of course I can only google it but here is an article:

Abstract:

The images show that free-flying butterflies use a variety of unconventional aerodynamic mechanisms to generate force: wake capture, two different types of leading edge vortex, active and inactive upstrokes, in addition to the use of rotational mechanisms and the Weis-Fogh 'clap-and-fling' mechanism. Free flying butterflies often used different aerodynamic mechanisms in successive strokes. There seems to be no one 'key' to insect flight, instead insects rely on a wide array of aerodynamic mechanisms to take off, manoeuvre, maintain steady flight, and for landing.

download at http://users.ox.ac.uk/~zool0206/flow.html

Note, also:

The Hawkmoths (family Sphingidae), which can fly up to 30 mph (50 kph). The speed varies among butterfly species (the poisonous varieties are slower than non-poisonous varieties). The fastest butterflies (some skippers) can fly at about 30 mile per hour or faster. Slow flying butterflies fly about 5 mph.