I've been away a bit, playing with CAT (18 degrees C) and carby heat (two thirds applied all day), winter rain, export boat deadlines, all fun stuff.
here is a reply.
Thank you for that EAGLE and others. With the hindsight of modern light rotorcraft behaviour and design I’d be delighted to discuss those old machine characteristics sometime. The sycamore (did they have tapered blades and or retreating leading edge?) must have been a funny animal. I dreamt about one once which in my comatose state turned out to be a Dutch windmill cavorting around. Maybe not so strange?
It seems that we are at cross purposes. You guys are talking about a characteristic which was described by ‘eagle’ and all the old text books as “settling with power”, with old style blade design and perhaps is also an inherent characteristic of some new designs, where you hurtle downwards at speeds up to 80 knots and unable to steer the damm thing around.
I am not.
Thomas Coupling quite rightly points out the confusions that arise in definition and I agree. We have all been subject to it, maybe because of references to the older generation blade / disc designs and the slowness of the text books / instructors to adapt to the change of new designs???????????
Of course we must also differentiate between types of helicopters as Nick Lappos points out in a recent article of his in the latest issue of a leading magazine.
Very few people experiment with these phenomena to the fullest extent which is fair enough if it generates a known and dangerous situation on types known for it.
Even on light types most people go to the edge of the ‘predicted’ (whatever that might be in the imagination)and teach students to stay away from that big dark phobia out there. Thus guaranteeing that neither the students nor they will ever progress for their own sake to really know their machine. That is dangerous for those who live in the low level environment.
However I am going to stick to my guns on this issue as it is one that we in the mustering fraternity meet often and unless one is prepared for it then it may hurt.
Old mate with his macho talk can bug off; perhaps he could study the video of the Seaking crash on the deck of a ship with its obviously unloading disc and tell me that there have never been any problems with incipient vortex ring state in his navy.
What I do, is demonstrate how VRS will hurt and then make sure the student can demonstrate his/her recognition of the incipient stage and then, their ironclad recovery from it, or, they don’t progress; simple.
I am talking about a situation that bites you in the backside if you are not careful which is a devastating descent for a short while, which happens with VRS. It is well described in the B/CA mag January ’84 issue by Dan Manningham, where he talks about “power settling” as a vortex ring state. Excerpts as follows:-
“Engineers refer to power settling as ‘vortex ring state’ because some air cannot escape from the rotor disc and becomes trapped in a ring shaped bubble that envelopes the outer rim of the rotor disc isolating it from the flow of undisturbed air. The vortex ring state occurs when the descent is nearly vertical and at a moderate rate. ---- but at some combination of moderate vertical descent speeds and moderate power setting, the upflow from the descent is approximately the same as the rotor induced downwash, setting the stage for vortex ring interference at the rotor tips. 300 to 600fpm is the range at which most helicopters are susceptible to power settling. Once the vortex ring has been created the helicopter will settle due to a loss of lift at the rotor tips. But if power is added to arrest the rate of descent, the increased energy at the tips will just add to the vortex ring and further degrade total lift.—---If the condition is encountered at a very low power setting, a rapid power application of full power may destroy the vortex bubble.. Nevertheless, if altitude is available the best technique is to push over to increase forward airspeed. At 15 to 30 kts the rotor will no longer fly in is own vortex-----“
This is what I demonstrate, as below.
One of the things that worried the heck out of me when I first started exploring the issue was what might happen to the relative AofA at the emerging recovery stage.
About then we started to hear about the horrendous R22 stalled blade stories. They had as their Highest Common Denominator, Low RRPM, and consequently not enough power to overcome drag and or retain blade rigidity.
Quite obviously while the machine is totally engaged in VRS in its totally isolated bubble of descending air then it is as safe as a house, the blade tips are not stalled. It is controllable in a disc sense because the blades are still operating at normal AofA attitudes. The disc has no idea that it is re-ingesting air, only we do because we are sinking with it.
