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CMP Piper Cub 91" W / Satio 1.25 gas conversion

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CMP Piper Cub 91" W / Satio 1.25 gas conversion

Old 06-01-2017, 05:17 PM
  #1  
AA5BY
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Default CMP Piper Cub 91" W / Satio 1.25 gas conversion

Several years ago, I bought a CMP Cub as part of the 'laying in' of planes in preparation of retirement. Plans to do a winter kit build of a BUSA 1/4 Cub went by the waysides and as spring sprung, the realization that a new plane wasn't going to happen this year unless it was an ARF, and thought turned to the CMP Cub.

A four stroke was a priority and preferably gas. I had a four stroke that had been on the shelf several years, but it was glow. I decided to convert it to electronic ignition with the hope of converting to gas, but I'd live with glow w/ignition if necessary. Adding ignition was no problem and tuning it for gas wasn't much effort so the plane box was broken open and I found the CMP Cub to exceed my expectations of quality and fit.

Commissioning the ARF was a delight rather than a hassle. Things only got better with flight. The balance was perfect with no additional weight. Elevator is neutral with power on / off trim the same. The Saito conversion not only sounded super but was a good power match for the plane. A 17 x 4W APC prop was icing on the cake, providing good take off and vertical thrust while offering excellent idle braking to slow the plane for those no head wind landings and keeping the Cub from flying too fast and beyond scale appearance.

The single problem with the plane was the O rings didn't work for the undercarriage suspension and springs were opted for.

The initial setup proved to be good, with 40% aileron differential, 15% rudder mix with ailerons, 1/4" aileron reflex at low throttle and 40% expo all around. I do sometimes add additional rudder for sharper turns, but the rudder mix combined with the differential keeps the Cub from yawing adversely.

It flies very well. Climb rate is excellent and surprising because of the slower speed. It climbs vertical for respectable wing overs. Rolls are slow but without much loss of altitude. Turns don't drop much altitude and depict a low wing loaded plane that flies on the wing. Spins are fantastic. Landings are graceful and slow and in fact are going to require raising the idle slightly due to the strong braking action of the prop.

The single problem with turning the engine for gas was getting the idle low. When leaning the LSN enough to obtain a smooth idle with the barrel nearer to a normal idle position, the engine would stumble on spool up. That was solved by programming the throttle servo to have a one second transition time. It still accelerates in about the same time, so there is really no notable flight difference, but the difference in the slow servo speed keeps the idle needle from withdrawing too fast and causing an engine stumble.

I really couldn't be more happier with this combo. .
Old 06-20-2017, 06:19 PM
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beachsideflyer
 
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Are you running two stroke oil premix in the Saito?
Old 06-22-2017, 02:30 PM
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AA5BY
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Originally Posted by beachsideflyer View Post
Are you running two stroke oil premix in the Saito?
Yes, at 15:1.

I have had a mishap however and broke the big end of the rod but do not know specifically why.The engine had been inspected after the 6th flight and all was well and the engine was being well lubricated and the big end was good. It broke right after takeoff on the 9th flight after reaching altitude and cutting the throttle to cruise. Neither crank pin or rod was scored and the cylinder remains good.

Though the bearings didn't feel bad in place, they were replaced and the back bearing after being cleaned up did sometimes chatter when spun. The suspect is bearing chatter fatigued the big end. The engine was run on glow for five years and had sit up for five and the bearings had some rust. It does not appear to be a lubrication issue.

At any rate, it is back together for another go and has two flights since. I did shim the jug .013 to drop the compression. We'll see.

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