Porpoising (?) T-Rex 450 Pro

[Scallybert]

This heli: hovers well; when hovering, responds hapilly to sharp elevator input; slow forward flight is good; fast forward flight is a bit more strange.

FFF is sometimes pretty (very ?) good; but if there's wind, it's a bit like a jeep bouncing over a rough road [*]. It's perfectly controllable, but not very tidy.

I think what I'm seeing is oscillation in the pitch axis. Opinion at the flying club is that it's porpoising/dolphining. Something common with FBL systems ?

Increasing cyclic damping prompted more severe oscillations... [Lucky I had another bank to switch to... ]

So, what causes porpoising ?

Is there a rule of thumb covering what to do ? Reduce overall cyclic gain ? Reduce specific elevator gains ? [Elevator Hiller ?]
[*] Well, that's what comes to mind. I dn't actually know much about jeeps...

[georgi UK]

=] hi, are the Houses of Parliament intact? lots of fireworks going off there!

the gyro is overshooting, like a tail gyro wag but in the vertical axis, essentially the same thing. can you post a log with software settings.

[Scallybert]

Originally Posted by gmcallister

=] hi, are the Houses of Parliament intact? lots of fireworks going off there!

I think so. However, we're in Hackney, (Hackerney...) and they are slightly out of sight - though I might be able to see Canary Wharf...

the gyro is overshooting, like a tail gyro wag but in the vertical axis, essentially the same thing. can you post a log with software settings.

Ah, a log... Is there a guide to creating/using logs ? [I'm guessing I'd have had to turn something on before I went flying.]

Settings (from memory) are 'sports', with: Hiller Decay halved; Rate ~7/8 of default; Cyclic Gain is locked to 70% (in one bank, that exhibits the problem).

So, analogously to tail wag, just backing off the cyclic gain should help ? More specifically, the elevator gains ?

[georgi UK]

=] the Bow Bells in earshot?!

I was originally looking for the servo arms being too long (as in a tail wag) but to have such a high cyclic gain will create the same prob. no question.

40% or below will suffice, all that happens is the gyro becomes more restricted, there is no gain or pun in doing this!

FAQ says how to upload files here.

[Thunder Fighter]

I am seeing a similar thing with my mini Protos (350mm blades), and have just been flying around it. With the gain at 60%-70% it is worse but I am not seeing any bounce on sharp inputs, dropped the gain to 55% and it is a little better but still there. Your description is spot on, it looks untidy and bobbles in fast forward flight and in wind. I was raising the gain as it didn't feel as locked in with the gain at 50% where I started.

//Dennis.

[zeeflyboy]

Try increasing/decreasing elevator damping gain and see what happens.

[Scallybert]

Originally Posted by gmcallister

=] the Bow Bells in earshot?!.

I think the wind would have to be very strong & in the right direction...

I was originally looking for the servo arms being too long (as in a tail wag) but to have such a high cyclic gain will create the same prob. no question.

40% or below will suffice, all that happens is the gyro becomes more restricted, there is no gain or pun in doing this!.

The RJX head has quite long arms on the grips, so actually has a similar swash travel to an FB head - so the same servo arms as FB seem suitable.

I've basically approached setting the cyclic gain the same way I would with a tale gyro. ie up the gain until you get oscillation, then back off a bit. The oscillation I was provoking through cyclic input was at >70% gain - so I figured I was ok. I was thinking that the bobbing around I was seeing was due to too little gain, rather than too much - probably...

Originally Posted by Thunder Fighter

I am seeing a similar thing with my mini Protos (350mm blades), and have just been flying around it. With the gain at 60%-70% it is worse but I am not seeing any bounce on sharp inputs, dropped the gain to 55% and it is a little better but still there. Your description is spot on, it looks untidy and bobbles in fast forward flight and in wind. I was raising the gain as it didn't feel as locked in with the gain at 50% where I started.

Yes. I've got a set-up that seems to hover very tidily. I think I'll copy that bank to the other one, then significantly drop & try various gains (from the Tx) on that bank.

Since the problem seems to be with the pitch gains, rather than roll gains; when I've got the pitch gains set in that bank, I might try and tweak the effective roll gains back to the higher values in the other bank. Or not.

What was interesting was that trying to use damping gain to damp out the bobbles provoked oscillation. No different to overdoing the gain on a rate tail gyro, I guess.

[Scallybert]

Originally Posted by zeeflyboy

Try increasing/decreasing elevator damping gain and see what happens.

Increasing that (rather too much) was today's experiment - it got moderately exciting..

[Scallybert]

So anyway, down at the field last weekend, and I thought that I'd fixed the problem by reducing the cyclic gain.

However, it seems taht I couldn't reproduce the problem with the previous gain (& settings), so I haven't actualy 'fixed' it.

My guess is that the problems were maybe related to the wind conditions, and these weren't present this time out.

Anyway, I'm now running with the gain at 45%. It does seem that maybe the hovering was more precise with the previous 70%.

[Thunder Fighter]

I think it is related to wind, when I have flown indoors the heli didn't display the same issues. I haven't been able to test anything as I have been traveling.

//Dennis.

[Luvmyhelis]

Man, 70% gain? At 60 mine is bobbing almost out of control violently.

Another thing to contemplate. Is the heli balanced with the battery you are flying? If not the heli will tend to bob at different throttle rates since the Sk is trying to also overcome the weight dispensation issue. Indoor flying tends to have the heli fly at one rate of head speed whereas outside this is not as consistent. Just a thought.

[Thunder Fighter]

Mine is balanced on the mainshaft and I did try different head speeds indoors. It could be a geometry issue as I was also able to get the main gain up to 70% without wobbles on stops and at 50% it doesn't feel really locked in. I am at 55% now and in wind it still bobbles/porpoises.

//Dennis

[Scallybert]

Originally Posted by Luvmyhelis

Man, 70% gain? At 60 mine is bobbing almost out of control violently.

IIRC, I;ve got the 'hovering' FM at cyclic 60% now, and the FFF FM at 45%. It was windy yesterday, and the cyclics seemed to be behaving themselves.

I managed to get the tail out of shape, though...

I guess headspeed and other mechanical/servo factors will have a big effect on how high a gain you can run.

I'd expect headspeed to effectively increase the gain (so you may have to reduce the gain), whereas any delay in the servos/etc is going to hurt things.

Another thing to contemplate. Is the heli balanced with the battery you are flying? If not the heli will tend to bob at different throttle rates since the Sk is trying to also overcome the weight dispensation issue. Indoor flying tends to have the heli fly at one rate of head speed whereas outside this is not as consistent. Just a thought.

Yes, I did check the balance - but it can't do any harm to look again. I guess there may also be some variation between packs - which may not all be identical.