Skultuna,Sweden2013-05-28

HK250GT FBL – Build and setup

Some things were tricky to figure out, so I put together this document in case someone needs a bit of help. I wanted the heli FBL (flybarless) to make it similar to my other heli (Align T-Rex 700E, 3GX). My idea was to build an inexpensive machine that I could toss around and learn stuff with, without worrying so much about crashing it.

Components and spare parts

Basic components (HobbyKing)

1 / HK250-GT / HK-250GT Electric Helicopter Kit (Alloy/CF w/ Blades)
1 / T2627-3800 / Turnigy 2627 brushless outrunner
1 / E2.3-0.4-14T / Pinion Gear 2.3mm/0.4M 14T (1pc)
1 / 155000109 / Flybarless Rotor Head Assembly Trex 250 / HK250
3 / 9378000004 / Turnigy XGD-11MB Digital Mini Servo 2.2kg / 11g / 0.12
1 / HKSCM9-6 / HKSCM9-6 Singlechip Digital Micro Servo (6V) 10g / 1,6kg / 0,09s @ 4.8V
1 / 297000003 / ZYX-S 3-Axis Flybarless System w/USB Program Adapter
1 / R100SAT / OrangeRx R100 Satellite Receiver
1 / 297000005 / ZYX-S DSM2/DSMJ Satellite Receiver Cable
1 / 9192000027 / Turnigy dlux 30A SBEC Brushless Speed Controller w/Data Logging (2s~4s)
1 / TGY_DEL_PB / Turnigy dlux Programming Box w/Data Logging Feature
6 / N850.3S.25 / Turnigy nano-tech 850mah 3S 25~40C Lipo Pack (63x32x25mm, 69gr)
1 / 601Ax5 / Male XT60 connectors (5pcs/bag) GENUINE
1 / XT60 / Nylon XT60 Connectors Male/Female (5 pairs) GENUINE
1 / HK-205BLW / 205mm Carbon/Glass Fiber Composite Main Blades
1 / Voltage-Alarm / Hobbyking 2-8S Cell Checker with Low Voltage Alarm
1 / 450-0010 / 450 Size Heli Servo Mounting Parts
1 / 9297000022 / Turnigy Ball Link Pliers for 250 Size Helicopter

This is a minimum list, no spare parts. I also got an OrangeRX (order nr R610) to bind the R100SAT satellite receiver to my transmitter (you can bind it with any DSM2-compatible RX, but more about this later). I velcroedavoltage alarm on the heli to get an early battery warning; not too keen about losing power in mid air (the ESC was setup to reduce motor power when voltage is getting low). The stock plastic main blades are quite soft, but still usable for mild flying. I put carbon ones on from the start; cheap but in general very good, stiff and durable and often no need to balance them. The 450 size heli servo mounting parts is not a typo, they are perfect for fastening the servos in the frame. Don’t forget the ball link pliers, it looks like a cheap pair of tweezers, but it’s really useful.

Spare parts (HobbyKing)

9 / 9378000004 / Turnigy XGD-11MB Digital Mini Servo 2.2kg / 11g / 0.12
2 / HKSCM9-6 / HKSCM9-6 Singlechip Digital Micro Servo (6V) 10g / 1,6kg / 0,09s @ 4.8V
6 / HK-205BLW / 205mm Carbon/Glass Fiber Composite Main Blades
3 / GT250-147 / HK-250GT Main Shaft (3pcs/set)
4 / GT250-126 / HK-250GT Tail Blade
2 / GT250-143 / HK-250GT Main Gear
1 / GT250-145 / HK-250GT Tail Drive Gear Shaft (2pcs/set)
8 / GT250-74zzd3 / HK-250GT Ball Bearing 7 x 3.5 x 2.5mm (4pcs/set) (main shaft bearings)
8 / GT250-682Xzz / HK-250GT Ball Bearing 6 x 2.6 x 2.5mm (4pcs/set) (main rotor blade grips bearing)
2 / GT250-52zzx4 / HK-250GT Ball Bearing 5 x 2.5 x 2mm (4pcs/set) (rear shaft bearing)
1 / GT250-63zz / HK-250GT Ball Bearing 6 x 3 x 2.5mm (4pcs/set) (inside tail rotor slide)
2 / GT250-133 / HK-250GT Tail Support Rod
4 / GT250-132 / HK-250GT Tail Boom
6 / GT250-121 / HK-250GT Base Plate & Landing Skid
2 / GT250-130 / HK-250GT Skid Pipe
1 / GT250-140 / HK-250GT Canopy Spinners
1 / GT250-125 / HK-250GT CF Horizontal/Vertical Tail Fin
1 / GT250-137 / HK-250GT Metal Tail Linkage Rod Fin Band
2 / GT250-116 / HK-250GT CF & Metal Anti-rotation Bracket
3 / GT250-113 / HK-250GT Metal Tail Drive Gear Set

