PSG Pro Scale TT

PSG Pro Scale TT

There I was wandering around the stands at the 2015 Rotor Live when in the corner opposite the bar, we came across PSG Dynamics, which was a new company to me. What caught my eye at first was a prototype of a 700 size pod and boom model. I then learned that PSG is a German specialist engineering company with a history of producing high quality parts for the RC model car market and also a range of parts for model helicopters, which led them to the prototype helicopter on show. I then spotted a set of mechanics suitable for a scale model and a 7 blade scale rotor head. By the end of the show a plan for a rather special scale model helicopter had started to form in my mind, which would include a set of PSG Dynamics scale mechanics… Thus a few months later, I ordered a set of the Pro Scale TT mechanics.

PSG Pro Scale TT mechanics
The reason I was so keen on the PSG Scale mechanics is the obvious quality of the engineering, combined with designs including some unique features that would suit many scale models. You will see the Pro Scale TT mechanics that I bought have a compact 2 stage gear reduction for the rotor head with a toothed belt driving a vertical layshaft that drops down at the rear of the frames and the tail drive is taken off this shaft. Two versions of these mechanics are available, the TT here that uses bevel gears for a tube drive, or with a pulley for a toothed tail belt drive. The clever part is that the tail drive components can slide up and down the shaft to suit the tail position on the model you are fitting them into.

The alternative PSG 700 Scale mechanics are more conventional in layout being similar to the T-Rex 700, but with the same 2-stage gear train and the tail drive is via an angular gearbox that can accommodate rigid and flexible tail drives.

Slim versions of both mechanics are also available that place the swashplate roll servos behind the main shaft with bellcranks mounted outside the side frames for the control rods. This reduces the width of the Pro Scale mechanics from some 108 mm to about 90 mm. Motor down versions of both mechanics are also available and conversion kits are offered for both the Pro Scale and 700 Scale mechanics.

A first for me, and is what makes them so desirable, is that PSG offer a very wide choice of gear ratios for their mechanics, both for the motor to rotor head ratio and the rotor head to tail drive ratio. This is a great service for scale modellers as most mechanics offer a range of motor pinion gears so the motor to rotor head ratio is chosen by varying the motor Kv and pinion gear, but the tail drive ratio is usually fixed. The general rule for scale models is the larger the rotor disc, the slower the desired rotor speed and multi blade rotor heads also reduce the desirable rotor speed. This usually results in a slow tail rotor rpm, so longer tail blades are needed, or even speed up gears. The PSG mechanics overcome this problem by selecting the right gear ratios for the rotor head AND tail rotor to suit your application.

When you order a set of mechanics from PSG, the order form asks you about the planned use. This is because PSG mechanics are supplied ready assembled fitted with the gears to suit your application. I selected a main rotor rpm of 1130 and tail rotor rpm of 5932 rpm; my specifications were:

  • Pro Scale TT
  • Left rotation
  • 2 m diameter 4-blade main rotor head
  • 2-blade tail rotor
  • KONTRONIK PYRO 850-50 motor
  • 12 S LiPo power

With the order completed I paid the pro forma invoice and PSG suggested I sent them the motor so they could fit it, I did this and waited. It did not take long for the box to arrive and I was very impressed to find the mechanics nestling in a foam lined presentation box – I knew these are special mechanics but did not expect such a classy presentation!

What Makes Them So Special
As mentioned the main rotor is driven by a 2-stage gear reduction mounted within the carbon side frames and in front of the main shaft. You will see that both sets of gears are Herringbone gears, which is also unusual. The first gear is 16 mm deep and is effectively two high quality spiral cut gears on top of each other to form the ‘V’. Thus you have twice the contact area of a single 8 mm gear and as the engagement starts at the outer edges, the mesh ‘rolls in’ making it very quiet, however the gear alignment has to be precise. Alignment of the first gear set is achieved by positioning the steel pinion gear on the motor shaft and the mesh is set by sliding the motor mount. The second stage reduction uses a larger 22 mm gear set – this set rotates at slower rpm, so there is more torque and hence needs to be stronger. Interestingly the steel pinion gear leads with the centre. Gear alignment here is achieved by the main gear being fixed and the steel pinion gear is shimmed for correct alignment. The mesh is set by the accuracy of the bearing blocks and the holes in the side frames. A combination of precise engineering and assembly results in perfect gear alignment and mesh that has the minimum of backlash.

