How Does a Vario 212 Rotor Head Relate to the Real Thing
Stabilization of the helicopter has been a long sought after quality but unfortunately it often robbed the very control desired by pilots. Cutting edge full scale aircraft now use computers for stabilization and control. Historically many tricks have been tried to meet this end, some of which have made possible the development of present day rotorcraft.
One such invention made by a gentleman named Arthur Young is the stabilizer bar which operationally is still found on some current day Bell helicopters. Many scale modelers would covet such a system which up until now was not possible. Vario presently offer the best scale attempt towards this end. I hope you will enjoy more technical information, in place of descriptive marketing objectives, since this is my intention.
A bit of chronology concerning Arthur Young is in order before delving too deeply into this arena. He was as I understand it born into financial stability which allowed him the opportunity for education and experimentation. His contribution to the helicopter rates well with those of Stanley Hiller and Igor Sikorsky. Maybe even more so since he did keep the competition on their toes after WWII. He spent much time tinkering with remote controlled helicopters in his barn attempting to design different rotorcraft ideas. Remote control in those days meant connection by wires using magnetic solenoid devices to control the cyclic. One concept he spent substantial but unsuccessful effort at related to a rotor driven by propellers located at the blade tips.
A major hurtle to overcome in the early development days was that of over control by the average pilot. Mr Young to this end invented the stab-bar common to the Bell series of rotorcraft. Once he had a successful and functional remotely controllable model of the system configured, he demonstrated it to Larry Bell of Bell Aircraft, and about two months later on November 24, 1941 an agreement between the two was made. This system includes the well known 47, 204/205/212 series of rotorcraft still in service today. The prototype Bell 206 helicopter actually had a stab-bar mounted to it. Early 206 production masts had splines cut into them for the fitment of hydraulic friction dampers…so it almost happened again! Because here is not the place for such detail, I encourage you to search the web for additional information on this fine humanitarian. Try looking under www.arthuryoung.com. Now let us investigate Arthur Young’s rotor system from a functional stand point. His design was based on a means to automatically offer a self correction for unwanted rotor disturbances. In basic terms it acted as an inertial damper (gyro) when considered from a purely general functional standpoint. It also made reference to the horizon tending to return to the horizontal plane with time. Back in those early days many rotor hubs or rotor heads were centre hinged loosely to the mast, some were even fully gimbaled. Cyclic control was a function of redirected rotor thrust only, since the centre gimbaled disk was fluid and free to tilt independently of the mast and helicopter. The helicopter was manoeuvred solely by the thrust vector pulling at the end of the mast rather than being aided by a bending moment supplied at the rotor hub. This contributes to the classic and smooth Bell ride. It is also one reason why the Bell mast is so tall supplying a longer leverage to the helicopter c of g, which is needed for adequate controllability. Command response is slow which means more time to think, inclusive of a certain conditioning to human reflexes. This rotor can not control the helicopter under zero or negative G force, a fact which is sometimes its undoing in the hands of inexperienced, or the those of cocky and disrespectful pilots. This is because the rotor supplies no control to the fuselage at zero g by being free to centre teeter. What transpires, actually happens when the pilot tries to regain control feel at zero g or below by moving the cyclic, this then causes the unloaded rotor to easily reach its mechanical flapping limits. The rotor hub yoke finally hits and shears the mast in flight, most likely resulting in death and total loss of the aircraft. This is commonly known as mast bumping.
The Bell semi-rigid seesaw head is also called a stiff in-plane rotor since no provision for dynamic blade hunting (lead/lag) is provided. The blades are aligned to the best chordwise balancing position and fixed or held there by a mechanical means called either a drag brace or blade latch. The 212/205 drag brace can be seen on the trailing inboard edge of (picture # 2)
Picture 2. The real thing with friction dampers to ensure “mast following“. Note the drag braces on the
inboard blade trailing edge since this a “stiff in-plane” rotorhead.
