Approach angles
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>"It seems that a helicopter would be somewhat less prone to VRS the more heavily it was loaded"
err.... is this right??? if a helicoter is heavier, then it would be operating at a higher pitch angle, and therefore more tip vortices, and therefore more danger of VRS, not less???
err.... is this right??? if a helicoter is heavier, then it would be operating at a higher pitch angle, and therefore more tip vortices, and therefore more danger of VRS, not less???
controller - heavier helo = more MR thrust required = higher AoA = increased downwash velocity, therefore more RoD required to catch the vortices and get VRS.
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the controller,
Do not think of pitch angles, they are the result, not the cause, and they are quite confusing, and actually, I would bet not one ppruner in 100 can tell me the real angle of attack (pitch) angle the blade assumes in ANY flight condition.
It is more intuitive, and more accurate, to think of the rotor making a stream of air run downward from it. Think of that stream as what supports the helo, as if it were swimming upward in a sea of air, and by pushing air downward, the helo raises itself. If the helo is heavier (for a given size rotor) it must toss more air downward, so the stream is pushed faster.
When VRS is encountered, it is because the rotor is lowering itself downward faster and faster until it catches up with some of its downwash. When the rotor is shooting downward at more than half its downwash velocity, it begind to catch up and the ring starts to form. By 75% of the downwash speed (maybe 800 fpm in a light helo) the VRS is truly encountered.
If the helo is heavier, it takes more descent speed to catch up with the faster downwash, so VRS is HARDER to find in a heavier helo.
If you are at a higher altitude, the air is thinner, and lighter, and so it must be pushed faster by the rotor, so VRS is HARDER to find at higher altitude.
Why do instructors teach otherwise? because they mix up VRS with running out of power to hover, where the aircraft falls through as it tries to hover. Falling through is EASIER at high weight, because it takes more power than normal. Falling through is EASIER at high altitude, because the engine makes less power, and the rotor needs more.
Don't mix up VRS with "Falling Through" or "Over Pitching". They look similar enough, but they are caused by two different things.
Do not think of pitch angles, they are the result, not the cause, and they are quite confusing, and actually, I would bet not one ppruner in 100 can tell me the real angle of attack (pitch) angle the blade assumes in ANY flight condition.
It is more intuitive, and more accurate, to think of the rotor making a stream of air run downward from it. Think of that stream as what supports the helo, as if it were swimming upward in a sea of air, and by pushing air downward, the helo raises itself. If the helo is heavier (for a given size rotor) it must toss more air downward, so the stream is pushed faster.
When VRS is encountered, it is because the rotor is lowering itself downward faster and faster until it catches up with some of its downwash. When the rotor is shooting downward at more than half its downwash velocity, it begind to catch up and the ring starts to form. By 75% of the downwash speed (maybe 800 fpm in a light helo) the VRS is truly encountered.
If the helo is heavier, it takes more descent speed to catch up with the faster downwash, so VRS is HARDER to find in a heavier helo.
If you are at a higher altitude, the air is thinner, and lighter, and so it must be pushed faster by the rotor, so VRS is HARDER to find at higher altitude.
Why do instructors teach otherwise? because they mix up VRS with running out of power to hover, where the aircraft falls through as it tries to hover. Falling through is EASIER at high weight, because it takes more power than normal. Falling through is EASIER at high altitude, because the engine makes less power, and the rotor needs more.
Don't mix up VRS with "Falling Through" or "Over Pitching". They look similar enough, but they are caused by two different things.
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A couple of pages back from Nick ...
Couldn't resist possibly my only ever opportunity to prove Nick wrong - see here
Regarding listening to your flight instructor, please remember your Mother told you to bundle up, so you don't catch a cold. She was off base (viruses do not care how cold you are!), but the advice was sound, and the intention was excellent!
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Nick
Quote : "Do not think of pitch angles, they are the result, not the cause, and they are quite confusing, and actually, I would bet not one ppruner in 100 can tell me the real angle of attack (pitch) angle the blade assumes in ANY flight condition."
My model/simulator does exactly do this, inch per inch, degree per degree or 0.001 sec at a a time if you which, and it took me ages to understand/verify/believe the results. So allow me to think that I do understand most of it, at least for the R44-I.