PSG Pro Scale TT mechanics arrived with my motor already installed

So, how is the wide main rotor rpm achieved? Well the chosen main rotor rpm is achieved by optimising the primary ratio to the motor Kv (motor pinion and first spur gear) – in my case 18T:71T = 1:3.9444. PSG then choose the secondary gear ratio from a range between 1: 9.04 and 15.53 using pairs of gears that can be fitted with the same distance between centres. For example, a 24 T and 55 T produces a ratio of 1:2.29, while a 16 T can be used with a 63 T that gives 1:3.94, but the distance between the gear centres is the same for both sets. A further 3 ratios between these examples are available. In my case I have the 500 Kv motor with an 18 tooth pinion gear driving a 71 tooth spur gear mounted on the intermediate shaft, a 16 tooth pinion gear then drives the 63 tooth main gear giving the overall ratio of 15.53:1. To calculate the expected rotor rpm, PSG use a cell voltage of 3.9 v with an efficiency/Governor setting of 75%: 12 x 3.9 x 500 x 75% ÷15.53 = 1130 rpm. The 24 T and 55 T gears mentioned would produce 1942 rotor head rpm.

I removed a side frame to show you more

The tail rotor is driven from the intermediate shaft via a pulley attached under the first spur gear, which drives a second pulley on the lay shaft at the rear of the mechanics. The choices of pulley range from 15 to 32 T, so there are plenty of options and in my case a 28 T pulley drives a 21 T pulley. The intermediate shaft runs at 4449 rpm, so the tail shaft rpm will be 4449 x 28 ÷ 21 = 5932 rpm, while the range of available tail rotor with this set up is 5932 rpm to 9492 rpm. The tail drive bevel gears are both 19 T. Finally, a one-way bearing is fitted in the hub of the first spur gear that provides a driven tail rotor during autorotation.

And the stronger main drive gear

Rotor Head and Swashplate
PSG also supply a range of rotor heads to suit 500, 600, 700 and 800 size models, with 2 to 5 blades. I ordered the PSG 800/4P Pro Kit, which as its name suggests is for an 800 size model with 4 blades! The Pro kit includes a matching swashplate and links, while the Basic kit is the rotor head with follower arms and the Universal kit includes the linkage set, but not the swashplate. It is worth noting that the 500 size rotor heads are bored for an 8 mm main shaft, the 600 for 10 mm shaft and the 700/800 for 12 mm main shafts. The quality matches that of the mechanics with nothing but the best used, as an example the aluminium used throughout is ENAW 7075-T6, which is a hard aircraft grade making it the best for this use even though it is time consuming to machine.

The rotor head has a solid hub into which are screwed the steel blade axles so it is a rigid rotor head. You will see the axles have a shanked section that is an interference fit into the hub and so ensures perfect alignment, which is how it should be done!

The blade grips are machined from solid including the pitch arms and these are supported by 2 radial bearings on the 8 mm shaft with a thrust race to take the end loads and an M4 screw securing the assembly. PSG specify using Loctite 243 for all metal fixings and recommend all parts are degreased before assembly. The bearings are all a good fit and I used a little bearing lock to secure them in place and on the shaft. ‘L’ shaped followers, again machined in one piece, with push rods are used to connect the grips to the swashplate providing the usual 90º phase offset. Twin radial bearings support the followers on the pitch arms that provide a slop free and precise linkage. A nice head button finishes off the rotor head.

As you would expect the swashplate is equally high quality and completely slop free. Another sign of quality is the use of a spherical plain bearing in the swashplate. This is clamped between the upper and lower inner ring sections that are secured with 5 screws. The bearing has a PTFE composite material on the sliding surface so the ball moves smoothly without slop. A large radial bearing carries the inner ring in the outer with 3 screws holding it in place. The result is a swashplate that will last a very long time…

A set of 3 mm pushrods is supplied with heavy duty ball links that are secured to the swashplate with 3 mm screws. The servo arm links are smaller and are secured with 2 mm screws. The elevator servo pushrod is supplied with a 7 mm sleeve that runs in the anti rotation guide that’s part of the upper bearing block and so holds the swashplate precisely throughout its movement.

Pushrods screw into the follow arms and are locked in place with a nut

Adding the Servos
The mechanics include a comprehensive servo mounting kit, which is the best I’ve seen, and I’ve seen a lot! Mounting the roll servos is straightforward using the supplied M3 screws and carbon plates – I fitted XPERT SM-5401 HV servos using the grommets but without the brass ferrules.