Spare parts (e.g. AmainHobbies)

3 / AGNH25028 / Align 250 Tail Drive Belt
4 / AGNH25015 / Align 250 Feathering Shaft Set (2)
1 / AGNH25107 / Align 250FL Pitch Gauge (Silver)

I got the belts and feathering shafts because the original parts did not seem trustworthy. Having said that, I have a couple of flights behind me now with the original feathering shaft, and so far it’s been OK. But I wouldn’t take chances with the tail belt, it’s quite weak. You might even consider a Kevlar belt (google for it), but the Align ones are good enough I think. You don’t really need the AGNH25107 pitch gauge in my opinion; soon enough you get used to eye-balling the blades and make the adjustments. But perhaps in the beginning it can be of use. Get a digital pitch gauge for blade angle setup, they are so much better than the mechanical ones (and you can zero them on any angle, which makes things easier).

If you’re going be flying this heli for a longer while, then sooner or later (if you are learning like me) you are going to need all these spare parts. So far I have worked through (i.e. exchanged due to crashes) the following (not a complete list):

  • A large number of main rotor tail grip bearings
  • A large number of main rotor shaft bearings
  • A number of ball links (disappeared in crash or broken)
  • 6 feathering shafts
  • 6 HKSCM9-6 servos (this was when I had them on the swash; cheap but strip easily)
  • 6 XGD-11MB servos (better for the swash, strip much less easily and still cheap)
  • 7 main rotor shafts
  • 1 tail rotor shaft
  • 3 tail booms
  • 5 base plates/landing skids (they are really flimsy)
  • 3 tail boom support rods
  • 2 tail rotor blades
  • 3 main rotor blade grips
  • 3 main rotor radius arms
  • 1 tail linkage rod fin band (the plastic original)
  • 1 anti-rotation bracket
  • 5 main rotor blades (they are really sturdy,often these blades don’t break in crashes)
  • 3 metal tail drive gear sets (unfortunate design, Align is much better on this detail)

NOTE: I also ordered the whole heli kit twice for spares. This is really a lot more economic than buying parts one and one, so consider this when going through the spare parts list (sum things up and compare to the price of the whole heli kit). Don’t worry about a lot of pieces piling up, won’t happen! I got a few swash plates and chassis left over, but that’s really all.I also ordered two FBL heads when I bought the heli (one for the conversion, one as spare), which was lucky because in one crash I lost half the head. You will need to order a new head every once in a while.

Experiences from flying & crashing

The machine is quite brittle; the slightest you touch anything with the main blades you get a bent main or feathering shaft, or the feathering shaft breaks. The bearings in the main rotor blade grips are very sensitive as well (if you disconnect the rods to the main rotor grips and move the grips they will bind and be unsmooth if the bearings are not 100% OK). So don’t crash if you can avoid it, and be sure to check all the bearings when you do! 

