Scale RCM 1/3 Scale Champ Build

[TonyHallo]

While I'll be, never heard of it, thanks Nick, might just make one. The kits are designed for 4" and 6" spinners, the plane doesn't have a spinner.

Edit
Just ordered two 50mm cooling fan on Amazon, $13. I think that the fans will be mounted to a sheet of 1/16" aluminum that will rest to the prop hub. Need a 3s battery.

 

[AKNick]

While I'll be, never heard of it, thanks Nick, might just make one. The kits are designed for 4" and 6" spinners, the plane doesn't have a spinner.

Shouldn't be hard, get a couple 12V fans and a 3S Lipo mounted on a V shaped wood stick that ties to the prop.

 

[AKNick]

Joe Lewis (aviator plus GP Engines exclusive dealer) told me back to back flights are never a good idea. I think because of heat soaking the motor/carb. Gotta let it cool down.

I do it all the time… just always mindful of the temps, cool down laps, and point the cylinders to the breeze after it lands.

 

[TonyHallo]

I have two temp sensors on each cylinder and they rarely get over 230f. It takes close to 9 minutes to fly the routine I'm using for the scale contests. I use full throttle twice during the routine, once during the climb in the Wing Over a short blast on the Overshoot. Other than that it is at half throttle. The 20 ounce tank is over half full after landing. I don't use much fuel so the temps are not a problem.
I suspect since I loose compression that the cylinders cool faster than the push rods. The cylinders are finned and are in the airstream path, the push rods are tucked under the engine baffle away from the airstream. The Champ baffle is shown on the top photo, the push rod tubes are right behind the servo screws below the cylinder.
I hope to fly the Super Cub soon. I used the air dusts that were made for the 30% Cherokee before reverting back to a two stroke engine. The ducts of the Cherokee are shown on the bottom photo. The duct hugs to cylinder and the push rod tubes are on the cold side of the baffle. If this install behaves differently, I'm thinking I might try increasing the intake valve lash a little. Current settings are .004" intake/.008" exhaust. I'm theorizing that the aluminum cylinders cool quicker than the steel push rods housed in tubes providing an additional level of insulation. Aluminum has twice the rate of expansion of steel, is it possible the intake valve is held open?

 

[dhal22]

That gets expensive!

No, full image.

Made it to the field yesterday for one flight, got rained out. The landing gear seems to be better, still need more work. Something odd about the engine, it starts cold and runs fine but after landing it will start hot but once it starts to cool it will not start until it cools below 100f. At one point during the cooling it looses all compression. It is still making the same power as new, swinging the 26x8 at 5600-5700 rpm, just doesn't like a warm start. I have found if I kill the engine after landing with the choke, it will start on the first flip after cooling down. If there are others at field it is no big deal but yesterday I was by myself and trying to squeeze another flight before the rain rolled in. I tried starting when the cylinder head temps were 120f, it started, ran at a high idle and quit. I gave up and headed home. Still wouldn't trade it for a two stroke!

Just now reading this. Hmmmm.

 

[dhal22]

That is the Roto 85?

 

[TonyHallo]

Correct

 

[Snoopy1]

Ok Tony I have read your engine problems several times and it has me stumped that as the engine heats up you lose compression. Just did some quick numbers and cannot see how the small difference in heat between the cylinder and the pus rods could be greater than 0.004. I can be wrong.
My thoughts are that the engine company got chemistry on the piston wrong. Or the sizing of the piston to the barrel at running temperature.

 

[TonyHallo]

Wouldn't you expect the cylinder to cool faster than the piston? It runs fine, just doesn't want to start if it is cooling down. I have the fans in hand and looking for the aluminum sheet, I hope to pick it up today on the way to the field.

 

[Snoopy1]

you would think so, I will try to explain. Ok in the air and you are flying at full throttle for a brief time. At this time all the dimensions of the cylinder piston and everything is at its designed Temperature and all dimensions of the engine are correct. (Clearances ) At this instant the piston is at a much higher temperature than the cylinder but all the dimensions are correct. Now you close the throttle 60% at this point the cold fuel and reduced air coming into the combustion chamber will cool the piston very quickly and will also cool the piston down quicker because it’s is much hotter than the cylinder walls. I hope this makes sense.
Please note this only happens if metallurgy selected for the piston and cylinder barrels and tolerances are not quite right.
let’s not forget that it is in a way a sealed chamber and the only points of leakage can be through the valve seats and the piston ring. Still think your idea of changing the valve clearance is the easiest and the quickest to do.