The four- to six-place Helio Courier was developed from the Helioplane proof-of-concept aircraft (which now resides in the Smithsonian Collection), and has a long list of unique “Short-field Take Off and Landing (S.T.O.L.)” features. With large high-angle flaps, droop ailerons, and full-span automatic leading-edge slats, the Helio is tough to match in the short-field arena. Both the inboard and outboard sets of slats are controlled by the angle-of-attack of the airplane. When the pilot rotates to the landing attitude, the slats can be heard thumping against the forward stops in the deployed position. The deep chord ailerons are capable of effective control response at speeds clear down to stall! The gear can be equipped with a crosswind swivel device, so that the airplane can be landed with a slight side load. Helio Couriers and Twin Helios were both used in the Vietnam War.