I do Agree completely that understanding VRS via angles, is the hard way, impuls theory is a lot more intuitive/simple for explaining this behaviour. In the simulator, both models are of course connected via the Math, but zillions of calculations happen to do this quantitatively, so don't assume that via one simple angle of attack drawing, the case gets explained in general.
d3
My model/simulator does exactly do this, inch per inch, degree per degree or 0.001 sec at a a time if you which, and it took me ages to understand/verify/believe the results. So allow me to think that I do understand most of it, at least for the R44-I.
I do Agree completely that understanding VRS via angles, is the hard way, impuls theory is a lot more intuitive/simple for explaining this behaviour. In the simulator, both models are of course connected via the Math, but zillions of calculations happen to do this quantitatively, so don't assume that via one simple angle of attack drawing, the case gets explained in general.
d3
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Tongue-in-cheek!
NickLappos quipped in a parting comment in this thread on 10 Nov:
“Regarding listening to your flight instructor (when he preaches limits of not exceeding 300 fpm when IAS is below 30kts), please remember your Mother told you to bundle up, so you don't catch a cold. She was off base (viruses do not care how cold you are!), but the advice was sound, and the intention was excellent!”
So imagine how shocked I was this morning to learn from the BBC News that “Mothers 'were right' over colds” and that “Scientists say they have the first proof that there really is a link between getting cold and catching one”. (http://news.bbc.co.uk/2/hi/uk_news/wales/4433496.stm).
I guess you pay your money and you take your choice!
“Regarding listening to your flight instructor (when he preaches limits of not exceeding 300 fpm when IAS is below 30kts), please remember your Mother told you to bundle up, so you don't catch a cold. She was off base (viruses do not care how cold you are!), but the advice was sound, and the intention was excellent!”
So imagine how shocked I was this morning to learn from the BBC News that “Mothers 'were right' over colds” and that “Scientists say they have the first proof that there really is a link between getting cold and catching one”. (http://news.bbc.co.uk/2/hi/uk_news/wales/4433496.stm).
I guess you pay your money and you take your choice!
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Experience versus science
Interesting topic again.
Nick stated : "viruses do not care how cold you are!"
The article states (imho): "Cold may reduce our defensive system effectiveness, when infected"
I would infer : "if you are not infected, cold (for not too long time probably), will not infect you"
Back to the topic : "in clean air VRS will not occur, if staying out of the area's shown in Nick's graph", so exceeding 30kts, 300 fpm should pose no problem. I also believe this.
But similar as in the case of a cold, you may be affected, meaning that real speeds are not what they seem, since at slow speed moving air masses may significantly change these speeds, even if this happens only momentarily, too short to be picked up by slow instruments.
Furthermore air may not be clean, for instance main rotor takes in some disturbing turbulence from tail rotor, a case that is not covered by the VRS model (as it assumes clean air)
So using "mothers rule" to keep for instance within 30/300 is probably not only well intended, but also correct if one has to assume some extra disturbances will occur, disturbances that may not be anticipated by the pilot (similar as not knowing you were already infected)
Doing a steep approach over a nice big runway, is different from doing a steep approach into an area with obsticles around creating non laminar air flows.
My 2 cents...
Added : I also do believe that (unanticipated) tail winds are one of those disturbances, see my earlier remarks/questions. It would be interesting to know what Nick's graph becomes when flying backwards for instances. My feeling is that margins may get slimmer because of the possibilty of tail and tail rotor influences.
d3
Nick stated : "viruses do not care how cold you are!"
The article states (imho): "Cold may reduce our defensive system effectiveness, when infected"
I would infer : "if you are not infected, cold (for not too long time probably), will not infect you"
Back to the topic : "in clean air VRS will not occur, if staying out of the area's shown in Nick's graph", so exceeding 30kts, 300 fpm should pose no problem. I also believe this.
But similar as in the case of a cold, you may be affected, meaning that real speeds are not what they seem, since at slow speed moving air masses may significantly change these speeds, even if this happens only momentarily, too short to be picked up by slow instruments.
