Bell 429 by ROBAN/Scaleflying.de

Mechanical Assembly

From right to left: mechanical assembly, wrapped parts of the motor shaft support bearing, main rotor head, tail rotor assembly with bevel gear, tail boom with tail rotor shaft (wrapped), main and tail rotor blades – wonderful!

I was especially glad to have the latest, improved version: motor shaft support bearing, helically toothed main gear, angular ball bearing on the main rotor shaft, longer servo-swashplate levers, hence longer servo arms, longer rotor blades, reflexed main rotor blade airfoil.

But should I really dismantle that? Enjoyment gave way to annoyance. All too often I didn't want to concern myself with the helicopter. At times when I was more sober I watched the instructional videos by Scaleflying.de, read about gear assembly, and inspected the mechanical assembly and the rotor heads again and again.

Eventually, there are nine months between this picture and the next one. In the summer, things started to happen really fast. The rotor heads were checked and turned out to be securely bolted (with threadlocker). The mechanical assembly was partially dismantled in order to reverse the bevel gear, then reassembled with threadlocker.

The result is really to my liking, even if I'm still not confident – about the screwlock, the bevel gear backlash, the bearing clearance. By the way, the tail boom is really a bit inclined upwards; that's how it runs in the tail cone. The ROBAN 800 mechanical assembly is basically the same in all of their models, but the carbon frame sides are different. In the B 429 model, the tail boom is just inclined and quite high in the body. And there is no 45° bevel gear in the tail boom like in the Tiger model.

The two carbon tail-boom struts are bolted and clamped with aluminum fittings. In one of the videos by Scaleflying.de they mention that these struts are affixed differently, depending on the mechanical-assembly frame in the respective model. They have to be bolted to the carbon frame sides where washers are under the bolt heads. That's how it was and now the whole assembly goes through the rear opening in the body, so all seems to be correct.

The motor moves a cogged belt which is visible outside the carbon frame side. Above the big pulley is a pinion which moves the big white plastic cogwheel visible here and thereby the main rotor shaft.

The three 120°-swashplate servos are linked by one 90° bell crank each. In this latest version of the mechanical assembly, the bell cranks are bigger so the servo arms are longer than before, and all three pushrods have the same length now.

The KONTRONIK Pyro 700-45 (kv 450) has been chosen due to the positive experience with the Pyro 600-09 (kv 930) in my HIROBO Schweizer 300. It has about half as big a kv and a 12s LiPo is used instead of a 6s.

A more powerful motor is not needed according to Scaleflying.de. If I would like to use one, though, its diameter should be not much bigger so it would still fit in the frame's recess.

The motor sits on a carriage which is pulled forward (left in the picture) by two bolts to tension the cogged belt.

The two tensioning bolts are visible here as well as the whole cogged belt gear train. When the belt is tensioned correctly, the four bolts clamping the motor are tightened. I used shiny bolts and washers (well visible here) while (quite short) black ones were included in the kit.

The T-shaped part is the motor shaft support bearing. It has three "legs" which are clamped – together with motor and carriage – by three of the four clamping bolts. That's quite a fiddling but doable. I just hope that I didn't cause any tension on the motor shaft when tightening the bolts.

If I didn't, the support bearing prevents permanent cantilever load on the motor shaft so it's a very good thing. A precondition is choosing the motor version with the longest shaft available – 38,5 mm. And the cogged-belt pinion has to be mounted with its set screws towards the motor – not like shown in the standard instructions but like shown in the special instructions for the support bearing and in the video.

By the way, I had hoped that the cogged belt would run centered between the pinion's lateral disks. However, the test run showed that it follows gravity and runs against the lower disk. I had to loosen the pinion again and shift it one millimeter upwards (towards the motor) so during operation the cogged belt runs centered on the big pulley.

The cogged-belt gear train's numbers of theeth are 22 and 78 – 3.55:1 reduction ratio. The following gear train with helically toothed steel pinion and plastic cogwheel has 20 and 78 teeth, respectively – 3.9:1 reduction ratio. Hence the total reduction ratio is 13.83:1 from motor to main rotor. Assuming the recommended 1100 rpm main rotor speed, the motor has to spin at 15210 rpm – no problem considering 30000 rpm maximum rotational speed (and 12s LiPo).

Below the big cogged pulley is the first (white plastic) gear of the tail rotor gear train. The second behind it moves the vertical shaft of the bevel gear below it. The latter's gear ratio – as well as the bevel gear's on the tail rotor – is 1:1.

