Black Hawk Accident was NOT settling with Power
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Black Hawk Accident was NOT settling with Power
The speculating is terrific, but often self defeating. We "learn" what happened before we know what happened, then we carry that "knowledge" around as truth while the real cause is announced weeks later, and we ignore it.
Some facts:
It was not settling with power. The rate of descent needed to get into vortex ring state is over 2500 fpm for a Hawk, and I saw nowhere near that on the tape. See RW-1's page for an explanation:
http://www.dynamicflight.com/aerodyn...ettling_power/
It was not LTE. A Black Hawk can hover in a 45 knot wind at 10,000 feet DA. The tail rotor is notoriously good, unlike many light helos and Bells.
It could have been a bunch of things, from failures of engines or systems, to loss of reference to simply "over-pitching" or running out of power.
In fact, the coning we see could be from a drop in rpm, and if great enough, it could get the tail rotor on the stops (not LTE, because the rpm drop is the cause). The gentle right turn is consistant with that rpm loss scenario, and the aircraft could start settling against the slope, set off the pilot's personal anxiety alarm bells (thus the cyclic wiggling) and then the main blades hit the slope and that was all she wrote.
One thing, that intact fuselage when the dust settled shows a safe machine, which held its shape and protected its occupants. In a Huey, the transmission would have unplugged and landed in the laps of the pax to help stir things up. Says a lot about US Military crash requirements.
Some facts:
It was not settling with power. The rate of descent needed to get into vortex ring state is over 2500 fpm for a Hawk, and I saw nowhere near that on the tape. See RW-1's page for an explanation:
http://www.dynamicflight.com/aerodyn...ettling_power/
It was not LTE. A Black Hawk can hover in a 45 knot wind at 10,000 feet DA. The tail rotor is notoriously good, unlike many light helos and Bells.
It could have been a bunch of things, from failures of engines or systems, to loss of reference to simply "over-pitching" or running out of power.
In fact, the coning we see could be from a drop in rpm, and if great enough, it could get the tail rotor on the stops (not LTE, because the rpm drop is the cause). The gentle right turn is consistant with that rpm loss scenario, and the aircraft could start settling against the slope, set off the pilot's personal anxiety alarm bells (thus the cyclic wiggling) and then the main blades hit the slope and that was all she wrote.
One thing, that intact fuselage when the dust settled shows a safe machine, which held its shape and protected its occupants. In a Huey, the transmission would have unplugged and landed in the laps of the pax to help stir things up. Says a lot about US Military crash requirements.
rjsquirrel
Your post must be a joke.
Helicopters get in VTR in a low speed situation at much lower sink rates than 2500 fpm.
As to LTE, that is why the 60 is not seen in Afghanastan.
One last thing, a Bell 407 filmed the crash of the 60 from above. This must mean that it is not too bad to be in a Bell.
The Sultan
Your post must be a joke.
Helicopters get in VTR in a low speed situation at much lower sink rates than 2500 fpm.
As to LTE, that is why the 60 is not seen in Afghanastan.
One last thing, a Bell 407 filmed the crash of the 60 from above. This must mean that it is not too bad to be in a Bell.
The Sultan
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the sultan,
You obviously don't know what settling with power is. As described in the texts (such as the one pointed to) the downward motion of the rotor must get close to the velocity of the downwash to enter vortex ring state (also called settling with power). Some pilots confuse running out of power and then settling as a result as being "settling with power". They are wrong, and their mistake could lead some people to believe that true VRS is common as an accident cause. It is not common, it cannot occur at low rates of descent, unless you are in an ancient helicopter with very low disk loading, like an R-4. For a Black Hawk, the VRS boundary descent rate is about 2500 fpm, for a Huey, about 1200 fpm.
The H-60 is used all over Afghanistan, just not where you were, I guess. There are dozens of H-60 types used regularly over there. The incident that made all the press was an overloaded 60M, operated way above its flight manual limits (a genuine combat emergency, not a crew error).
One would have thought that you might be able to differentiate between a news helicopter cruising at 40 or 50 knots and a heavily loaded rescue helicopter in an OGE hover. You don't understand, obviously. Work on it, OK?
It is not too bad to be in a Bell. I've got a thousand hours of combat in one. The reference to a Huey is correct, I've seen enough of them toss their transmissions after a blade strike to know. The older helicopters, Bell included, are not designed to the same tough standards as the more modern ones. I understand that the newer Bell models are quite tough.
You obviously don't know what settling with power is. As described in the texts (such as the one pointed to) the downward motion of the rotor must get close to the velocity of the downwash to enter vortex ring state (also called settling with power). Some pilots confuse running out of power and then settling as a result as being "settling with power". They are wrong, and their mistake could lead some people to believe that true VRS is common as an accident cause. It is not common, it cannot occur at low rates of descent, unless you are in an ancient helicopter with very low disk loading, like an R-4. For a Black Hawk, the VRS boundary descent rate is about 2500 fpm, for a Huey, about 1200 fpm.
