Trex 500 with DDFP Tail

I thought I would post this, as it’s not being done too often. I put a Direct Drive Fixed Pitch tail rotor on my Trex 500 test helicopter. I finally got it working really well. However the current code is somewhat broken. The tail is active without arming the flight controller. And it goes to full tail thrust when you arm.

Other than that it works well.

This is like the second flight test with it, before I got it tuned properly

This was the final flight test after tuning it, but the camera ran out of SD card and the video quit at 3 minutes into the flight

The final flight of the day was a 1.7 mile total distance auto flight with a target speed of 20m/s. It performed beautifully on the auto flight with perfect yaw control in 15-20 mph wind out in the open field.

This is couple photos of the modification. The motor is a Tiger 3510-630. The prop is a APC 9 x 4. The ESC is a Lumenier “Silk” 40A. Amazingly, it takes less power to run this tail than it did the stock tail. My hover amps went from an average of ~22 down to ~21 (4S battery).

New photo by Chris & Kristin Olson New photo by Chris & Kristin Olson

I probably should’ve put this in a blog instead of the Trad Heli support section. But I now have the DDFP tail working properly on my Trex 500. Hopefully the changes in the code can get into master so it works for everybody.

Doing some further tuning of the DDFP tail on my Trex 500DFC. After a lot of experimentation I’ve found the combination to get it to perform equally well compared to the stock tail is to use a motor and prop combo that throttles the tail motor at ~1600-1700pwm in hover. That seems to provide equal tail authority in both directions. Holds a heading absolutely perfect. And in FF flight the tail motor rotor throttles down to ~1400-1500 pwm which gives the tail equal accuracy in yaw being hit by crosswinds from either direction once the vertical stabilizer starts to assist.

Have basically tested it and tuned it extensively flying auto flights with it in quite strong wind to get it to this point.

Again, I’m flying modified code here provided by Rob L. to test this, as the code for DDFP is basically broken in the current stable release. And there’s a couple changes Rob would like to make to it yet. But I am very, very pleased with the simplicity and performance of it on the 500. I am still using the same components I started with, although I have cut down the APC prop a bit at a time until I found the combination of rpm and throttle that makes it really lock that tail right in.

This would be a very nice tail drive for a scale model with the tail rotor mounted in the top of the vertical stabilizer or something, vs using one of those chincy angled tail drives, or a cable.

Now that I’ve found the proper combo for the 500, I’m going to try to get it working on a 600.

Sorry for the maybe silly question, but what are the benefits of this over the traditionnal servo setup?

Anyway, great work! Definitely reminds me i need to find some time to finish setting up my trad heli!


The tail drive is probably the most fragile and failure-prone part of most RC helicopters. And usually the source of a lot of the vibration. Amazingly, the DDFP tail seems to be more power efficient than the stock tail drive. The stock drive runs the tail rotor at about 10,000 rpm on this 500. The DDFP runs at about 3,500 - 4,000 rpm in hover and ~2,500 - 3,000 rpm in forward flight.

I’ve actually been very happy with it. A friend of mine landed his 550 in the grass and the grass contact with the tail rotor blades blew some gears in the tail drive. So we put a DDFP on his 550, put my modified build in it and it works great on a 550 size too.

The code I am flying right won’t start the tail rotor motor unless there is a throttle signal present on the Channel 8 RSC. So the tail rotor can still be started prior to arming by flipping on the governor switch. So it’s not totally “safe” since you could start the tail rotor motor on the bench accidentally. But it is functional enough to test the code at this point, to make sure the system properly throttles the tail rotor motor to maintain good heading lock.

Have had no tail blowouts with it, even in FF flight. So I think it is a nice mod to improve the reliability and reduce vibration in smaller size helicopters, possibly up to 600 size (still going to test it on a 600).

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thank you very much for all the info :slight_smile:

I’m doing some research and would like to know a measured value of counter torque on RC or UAV heli of various sizes.
I’ve been thinking that with DDFP, the tail boom no longer needs to be very stiff for TT or belt and can be made longer which reduces work for tail (up until weight and CG become unmanageable) while reducing yawing speed which is not important for UAV anyway.

Do you happen to have/know any?

Thank you

I do not know of any values that have been measured. It will be a calculated value that is directly proportional to the torque absorbed by the main rotor. For the purposes of your experiment you would need to know the maximum torque the heli’s powerplant can produce, subtract the torque converted to lift to arrive at the resultant drag torque, measure the arm and use the classic torque = force x distance formula to arrive at the required thrust for the tail rotor.

That’s not something I think I can measure or given by manufacturer.
VoltAmp/(2piRPM/60) = torque ??

L/D ratio?

No, because the torque at the mainshaft is going to be a function including the transmission gearing. And I don’t know that your formula will take into account losses in the motor core.

The L/D ratio should work for that part in approximating what is turned into lift vs the drag that is converted to counter-torque. I think the problem is in calculating what the actual output torque is of the mainshaft. With full-size heli’s this is a known because it is tested with dynamometers during certification of the gearbox and engine(s).

It could maybe be measured if you construct a turntable with a heli on it and operate the heli at maximum power and collective pitch. Multiple the force on the arm (tail) by the length and you would have measured torque. But I don’t know of anybody that has done that.

I know it is an old thread, but is the DDFP code merged to 4.1 arducopter and works properly? I have trex500 which I cant get to work stable. I think problem is with vibrations. I can’t get spare parts for tail so i have deceided to make DDFP tail as i can make a mount by myselfe.

Yes it works. Just had another user provide me a log for review and he was using a DDFP tail. This is the thread

Thanks, good to hear that.