The two plastic gears' numbers of teeth are 40 and 32, what makes for a 1:1.25 transmission ratio. Together with the cogged-belt gear train's 3.55:1 reduction ratio, the total reduction ratio is 2.84:1 from motor to tail rotor. At 1100 rpm main rotor speed, the tail rotor spins at 5363 rpm. With its 290 mm diameter (280 mm by the instructions) and its four blades it should have quite some power (and the simulator confirmed that).

The bevel gear has been "reversed", that is the bevel wheel on the vertical shaft has been relocated from top to bottom, to reverse the tail rotor's sense of rotation. Now it turns the right way round, like on the original helicopter and like it should be "aero­dynamically".

This bevel gear still worries me even though all looks good and works correctly. An instruction sheet was included in the kit, which points out that the bevel gear has to be adjusted, and there the bevel gear is shown like here. Probably the picture shows another helicopter model's bevel gear, though. Anyway, the shaft's flat portion for the wheel's set screw was not symmetrical and had to be extended (with a grinding wheel on a Dremel). So the tail rotor's wrong sense of rotation must have been intended by the designer. I asked Scaleflying.de by e-mail whether there is a special reason for that or any reason to not reverse the gear, but they just didn't reply.

Reversing the bevel gear was an opportunity to inspect all details. To this end, I removed the whole gear from the carbon frame, what was quite elaborate. I could have done it easier by removing the lower bearing plate and unloosing the wheel set screws. The shaft could be pulled out then as well as both wheels and the brass spacer sleeve. Originally, the bevel wheel was on top (left in the picture) and the spacer sleeve below. Now it's just the other way round and all works well like before.

Above the bevel wheel were two shims to adjust the backlash. Of course, the shims were put under the wheel when it was reversed. As far as I can see, the gear was adjusted correctly then. I just can't rely on that because Scaleflying.de would not be liable for any damage. That's not a theoretical problem – my clubmate lost his Tiger model twice in crashes because the 45° bevel gear in the tail boom failed. Of course it was his fault, meaning he got no compensation. (Now he installs a flexible shaft instead.)

For three reasons I believe (or hope) that my B 429 won't crash due to gear failure: (1) The 90° bevel gears seem to be not so prone to fail or in any way critical, perhaps because they are easier to adjust or factory adjusted – who knows. (2) When inspecting the bevel gear I realized that both bevel wheels have to be adjusted so that their teeth are not offset to each other. (The teeth of the failed 45° bevel gears are stripped, but on one of the wheels only partially, that is not their outer part.) If that's not correctly done, also the backlash can't be correctly adjusted at all. (3) The 90° bevel gears have rigid open chassis so correct adjustment is visible. The 45° bevel gears have somewhat sloppy split closed housings so that the final adjustment is not visible and perhaps a correct adjustment is not even really likely.

ROBAN should really do like HIROBO: By all means, such a gear like the 45° bevel gear has to be prefabricated – assembled and adjusted. Perhaps they could design it in a way that no adjustment is needed, or at least they could use a fixture to easily and reliably adjust the gears – in the factory. They ought to accept responsibility, that is assume liability, but they won't possibly get that right.

However, I still don't know how to adjust the gears correctly and can guess only. Mr. Illig from Scaleflying.de shows in his video how to tension the cogged belt. But he just pokes around in the bevel gear with a screwdriver (there is no help for it, there's nothing to be seen) and tells that the bevel gear has to be adjusted – but not how. As far as I can see, the bevel gear was adjusted correctly, that is I couldn't do it any better.

That applied to the bevel gear on the tail rotor as well. It's made from the same parts as the front bevel gear; the bearing plates are just closer to each other. It's easy to check that the two bevel wheels are correctly positioned to each other. And the correct backlash can be checked by turning the shaft to and fro. The wheels were greased already.

The tail rotor has been converted to the reversed sense of rotation: the pitch lever links removed from the link balls, the blade holders reversed so the pitch levers "lead", the link balls unscrewed and screwed in from the other side (with threadlock of course), the pitch lever links snapped on again.

Now the tail rotor turns clockwise (as seen like here in the picture), the tail rotor blades moving backwards at the top, with the main rotor's wake and not against it. That's how it is on the original helicopter and that's (almost always) aerodynamically correct.

The tail servo (XServo X75 BLHV) is clamped to the tail boom – a simple and neat solution. In the instructions and in pictures I saw at least four different ways to mount the clamps, the plate and the servo and how straight or oblique the whole thing is clamped to the boom. This was one of these ways and it seemed to me the way actually meant by the designer.

