Butterfly eVTOL Development Moves Ahead With Toray/Overair Partnership There is a company currently working on development of an eVTOL called the Butterfly. Overair is actively developing the prototype and they have recently been in the news thanks to a partnership forged between themselves and Toray -a prepreg specialist-. For the purpose of the development of the Butterfly, Toray is working with Overair using the Toray T1100/3960 prepreg system, a material that is formulated for high-performance aerospace applications where the optimal ratio of strength to modulus properties is critical. Toray's 3960 is a highly-toughened 350°F/177°C cure epoxy resin with a glass transition temperature (Tg) of 400°F/204°C. It is optimized for increased performance and allows for autoclave and out-of-autoclave curing.