I used HKSCM9-6 on the swash in the beginning, but boy did I exchange a lot of servos then, they are cheap, fast, precise and strong but they strip so easily when on the swash. I have used a couple of XGD-10HMs but the ones I really recommend is XGD-11HM, strong and precise and the last two gears are metal, so they take crashes a lot better. When you crash them a few times, they will build up slop. In the end this will force you to reduce the proportional and differential gain in roll and pitch axis (to avoid oscillations), but then it’s time to exchange them for new ones and get rid of some slop. The ZYX-S does not like slop in head and servos.

The HKSCM9-6 works well on the tail and did not strip when on the tail. I also used a TGY-212DMH, which during one period allowed me to increase the tail gain by 5% and still have a stable tail. But in the end I was using the HKSCM9-6 on the tail all the time; once the slop returned it was no use with the 212DMH (it didn’t improve the situation).

The tail is a constant source of work, exchanging parts to try and reduce slop. If you have slop, then you get a lot bigger tendencyfor tail wagging, and then your choice is to go home and repair or reduce tail gain (with a lot less locked-in tail). I exchanged the tail slider and tail rotor hub with grips for Align parts, which worked very well for a while, but then I got slop back into those parts as well, and was forced to reduce gain again. Constant work!

One bad thing in the tail is the little arm that holds the control arm for the tail rotor deflect mechanism, encircled in red in this picture:

This little arm is screwed on the side plate with a single screw, a short way into aluminium that isn’t of the best quality, so it will come off with time! I was just lucky it happened gradually for me, so the heli got progressively more and more nervous in the tail, I landed it and went home since it felt so unstable, and then I noticed this loose little arm. More threadlock and tighten it as much as you dare (careful not to strip the soft metal); this is a problematic point. Align has this arm integrated with the sideplate as a single piece of metal, a lot better solution. But I didn’t change it, it only happened once, and I made this into yet another thing to check every once in a while.

The build

There is no manual for the heli from HobbyKing, so I downloaded the T-Rex 250 manual ( Most of the manual is spot on, there are a few details that don’t match (mostly the tail belt drive in the front). Recommend checking out HK250GT build videos on YouTube (search for this: nuttcaze hk250gt).

Some old pictures when I still had HKSCM9-6 on the swash:

View from the left.

View from the right.

Bottom view.

The FBL head.

Tail servo rod made from 700 tail boom support, this made a bit of difference (absolutely zero flex and no guide rod to cause friction). But I lost it in a crash, so went back to the stock one.

Re-enforced canopy with mounting holes moved back (had to move them back a bit, got tight with both battery and voltage alarm).

LiPo monitor at work

All ready to go!

The motor shaft in the recommended motor is too long,which makes it impossible to mount the lowest aileron servo. I used a metal saw and cut off just enough of the shaft to go free of the servo. The pinion included in the kit doesn’t fit this motor (too big hole), so use the pinion listed in the components list above instead. Just press it on the shaft; it will stay on due to the tight fit. The motor mount holes need to be widened, did that with a drill just moving it back and forth in the motor mount holes.

There is nothing in the kit to lock the tail rotor assembly to the tail boom. Don’t rely on just screwing the tail tight together or using glue; it will slip with time.There is a hole in the tail assembly and in the boom. I drilled the hole in the tail rotor assembly a bit bigger to fit the hole size in the boom, cut off a fitting screw so it just went through the assembly and one millimeter into the boom (mustn’t go far in; can touch the belt): I filed down the head of the screw a bit and used the tail fin to hold this screw in place. No more movement.

You cannot rely on Loctite being used in pre-assembled parts. For example: the FBL head had several screws and parts which were completely loose or missed Loctite. The fascinating thing is that quite a number of parts have Loctite (which makes you wonder why not _all_ parts have it). Anyway, it is really necessary go through all screws to be safe, don’t ignore this. Ball links and arms were loose in the FBL head for example, and the feathering shaft was not even tightened. Would have exploded in mid air...

Mount the right aileron servo in the lowest servo hole. This avoids that the servo arm destroys the main gear in case it touches (and it might if you use the whole servo throw). Someone recommended mounting the servos inside the heli body to avoid that the servo rods touch the canopy. I don’t recommend that since you would have to take the whole body apart just to exchange a servo (and exchange them you will).

