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Mar 08, 2018, 09:36 AM
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RX120-2S - Printed size 120 helicopter with power from 2S battery.

I want to thank Mike Jennings (MikeHRC) for the translation!!!

Russian version

Today I want to tell you about the next printed helicopter, with blades of 120mm and power from a two-cell battery.
This time it will be a helicopter, mostly based on components from helicopters KK120 and KK110.

I have long wanted to create a helicopter of this class, powered by a two-cell battery, since an increase in the supply voltage allows you to reduce operating currents, reduce the load on the battery, and increase the efficiency of the power part, which has a very positive effect on the dynamics of the helicopter. All this was confirmed by messages from users and numerous video flights of such helicopters as KK120, NSR100S and Align T-REX150. However, these helicopters have a number of drawbacks and, of course, in my opinion, the most important of them is the location of the tail motor at the end of the tail boom.
So I decided to create my own helicopter with the well-proven tail drive through the shaft, and the location of the tail engine inside the frame, and therefore powered by a 2S battery. And that's what I got.

The frame of the helicopter is already well familiar to the frame from the one-pot version of the RX120, redrawn for skis from the helicopter KK120. Rama even turned out a little easier than a single-bank version. The location of the main elements on the frame remains the same.
A canopy from the model WL V977 has perfectly fitted this helicopter. From the inside, it was glued with tape and trimmed from above and on the sides so as not to tear at the front traction and on the skis.
This trial version of the helicopter, in the final version will be made very small and minor modifications of the frame, and I hope to get a global change in electronics. In the meantime, I'll tell you what this helicopter consists of.

As a flight controller, I use the controller from the K110 helicopter. This flight controller is powered by a single-cell battery, so I had to use the WEB , in order to reduce the supply voltage coming from the battery, down to 5 volts. This is the smallest WEB with the appropriate characteristics, which I managed to find. It weighs 1g. The output voltage turned out to be 4.8V, but it's even a plus, as rumours are circulating that the 5330 servos that I use in this helicopter are not very good at 5 volts.

As the main engine, the Oversky HP06V2 motor is used at 8200kV. The given motor has excellent characteristics and very pleasant price . The reason for such a low price as it turned out to be simple.
This motor is specially designed for Trex 150, it fits the helicopter perfect. The hopes of Oversky, and not only of them, on the model T-REX 150, apparently did not materialize.

As a tail motor, I used the BR1103 engine at 8000kV.
The engine performance at a very high level, and the price is more than humane!

The tail engine is traditionally located between the rear servos. The motor motor mount has been slightly modified so as not to interfere with the installation of servo drives. Two side fasteners were cut off, and the engine itself was fixed to the remaining pair of fastening ears.
As in my previous helicopters, the drive shaft of the tail rotor is fixed directly to the rotor of the engine. The shaft is fixed with a small bolt on M1, which is visible at the end of the rotor hub. For this bolt in the hub, a hole was drilled and threaded. At the edge of the shaft there is a slot, into which this bolt rests.

Servo drives are used by all well-known servo drives 5330.
Mounting them on the frame has not suffered any changes in comparison with the models RX100 and RX120.

I completely took the head and blades from the helicopter KK120.
With blades, with the help of alcohol, erased unnecessary inscriptions and the logo of HK, as it did not combine with the blue canopy.
The blades are quite heavy and give a different feeling in flight compared to the narrower and lightest blades from the X12 K123 or WL V931.
The helicopter with them is more imposing, the rotor's inertia is increased.

In the Rotor grips I replaced the bearings. I do not like stock bearings at all. They are without a separator type and do not have good quality. With these bearings, I already had problems with the trimming of the grips with a tight tightening of the feathering shaft. This led to vibrations and a "butterfly". Therefore, I immediately replaced them with bearings from WL V977 grips. They are with a plastic separator, much better and more reliable.

I also improved the plate of the swashplate. Replaced the brass "fish eye" with nylon, which he ground himself. The inner part of the plate was pasted onto the elbows in the bearing and maximally pinched the lower edge under the bearing itself so that it would not rub against the main shaft with strong plate deviations

The hub has also been slightly modified, but this is almost not noticeable. Over time, the sharp edges of the traction limiters, erase the drafts themselves, coming from the plate to the trunnions. I did not remove the chamfer from the edges in order to blunt them, but just slightly hampered this edge with a usual thin screwdriver with a well polished side surface. For these purposes, any thin metal rod with a smooth surface or the tip of a thin drill is suitable.
Visually, the effect of the jam is almost invisible, but now the edge has become blunt and the thrust will no longer be so worn about it.

The main shaft I made from a carbon rod with a diameter of 2.5 mm.

