbing. I circled for 18 minutes. That's right, 18 minutes, landing at 18:40 on the clock. Battery voltage? 11.05V . What does this say about possible flight times? This could actually work for FPV with just one battery!

1 Trackbacks

  1. I killed the motor. Apparently ramming into pavement at full throttle (don't ask. From now on, when something goes wrong on takeoff, I abort) is not such a good thing for a long thin shaft and a firmly bolted prop. A few flights later, the motor died

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