Using a Century NX50 for Precision FAI Aerobatics ?
Phil Noel (copyright)
There are a number of "features", on any model, that can enhance its' relative smoothness and precision. One does not necessarily need an expensive, high-zoot 90 sized heli to start getting competitive in this discipline. Using the Century 50NX will give you a relatively low cost means of getting into precision aerobatic practice and competitions, while performing at a surprisingly competent level.
I have used a Raven 50 from Century, for a number of years, as an FAI practice machine, and have recently changed to their newly released 50NX. Like the high-zoot Avro Blitz & the Kyosho Calibers, it uses a smooth, two stage power system. It is an all gear system, rather then a belt system, so belt tension is not a consideration. It also has its' auto bearing in the primary hub, rather then in the maingear hub, so it is under much less strain.
I also like its rotor head hub, which is not only a precision machined piece, as is the Kyosho Calibers and Hirobo Evos, but it also incorporates a clamping device machined into its base, as seen on the Blitz and a number of the other recently released 90 sized FAI machines.
It has a great 4mm flybar system, one that is rarely seen on the 50 sized machines. It is not only more rigid then the more common 3mm units, but allows me to experiment with a number of different flybar lengths and FAI type of paddles, of which all seem to be made for 4mm flybars. It also has a rather heavy duty flybar control yoke system (straight from their big 90 nitro and gas powered Predator) that will not flex under heavy cyclic loads.
Another plus, is that the 50NX’s slop free metal swashplate and eCCPM control system insure the transfer of even the smallest command will always be linear and precise. The three identical dual bearing bellcranks that transfer the servo commands to the swashplate, will probably remain slop free for the life of the heli, as will the heavy duty dual bearing supported metal swashplate that comes stock with the heli. This all means less maintenance over many hours of practice.
The efficient power transfer of its torque tube drive system (again not a standard option on nay other 50 near its price range) and the better auto performance it provides, due to the lesser drag of this system. The long stock tail boom also lets me use the 615mm FAI blades that are made by Rotor Tech. This combination makes for auto performance that can rival many 90's.
The tail control precision of its' triple bearing tail rotor with a steel hub, which features integrated machined spindles for each blade grip, along with two radial and a thrust bearing in each grip, is usually only found on high priced FAI 90 machines. So it is a big plus to find such a tail rotor system here without having to upgrade.
But all that being said, I think the main thing that has separated any of my FAI helis from my 3D ones, is the set-up of the flybar systems, the selection of mainblades and the different settings of the pitch and throttle curves for hover, compared to upstairs.
Tuning the flybar lengths and experimenting with flybar paddles can make a world of difference - ditto between the differences between similar length 3D blades and those designed for FAI use. e.g. the difference between the Rotor Tech 615 FAI and their 610 3D blades. And due to all the fuel burn one requires to practice and become proficient in FAI, I find that a helis with a low "component wear factor" to be a big plus...as it will stay as consistant in flight performance from its' 10th flight to its’ 400th.
I really feel, that teaching anyone who wants to try FAI how to set-up any heli, be it from any manufacturer, to get the best out of it for FAI precision, is even more important the heli itself, but with the 50NX, one is certainly starting with a platform that seems ideally suited from the get-go.
So, like most anything, how much money one throws at something does not necessarily make him more competitive...his thumbs and his understanding of how to optimize the set-up of his heli for FAI precision, are much more relevant.
On that note, let’s look at how one can do so with the 50NX. Also please note that I am assuming we are starting with a relatively linear mechanical system set with an -11/-5.5/0/5.5/11 collective pitch arrangement. If the mechanical system is linear, it will be easy to consistently set for what we want, simply by using the software in the radio.
First, let us look at what we can do with the rotor head. If you are serious about FAI type of competition, then get a good set of FAI blades. These are very different from ideal 3D blades. A good all around choice here is the Rotor Tech 600mm FAI units (CN266001) or even better, their 615mm units (CN266101). The next item, that will make a lot more difference then most think, would be to get a set of the Rotor Tech CN262460 FAI flybar paddles. The weight of them is perfect for stability in gusty winds, while the fat airfoil and large area insure a fast and predictable cyclic rate that really works well in the aerobatics.
Now you will have a rotor head with all the potential of being solid in the hover, predictable through maneuvers, and which tracks on rails in fast forward and fast backward flight, at all airspeeds. It will be without any left/right twitchiness or fore/aft pitchiness at anytime, even in relatively gusty winds or at high forward and backward speeds. Note, I said it now had all the potential to be such. That is because the rest is going to be in the collective pitch curve and the cyclic pitch set-up to come later.