The H-60 is used all over Afghanistan, just not where you were, I guess. There are dozens of H-60 types used regularly over there. The incident that made all the press was an overloaded 60M, operated way above its flight manual limits (a genuine combat emergency, not a crew error).
One would have thought that you might be able to differentiate between a news helicopter cruising at 40 or 50 knots and a heavily loaded rescue helicopter in an OGE hover. You don't understand, obviously. Work on it, OK?
It is not too bad to be in a Bell. I've got a thousand hours of combat in one. The reference to a Huey is correct, I've seen enough of them toss their transmissions after a blade strike to know. The older helicopters, Bell included, are not designed to the same tough standards as the more modern ones. I understand that the newer Bell models are quite tough.
Has anyone considered an engine failure?
The little kick to the right got me thinking?
ALSO, lets all remember that this is the PPRUNE and if you don't know what that stands for then you should not be here.
Lets speculate until the cows come home!
The little kick to the right got me thinking?
ALSO, lets all remember that this is the PPRUNE and if you don't know what that stands for then you should not be here.
Lets speculate until the cows come home!
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Speculate ??????
Don't speculate!!!
That does nothing but create rumour and gossip. Go to the ENQUIRER website for that.
We are supposed to be professional, thinking individuals.....
Let's just be glad those 4 crewmembers survived. That Sikorsky fuselage gets my praise.
I will wait for the investigation and actual account from those involved.
D.K
That does nothing but create rumour and gossip. Go to the ENQUIRER website for that.
We are supposed to be professional, thinking individuals.....
Let's just be glad those 4 crewmembers survived. That Sikorsky fuselage gets my praise.
I will wait for the investigation and actual account from those involved.
D.K
Why not Speculate Donut King ???
Speculation here will not influence a BOI or investigation. In terms of professionalism it is entirely professional, IMHO, to hypothesise about these cases; it can be very educational and may even prevent an occurrance in the future.
There is an age old tradition in aviation of going over situations informally, either in the crew room or the bar, why not continue it here?
As long as there is no finger pointing or disrespect I see problem.
Gibbo
Speculation here will not influence a BOI or investigation. In terms of professionalism it is entirely professional, IMHO, to hypothesise about these cases; it can be very educational and may even prevent an occurrance in the future.
There is an age old tradition in aviation of going over situations informally, either in the crew room or the bar, why not continue it here?
As long as there is no finger pointing or disrespect I see problem.
Gibbo
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I also don't think it could have been SWP but, I strongly disagree with the statement that you need a rate of descent in excess of 2,500 fpm to encounter one (even in the "hawk"), especially at higher altitudes, my home airport is at 7,340 FT AMSL and you can easily get into a SWP situation with say a 700 fpm true vertical descent (no wind), in any heavy helicopter at this altitudes.
When it comes to SWP its mostly about weight, disk load, and power, and there is not a lot of differece between say a 412 or a UH60, actually the 60 may be even more vulnerable due to higher weight and a lot more power.
I think its irresponsible and ignorant to make such affirmative statements, considering many newer pilots are reading this forums to build up their "decision making criteria" and settling with power is affected by so many variants.
2,500 fpm descent to have SWP, come on. In the position they were a good strong downdraft was all that was needed.
Anyway I personally think it was more like engine related.
When it comes to SWP its mostly about weight, disk load, and power, and there is not a lot of differece between say a 412 or a UH60, actually the 60 may be even more vulnerable due to higher weight and a lot more power.
I think its irresponsible and ignorant to make such affirmative statements, considering many newer pilots are reading this forums to build up their "decision making criteria" and settling with power is affected by so many variants.
2,500 fpm descent to have SWP, come on. In the position they were a good strong downdraft was all that was needed.
Anyway I personally think it was more like engine related.
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Blenderpilot,
I have to agree with squirrel. You and sultan are mixing up the fundamental problem of having too little power with the other aerodynamic condition specifically related to reingestion of the downwash known as Vortex Ring State or settling with power.
What I think you are concerned about is a true issue for pilots - the behavior of the machine when you are severely performance limited. That causes most helicopter accidents that occur on landing, is especially a problem at altitude and high temperature, and has nothing at all to do with settling with power.
It is a very common mistake for pilots to lump these things together, and it is passed from instructor to student as gospel, but it is still wrong.
In the case you describe, the engine power available is very little more than the hover power required, so there is almost nothing left to maneuver. Any disturbance, or any downdraft can absorb all the extra power that you have, and you will descend. That is because you don't have the power to climb, since there is no more power left. Most landing accidents at altitude are caused by this. You are right to be concerned if it is glossed over.
