PPRuNe Forums - best approach speed and techniques to avoid vortex ring condition

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- - best approach speed and techniques to avoid vortex ring condition (https://www.pprune.org/rotorheads/595532-best-approach-speed-techniques-avoid-vortex-ring-condition.html)

Quote:

Originally Posted by AnFI (Post 9794767)

Graph labelled Vortex Ring State, does not show VRS, re-inspect

Sorry I do not understand your comment.

High ROD doesn't matter a zack provided Your above ETL :eek: & why not a decelerating approach....why fix a speed to it? ie. start at 120knots & keep slowing down till You stop, arrived over the H :ooh:

This low timer says: best thread in a while, learning/confirming a lot :)

the graph is labelled 'Flow states in descending forward flight' at the bottom but the very top line says 'Vortex Ring State'.

Fully re-inspected:ok:

Quote:

Originally Posted by NickLappos (Post 9794675)

The terrible thing about rotors is that as you go from a stable OGE hover downward, the power needed to hold a steady slight rate of descent is HIGHER than the power needed for steady hover.

I fail to grasp that. What is wrong with the following description:
a) we hover OGE at 80% power
b) setting power to 77% will start descent, with an accelerating(!) rate of descent,
e.g. VSI needle will slowly wander(!) towards max descent
(like free falling with the rotor downwash compensating almost all of earths gravity,
thus lift force < gravity force, hence we will accelerate downwards, I get that part)
c) we restore power to 80%. Momentary sink rate is preserved,
won't increase and won't decrease, e.g. VSI needle negative, but steady.
d) we raise power to 83%. Sink rate will reduce, VSI needle still negative, but starting to climb.
e) when we reach sink rate 0 we could reduce power to 80% and HOGE, or leave it at 83% and start to climb.

You were insinuating c) to be wrong. why is that?:confused:

thanks Reely340 for so clearly stating the question!

What I am stating is that in c below, the application of original power of 80% will not steady the descent, because in a slight descent of say 250 fpm, the power needed is more than 80%, it might be 85%. So if you restore the power while descending at 250 fpm to 80%, the aircraft continues to have a power deficit and will increase its descent rate. To arrest a descent of 250fpm , you must pull 85% torque. (all these numbers are hypothetical, but illustrate the issue.

If I could draw a power required curve for you, it would look like the line for -500 foot rate of descent crossed over the level flight one, so a steady descent of 500 fpm takes much more power than steady OGE hover.

Quote:

Originally Posted by NickLappos (Post 9794981)

So if you restore the power while descending at 250 fpm to 80%, the aircraft continues to have a power deficit and will increase its descent rate. To arrest a descent of 250fpm , you must pull 85% torque. (all these numbers are hypothetical, but illustrate the issue.

Interesting, :ok: very important to memorize, but why is that so?

If reapplying the (ficticious) original HOGE power of 80% after inducing a 250fpm descent does NOT arrest descent rate, e.g. sink rate keeps increasing, this would mean that a descending rotor produces less lift than a hovering rotor (pitch being identical).
I can imagine that as pitch were identical, AoA is higher with the rotor descending.
But this condition should produce MORE lift than in HOGE, right?

I'd say this: if (blade) pitch during -250fpm is kept identical to HOGE pitch, AoA will be higher due to descent. Higher AoA means more drag. More drag at same power setting results in rrpm decay. Rrpm decay is compensated by closed loop rrpm control via increase in torque. Now the increased drag from increased AoA is nicely compensated, pitch and rrpm still at HOGE setting, e.g. same lift as in HOGE, but TQ has climbed to a higher level to reestablish equilibrium. How does that sound? :8

If above is true, merely wanting to control/arrest a desired descent rate will produce requirement for more torque. I think I got that, not?

I can see now, that a piston powered helo (the ones I'm flying) can effectively run out of power and settle_with_power from trying to achieve too high a sink rate on approach.

But wouldn't that mean, that people descending with insufficient turbine TQ margin would rather overtorque their engine,
(fuel control trying to maintain rrpm), till the gearbox fails, than "falling through" from settling-with-(lackof)-power?
I always picture turbine helos as excessively overpowered with the gearbox as the limiting factor, in non hot-and-high conditions.

