How to Convert a Trex 450 for Flybarless Flight with... (rev 1.0)

How to Convert a Trex 450 for Flybarless Flight with the SK360
(rev 1.0)
A Trex-450 can be converted for flight with the SK360 Digital Flybar using stock rotor head
parts. This document starts with the assumption your rotor head from the swash up has been
removed and taken apart. Figure 1 shows what the rotor head will look like after the
conversion.
The two main features of this conversion are the anti-rotation mechanism on the bottom, and
the reducer arms at the top. The reducer arms are required because the standard blade grips
for the SK360 have their ball link close to the axis of the spindle shaft, which results in too
much cyclic blade pitch for flybarless operation.
You could just reduce the servo travel, but that amplifies any slop in your swashplate, and
you’d only be getting half the torque and half the precision out of your servos. By using the
full range of servo travel, you can get much better stability.
Figure 11 : The assembled Flybarless rotor head
Copyright 2008, by Skookum Robotics, Ltd
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Step 1: The first step is to make the linkage rods. Two 36mm lower rods and two 14mm
upper rods will be needed, measuring from the inner edge of the connectors (See Figure 2).
Figure 22: Upper and lower linkages
Step 2: Assemble the reducer levers, which are made using two of the Mixing Levers from
the stock flybar cage. These can be plastic or aluminum, but metal is better here. The reason
is that when you crash, something is going to break, and if you use plastic levers here, they
will be what breaks. The problem with that is the bag of flybar-cage parts they come in is
relatively expensive. Looking at figure 3, note which side the balls are mounted on.
Figure 33: Reducer Levers
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Step 3: Build an anti-rotation mechanism. For this step you’ll need either a second set of
the stock Mixing Levers or else a set of the stock Flybar Control Levers. Mixing levers will
be a bit easier to modify.
At the bottom of Figure 4, an unmodified Mixing Lever is shown. Here the short end has
been cut off, which is optional. The flat at the long end has been filed or cut to just barely let
the stock Flybar Control Lever Link fit onto it, with the standard Control Lever Link
mounting bolt. Notice the brass bushing is not used, which means a good fit is required.
The arm is now mounted to the far side of the stock Washout Base, so the arm doesn’t stick
out too far. The reason a long arm is used is to allow the full range of collective pitch motion
of the swash, without the anti-rotation mechanism binding or the washout base moving.
Figure 44: Anti-Rotation Mechanism
For this flybarless conversion, the Washout Base must be fixed in place so it can’t slide
down off the guide pins at the base of the Rotor Head Block. One crude but workable way is
to bend both guide pins slightly to one side (See Figure 5). A second way is to drill and tap
the side of the Washout Base for a set screw.
Figure 55: Modifying the Rotor Head Block (optional)
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Step 4: Assemble the rotor head. In Figure 6, notice that the reducer arms are mounted
level. The idea is to get all the lever joints to be as close to 90 degrees as possible, for even
blade pitch changes.
Figures 6 and 7 show metal blade grips, but for sport flying plastic blade grips can be a good
idea. This is because they’re inexpensive, and will break before anything else does, acting
like a mechanical fuse (something has to break, and your choices now are grips, reducer
arms, or servo gears).
Damping should ideally be firmer than stock, although not completely stiff. Adding thin
shims to either side of the Rotor Head Block when you mount the grips will accomplish that.
Figure 66: Completed rotor head, end-on view
Tip: When setting the blade tracking, adjust the lower linkage rods not the upper ones. This
gives much finer control over tracking.
Copyright 2008, by Skookum Robotics, Ltd
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Figure 77: Completed rotor head, side view
Step 5: Mount the SK360 Digital Flybar. The best place to mount the SK360 is between the
frames on the main frame bottom plate, with the gain dials facing backwards. This keeps it
away from any heat sources, and protects it in case of a crash.
To avoid vibration problems, make sure the gyro has at least 2mm clearance from both side
frames, and that the wires leading to the gyro are a bit loose for their last few cm. Try to
have its case parallel with the side frames of the Trex.
Figure 88: Mounting of the SK360 Digital Flybar
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Step 6: Configure the SK360 using the setup software. An SK360 setup file for this Trex
450 conversion should be included with this document as a starting point.
Follow the steps in Section 9 of the manual, to be sure the SK360 is setup properly. The
included setup file should give you a boost here, but don’t be afraid to change anything if
you need to. Then follow the procedure in Section 5 of the SK360’s manual to be sure both
the mechanical and gyro setup are correct, and you have the right range of cyclic pitch.
Note that if you’re a sport flyer, it’s a good idea to set about 40% expo on aileron and
elevator in your transmitter. Scale flyers may also want to reduce the rates on the Advanced
tab in the SK360’s setup software, from 180/180 to 120/120.
Finally, go to sections 11-13 for final checkout and some basic tuning tips.
Step 7: Fine tune the SK360 for maximum performance with the Trex 450.
If you’re using digital servos on the swash, you can safely increase the damping gains on the
Advanced tab to 20/16. This will help steady the heli in fast forward flight and during piros.
If you’re a sport or scale flyer, you may want to reduce the “Hiller Decay” on the Advanced
tab to as low as 40%. That will help the heli hold steady in wind and trim itself level. For 3D
flying Hiller Decay is better off at 100%, to damp out oscillations during repeated tick-ticks.
If your heli still consistently drifts gently left, right, forward or back in a hands-off hover,
check to be sure the swash is level. It’s safe to trim the swash mechanically with the servo
linkages, or in software, whichever is most convenient.
If you’re a 3D pilot, for good performance during piros and fast collective changes, you must
make sure all of the following is right:
1) The heli’s centre of gravity should be directly under the main shaft.
2) With the SK360 in setup mode, the swash should be level at max positive, zero, and
max negative collective pitch. If it isn’t, adjust the individual servo throws (servos
vary!)
3) Adjust the tail drag compensation. Start from a hover, and then hit max positive
collective. If the heli’s nose goes up, lower the tail drag comp for elevator by 1 or 2
steps, and if the nose goes down increase the value.
Copyright 2008, by Skookum Robotics, Ltd
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