There are various model helicopter designs but most all have one thing in common, they use a flybar. Early helicopters employed a flybar only to control the main rotor cyclic pitch and to this day some still offer a pure Hiller type system. This eventually evolved into a Bell/Hiller hybrid. The hybrid system basically adds another more direct cyclic input into the Hiller equation, which by nature by passes the flybar.
Early developments of the RC model attempted to copy existing full-scale systems through trial and error. Little bits of one system were adapted to another in the hopes that model control would improve. It took many years and individuals to prefect model rotor control to the point it is today. There was no great single-minded effort that decided what the ideal system might be. The modern day RC model is the collection and adaptation of the ideas of many. We still see design evolution with the best ideas copied from one manufacturer to another in search of a better formula. Unfortunately at this stage many products are at the top of the ladder leading to present day gains being just noticeable at best, and more often than not subjective in results.
The Hiller cyclic control system our models enjoy was initially copied from a full sized helicopter designated the Hiller12E manufactured of course by a company named Hiller. Stanley Hiller the inventor of this system coined it as a “servo rotor”. This makes sense in that the swashplate rotating control rods vary the paddle pitch angles during cyclic input. This causes the servo rotor (flybar) to follow cyclic commands. The main rotor blade grip pitch horns are attached to the servo rotor in such a manner that allows cyclic blade angle changes to follow the servo rotor when it teeters. Basically the swashplate flies the paddles and the paddles through associated connecting levers or links will fly the main rotor blades. This allows for the use of smaller less powerful control forces to be applied to a swashplate. The main rotor is slaved to the flybar. Think of it like power steering.
The flybar like all rotors has inertia and exhibits gyroscopic properties. What concerns the system most is rigidity in space. If the servo rotor is made to be heavy, the paddles will have to work harder to move it. The results in the case of an overly heavy flybar cause a requirement for larger paddles or higher paddle pitch angle values. If the formula is not balanced correctly the helicopter main rotor will either respond sluggishly or aggressively (light flybar) to pilot commands. The piloting control response from excessive flybar inertia may become damped to the point that it is dangerously washed out.
Looking a little further into the mechanism it is important to note that the manner in which the rotating linkages connect from the servo rotor (flybar) to the blade grips. If gyroscopic forces are adequate for say a hover condition and one were to disturb the fuselage by pushing sideways on the skids, this would cause a corrective self righting cyclic input due to the geometry of the rotating linkages, remember the flybar has stayed in position due to the gyroscopic law, while the helicopter along with its linkages has moved. This is what channels the corrective and automated cyclic command to the main rotor blades. The tendency in this case is to return the main rotor into its earlier position. This is the stabilizing factor that flybar control offers. Bell helicopters with regard to this feedback feature have designed a similar system called a stabilizer bar, the key difference being in that it has no servo paddles.
The Bell system attaches the blade grip pitch horn through the pitch link to the stab bar by means of a lever, so when the bar teeters the blade follows. However also attached to this lever is a control rod from the swashplate. In effect the lever mixes cyclic pitch change with the gyroscopic properties of the stab-bar. The problem at this point is we need to upset the gyro qualities if we expect any amount cyclic control. This is similar in reasoning to the previously mentioned Hiller stabilizing factor that is channelled through the rotating linkages.
With the flybar we have paddles to cyclically cause teeter movement through a slaving action which is then transferred to the blades, but at this point in the explanation with the Bell system we do not. If we removed the paddles on a pure Hiller system we’d basically be in the same boat having a helicopter that is too stable but with no pilot control! The Bell stabilizer bar teeter is attached to the mast through the use of friction dampers. When a cyclic command begins at the rotor, the bar is pulled along to an extent. Since the bar is following the mast by this method through the rotating controls connected to it, cyclic pitch change is allowed. Another manner of thinking about the result is that the stab-bar is not allowed to pull out piloting cyclic commands as readily. Bell term the quantity as the mast following rate.
The model flybar system (Hybrid Bell/Hiller Cyclic Control) uses features from both the fore mentioned methods but with modification. The parameter using the direct connection we see on most modern day helicopters is the key and in a pure form it is commonly referred in the modeller community as a flybar-less rotor system. Swashplate cyclic control in this case is fed directly to the blade grips through the pitch links. The typical modern day helicopter uses rotating mixing levers to blend these direct blade inputs with the slaved flybar inputs. Further more the flybar damping is also blended through the mixing levers. These levers are typically located either on the flybar or at the blade grip pitch horns.
Hopefully some of what I have written so far is understandable so we shall now look at the difference between the direct and indirect (flybar slaved) inputs as they apply to the blade position or location in rotor disk rotation. All rotor airfoils react to cyclic change in a belated manner. It takes time for the blade to climb or descend during it journey round and round. The reaction time is approximately ¼ of a revolution. This just happens to coincide with the gyroscopic law in that a gyro will respond to a disturbing force 90 degrees later in the direction of rotation.
With the direct inputs the rotor response is at 90 degrees to the cyclic pitch change. The paddles respond in the same manner but it takes another 90 degrees for flybar teeter induced cyclic pitch change at the blades to be felt at the main rotor disk. The reaction from paddle pitch change to disk deflection is 180 degrees due to the slaving effect.
A pure Hiller system will feel less locked than a flybarless head would, however a pure Hiller system can still be made to fly aggressively and complete 3-D manoeuvres. A pure flybar-less head can fly 3-D too, but unfortunately it will have bad habits with regard to control cross couple. In other words the helicopter will eventually do something you have not have commanded. This is why a flybarless head is generally regimented to scale flying where cross couple is easily controlled and anticipated. Pitch ranges or cyclic rates are held to a minimum for scale flying. It should be noted that modern day dual axis gyros have been used to replace the flybar damping and stability. In this case, hopefully one might better have cake and eat it too.
