Hi Walter,
I'm glad you made your post. You were the inspiration for Will Moore's Big ECL twin bomber You proved to all of us how well a twin ECL plane could fly. :-)
It is interesting that you are getting such good performance with the smaller 9.6" dia. props. What RPM are you turning them? We are turning our 12 x 6 APC-E props around 9,100 RPM with good results. Will's counter rotating props have worked very well, and we think they help. They sure don't hurt anything. ;-)
I think one of the big benefits to the twin ECL setup is the same benefit we get on FS twins; the accelerated air over the wings increases lift. This means a lower AOA, which should mean less power req. for the same AS. This is just what we need in CL. Our speeds are somewhat fixed around 55 MPH, so any increase in lift is a solid PLUS. This benefit may be why you can use such small dia. props and still get good performance? Will's two 12x6 props pull his big 74 oz plane with authority, even using the smaller Hacker motors.
Igor's point is a very good one. We get a very BIG (exponential?) increase in power by increasing prop dia. An increase in Prop Disc area is important, and will help, but the real benefit comes from increasing the dia. of the disc. .... For example:
A 9" dia prop has an area of 64 sq. in. Two have 128 sq. in. of area. BUT, at 9K RPM, the tip speed for each prop is only 240 MPH. ..... A 13" prop has a similar disc area, 132 sq. in. BUT the tip speed is 348 MPH. That is more than 100 MPH faster. Since most of our thrust comes form the outside 20% of the prop, our increase in area occurs where it is really producing a lot more thrust because of the rapid increase in prop speed out there. ..... I think this is what Igor was referring to in his post?
You are right about the LG issue. The smaller props do make it easier. I'm willing to have "Stork" gear for all the extra power the larger props give us, but it does look a little dorky compared to the sleek, low slung look of the wing mounted lower gear models. There are other issues that may limit how big a dia we can go to. The torque and P-factor problems are magnified as we grow our props. I use 28" props on some of my planes and often have to use R RD, so maybe we will have to start using Rabe RDs on our BIG prop ECL planes in the future?
Again, thank you for being an ECL pioneer and showing us the way! :-)
Regards,