The tail boom is made from very thin aluminum and its front end was a bit crimped in, probably during transport. With some force, it had to be stuck and turned into the receptacle on the front bevel gear. Fortunately, I didn't crush anything in the process.

Also the swashplate servos (XServo X70 BLHV) are well linked. They are mounted in a special frame so that all three pushrods to the bellcranks have the same length.

The ball links are made from a quite hard plastic and (hence?) have a little bit of slackness. That's strange – so far I had only links made from ductile plastic and had to press them with flat pliers so they are not too tight.

There's an own place for the gyro system (Micro­beast Plus HD) so it's close to the servos and exactly parallel to the main rotor disk. The servo leads are protected from rubbing on the sharp edges of the carbon frame by wrapping them in slashed pieces of fuel tube.

There was some bother about the servo arms: The instructions recommend metal (aluminum) arms. An additional instruction sheet specifies at least 13 mm lever arm for the new-version swashplate linkage. Aluminum servo arms are hard to get in any case, especially 13 mm lever arm, and I found only one offered in the Web. Unfortunately, it doesn't fit the servo shaft (not properly deburred, swollen when anodized, but also servo shaft too big!) and has a 3 mm threaded hole instead of 2 mm. In a video by Scaleflying.de, Mr. Illig says that metal servo arms are hard to get but using plastic arms is OK. So now the supplied plastic arms are on the servos – with 13 mm lever arm.

The mechanical assembly is an operative unit. Earlier versions even had a "porch" for the ESC with two more mounting lugs, but obviously that has been abandoned. Now four lugs must do and the ESC is screwed to the top of the cabin ceiling in front of the mechanical assembly.

After it's completion, the mechanical assembly has been test-run with the 12s drive batteries. The ESC was set up as a governor and the set rotor speed has been defined. After a short break-in of gears and bearings all ran smoothly and with low friction. Amperage – as indicated by the ESC's telemetry – is a measure of friction moment and suggests about 94% overall (main and tail rotor) gear efficiency.

Body

The struts of the landing gear halves are stuck through holes in the fiber­glass body into a ply­wood structure inside, then fastend by stick-through bolts. Actually, that's a good solution but the right rear strut went not deep enough into the hole so finally some pieces of epoxi chipped off – fortunately only below the strut where it's not visible.

Just because they are well visible here: door hinges and handles are not really proto­typical but not too bad either. They are a bit coarse but stand out just as little as the badly glued-on foot­steps.

My problem is visible here: The foot­step's midlle attach­ment is glued-on flush but the other two at the front and the back stick out a bit. The foot­step is so rigid that rubber bands, adhesive tape, and later the glue slackened. Now some Canopy Glue will some­what fill the gaps.

This is a bottom view of the gaps; later they will stand out very little, see previous picture.

The doors are flush-mounted and look pretty good. The interior equip­ment, mostly seats, has to be glued to the beige-colored ply­wood cabin floor – that's all. Looks pretty good as well.

There are tabs on the bottom of the seat pedestals, which are stuck into slots in the floor (hardly visible in the picture). So it's perfectly clear where the seats go in the cabin; they can be glued on only there.

The box in the middle has a fuction: It's a handle on a slider which locks and unlocks the hatch in the cabin floor in front of it. The box is grabbed to move the slider back and forth.

Three seats are on the cabin floor hatch, and they are used as a handle to remove the hatch and get at the battery com­part­ment.

In the middle is room for two 6s 5000mAh LiPo batteries and on the sides for two receiver batteries. Behind them (left in the picture) is room for other things to be placed here (master switch for the drive batteries, dual switch mixer for the receiver batteries, telemetry sensors) as well as cables and connectors.

The cables to the ESC on top of the cabin ceiling can be threaded through the vertical ply­wood duct visible in the back­ground (behind the corner of the hatch opening). In front of the battery compartment (right in the picture) are the pilots' seats, which are seen from above in the next picture.

This picture is about sticks, though, that is the cyclic and collective control sticks.

In the full-size heli­copter, the pilot is in the right seat (left in the picture) and his collective stick is on his left, not on his right like here in the model helicopter. The model's seat would have to be more to the right side and the collective stick between seat and the center console (but it's not really easy to glue them that way due to the tabs and slots). There is even another, smaller console on the center console's right side (below the panel, not visible here) so a pilot manikin's left leg would have not enough room.

Seems the model's designer missed some­thing, but then again he went to great effort: The switch box on top of a cyclic stick is asymmetric and here both sticks are inversely asymmetric because the pilot uses his left hand and the copilot his right hand in this arrange­ment. Actually, both cyclic sticks should be equal because both pilots have them in their right hands (and the collective sticks in their left).

