Butterflies display a range of flight behaviors, from fluttering flight to long distance directed migration. Butterflies also look like no other flying animal, with unusually large and broad wings relative to their body size. Wing shape influence aerodynamics and butterflies have been suggested to use almost any aerodynamic trick in the book to boost flight performance. However, quantitative measurements of the aerodynamics of free flying butterflies are lacking. Here we present direct measurements of the forces, from particle image velocimetry, generated during take-off flights in Silver-washed fritillary (Argynnis paphia). We find that the butterflies use upstroke wing claps to thrust themselves forwards while the downstroke provides the majority of the vertical force. We also suggest a mechanism for effective and efficient wing claps, using flexible wings, which we test using a robotic clapper. Together our findings suggest butterflies evolved an efficient wing clap, which may explain their unusual wing morphology.