Technically, a snap roll only requires elevator (to induce the stall), and rudder. However, in an article written by Scott Stoops in a recent r/c magazine, he says that it's OK to "help things along" with some aileron. According to him, if it looks like a snap to the judges, then it's a snap. Now, I don't compete, and have no desire to, but I do like the look of a vicious, nasty, servo-stripping snap roll in an appropriate plane for the purpose, and to my untrained eye, adding aileron helps that cause. If you have RealFLight G3, try flying the Extra Special (a profile Extra) and play around with snaps....that thing can do some gut-wrenching snap rolls! My favorite is to get some altitude, head straight down at full throttle, and then simultaneously pop full up elevator, full left rudder, and full left aileron while dropping the throttle to zero at the same time as I apply rudder. Just looks so bloody violent! I love it! Not sure I'd try it on a real plane, but with adequate servos behind me, i just might one day!