Skylane
Posts: 136
Joined: 10/11/2002
From: Tallahassee,
FL, USA
Status: online
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As is often the case, there are several variables to consider. I started flying my Cutlass Supreme with an OS 61 FX. However, I had trouble getting the CG right and I had to keep adding lead to the tail. Finally, I bought an OS 55 AX which is 5.1 oz lighter than the 61 FX with nearly the same power. I then was able to remove about 3 oz of lead from the tail and the resulting CG was perfect. The plane was now 1/2 pound lighter and it flies great. I am currently using it in SPA Sportsman.
Now, consider the Super Kaos. I flew one in competition in 1979 using a Kraft 61 - now RJL 61 and available from www.mecoa.com.
RJL 61: wt with muffler = 20.3 oz, rated hp=1.8 @ 16,500 rpm
OS 61 FX: wt with muffler = 23.6 oz, rated hp=1.9 @ 16,000 rpm
OS 55 AX: wt with muffler = 18.5 oz, rated hp=1.75 @ 16,000 rpm
Now, these days we prop our engines to turn a lot less than 16,000 rom. My 55 AX on 20% fuel turns 11, 600 rom with an APC 11x8. But, use the hp figures as a relative guide.
The 55 AX puts out a bit less than the 61s, as others have stated. But it is also lighter. However, the 55 AX compares favorably with the RJL 61 which I know flies a Super Kaos very well.
So, you have options. Slightly more power, but at a weight penalty. Either will work, but the lighter plane may well fly better. I know that that is the case with my Cutlass. A lot will depend on the individual airframe and equipment location. In the case of the Cutlass there was little choice as to where to put the RX, battery, and servos and it has a relatively short tail moment. Hence, the lighter engine works well. If engineered from the beginning, one could move servos, RX, and battery to the rear to help offset a heavier engine. Also, it is worth considering that the OS 61 FX really liked a 12x8 prop and you should consider ground clearance if using a nose gear while you are still at the building stage.
As I said, there are many variables and several different combinations can be made to work. Personally, in the older classic pattern planes that were designed around the smaller 60s of that era I find the 55 AX to be a good solution. But I will be using my 61 FX in my new Deception I am building and also probably in my new Compensator (I haven't decided on this one yet - I'll have to wait and see the all up weight and how the CG comes out.)
Jeff
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Skylane N9214G
SPA 243
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Super Kaos and OS 55ax?? - 
