Variable Pitch Direct Drive Tail for 700/800 class heli


I would like to try the direct drive tail on a 700 or 800 size heli. Could you please give me some suggestions how to do it, especially what size/type of motor. I guess I can't get a small enough ESC and motor for 10s or 12s ? Only option to use extra battery or use the lower half of the main battery ?

Did you notice an increase in efficiency ?

Thank You

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  • On my older 600-700 electric, I have used 8S for the main rotor, and the tail rotor.  The tail rotor was only rated at 6S, but the winding insulation can take the extra voltage.  I think the voltage rating of the motors is done more because of either maximum RPM tolerance, or more likely, maximum load placed on the motor with props typically used for multirotors.

    I used the ESC governor to limit the RPM to a reasonable level (6-8000 rpm) and the load is typically quite low in this application was we are typically choosing oversize motors to find a suitable shaft size, and also to handle very brief overloads when we push the tail rotor hard.

    I usually rework the motors a little bit to fit a suitable shaft without using a terribly oversize motor.  On most of the motors, you can pull the bearings and install 1mm larger ID bearings in the same bore.  Then you just need to ream out the end bell to fit the larger shaft which is easily done.  It's a big of work and requires a lathe.  Just depends on how much effort you want to put in for the best setup.

    • Then perhaps one of those T-Motor 8S would run ok on 10S. They do seem to be very well constructed motors - I should hope so at the price. I've got three of different sizes: one on the 500 tail, another for the 600, and one for my (under-construction) XUAV Talon. OMG, now a plane? Was what my wife said...

  • Salut Ultrafuge,

    Yes, there's always the problem of finding a small motor that will run on higher S battery. I'm setting up a TRex600 now, it'll be a heavy machine for photo-taking using a low HS with asymmetric SpinBlades, so I'll use just 8S for the main motor. I found a few different T-Motors that run on up to 8S and will try this one for the 600 tail:

    The smallest HV ESC I could find was the HobbyKing YEP 80A HV, a bit of overkill but should be ok.

    Another problem is the diameter of the tail shaft - most small motors will have a smaller size than the tail assy of the heli. The Trex 600 uses a 5mm shaft so the above motor is ok, and I found that some TRex 450 mainshafts (not the DFC) are 5 mm and just a little longer than necessary.

    Similarly, I set up a TRex500. There I used a MN3110-15 T-Motor - kv780 for the tail, but had to use a separate battery since the main is on 6S. And I had to buy some 4mm hardened shafts since there didn't seem to be any stock item that could be modified.

    You will need to cut a custom port-side face-plate for the tail box to mount the motor, the attached photo shows the TRex600 tail box (but with another motor). And use a piece of brass tube and/or shaft clamp to prevent any side-movement of the shaft, since when you replace the original shaft you should not use the circlip on the new shaft since that would require slotting the shaft which produces a weak point - my first try resulted in a sheared-off shaft. For bearings there are the 2 motor bearings plus the stb-side tail-box bearing, I eliminated the stock port-side bearing - too difficult to align 4 bearings and mount one in the CF custom plate.

    I'd not recommend using 1/2 of the main battery for the tail.

    Efficiency... no gigantic increase noted - for me the DD tail is somewhat, maybe much better vibration-wise for eliminating photo/video problems. Other than that, I think my dislike of gears in general is my motivation for these DD-tail machines!


    • Hi Peter,

      Thanks a lot for the very informative answer !

      In my area 'nobody' likes asymmetric SpinBlades. The ones I got I sended back, because of bad quality manufacturing (leading edge had no nose because of the man with the grinder). I compared Edge 693 FBL blades with Helitec reflexed airfoils and according to my logger they draw the same amount of Watts with identical HS, but the Helitec have a lot more lift. Therefore by adjusting for the same climb rate the comparison is much better and I can fly the Helitec with 15% less HS than the Edge with the same performance.

      I'd not recommend using 1/2 of the main battery for the tail.

      My heli runs on 10s with the best efficiency, the idea was to use a 5s 5Ah on top and a 5s 8(?)Ah at the bottom which would feed the tail as well. Could you please tell me

      - what size of extra battery and main battery do you use or in other words if you have 100% Wh on the main, how many Wh do I need for the tail ?

      - why do you not like the idea of using the bottom half of the main for the tail ?

      Thank You

      • I've not seen any definitive arguments against dividing batteries, but just suspect that you might lose some energy when cells of A are lower than cells of B, and B might then charge A with consequent losses. And at the end of a flight the two batteries might be at quite different charge and need separate recharging. So, I can't definitely say you will find some significant disadvantage. Just maybe.

        On my 500 I use 2x4Ah in series for the main and 7V servos (CCBec), and a single 2.2Ah for the tail, VTx and cams. There is always about half a charge left on that one when I run the 6S down to maybe 75-80%. I prefer to keep a good reserve on the 3S system - the weight saving I could get by using a smaller 3S battery is not much, and also I  have some extra juice in case I want to run the video system for awhile to test things before takeoff. The heli weighs a  bit over 3kg and I can get 15+m flight time just cruising and photo-ing. Not a fan of aerobatics!

        My SpinBlades have been ok, but I've not compared them against anything but some cheapos used for first liftoff. The Helitechs are essentially scale blades? Their airfoil looks interesting, might have to try a pair if I should damage the present set of SpinBlades.

        • And at the end of a flight the two batteries might be at quite different charge and need separate recharging.

          Yes you need to charge them seperatley, but the top battery will not draw current from the bottom battery, this can only happen if you connect them in parallel. It is important for parallel connection to connect the balancer cables as well.

          On my 500 I use 2x4Ah in series for the main and 7V servos (CCBec), and a single 2.2Ah for the tail, VTx and cams. There is always about half a charge left on that one when I run the 6S down to maybe 75-80%.

          If I got that right, than the tail consumes a lot less than the usually claimed 25%. Good to know.

          The Helitechs are essentially scale blades?

          No only the multiblade sets are for scale. The 'S-Schlag'=reflexed airfoil have standard or more chord length - see here Helitec 600mm reflexed

          • Seems to be. I wonder just how much those extra gears draw for the torque tube setup, might be a significant part of that claimed 25%. Of course, I don't use much power (energy) on the tail compared with aerobatic flying - that may also be a factor here.

            You're probably right about that series vs. parallel thing, if you should decide to run the tail on half the main, send a signal about its success or something less than success. Will be good to know.

            Thanks for the link, I went looking for these but didn't find the main site. Reasonable price, very German-looking with the squared off ends!

    • Peter, you will find those 4mm shafts in old PC - DVD drives.

      • Hi Fred,

        Long ones? I did find a good supplier of hardened SS shafts of all diameters, and order those when I need them.


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