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THE ROHR TWO-l75 (l974)

by F. Marc de Piolenc

This article appeared in its original form in the TWITT Newsletter. TWITT (The Wing Is The Thing) is an organization devoted entirely to all-wing and tailless airplanes. For further information contact:
P.O. Box 20430
El Cajon, California 92021

In 1974, Rohr Chairman Burt Raynes resolved to move Rohr into the light airplane market. Challenging the marketing base of the Wichita Giants would be an extraordinary step in itself. In order to succeed, Rohr had to offer a product so undeniably superior to its competition that prospective buyers and dealers could not resist it. Raynes summoned Walt Mooney and told him to come up with a quantum leap in light aircraft technology. It must have better performance, greater safety, accessibility and comfort, greater economy and lower production cost than any competitor. It must also, in Mr. Raynes' own words, "drive Cessna nuts." Walt was given an adequate budget and promised the services of any Rohr employee he needed. Within budget and on time, Mooney et al. built three airframes (two flying prototypes and a static test article) and two scale models (1/10 and 1/2) for water landing/takeoff feasibility tests (!). By the time the project ended (for reasons having nothing to do with the merits of the airplane), one prototype had accumulated 23 hours in the air.

Walt took full advantage of Burt Raynes' license to grab the best people Rohr had to offer. The key players on the project team were:

  • Walt Mooney, Designer and Project Manager
  • Bill Chana, Engineer and Project Administrator
  • Mike Voydisch, Propeller and Duct Design
  • Don Westergren, Test Pilot
  • Bob Fronius, Shop Foreman

  • At first glance, the Rohr Two-175 looks like an exercise in novelty for its own sake. It is a low wing Delta of stressed skin, fiber-reinforced plastic (FRP) construction, propelled by a buried pusher engine driving a shrouded propeller. Its nosewheel fairing doubles as a "rhino rudder." Both the wing and the vertical tail fold for transport and storage. Seating is side by side; access to the cockpit is through huge polycarbonate (!!) panels that open in gullwing fashion. The single stick is mounted centrally, accessible to both pilot and co-pilot. The landing gear is fixed and springs. Oh yes: the vertical tail is attached to the top of the propeller shroud. It is very hard to find a single conventional feature in the aircraft. In context, the bizarre features listed above seem not only sensible but ingenious. Take the Delta wing: the whole machine was intended to fit a standard FHA single-car garage, which limited span to 20 feet. Keeping the wing loading and structural loads within reason led to a deep root chord and sharp taper, and voilà! A Delta with folding outer panels. The vertical tail alternatives were a fuselage-mounted tail which would have to be huge to compensate for its shorter moment arm, or a small-area, high AR tail mounted on the prop shroud. The second choice reduced wetted area, so that's what Two-l75 got...with a hinge so it would fit that low-ceilinged FHA automobile hangar. But why composite construction? Surely that was a big technical risk in 1974. The point of that choice of manufacturing technique was to reduce the parts count and the number of manufacturing operations, which it did. The combination of adhesive bonding and molding major sub-assemblies in one piece kept parts count down to a miniscule fraction of that of a two-seat metal airplane, even counting each ply of laminate as a separate piece. Wing cores for the prototype were cut by the hot wire method familiar to builders of modern FRP homebuilt airplanes, but a faster and more elegant method had been worked out for production. The wing, which was the airplane's primary structure, would be built in clamshell molds. First, foam "sugar" would be placed in the mold. Live steam would then be injected to expand the polystyrene beads to the mold contour. The mold would be opened, the core temporarily withdrawn, and "B" stage fiberglass prepreg would be laid up on the inside surfaces of the mold. The core would then be reinserted and the mold would be closed, compressing the core slightly and ensuring even pressure on the laminate. Heat would then be applied in the usual manner to cure the assembly. The use of prepreg would avoid the weight gain problem of a wet layup and would prevent the laminate from becoming resin-starved. The process as a whole is simple, repeatable and cheap. Polycarbonate is both stronger and harder than Plexiglass, both very desirable features considering the huge "glass" area in the cockpit, but the conventional wisdom said it couldn't be molded. Bob Fronius and his team worked out a way and molded three sets. The landing gear needed only to be long enough for the 'plane to rotate without dragging its tail; prop clearance was not a problem. So why not have a fixed, unsprung gear, with the mains faired directly into the bottom wing surface and the fin-like nosegear fairing attached to the steerable nosewheel strut? Conventional wisdom said it wouldn't work. Of course it did.

