I was told when converted my first motor that I needed to put it on a plane that was 1000 sq in or larger. What would happen if I put one on a plane with less than 1000 sq in wing? The reason I ask is I have a Extra with a 72.5" (837 sq in) wing that I would like to put my 33cc homey on. The plane calls for a 120-140 4c and a 33cc seems just about right for a 120 size 3d plane. Am I right or wrong?

Thanks

Its not so much a firm 1000 sq. in. area, as it is the wing loading for the particular airframe. If you are not pushing your luck on wing loading, you'll be fine. Your example is pretty close to a Carl Goldberg Extra 300 at 852 sq. in., which flies great at 9.5 lbs or less. So there is a sample weight vs wing area (or wing loading) to shoot for. If your conversion engine will put your plane well over this weight, it's a crap shoot. The engine will pull the weight around no problem, the airframe just might hate you for it, and let you know all about it during stalls, and landings I personally enjoy test flying overpowered, overweight planes for their size....but they really never fly GOOD, ya know? Sometimes you get lucky with larger warbirds that can literally weigh 20 or 28 lbs and still fly great either way. But smaller aerobatic models usually are not so forgiving.

Chad

Thanks Chad, According to the box it's 11.5 lb with 31.58 oz sq/ft wing loading. The 33cc homey weighs aprox 53 oz. How do I calculate the wing loading for this? Also what wing loading should I try and shoot for? The 31.58 oz or lighter if possible?. Forgive me but I'm in uncharted waters so to speak....


EDIT:

DUH!!! Sorry for being lazy I just looked up how to calculate wing loading. I am concerned though that the example you provided is two pounds less. I really don't want to have to land a rocket.

If it were me, I would aim for anything less than the box says.

The box figures are usually smallest motor size possible and maximum weight.

I have a clipped wing cub with 1380 sq.in of wing.
I had a ST-90 on it with 1 1/2 pounds of weight on the fire wall.

I put a 25cc Toro on it from Brillelli. But by the time I moved the batterys and receiver 8" away from the EI
I was not able to shead any weight. It flew ok untill it tip stalled on a dead stick.
I was lucky it only ripped the landing gear off and shattered the front 8" of the plane.
\No damage to the engine though.

I extended the nose 3" and lost all the added weight.
Now it flys slow and doesn't seem to want to slall at all.

Here a pound and a half sure made a difference.

Chad has it about right...

I'll add that it seems to me that there are two issues of concern. The wing loading as mentioned and the thrust / weight ratio, which defines the vertical performance required of an aerobatic flyer. If one looks at the suggested list for weed conversions, they don't include aerobatic models because the thrust / weight ratio is poor.

I followed the guidelines and put a converted weedie on a Phaeton 90 that has nearly 1500 inches. The generous square inches handle the extra engine weight without problem even with heavier tex covering but the thrust / weight ratio is poor and while the plane flies excellent on the level and has a great glide slope for landing, vertical is limited to realistic rather than performance.

I and another at our field both have Hanger 9 PT-19s. I've a 1.2 4c on mine and he has a Ryobi 31. Both fly fine on the level and land well but mine has a better thrust / weight ratio and will pull larger loops. Mine also will do lumcevaks and will easily recover from any attitude but his is precarious about coming out of some positions, so he doesn't do them, which is a shame because the PT-19 is capable of doing some of the prettiest lumcevaks.

So, while the conversion might be ok with the wing loading numbers, the Extra is an aerobatic plane that also needs good thrust / weight numbers for the performance flying it is designed for.

Another example of this is the P-51 Mustang. Commissioned by the UK but built in America, it flew ok but lacked performance needed as an aerobatic fighter plane. That was solved by replacing the Allison engine with the Rolls Royce Merlin, increasing the thrust / weight ratio.