Heli-Baby NT

Heli-Baby NT

Modellers of a certain age will instantly recognise this model but then look again and wonder, while others will ask what it is, which is exactly the response the Heli-Baby NT should create. The first group will ask, “is that a Heli-Baby (or Heli-Boy)?” and the answer to everyone is that it is a 21st century model created in celebration of the 40th anniversary of Dieter Schlüter’s Heli-Baby, which arguably was the forefather of all current RC model helicopters.

The Schlüter Heli-Baby was first shown in 1975 and was available in the shops in 1976. Earlier Schlüter RC helicopters were much larger, such as his first model, the Cobra that used a .60 i.c. engine, while the Heli-Baby was much smaller, powered by a .40 i.c. engine, which made it much more accessible to everyday modellers. For more information on this ground breaking model, I suggest searching the internet where you will find a wealth of information including reviews of the model from 1976 and later. Many modellers across the globe love to keep Heli-Babies flying and to this end Gerd Guzicki of Minicopter has been supporting enthusiasts by suppling many of the parts and even a limited number (now ended) of full ‘Reproduction Kits’ of the original design. A great place to start looking is Minicopter’s dedicated website: http://www.helibaby.de/

The Heli-Baby NT
Gerd’s long-time affair with the Heli-Baby led him to the idea of producing this celebration ‘Heli-Baby’ using the original canopy and pilot seat along with the same upper side frame outline, but using a 21st century rotor head and tail assembly and thus creating a fun electric model that looks like the original Heli-Baby but performs like a modern sports machine. Thus the Heli-Baby NT (New Technology) was born.
The original Heli-Baby used one-piece metal side frames that carried the engine and clutch driving the main gear at a ratio of 6:1, while the tail drive was taken off the clutch bell, driving a flat tail belt at engine rpm. The NT uses ‘stacked’ carbon fibre side frames where the narrower upper section carries the main shaft and motor mount, and the lower frame set is spaced at 60 mm to carry a 6S LiPo pack. Like the original, the NT gear ratio is 6:1 and the tail belt has a direct drive from the motor, but uses a reduction ratio of 13:16 and a toothed belt drive to provide a main to tail ratio of 1:4.875. Note, this is important because the tail rotor on the original ran at engine speed, which was up to 12,000 rpm with wood tail blades! The NT will fly very well at a head speed of 1750 rpm and less, which is a maximum of 8581 rpm at the tail rotor making the tail very powerful and perfectly safe.

The original Heli-Baby used a fixed pitch teetering rotor head and wood rotor blades, while the NT rotor head and tail rotor assembly are from the Minicopter Diabolo 550, which is a bang up to date model fully capable of hard-core 3D. The original used plywood bulkhead and floor, while the NT has a carbon fibre bulkhead and composite floor.

The Heli-Baby NT is a full build kit with only a few parts pre-assembled. It is a ‘bare’ kit so you will need a motor, ESC, 4 servos, 3-axis gyro, receiver, and rotor blades, all of which Minicopter can supply. This is their recommended list of parts needed:

Recommended Equipment:
Motor: Plettenberg Orbit ‘Heli-Baby’ Ord. No. 5110
ESC: Hobbywing 50A V3 Ord.-No. 6090

Servos: Graupner DES 708 Ord.-No. 6081 (set of 3)
Flybarless system: MicroBeast or similar
Receiver: 2,4 GHz

Telemetry: f.e. Iisi TXE50 Ord.-No. 1751
Flight battery: 6S 25-30C up to 4400 mAh
Main blades: NHP 500 mm Ord.-No. 1023
Tail blades: Zeal 80 mm Ord.-No. 1052-1054

Other equipment can be used, although the motor is special for this model and so an alternative will be hard to find!

Building the NT
All builds start with a look at the Manual and in this case it is a PDF file downloaded from the Minicopter’s website: www.minicopter.de. You can print out the manual or do as I did and work from the screen, which, due to the excellent resolution of the photos and clear and concise building notes, is easy.

