Prop Dimensions

Just a general query regarding prop sizes. Both my Chipmunks have OS GT33 engines with 18x8 Menz wooden props which pulls them around happily or so I thought.

Then last week talking to Alan F who is also using an OS GT33 but swinging a "club" of an APC 20 inch. His plane flew well too. When I asked why such a size prop his answer was to give the engine some loading. This gave me something to think about.

Have I under propped the Chippys? Not sure of Alan's model weight but my Chippies come in at about 14lbs which I would think is heavier than Alan's model.

Suggest you start by establishing the RPM.
e.g. The Mintor 33cc in my Super Stearman weighs 14lbs and turns a 19x 6 @ 6300 rpm without excess prop or exhaust noise so I estimate you should be getting a bit more than 6500.

An 18" X 8" might be the choice for running in as it would keep the engine lightly loaded. The GT 33 has lots of "grunt" compared with other engines in the same size range, perhaps going to 20" X either 8" or 10" would be possible, all depends on how you want to fly the model, as long as the tacho shows that the revs are in the "comfort zone". Go for it.

Phil, the OS break in prop is 18*10 so I think you are running it light at 18*8. My 20 * 8 prop size is fine for the motor but the APC is heavy so motor pick-up isn't as crisp as I would like. OS suggested range is 18*8 - 20*10

The Votec is 13 1/2 lb

Ordered a 20x8 Menz to fit to Pennzoil in first instance to how it performs.

PhilF51 wrote: Ordered a 20x8 Menz to fit to Pennzoil in first instance to how it performs.

I'll be interested to see how you get on with it Phil. I might look for something lighter than the APC in the future.

kevinross wrote: Suggest you start by establishing the RPM.
e.g. The Mintor 33cc in my Super Stearman weighs 14lbs and turns a 19x 6 @ 6300 rpm without excess prop or exhaust noise so I estimate you should be getting a bit more than 6500.

Kevin - 14 Ibs is a hell of an engine weight !!!
Colin.