Trex250Build

Align TREX250 Remote Controller Electric Helicopter
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Building tips:

First let me say this: It is small and all the parts are small. Very small. Magnifying glass is a recommended tool. On this page I have assembled some of the experienced I have accumulated when building the machine, crashing it several times and finally tuning it to be a reasonably smooth running helicopter.

Build User Manual:

The TREX250 build / assembly user manual is available in PDF for ease of access.
Build Manual

Thread-lock:

Although thread lock gives screws more holding power, you can still loosen and remove them for maintenance by applying a little added pressure. Stronger red and green thread locking compounds require heat to remove.

Green: Permanent and require heat to undo. Green is not required for the TREX250 and you just end up stripping the screws when opening (after a crash). So don't use Green on this machine. (Except on one part ONLY. Fixing the pinion to the engine shaft)
Red: Semi permanent. Still risk stripping screws when opening. My experience is; Red is not needed anywhere on the TREX250
Blue; This works fine on the TREX250. I use this with success on all metal to metal parts

Rotor Head (mixer and washout arm free play)::

A beautiful piece of mechanics. In my experience it has a flaw in the design. There is free-play or slack in the wash-out arms caused by free play between the screw and the bearings. You can experience this free-play yourself by twisting (positive/negative pitch) the main rotor blades by hand, The free play represents several degrees of pitch which is essentially outside your control. Not good.

The effect of this slop is demonstrated with actual pitch curves measured on my TREX250. The blue curve (Noforce) is the weight or the pitch gauge pushing the blade until there is no slop anymore. The Red curve (Force) is me holding the weight of the pitch gauge and attempting to align it in a mid point between the two slop extremes. As can see the slack represents multiple degrees on pitch-slop. Slop that cannot be controlled by the servos. The rotor-blades kind of flutters, certainly so when you transition between negative and positive pitch.

The slop is originating from two principal sources on the head. The -mixing arm- and the -washout arm-. On my machine the main slop was coming from the -mixing arm- so I concentrated my efforts on this one. My solution was to add an aluminium washer before entering the socket button head screw forcing the mixing arm to the flybar seesaw holder. For this to work a longer screw is needed.
All these parts are found in the spare parts bag for the Align TREX250 called:
- H25033T : HARDWARE BAG - TREX 250 (loads of useful screws). Buy one when you buy the helicopter!
- H25054T : 250 SPECIAL WASHER - TREX 250

The the screw was tighten (with Blue Thread lock) to the point of just stripping the screw. Result, slop is almost gone, the mixing arm may move a little tight at first, but this is better than the massive slop.

The slop on the washout arm was reduced by similarly tightning the screw marked with red arrow to just before stripping the screw (and adding blue thread lock)

The final rotor head can be seen in the picture to right.
Arrow (A), showing the added washer. and arrow (B) showing the screw tightened to the max.

Main drive gear:

The instruction manual states that the pinion gear should be fixed to the engine drive shaft (using red thread lock !!) with the top of the pinion flush with the end of the drive shaft. This is kind of fine, but in order to make the main drive gear run in the middle of the pinion gear I found it necessary to to add a spacer in addition to the one specified in the build manual. With the main gear added and secured with the main gear screw (jesus bolt) I pulled the rotorshaft hard up, on the same time pushed the -lock collar- hardish down and secured the setscrew on the -lock collar- The result was a nice no slop tight fitting rotorshaft.

Helicopter overall balance:

A tip from another TREX250 pilot led me onto checking the balance of the overall helicopter, with the battery in place. Hold the helicopter using two fingers on the -head stopper-. Check if it stays level. Mine didn't and was tilting backwards, "tail heavy". To compensate for this some weight was added and decided to glue lead underneath the battery tray. Lead weights are readily accessible if you reload your own ammunition. I found a .357 mag 168grains (about 11 grams) lead bullet to be the perfect fit. Cemented the bullet onto the battery tray using epoxy glue.

The added benefit of this configuration is that the power wires from the battery to the ESC are held in place by the bullet.

Electronics and Component placement:

With no former experience in building a helicopter I searched internet for guidance and advice and found plenty. But still there were numerous un-answered questions, typically on small details here and there. On the electronics placement front the experienced pilot typically use words like "tuck it away" for the electronics. I thought about this and decided against tucking things away, I wanted things as accessible as possible on the final product. This turned out to be a very smart decisions as small adjustments and corrections following a crash is necessary as I learn more and more in how to control the machine. There are four considerations regarding component placement:
1/ Antenna
2/ Gyro placement
3/ Receiver placement
4/ Electronic Speed Controller (ESC)

Antenna placement. There are a number of debates and opinions on how to place the antenna's for a 2.4GHz radio system. However, keep this in mind. The TREX250 is a small helicopter. It simply is not possible to fly this particularly far away from where you are standing. So the Tx power is plenty to overcome minor misalignments. In order to maintain a reasonable 90 degrees between (as recommended by the manufacturer) and on the same time keep it away from the noisy (electric noise) ESC and motor I strapped the antennas to the -Tail Boom Braces- as can seen from the pictures below, Works fine, no problems. I've flown the machine to about 30-ish meter of altitude. No problem.

Gyro placement. This was placed on top of the -Gyro Mount tray-. But reversed so the LED's and control button is easily accessible. Reversed in the sense that the Gyro must be programmed to understand that its "front" is pointing backwards. So rather than tucking it away inside the gyro mount tray compartment it was placed as far out as possible to enable continual access to the programming interface.

Receiver Placement. The receiver (Futaba R617FS 7channel) was glued to the underside of the gyro mount tray using double sided tape. It is placed such that the connectors can easily be accessed for maintenance. The added benefit of this placement is that you can see the green power ON LED in the receiver shining during flight (tail in flight). Great confirmation that everything is in order -and- it works nice as an orientation light.

Electronic Speed Controller. The only place left to locate the ESC is on the side of the machine. One plastic tie wrap secures the ESC nicely to the chassis. The benefit of this location is that the power wires to the engine can easily be accessed from underneath and thus disconnecting the engine of servo testing/adjustment or similar activity is required.

Tracking:

The tracking is adjusted by modifying the length of the linkage rods connecting the swash plate to the mixer arm. It is a bit of a trial and error process. I stained the tip of one of the rotor blades with a blue felt tip pen. Then spun the rotor at a moderate RPM and lowered my eye sight to the level of the rotating "dish" made by the rotor blades. If you don't spin the rotor to fast it is easy to spot the blue colour and then adjust the pitch up or down to bring the blade into the same rotating plane as the other blade. Before starting to adjust the tracking make sure a zero pitch reference blade is established and adjust the "other" blade into the same rotating plane as the reference blade. The picture below demonstrate the finished results, with the main rotorblades spinning in the same plane. The picture also show the effect of the red flybar paddles. I painted them red to increase the visual print of the machine in flight.

Some shots from the assembly process, more to come: