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Loss of lift at landings


GRoss
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Hi everybody!

 

Just wondering about the loss of lift/engine power at landings, that I sometimes experience - it only happens occasionally, but when it does I always crash (but once).

 

Pretty much what happens is I find a spot to land in, hover above it and start my descent with landing gears down - sometimes I get weird vibration inside the cockpit and it seems that the rotor blades stop generating pretty much all the lift, because even with collective at max I cannot stop the descent. The only time when I got out of that situation is when I started jerking around the cyclic (the stick) and the chopper went back to normal.

 

Any ideas what is it? Is it some kind of stall or something?

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You have just made acquaintance with Rotor Vortex Ring State. ;)

 

Basically you descend too fast with too little forward speed, and your rotor blades end up inside their own downwash which results in loss of lift and vibration. Increasing collective only makes the problem worse once the RVRS is "properly" entered.

 

To avoid this, ensure that you do not have a greater than 5m/s sink rate when at less than 50km/h TAS (True Air Speed - note, not the speed on the HUD, that's Ground Speed). If you find yourself having entered the RVRS, put your stick forward to build forward speed so you can get outside of the "ring". I typically try to ensure I never go at a higher than 3m/s sink rate when slow and low in order to have some margin. This is especially important when low, since you will then not have enough air below you to successfully exit the ring through increasing forward motion.

 

http://en.wikipedia.org/wiki/Vortex_ring

http://en.wikipedia.org/wiki/Settling_with_power


Edited by EtherealN

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Thanks a lot for the explanation! - makes sense now!

 

Gotta say though - it seems like flying a helicopter is much harder than flying a modern jet, especially on with coaxial rotor design... fun! 8D

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... it seems like flying a helicopter is much harder than flying a modern jet, especially on with coaxial rotor design... fun! 8D

 

Indeed! Helicopters are quite harder to fly. Then again, if you manage to do it, you did something "exceptional" compared to all the other virtual-pilots!

 

Descending using the 'D' key will help you maintain a descent rate no more than 2m/s.


Edited by isoul
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I used to fly helicopter logging in Alaska. The thing that I had to watch was not to dump the collective to drop the hook through the trees. The first time I settled was the only time I needed to be shown. Jettison the line and hook, not hit trees while recovering, go back to the barge to clean out pants.

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Out of curiosity, did you read the manual's helicopter physics section?

 

Hi everybody!

 

Just wondering about the loss of lift/engine power at landings, that I sometimes experience - it only happens occasionally, but when it does I always crash (but once).

 

Pretty much what happens is I find a spot to land in, hover above it and start my descent with landing gears down - sometimes I get weird vibration inside the cockpit and it seems that the rotor blades stop generating pretty much all the lift, because even with collective at max I cannot stop the descent. The only time when I got out of that situation is when I started jerking around the cyclic (the stick) and the chopper went back to normal.

 

Any ideas what is it? Is it some kind of stall or something?

[sIGPIC][/sIGPIC]

Reminder: SAM = Speed Bump :D

I used to play flight sims like you, but then I took a slammer to the knee - Yoda

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....Descending using the 'D' key will help you maintain a descent rate no more than 2m/s.

 

To preempt a catastrophe of truly epic proportions followed by uncontrolled bursts of profanity seldom uttered from the mouths of mere mortals......Do not attempt landings with Auto-Descent toggled on!

 

Just thought I'd mention it in case.

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Out of curiosity, did you read the manual's helicopter physics section?

 

RTFM? Say it :lol:

 

To preempt a catastrophe of truly epic proportions followed by uncontrolled bursts of profanity seldom uttered from the mouths of mere mortals......Do not attempt landings with Auto-Descent toggled on!

 

Just thought I'd mention it in case.

 

This is so true. I didn't know that stabilization would disengage on 4m. So... I was like feather on the wind ;)

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Descending using the 'D' key will help you maintain a descent rate no more than 2m/s.

Be sure to keep in mind that this disengages route mode, as they are two different positions of a three position switch. Then again, I don't see why you would want to be in route mode while in a hover or near-hover.

I only respond to that little mechanical voice that says "Terrain! Terrain! Pull Up! Pull Up!"

 

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-Robert Goddard

 

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To preempt a catastrophe of truly epic proportions followed by uncontrolled bursts of profanity seldom uttered from the mouths of mere mortals......Do not attempt landings with Auto-Descent toggled on!

 

Just thought I'd mention it in case.

 

A good and important addition to my hasty answer!

 

I knew that... I have crashed once in order to learn it(the hard way)!

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If you get stuck in VRS, Max out the collective (Pull hard) and push the nose forward. If you have enough height, you'll make it. Just remember not to max out the collective for too long or you'll loose rotor RPM and die!

 

Wait I thought you should floor the collective in VRS. Doesn't maxing collective just make it worse?

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Increasing collective makes it worse. To exit settling with power (vortex ring state) the proper procedure is to lower the collective (doesn't have to be to the floor, but certainly don't ADD collective) followed by movement of the cyclic to gain airspeed (doesn't have to be forward but that's usually the easiest way to go so it's used most often). You just need to get the airspeed above ETL (effective translational lift), which is around 30-35km/hr (roughly 16-20 knots) or so.

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The RVRS is my best friend. Always showing up when I don't need assistance, but nonetheless always let's me know the presence is near.

 

Ironically enough, my best friend always shows up when I'm shouting the beginning of a sentence, usually starting with: "FFFFFUUUUUUUU....."

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haha... thanks again for the explanations! ...and as somebody has asked - I did not read that section of the manual - its waay to big, and I'm just trying to learn the basics here still + having pretty much no free time does not help 8((

 

..I am not sure what do you mean by that though:

 

To preempt a catastrophe of truly epic proportions followed by uncontrolled bursts of profanity seldom uttered from the mouths of mere mortals......Do not attempt landings with Auto-Descent toggled on!

 

Just thought I'd mention it in case.

 

Isn't auto-descent supposed to help and all?

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auto-descent (as well as auto hoover) needs the doppler radar to work properly, which in turn requires height I think above 5meters. So auto-hover and auto-descent will diseangage if you are below 5m, which leads to 'a catastrophe of truly epic proportions followed by uncontrolled bursts of profanity seldom uttered from the mouths of mere mortals......' if you are not prepared for it.

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Isn't auto-descent supposed to help and all?

 

In the case of landing, no. Have to do that the old-fashioned way.

 

Edit: Sniped :D

Novice or Veteran looking for an alternative MP career?

Click me to commence your Journey of Pillage and Plunder!

[sIGPIC][/sIGPIC]

'....And when I get to Heaven, to St Peter I will tell....

One more Soldier reporting Sir, I've served my time in Hell......'

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What he means is that, when using auto-descent, all of the stabilization channels will disengage when the radar altitude drops below 4 meters. This will leave you with an extremely unstable helicopter just a couple of meters above the ground. Careful use of the collective is the better solution.

I only respond to that little mechanical voice that says "Terrain! Terrain! Pull Up! Pull Up!"

 

Who can say what is impossible, for the dream of yesterday is the hope of today and the reality of tomorrow.

-Robert Goddard

 

"A hybrid. A car for enthusiasts of armpit hair and brown rice." -Jeremy Clarkson

 

"I swear by my pretty floral bonet, I will end you." -Mal from Firefly

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Start reading ... the basics are in the manual.

Sorry to say but either you find an instructor, or you do a lot of reading when it comes to aircraft. Or both ;)

 

haha... thanks again for the explanations! ...and as somebody has asked - I did not read that section of the manual - its waay to big, and I'm just trying to learn the basics here still + having pretty much no free time does not help 8((

[sIGPIC][/sIGPIC]

Reminder: SAM = Speed Bump :D

I used to play flight sims like you, but then I took a slammer to the knee - Yoda

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Just thinking... wouldn't it be best to keep lateral speed at/above ETL threshold until you got within ground effect proximity? I haven't tried it but lately I've been getting more educated in the "optimal safety profile" for things like takeoff. The theory seems to be "OK what if the engines failed now... ok how about now? ...now?" A state of beautiful persistent paranoia.

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