The head flapping axis is also under slung or cradled to the feathering axis which reduces fore/aft blade bending moments caused by Coriolis effect when rotor deflection occurs. The head is preconed to reduce bending moments and extend component life. After all, it cannot not fly inverted or safely under negative g loads. The interesting thing about this head as compared to our models is that it has no thrust bearings. Centrifugal loads are carried by tension-torsion strap packs. These are nothing more than wire wound devices connecting two bobbins which are free to twist for pitch change even while under extreme tension. One end connects to the blade grip while the other is attached to the rotor hub. Vario manufacture this device in Kevlar and functionally implement it into their scale BO-105/BK-117 rotor head.
Since our models are so very aerobatic they cannot have such flapping freedom at a centre teetered location and preconing would be functionally useless. We as modelers use a cushioned teetering restraint usually made from rubber, which is the contemporary or present day mulit-blade thinking Bell and others now follow by various means. Production or development wise, the Bell two blade system has all but been discarded. As modelers we allow for slight dynamic blade lead/lag (hunting) either in the form of rubber cushioning or by means of a slight freedom at the blade bolts. We have no need of modelled down drag braces as this would complicate blade balancing and alignment issues. Flexibility can also be built into a rotor blade as an allowance to dynamic movement. So hopefully now, among other things we know physically why we should not have a free centre teetering, full functional, exact scale replica Bell rotor head installed on a model and why we build the way we do.
The stab-bar mechanism is ideal for the scale model helicopter offering accurate appearance with a similar user friendly function to that of the model helicopter flybar. Mechanical design also cooperates with the present day model rotor control system layout, but in a simpler less complicated way if desired.
We have a less intricate system using two instead of four rotating control rods connecting the spinning portion of the swashplate to the rotor head. Since paddles are not used with a stab-bar, the washout is not required. We do however need a means to drive the rotating portion of the swashplate. Both the rising/falling swashplate and the fixed type with a collective scissors affair may easily be adapted. Scale wise a collective scissors system using the fixed swashplate is the most accurate copy. Because the rising/falling swashplate is used on both (electronic) ECCPM and the conventional MCCPM (mechanical) either mixing means will work with this type of rotor head. The hardest part to accurately modelling the Bell system comes in the manufacture of tiny frictional dampers which cause the stab-bar to follow the mast or cyclic manoeuvrers. Two methods for modelling are available, omit and compensate, or manufacture the tiny dampers in some form. Arthur Young’s model certainly had no dampers and it was only after scaling it up that the need became apparent. Again, the dampers are firmly fixed to the mast connecting directly to the stab-bar to resist but still allow full stab-bar teetering action.
A little more basic information on how the stab-bar operates. It is as stated, similar in function to our popular flybar adding stability to the rotor and helicopter. With the flybar we have paddles to cause the bar to follow cyclic inputs being dependant on the flybar inertia inclusive of paddle size and shape. Inertia increases at the bar will slow both the stab-bar and flybar making the cyclic more sluggish where as increasing the paddle effectiveness of the flybar will speed matters up, and so on. The stab-bar and flybar behave much like a gyroscope having rigidity in space. It should be stated with the conventional model helicopter that until the cyclic input (roll/pitch rate) exceeds that of the slowest component, the flybar, it may actually appear to initially lead the rotor disk during large inputs. Rest assured when hard strong inputs are made and the rotor/helicopter rolls or pitches, the bar will lag behind. This is because its damping or lagging behind is dependant or based upon the rotor or more specifically the mast actually moving in an angular method. If the rotor quickly deflects and the mast basically stays put then the flybar will lead since there is no induced mast deflection for it to follow. It will do the same thing on the ground for this very reason when the cyclic is deflected. The stab-bar mast following time as it is called, in the case of a true Bell system is controlled by hydraulic friction dampers connecting the stab-bar teetering to the mast, instead of using aerodynamic paddles. The mast through the friction dampers pulls the bar behind it when a pitch or roll rate is established. If this did not happen the