BD3SX and E7SE on Head Speed 1300 - 1600 elevator wobble.

[johncclayton]

Hi

I've read the sticky note on setup and am having issues tuning out the elevator wobble.

This is the third heli I've setup on BD now. The E7SE had a Spirit FBL in it until a few days ago when I switched back to BD because on Spirit FBL I can't get rid of aileron wobbles - and I went through an incredibly extensive month long experiment on an X5 with the same issue and have ZERO desire to repeat that on the E7SE.

Ok, on with the show.

Important: I wish to run low head speed - range around 1300 - 1600. I'm tuning at 1600. Motor 4525-520, Kontronik Jive 120 HV+ mode 4. The Heli is new, has about 12 flights in total on it. 116 tails, 696 mains.

Yesterday I started with head gain 50, and played with elevator filter (2-8), p-portion (45-55) and d-portion (10-20) in that order. I admit that I didn't play with elevator filter each point at a time, so I may have missed something there.

In general I tried the lowest and highest extent (via RTT) of each setting in order to learn the resulting flight behaviour. My observations:

1. a higher p-portion tends to introduce a shake in the entire head, whereas bringing it to 7 leaves the head loose and sloopy, not locked in at all. I left it at 35 in the end.
2. a low d-portion (2) reduces the fequency of wobble and the amplitude seems larger but the heli feels real loose. higher d-portion (16) seems to let less wobble come through, but what does appear is higher frequency/lower amplitude - I could not get rid of all the wobble even at 20.

Regardless of what I do elevator wobble is present - I would characterize it as a slower wobble - much more an oscillation than a high frequency vibration. To try and put that in perspective - when I run the BD on ID2 (1900) with head gain 55 and ail gain at 15, descending from a hover causes very quick aileron shakes / vibes. I intend to leave the ID2 tuning for another day in the hope of keeping things a little simpler.

My test method is to hover and give quick elevator inputs, then get the stick back to centre without letting it go. During tic tocs if I am extremely careful with how I mix elevator and collective input I don't see any wobble during flight - but its trivial to cause this during tests & hovering.

The tail is super locked in (I like it more than the Spirit FBL i had on there).

Questions:

1. I've tried head gain at 40 and ele gain at 11 and I still see the wobbles. Should I simply continue to go lower on the main head gain, e.g. 30 and try again?
2. Whats the expected gain settings for low head speed setups? I intend to get it right for 1600 and drop the head speed somewhat. When everything is good I also wish to fly on 1050 with 6S too. I though low head speed setups needed more gain in general?

Looking forward to investigating and solving this...

Thanks.

John Clayton (Switzerland and loving it)

[rhodesengr]

It may not be the FBL. A lot of the heads out there will wobble as you lower head speed.

So the question is this: has any FBL given you wobble free operation at the low head speed you want?

My T550 starts to wobble below 1600.

I am no "low head speed" expert so you may want to post or read in the low head speed section here. But, I think you could try softer dampers. Also, I think I have read that low speed wobbles are a resonance and might go away if you go even lower.

[johncclayton]

Thanks for the dampers idea - appreciate it. The spirit fbl flew well on 1050 through 1340 - so I suspect it's not the Heli. But I could not get spirit to fly on 1800 without aileron wobble unless I used silly low gains which caused everything to be loose.

I will also post on low head speed forum.

[jrwalte]

Have you tried increasing head gain? Usually low hs requires higher gains and a slow wobble can be gain too low.

[DEMONjoe]

Here is what you need to do to find out if it is the FBL system (at least in the DEMON case) or not: get your head speed to a reliable value for a 700 size heli, which is 1900-2100. If it works there, the heli is fine.
The rest is simple physics, which especially in low rpm flying is too often not taken care about. A heli is designed for a certain rpm, and this is usually the higher speeds, so this is where physics work. If you go low, and 1300 already is very low, 1050 almost ridiculously low, you will need to change parts on the heli to get back to where physics can work.
So in short words: if physics are not given, then system cannot do anything to compensate this loss.

Not trying to tell that low rpm does not work. Simply do not expect a heli to work in standard config in all rpm ranges.

[johncclayton]

this makes sense of course, yet I lack a physics degree - so that does somewhat limit my capability in terms of working some of this out.

on the other hand, I can [sometimes] follow instructions and saw that in the mixer tab I had 16 degrees of cyclic throw instead of the recommended 8-10.

I brought the value down to 10 and the whole thing works a TON better. Still a bit of wobble and I'll now go from the beginning with the tuning procedure to attempt to remove it.

[DEMONjoe]

Oh yes, great. This is a good finding, as this impacts physics too

[popokatepetl]

I do fly various head speeds on my helicopters and every single helicopter has this tendency. If it does this at 1300 then go bit higher or lower with rpm and it's going to dissapear once of a sudden. You just have to find the sweet head speed for your helicotper and for your head.

What I want to say is set the head speed once the elevator wobble dissapear It can be higher or even lower rpm.

[rhodesengr]

Originally Posted by popokatepetl

What I want to say is set the head speed once the elevator wobble dissapear It can be higher or even lower rpm.

in other words, a resonant effect. Move off the resonance and it goes away.

[s car go]

I have a logo 600 setup for low head speed. I can continuously vary my head speed while in flight from 800 to 1800. I often fly at 1200. I get 15 minute flights. I can still fly at 1000. 900 is about the lower limit.

There are speeds where I get some instability with the BD3sx. I experience some minor elevator oscillations. I just have to move the head speed 50-75 RPM to get out of it. I attribute that oscillatory behavior to a resonance point between the mechanical system and the stability system. Its very mild and makes it look like my helicopter has Parkinson disease. I just bump the head speed up or down a bit to clear it up.

In industrial systems it is common to have critical points of instability or resonance as motors are brought up to speed. You cant really eliminate them you just accelerate through them as fast as possible and stay away from them during normal operation. I think its the same with these helicopters.

[johncclayton]

Just wanted to throw out a big thank you for all the replies. Fantastic stuff.

I'm from NZ but live in Switzerland - and my brother is coming over next week to fly helis with me for a week or two - we'll tune the heck out of the heli's and see where we get.

Beer may be involved after we're done flying.