I just have to say..for weeks now since I bought my new ul1 I been doing trials and errors to get this thing run fast and stable..NOTHING HAS WORKED!!I went through a dozen diffrent props and tuning adjustments nothing more than 38mph and very unstable at top speed!I ordered an x642 prop and the airfoil and now I'm getting 48mph with my heavy phone as a gps and full throttle stability all around the lake!! Took the phone out..even faster!! And still just as stable! I opened the package and I laughed at this airfoil but I'm here to tell you guys IT WORKS!!!Well worth the $20 and no more bashing my head against the wall!!