helicopter would be too stable. This is because the angular displacement between the “rigid in space” stab-bar and the mast becomes too large causing or supplying an input to the blade opposite in nature to the cyclic control input. This mechanically happens through the rotating control geometry. As can be reasoned excessive stability would result by removing the paddles from your conventional model helicopter and adding weight in place of. Since the full scale, similarly to our models, have a desired maximum cyclic response (roll/pitch) rate, the stab-bar actually has mechanical stops limiting just how far behind the bar can lag….or how much cyclic pitch it will be allowed to remove. In reality these physical limits are normally never met and the rate is dictated more by the geometry of the mixers and damper hardness. The intricate feel of the full scale is also controlled by the progressiveness of the hydraulic friction dampers. Eg: the further and/or faster they are moved the stiffer or harder they become…. now where did I hear that comment before! This increases control authority during rather large or quick short cyclic deflections and allows a softer feel around a slower closer to centre cyclic stick movement. The mixing (feed back) ratio much like on our model mixer arms, is controlled by the non adjustable geometry of yokes connecting the stab-bar to the pitch control arms located on the blade grips. The yokes can be seen in the diagram situated inside the two halves of the stab-bar. The full scale system has an internal cable attached to each stab-bar weight so that if the hollow pipe portion of the bar cracked or failed it would not throw a weight.
A alternate way of understanding how the system works might be demonstrated better by lifting the model by the level flybar, then moving or rocking so as to hold the model to one side using the flybar teetering hinge point. At this time note how the rotating linkages channel a correcting cyclic input to the blades opposing to the direction you selected or deflected the model to. Remember it takes another 90 rotational degrees for the blades to respond for the effective or actual corrective measure (high or low flap). No one said it will be easy understanding the system and I certainly hope to make you think a bit more about what I have said. Don’t feel bad if you have to scratch your head a tad while messing around on the bench with your model. I suggest you pretend the paddles are not even configured into the system when assessing the damping nature of the flybar.
Now let’s get to the main point of this article, which is how Vario managed to design a production scale model helicopter rotor head using a stabilizer bar free of paddles, friction dampers or flybar/direct input mixers. This company continues the lead in scale model rotorcraft design and development having such credits to its name as the NOTAR, BO-105/117 scale flybarless head, Astar full scale head and many more exacting replicas. Details of Vario rotors are not just skin deep, often implementing fully functional, miniature mechanical copies whenever possible. If it is not feasible due to operational or physical restriction to make an exacting copy, the next best thing can be expected. This company even supplies a two axis main rotor piezo gyro for flybarless rotor systems. If you want the foremost in scale operation and can afford it, this company would be my first choice. If you have an existing machine from another origin certain components will readily adapt, thus more economically increasing the realism. The worst relevant situation might require slight modifications to the mast and the two rotating control rods at the swashplate. Read on and we’ll see if it lives up to these claims.
The Vario head (picture #5) is conventional in its basic design having a through spindle cushioned at the plastic head block ends for teetering. Each blade grip contains two radial and one thrust bearing. The stab-bar is mounted to the top of the rotor hub and connected to the blade grips by two yokes and two levers. This mounting position, in a purely scale sense is incorrect for the Bell 47 since this aircraft has the stab-bar located under the rotor head. It is however at the correct location for the 204/205/212 standards. The yokes also attach to the two rotating control rods from the swashplate. These rods transfer both collective and cyclic control to the stab-bar lever and yoke assemblies. Both rotating control rods are straight requiring no bends to get around obstacles. From the yoke/levers, the input goes to the blade grip pitch links. If the bar teetered (or better said if the helicopter teeters around the bar) the yokes could supply a cyclic stabilization or reduction factor. Unfortunately the Vario bar does not teeter. If building Bell variants in fine detail it might also be handy to know that the tail rotor is mounted to the left side on the shorter 204 and turns away from the main rotor down wash.