Theservo rods may touch the canopy if you use the original holes in it. I used glass fiber tape to strengthen the canopy up a bit and made new holes a bit farther to the end (refer to the picture of the canopy).Also note that the canopy becomes like glass in sub-zero (Celsius) temperatures, I broke off a part of it with very little force. I tried a fiber-glass canopy from HK (order #140000001) but it was way too small, it fitted perfectly _inside_ the original one!

The listed servos (both the HKSCM9-6 and the XGD-11HM, see below) fit very well in the frame. There are no plastic nuts to fix them in the kit, and they won’t stay in place without nuts. I took some superfluous servo arms, drilled the holes big enough for the silver flat head servo screws to enter, cut the servo arms off in suitable sizes and used them for nuts. Later I bought a 450 servo mount kit (HK product ID 450-0010), it was perfect for the job.

I replaced the servo push rod with a one that I made from a 700E tail boom support rod; this really helped (refer to the picture of the tail above). Then in a crash I lost it, so I used the stock one again, but with two guides instead of one, this worked pretty well too actually.

Setup

DX8

Setup a new Heli model for the HK250GT. Select 1 SERVO NORMAL for Swash Type. This means no swash mixing at all in your transmitter (important key factor). Look at the Monitor menu in the DX8, make sure that pitch input really just affects the pitch channel, elevator input only the elevator channel and the aileron input only the aileron channel.

The travel was all 100% for all channels.

The Elevator was reversed, all other normal.

I used 30% expo on aileron and elevator, 20% expo on rudder.

ZYX-S

I chose the ZYX-S because it seemed like a good idea to save a few grams by not having a full receiver in addition to the stabilization gyro. I thought it was quite tricky to get it working, but now I understand why I had problems (just keep reading).

First connect the satellite RX to the OrangeRX (or whatever receiver you use to bind with). Connect the bind plug to the RX, power up the TX in bind mode and power up the RX. The bind should occur. Turn off RX & TX, remove the bind plug and power up RX & TX again to verify the bind. It is OK if the LED on the satellite blinks slowly; it does that when it believes that it has rebooted (this allows detection of in-flight reboots). If you turn off only the RX and turns it back on, then this occurs. Not a problem, just be aware of it. But if the satellite didn’t blink when starting to fly, and it blinks when you land, then you have had an in-flight reboot which is not good, might want to check your BEC then.

Connections to the ZYX-S:

ZYX-S label / Connect to / Comment
PIT/ELE/AUX / Not connected
AIL / ESC (throttle and power)
RUD / RX satellite via special cable
CH3 / Pitch servo / NOTE: Setup before connecting!
CH2 / Aileron right servo / NOTE: Setup before connecting!
CH1 / Aileron left servo / NOTE: Setup before connecting!
CH4 / Rudder servo / NOTE: Setup before connecting!

NOTE: You must do basic ZYX-S setup before connecting the servos; else you run the risk of damaging them. So initially, just connect the satellite and power to the ZYX-S (and the USB connection to the PC of course). And don’t have any blades mounted (might disconnect the motor to be absolutely safe), because you will be fiddling with all settings and it’s easy to get the thing going inadvertently (please don’t ask me how I know this :-).

Start the ZYX configuration program (you can download it from the ZYX-S files section at HobbyKing). Use the Windows Device Manager to figure out which serial port the USB plug identifies itself as, this is what I got:

Select the correct serial port, then press the Connect button and you should get a green light. NOTE: You need to have the TX going and the already bound satellite connected to the ZYX-S, else you won’t get it into configuration mode (can force it by pressing the little Set button a couple of times, but you need TX input to check things). Once connected, select your skill/ambition level (I startedwith 3D soft). Then press the Setup button.

The ZYX-S Transmitter mode I got working was DSM2-1 (it should have been DSM2-2 with the DX8 I guess, but I couldn’t get that to work):