This article is reprinted from the Febuary 1998 issue of Model Helicopter World.
Model Helicopter world is an excellent radio controlled helicopter magazine published
In the UK. Model Helicopter World can be reached in the US by Contacting
Traplet Distribution USA (626)-836-6931, and in the UK via email at mhw@traplet.co.uk
Note from HHI: The Quick .60 Head is available for immediate delivery
The Project
The other day while rummaging through the shop for ball links my lonely and neglected flow powered Schluter BK117 began giving me haunting looks. The machine has occupied the work bench all summer without so much as turning a blade. At this point I am thinking what a waste of technology. The reason it has not flown is due to the four bladed rigid rotor head. I had spent many hours mechanically repairing poor bearing fits in the blade grips only to be rewarded with a machine having the unpredictable disposition of a rabid dog. I will take a flybar stabilized rotor system any day - thank you very much! My admiration goes out to those who have taken the time to learn to fly this type of model rotor system.
I began looking for a replacement rotor head since guilt had seen the better of me. While I like the through spindle rotor head designs, I wanted a soft rotor disk for a scale machine. I began researching matters and came across an advertisement for the Hobbies & Helis "Quick" metal rotor head. This is designed specifically for most .60 size helis and has teetering damping with adjustable mixing. It is an underslung teetering/flapping design, which is known for its smoothness and was very well suited for my application. I checked `around town' to find out what kind of success people were having with this head and was unable to get any firm specifics so I thought more information would be of interest to people. Since I fly the X-Cell gasser, this gave me a reliable platform for evaluation. I am running the 45-degree swashplate system on the BK-117 and time will have to be spent manufacturing suitable linkages and a flybar washout system to properly phase the blade inputs. I personally like the teetering type since the mechanical design is less complicated than the swing head, requiring no extra connecting rods from grip to grip for the flapping action. As far as which type is better, I cannot comment since I have only used the through spindle and under slung teetering systems. There are several alternate teetering and swing heads available from various manufacturers, I just wanted you to know there are options out there if you are in the market for a similar semi-rigid rotor head design. Hobbies & Helis were waiting for the next shipment when I ordered the rotor head, so a couple of weeks wait did not seem too long as I had other machines to fly.
In preparation for the head change, I measured and recorded collective and cyclic pitch angles at the paddles and blades, which also included the mixing ratio for flybar tilt. While this is probably unnecessary it does give me some comparative numbers to digest later. What is going to be interesting will be what over all effect the increased blade flapping and non-offset mixer hinging will have with all else being basically equal.
The Head
The finish quality is nice with gold on black coloring. The blade grips are well supported by two radial and one thrust bearing. The spindles are housed in a bearing block, which is attached and `sandwiched' between two head plates. The head block side plates (hub plates) have an area milled off on the under side which partially exposes a counter sunk screw. I see no reason to remove as much material as has been done, even though the structural integrity of the head has not been compromised. This design distributes the load over eight bolts per grip so structural safety is not, I repeat not, a concern. The reasoning for material removal was for Hiller pitch control arm clearance, but the arm comes nowhere close to the screw. I'm fussy about these things and am not afraid to admit it.
The real beauty of the hub plates is in their basic design. A channel accurately milled into these plates and secured as mentioned earlier positions each blade grip bearing block. Using such a method ensures accurate alignment when assembling the rotor head following maintenance. This could be the case when greasing the thrust bearings or should damper replacement be necessary. I would advise thread locking the 16 hub plate screws in addition to the 2 damper assembly retention screws. Mine had no thread locker in this area and further checking revealed all other fasteners to be securely attached. Additionally, it would be wise to grease the inboard thrust bearings at this time.
Each spindle bearing block transfers centrifugal load to the head plates from the spindle and blade grip. A 4mm bolt ties each blade grip and spindle together. Early heads had used 3mm cap screws. The reason for increasing the size of the bolt to 4mm was to take full advantage of the larger stress rating of other head components in the `loading path' of blade centrifugal force. I have not heard of anyone throwing a grip on the earlier design, only speculation on this point. I might add that the grips are bushed for the blade bolts.
The massive head block is machined from bar stock aluminum and houses the teetering elastomer, the flapping bearing shaft and the flybar carrier bearings. Mounted on the exterior are the cyclic Hiller multiplier levers and the pins to drive the washout unit. Mounting of the block to the mast is of a redundant nature using the usual stainless steel bolt along with a clamp up at the lower end. The washout drive pins screw into the head block with thread lock.
The Hiller multiplier is not often seen on a model helicopter and allows easy adjustment of the Hiller ratio, independent of the adjustable direct inputs to the mixing lever. Both levers are dual ball raced. The multiplier lever pivot bolts are a dual purpose by supplying the clamp up force mentioned earlier to the slotted head block. Very crafty indeed.