If the bar did teeter two things might cause this, either a resulting helicopter angular movement from the servo control outputs or from an outside disturbance to the helicopter assembly. One major piloting difference between a flybarless head and the flybar/stab-bar type is that one will not have to worry about the helicopter rapidly tucking under when lowering collective in fast forward flight as the bar automatically corrects for this. It will also trim out much better in cruise making it slower and much easier for the model aviator to keep up with during changing flight conditions. Setup correctly, it will give smooth, stable scale like flight appearance. Why Vario did not take this rotor head design one step further I know not.
Vario supply specific counter clockwise rotating blades but any quality blade of the correct span and rotation may be used. Since a most likely choice would be a scale application for this head then stiff, light carbon blades would not be the best blade choice. I chose to use Leisure Tech blades because they offer some flex, work well with good visibility, and are nicely priced. I also happened to have some in my possession. Anyone can fit a Vario head, with Vario blades, to a Vario machine and with no difficulty, but since I make my own purchasing decisions I decided to fit it to my X-Cell machine. This is even though I have access to Vario ships. One day I might try it on a Vario helicopter should time permit, though I’d certainly expect the same basic results. The super reliable Blois G-23 powered X-Cell “gas-tank” was being used as a video test platform to aid a fellow club member and so it continues on with this analysis tasking.
Mounting the head to the X-Cell required two Niftec control rods. Depending on the length of the mast used and its placement in the X-Cell bearing blocks will determine the required rod length. I used 55mm length with the long Miniature Aircraft mast. The Vario head can have the pitch control arms on the blade grips leading or trailing. This might reverse delta values on other rotor heads, but because this head has none, it doesn’t matter. The Vario grips are meant to be flipped offering nut holding indents on both sides. The only thing I did for this reversal was to use 4mm thick spacers between the reduction levers and the blade grips instead of the three tiny .5mm washers. You don’t have to flip the grips but the swashplate will then be lowered for collective increases and raised for decreases (bass-ackwards). Aligning the head retention bolt posed no problem since I simply bolted it on. As the spacing between the blade bolts is slightly longer compared to the stock X-Cell head I am happy to report that no tail rotor clearance issues surfaced. I was initially concerned about the collective range not matching up with the X-Cell control system due to the intended ECCPM nature of the beast. Full travel of the X-Cell fore/aft collective idler yielded a collective range of approximately 21 degrees. This value as you can see is quite acceptable and can be altered slightly by selecting a different hole on the reduction levers. This mechanically also affects the cyclic rate in a very, very minor way. The cyclic range indicated a center value dependant on the X-Cell swashplate limitations of +/- 4.5 degrees. Prior concerns with this machine (and others) to washout arm clearance are no longer relative since it is after all no longer installed.
Here is where I had a problem with this rotor head. It is advertised as having a damped stab-bar system similar in action to the full size. When I ordered the head from Vario, I fully expected an operational stab-bar (picture5). It was after all my sole reason for the purchase. I have been told after voicing concerns that this incorrect technical specification will be corrected. The Vario stab-bar does not teeter being functionally a decorative item only. The levers on the blade grips are implemented to reduce control inputs allowing for standard collective and cyclic values based on modern model helicopter swashplate control design. Removing the paddles with the washout control rods, and then locking the flybar teetering would essentially accomplish the same thing on a standard head. What we have here is a high quality flybarless rotor head with ideal control geometry designed to match up with the rest of the helicopter control system.
What I physically like about the rotor head putting aside my previously mentioned disappointment follows. The precision head block is aluminum and fits to the mast with a through bolt and a secondary clamp up arrangement exercising a compressible collar surrounding four lower head block slots. The single through spindle uses heavier thrust bearings than most and does not have a turned down area on the spindle ends. The thrust bearing is also higher quality than what you’d normally see in some other products and it does not care which way it is installed. The rubber spindle dampers are housed in plastic inserts which are hand pressed into the metal head block. This removes any chance of the metal damper adjusting washers contacting the metal head block and causing radio glitches. The 3 X 7mm rubbers carry all the helicopters weight. All moving pivot points are slop free being fully supported by ball bearings. The plastic blade grips have threaded brass inserts moulded into the pitch horns. The supplied control rods are very strong being thicker than most other 60 size. A flybar can easily be adapted to the rest of the head if desired at a later point in time. The rotor head definitely has the good looks of the full scale system and overall the structural integrity is rather heavy duty with a low friction operation. Building any head from scratch ensures that everything can be assembled and thread locked correctly, then double checked, so as not to be exposed to any rapid mass production pitfalls.