As you already have a very precise tail rotor system in the 50NX, one only has to go to some optimum tail blades to insure maximum precision in any yaw or pirouetting maneuvers. Here I recommend you replace the relatively flexible stock 85mm plastic tail rotor blades with some stiff, well designed C/F tail rotor blades. By well designed, I mean a set that have their chordwise CG relatively forward so they do not lead or lag while spinning. There may be others, but to date, the only ones I have found are again from RotorTech. These are the CN260923 & the wider chord CN260926. The ones you use will be relative to the power you have available to you, the mainblades you choose and the head speeds you like to use.
Also, a worthy change for more control precision and to optimize the gain setting on your gyro, would be to install a C/F tail rotor control rod system. I found the CN2264 to be an easy change and compatible with many different helis.
For the hovering maneuvers, you want the heli to fly with a relatively soft collective, so that any altitude correction does not look bouncy. You want them to look smooth and relatively seamless. You want to have enough collective to make the necessary corrections and to perform the hovering vertical climbs at an appropriate rate. You also want enough cyclic to make smooth positional corrections and smooth starts and stops, without looking twitchy or without inducing over correction. For this you do not want all the collective pitch range, or all the cyclic pitch range, needed for the upstairs aerobatics.
I used to hover the old way, which was at 1/2 stick, but with the advent of the more demanding aerobatic maneuvers in the schedules today, ones that entail more inverted flight, I found setting my FAI machines for hover at the same stick positions as in 3D and also for idle up modes upstairs to be better. It meant only to have to learn the feel of one stick position for anytime I wanted the heli to be in a right side up hover. So teaching myself to hover at 3/4 stick made it easier for the upstairs stuff also. This way, the point where I felt comfortable, when the heli was upright, and stationary, was always at the same stick position.
Considering this, for the hovering maneuvers, I suggest that you set your top stick to be less then your upstairs setting. As an example, if your upstairs aerobatic mode is set for 11 degrees, then make your top stick collective pitch for the hover mode to be 8 degrees. As all the hovering maneuvers are flown right side up, one does not need negative pitch settings that will make you climb quickly inverted. You just need enough negative to allow you to descend at a reasonable rate in the highest winds that you would have to compete in. So you probably will not need much more then -3 on the low end of the stick. As you can see, you now have a limited pitch range of only 11 degrees (-3 to +8). As for the cyclic settings, if your upstairs mode is set for 7 degrees of cyclic, set for only 4 or 5 degrees in the hover mode.
Your hover mode should also be set for a rotor speed that is also conducive to smooth corrections and point to point movement. Setting to hover at a lower rotor speed then you would set for the aerobatic portion will also be a big help. Consequently if you set for 1900 upstairs, set for only 1500 to 1600 for the hovering mode.
With a 1600 rotor speed and a set of 615 mainblade, you will find that your heli will hover at a 4.5 to 5.0 degree of collective pitch. This is the pitch setting to use at the 75% (3/4) stick position. From there, you can set a straight line to the 0% (bottom) stick position. You should have a setting something like this -3/0/3/5/8. You will note this will result in a little hump at the 1/2 stick to the 3/4 stick position. This will introduce a slight exponential setting in the collective, at hover.
After all of this, you will find your stick movement workload is much diminished while attempting any of the hovering maneuvers. So you can spend more of your brain’s data processing power, and your finger coordination command center, in stopping vertical and horizontal ascents and descents where you want them, your forward and backward movements in hitting the flags and in controling your pirouettes to piro rates you desire, instead of also having to try and keep the heli from bobbing and twitching.
The upstairs aerobatic set up is much less complicated. I personally like the collective set-up to be such that at the higher rotor speed needed for the faster cyclic response, I will still hover at my 3/4 stick setting (so this may only be 4 or 4.5 degrees) with the top setting being what the power system can take without bogging, with some reserve for minor correctional cyclic input at full throttle opening (maybe 10 or 11 degrees), with 0 at half stick and a mirror image inverted (below half stick). Here one may start with a collective setting of -11/-4/0/+4/+11. If you have a radio with more powerful software that has 7 or more points that can be set, one may do a humped curve right side up and inverted also, e.g. -11/-4-/-2/0/+2/+4/+11, to introduce a bit of exponential for a softer collective response through any of the hovering parts of the scheduled maneuvers .
Of course, throttle settings must be used that will maintain the desired rotor speeds at all these various pitch settings. But here I am assuming that your engine carb needles have been optimized and you are using a governor.
All of the above should give you a good place to start, and allow you to tweak it further in the future, to suit your own personal prefered feel. Much of it is also applicable to other heli airframes that are relatively solid and consist of basically well designed systems. So now, go out there, burn some fuel and enjoy your practice sessions and get competing.