    Deltas need high static thrust for takeoff, when their high induced drag puts them at a disadvantage vis-à-vis conventional planes. Getting the necessary thrust with reasonable horsepower and prop diameter called for a shrouded prop turning at 4400 rpm, faster than any conventional reciprocating airplane engine's output. Fate intervened: Lycoming had two special high speed engines which had been built for a contract that didn't quite pan out; Lyc let Rohr have them cheap. These gave 150 horsepower at 4400 rpm. They had special cam profiles, but were otherwise conventional. That stroke of luck left the problem of cooling a buried aircooled engine for long periods at zero forward speed. A generously-sized dorsal scoop and an exhaust-driven ejector nozzle did the trick. In typical 108°F weather at the test site, the engine never overheated. The cooling system was in fact a bit too effective, and the inlet area would likely have been reduced or louvered if the program had continued. A short extension shaft drove a four bladed propeller. Mike Voydisch's prop, shroud and six-bladed stator gave a 1280 fpm climb with one man on board. Stator and blade profiles and pitch distributions are critical. The wing and fuselage underbody formed a "chassis" which was the major structure of the airplane; the rest (except for the duct and tail) was a fairing. Plugs for non-structural panels were carved out of body clay, Detroit style. Even the engine mount, though otherwise conventional, attached to the structural "pan" instead of the firewall bulkhead. Fatigue tests on the FRP structure showed that a structure that could resist a specified static load had an indefinite life under alternating loads. NASA spent a lot of money some years later to find out the same thing.

    A few details of wing design: the outboard leading-edge droop/extension (Walt Mooney calls it a "dog tooth") was added to correct a pitch-up tendency at high angles of attack. The airfoil section is a symmetrical french curve special laid out by Walt according to exacting scientific criteria: if it looks right, use it. There is no spar, and the structural joint between the center-section and the outboard panel is made up of two FRP piano hinges adhesive-bonded to the stressed skin of the wing. The FRP hinges were developed for the Two-175 because of questions about the reliability of adhesive bonds to metal. Torsional loads were transmitted by trunconic bosses molded into the root of the outboard panel near the leading and trailing edges; these fit neatly into recesses in the center-section. Folding the wing was a matter of removing the lower hinge pin.

    Don Westergren, the project's test pilot, noted that despite the machine's unorthodox layout it had a very normal feel. He got used to the machine quickly and felt comfortable with it, despite the fact that the wing was completely outside his field of vision, requiring the use of the instrument panel as an attitude reference. Only in landing did the Delta's special character demand special technique: approaches must be flown at constant attitude and the machine allowed to flare itself in ground effect; hauling back on the stick C-l50 style would have "buried" the tail. Don had a chance to test engine-out performance somewhat earlier than intended when a driveshaft coupling failed shortly after takeoff. Don 180'd and put the 'plane down on the runway without damage. The pivot point of the nosewheel fairing/rudder had to be moved forward early on. Otherwise the tests were uneventful and there was little down-time for modifications.

    Early in the program somebody realized that the plane was capable of floating on its sealed, foam-filled wing. Tests with scale models showed that the airplane could take-off and land on water with the help of hydro-skis. This opened up the possibility of snow landings as well. With retractable skis, the Two-l75 would certainly have had the cleanest seaplane configuration ever seen.

    So what happened? Rohr got into financial trouble with other projects and the Two was a victim of the ensuing corporate belt-tightening. Reusable equipment was salvaged from the airframes and they were destroyed, as was every speck of technical documentation. The plane lives on only in Walt's collection of color alides, a few 3-views and memory.