What I did not find in the way of excellence ensues. The head did not come with a swashplate driver so one will need to be purchased. (I used an old Schluter Champion unit). The open radial bearings unfortunately are not the shielded type. This is minor, since inboard and outboard washers tend to partially seal the bearings from contaminates. The tiny bolts holding the reduction lever yokes to the stab-bar were too long and contacted the steel center bar holding the mock up weights. This has the effect of slightly loading up the tiny bearings. I ground the bolts length slightly and used low profile self locking nuts (picture# 6).
You could file the standard nuts down for the same reason. The yokes need trimming to allow full travel which is mentioned in the sparse assembly instructions. Anyone with a bit of experience can safely build the unit but it is the less seasoned type of individual who may have difficulty. No cyclic pitch values are given here so in this matter you are on your own. The blades can not be easily folded back over the tail boom and maintained with a foam support during ground transport. Feed back forces to the servos from the rotor are higher even with the reduction levers. This is because a flybar can teeter to slightly absorb feathering rotor forces.
Finally the time came to carefully test the system. First, the collective was set to the specs I had been using prior to this installation thus removing the need for throttle curve alterations. There was not much to set differently other than using the hover pitch trim knob, due to exceptionally well matched geometry. Even though I use the GV-1 governor on most of my helicopters, I still like the collective control to feel exactly the same in the air. The cyclic setup is the big question since quite frankly I did not know exactly what to expect in fast forward flight and during rapid hovering manoeuvres. Hand braking the rotor is not difficult even though the bar has no head button. I use the “poor man’s rotor brake” at shutdown time by operating the throttle hold in conjunction with the application of collective pitch. Tracking this rotor must be done by adjusting one or both of the long rods connecting the swashplate to the reduction levers.
The initial run up was uneventful with tracking checks. In the stationary hover it felt different from the flybar rotor system. Gone is the locked in feel and stick input timing is different. The feel of the cyclic differs with changes in rotor rpm…more so than with the flybar stabilized system. It takes but a few moments to get acclimatized to, since any over control needs immediate pilot correction. The wind condition was very slow during this point in testing (picture# 7). A good analogy would be comparing the feel of a street bike to that of a dirt bike since both can be ridden correctly with time in. They drive distinctly and are meant for two entirely different situations.
The forward flight evaluation was initiated in several cautious baby steps guarding against the unexpected. Wind conditions were low and with no gusting. I am still pleasantly surprised at the ease of control during moderate forward speeds. Approaches for landings exhibit no pitching moments whatsoever. Banked turns are easy even though a bit more attention is needed in order to make things look balanced. It appears at this point, that the 200 gram Leisure Tech symmetrical blades and the Vario rotor head compliment one another with the X-Cell not complaining one bit either. This is a fortunate situation as many scale Bell fuselages are available for this particular ship.
The next few flights met with higher winds, gusting at times. I have confidently flown this same machine with a flybar under similar conditions. When flying the Vario head under these same conditions my hands are certainly full. The machine was so hard to keep up with that I put off further forward flight testing. It is amazing just how much stability a flybar employs when you need it. In all fairness one would probably not fly a scale machine under these circumstances.
It does fly well for what it is……………..a flybarless or non stabilized rotor head.
Just prior to landing the machine exhibits a notable roll cross-couple, nothing radical but something you should be aware of. All in all not a bad beast and one that should give good service life. For a scale 212 this head will look accurate, since most people fly scale in good wind conditions its nasty flight habits should really pose no problem.