So, with my wife Anita and daughter Rebecca on a road trip to the shore, I had a day all to myself to break in my O.S. 75AX and fine tune my propeller selection using a digital tachometer to get the engine numbers…
And talking about numbers:
- Displacement: 0.75 cu in (12.3 cc)
- Bore: 1.02 in (25.8 mm)
- Stroke: 0.93 in (23.5 mm)
- Practical rpm: 2,000-16,000
- Output: 2.4 hp @ 15,000 rpm
- Weight: 19.4 oz (550 g)
- Glow Plug: O.S. #8
- Fuel Used: Byron AeroGen2 15% Nitro / 18% Oil
- Recommended Props: 14×6, 14×8, 15×7
First I used the Evolution 14×6 prop to break in the engine and I ran three tanks through the engine running the engine slobbery rich for two minutes (4-stroking!) and then leaned it out to max rpm for 2 minutes. I then richen it up and repeated the cycle for the whole tank. I then let the engine cool off for 20 minutes while I grabbed some lunch!
Fired up the engine again and tweaked the needle again for max rpm and then backed off about a quarter turn. My Master Airscrew 14×6 prop earned about 9,200 rpm and idled a smooth and constant 2,900 rpm. The Evolution 14×6 was very close the Master Airscrew with a 9,100 top end and a 3,000 rpm idle.
I also tried a few different glow plugs which, for the best idle and overall good throttle transition turned out to be the O.S. #8 plug. Next I switched to what I thought was my most promising propeller, the wood Xoar 14×4. Remember I am proping the engine for fun fly so I am looking for a good idle and transition and a powerful climb, not max rpm. Of course this will require a trip or two to the flying field to see what the ultimate winner is.
The Xoar 14×4 tached up to 10,800 and had a very low idle of 2,100. The low end needed to be tweaked a little as the engine started to load up a little. I leaned it out about 1/16 turn. The engine died with at an idle of about 1,700.
So with the fun fly prop looking good I dropped the diameter to see what a sport flying prop could do. I went to the APC 13×6 and was pleased to see the engine rev up to 11,350 without any adjustments. Tweaking the needle a little got a solid 11,550 rpm with excellent throttle transition from a 2,400 idle. I think this is the prop I will be using for the first test flight.
Fuel consumption seems pretty good also, as it took only 3/4 of a 10 oz. tank to do all the prop tests. I used both the O.S. #8 and an Evolution glow plug but I could not see any obvious differences! My Ni-Cd glow driver battery died so now I am charging it up as well as a second backup driver.
Next stop–CCRCC Farmington Flying field!! See ya there.