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What is the optimum rotor speed to avoid tail movement

I have 2 Trex 450 :–

1st one — flybar running Pixhack (Arducopter 4.0.5),

2nd one — FBL running APM2.5 (Arducopter 3.2.1)

After turning the knob (CH 8 controlling motor speed) to around 1560 pwm (range in both heli is roughly 1010 – 1950 pwm) and taking off in Stabilize mode, the heli is rather stable especially in light wind condition. However, changing the collective will make the tail vibrating up and down (not left and right).

I have added one more bearing to the torque tube to both heli, and a suitbale vibration damping mount is installed to APM ( Pixhack has a built-in damping structure), so vibration is not a factor. It is also shown in log which is within spec.

Can anyone shed some light on how to adjust an optimum rotor speed to avoid tail hunting, or is this inevitable because of resonance phenomena due to changing the collective (thus speed of rotor) will coincide with the natural frequency of the heli ?

Which is the parameter to reduce the tail gyro gain ?

@wcfung1 a log would be very helpful to help you with this issue. I would prefer the log be from the heli with AC 4.0.5. Please set the LOG_BITMASK to 131071. AC 3.2.1 is so outdated and so much has changed that I would rather not analyze the data from that heli. What we learn from the heli with 4.0.5 most likely will be directly relatable to the one with 3.2.1.

Hi Bill,

Thank you for yr quick response. My Trex 450 flybar had been equipped with Revo FC for the past few years

until last year when I replaced the Revo with Pixhack. The only reason for the replacement is to solve the Toilet

Bowl Effect during loiter which I cannot solve for years.

I here attach following :-- (All obtained last year and refer to the flybar 450 with REVO FC)

  1. the param (log bitmask was not set to 131071 as you requested, so no FAST ATT),

  2. the DF log, and

  3. the Binary log .

  4. photo of heli

T log is not available right now until next weekend, weather permitting. Let me know if you need it.

In fact, I am more concerned about the TBE during loiter than the optimum rotor speed in relation to tail stability.

I could not get any useful help from the forum on the former problem for years.

I have been referring to different post discussing TBE, and I was sure the built-in compass in the REVO FC works.

Vibration is taken care of by adding an extra torque tube bearing, plus the Revo built-in damping structure.

Among the efforts to trouble shoot TBE, I wanted to check the Auto Analysis of flights, but the analysis failed to execute

on all REVO(no SD card slot), be it flashed with plane, heli or multicopter FW, even though I set LOG_BACKEND_TYPE

to 4 (Block). Do you have any clue ?

May the GPS module (mounted on anti roll bracket) interferred by the rotation of the main blade above be

a factor contributing to TBE ?

Appreciate your help to find out the casue of TBE. I always admire you guys as developer equipped with the required knowledge to cope with different problems in UAV.

Revo__very steady in STAB mode__May 2020__dual torque tube bearing__4cell.param (17.3 KB)

2020-07-25 14-53-47.log (3.83 MB)

2020-07-25 14-53-47.bin (1.94 MB)

I’m sorry that I haven’t looked at this yet. I will try to do so in the next few days.
Thanks for your patience.

HI Bill,

Take your time. No rush.



Hello Frankie

Nice setup, I would take the GPS a bit toward the tail, about a 1 cm to get a better reception.
Advise you to add RPM sensor (by hobbywing), a tiny gadget, small cost, that will give you the RPM reading.

looked briefly at your data… did you check the data yourself ?

seems that your pitch control is not correct, start with the rate, you have to fix it first.
roll looks fine, yaw needs tweak.
Also you are getting vibration errors and failsafe.
Check also the rigidity of the rotors moment, they should move real easy. a big part for the TBS.

Thanks for the reply.

Moving the GPS 1 cm towards the tail does not seem to improve the reception, as it is still under the envelope

of the rotating blades.

Yes, now in the process of adjusting the rates.

Could you please elaborate in greater detail about the rigidity of rotor moments. Not sure if I understand it.

Thank you,

I think that the movement will make reception better, also depends on your GPS type, I see too much warnings in your data.