The build starts with the main side frames that are from 2 mm (1.9 mm) carbon fibre sheet that are initially joined with 3 aluminium bearing blocks. It’s interesting to note that the bearing blocks are a mere 5 mm thick, making them that bit lighter, and also that the 3 mm holes in the blocks and frames for the screws are 3 mm (note 3.2 mm), which means the parts line up accurately. A special note is not to sand the edges of the frames because it’s not needed and so removes the risk of scratching the lovely finish! When tightening the M3 screws, check the frame alignment – I inserted the main shaft to help with this. You will see in the photo that a long pitch lever is attached at this time, this is double ball raced making is very precise.
A 10 mm main shaft is used that has a small indent to locate the shaft collar; this is not the more usual split collar that clamps to the shaft but uses 3 M3 grub screws to secure it and one of these has a pointed tip that locates into the indent. I like this as it’s precise and very secure, although the collar was a very tight fit on the shaft and needed a little easing. A note here is to fit the collar the right way round as one side has a shoulder that needs to face down as it rests on the upper ball race, also note that the end of the main shaft with the through hole is the top!

The 60 tooth main gear is machine cut from Delrin (?) and is supplied with the teeth wrapped in foam to protect them – that’s a first for me! The auto hub is a very high quality item with 2 ball races and a 12 mm Torrington bearing – the gear is secured to the hub with 4 screws. The main shaft slides through its bearings, then a 0.5 mm washer and the main gear unit are slid on from below. The shaft should protrude ‘a small distance’ and an aluminium thrust washer is secured with a M5 screw into the bottom of the shaft. The aim is for no end float, but without any preload on the bearings. A selection of shim washers is included to adjust this – I did not need to use any.

Minicopter recommend the Plettenberg Orbit ‘Heli-Baby’ Ord. No. 5110, which is 42 mm diameter and 46 mm long with a very long (50 mm), 5 mm shaft. The motor spec is not provided, but from what I can see it is a 10 pole, 12 windings outrunner and the gearing suggests a Kv in the region of 500.

A brass spacer followed by the 10 tooth pinion gear are fitted onto the motor shaft and the motor is then screwed to its mounting plate, which is screwed to a carrier plate. This assembly then fits between the frames from below with the motor shaft though its support bearing and the carrier is secured into the frames.

I found the gear mesh a little tight and as advised loosened the main shaft fixings and pushed the shaft forward, which eased the mesh a tiny bit. Next another brass spacer goes onto the motor shaft followed by the tail belt drive pulley. You might think that with the motor secured to an extension plate it might flex, well I checked and could not flex it, so it is more than strong enough for this application.

Lower Frames
These are made up of 2 carbon side frames attached to 2 machined aluminium base plates using M3 screws and nuts – use thread lock as plain nuts are supplied. Gyro and controller platforms are added to make a strong box frame. A note here is that the manual shows 16 M3 hex screws for these, but 12 were supplied along with 4 dome headed screws that didn’t have a home. So I found 4 replacement hex screws and kept the dome headed ones for when I discovered where they should have been used!

The factory assembled undercarriage is conventional tubular skids, however the cross struts are from bent 5 mm steel rod pre-glued into the skids. It has a wide 29 cm footprint that is in keeping with the original Heli-Baby. It is attached to the bottom of the frames using countersunk screws through the baseplate into ‘U’ brackets that clamp the wire cross struts.

The upper frame assembly is joined to the lower with 8 screws, spacer tubes and 4 threaded spacers to form a ‘stacked’ assembly.

Canopy Frame and Servos etc.
The photo shows carbon and plywood rear canopy plates along with composite and plywood bottom plates, which are alternatives… I used the carbon set. First a plastic block for the tail servo mount is fitted between the frames followed by 2 threaded spacers for fixing the rear canopy with M3 screws. The tail servo mount fits through this and is secured to the plastic block behind the bulkhead. You are advised that the hole for the servo mount will need easing.

Four angle brackets secure the bottom plate to the back plate and lower frame; these allow some adjustment so take your time to ensure the plate is parallel to the bottom of the frames.

