Tail Travel question

I built a Tron 7 helicopter, a 700 size helicopter. I am using RotorTech Ultimate 106mm tail blades. The build manual specified to use 15mm to 16mm servo arm for the tail servo. However, with the 16mm tail servo arm, I could not reach the extremes of travel because the Neo tail travel does not go beyond "120" on either left or right travel. I replaced the 16mm servo arm with a longer Mikado 17mm servo arm. Now I can reach the left and right tail slider mechanical travel extremes, but the Neo travel numbers are a bit high, 112 Left and 116 Right. The tail is holding pretty well to whatever maneuver I can throw at it (backwards inverted hurricanes and funnels), but I am concerned about these high 112/116 travel numbers.

I am flying with YGE governed head speeds of 1900rpm on Bank 1, 2000rpm on Bank 2, and 2100rpm on Bank 3. The tail gains ended up at 80, 77, and 70, respectively. I experience tail oscillations on backwards inverted hurricanes if I increase the tail gains any higher. The auto tail optimizers ended up at 18/31, 16/30, and 34/31, respectively.

Questions:

1) Should I try using a longer tail servo arm (18mm or even 19mm) to obtain tail travel numbers closer to 100/100?

2) Should I just limit the tail slider travel anyway as perhaps the mechanical extremes of travel may be a bit too much and possible causing tail blade stall effect?

3) Are the tail gains of 80, 77, and 70 a bit low for a 700 size heli? I assume increasing the tail servo arm to 18mm or 19mm would increase the mechanical gain and therefore result in having to decrease the Neo tail gains down even further.

1 - all okay here, this uses best resolution and force.

2 - a stall effect will usually give a blowout on a fast full collectice climb. As soon as this does not happen - all okay.

3 - they depend on servo type (Futaba usually allows the most gain), lever ratios and more. In general it is not needed to max out the gain until the last digit with the Vbar. Also - if 70% or so is okay - nothing needed here.

regards (EN) / Gruss (DE)

Rainer Vetter

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Ok. I'll leave things be then.

There are several folks in the VBar Facebook forum who claim the VBar Neo works best when the tail travels are set to 100/100. I have not seen credible evidence to support this claim. Is there any truth to this?

Well - our control loop doesn't matter about this.

To understand it:

At the end of the calculation, where the output signal is calcualted for servo throw is a simple multiplier.
It just multiplies the signal (center shifted with trim flight result) with the trow values.
So no magic here.

But - consider the physics:

If a servo has to move more (throw 120) for getting the same result on the blades it has:
- a bit less speed
- a bit more force
- a bit more precision/resolution

If the same servo has to move less (throw 80) for getting the same result on the blades it has:
- a bit more speed
- a bit less force
- a bit less precision/resolution

There might be also the internal servo control loop included with different results on both scenarios too.

Well - if it should be a difference (which i can't imagine) - just try it. But as best - at the same day, with the same helicopter testing back to back ;-)

regards (EN) / Gruss (DE)

Rainer Vetter

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Posting and answering - please use a PC and a standard keyboard. Answer all questions fully, take the needed time, don't forget probably requested attachments.

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Thank you very much for clarifying this point.

Hi John,

to put some more numbers to it, and to irritate the fb community

90 or 110 means 90 % or 110 % of the servo pulse/ servo travel (which it was designed for, like +/– 45°).
At this point it has it's speed, force, precision/resolution as per the spec (hopefully).
In a linear world, and not taking lever principles into account, you'd gain or loose 0.05 s -> 10 % -> a wee 0.005 s of time over the _full_ deflection, or (that sounds worse) of 5 kgcm of torque, +/– 0.5 kg ... for resolution, I find no reliable values.

In real life, I find you notice similar end points (like your 112/116) compared to say 60/120, but also only if you are an accomplished enough pilot to feel the difference between what you do on the sticks and what the model does, and if you notice the smallest differences.
And I find, the more demanding the situation, the more you notice if you are within the 100 points limits (say +/– 10 %), compared to if you would limit to say 60/50 with a servo arm too long, or exceed to 120/115 with a servo arm too short.
But it's also no exact science, because there are servos which have a different travel per servo pulse, different torque, different response characteristics/control loops. Then, there are helicopter designs which allow for more or for less travel on the tail rotor shaft, rpm and tail blades have a saying in this equation, on small / high rpm models you probably want to use all the travel/tail pitch you can get, while on bigger models, you want to stay within the stall angles ...

In the end, don't be too focused on reaching absolute values

—Eddi

Born to fly ...
forced to work.



1 mal bearbeitet. Zuletzt am 11.10.2021 11:09 von Eddi E. aus G..

Thank you!!