Dr. Bertelsen's « Arcopter » concept

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On January 23, 1958, the staff at the NACA Langley Aeronautical Laboratory entertained a visitor, a practicing physician from the small town of Neponset, Illinois. His name was William R. Bertelsen, and he had brought with him a small, powered model of a radical deflected-slipstream VTOL, called the “Arcopter” because of its arc-shaped wing. On hand that day were many luminaries of that era’s of aeronautical research: Richard Kuhn, John Draper, Kenneth Spreeman, Frances Rogallo, Charles Zimmerman, Robert T. Taylor, and others. Dr. Bertelsen hoped that his Arcopter concept might be included in the ongoing wind tunnel studies of various deflected slipstream prototypes. He started the glo-plug engine and flew the model, demonstrating its ability to hover out of ground effect. He made the model translate forwards and backwards, manipulating a canard vane by means of a slender piano wire. To place this event in the context of the times, the Arcopter demonstration at Langley took place three months after the launch of Sputnik (October 4, 1957), and less than one month before the rollout of the Ryan VZ-3RY Vertiplane in California (February 7, 1958). The first flight of the VZ-3RY at Moffett Field was still a year away (January 21, 1959).

The group at Langley was largely impressed with the Arcopter, but they also made it clear that the near-term chances for a study at Langley were zero. Langley Aeronautical Laboratory was about to become NASA Langley Research Center, only ten months hence, on October 1, 1958. Funding for aeronautics was being reduced to a trickle as the space program became an overnight leviathan. A photo of the inventor and the model that was flown for NASA Langley appears in Figure 24 below.

The Arcopter is distinctive because of its unique wing and flap geometry. Dr. Bertelsen reasoned that the arc shape should be the most efficient configuration for turning the circular slipstream of a propeller. It minimized the wing surface area, thereby also minimizing losses due to skin-friction drag in hover. The flaps were permanently extended on the proof-of-concept model. On a full-scale aircraft, they could rotate clockwise to retract into the main arc wing during transition to high-speed cruise.

The model’s wings were made of molded acrylic plastic. Span was 12 inches and the diameter of the propeller was 14 inches. Power was from a rear-mounted “Torpedo 35” glo-plug model engine [Fig. 25].

The aluminum landing legs doubled as handles for sensing forces and moments. Note the canard control surfaces for pitch, yaw, and roll control just behind the propeller in Figure 25 above. Since the canards were always bathed in the slipstream of the propeller, they
remained effective in hover [Fig. 26].

Locating the engine in the rear balanced the model perfectly so that the arc wing did all the heavy lifting. The horizontal canard planes were basically unloaded, except for simply pointing the nose up or down. The canards also served to reduce the torque generated by the large propeller, although they did not eliminate it completely. Plans for a full-size prototype call for a dual-rotating, controllable-pitch propeller to eliminate torque and augment roll control with differential pitch [Fig. 27].

The Arcopter configuration takes advantage of a large-diameter propeller (14 feet) for maximum efficiency and features centerline thrust. No cyclic pitch mechanism is required. The twin engines could be modified automotive piston engines to minimize cost. However, the most important aspect of the Arcopter system is the force focus, which directly addresses the pitching moment problems that plagued the VZ planes, i.e., the Ryan Vertiplane and the Fairchild Fledgling.

Dr. Bertelsen had studied the works of Richard Kuhn, John Draper, Kenneth Spreeman, et al, on large-chord slotted flaps (references 1 and 7) from 1955 and 1956. He was well aware of the trim problems associated with the deflected slipstream approach that these NACA researchers documented during exhaustive trials in the Langley wind tunnel. To minimize the trim problem, Dr. Bertelsen lowered the center of gravity and mandated a radial flap extension path [Fig. 28].

It can be seen that the radial deployment of the flaps keeps the individual flap resultant force vectors focused through the center of gravity during all stages of extension and retraction. Therefore the vector sum of the thrust and wing/flap vectors is a vertical lift force that continuously coincides with the center of gravity. For pilots it means that the aircraft will remain in trim, with a zero pitching moment as the flaps are extended and retracted. As a bonus, there is static angle-of-attack pitch stability, even at very low forward speeds (unlike the VZ planes). This beneficial focus of forces is referred to as the “Bertelsen Effect” in the original U.S. patent #3,597,614, filed in 1961.

The Langley Arcopter demonstrator model was not the first Arcopter to lift its own weight. A smaller model with wings and control surfaces fashioned from X-ray film flew in 1957 [Fig. 29]. This model was powered by a Cox “Thimble Drome” 049 glo-plug engine. Fig. 30 below is a photograph of the model flying in the waiting room of Dr. Bertelsen’s medical office.