It is the elevator servo fitting kit that’s the interesting part – the servo fits between the side frames and its position has to be right so that the control rod is perfectly parallel to the main shaft. Different servos have different heights so positioning the servo correctly can result in using spacers that never seem to be exactly right… The PSG system allows the servo to be slid on the mounts and when in the correct position screws secure it in place. The servo is attached to blocks that slide in 2 ‘U’ shaped fixing blocks attached to the side frame – it is a ‘tongue and groove’ arrangement with an M4 screw locking them in place. Thus you can slide the servo into the perfect position and then secure it – excellent.

The last bag contains 4 frame spacers that can be fitted in a number of positions around the frames to best suit your installation. The spacers are drilled and tapped, which just happen to match the mounting holes on a KONTRONIK KOSMIK ESC, as do holes in the side frames… A coincidence? I think not…

And Finally an Angular Gear
The last box I ordered is the PSG Angular Gear, which is an intermediate tail drive gearbox for models with a high tail gearbox. Again this is heavy duty using ENAW 7075-T6 aluminium with ‘cyclo-alloid splinted gears’ which are spiral bevel gears with helical cut teeth that are specially treated. You will see that an idler gear is used between the input and output gears transferring the drive and this allows the output shaft to be rotated making it a variable angle gearbox –this is about the only such gearbox I know of.

This results in a very quiet and smooth running mesh making it suitable for larger models with rotor discs up to 2 m diameter. The standard version has a single transfer gear and the angle is infinity variable between 15º to 80º with stops at 40º and 50º. A HD version is also available that uses 2 intermediate gears, which provides a more balanced and stronger power transfer. This version is also variable but in 9.5º steps from 19º to 76º. The input and output shafts are 8 mm and the gearbox is designed to be mounted in a 25 mm o/d tail tube with 23.4 mm i/d (0.8 mm wall thickness).

Bottom view shows the meshing, spare shim washers are supplied

Again the quality is superb and I can not imagine it ever wearing out! The one consideration with this intermediate gearbox is the design makes it quite bulky – the length and height isn’t usually an issue however both versions are 44 mm wide, which some models will not be able to accommodate…

This brings me to a couple of useful accessories that PSG offer to help with larger models needing a robust tail drive, namely the tail drive components. First PSG offer 6 mm and 8 mm tail drive tube supports to fit their tail boom (23.4 mm i/d). Again these are well engineered with 2 ‘O’ rings that ensure the support can not twist in the boom.

Then there are heavy duty Claw Couplings for the 6 mm and 8 mm shafts, which I particularly like. These couplings use star shaped dampers that are 20 mm in diameter, 8 mm thick and are a tight fit in the couplers. These support the drive tube really well, which is particularly useful for shorter unsupported sections of the tail drive, i.e. from the intermediate gearbox to the tail gearbox, where it is often impossible to support in a bearing.

6/8 mm Claw Coupling from the mechanics to the 8 mm shaft, with another into the Angular Gearbox and another for the short 8 mm shaft to the tail gearbox (with a simulated 6 mm input shaft

Summing Up
When I first saw PSG Dynamics’ products, I was very impressed and after a close examination I am even more so. The versatility they offer is unique and the attention to detail in the design, quality of materials used and accuracy of manufacture make them very special. Add to this that every set is assembled to meet your requirements make the whole experience of buying them that bit more special, however be warned, such quality does not come cheap.

The rotor head, is again extremely well made, although it’s more functional than scale…

When looking at the Angular Gearbox, it may seem over engineered but using intermediate gears is the only way to provide a variable output angle AND maintain perfect gear mesh. I think it would be last assembly in the tail drive to fail… The drawback of such robust construction is the physical size of it.
At the time of writing, PSG were not offering a method of mounting a 25 mm tail boom to the mechanics, which in many scale models would not be needed, although in some cases it would, and in addition it could help in aligning the tail… Equally PSG were not offering a tail rotor gearbox. However, they are now taking pre orders for the ZENYT-Mechanik which is a complete 700 size helicopter kit, so perhaps we will soon see a boom mount and tail rotor gearbox available for the scale mechanics…

I guess this leaves the question of what to fit these mechanics into… Well, as they say that’s another story, which will be told when it’s finished!