Furthermore air may not be clean, for instance main rotor takes in some disturbing turbulence from tail rotor, a case that is not covered by the VRS model (as it assumes clean air)
So using "mothers rule" to keep for instance within 30/300 is probably not only well intended, but also correct if one has to assume some extra disturbances will occur, disturbances that may not be anticipated by the pilot (similar as not knowing you were already infected)
Doing a steep approach over a nice big runway, is different from doing a steep approach into an area with obsticles around creating non laminar air flows.
My 2 cents...
Added : I also do believe that (unanticipated) tail winds are one of those disturbances, see my earlier remarks/questions. It would be interesting to know what Nick's graph becomes when flying backwards for instances. My feeling is that margins may get slimmer because of the possibilty of tail and tail rotor influences.
d3
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fpm v m/s
Whooooa there!
Just catching up on this very good thread and had to comment on something quite important: 1m/s equals 60m/min equals approx 200fpm in my head. The exact figure according to Online conversions is 1 meter/second = 196.8503937 foot/minute. (Nice place for conversions by the way).
Oops, just read D3's post again and maybe he already took half away from it to get the critical Vi, in that case forget this post except the part about online conversions...
Cheers!
/2beers
Just catching up on this very good thread and had to comment on something quite important: 1m/s equals 60m/min equals approx 200fpm in my head. The exact figure according to Online conversions is 1 meter/second = 196.8503937 foot/minute. (Nice place for conversions by the way).
Oops, just read D3's post again and maybe he already took half away from it to get the critical Vi, in that case forget this post except the part about online conversions...
Cheers!
/2beers
It's not just an adventure....
it's just a job!
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Hi Nick,
Great comments. I know this thread is old but I am just catching up.
Firstly. When I was flying CH-124A's I believe the Maximum Normal Landing ROD was 8ft/sec and the absolute demonstrated Max was 10ft/sec (Emergency: 12ft/sec). Maybe I'm getting old and can't remember but I know on the CF's DDH's, we only registered a hard landing at the LSO/FFS station, above 8ft/sec.
Secondly, one factor about Mr. G's accident in the SeaKing was a lack of vertical reference training, ie, had he been trained in VRef, the accident may not have happened. I have spoken to both Pilot's as I know them personally and Mr. G. and Mr. B. (the Pilots involved) had no VRef training. Fatigue was also a cause factor in the accident investigation among many other factor's, all non-crew related.
Great Posts!
Cheers,
OffshoreIgor
Great comments. I know this thread is old but I am just catching up.
Firstly. When I was flying CH-124A's I believe the Maximum Normal Landing ROD was 8ft/sec and the absolute demonstrated Max was 10ft/sec (Emergency: 12ft/sec). Maybe I'm getting old and can't remember but I know on the CF's DDH's, we only registered a hard landing at the LSO/FFS station, above 8ft/sec.
Secondly, one factor about Mr. G's accident in the SeaKing was a lack of vertical reference training, ie, had he been trained in VRef, the accident may not have happened. I have spoken to both Pilot's as I know them personally and Mr. G. and Mr. B. (the Pilots involved) had no VRef training. Fatigue was also a cause factor in the accident investigation among many other factor's, all non-crew related.
Great Posts!
Cheers,
OffshoreIgor
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Igor,
Thanks for the details, I know the normal civil 61 is 8 ft/sec (derived from US Navy requirements as the HSS-2). The other numbers 10 and 12 are with some damage, at known points, thus the inspection after you write up the hard landing. I flew off the first 124's with de-ice blades at St. Hubert back in 1974, it was a fun summer, working with a great Canadian pilot, Ross Lennox, who flew for P+W Canada back then. Had a few drinks with the Snowbirds at St. Hubert Officer's Mess, and a big headache the next day!
If you have an accident report or any details on that CH-124 can you share them?
Nick
Thanks for the details, I know the normal civil 61 is 8 ft/sec (derived from US Navy requirements as the HSS-2). The other numbers 10 and 12 are with some damage, at known points, thus the inspection after you write up the hard landing. I flew off the first 124's with de-ice blades at St. Hubert back in 1974, it was a fun summer, working with a great Canadian pilot, Ross Lennox, who flew for P+W Canada back then. Had a few drinks with the Snowbirds at St. Hubert Officer's Mess, and a big headache the next day!
If you have an accident report or any details on that CH-124 can you share them?
Nick