Minicopter suggest using standard servos and in particular Graupner DES 708BB MG that are standard size and spec’d at 88 N/cm and 0.11 Sec/40º (16.5 Sec/60º) and an impressive holding torque of 88 N/cm at 4.8 v. A Savöx SH-1290MG is recommended for the tail control. These are fitted with M2 screws that allow some flexibility for other servos with slightly different hole centres.

Rotor Head Assembly
If I’m not mistaken the swashplate and rotor head are the same as that supplied with Minicopter’s Diabolo 550 and so is a very high quality item.
The swashplate is very elegant with the inner ‘ring’ having just 2 sculptured arms for the blade pitch links, only one of which drives the inner ring and is why Minicopter calls this ‘Single Rod Control’ and is subject of a Patent Pending. This idea is different and makes perfect sense as it eliminates stress that can be induced with a design using 2 fixed drive links when the feathering shaft moves in its dampers. The photographs show one rod is a fixed length using 2 ball races to mount to the blade pitch arm and so drives the swashplate using a heavy duty ball link. The other link is more conventional but it uses a turnbuckle to join the heavy duty links, so there is no excuse for anything other than perfect tracking!

Minicopter FBL rotor heads are all similar in design; the Heli-Baby uses an 8 mm spindle carried in hard dampers in the aluminium head block. Two high quality radial bearings with a thrust race between carry each machined blade grip. The difference to many designs is that the bearing stack is assembled onto the spindle with a collar and the blade grip slides over the stack and is secured to the collar with M3 screws. Again the fit is precise with very tight tolerances – the manual advises you may need to heat the blade grip if it feels too tight on the bearing stack!

The fit of the rotor head to the main shaft is also a very precise interference fit – it is so accurate that you must not twist the hub as it slides on, but use a drop of oil to help if needed. A special screw is used to secure the head block and you’ll see that the hub does not clamp to the shaft – it is so accurate, it doesn’t need to be clamped – Minicopter consider that a clamping screw is only need if the fit is loose and that means it isn’t as accurate as it should be!

The last comment on this section is the swashplate to servo links; all 3 are direct and parallel to the shaft.

The tail gear housing is supplied with the pulley, spacer and shaft as a matched set, so there shouldn’t be any shimming required making assembly that much simpler, and you get it, more precise! The pulley is secured with 2 setscrews, so it’s very secure. The metal boom clamps are slid onto the boom, and with the belt fed though it, you slide the housing over the belt and boom and the pulley is inserted into the belt followed by the spacer and shaft. With the set screws securing the pulley, the boom can be slid between the frames and the belt over its drive pulley (make sure it is the right way round…).

The tail rotor is again all metal with the grips supported on the usual 2 radial bearings and a thrust race. As with the main rotor head, the bearing stack is assembled onto the hub, the grips slide over and are each retained with 2 M2.5 screws. The pitch slider is conventional with articulated links to the grips and a single ball operates it via the ball raced bell crank.

For many years Minicopter have utilised ‘propeller moment weights’ (AKA Chinese weights), on the tail grips and do so on the Heli-Baby NT. In this case they are short aluminium tubes either side of the tail blade on the retaining screw and their function is to reduce the load on the tail servo. Final jobs for the tail are adding the plastic fins, bent wire tail protector and tail pushrod. Take your time with the pushrod as it will need to be bent for it to run smoothly, you may well find that the tail ball links need ‘sizing’ to reduce friction.

Well, you need an ESC and I chose a KONTRONIK JIVE 100+ LV knowing it will work well and I have found the BEC to be completely reliable to drive the servos etc. I like the convenience of the EMCOTEC SPS (Safety Power Switch) that eliminates the spark when connecting the 6S LiPo packs, so an 18 – 70 V 60 A / 120 A was added to the list. Minicopter show a MicroBeast gyro fitted on the lower left side frame and accommodate it with access for the servo leads etc. As it happened I had a JR TAGS MINI spare, which is about the same size and can be configured for side mounting, so I used that along with a RG731BX receiver using the XBUS connector so the servos and throttle connect to the TAGS Mini. These were fitted and carefully set up.

