I have a 40% sport scale homebuilt kit here. It is a low wing aircraft with side-by-side 2-place seating. It is the Piel Emeraude airplane.

http://www.fly-imaa.org/imaa/hfartic.../v9-3-19a.html

126" wingspan 35-40 pounds RTF. Recommended engine size is 3.1 - 5.2 cubic inches. (50cc - 85cc) I don't build heavy, so let's assume it weighs a maximum of 40 pounds RTF.

I want more than scale performance. I want knife edge performance. Not slow high-alpha knife edge. It's not a 3D plane. I would really like medium to high throttle knife edge performance.

Consider that I fly at 6000 feet and there is about 10% - 15% drop in engine performance here.

What engine would you put in it?

Thanks

BME 116 or BME 150LT

I'd go with a 140 to 160. 40 pounds is 40 pounds. I know the guy that did the review in High Flight. He's not one to under or over power a plane but he flies at lower elevations.

Wow. Thanks guys. I was thinking of going with a 100cc twin or even a 90cc single. But your recommendations of 140-150-160cc have me tossing my initial idea right out the window.

You think 150cc will give good sport power and not unlimited, right? I don't need unlimited.

I'm guessing too but here's why I'm guessing this way. A KE pass is the same as a slip. You use a slip to slow a plane for landing. A plane that can barely hover looses speed in a long KE pass. At 40lbs a 150size motor would be needed to maintain a KE pass. At 35lbs a 100 cc class engine should do the trick.
It would be interesting to know if a fullsize top of the line aerobatic airplane can hold a KE pass indefinately. I am pretty sure they couldn't just a few years ago. Not untill they were allowed to hop up the IO 540 over about 330 HP.

Nope
Full sized Extra 300 series ("L" configuration) fitted with original IO540 - definitely not
Same plane - retrofitted with Thunderbolt Engine upgrade (330 Hp version of the I0540) and MTV 9-B-C/C200-15 Prop - not indefinitley but certainly for a lot longer.

But then again - it doesn't prop hang, torque roll or have unlimited vertical indefinitley either. Fast rolls around any of the Axes and spins are good fun though

Pilot weight - a little over 125kg. - Aerobatic fuel plus about another 20 gallons on top.

Me thinks photos 2&3 are of Cap 10's

Me thinks you are correct.
But the Piel Emeraude was a predecessor to the 10 (with most similarities stopping at the cosmetic points).

Everything is going to depend greatly on the wing loading. If it's in the 36-38 range you might get away with a 120. For true scale the 120 might just do it. For anything beyond scale it will need more engine. Don't forget the density altitude, right? Even with a 120 you're going to need some runway.

One of the best "vintage" aerobatic trainers by the way. I used to love the lines of the Cap 10 (full scale) back in the day. Cool airplane!

The Emeraude led to the development of the Cap airplanes. Amazing huh? The Mexican Air Force still uses them for training. I am planning to bash mine into a Cap 10B. That explains the desire for more power.

I was thinking a 120 would be pretty snappy, but I really have no idea. 40 pound airplanes aren't something I have ever flown. After asking a couple local friends, seams all the Caps, Yaks, Extra in the 40% size are using 150cc - 170cc engines, some of them are on pipes too. Those do have unlimited power at this altitude.

I'll start looking at 120cc - 170cc engines. Thanks guys. [8D]

Happy to help, as always.