Hoverfly Fast Descent - Wobbles?

[jcmonty]

So, I have been noticing lately that my Y6 (HFP, 4.3.860, Avrotos, 12x3.8SF top, 13x6.5E bot, 4.0kg, Man/AL gain 35% EPA, props are recently balanced) has had issues descending. When descending at slow to moderate speeds, I will get oscillations and wobbles. Generally, I have to arrest the descent or try to get out of my prop wash to eliminate the wobbles.

I know this is a common problem for multirotors (and rotorcraft) in general, but I previously had no real issues with descent wobbles with the HF. Also, I have seen plenty of evidence that descent wobbles can be eliminated. The impetus to the increased wobbling was increasing the AUW to the aforementioned 4.0kg (added my T2i on a static mount) and lowering my gain EPA about 20% due to the camera having rapid oscillations. All other fly qualities are spot on, including flying in wind.

What are my options to eliminate or lessen the wobbles? I really can't up the gain much without degrading the video quality due to those high frequency oscillations. I have considered changing props to something like 12x4 Xoar and 12x6 Xoar, but I am not really sure if that will help. I generally don't use any descent footage, but it's a bit nerve wracking hear and seeing the craft wobble at 100+ft in the air. :cold:

Thanks for the help
-Jonathan

 

[DucktileMedia]

Stiffer props would help a little but i think most multis do this. Although my XY8 hoverfly pro doesnt do it that much and I am using 12" APC's top and bottom. Hmm.. What caught my attention was the fact you have a Y6 and HFP as I am considering adding 2 more motors to make another octo.

 

[jcmonty]

What apc s are you using? I really think its a combo of the low gains and props that I am using. Of course if I switch props I will have to adjust my gains as well.

 

[DucktileMedia]

slow fly 12" They work great even on my XY8. I have Xoars but havent put them on yet. These are my test props but they work so well I never change them.

 

[workshop]

You are over propping those motors. Switch to 11x4.5 and / or 11x3.8 and you'll get better performance all the way around. Set up a single motor/ prop combo in MotoCalc or similar program and you'll see where the power curves are.

Jeff

 

[jcmonty]

Interesting. On the one hand I see your point since props can react faster due to the moment of inertia difference. But overall performance I have a more difficult believing since all of the data that I have seen and calculations I have done suggest otherwise. I am more efficient with these props for sure (well I*can improve*on the bottom props) ecalc supports my observations

also keep in mind that since its a coaxial you get unloading of the bottom and even the top a bit.

Of course I would love to see any data or testing that you have done that proves my thinking wrong

 

[workshop]

I am not trying to prove anybody wrong; these are areas of gray. I am offering my help so you can get the best shots possible.

I have built over two dozen professional multi-rotors over the last two years. My machines carry anything from a FLIR Quark to a RedOne Epic. I am the Technical Director for RadFlight and our pilots shoot movies (Hunger Games) and car commercials (Toyota, Honda, Scion). We use HoverFly Pro.

Jeff

www.radflight.com

 

[jcmonty]

No offense intended, and I apologize if I came off like that (darn smartphone limitations..) Impressive credentials, and you definitely have more experience than me in this department! I appreciate your input and help.

The question I should have made more clear was why you are recommending that I am overprop? In what way do you think my props are ill-sized for these motors. I am sure that you will agree that efficiency will suffer a bit with smaller props - which I concede is not the point of the topic. Also, my props are not stressing the motors or the propulsion system beyond what the manufacture specs that. So, that brings me back to the inertia factor and how the rotor can respond to changes (i.e. descending in prop wash). Can you share any more info on your comparable setup(s) that point to my props being too large - like weight, config, props, motors etc?

I still feel like it's a gain issue, but I can't seem to get rid of the oscillations on the camera mount with higher gains. I may have to try stiffer isolators or more of my current ones. I don't have a jello or vibration problem that I can tell.

Thanks again!

 

[Webheadfred]

I agree that you may be over propped a bit. I've come to this conclusion after playing around with various prop sizes and pitch. Not that your motors can't handle the pitch and the size. It's kinda like a lower gear on a drag racer, instead of starting in third gear; you'll eventually get to where you want to be, but not as fast as you need to be there.

I do get better flight times with 13x6 or 12x65 type props on the bottom and 11x47 on top. These are APC slow fliers. But since the prop is spinning slower and takes more bite out of the air, the response is a bit slower when the flight controllers asks it to spool up. Descending through its own wake and turbulence requires very fast response times for the blades to adjust to the wake as it descends. By using a bit lower pitch and size, the blades can respond at a faster rate to counter a lot of the turbulence. I also get to a 'sweet' spot in RPM which is much smoother as far as video is concerned. The draw back obviously, is reduced flight time.

 

[workshop]

No apology necessary. You wanted some background on how I formed my opinions.

