Vortex ring

[d0ppler]

This is probably a stupid question, but why arent the upper rotor creating a constant vortex ring for the lower rotor on the Ka-52?

[tusler]

This is probably a stupid question, but why arent the upper rotor creating a constant vortex ring for the lower rotor on the Ka-52?

 

I am just guessing here, but I would think the air on the lower rotor is more chopped up and turbulent compared to the clean air the upper rotor gets. So the engineers have probably done something like rig the lower rotor at a increased blade angle compared to the upper so it lifts works out to be equal to that of the upper rotor, this would apply in a hover. Once you start forward flight I imagine the lower rotor dynamics change because you are getting some clean air through it now.

[Vortex]

Because it's creating a downwash. VRS needs upwash.

 

VRS occurs because the inner part of the rotor has a slower air speed, as such it stops creating lift when air is being forced up through the disk when you descend rapidly.

 

As this area of non lift producing lift gets bigger along the disk, less lift is produced. The faster you fall the bigger this area of the disk becomes, so as you can see it's a situation that only gets worse if you don't remove one of the three ingredients needed in VRS.

 

This picture is worth a thousand words

 

 

See the air going up the middle of the mast, that airflow gets wider and wider on the rotor, until it meets the tip vortices by which stage yuo are falling like a rock from the sky. In a normal hover even the inner rotor creates lift.

Edited December 31, 2008 by Vortex

[rmakowsky]

Sounds about right, vortex ring is the rotor wash returning through the rotor system creating very high induced flow (downward air movement), while in the case of the lower rotors they have a higher induced flow of air but not as high as encountered in vortex ring.

 

Also seems that the dual rotor system is more susceptible to vortex ring than a single rotor (on same mast) system.

[d0ppler]

Thank you guys for the enlightenment!

[press]

So the engineers have probably done something like rig the lower rotor at a increased blade angle compared to the upper so it lifts works out to be equal to that of the upper rotor, this would apply in a hover.

 

This increases the torque. Torque is what turns the helicopter....yaw movement. So...I don`t think this is the answer he`s searching for.

 

Keep searching :music_whistling::thumbup:

[Reticuli]

This is probably a stupid question, but why arent the upper rotor creating a constant vortex ring for the lower rotor on the Ka-52?

 

I would think because both main rotors are propelling air downward normally.

 

Vortex ring requires sufficient decent rate, high enough collective, and low enough lateral (front and/or side) velocity that the downwash is getting sucked back up into the main rotor(s) -- usually the inner portion, like a doughnut, hence the "ring" -- rather than being used to push against the surrounding air medium, thus self-perpetuating a sudden reduction in lift.

 

I don't see why having more than one main rotor would make that more of a problem. I get a hunch there's a little more behind your question, though. What were you thinking might be contributing to it? It's an interesting question and since I'm no expert on the matter I can't say there wouldn't be some issues involved.

 

As a general rule, the correct procedure is to lower collective to regain cyclic authority, and use front/side motion to get out of the ring state. It doesn't seem to be perfect in DCS, but it's close. I believe the correct way to prevent it is to either keep your decent rate very low in a hover or at very low speeds, or if you need to decend very rapidly, fly down instead.

 

BTW, that pic looks familiar. Is that from the FAA manual? Think I have that one.

[Yo-Yo]

... since I'm no expert on the matter I can't say there wouldn't be some issues involved.

.....

It doesn't seem to be perfect in DCS, but it's close.

 

And what's wrong in VRS in DCS?

As a general rule, the correct procedure is to lower collective to regain cyclic authority,

 

That's not exactly so... one of the main reason to lower it to lower the VR velocity because if you try to escape by increasing lateral speed the rotor passes through highly turbulenced area of VR and unexpected flapping can cause blades intersection.

[Vortex]

And what's wrong in VRS in DCS?

 

I don't know, I was hoping you could tell me. Take a look at my thread, I posted a track and no one has come back with an answer yet.

 

http://forums.eagle.ru/showthread.php?t=36020

[Reticuli]

And what's wrong in VRS in DCS?

 

 

That's not exactly so... one of the main reason to lower it to lower the VR velocity because if you try to escape by increasing lateral speed the rotor passes through highly turbulenced area of VR and unexpected flapping can cause blades intersection.

 

O.k. so that's a quirk of coaxials. Try and laterally get out of it too quickly without lowering collective and you bump the rotors and crash. O.k., though I don't think I've noticed bumping coming out of VRS, yet.

 

My FAA and other manuals say you also get difficiencies of cyclic authority until you lower collective in VRS. At least that's how non-coaxials are said to behave. And normally any non-vertical motion will assist in getting you out of it, though obviously if you have limited cyclic authority you have to lower the collective anyway to regain it and if that presents other problems (like bumping) you might not want to do initial lateral motion even if you could.

 

I'm just saying, according to the manuals I have, there seems to be too much cyclic authority. Right now, I get out of it just by nosing down, given a high starting altitude, without even needing to lower collective, while everything I've read says that ought not to be sufficient since you wouldn't be able to nose down in the first place. Lowering collective definitely helps right now in the sim, but doesn't seem absolutely necessary...assuming one is careful enough to avoid bumping. Do the dual main rotors give you more cyclic control to move the helo when in VRS even when you don't lower collective? And I seem to be able to get into them even when I'm not in a hover and already have significant, if not high, speed.

 

Maybe I should try increasing the collective when VRS hits and see if I lose cyclic. Maybe the fact I already know not to do that is preventing me from experiencing what is in so many manuals and might already be in Blackshark...

Edited January 3, 2009 by Reticuli

[uhoh7]

I haven't been in the vortex more than about 200 times.

 

It rarely kills me now, even low. I probably am doing it wrong but I pull up on the collective and usually go lateral if i have room. Rpm warning goes off, of course, but I get out. It helps to catch it early.

[Vortex]

^^^^^ Raising the collective simply unduces a greater VRS and you will lose altitude faster, negating any height you think you gain by "catching it" early. If you want to catch it early learn the signs of entering VRS and you will also learn the signs of leaving VRS. Once you establish you have left VRS you can then establish a positive climb away.

 

 

I'm just saying, according to the manuals I have, there seems to be too much cyclic authority. Right now, I get out of it just by nosing down, given a high starting altitude, without even needing to lower collective, while everything I've read says that ought not to be sufficient since you wouldn't be able to nose down in the first place.

You still have cyclic authority in VRS, it's just very "mushy" response wise. In my training using the cyclic to exit VRS, I never needed to lower the collective. If I raised the collective I was still able to escape, but I lost ALOT more altitude and falling like a rock during VRS, literally.

[Reticuli]

What are the precurser signs in DCS? It's not like you have your inner ear or your body's ability to feel a change in vibration to help you. I can't even hear much of a change in rotor rpm when I get a low rpm stall going, though maybe I need to turn the sound up.