Impossible to be certain. "Like a Mustang P-51" can mean hundreds of different designs, with flying weights from 5.5 ounces to 55 pounds, some scale and some nothing like scale airfoils or wing loadings.

Can also depend on where you have the center of gravity along the wing chord, but as a rule of thumb I'd say it will bring the nose up initially, but if overdeployed (too much throw or at too high of a forward speed) will snap it down. Get up to 50 feet, slow doen to a crawl and apply flaps to see how it reacts. Start with a tiny amount and work up.