Mention of Crab's name this am reminded me of some unfinished businees in this thread, so here goes.
Plenty of ways or “situations” to get into VRS as we see by reading this thread and ‘visualizing the old balloon’ trick simplifies things. The easiest way is to hover in formation with an imaginary balloon. Forget about ground wind speed or direction, just stay in station laterally and then descend in a SWP mode into your own downwash at a rate to induce recirculation, bingo.
In effect your whole machine complete with its recirculating vortices as separate from the surrounding air mass is inside the balloon, in freefall, falls quick eh? Then all of a sudden the balloons skin cannot hold on any more and away it blows. Whoosh, the aircraft goes straight into Crab’s definition of windmill effect. Simple, that’s the sequence of ab initio training for VRS that I do. I may do that three or four times in the one exercise starting from say 2,000 feet. Other low level exercises complete the picture.
In that scenario you simply cannot fall thousands of feet, I have done it hundreds of times in R22, R44’s and B47’s. In each of those the freefall ‘bit’ accounts for no more than 120 feet or a bit more, that is the A/C is either balanced and or steered so that it remains level within the vortices and won’t automatically tip toward a C of G displaced from the C of L to take it out into fresh air during that ‘bit’.
At the point where the whoosh occurs is the one place that I worry about relative A of A and whether it may be enough to stall the rotor blade. R22 rotor blades stalled are not a pretty sight, we all know that. Perhaps as the power is still on with N/R top of green, allows them to resist any bending and maybe they do stall momentarily, I don’t know.
I do know there is no mention of limitation for the manoeuvre in any of those POHs, so the test pilots and gurus must have considered and accepted the procedure.
Let me turn to the statement “deep VRS”. This confuses me and may be something that confuses less experienced people if not is frightening as a phobia. As I explained above it is to me a furphy. Do some aircraft develop another flight characteristic that is frightening and thus would be dangerous from a low level entry through SWP into VRS? Let’s look.
The lift for most helicopters occur on the outer ends of the blade / disc not in at the roots of the blades, in fact various sections of the blades are driving or driven in normal flight and in the hover. One only needs to damage a small portion of blade tape on the outer end to see how much lift is lost and therefore how much lift occurs at the outer ends of the blade.
It is simply impossible for circulating vortices to remain attached to the disc in the face of say a 6,000fpm descent or more of directly opposing airflow. What I suspect happens in the case where aircraft that have entered as in a classic SWP thru to VRS manoeuvre and then fall for up to 12,000 feet or more have stalled the rotor blade tips.
A stalled airfoil always needs airflow to come from a different direction to displace the stall, which of course is an attached vortices that destroy lift and is on the upper side of the blade /airfoil out of the upcoming and relative airflow.
In VRS the blades / disc are always steerable, to wit the recovery, cyclic is moved forward or any which way, therefore they are not stalled. Stalled blades won’t be steerable.
This is a common phenomenon in various types of fixed wings; they get stalled, deep stalled where the tail plane is also stalled and the pilot can’t get out of the stall, because he can’t change the direction of the stalled airfoils. He must then either jump out or crash with it.
Perhaps a quickly developed blade stall not VRS, is what happens with the Sycamore type for example I think was mentioned, tapered blades rigid enough to resist bending but the pesky things are carrying their stalled condition for a long time, up to twelve thousand feet in fact and at a very high rate of descent, some of the accounts are. Sure as god make little green apples they won’t be carrying recirculating vortices will they?
That to me is not a condition of “DEEP VRS” it is a situation of stalled airfoil. Perhaps this deep VRS is nothing but a long held misconception covering a much more dangerous flight characteristic.
Of course anyone can go out with smoke generators and prove me wrong; it would be easy to see. Someone else can do it in those nasty drop-a-lot turnouts though.
VRS is a condition which must be learnt, it is possibly the major cause leading to the overpitched condition which claims most low level accidents. Pilots whistle around the corner all of a sudden fall out of the sky, get a fright and pull too much pitch instead of recovering, N/R and engine RPM and power decays, blades stall, A/C falls, end of story.
It’s easy enough to get into and with education for quick recovery you should never fall more than 80 feet in a light machine; remember if entry is at 50’ only the last 30’ hurt.
It is also easy enough to whistle around the corner chasing belligerent cattle and dammit straight into overpitch, those not trained to recover properly crash (happening regularly lately). Those better trained recover – and with more training again, they recover - and - manage to keep control of the belligerents. A good point is that nowadays we have lots less in the way of belligerence in the cattle, in itself a trap for when it suddenly looms and has to be controlled. “Situations” may develop that haven’t been seen.
Or people stuff up a simple quick stop, overpitch and crash, one of those at the Finke Desert Race recently.
As CAGS points out the other low level malady although much less likely with mustering pilots is the simple settling with power. He will surely remember the infamous Sydney city to surf B206 accident of quite some years ago where a fully serviceable machine was flown into the ground at a steady rate of descent = power settling.
VRS = dramatically fast ROD, is not a death dealing phobia, shooting the tube I call it, learn it and live.
My point of view is that when we do intentional training we first set up a SWP situation which is a slower but controlled descent rate, then increase descent rate until the tip vortices begin recirculating where we lose control of the descent rate and the aircraft thus enters VRS and falls – with those clinging vortices until they are either blown away by the fast resultant airflow or we tip the aircraft over so that it enters new clean air of its own accord and thus leaves the vortices behind.
Here this time we have not talked about another quick way of recovery from the very early onset of VRS, aircraft first sensed as sinking – quick flick up of collective to defeat the recirculating vortices – but be aware that immediately that may put you in an over pitched situation and you need to get the collective down sharply to recover that condition.
One thing I find disturbing and Shytorque has described it well, is that many students – now experienced pilots - have not developed that sense of preservation of being in control of the aircraft instead of the other way around, and will not show awareness of, or are sensing quickly enough the initial stage of the falling of it and the recovery necessary. Whether it is because they are not prepped into this chain of logical thought as students or not, I don’t know. But the correct circuitry of thinking – if I can put it like that - must be able to be brought up at random, because it can grab you at any time, real easy.
Crab, are you happy with those thoughts, have you done any in a R22? If not can you have a go at going up and developing this “DEEP VRS” thingy and let us know how you go because I am buggered if I can?
Last edited by topendtorque; 18th Jul 2012 at 23:36.