Understanding Model Helicopter Rotor Heads
Posted on December 3, 2008 by aaronredbaron
Model helicopters have come a long way, and today’s modern machines are capable of astounding performance. Models differ from their full sized counterparts in many ways, and one of the biggest differences is the rotor head design itself. A mechanical stability system called a flybar is employed in most instances to add a degree of stability and make control response more predictable. Although considered by many to be black magic, all flybar equipped model helicopter rotor head designs take advantage of concepts invented in the early days of helicopter technology by Arthur Young of Bell Helicopters, and Stanley Hiller Jr. of Hiller Helicopters. Understanding exactly how these systems work can be a mind boggling quest into the mathematical world of aerodynamics, processional forces, and trigonometry (to begin with). How then can the average hobbyist gain a functional understanding of how these systems work? The answers are buried in the history books. Taking a look at the Hiller and Bell concepts can be very beneficial in understanding how the flybar equipped model helicopter head does what it does so well.
The Bell Stabilizer Bar
Arthur Young (1905-1995) was a remarkable man whose true passion was philosophy. But, after running into a frustrating roadblock on his comprehensive theory of the universe, he chose to go after a more tangible goal. Seeing much potential and much failure in the fledgling helicopter concept, Arthur set about building models to test his theories. In 1941, after 12 years of experimentations, the stabilizer bar on his flying (tethered) model was perfected enough to win the praise of the Bell Aircraft Corporation, leading to funding of the program which was to eventually develop the famous Bell-47.
Arthur’s concept was to mix direct inputs from the pilot with a stabilizer bar which acted as a gyroscopic stabilizer. This stabilizer bar was mounted at 90° to the main blades and fitted with weights at the end, but was aerodynamically blind (airflow played no role in the stabilizer’s response, it only acted to gyroscopically stabilize). The definition of a gyro is “a rotating mechanism in the form of a universally mounted spinning wheel that offers resistance to turns in any direction.” (quoted from www.answers.com/topic/gyros) Imagine the spinning stabilizer bar as a disk or the wheel from the gyro definition. When the Helicopter moves, this disk wants to remain in its position relative to the ground. The disk (stabilizer bar) is free to tilt, so that as the helicopter changes attitude the disk is free to remain in its plane. When the Stabilizer bar tilts, it induces a pitch change to the blade in such a manner that the helicopter wants to self correct. Mixing was accomplished with levers which were mounted on the stabilizer bar in such a way that pilot inputs and/or stabilizer bar movement could induce a change in the blades pitch angle. The overall effect of the stabilizer bar was to make the helicopter very stable. It tended to resist changes in attitude, such as being pushed by a wind gust.
The Bell stabilizer bar was patented and proved successful for several helicopters including the Bell-47 of MASH fame, and the UH-1 Huey which earned a place in History from its role in Vietnam. By the 1960’s, Young’s stabilizer bar was obviated from new Bell machines in favor of simpler direct linkage designs. After his success with Bell, Arthur Young went back to philosophy and wrote several books.
More information about Arthur Young is available at www.ArthurYoung.com
The Hiller Rotor-Matic Paddle System
An American whiz kid came up with concept of control paddles, and his achievements are nothing short of astounding. Stanley Hiller Jr. (1924-2006) had built his Hiller Industries into a improbable depression era success story selling gasoline powered tether cars before he tackled the conundrum of the helicopter. In 1944, Stanley Jr. completed the first successful helicopter flight in the western United States with the first successful co-axial counter rotator helicopter in America, the XH-44 Hiller-Copter, having never flown or even seen a helicopter fly. After investigating several rotary winged concepts, youthful Stanley Jr. invented and patented the Rotor-Matic control system which used paddles mounted at 90° to the main blades on a flybar to boost pilot inputs and stabilize the pitch and roll axis. His inventive solution to rotary winged stability issues raised eyebrows worldwide with its inherent stability.
Although the Rotor-Matic system looked somewhat similar to the Bell stabilizer bar, the function was altogether different. Hiller’s concept with the Rotor-Matic control system was to direct all control into manipulating the pitch angle of a paddle shaped airfoil. When control was initiated, the flybar assembly to which the paddle is mounted is able to tilt, the paddle flies into a new position, and the linkages are connected in such a way the flybar movement changes the main blade’s pitch angle. Hiller’s design also benefited from the effects of a gyro, and was able to hover for extended periods without pilot corrections. Adding the Rotor-Matic control system to his design, Hiller was able to introduce a significant level of stability, while at the same time boosting pilot inputs (sort of like power steering). Although Hiller’s system did offer great stability, there was a delay in the controls which had the effect of making the helicopter less responsive.
The Rotor-Matic system was employed successfully on the Hiller 360, which was to evolve into the famous UH-12. The Rotor-Matic system was used in a variety of experimental designs, but never gained widespread appeal. Throughout his time at Hiller Aircraft, Staley Hiller Jr. experimented with a slew of vertical flight innovations which would cement his place in history, among them the first tilt-wing VTOL airplane, the first helicopter to counter torque with compressed air (similar to the modern NOTAR tail control system from McDonnel Douglas), and the worlds first hot-cycle pressure-jet rotor, as well as others.
More information can be found about Stanley Hiller Jr. at the Hiller Aviation Museum. www.hiller.org
Putting It All Together; Bell and Hiller In One Rotor Head
Although both the Bell Stabilizer Bar and the Hiller Rotor-Matic control system were proven viable with their early applications, both systems were circumvented from modern designs as helicopter technology progressed. These two systems were never employed together in a full sized helicopter; it wasn’t until the 1970’s that both concepts began to appear on model helicopters. In the late 1960’s the radio control hobby was just beginning gain traction and take off. Radio control systems could be purchased and airplanes built to fly with some degree of reliability by average hobbyists. The idea of a radio control helicopter was probably very tantalizing to the early pioneers, and although a scattered few set about to build flyable radio control helicopter models, the majority of those early endeavors produced a few seconds of uncontrolled hover, at best.
The first successful RC helicopter flights were flown by Dieter Schluter with a simplified version of Hiller’s Rotor-Matic system. Around 1974 the Kavan Jet Ranger arrived which was the first to combine both the Bell and Hiller concepts. In effect, Hiller’s paddles and the ability to control their tilt were added to the Bell system. By adding Bell’s concept of direct input mixed with the gyro effect to the Hiller paddle system, models were now able to take advantage of the best of both worlds and the modern Bell/Hiller model helicopter rotor head was born. The paddles increased the Bell systems control power, while the direct linkages through mixing arms provided more direct, responsive control.
Without diving deep into the specifics, in a nutshell the benefits of the Bell/Hiller combined rotor head are; The gyroscopic effect helps tremendously with stable hovers, the paddles add predictable control response and boost control power while also making forward flight more manageable, and the Bell direct input provides a direct responsive feel. Despite limitations stemming from physical relationships in the linkages, the Bell/Hiller rotor head can be fine tuned by adjusting a variety of variables. The ratio of direct input to paddle input, the size, weight, and shape of the paddle, flybar length, and many other variables can be manipulated to make the helicopter more stable, or more responsive, etc.
Flying Without The Flybar
Although models can be flown without a paddle equipped flybar or electronic stability system, without stability augmentation models present some strange tendencies; they tend to want to pitch up and to the side severely in forward flight, and have very quick response with next to nonexistent inherent stability. Some of these issues can be reduced at the expense of agility through properly weighted blades. The Kavan Jet Ranger was offered in a Flybarless version, GMP had a Flybarless version of the Legend, and more recently Jack Dunkle of JMD Models has been very successful with flybarless scale models using weighted blades with no electronic stability. However, Jack’s models are expressly built to fly smooth and stable at lower than average headspeeds. Electronic stability systems designed to manage a model helicopter’s rotor head response are most often referred to as Flybarless Rotor Control systems. Flybarless systems offer user definable stability and control response, while at the same time improving the rotor heads looks and efficiency.
Mikado was the first company to release complete helicopter kits with a matching electronic stability system (Mikodo’s V-Bar), and now Gaui also offers their own matching flybarless stability system (the GU-365) for their 200 and 550 Hurricane models. These cutting edge systems offer hot dog and scale pilots huge advantages, but the concepts Hiller and Young developed more than half a century ago still offer manageable control for uncounted hobbyists worldwide. Until the electronic stability systems mature to the point they are cost effective and user friendly enough for the average beginner helicopter, we will continue to see Bell/Hiller rotor heads dominating the model helicopter market.