While flying the glo-plug models, Dr. Bertelsen found that the turning angle of the slipstream was reduced somewhat when descending into ground effect. However, unlike the VZ planes, the Arcopter seemed to be able to lift more weight in ground effect rather than less. He realized that there was a potential for a new class of aircraft designed to operate in ground effect full-time, lifting big payloads with minimal installed power. When it became clear that no NACA support for Arcopter development was forthcoming, Dr. Bertelsen shelved the Arcopter idea and began his alternate career as a pioneer of ground-effect machines, otherwise known as air cushion vehicles or hovercraft.

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Arc wing development then languished for the next fifteen years. In the mid-1970’s son William D. Bertelsen began working with his father to revive the arc wing. Due to the chronic shortage of funds, the only avenue open to the Bertelsens was to develop an ultralight version of the arc wing. This they did, beginning with gliders and kites. The static pitch stability of the arc wing was documented by flying a ten-foot-span kite without a tail [Fig. 31].

The ultralight arc wings were tension structures. W. D. Bertelsen found that the angle of attack at which the wing stabilized could be controlled by varying the tension on the trailing-edge anchor straps. More tension increased the stable angle and less tension decreased the stable angle. In 1975, a 4-foot-span version of the fabric arc wing was tested in the 5-foot by 5-foot wind tunnel in Aero Lab B at the University of Illinois in Champaign-Urbana, under the supervision of the late H. S. Stillwell. The test series was significant to the extent that it provided the first official documentation of the static pitch stability of the arc wing. W. D. Bertelsen
presented the results of the all the ultralight arc wing experiments in a 1979 paper for a NASA conference on the “Science and Technology of Low Speed and Motorless Flight” at the Langley Research Center, March 19791.

In the Fall of that same year, W. D. Bertelsen entered the University of Illinois to study aeronautical and astronautical engineering. In the Spring of his senior year, he headed a student team to design a 400-knot utility VTOL based on the Arcopter for the 1982 Bendix senior design competition2. A mockup of their proposed aircraft is shown in Fig. 32 below.

This rendition of the Arcopter featured a twin-boom tail structure to enhance directional stability during highspeed cruise with flaps retracted. Note the dual-rotating propellers. Power was to be provided by two rear-mounted GE T-64 shaft turbines. The Bendix design project afforded an opportunity to measure the efficiency of the Arcopter wing and flap system in converting propeller thrust into lift. Dr. W. R. Bertelsen built an electric-powered model of the wing and flap system [Fig. 33].

Wingspan and diameter of the dual-rotating propellers was 24 inches. With this apparatus (consisting of a wing and two flap elements) it was found that 72% of the propeller thrust could be converted into lift by inclining the thrust axis +30°. Another test was done with the wing and only one flap element. This second series showed an increase in lift for the same inclination of the thrust axis. With just the wing and one flap, 82% of the propeller thrust was converted into lift. The data from the electric model bolstered the claim that a full-scale UI VTOL could meet the Bendix design criteria. These included taking off from an area only slightly larger than the aircraft with a 3,000-pound payload on a 90° F day. However, the UI VTOL did not win the 1982 Bendix competition.

To date, the Bertelsens have not received any outside funding for further development of the Arcopter VTOL concept. Part of the reason may be because the Arcopter idea seems to be a difficult one to communicate on paper. There is also the stigma of the failed VZ airplanes. But the deflected-slipstream approach remains the only strategy that offers vertical takeoff with high-speed cruise at low cost. With the advent of computational fluid dynamics and lightweight composite materials, the deflected-slipstream VTOL merits another look.



Article reproduced from History of Deflected Slipstream VTOL Aircraft by William D. Bertelsen and William R. Bertelsen

Useful references:

• [1] Bertelsen, William D., “Introduction to the Arcopter Arc Wing and the Bertelsen Effect for Positive Pitch Stability”, Hanson, P.W. , compiler, Science and Technology of Low Speed and Motorless Flight, 1979 March 29-30, NASA Langley Research Center, Hampton, VA, NASA Conference Publication 2085, Part I.
• [2] Bertelsen, William D., editor, “UI VTOL: Arc WingVertical Takeoff Subsonic Utility Aircraft”, submitted to the AIAA/Bendix Student Design Competition, Department of Aeronautical and Astronautical Engineering, University of Illinois at Urbana-Champaign, IL, June 1982.