The tail of the helicopter also did not undergo any changes compared to the previous models of the RX100 and RX120. It uses all the same bevel gears from Walkera V120D02S helicopter and trimmed propeller from WL V931.

Well, now the fun begins. Here and there are the biggest changes in comparison with the one-pot version.

In the photos above, you can already see the familiar laying of wires from the servo drives, and the tail engine, it has not changed, in contrast to everything else. Since the power of the helicopter has increased, compared to the one-bar version, I thought it would be better to use the main gear from the KK120 model. It is more durable.
As in previous models, I decided to use the sliding fastening of the main gear on the shaft.
I cut the slot inside the gear and put a brass tube strap on the hub. However, here I was waiting for some disappointment, the gear was not very tight on the shaft. The hole under the shaft in it was slightly larger than the gear of the WL V977 helicopter. Only in a small section of the seat under the shaft was the right diameter and sat tightly on the shaft, but its area was small for good adhesion to the shaft. I was already thinking of using a gear from the WL V977 helicopter, which fit perfectly under the sliding mount, and sat tight on the shaft, but then decided that it was possible and necessary to refine the gear from K120.
Taking a metal shaft, I put a gear on it from K120 on the superglue. After waiting for the glue to dry, I carefully knocked the gear off it. The glue remained inside the landing hole and just added the missing material for a tight fit on the shaft. Gear sat tightly on the shaft of the helicopter and the sliding mount began to work as needed.

I tried to shift the skids as much as possible, thereby displacing the battery, which is inserted into them, in order to correctly distribute the helicopter. Therefore, they had to be modified a little, making a small cutout for the main gear so that it could freely slide off the shaft in case of accidents.

As the main ESC I used Diatone D6. This is practically a copy of the XP7A controller. With the selection of the main controller, there are no special problems if you connect it to the PPM signal. Here you can choose any modern controller. With the connection of the main controller to the PWM, things are worse. Variants of the choice of the regulators are limited to the presence of old versions of the BLHeli firmware, version no higher than 12.1, which can still understand the PWM signal. Of the available options there is a regulator ХР7А, and Diatone D6 which I had in a single copy, I was not able to find on sale anymore. The 7 amp regulator should suffice, since the operating currents at power supply from 2S will be lower than when powered by 1S.

The most important problem in the construction of this helicopter was to find the right tail regulator. Since we are limited only by the signal PWM, for the tail regulator, then the choice of such a regulator is greatly narrowed. On the one hand, we are limited to the availability of a specific controller for older versions of the BLHeli firmware not older than 12.1, which is a PWM signal, and on the other hand we need a controller with fast keys that can provide the desired speed of signal processing for the tail engine.
And just with these conditions everything is very much complicated. Regulators that meet such a requirement unfortunately do not. There is a partial solution to this problem by reworking the XP7A controller, by replacing certain resistors, and replacing its firmware with a faster version of XP7AFast. The same procedure can be carried out with the Diatone D6 regulator and also use it for the tail motor.
I, in turn, in this model, use a smaller and easier MU3a regulator to control the tail engine. For this regulator is also suitable firmware XP7AFast, but with some reservations.
In the regulator, which is installed in the helicopter, I just filled in the firmware ХР7АFast, version 11.2, and it normally worked. On older versions, up to version 12.1, his work was not stable. On the last 14 versions of the firmware, this controller also worked well, but it does not make sense to use them because the PWM signal does not already support them.
However, the most important discovery for me was the fact that on the spare controller MU3a, the firmware XP7AFast and even the usual XP7A, version 11.2 did not want to work stably. At the same time on the latest firmware 14 version, the controller works stably, but as it turned out in them the speed of working out of power keys is understated. Either this controller came from another batch with other components and did not want to work properly, or it should also be modified to work with the firmware XP7AFast stably.
In general, the confidence that any MU3a controller can work well with the XP7AFast firmware is not yet available. Just want to say that on the very early releases of the firmware XP7AFast, the regulator burned out 2 power keys, so I do not recommend experimenting with them.
But even if all this is omitted, then the speed of the stable running firmware XP7AFast may possibly not be enough for a clear tail operation. Therefore, the question with the tail regulator is, unfortunately, still open.

Another unpleasant discovery for me was the lack of batteries on the market of this type that is used in helicopters KK120 and T-REX150.
The most interesting is that at the time of design everything was fine, and when I started to assemble the helicopter and decided to order batteries for it, it turned out , that from the hobbing, nanotech was lost at 330mAh, and no alternative appeared. Hardly I found the right type of battery on the aliexpress, one was pleased that at a humane price. The batteries turned out to be pretty good.

As can be seen in the photos above, the battery is not installed in the basket until the end. This is done for the correct weighting of the helicopter.

