Tiltotor Thrust Control
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THRUST CONTROL OF VTOL AIRCRAFT – PART DEUX DANIEL C. DUGAN [email protected] Aero Engineer/Research Pilot´‡ NASA Ames Research Center Moffett Field, California ABSTRACT Thrust control of Vertical Takeoff and Landing (VTOL) aircraft has always been a debatable issue. In most cases, it comes down to the fundamental question of throttle versus collective. Some aircraft used throttle(s), with a fore and aft longitudinal motion, some had collectives, some have used Thrust Levers where the protocol is still “Up is Up and Down is Down,” and some have incorporated both throttles and collectives when designers did not want to deal with the Human Factors issues. There have even been combinations of throttles that incorporated an arc that have been met with varying degrees of success. A previous review was made of nineteen designs without attempting to judge the merits of the controller. Included in this paper are twelve designs entered in competition for the 1961 Tri-Service VTOL transport. Entries were from a Bell/Lockheed tiltduct, a North American tiltwing, a Vanguard liftfan, and even a Sikorsky tiltwing. Additional designs were submitted from Boeing Wichita (direct lift), Ling-Temco-Vought with its XC-142 tiltwing, Boeing Vertol’s tiltwing, Mcdonnell’s compound and tiltwing, and the Douglas turboduct and turboprop designs. A private party submitted a re-design of the Breguet 941 as a VTOL transport. It is important to document these 53 year-old designs to preserve a part of this country’s aviation heritage. INTRODUCTION During the design phase of an aircraft, the control of Vertical Takeoff and Landing (VTOL) aircraft, thrust should be a straightforward process. If the aircraft is to including helicopters, often use a conventional collective be an airplane, throttle(s) would be the logical choice – “stick” or Power Lever – or throttles of various designs. The forward for increased thrust and aft to reduce thrust. This convention “Up is Up and Down is Down” has served the holds true for jets or piston powered aircraft with some VTOL community well for many decades. There have also variations in the throttle design. Most use a conventional, been successful designs that use a type of throttle where vertically oriented throttle(s) on a quadrant to be operated by “Forward is Up and Aft is Down.” All seem to work the pilot’s right or left hand. Many Cessnas, however, use a relatively well. One notable and unsuccessful adaption was push-pull rod with a knob to grip it and it is moved forward found in the MV-22 Osprey. The original collective design into the instrument console for increased power and aft to was replaced before the early Full Scale Development (FSD) reduce power. models were built. In this case, “Forward and Down was Up,” and “Up and Aft was Down.” Many thought that this was a recipe for disaster and they were proven correct. In one emergency situation, control of an MV-22 was lost Presented at the Fifth Decennial AHS Aeromechanics during its first flight due to a lateral control out-of-phase Specialists’ Conference, San Francisco, CA, January 22-24, condition. The instinctive reaction of the pilot caused him to 2014. This is a work of the U. S. Government and is not abruptly move the Thrust Control Lever (TCL) the wrong subject to copyright protection in the U.S. way, which resulted in the loss of the aircraft after he had 1 managed to put it safely on the ground for an instant. After for increased lift. In both of these prototypes, the pilot stood that mishap, a long and expensive redesign was undertaken upright while controlling the platform. There was a cross which resulted in a pure forward and aft longitudinal shaft included in this design. In the final configuration, the movement of the TCL with no arc. VZ-1E (Figure 3), the duct was deeper still and the pilot was seated and used conventional helicopter controls. The first Some designers of past VTOL aircraft did not want to two variants were kinesthetically controlled by the pilot by deal with the Human Factors issues so they included both leaning in the desired direction of flight. The VZ-1E, throttles and a collective control. Others designed unique however, had vanes in the duct to permit control of the flight controllers, but the convention was always Up and Down or path. Forward and Aft. Various schemes were sometimes used to “gang” the throttles together and even operate them as a type of collective until the forward flight or airplane mode was reached. The designs presented in this follow-on paper include many widely varying types. From the military’s “flying platform” concepts, which were actually flown, to designs that never left the drawing boards. None were ever truly