The bolt that holds the rotor affects the moment (or torque) on lead-lag movement, it should not be too tight.
The blade should move when you lightly bang the heli :slight_smile:

check the folowing video :

1 Like

How do you like the 450 flybar with AC4.0.5 firmware? Does it seem pretty stable, or is it a handful to fly, I have a couple 450s with FBL thinking about trying fly barred?

@wcfung1 HI Frankie,
I looked at your log and parameters. Regarding your pitch motion when you apply collective, this is very common for helicopters that have fairly stiff rotor systems like RC helicopters and have a CG offset. Here is your log for the pitch rate desired and actual pitch rate.

You can see how they are offset quite a bit. Since you don’t have any ATC_RAT_PIT_ILMI, then you see that the bias is carried in the desired vs actual. If you had a non-zero ILMI, then there would be a lot of integrator carrying this bias and the desired and actual pitch rate would lay on top of each other. But just putting in a non-zero ILMI will not stop the coupling from the collective. You need to get your center of gravity under the main shaft.

I would also recommend the following
Increase ATC_RAT_PIT_I to 0.1
Increase ATC_ANG_PIT_P to 4.5

I would recommend doing some testing to see if you could increase ATC_ANG_PIT_P and ATC_ANG_RLL_P to higher values. Stop if you start seeing oscillations and reduce it until they go away. I wouldn’t recommend ANG_P values any higher than 10.

Let see how the aircraft looks in stabilize before we tackle the TBE. I think you will see an improvement in loiter and auto modes with the ILMI being non zero.


Hi William,

At one stage, it was quite stable in Stabilize mode, but I have hard time tuning it to hold in position

in Loiter mode.

Couple of months ago, I replaced the Revo FC with Pixhac with the same setting. Things start to get worse

even in Stabilize mode. Now getting help from developer in the forum.



HI Bill,

Really thank you for your time. I have adjusted the param according to your suggestion,

Will wait for next weekend to test fly if weather permits and let you know the results.

I guess you suggest me to check the CG is because of the actual pitch rate being off from the desired pitch rate.

It is my mistake to check the CG by lifting the top part of the head to see if the heli tilts. Now I found out by putting something thin underneath and across the landing gear (directly below the main shaft) that the heli is nose heavy, so I adjust the position of the battery.

Hopefully, I will see some improvement later.



Hi Bill,

I test flew the 450
flybar using your suggested values, and have following findings and
questions :–

  1. In stabilize mode,
    the tail stopped pitching up and down as I (have a helper) increased
    the throttle further. In another subsequent flight, I used 4 cell,
    instead of 3 cell, the tail also stopped pitching up and down at a
    lower throttle setting than that with 3 cell. So this is solved.

  2. When I switched
    from stabilize to loiter mode, the collective increased quite
    drastic and the heli

    ascended to a higher
    altitude. In fact, the increase in collective is a result of
    switching from stabilise to altitude hold. So from alt hold to
    loiter, there is no collective increase. Which is the param that I
    should adjust to cancel this drastic increase in collective from
    stabilise to alt hold ?

  3. In your last
    analysis, you graphed my log (on pitch) to compare P des and P,
    instead of comparing P out. Why not P out ? And what is P ? It
    yields quite a different result if P des is compared to P out.

  4. When doing the
    test fly, the change in collective (in stabilise mode) is very
    sensitive to slight change in throttle stick (for collective pitch) input. Which is the parameter to adjust to reduce this sensitivity ?
    See attached DF log.

  5. Referring to the
    same attached log, since I was on stabilise mode all the time,
    should I just compare the Desired versus Actual under ATT, not under
    RATE ?

    Is it meaningless to
    compare the Desired versus Actual under RATE if I am on Stabilise
    mode all the time ?

    When comparing both
    roll and pitch under ATT, they are offset. Besides CG, what may be
    the other reasons for the offset ? Considering the difference in moment of inertia between pitch and roll axes, I set ATC_ANG_RLL _P to 4.5 and ATC_ANG_PIT_P to 3.5., and the heli was quite stable with these settings.

Thank you for your
continued support. (will do testing involving RATE controller next time).

2021-02-24 Pixhack heli.log

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