Final Bits
The plastic seat needs careful trimming; the plastic is quite brittle, so take your time, it covers the servos etc. and is secured with 6 Ms screws. The collective and cyclic sticks are option extras and well worth the extra. The original had a bearded Action Man pilot, which I didn’t have and so used the chap in the photos!

Now I came to be hardest part of the build, joining the canopy halves. These are pre-cut for you and the join has an overlap. A small tube of UHU ALLPLAST glue was included, which is fast drying but rather ‘stringy’. I used clamps to hold the top and bottom of the halves together, with tape along the rest of the join. I then squeezed glue into the join. This was not a particularly good option as glue seemed to go everywhere and then needed cleaning off the plastic. I understand that most owners, new and old, all have similar stories! I have since been advised that masking the join lines helps but excess glue can then bond to the tape. I also heard that ‘Canopy Glue’ is easier to use and also dries clear. I then managed to crack the canopy and so ordered a replacement and made a better job of this one using masking tape and Canopy Glue applied with a small syringe. The frame lines are moulded into the canopy and can be painted black or as I did using a permanent black marker pen.

The canopy fits around the rear frame and a lip on the bottom edge locates on the bottom plate; a strong ‘O’ ring holding the ‘ears’ tight at the back. With the carbon rear frame, the fit was extremely tight to the point where I had to distort the lip of the canopy to force it over the frame. The result is a very tight fitting canopy that certainly won’t come off in flight! Minicopter have since modified the frame plates making it easier to fit the canopy and new kits will be supplied with these.

The lower frames are spaced at 60 mm and with a height of some 50 mm, the battery compartment is huge, although the base plate screws do restrict the corners. The recommended battery is a 6S 25-30C up to 4400 mAh, so I used OptiPOWER 6S 3500 mAh 30C and OptiPOWER ULTRA 6S 3500 mAh 50C packs. The packs can slide right through the frames to the seat, so they need positioning for the correct CG. I established where my packs needed to be and added a cross member between the frames to prevent the packs moving too far forward; the supplied Velcro strap holds them in place. The TAGS MINI was set up according to the manual and I assigned 2 gyro gains so I can change the rotor head and tail gains from the transmitter.

First flights were a little lively so I used the dual rates to reduce the response and also added some Expo to make the Heli-Baby more docile. The rotor head rpm was quite high at about 1900 rpm which felt a bit too high, so I reduced this. After this it was a matter of adjusting the gyro gain, and I found I could run the head gain at a high setting, but the tail rotor would not settle. This turned out to be because the tail rotor linkage was a bit sticky, which combined with the tight ball linkages produced a slow wag. Now this is sorted out the tail holds really well. Next I started to reduce the rotor rpm because lower rpm means less power used and hence gives longer flight. The main rotor blades are quite short at 482 mm, and only weigh 109 g each, so it is a trade-off between very slow rpm and a lot of blade pitch versus high rpm and less pitch – after a lot of flights I settled on 1550 rpm (70% throttle in governor mode), which using the 3500 mAh packs, gives 8 minute flights and a good safety margin. The motor, JIVE and battery packs are barely warm showing it is very efficient.

I have tuned the model to fly sedately and smoothly, which, to me, suits the model well. It is also very quiet in flight. Everyone who has seen it, like it and those who don’t recognise its heritage, see it as a cute Sunday afternoon model for stress free flying.

Summing Up
The Heli-Baby NT completely catches the appeal of the legendary Schlüter Heli-Baby, but with the benefits of 40 years of progress to produce a fun model that could well last another 40 years! It is a very simple design, which again adds to the appeal as there is little to go wrong and the quality of the parts promise a long service free life. It is fitted with a freewheel hub, which is good because the rotor disc cannot drive the motor and so autorotations are technically possible – no I have not tried, and hope I don’t need to… Mild and simple aerobatics such as loops and rolls are possible, but to me the model is not about that kind of flying, remember the Heli-Baby was a fixed pitch model that was a challenge to hover and circuit flying was only for the highly skilled, so this NT version already performs beyond the dreams of owners 40 years ago!
The 1976 Schlüter Heli-Baby was a classic and Minicopter’s 2016 Heli-Baby NT is another.