I am getting back into the hobby after many years. I am building a Top Flite model of the P-51 mustang, one of the older kits(60"span) I am using a 72 four cycle engine and want to use a four bladed prop but have no idea what would be the best size.
Any help will be appreciated. Thank you.

John Murray

A 72 four stroke will swing a 13x6 two-bladed prop. To convert this to a four-bladed prop, subtract one from the diameter and two from the pitch. So you would need a 12x4 four-bladed prop. Here is a conversion chart from Zinger:

http://www.zingerpropeller.com/_2_blade_.htm

There might be some other conversion charts out there.

Here is a comment from O.S. Engines: http://www.osengines.com/faq/product-faq.html#q155

IMHO, I don't think a 72 four stroke will be enough to power your 60" span Mustang. You might fly at scale speed, but you might not have great vertical performance.

About 10 years ago I built a Midwest Mustang Sport .40. It had a 54" wing span, a "Fox .40" 2 cycle and flew great with a 2 blade 10-6 prop on 10% fuel. I went to a 2 blade APC 9-7 prop and 25% fuel and it screamed!!!! I flew this plane often for 4 seasons and killed it the first flight of the 5th.
A little off the subject... This was the first plane I ever built or flew (not counting Cox control line) I learned to fly it on a buddy box at the field in Oyster Creek TX.
As far as the 4 blade prop, I'm now building a 102" Nosen P-51 and plan to use a 2 blade on it. To me, it doesn't seem worth the trouble to swing a 4 blade prop unless one is going "all the way" with the scale look.
I hear myself talking,,,,,, all the while, I'm planning on putting a scale cockpit in it....................