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Mar 08, 2018, 07:35 AM
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RX120. A printed size 120 helicopter.


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

Russian version http://forum.rcdesign.ru/blogs/198531/blog21630.html


So, I want to present to you my version on the theme V977 - K110.

WLToys V977 and XK K110, these are popular helicopters that do not have strong competitors in their class. They can be said to be brothers, with only minor differences.
Like any large and affordable product, there are a number of advantages and disadvantages. I will not list them all, in my diary you can find a topic on 977 with a detailed description. Here I want to tell you what I managed to create on the basis of available spare parts from these, and not only these, helicopters, as well as using 3D modeling and subsequent 3D printing.




It all started with v977, on which I'm learning to fly for the second year. During this time, much has been altered and rethought in the design of this helicopter, but the stock frame showed its limitations and did not allow to bring improvements to a new level.
I decided to create my own frame, the main feature of which is the location of the tail engine inside the frame. Implementing this idea was not quick, there were several design options, but in the end I chose the most simple and successful in my opinion.







The decisions to transfer the tail motor into the frame were caused by a number of factors.

Firstly, this is due to the stability of the helicopter. The tail motor at the end of the boom is not the best solution in terms of the security of the engine itself in case of accidents. Once, in crash, I bent the hub of the bell at the tail engine. I have straightened the shaft, this motor works fine to this day, but this is not an option.

Secondly, this is due to the flight characteristics of the helicopter. Also, having the tail motor at the end of the tail boom, does not help the implementation of many aerobatics figures.

Thirdly, the weight of the tail engine adds inertia to the tail.

I built the frame in an accessible and simple FreeCAD editor.
Subsequent printing was performed using SLS technology, material PA2200.
This material is very interesting, but it has its own peculiarities, they are described below.

Here's what I got:




Blades from V931, skis from V977, canopy from V966.




Control board from V977. The main engine is HP06S 16200kv, the tail HP03S 13500kv. Regulators MX-10a and MX-3a




Standard servos from V977 / K110. Head from Heli-Factor.




The servo holder is attached to the frame by two screws. Locking ring turned from a solid nylon. The main shaft is made of carbon rod 2.5mm




The tail beam mount. Inside the mount is a slot, and the same slot on the boom itself, not allowing it to turn in the case. The boom is tightly inserted into the mount. The boom itself is made of a 4 * 2mm carbon tube, 153.5mm long.




The tail motor is located between the rear servos. The lower part of the motor, in which the mounting holes are located, has been slightly modified so as not to interfere with the installation of servo drives. Two side lugs were cut off, just on the fixing holes, and the engine itself was fixed to the remaining pair of fixing holes.

On the hub of the bell is visible a bolt with 1mm thread. This is a regular bolt from feathering shaft. This bolt locks the drive shaft of the tail rotor. For this, a hole was drilled in the hub and threaded. At the edge of the shaft there is a slot, into which this bolt rests.

The drive shaft of the tail rotor is a shaft from the Walkera V120D02S.
This shaft serves as the drive shaft, and the shaft of the engine itself. I decided to simplify the design and not make any adapter couplings in the shaft-motor assembly. This allows you to remove unnecessary vibrations that may appear in the coupling, although it's a little difficult to assemble the tail.




The main gear is inverted and does not have any splines. Tightly worn on the shaft and nothing on it is not fixed. Between the gear and the bearing on the shaft is a metal retaining ring, which rests against the bearing and prevents the shaft from moving upward.




The tail case is made for bevel gears and bearings from Walkera V120D02S




The tail fin is attached to the boom with a screw, which partially enters the body of the boom itself. Under the screw in the boom made a small groove. The tail case is attached to the boom as well as the fin, plus it has a slot, the same as the case of the tail boom, which does not allow it to rotate. The tail rotor shaft is 1.5 mm in diameter, made of carbon fiber. The shaft is made with a slot under the pinion.




The tail screw from V931 is cut to 44mm. The hole under the shaft is bored.


The weight of the helicopter without a battery went out 46.3g

Well, now about the result. The result is very positive. Despite the fact that this is a trial version, so to speak, the first attempt, she assembled without problems and flew very well.

The tail really does not know! The tail is sharp, sharp with clear stops, without swinging after braking. The tail has no inertia. It really is felt, and it was not so pleasant to fly on the second carcass with the motor on the tail.

I was worried that after transferring the tail engine to the frame the center of gravity would shift, so I shifted the skis slightly towards the tail in order to shift the battery that is inserted into them. But my fears were in vain. The center of gravity did not go anywhere. In particular, the heavy metal drive shaft of the tail rotor is to blame for this, it weighs 2.3g. In the future I will try to make it carbon-fiber, and in cases of need to compensate for the displacement of the center of gravity by the position of the battery.

Briefly summarize, the tail motor at the end of the tail boom is evil!

Of course, I did not take into account all the nuances, the lack of design education makes itself felt, but basically it's minor edits that do not affect the flight itself.
But there is one nuance that I did not take into account and which should be taken into account in any design. Nuance is associated with the material.

SLS printing Im very pleased with the quality and good accuracy. Dimensions a little walk within one two dozen in size, but this is not critical and does not affect the assembly at all.

As for the material PA2200 - the material is interesting, very light, while it is thin, it does not break, it can be bent and spring back. But with a set of thickness becomes more fragile. The frame from it came out weighing only 3 grams, while the stock frame from 977 weighs 5 grams. But behind its ease lies another of its features, of which I did not know. It is quite flexible. By its elasticity, it can be compared with plastic, which is used in stock skids from the V977.
This rigidity is sufficient for the servo holder, for the tail fin and tail case, but for the frame itself this rigidity is already scarce. Therefore, when using the RA2200 material in printing, I can recommend to strengthen the construction by an average of one and a half times. In doing so, you will get good stiffness and weight. Well, the fact that the material is difficult to break, will only be a plus. Of the minuses of the material, one can note the porous surface and the march, in the case of white.

In fact, there are a large number of different materials for printing on SLS technology, including various polyamides. There is such a polyamide as the RA3200, and according to the specifications it is tougher than the RA2200. I will try to find an affordable print from PA3220 and from it to print the final version of the frame. Well, if I do not find it, I'll just strengthen this frame and print it with the same RA2200.

Nevertheless, I found a temporary solution, which allowed to assemble and test this frame construction. I permeated the frame with cyanoacryl (superglue). This increased its rigidity to quite acceptable, but made it fragile. Therefore, I also advise you to act only with the greatest need.

Also I would very much like to hear your adequate opinion on all this. Any useful advice is gladly accepted.

This is not the latest model. In the near future will be the final version of this model and yet, I hope not one model, another size, the frames of which are already almost designed.

I built my model here www.meltwerk.com (not advertising)

I also wanted to say thanks for the help in implementing this project to Alexander Babay and Vladimir kycerjaska


And this helicopter flies like this.
The weather is not very flying. The wind blew it off, so it did not turn out to be very interesting.

What can I say about the flight - the helicopter is flying differently!
A light tail is really cool! Although the pilot of me no, but even I feel a positive difference.
The helicopter flies much more pleasantly, more easily and not through the tail. The flight became more airy than it was.
I think that experienced pilots who know how to fly complex figures would appreciate all the changes in handling, much better than me.

The video is a little twitchy, the operator first shot helicopters and was not prepared)


Printed helicopter RX-120 (5 min 10 sec)
Last edited by remlen; Yesterday at 02:47 AM.
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