RCU Forums - View Single Post - Phoenix Models Sukhoi SU-31 CG and engine Q's
Old 12-05-2005, 08:30 PM
da Rock
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Default RE: Phoenix Models Sukhoi SU-31 CG and engine Q's

My instructions had a cute little picture that shows the model as having 0 up/down thrust and 2 degrees of right thrust. It actually didn't say anything anywhere about tuning thrust into the model or not tuning thrust into it, either way. And most people would expect that a model that comes with a firewall type engine mount is going to have the suggested thrusts built into the firewall if the model is prebuilt and advertised as an ARF.

The instruction booklet also had a paragraph about installing the engine mounts and has a picture to illustrate. The picture shows the mount being centered on the pre-drilled access hole in the firewall. It shows the hole offcenter to the right and down. I happened to take a digital picture of the firewall of my airplane to use as sort of a blueprint. I then printed it out at the exact size and did my locating with it. Before drilling, I plugged in the wing and did incidence measurements of the wing and stab and worked out the C/L of the fuselage. They were close to zero zero zero, so I use the zero fuselage line and measured the firewall's thrusts. I have a very accurate incidence measuring gauge. My model's firewall has a measured 3.5 degrees downthrust and a little over 2 degrees right thrust.

If you look at the picture below you'll see my picture and where I drew center lines to locate the center of the firewall. I figured to have the engine's prop shaft wind up squarely in front of the center of the fuselage so as to be centered in the cowling. The cowling would be lined up with the fuselage so the fuselage sides were excellent reference lines. To build your plane so that the prop is centered in a cowl like that, if your plane has right thrust in the firewall, you move the motor mount off the the right (when viewed from the front) based on the right thrust angle and if it has upthrust you move it down etc. If you look at the picture of the Phoenix firewall and hole, it's obvious they positioned the hole for right thrust and up thrust.

Problem is, they show the model as having no up/down thrust yet have one picture showing the engine mount install based on up thrust and the actual model has the "locating" hole positioned for up thrust. And the model has downthrust. Anybody following the booklet for my model is going to have an offcenter prop.

And your booklet and model are bigtime different than mine. Wonder how many different runs they made of this model? Good thing your instructions matched your plane. Mine flies great with right and down, too. So wonder what else differs between our two?
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