Century Swift ARTF

Written by Ashley Davis Sunday, 23 April 2006 00:00

Swift ARTF

Introduction

The Swift comes as an almost ready to fly kit. As such the build of this helicopter is much simpler in that it mainly consists of bolting two preassembled components together. The two main components are the main frames (including rotor head) and the tail boom (including tail rotor).

Having said this there is some building work to be completed. The undercarriage has to be assembled and bolted to the main frames. Additionally the tail boom supports need to be installed.

The above statements do make it sound like building this helicopter is very simple. In terms of building a helicopter from a kit of parts this is true. However, all helicopters are complicated pieces of engineering and just bolting these components together does require care, attention and some knowledge in order to produce a smooth running machine. The following pictures and instructions are meant to complement the manual supplied with the kit and are not meant to be used as replacement for the supplied manual.

Let's start with a description of exactly what components will be used in this build of the Century Swift.

Included in the kit are the following components:

* Phoenix 60 amp electronic speed controller
* Century sports brushless motor
* FlightPower 3700 mah 4S lithium polymer pack

For this build I added the following components:

* Hitec 635HB high torque analogue servos (x 3)
* CSM 560 heading hold gyro
* Futaba 9254 digital Servo
* JR 770 PCM receiver
* Overlander 2000 mah 4.8V nickel metal hydride pack
* GWS voltage monitor
* Medusa 6V BEC
* FMA 4CH optical isolator

The last two items above are really optional components which I added as I wanted to run a power system where the receiver power system was isolated from the speed controller and the cyclic/collective servos. I should also point out that the configuration above was not my initial choice. Initially I went for a much simpler system but unfortunately this led to a radio lockout and a crash on the Swift's second flight. Having had this problem I decided to install a much better and partially isolated dual power system for the radio equipment. Since doing this I have not suffered any radio problems or reduced range on the radio system. I will explain this dual power and optical isolation in the setup article which can be found in the setup section of the site.

For reference Century recommend running the Swift's entire radio system from a separate receiver pack and not to use the BEC capabilities of the Phoenix 60 amp electronic speed controller. Given my own experience I believe this to be good advice.

Box contents and undercarriage

So, let's open the box and get started:

The first task is to build the undercarriage. The undercarriage build includes the installation of the carbon fibre battery tray. The undercarriage once built is then bolted to the main frames. This section of the build is extremely straightforward and really requires no more explanation. Below are some pictures of the undercarriage build plus a quick look at the provided manual.

Tail boom installation

Having fitted the undercarriage the next step in the build is to fit the tail assembly to the main frame assembly. This includes setting the gear mesh between the tail rotor drive gear and the main gear.The tail boom bolts to the mainframe using eight self tapping screws. The screws are self tapping into plastic and therefore care should be taken when tightening the screws as stripping the thread is not desirable. The boom should be installed such that the tail rotor is on the right-hand side of the boom when viewing the helicopter from the rear. The eight screws should not be tightened initially as the gear mesh needs to be set between the small gear on the end of the tail boom and the main gear inside the main frames. In order to set the mesh the manual gives a good hint, which is to use a piece of paper between the gears in order to set the correct mesh distance. Once you have the correct mesh the eight screws can be tightened to secure the tail boom. Once this is done the tail boom supports can be fitted. The tail boom supports go from the back of the undercarriage struts to the horizontal stabiliser bracket. Again the manual provides a good hint in that some electrical tape should be wrapped round the boom in order that the horizontal stabiliser bracket is a good tight fit onto the tail boom. Below are pictures of the tail boom and tail boom support installation.

The prebuilt tail boom does not come with the vertical stabiliser installed. Fortunately this is very easy and consists of removing the M3 bolt that goes through the tail gearbox. The vertical stabiliser can then be positioned in place and the bolt replaced and tightened. The last thing I did in the tail boom installation was to remove the tail blade grips and apply Loctite to the screws holding the tail grips in place. I also discovered when I did this that one of the screws was actually already loose and would have resulted in a tail failure had I flown the machine. As with any preassembled kit it is good practice to thoroughly check all of the screws and bolts to make sure they have been installed and tightened correctly.

Lastly the tail control pushrod should be installed into its holding bracket and connected to the tail pitch slider control horn. On my machine I found that the bracket was too tight and required a small amount of work with some sandpaper to loosen it enough that it wasn't causing friction on the tail control pushrod. The bracket should not be secured in place at this point in the build as its position needs to be set when installing the tail servo.

