HK250 GT Build How-To

Some notes on the things I found and modified as I was building my HK250 GT (Align Trex 250 clone):

One way bearing binding.
The securing washer on the bottom of the main shaft binds against the one way bearing. The length of the main shaft below the shoulder is not long enough to allow the bottom screw to be tightened against the shaft without binding on the OWB.. hence the main gear is not decoupled from the main shaft in autorotation.
These little helis are notoriously difficult to autorotate anyway because they have so little blade inertia that the flare at the end of the descent must be flown perfectly. There is only enough energy in the blades to flare once and slow the descent for landing whereas with bigger helis there is enough to bring them to a low hover for a couple of seconds before the blades slow down and stall. Unless you're well practised at autorotations and think you can pull it off with this heli you may not want to worry too much about the binding problem. With the radio and servos being powered from the same source as the motor, if the motor fails, the chances are you won't be in control of the heli anyway.

Mounting the motor.
I used the M3 size screw holes in the motor as they are the correct distance apart for the motor mount on the heli. The slots in the mounting plate need to be widened ever so slightly to allow M3 screws to move freely.

This motor has quite a long shaft and I didn't want to cut it in case I ever want to use it on another machine so I used cut down servo arm centres as spacers to drop the motor down to allow the top of the shaft to be free of the lower servo case. Cut down M3 screws to length so they don't damage the motor windings as they screw into the motor chassis.

Turnigy 2627 4200Kv motor pinions.
The 9T and 11T pinions that come with the Turnigy 2627 4200kv motor I've chosen for the HK250 definitely measure 0.4Mod and are therefore suitable for use with the HK250GT main gear. Be aware that the standard pinion for this heli is a 15T so with a 9T or 11T pinion the head speed will be lower for the same motor Kv rating. You may want to buy a 15T 2.3mm shaft 0.4Mod pinion to get the highest head speeds if your motor has a lower Kv rating (3100Kv for example).

I'm going for a 13T pinion with the 4200Kv motor. Early tests show the max headspeed at zero blade pitch with a 2S battery is around 3000RPM. With a 3S battery that should bring it well up over 4000RPM which is where this heli likes to be. (The Align Trex 250 manual recommends not using head speeds greater than 4500 RPM although there are people out there on the forums who run higher.. can you imagine what happens when something lets go at that kind of rotation!)

In case you read the comments on the HobbyKing site, someone thinks the pinions supplied with the motor are 0.5Mod which I believe to be wrong. If you want to measure your pinions and find out for yourself read my Pinion Modulus How-To.

Cyclic Servo mounting
Put the servo mounting tabs outside the frame so that the servos are easier to change. That way there is no need to open the whole frame to change one. The geometry of the linkages to the swash plate is not perfect (it should be 90degs to the servo arm and 90degs to the frame at zero blade pitch) with this system but it's not too bad. I used the second hole in from the thin end of the servo arm to optimise the geometry.

Setting up the Head.
The set up required shortening the diagonal ball links to the upper mixer arms almost as far as they would go to raise the swash plate up the shaft. Out of the box it was too low in its range at zero pitch.. it allowed more negative pitch movement than positive. These kits are NOT set up at all, you really need to go through the full set up for the head starting at the servo arms and working up from there.

Be careful when adjusting the ball links... the standard links supplied by HobbyKing are designed to fit onto the ball one way only. If you look closely you will see that one side of the hole in the plastic link is larger than the other. Adjustments to the length of the link can only be full turns so that the correct side faces the ball when you re-fit it. How do I know? Because I wrecked a link putting the smaller diameter side onto the ball. After doing that the link was so sloppy on the ball that I couldn't risk using it.

Get some ball link tweasers, the standard ball link pliers used for 450 size helis and upwards are too big and will stretch the little 250 links. Pretty soon you'll find the links are too sloppy on the balls. I tried one proposed solution for sloppy links which is to heat the plastic a little - allegedly this makes it puff up and fit more snugly. Well it didn't work on mine.. the plastic just melted and destroyed the link. Your mileage may vary with this technique!

Tail boom fixings.
1) I used an M1 screw, cut down to length, to secure the tail box onto the boom. There have been reports that the normal fixings are not enough to stop rotation. This modification ensures that the tail box can't rotate around the boom or slide off the end.

Cut the screw so that it is long enough to pass through the existing hole in the left hand side of the tail box and just screw into the hole in the boom but not touch the belt. The head of the screw needs to be fairly flat so that the tail stand can screw down over the top of it without bending.

2) At the helicopter end of the boom, I lengthened the boom slot to allow easier installation of tail rotor belt using a Dremel cutter wheel. The down side is that with a longer slot it is only the friction of the tail boom clamp that holds the boom in position against the tension of the belt drive. Will it hold it? Is there an easy mod to secure the fore/aft position of the tail boom? - I'm working on it. Overall, it would be better not to lengthen the slot any more than is absolutely necessary to get the belt over the drive pulley.