Where does all this lead you may be thinking. As can be reasoned with this hybrid mechanism, the unit as a whole will have qualities of all its parents. The interesting part is various model helicopter manufacturers will design in differing amounts of each parameter. Mixer ratios, flybar length, paddle selection along with maximum available paddle and cyclic blade pitch angles will all contribute to how the model will fly. Most important for a seasoned flyer is how flexible it will be for agility adjustment while still maintaining a very predictable nature.
A good comparative example might be the model with ample cyclic blade and paddle angles as compared to one with lower values. The former may be tuned in its present condition while the latter will need parts exchanged to meet a similar end. There will be minor comprises in handling at times but nothing the average sport flyer cannot live with. If the swashplate tilt, blade/paddle angles, and rotor rpm are maximized with the maximum cyclic rate still found to be lacking, then we are left with working the flybar inertia, paddle size and paddle shape.
It is possible to select a paddle too aggressive for the flybar inertia. In this case flybar weight may be added and slid in and out on the flybar rod to customize the maximum cyclic control response. Sometimes the flybar pitch range can be reduced with adjustable washout arms to reduce the Hiller steering parameter. The balance or feel between small stick movements and large ones will be altered however. What one person prefers for this control balance may differ from another, however humans can easily adapt provided an over control (too sensitive) situation does not exist. Expo in the radio can be used to tame the center area down a tad after the feel is tuned as best it can with the flybar.
You’ll find that a helicopter can have a locked-in feel during the hover where moderately large quick stick deflections are instantly felt by the helicopter but in a manner as not to cause the helicopter to pitch and roll to the eye. An analogy might be driving a small car with rack and pinion steering as compared to driving a heavy SUV. In this case the direct inputs through the mixers are helping you out. With the Bell/Hiller mixers (as the modelers tend to call them) set correctly we are able to fine tune this “around center feel”. Be advised changes to the cyclic are like adding small amounts of sugar to your coffee.
The BH mixer has another purpose in life and that is to allow post swashplate collective inputs to transparently flow through any cyclic values and into the blade grips. This applies to the swashplate that rises and falls for collective pitch change. When you more closely ponder the device, it also functions as a rotating CCP (cyclic/collective pitch) mixer. Existing +/- cyclic command values are added to any collective change. It has locally a similar function by nature as rocking and sliding servo trays!
The angular ratio of flybar teeter to cyclic blade pitch change is sometimes referred to as the flybar authority or Bell/Hiller ratio. I prefer the former European terminology since this ratio has nothing to do with the direct inputs which some term the Bell inputs, thus creating confusion. Even on a pure Hiller system there is still a flybar authority ratio. There is however a ratio between the maximum direct input cyclic blade angle and the additional cyclic blade angle created by full flybar teeter. The relationship between the two ratios will change how the model feels. This can be adjusted at the mixers and it often has an effect on the total collective range. The servo endpoints will need adjusting in some cases to acquire both the original collective and possibly to remove cyclic binding at the head area. Some helicopters allow you to move flybar mounted mixer fulcrum closer to the mast thus lowering the flybar authority while some blade grip mounted mixers allow you to alter the mixer lever arm length.
Sometimes an allowance or provision is implemented for both mixer arm length and fulcrum position on the flybar. In this case for a given swashplate tilt, direct blade angles can be altered together with the flybar authority. To complicate matters further, if the mixers are mounted to the blade grips as with the Bergen type machine and the mixer fulcrum position shifted on the blade grip pitch horns, then the flybar authority will not change. What will change is delta values (flap/feather coupling). Generally if you have a very sensitive and pitchy rotor in forward flight, flybar weight is needed or the paddle deflection angle and possibly a paddle size reduction. Flybar weight is a very easy method to tame down an overly sensitive helicopter. This ignores very poor main blade selection since it may cause correct tuning results to be very difficult if not impossible. If you change the ball stand off lengths on the grip mounted mixer in order to alter the flybar authority then both collective and the direct cyclic rates will also be modified.
Modern radios have various features such as expo to tame the beast around center stick. I tend to use this feature last after working the rotating control throws, rotating mechanical mixers along with flybar weight as best I can. I’ve also found with a paddle size increase that one arrives at a point where accurate handling in forward flight goes for a sharp dive. Axial rolls are the most obvious degradation. The faster maximum roll rate will cause issues for the exceedingly greedy.
Finally the flybar length may be altered to change the maximum cyclic power. Adding length with all other parts remaining the same will offer an increased maximum cyclic rate. This is due to the more dominant and higher local velocities the paddles see. Granted flybar inertia will be higher with the more outboard paddles and added flybar rod mass but not so much to fully cancel out the aerodynamic addition.
The 3-D flyer will benefit from a light and agile flybar in another way, as remaining flybar damping will help during abrupt and large cyclic changes. In the case of a rainbow, tic-toc or reversing roll when opposite cyclic and/or collective are pounded into the machine suddenly for directional change, it goes something like this. Flybar stability or gyro damping qualities momentarily pull cyclic commands out at the blades preventing bogging the engine down and/or stalling the blades until such time as the machine has changed direction and velocity. There is a point in time where the servo rotor notably lags behind the main rotor disk deflection. After this time rotor inflow has stabilised reducing the local peak angle of attack to an acceptable level thus allowing for rather large cyclic pitch values!
Tuning the flybar to a helicopter maximises the main rotor blade performance. This is the most economical manner in which to approach a desired performance level. Some people unwilling to wet their feet in this regard often spend a small fortune trying to find that special rotor blade willing to cooperate with an existing flybar setup. Every helicopter type is different and so various setup methods apply, don’t be afraid to experiment.