According to the owners of the helicopter HK K 120 I realized that there is a problem with taking the battery out of the basket, and I decided to deal with it right away.
Having examined the basket itself, the battery and the connector, I understood why the battery of the KK120 is getting badly.
There are 3 reasons for this.

1 - hooks on the connector itself.
They need to be cut off.

2 - location of the connector.
He turned the notches for hooks down. It turns out itself a connector at this location, is below and further from the plane of skis, to which it is attached.
When removing the battery, even with the cut off hooks of the connector, it still gets bad, because the connector itself is far from the plane of its fastening, and begins to bend inward (upwards, if the helicopter costs). This makes it more difficult to disconnect the connector. The fixing plane of the connector must be aligned with the plane of the pins inside it, or as close as possible to it, then it will be free to disconnect. And to do this, you need to turn the battery over to move up that part of the connector that is on the battery, and accordingly the nest on skis can also be displaced.
As you can see in the photo, the battery at me is developed on 180 degrees in a basket, in comparison with an arrangement in model K120. This makes it easier to disconnect the connector.

3 - on the plastic shell of the battery can be obloy, which can cling to the basket and interfere with the free movement of the battery in it.
In my case, this problem is not relevant, since the battery enters the basket not to the end, and the flash does not interfere with its movement.

The connector itself, I fixed on skis with a strip of textolite, pre-inserting and soldering it to it. The strip itself is screwed to the skis with two screws.

As a result, the battery from the basket, in my helicopter, gets pretty easy. It is much easier to change the battery than in a single-bank version with jst connector.

Well, the most interesting thing is the flight.

Making a helicopter fly well was not easy. The dynamics of the two-cell helicopter is significantly different from the dynamics when powering the power unit from one cell. The most important difference was the dynamics of the tail engine. As the supply voltage increased, the tail power and the reaction rate increased. And just with the increase in the rate of reaction of the tail motor to the signal from the flight controller, many difficulties are connected.

The flight controller from helicopter KK110 has its PID settings, under the characteristics and capabilities of a particular model. In particular, at the speed of tail signal, taking into account the mass and inertia of the tail, as well as the power and speed of the tail engine.
Since in my model the tail does not have a heavy engine on the end, and accordingly has less weight and inertia, then it works out much faster. And if you add power from 2S, then the speed will increase. And if in the one-pot version of PID the settings of the flight controller were still suitable for the tail, then on the two banks they are not quite suitable. As a result, the tail is rearranged. To avoid this, the Motor Gain parameter must be lowered in the tailgate settings. However, a decrease in this parameter does not pass without a trace. Too low the value of this parameter leads to the fact that the tail lacks the reaction speed. In this helicopter I had to lower it to a minimum value of 0.75 to calm the tail and to minimize the overshoot effect. The tail began to work stably,
Therefore, for stable operation, the tail had to slightly increase the gain in the settings of the regulator and to measure with its small tremor in operation.
All these dances with a tambourine are certainly not a solution, but only half measures. Unfortunately, they do not give an ideal result. The settings for all PID parameters for both the tail and all the others must be done directly in the flight controller. It is almost impossible to adjust them correctly at the regulator level.

To my regret, the flight controllers from helicopters KK110 or WLV977, there is no possibility to configure PID, although this possibility could be, this is not very difficult to implement and has already been implemented in other flight controllers of this type.
Therefore, in the final version I plan to use another flight controller with the ability to configure the PID, and I hope that the tail will work much more stable and clearer.

In terms of power and mechanical parts to the helicopter no complaints at all. The main motor perfectly keeps the speed and load.
For me, this power level is already redundant, and it will be quite difficult for me to handle it. I even had to tighten a little step in the console settings, up to 90%.
Compared to the one-pot version, the helicopter fires at the top much faster.

The weight of the helicopter with the blades from the KK-120 was not small - 58g, but it is still slightly lighter than the helicopter donor KK-120.
The weight of the battery is 18 g, the total flight weight is 76 g. However, this weight is not felt at all, since the power unit can easily cope with it.

Well and at last a group photo of all my models.

According to the flight video with blades from the KK120 XK, it is possible to evaluate the dynamics of a helicopter with power from two cans. In my opinion it is more than impressive.

Flying on heavy and wide blades from HC K 120, I decided to try to put on the helicopter good old blades from HK K123. They are the same length, but already easier. And then I was pleasantly surprised. With the blades from the KK-123 helicopter, the helicopter played with new colors !!! The flight became more airy, and the reaction and speed over the pike increased. The helicopter became more brisk and light. To fly with light blades from HK K123 I liked much more. I again increased the pitch to 100%, and received on the narrow blades the same response to the step as on the wide ones.
Also, according to the sensations, the speed of the head slightly increased with the light blades, and the load on the battery decreased.

Last edited by remlen; Dec 10, 2018 at 02:38 AM.
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