Nice post I made but it still feels wrong, looking at the bigger picture:

Going 50 knots I know, that
- climbing at 400 fpm needs a lot of power
- climbing at 200 fpm needs less
- straight cruise needs even less power
- descending at -300 fpm definitely needs less power then straight cruise
- autorotating at -600 fpm needs no power at all.

Why would that change if hovering?
Steadly lifting something upwards, needs more energy
("stored" as altitude energy, converted into kinetic energy upon free fall and heat upon impact ;) ).
Steadily descending vertically should "produce" energy, or need less than hover.

*sigh* I definitely need an aerodynamic explanation.

In a vertical descent the rotor tips are experiencing a flow from beneath, increasing the tip vortex and creating additional drag - so to re-establish the hover from a descent you must first overcome this extra drag with power.

At the root end, any increase in RoD will increase AoA markedly, very probably past the stall so the effectiveness of that part of the blade is also reduced so more drag there as well to be overcome with power.

tip vortices, induced drag. Slick explanation, makes sense, thx a lot!

Quote:

Originally Posted by albatross (Post 9794605)

Used to do a thing called "laser survey"
Hovering at 3-6 thousand feet maintaining position +_ 12 inches over a 10 amp gyro stabilized laser pointed veritically and impacting on a 24 inch screen on the bottom of the helicopter. The laser dot could be seen on a 4 inch tv screen in the cockpit...just keep the dot in the middle of the screen...that part took a lot of practice! LOL Some guys never could do it but with 4-6 hours of training most could ...some could never stop climbing, some could, some could also descend while keeping the dot centered and one eye/hand genius I knew would happily do hover turns as he climbed, hovered, descended and smoked a cigarette.
Sometimes ( a lot actualy ) you would be sitting there in a hover and the vsi would flicker downwards.
Add a little power ..vsi would flicker to 1-200 fpm and then WHAM you would be in fully developed Vortex Ring State...lots of fun but we could never figure out how it developed so quickly. Had lots of time to explore it as we plummeted downwards with the VSI pegged and the cyclic like a wet noodle..( once you lost the laser you had to go back to groundlevel hover over the laser to get the point back in the screen then climb vertically back up. The surveyors used to laugh when the helicopter rapidly disappeared verticaly downwards out of the field of view of their transits...the told me that they detected no downward movement of the helicopter before the abrupt downward departure and they had the helicopter in the crosshairs of a 50x theodolite.
Basically we were using the helicopter as a very tall stadia rod.
So anyone have an idea as to why the helicopter would suddenly enter Vortex Ring State from a stable hover? We were very light..usually only the pilot aboard and perhaps 3/4 fuel - AS350D in my case but we also did it using 500D, Gazelle and Alouette.

One thing I did not mention was that the above scenario used to happen after you had been in a stable hover at 2-6000 agl in light or no wind conditions for 3-4 minutes ..so you had been hovering +_ 12 inches creating a downflow of rotor down wash unaffected by wind thereby creating a column of down flowing air. That might be a clue as to what caused the sudden downwards departure into VRS. I understand that this case was not something you normally do in ops but it was a very interesting scenario.
Perhaps I was creating my own local downdraft?

Have experienced the same as you Albatross while sitting on the end of a longline for extended periods holding a load in position with no change in power.
Normally did if by default anyway but used to move the machine around to stop VRS/settling from developing as we never had the luxury of height for the recovery. ( normally moving around s bit anyway)
I wonder if a prolonged stationary hover gave the induced flow a chance to accelerate to a higher level than normally would happen and then a little decent would start due to reduced A of A, a small pitch pull to hold the height and then VRS.
Essentially what Nick said, settling first then VRS.

According to the books I have here, the downwash velocity about 2 disc lengths below the machine is twice the induced velocity, so maybe getting anywhere near that may be a problem.

Phil

Makes sense FADECDEGRADED.
Thanks PACO for your imput.
It was a long time ago but it sure was interesting.

It became predictable and was never a problem but the first time it happened it sure got my attention!