Vortex Ring Autorotation
 

A question that has always troubled me.

If you are in fully established vortex ring state, why would entering auto not instantly exit the state?

Once the lever is fully lowered and the green arrow is going in the right direction, surely the vortex must disappear?


I am genuinely interested.
I am also very interested to know whether anyone has any wind tunnel smoke videos of a rotor in normal flight and in auto and vortex ring so I could see the actual effect visually?

I am having trouble visualising how the state could survive auto.

thanks.

Tourist, I believe it does exit VRS once the collective is fully lowered and the disc enters autorotative state.

However, as the IAS is very low and ROD very High, raising the lever again would put the rotor straight back into VRS.

The recovery therefore is to lower the lever fully and apply fwd cyclic to restablish IAS before raising the collective.

That's my understanding. Now where's a Test Pilot when you need one..........

You need low IAS, a high rate of descent and power applied to create VRS. Therefore to enter AR will eliminate VRS. But as stated above, if you apply power later on and you have still a low IAS and a high rate of descent you will find you immediately again in VRS...

Tourist - if you do search on pprune you should find posts by Nick Lappos about this - it is also referred to as windmill brake state.

So yes - fully lowering the lever will get you out of VRS but you need lots of height and the Nr will increase as the RoD flow increases through the rotor.

Dear Tourist
May I present you with the LINK to a post from years ago, written by Nick Lappos.
BTW, the whole thread is worth to read. Learned a lot from it and the banters between Lu Zuckermann (RIP) and Nick (not) are almost classics.

That's all agrees with my knowledge, however as we all know from actually flying in fully developed VRS, it actually takes quite a while to get full control back to the cyclic after you enter auto. You can be holding a lot of sick forward for quite a while before the nose drops.

The other thing is that fully developed VRS has a higher rate of descent than auto so fully lowering should instantly reduce your descent?

Thanks Rotor, that is indeed an interesting thread. I've started reading it but have not found the VRS section yet.

But then how do zero speed autos work?

Zero speed auto has no engine. To get back into VRS you need power.


Guessing - as I am no test pilot


I suspect that if you could hold the lever up long enough in a zero speed auto then you could get back into VRS, but you bleed energy so quickly that without an engine you can't hold the air flow long enough.


To have truly the same situation, you are talking about Auto from high hover. If you are high enough, you put the nose down and get some speed, so you avoid going straight down. If you are low enough, then straight down and cushion with the lever. In the latter case there is no VRS as the air does not get to recirculate (ground is close and it disperses the downdraught). Between the two heights, - you bend / break the machine and damage / kill those on board (hence the HV curve).

The link should take you directly to post 177.
He explains nicely, why you will not get into VRS while flaring or in a quick stop.

Not true though is it. When you raise the lever you are taking power from the stored energy in the head. The air does not know or care where the power comes from....

For some reason your link takes me somewhere else, but I tracked it down, thanks.
I don't think that is relevant to my question though.

If fully lowering the lever instantly gets you out of VRS, then you should have instant full control over pitch.

You could then rapidly lower the nose 30 degrees or so exactly like a recovery from a zero speed auto and climb away once past lets say 30kts.

Plus of course VRS is not instant to establish and you are descending away from your previous vortex so should be able to pull hard for at least a couple of seconds exactly as you might in the very bottom section of a zero speed auto to the ground.

I'm not saying all this is actually what happens, I am just trying to establish what is actually going on in my own mind.

Thanks crab, lots of interesting links to look through there, but unfortunately most of the interesting stuff Nick Lappos links too are so old they don't work.

Tourist


Exactly my point - if you could take power long enough you would get VRS. in light helicopters anyway, not enough stored energy for you to reverse the flow long enough to get into meaningful recirculation.

Zero speed autos are just that - no IAS. This could mean going backwards over the ground, at a set recovery height (I think 1200' for the Gaz piece) you select significant nose fwd to regain a positive trajectory over the ground if you wish or leave it to land with no fwd speed at all. Switching the engine off during the descent makes it an EOL and no longer an auto.

I get what Tourist is asking: If the RoD in VRS is *higher* than an autorotative descent, wouldn't lowering the pitch completely and entering a zero-speed auto reduce your RoD and return cyclic control?

Answer: P-r-o-b-a-b-l-y...but there are a lot of variables. If you've allowed FDVRS to develop, then transition back to zero-airspeed auto is going to take some time. And when you're plummeting to earth at 2000 fpm or greater you don't really have a lot of time to play with. If it's "just" IVRS, then lowering the collective fully and pushing the nose down is going to give you a serious rate of descent.

These discussions are largely theoretical. Prouty admits that the air and airflow through a rotor is so chaotic that wind-tunnel testing of VRS models is not really super-accurate. The airframe of the little model helicopter does not move in the environment like a full-scale helicopter in the real world. The wind tunnel air is pretty constant, unlike that produced by Mother Nature.

Secondly, nobody gets into FDVRS way up high with lots of altitude and time in which to recover. If you ever did, then it really doesn't matter which technique you choose.

Mind your head
 

Whilst entering auto from a VR state may theoretically remove one of the contributing factors, I'd suggest that you consider the design of the particular rotor head. Entering auto from level flight, the rate of descent builds up slowly. Imagine the shock loading on the head if you already have a rate of descent of 2,000 ft per minute and suddenly flatten the blade pitch to present its full surface area to the RoD airflow. It was the great Bill Barrell who advised me (many years ago) that the technique was inappropriate for teetering heads particularly. I think his exact words were "Don't be a f#*king idiot!"
:=

I would think it would wind the Nr up very quickly if you introduced that much airflow suddenly into the disc from below with zero collective pitch!

Tourist - you don't carry out the EOL from a zero speed condition, you adjust to normal auto speed before that and only use the zero speed to help you reach your aiming point. A constant attitude EOL is still done from 30-40 kts IAS so lots of forward speed.

However, raising the lever in a zero speed auto will simply decay the Nr, way before you get to any chance of opposing the upward flow through the disc and revisiting VRS - you would have slowed and stalled the whole rotor and properly fallen out of the sky. It would reduce your RoD a bit, but only temporarily.

There is nothing in the rules that says you cannot finesse entry into autorotation by lowering the correct amount to stop the Nr going crazy.

Not when I went through Wallop. I still remember with some horror the zero speed EOL. It was near enough vertical in the Gazelle.

Doing the helo conversion one of the demonstrations was VRS. A climb to 14,000 in the Huey (with parachute) and damned if the instructor (Jerry Hardy) could get the aircraft to comply, pulling collective to max permitted merely reduced the ROD.