This head has no `Delta' effect (nor is it required) when flapping and the pitch horn input is located at the center of the head, or not offset hinged. The damper stiffness can be adjusted if you so choose by selecting a harder elastomeric material. Damping occurs only on the teetering axis. Due to the geometric nature, under slinging of the feathering axis to the flapping axis will cause any in-flight dynamic blade oscillation (lead/lag) to be reduced to an amount, which the blades can surely handle. Coriolis effect, induced by blade flapping is largely cancelled out in the way the blade mass is shifted during this action. In other words the blades are quite capable of absorbing the lower fore/aft bending moments of this head design if tightened down completely. If you plan on hovering/flying upside down most of the time, this may not be the very best design in a soft damped state, since underslinging works correctively only under positive coning or rotor G's. The blade bolts could certainly be loosened up slightly to allow for the lead/lag oscillations and /or the damping stiffened up, to allow for a best compromise situation, similar to a stiff non-underslung spindled rotor head design. The supplied blade bolts are threaded most of the full shank including where the blade makes contact and therefore should be replaced with bolts that are threaded only at the end for nut engagement and bolt tension. I have noticed this time to time with various manufacturers and simply shake my in disbelief; it is very easy to rectify with proper placement hardware.
Not included with the head are the flybar and the direct input rods from the swashplate to the mixing levers and paddles. The unit comes with metal mixers and a flybar yoke with paddle pitch control arms. All the moving parts are properly supported by ball bearings with no bushings.
Now that I have nit-picked, the bottom line is that this head is structurally capable of carrying the .60 size blade stress.
Installation
The flybar was balanced during installation and no weight was added also the head required no additional balancing by being spot on. The rotating control rods supplied with the head were matched to their opposite mates using a caliper. The balls on the head are slightly smaller than the stock X-Cell balls but enough properly fitting rod ends were supplied. I followed the simple set-up instructions and the pitch curve remained very, very close to the original after I bolted a pre-balanced set of blades to the unit. To be honest, I have not changed the pitch/throttle curves, relying on the pitch trim knob for a slight adjustment.
In the Air
I picked the machine up into a hover after playing with it on the ground and it was very smooth, especially when making cyclic control inputs. I'm smiling at this point because my fingers seem to have become better at air taxiing about. The machine has become less sensitive, but still retains its good cyclic authority. This type of head is completely resistant to ground resonance, even on a hard surface and at any RPM. One characteristic of this head is the ability to run at high or very low RPM without any shake, wobble or nodding. Accurate placement when landing on a `postage stamp' size landing area is easy, even in gusting conditions. This is all a big plus in my books.
There was some question of interference between the multiplier levers on the head and the stock plastic washout arms during the initial static inspection. With the flybar level and then deflected with full lateral cyclic held in, seven additional degrees of (Hiller) blade pitch was recorded before binding occurred. This gives a total of 13 degrees from a level rotor disk in addition to any collective settings. This is certainly more than enough pitch by the indicated numbers. It is often very hard to convince oneself of this fact even after reviewing these basic calculations. I decided to put a dab of soft paint where such contact might occur and fly the machine for later inspection. Well the good news is no contact was being made. In reality this flybar, like most, does not tilt very far under dynamic conditions. A little monkeying around is to be expected with a large component change such as this, and I was really surprised that more effort was not required.
All things considered, this head will fit most any .60 size machine with a little creative thinking. In several other installations the above static interference will never happen. I initially used a very light flybar system with the condensed X-Cell control system (short mast) which by nature is more critical to this particular situation. In all fairness, if you are not as greedy for collective and cyclic control travel as I, and simply installed it as supplied, there would be no question here whatsoever. As you might imagine, I like to tinker and also to be very sure of things.
After feeling confident with the machine, I went upstairs and found the cyclic to be strong, which is something a gasser likes. It can, however, drag the engine down, so provisional mixing between the cyclic and throttle is recommended, or a governor might also do the trick. Consideration of top pitch when using hard over cyclic should be given for the same reasons. There is a small amount of pitchiness that can be attributed to this head installation when using a very light flybar and sharp Schluter paddles. On other rotor heads, I would not venture to go this light and sharp. The machine will also tuck the nose under slightly with a rapid collective decrease as is common with an extremely light flybar. As mentioned, the helicopter has a strong cyclic, but without any of the squirrelly fuselage following that can sometimes look `rachet-like'in appearance when over controlling with the light flybar. High `G' loops with full positive collective can be made pretty tight with relatively small paddles.
I have now switched to the stock MA paddles since weight can be hidden and fine-tuned internally. This has totally removed any minor flybar-less qualities induced by my previous set-up. Cyclic is still strong and I shall stay with these paddles for now. This is certainly a personal thing and will vary from person to person. This head shines best during hovering and close in maneuvers.... This is where fine details are easily seen. As for the forward flight, it rates up there with the best of them, however, it is more focused towards sport flyers than the 3-D jockeys for the reasons mentioned earlier.
I have not explored the full possibilities of this head yet, even though presently it has met my expectations as a sport flyer. It is a very flexible and user friendly mechanism and there will be no set-up limitations due to lack of adjustability. In a nutshell, I like mine and would certainly recommend it for consideration to the intermediate flyer that is in the market for such a component in this price range. If I had the choice to do it all over again, I would still choose this rotor head type for my future BK-117 project.
The "Quick" rotor head can be purchased through Hobbies & Helis for the retail price of $249 US Dollars.
By Stephen