Although the X-18 is generally associated with the name "Propelloplane"
, this term was in fact coined by Hiller to designate a whole series of earlier designs using tilt-wings in the same way as the X-18.
The AIAA's website
provides a very good starting place to these fascinating designs, whose sole misfortune was to be unproven compared to the then-in-use aircraft and helicopter configurations. The texts below were taken for the most part from that excellent website.
- First known Propelloplane was the Model 1035. This design was the first tilt-wing under an Army contract. Wing tips were fixed due to initial concern about tilted sections not bathed in propeller downwash. Studies on this design began in 1953.
- Second known Propelloplane was the Model 1045. This design was proposed to the Army in 1954 against its requirement for a utility helicopter. It featured a bubble canopy which provided excellent visibility. As with the earlier Model 1035, only the parts of the wings extending on each side of the fuselage tilted, while the central part stayed in place. Although convinced a tilt-wing would fulfill the mission, a conventional helicopter was advanced as the primary offering (the Bell UH-1, aka "Huey").
- Much bigger was the Model 1048/A Propelloplane. Hiller received the first government-funded tilt-wing study contract for that model, which turned company policy toward the transport size. The design used the existing Lockheed Hercules fuselage. The Office of Naval Research became fond of this 30-ton transport and encouraged more development.
- The Model 1051 Propelloplane, which was another tilt-wing design, possibly the one that was submitted in a competition held by the Office of Naval Research and was defeated by Vertol's lower-cost aircraft.
- The Model 1056, Model 1057 and Model 1058 designs, which were respectively a tiltrotor design, a tilt-wing design and a tilt-duct design in a Hiller study for the Navy Bureau of Aeronautics which compared several VTOL and STOL concepts. Ultimately, the tilt-wing concept was confirmed as the most advantageous to develop and build.
- The Model 1085 Propelloplane, a proof-of-concept research prototype designated as the X-18 by the US Air Force which utilized a modified fuselage from a Chase XC-122C Avitruc model with other parts coming from a Convair R3Y Tradewind transport. The Allison T40 turboprops and the propellers were reused from the Navy's unfortunate Convair XFY-1 "Pogo" and Lockheed XFV-1 "Salmon" VTOL fighters.