Why some copters show vortex r.s. , others not

[rainer-v]

on what it depends that some copters ( as the phantoms ) show fast heavy vortex ring states and others not . E.g. my old hisystems x8 kopter could fall from sky without any gas nearly in freefall and it behaved completely controllable. alao the mavic2 can descend with 10m/s ( modded ) and looks pretty safe . the phantom starts to wobble already with 5 m/s . on what it depends that the drones behave ao different in this important aspect.

 

[Andy.]

Thanks. You prompted me to google “vortex ring states” and I learned something I wasn’t expecting to on a Friday evening.
As somebody who has only just learned of the existence of Vortex ring states, I am in no position to answer your question/ discussion but who knows, perhaps somebody else here has something more constructive to add.

 

[hherbson]

*My opinion, which is not an expert opinion*
Since most drones have a low center of gravity, a huge factor in my opinion is the prop design and drone weight. With the quiet props on my MP, it comes down with less stability than the stock props. While falling with the motors off, it falls flatly with the stock DJI props. With the DJI quiet props, it shakes and flips from side to side, the way a leaf falls. I think this is due to the increased "chord" of the quiet props. This isn't VSR per se, but thought you might find it interesting.

Coming down fast with motors on I believe heavier drones are more likely to produce a vortex ring state since more power is still required to resist gravity, compared to a lighter drone. You may find this phenomenon is reduced if you also maintain some forward flight, which is what I was taught by a helicopter instructor. Forward motion (or motion in any direction) will help prevent a vortex ring state from achieving it's maximum intensity. When dewpoint and atmospheric conditions were just right, you could see the a VRS beginning to form at the rotors diameter, and knew to do something immediately, moving or climbing to break the building cycle, kind of like breaking a pilot induced oscillation.

Regards,

Kevin

 

[novi81]

Prop design and weight are essential for Vrs, even air density can be a factor. Forward movement or head wind neutralize Vrs. Basically you try to never descend fast down or sooner or later you WILL get in Vrs no matter electronic nanies [emoji3] I always descend while moving forward or in spiral, especially with Phantom. I have ordered Master Airscrew props for Mavic .It will be interesting to see how will they affect behavior or Vrs sensitivity. On Solo and Hubsan 109 they successfully eliminate Vrs

 

[Kilrah]

Flight system tuning and optimization.

 

[rainer-v]

yes i tried today with the phantom to increase the descending velocity. It behaved very unstable, but i used the new silent props.
Tomorrow I will try the factory props.

My Hi-System drones are pretty heavy, One is a X8 with 5 kg weight ( camera and battery 2 kg on the bottom ) and the other is a regular Octocopter with 7 kg. Both I’m flying since years and I never experienced not a tint of unstableness when going down very fast.
The Mavic2 now looks also pretty stable, even increasing the descending to 10m/s and going down straight.

 

[Rivendell]

A true vortex ring state is aggravated by applying power, not cured. To arrive in the state, the helicopter/drone’s vortices (induced downwards through the rotors by the power being applied by the pilot; helicopters don’t suffer VR when no power is applied, i.e. when in autorotation) meet, at similar speed, the opposite airflow coming upwards created by the rate of descent in the same column of air (hence vertical/near vertical descent is required). The airflows fight each other and create a disturbance around the rotor disc that creates an increased rate of decent. The natural reaction is to apply more power, which increases angle of attack. In the VR state, the increased vortices this creates leads to an increased rate of decent (rather than a fly away condition) and yet more power is applied. The blade eventually stalls at the root as this is where the greatest angle of attack will always be (and why all rotor blades have a degree of pitch ‘washout’ - you can clearly see it on your Mavic; the blades are ‘flatter’ at the tips than the root. This is because they are going slower at the root and to create enough lift - and even out the forces across the blade - they must offer a greater pitch angle to the airflow). Once root stall has occurred you’re in real trouble as it will only contribute to greater angle of attack and more stall and so on until a crash occurs. There have been many in helicopters.

Weight does make a difference. Lighter helicopters require less ROD than heavier helicopters to get into VR, but are easier to recover as the overall scale of the disturbance is less. In a light helicopter, a sharp movement of the cyclic stick sideways to place the helo clear of the column of disturbed air into cleaner air is often the taught recovery technique. In a heavier helicopter a reduction of power - if necessary into autorotation - combined with forward cyclic achieves a similar aim.

The scale or airflow disturbance around a drone in descent is far less and while VR/power settling is a good term to describe the rough ride the drone suffers in high rates of decent, it should always be recoverable through greater application of power. The manufacturer will design rotor speed versus weight parameters to avoid root stall.

I’m not familiar with the X8, but if you’re genuinely falling “without gas” (entirely possible in a helo) you won’t encounter VR. Phantoms are likely to ‘wobble’ more than Mavics in high ROD.

 

[rainer-v]

nice explanations. Yes my larger kopters can go down „without any gas“, which means they nearly are free falling.

Anyway the mavic2 looks completely stable with modded setting of desc rate -10 , which should be app 36km/h vert. speed.
Its an important savety feature for me , to be able to go down fast, at least in any urban environment where lo flying helis can come close ... the phantom4 is really bad in this aspect for me.