Vortex ring state

[Rappy1]

I thought i would bring this topic it up as i have noticed a few times on nitro helis i have owned and seen it happen to other people as well. Vortex ring state for those of you that don't now is induced flow entering the rotor. On the full size ones this can happen if you start to slow up while decending and the rotor wash re enters the rotor.
I am pretty sure this is what the r/c heli is entering when you are maybe 1m to 2m off the ground and all of a sudden the heli just drops. You normally panic at the sight of it dropping and add more power to try stop it from hitting hard on it's skids. Which should in theroy make the situation worse, as the blade starts to stall. In the past i have experienced it on the full size one when we were filming from high up and had to do a very fast zoom in while trying to keep the heli level and in the same spot which meant helping the camera by dropping the collective, to correct it was just a case of either trying to shake it out by moving the cyclic left and right or backing up.
I guess the best way to avoid with R/C is always to fly nose into the wind but in those cases when you either can't or have a wind change it would be handy to know away to recover. I will have to have a play around on the next windy day and see if i can find away to avoid the heavy landing which normally bounces you back up into the air about a foot. If anyone knows away to recover at the low hover from this please share as i am sure there are a lot of new-bes that would suffer from this.

[AaronS]

I have experienced it on a 50 sized AP setup. I was flying a underslung camera mount on a Sceadu 50 and it would wobble and the collective would not respond immediately during decent. smooth collective input and applying the collective early and slowly helped in my case, sort of like doing a decent with a fixed pitch heli. the Sceadu flew perfectly without the camera mount, and the same mount flown on the bigger 90 sized electric Ion is totally stable and no problem. On some smaller helis they will do it if the conditions are right. I have not experienced it to speak of other than that, with thousands of flying hours under my belt. I think a lot of it comes down to disk loading and power. our models are so lightly loaded and have so much power they can just power through a stalled condition which is harder to get into in the first place with higher headspeeds (compared to full size). I think large scale helis would be more prone to this. I wonder where the Doc is :rolleyes: Dr. TIm has flown enough big scale stuff he probably has some relevant knowledge on this topic ;)

[Motions]

I believe the correct term for this is Settling with Power. I have videos of this on scale heli models.

[heli-cuzz]

Are you referring to FF or FFF or just hovering?

I've had my heli shift altitude up and down about 1 to 2 feet on a super windy day doing FFF without any stick movements besides the last known position to perform FFF.

[AaronS]

I think the most common way to encounter this is when trying to do a vertical or near vertical decent. The helicopter hesitates and wobbles and does not respond immediately to the controls. The shift in the wind is something different. Think of it like this, when you transition to forward flight it takes less collective to maintain altitude. When you are hovering in the wind the rotor is acting like it would in forward flight, so if the wind drops, suddenly you need more collective to maintain altitude, and hence the drop. Or, if you are in a hover with no wind and a gust whips up, the heli will transition into "forward flight" and you suddenly have too much collective to maintain altitude, thats why it jumps up. This is also one of the reasons why fixed pitch helis get tossed around so badly in gusty wind. They cannot compensate as quickly, so you are constantly behind the curve trying to keep up with varying wind conditions.

[schwa]

This sounds like what happened to my Axe the last time i flew it at the park. I was just hovering it and all of a sudden it took off about 3 feet, i over compensated and drove it nose first into the ground. that was an expensive crash that one

[LEM]

I haven't had it happen to my Axe but have read of the vortex ring state on full size rotary wings (Osprey). It causes the blades to stall much like a plank can stall if airflow over the wing is not enough to over come drag and gravity. However, with a rotary wing it can be hard to recover. I think the USMC lost a couple Ospreys and some Marines due to unrecoverable stalls. I happen to know for a fact that the Osprey has software in its computers now to help keep it out of the known flight envelopes which can lead to the vortex ring state.

This is not to be confused with ground effect which happens only close to the ground Vortex ring can happen at any height if the conditions are right, as Rappy 1 mentioned a rapid vertical descent with no wind might cause it.

If your up to some technical reading here are a couple sites to check

http://www.safetycenter.navy.mil/MEDIA/approach/issues/jun03/vortex.htm

http://www.glue.umd.edu/~leishman/Aero/vring.html

And here's some info on the Osprey and VRS


http://www.globalsecurity.org/military/systems/aircraft/v-22-vrs.htm

[Rappy1]

They are freaky things to experience in the real thing as the more power you give it (your natural reaction) makes it worse. Lucky when it has happend we were up at high altitude +7000ft. I can see it being a issue you could end up with if you are doing the likes of photography with a larger rc heli maybe taking a shot of a building and only having a small area to land in like maybe a side street. The extra weight of a camera may make the problem worse.