ORIGINAL: flystunt

I am just starting a Wing mfg. P-5 build and have not settled on the optimum power plant. At 640 squares, this is not a large pattern airplane by any means. I do want an SPA legal airplane and will not pipe the motor (Probably a tongue muffler that weighs less than an ounce and it will have retracts, just left down at contests). Otherwise, I want to wind up with as light of a model as possible but yet have good power to weight. (The airframe should come out pretty light as I have ample amounts of 5-6 lb balsa for sheeting, control surfaces, etc. from my C/L stunt days)

I was perusing one of the Milwaukee hobby shops Greenfield News and Hobby and came across one of the new Evolution 60NXs in his display case. I had read what Mike had to say about this motor in the thread on the Horizon Hobby P-7. Needless to say, I was so impressed with the small size I brought it home. In comparing with the OS 55AX and the benchmark OS 61 SF, some interesting numbers become evident:

First, the Evolution is amazingly small? Side by side with the 55 AX the Evo is actually a touch shorter! Mounting lugs are the exact same dimension as an OS 46 case size! With minimal relief to the inner inside of the mount, both the 55AX and Evo 60 will bolt into a 46 mount pad. One difference worth noting: The Evo lugs are drilled for 8-32 mounting bolts while the 55AX is drilled for 6-32s. Funny Evolution would choose the larger lug holes and then market it as a bolt in replacement for a 46. I will wind up shimming mine with an aluminum tube if I use it.

Second: The carb opening on the SF is way bigger! If air going in is any indication of power (we know it is), the 61 SF is going to out muscle both the Evo and OS 55.

Third: Weights. Not advertisements, but actual measured weights (Ohaus triple beam balance) without muffler: OS 61 SF - 558.0 grams or 19.6 OZ. Evo 60 - 406.2 grams or 14.3 OZ. OS 55AX - 400.0 grams or 14.05 OZ. Now the OS 55 was missing its glow plug and the Evo is stock with the rear needle. I am going to remove the bracket that holds the rear needle and tube which should shave another 10-15 grams. All-in-all I bet the Evo and OS 55AX come out nearly identical in weight when I finish with them.

As much as I love the SF, I am wondering if the Evo or 55AX would be a better balance for the P-5s 640 squares. I am still stunned by the nearly 6 OZ weight difference between the new gen ''light cases'' and the tried and true SF. Pretty amazing! That delta can go a long way towards any power deficit!

Any thoughts? I know Crank mentioned the 55Ax as a good choice for the P-5. Other opinions?

Sean

pics are are side-by-sides with the Evo and SF and then the Evo and 55AX. Last is a size comparision between the Evo, 55AX, and a standard 46!