However inside and with that bubble the machine must be falling at the acceleration of gravity which will only last for as long as that bubble of recirculating vortices can withstand the friction from the upcoming relative airflow.
THERFORE: - complete VRS will NEVER last for long, a couple of hundred feet in the biggest helicopters I suggest.
But what happens next? The M/R blade has to transition from that safe-house of normal AofA operation to one of a very high AofA coupled with a high G loading, which is required to slow the already gained massive RofD, not a happy situation.
At that developed state there is one coupled, advocated recovery, 1) collective down and 2) cyclic forward.
Re;-
1) a) Collective down presents the disc in flat pitch, as long as it presents itself purely as an unstalled item then it should prevail with some lift to slow the RofD down. I thought about the windmilling airplane propeller in flat pitch, it simply acts as a brake and slows things down, so to should the rotor disc as long as it retains its flat centrifugal plane of integrity.
b) By experimenting with quick stops I found that it didn’t matter how severely I did them, by presenting a flat disc to the relative airflow, there was never a problem with blade stalling.
c) All the time in the VRS practice I was juggling with the collective to keep the machine in VRS as long as I could.
2) a) This was more interesting. I found that inside the bubble of air the machine immediately started to fall toward the C of G, especially laterally, unless it was loaded symmetrically and then it fell toward the right (I.E. the gyroscopic precession in the southern hemisphere).
b) As I was initially doing it in a G5 by myself (for those who are not aware the driver sits to the left of the G5) I loaded the left litter and the fall to the left was very pronounced. I found that I could apply right cyclic to hold it in.
I found it easy to manipulate the disc at any time in VRS, so the forward cyclic for recovery is immaterial, any which way will do.
c) A tilted disc certainly makes the transition into clean air quicker and reduces the RofD quicker than from a flat profile when the VRS state simply “blew away.” This to my mind was because the reduced AofA of a disc tilted towards the relative airflow gave me a far lower AofA and thus a far higher performance value to work with rather than struggling with the disc still at high AofA and not wanting to go anywhere sitting flat ar**ed in a vertical descent.
I concluded that applying forward cyclic (or in any direction) is an ‘after the event’ process, which merely serves to help flatten out the RofD quicker if one has applied it earlier.
I reckoned to get out of any “worse situation”, one simply must use the further resources that one has, I.E. 1) roll toward gyroscopic precession, 2) use T/R to present the airframe as a drag moment to help push the rotor disc over toward the relative airflow.
It seems with the “settling with power” discussion with the old style rotors that what is needed is to get the AofA down by getting the disc tilted toward the relative airflow.
I understand the exasperation with the ‘tradition’ of trying to push over “forward” with cyclic. That is usually the slowest response direction of the cyclic. Certainly some quick rolling moment is required. With 8,000fpm = 79kts up yer trousers leg, yer could stick yer arm out the window perhaps?
Or maybe it might have been easier to attempt recoveries as explained above?
I think I should leave you guys to your discussions about “Settling with Power” where you do not talk about a single ‘vortex ring state’ but instead talk about a phenomenon that I have been unable to replicate in the R22 and B47 despite many hours trying.
I assume that you refer to the old textbook and the old style machines where the blade stalls from the root outwards and every thing turns into a big rout after that, no inferences meant.
Certainly any suggestion that a ‘fully developed VRS’ is where the disc is carrying one big vortex downhill at 8,000fpm is absolutely ludicrous. Perhaps your phraseology needs rephrasing.
The beginnings of either phenomenon could well be induced by a high DA situation where not enough performance, in either the rotors or engine, is able to arrest all measure of descent. This can be a perfectly safe environment and is often encountered. The trick is to keep that descent below 300fpm and not in the discs’ own rotor induced downwash. Shooting down its own tube, so to speak.
But, shooting the tube can be done anytime, with distractions aplenty and changing wind cues invisible.
That's sort of where i live.