successful and adapted by the Armed Services, but a lot of Research and Development dollars were expended. In the previous paper, only two concepts discussed were successful – vectored thrust as embodied in the Harrier series, and the MV-22 tiltrotor. The tiltwings, tiltducts, and others were discarded. Some of these were evaluated in flight and the rest faded into obscurity. Figure 1. Hiller Pawnee Variants This paper will also present some heretofore little known designs from the Tri-Service VTOL Transport competition from 1961. There were many “players” in this competition, but the only entry built was the Ling-Temco- Vought XC-142. Five prototypes were manufactured and all but one came to an untimely end. It resides in the USAF Museum at Wright-Patterson AFB, Dayton, Ohio. FLYING PLATFORMS In the fifties and sixties, the U.S. Army pursued the development of “Flying Platforms.” The Navy, Air Force, and NASA were also interested in the concept and contributed funding for some of the research. The ducted propeller was often used to augment lift. Many were flown and tested but none proved practical and they were either scrapped or sent to museums. In some cases, it was difficult to determine the thrust controller, but a motorcycle type twist grip throttle was often used. VZ-1E “Pawnee” The Hiller VZ-1E was the third variant of this ducted fan platform and it used a conventional helicopter collective pitch lever. Two earlier variants used a twist grip throttle (Figure 1) The first variant, the VZ-1 (Figure 2), was funded by the Office of Naval Research (ONR) and was powered by two 40 hp engines. A 5 ft diameter duct enclosed two contra- rotating rotors. No cross shafting was included in the design. This was unsafe and unacceptable because each rotor was driven by one of the engines. The second Pawnee had a Figure 2. The Hiller VZ-1 duct diameter of 8 ft, three 40 hp engines, and a deeper duct 2 Figure 4. The De Lackner Aerocycle Figure 3. The Hiller VZ-1E UNIQUE VTOL AIRCRAFT De Lackner Aerocycle A number of unusual and innovative VTOL aircraft were A National Advisory Committee for Aeronautics (NACA) studied to document the type of thrust control used in their engineer, Charles Zimmerman, proposed that the rotors of a design and operation. VTOL aircraft be located on the underside of the air vehicle. Kinesthetic control was proposed for aircraft control. The Ryan VZ-3 “Vertiplane” U.S. Army ordered a number of the DeLackner DFH-4 Helivector or later known as the Aerocycle (Figure 4). As This was a one-of-a-kind aircraft built for the U.S. Army in one might expect, this configuration could be hazardous to the mid-fifties with its first flight in 1959 (Figure 5). It was the pilot with two rotors spinning in opposite directions just an unusual configuration with very large flaps and end plates under his feet. The rotors also kicked up ground debris and to capture the propellers slipstream and augment lift. The dust during takeoffs, landings, and hovering flight. The pilot aircraft was powered by a Lycoming turboshaft engine that was held in place by a safety belt and his grip on the produced 1000 hp. The concept was to allow flight from motorcycle type thrust control on the handlebars. This grip hover to low forward speeds, however neither hover nor controlled RPM only since the rotors were fixed pitch. A 40 vertical takeoff were ever achieved. It required some hp outboard motor powered the platform and the landing forward speed to make every short takeoff gear initially consisted of airbags replaced by skids later in the program. Differential prop pitch was used for roll control while pitch and yaw control was provided by the engine exhaust at the There were two accidents early in the program. Fortunately tail until the aerodynamic controls became effective with no one was injured but the project was eventually cancelled. forward speed. A throttle was used for thrust control. The aircraft was located at Ames Research Center in the early sixties. During one “unplanned” maneuver, the pilot ejected over the Bay’s salt ponds and the aircraft crashed. The pilot survived with back injuries and the aircraft was rebuilt and resumed flying in 1961. An earlier accident had occurred in 1959. 3 October 1958 where it remained until the Army moved it to Langley Research Center, VA. The Doak remained at Langley until August 1972 (1). The search for the type of thrust control used was long and frustrating. The aircraft is on display at the Ft. Eustis, VA Transportation Museum; however, the contents of the entire cockpit had been removed and references to the aircraft did not include the thrust controller. Finally, a grainy, black and white photo surfaced that showed an early, tethered hover flight before the aircraft skin was installed.