A helicopter has a power unit supported on an arch support member above an airframe. The power unit may be moved forward and rearward by a trim actuator to trim the helicopter and tilted about pitch and roll axes by control actuators for directional control. The control actuators may form a parallelogram linkage with the arms of the arch support member. Each control actuator may comprise a hydraulic cylinder having two cavities and a pair of pistons which further divide each cavity into a pair of volumes. The arch support member and landing gear assemblies may connect to transversely extending struts. The landing gear assemblies may be designed to progressively deform in case of a high impact landing.