The magnets (and the magnetic flux) in any given motor can only interact with magnetic field of the armature windings to a certain point. Although we can push a variety of voltages and currents through the windings, the magnetic flux of the permanent magnets is fixed and can only provide as much force as their size and chemistry dictate.

One can test for this point by graphing static thrust and motor heat while changing load (different propellers). Usually there is a point where thrust produced by the propeller tapers off and heat starts to rise; that's where the extra battery energy goes; heat.

Russell Duffey did scientifically valid tests on the Avroto and found that they peak out (thrust) with a 11x4.8. His results can be found in one of the Multirotor forums on RCG.

A note on "efficiency"... Efficiency is the least significant variable in planning a cinema system. The time between "Action!" and "Cut!" is measured in seconds not minutes. Reality TV games shows with running clocks (WipeOut, Unchained Reaction, etc.) are the only exceptions I can think of and even still, those shot times are just a minute or two. If one doesn't show up with ten sets of charged batteries, one is seen as "unprepared" no matter what the efficiency of the rig may be.

Jeff

 

[DucktileMedia]

Jeff your reel is badass! And what you say is very true about short flights. thats why I never get all worked up about flight time. But some people want to do FPV or arent doing cinematic shoots. But yes, get up and come back is how I do my work 99% of the time.

 

[yeehaanow]

To the OP- are you decsending in AL mode or manual? I find that if I have wobbles, switching to manual will pretty much solve the issue, and I run 14" apc props. I'm on a flat 6 though.
Also, I notice that it's worse in wind.

 

[E-Copter]

Hi,

the behaviour in descents can be also sometimes linked to the profile of the motor supports ( arms), as they " break" the airflux and create a disturbance in which the props are spinning themselve ( they work in their own disturbed airflow). You can, sometimes, reduce this phenomena by applying what is said right over here by Jeff, and also by adding some round foam around the arms, under the prop location, or putting some airfoil profile to help to keep the airflow of the propeller more " clean" if i can say so during descents.

Then, the last parameter are your gains and your offset setups for the IMU depending of CG, meaning depending of the batteries and the camera you are carrying.

I do agree to what jeff says about efficiency, what is needed is 100% operational machine, plenty of battery and a perfectly maintained machine that will accept dozen of start / stop / unplug sessions a day without getting into troubles

Best regards,
Fabien

 

[Sanderx]

Descending when slowly flying forward can reduce the wobble as well as you keep your machine out of the prop-wash.

 

[guy1a]

yes .. moving forward helps

https://www.youtube.com/watch?v=t9ZlheDgGu0&feature=youtube_gdata_player


Sent from my GT-I9100 using Tapatalk 2

 

[Earl E Adobtr]

Rusty recommends 11x4.7 and 4C for the Avroto with Plush 30's. He's tried all the variations and this turns out to be the best balance - of course with a Y6 it's going to be a bit of a challenge re-figuring top vs bottom. Also in multirotor forums there is a recommendation to add a "wedge" to the motor boom for the length of the prop. Once I implemented these two things on both my quad and hex (with HFP) the oscillations on descent stopped happening. One caveat, if it's pretty windy you shouldn't be descending fast and you should switch out of AL - pretty sure these were covered in prior posts.

 

[Jodde73]

I'll never go straight down. Just go slowly forward or do the "corkscrew".

 

[jcmonty]

Rusty recommends 11x4.7 and 4C for the Avroto with Plush 30's. He's tried all the variations and this turns out to be the best balance - of course with a Y6 it's going to be a bit of a challenge re-figuring top vs bottom. Also in multirotor forums there is a recommendation to add a "wedge" to the motor boom for the length of the prop. Once I implemented these two things on both my quad and hex (with HFP) the oscillations on descent stopped happening. One caveat, if it's pretty windy you shouldn't be descending fast and you should switch out of AL - pretty sure these were covered in prior posts.

I have considered aero-ing up my booms a bit, should help with drag a bit on that area as well. Perfect job for a 3D printer i think Is anyone actually using the 11x4.7 SF on their larger copters? They just seem too flimsy to me for all of that weight - especially if you consider fast maneuvars that may occasionally be necesary. Maybe the graupners are a better bet.


Thanks for all of the input. It's just a hobby for me at the moment, but I have been approached by some people to use for various shoots and projects. Guess I should just get some more batteries.

 

[jcmonty]

Forgot to post this (which I am sure that many of you have seen): http://www.1001copters.com/content/8-avroto-m2814-11s-v3
Test data for props with the Avroto. And yes, it's mostly about efficiency for those tests rather than this specific thread issue. Useful info though
Also: http://www.ae.illinois.edu/m-selig/props/propDB.html
Great wind tunnel data on some of the smaller props. I wish they had larger props, but you can somewhat extrapolate the data to large props with some analysis. Again not useful for the fast descent wobble.. but cool none-the-less :nevreness:

End off topic