Rotor head adjustment

The next step is to make the adjustments to the rotor head such that the machine is ready for flight. The Swift comes with the rotor head fully assembled but the flybar is not secured in place at the factory. I also found on my machine that I needed to take the set screws out of the main mast collar and Loctite them in place. For safety I also removed the blade grip bolts and applied Loctite so that these also cannot come loose. In general any screw or bolt that is metal screwing or bolting into metal should have Loctite applied. The adjustment required for the flybar is that it needs to be positioned correctly. The distance from the flybar hub to each paddle should be equal. The flybar has flats on it so that the grub screws in the flybar control arms can locate correctly. The flybar paddles should be positioned such that they are horizontal and perfectly aligned with the flybar control arms. If you sight down the flybar the flybar paddle and flybar control arm should be level with one another. Once the flybar paddles are level and the distance from flybar hub to each paddle is equidistant the set screws can be installed. The instructions highlight that you must use Loctite on the set screws. The instructions also highlight that at this point the various control rods installed on the rotor head should be checked to make sure they are all the correct lengths.

Servo installation

The installation of the servos and linkages is very well explained in the supplied manual. The pertinent points are that the ball links are keyed in that the century logo on all ball links should face outwards. Additionally the manual points out that the linkage rods should be installed such that 90° angles are formed between the servo horn and the control rods when the servo is at its neutral position. What the manual doesn't show is how the servos are bolted into the frame. Included in the box are some small plastic parts which the servo screws locate into in order to pinch the servos onto the main frames. The elevator servo also requires some spacers (also included in the kit) which move the servo out into the centre of the main frames so that a nice vertical control rod goes from the servo to the swash plate. I have pictured these small mounting parts below.

I chose to use servo discs in my installation and I noticed that some binding can occur between the ball link and the servo disc at extremes of the servo movement. Therefore if you also choose to use discs then I have included a couple of pictures below of how I cut the servo disc to alleviate the potential binding.

Section 6 of the manual describes how to set up the tail linkage and tail blades. This part of the manual I found to be incorrect in how it describes the setup of a tail rotor. The instructions detail making sure that the tail blades are at 0° pitch when the servo is in its neutral position. The effect of setting up the tail servo in this manner will mean that the gyro will continually have to fight the torque reaction of the helicopter as the tail system is not mechanically set up to compensate for this. Usually a tail rotor is set up such that there is a few degrees of pitch on the tail blades at the servos neutral position, there should be enough pitch to counteract the torque of the helicopter in the hover. This then allows the gyro to just compensate for changes in torque due to increases in either throttle or pitch on the main blades. Setting up the tail rotor in accordance with the manual will more than likely cause tail wag and cause the gyro to overcompensate when performing fast pirouettes with hard stops (such as would happen in a 540° stall turn as an example). I will cover how to correctly set up the tail rotor in the setup section of the site but for the moment just assume that the setup recommended in the manual will need to be changed during the initial flights of the helicopter.

Servos Update (June 2006)

Following the above servo installation I had some problems with swashplate interaction and decded to upgrade to digital servos. I picked Hitec HS6965HB servos which are extremely fast and very accurate. The pictures below show a much tidier servo installation with inward facing ball links and much straighter runs to the control horns.

Servo Ball links

The balls attached to each servo control horn need to be placed so that you get equal movement on the elevator and aileron servos. This is achieved by making sure that the aileron control horn balls for each servo are at 12.5mm from the center point of the servo. This then matches the 90 degree control horn to which the aileron servos are attached.

The rear elevator servo should have the ball at between 20-22mm from the servo center. This should then give you equal movement on the pitch when you use all three servos to raise and lower the swashplate. You may need to adjust the EPA on the elevator servo to reduce it's movement very slightly at maximum and minimum pitch. Otherwise you may get some slight interaction at full postive/negative collective pitch.

Also worthy of note is that the rear servo control rod will bind against the swashplate anti-rotation guide at full collective pitch, to remedy this just dremel away a small amount of material at the bottom of the antirotation guide.

See top right picture.

Linkage Lengths

Following a discussion on the Swift-Tuning forum here are some suggested pushrod lengths as the ones in the manual don't appear to be correct. All the following measurements are from ball link center to ball link center :

Aileron/pitch control arm to swashplate is 78mm

All the other linkages are then set based off of this one such that when the control horns are horizontal you have 0 degrees pitch on the blades and the washout arms and bell mixer arms are also horizontal.