The tail has to be bolted to the body with six 2mm bolts, a foam piece on the tail boom carrying the tail's weight. At least I don't think the small bolts are capable to do that on that long lever arm. The holes can or have to be drilled – with tentatively fixated tail – without support. The drive-in nuts on square plywood pieces were included in the kit. Because they are hardly accessible later they have been glued in already.

The front window has been screwed on.

The decals have been applied – following the instructional video by Scaleflying.de. My UAS operator ID (e-ID) as a QR code is hidden between the stars in front of the black exhaust nozzle.

The two dummy antennas on the top cowling are glued on. Glueing another five dummy antennas is saved until last so they are not broken off while I'm further working on the helicopter.

Especially without direct sunlight the model's red color seems too dark to me – compared to the original as well as to other models. Perhaps they used a paint that looks different in different lighting, but I think this shade of red doesn't quite match the original (but it does match the carpet).

Tail

On to the tail: In the tail cone runs the actual tail boom with the tail rotor shaft. That's why the horizontal stabilizer can't be sticked through in one piece, and two halves have to be glued into recesses in the cone instead.

At the stab tips are fins (screwed but glued as well) with very nice, small position lights (glued on). I was scatty again and fastened them the wrong way up, their longer side upwards, but that's not a problem and just one more non-prototypical trifle.

By the way, the tail is here upside down because the stab halves were aligned while the glue was setting. It's Canopy glue again because it bonds so well on smooth plastic surfaces and because it stays a bit elastic. And when it's cured it hardly stands out.

This big fin with tail skid and two lights has to be screwed to the tailcone's rear. Four indentations for screw holes are visible around the cable exit. The cables belong to the lights which are glued to the fin's top. The skid is simply screwed into a wooden filler.

The nice red anti-collision light was included in the kit, but no white rear position light – another mysterious deviation from "scale". I took one LED from a Multiplex light set and glued it on where the original's rear position light is. Drilling a hole for the cable was no problem since the whole fin is hollow plastic.

The big central fin is screwed on – one black screw head is visible in the opening. In the tail cone as well as in the fin were four indentations each for screw holes. In the tail cone, the fiberglass plastic has to be drilled out for the 3mm screws. In the fin – plastic with wooden filler – 1mm or 1.5mm holes suffice to let the screws go in without breaking. (All screws and bolts included in the kit seem to be brittle and are sheared off easily.)

The holes in tail cone and fin matched, so the fin was screwed on. Now it seemed to be badly bent forward, and compared to the original helicopter it was. So it was taken off again and three of the holes in the tail cone were made slotted holes (more forward and/or upward) using the drill bit's stem (side) like a milling cutter. (The upward slotted hole is visible in the picture.) When the fin was again screwed on it was perfectly upright. Seems to be a flaw in the molds.

The fins attached to the stab the wrong way up are visible in this picture. Now their upper and lower parts' sweep doesn't match the central fin's but that is virtually invisible. The main rotor is high enough to avoid any fin strike. (And at Scaleflying.de I saw a picture of another B 429 model with the side fins just as wrongly attached as here.)

All four lights are plugged and tested – looking good. The light controller included in the kit is not too bad. (Newer kits include a different controller.) It has three free connectors for rear position lights of which one is used here. It can be switched (with a transmitter switch) to different blinking patterns of which one is prototypical. (I had to find out by trial and error, no instructions.) Only the red anti-collision light is blinking then, in fact so smoothly that I don't want a different controller. Because it works only at voltages up to 6V I even had to slot a voltage regulator in ahead, but that was worth it for me.

The decals are applied (well visible) and the six holes for the fastening bolts close to the forward edge are drilled (hardly visible).

Electronics

Pending…

Completion

All rotor blades have been balanced beforehand. Since main and tail rotor have four blades each, the two lightest and the two heaviest of each rotor have been identified (with the blade balance). The pairs have been labeled with colored adhesive tape (red-green and yellow-blue) and balanced with the white adhesive tape included in the kit under the respective lighter blade's tip. Quite a few people claim that ROBAN blades vary a lot in weight, but not much tape was needed here – perhaps because balance can be done in pairs. The blade holders have been labeled with correspondingly colored adhesive tape so that the blades are facing one another by pairs. That way each blade will always go to the same holder and any individual blade angle adjustment persists. (The linkages from the blade holders to the swashplate stay bolted to the blade holders so no color tags are needed on them.)

The rest is pending…