Hi Bill,
We don't know what type of airplane or airfoil you are using. If you are using a flat bottom wing with a strong Ryobi you should be fine. If you are using a weak Ryobi or symetrical airfoil you will be marginal at best. If the engine will spin an eighteen inch prop in the seven thousand rpm range I would consider it a strong engine. Spiiining a sixteen inch prop in the high six or low seven range might cause problems. If you put one strong Ryobi on each wing it would add six pounds and give a wing loading of about 35oz./sf. The power loading would be about 1/3hp per pound. This would be minimum aerobatics capable. If the 20 lb plane isn't a floater, you should look for a stronger motor like a Homey 33cc Bandit chain saw. Easy conversion and lots of power.

Dave