The servo linkage lengths will depend on what servos you are using.

The swashplate to bell mixer link is 100mm

The bell mixer to blade grip link is 25mm

Linkage elevator servo to swashplate is 50mm

Linkage flybar control arm to washout arm is 54mm

These lengths should allow for approximately +/- 10 degress of pitch with 0 degrees at center stick (typical 3D setup).

Motor Installation

Installing the motor is a straightforward affair but there are a couple of problems that I discovered which are specific to UK supplied Swifts. The manual shows that the motor is fitted using two M3x16 screws. The motor supplied with the UK Swift cannot accommodate this size of screw and a 12 mm screw should be used instead. Century UK remove the 16 mm screws from the kit as trying to use these to mount the UK motor could damage it. US based kits come with the 16 mm screws and using these will not damage the US supplied motor.Before fitting the motor it makes sense to make sure that the pinion will fit onto the motor but there is no need to tighten the grub screw in the pinion at this point. Once the motor is mounted the pinion can be moved on the shaft to get a good alignment to the main gear. Once alignment has been done the grub screw in the pinion should have Loctite applied and be tightened. Now that the pinion is vertically aligned it needs to be horizontally positioned by moving the motor on its mounting such that a good gear mesh is created between the pinion gear and the main gear. Again the same principle can be applied as for setting the gear mesh on the tail rotor. Using a small piece of paper to set the spacing between the pinion gear and the main gear is the easy way to set the correct gear mesh.

Electronics installation

Placement of electronics and wiring is very important in electric powered helicopters. The idea is to try to separate as much as possible the electrical components such that they do not interfere with one another. As I mentioned in the introduction my initial setup had most of the electronic components quite closely located to one another and the result was a lockout/failsafe of the radio system which resulted in a crash. Having had this experience I decided to go for a much more comprehensive electronics installation. The following pictures show how I have installed the power system, battery elimination circuit (BEC), receiver battery pack, electronic speed controller, receiver and gyro. Additionally it is important to make sure that any wires are routed away from power carrying wires in order to avoid interference in the radio system. The electronic system installed on this machine uses optical isolation in order to separate the power systems of the helicopter. Essentially all of the cyclic servos and the electronic speed controller run off of one power system (Medusa BEC) and the receiver, gyro and tail servo run off of another power system (receiver battery pack). All of the components likely to cause electronic noise or interference are gathered together at the front of the helicopter (motor, electronic speed controller, BEC). The components not likely to cause electronic noise or interference are at the rear of the helicopter (gyro, receiver, optical isolator). This separation of noisy components from non noisy components in itself will provide for a much better radio system. The optical isolator acts as an additional safety measure between the two power systems. One last benefit from running this system is that the BEC is a 6V version and therefore gives me increased speed and torque on the cyclic servos.

When it came to mounting the gyro the Swift as standard doesn't appear to have any predetermined place. I therefore stuck a small square of carbon fibre to the top of the tail boom and place the gyro in this position. My only concern with positioning the gyro in this particular place is that it may get affected by static buildup on the tail drive belt. This could cause the gyro to reset in-flight but so far it has not been a problem.

Canopy and blades

The next part of the build is to cut out the canopy, cut out the canopy windshield and then install the canopy rubbers and windshield. The canopy windshield has a hole within it which needs the bottom cutting out of it. This hole is to direct airflow over the motor. Stickers can then be applied to the canopy after washing in soap and water or altrnatively it can be spray painted with appropriate paint. Century recommends Krylon 'Fusion' paint. Lastly the main blades require some installation work to fit the blade root reinforcement, which consists of some plastic inserts and a brass tube which goes in the blade root hole. The plastic inserts should be glued to the blade root and for this task I use slow setting epoxy as it creates a good solid bond. After the blade reinforcement has been fitted the main blades can be balanced using suitable balancing equipment. I will cover blade balancing in the setup section of the site. Once balanced the main blades can be fitted to the blade grips using the supplied blade bolts and nyloc nuts. This completes the build of the Century Swift. Below are a number of pictures of the completed model which may be helpful in performing a successful build. Following the build the helicopter will require setting up which is covered in the Swift setup article in the setup section of the site.