Topic: O.S .61 SF

Killer Kaos flies (restomod part 5)

2015-08-05 21.37.44

The O.S engine gave me a little headache. I ran it at home a little bit, and it ran strong and started like any O.S motor should, i.e. with a light flip of the prop. It tached 10.000 rpm on the APC 11×12 and idled smoothly. All was well and then I tried holding the nose up, and it leaned out badly. A lot of tweaking later, I was unable to fix it. I re-did the fuel plumbing and moved the tank a lot further forward and hoped that would solve it. It may have been optimistic having the tank as far back as I did without using a pump or header tank.

On the field yesterday I had exactly the same problems. I switched from an O.S 8 plug to an OS F, and that did nothing. I tried a smaller prop, an 11×9, and that helped a bit. I then tuned it some more and eventually it ran strong. I had one flight where I landed early because I could hear it leaning out a bit in loops, tuned more, and then suddenly it ran like nothing else mattered, not missing a beat going up, down, sideways or anything. Success! The third flight was suddenly bad again, leaning out. Landed in a hurry, tuned, and suddenly it ran great again.

I have a successful track record of making stubborn engines run good, and I don’t intend to let this one beat me. I suspect the pipe is slightly too long, and I think shortening it might do the trick. I will also try an 11×10 to load it up just a bit more.

The plane flew just like you’d expect. I had some wild deja vu’s while flying. I have lots of flights on the old version of it, so it behaved as I remembered, which is a strange sensation when I was mentally prepared for a maiden flight! I was also surprised at how well behaved it was on takeoff and landing, and how well it tracked straight and level. It has no vices whatsoever.

Here’s a before/after photo, with at least 13 years between the two:

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All the pictures from the project can be found here:
https://www.flickr.com/photos/aerowold/sets/72157651018400942

The restomod project blogposts:
Restomod part 1
Restomod part 2
Restomod part 3
Restomod part 4

Killer Kaos restomod part 3

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The finished radio bay in the fuselage

My favourite part of any build is the pushrod mechanicals, the electronics and engine installation and so I take my time getting it as perfect as I can. I ground out a huge cutout for the Tettra 400cc tank in the fuselage former. The tank would fit in front in the tank bay, but I wanted it further back in order to not have the CG shift in flight due to the tank being emptied.

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The Tettra 400 cc Crank Tank fitted. 

The servos were moved forward from their original position and the battery sits behind them, in order to fix a CG issue that the plane had back in the day. Because of that, the pushrods had to be lengthened. I wanted to stick with the original Carbon Dubro pushrods that were already in there, so I simply added a longer threaded rod that extends a bit into the pushrod inner and outer tubes. It adds a little bit of weight, but it adds it behind the CG where I need that weight, and as a bonus it stiffens everything up a little more. BLS451 servos were bought very cheap from a chopper guy switching to HV gear. They are great servos, so I get them whenever I see them up for sale. The throttleservo is a Dymond metal gear digital midi.

To top everything off, I fuel proofed everything using flat black enamel paint, which also looks cool. I’m very happy with how the interior bits and pieces turned out.

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The radio bay. Note Tettra switch mount bracket

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The aileron linkages. I used a pair of trusty old 9154s that have over 400 flights but are still pretty solid. I added plywood inserts to the ailerons so that the screws have something to bite in. I shortened some Futaba servo screws to fit, as I didn’t want anything visible on the upper side of the ailerons.

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The O.S .61 SF engine was fitted in order to get the throttle linkage hooked up. The intake is finished. It’s a lot of motor for so small a nose.