However, that is NOT Vortex Ring State, and it is NOT settling with power. To get into VRS, the rotor must descend fast enough to catch up with its downwash. That speed DOES depend on the disk loading, and that rate of descent IS around 2500 feet per minute for a Black Hawk. The downwash speed of an H-60 is about 45 knots, which is about 4500 feet per minute. At about half that rate of descent, the inboard sections of the rotor are eating up wash, and not producing any down wash. No downwash, no lift. THAT is settling with power. For a Huey, with half the disk loading of a Hawk, you get the first nibbles of VRS at about 1200 fpm rate of descent.
The reason why this is important is that you can't go around teaching folks that some mysterious stuff happens to the rotor at altitude, with moderate rates of descent. You must teach them that at altitude with moderate rates of descent, if you don't have enough power, you will get painted into a corner you can't climb out of, and the earth will smite you.
If you think a downdraft can cause VRS or settling with power, you are on the wrong page! That downdraft is dangerous, I agree, if you are in a heavy helo with little excess power, but that has nothing to do with settling with power.
I can mail you some charts and pictures and stuff, or post them on the web to discuss this more fully, if you'd like.
I have to agree with squirrel. You and sultan are mixing up the fundamental problem of having too little power with the other aerodynamic condition specifically related to reingestion of the downwash known as Vortex Ring State or settling with power.
What I think you are concerned about is a true issue for pilots - the behavior of the machine when you are severely performance limited. That causes most helicopter accidents that occur on landing, is especially a problem at altitude and high temperature, and has nothing at all to do with settling with power.
It is a very common mistake for pilots to lump these things together, and it is passed from instructor to student as gospel, but it is still wrong.
In the case you describe, the engine power available is very little more than the hover power required, so there is almost nothing left to maneuver. Any disturbance, or any downdraft can absorb all the extra power that you have, and you will descend. That is because you don't have the power to climb, since there is no more power left. Most landing accidents at altitude are caused by this. You are right to be concerned if it is glossed over.
However, that is NOT Vortex Ring State, and it is NOT settling with power. To get into VRS, the rotor must descend fast enough to catch up with its downwash. That speed DOES depend on the disk loading, and that rate of descent IS around 2500 feet per minute for a Black Hawk. The downwash speed of an H-60 is about 45 knots, which is about 4500 feet per minute. At about half that rate of descent, the inboard sections of the rotor are eating up wash, and not producing any down wash. No downwash, no lift. THAT is settling with power. For a Huey, with half the disk loading of a Hawk, you get the first nibbles of VRS at about 1200 fpm rate of descent.
The reason why this is important is that you can't go around teaching folks that some mysterious stuff happens to the rotor at altitude, with moderate rates of descent. You must teach them that at altitude with moderate rates of descent, if you don't have enough power, you will get painted into a corner you can't climb out of, and the earth will smite you.
If you think a downdraft can cause VRS or settling with power, you are on the wrong page! That downdraft is dangerous, I agree, if you are in a heavy helo with little excess power, but that has nothing to do with settling with power.
I can mail you some charts and pictures and stuff, or post them on the web to discuss this more fully, if you'd like.
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Well it seems you are truly informed on the subject, and have certainly clarified the situation to me at least! I was stupid of me to say that "all that was needed was a downdraft", honestly, I know better than that!
You mentioned a downwash speed of 45 KTS in a UH60, I suppose this varies with power power right?
I've been in situations doing ENG in an B206L4 at 8,000 FT AMSL, was hovering with about 80% TQ (no wind), I decided to descend vertically (slowly), so the camera could have a better view of an accident under a bridge which was about a mile away, about 5 seconds later with about 500 FPM the thing was "clearly" in VRS, it shook like crazy, it started wobling slightly, and applying power didn't help at all, on the contrary. At least I think it was VRS.
The helicopter aerodynamics book I have right here states that the actual critical rate of descent to achive VRS is dependant on GW, HD, and "other pertinent factors" but in any case you should consider anything of more than 300 FT/MIN, and nearly 0 airspeed a probable VRS area, I used to think this was excessive even for say 8,500 FT 25 degrees celsius we commonly operate here, now I really don't think this is excessive except when I have lots air below me.
I will definately like to have more info on this subject since I obviously have much to learn, I only have about 2,500 hrs in helicopters, but I have only flown little below say 7,500 FT, this has probably led me to be extra careful with collective application, and helicopter operation, up here the things just tend to drop to ground much faster!
Thanks for your input.
You mentioned a downwash speed of 45 KTS in a UH60, I suppose this varies with power power right?
I've been in situations doing ENG in an B206L4 at 8,000 FT AMSL, was hovering with about 80% TQ (no wind), I decided to descend vertically (slowly), so the camera could have a better view of an accident under a bridge which was about a mile away, about 5 seconds later with about 500 FPM the thing was "clearly" in VRS, it shook like crazy, it started wobling slightly, and applying power didn't help at all, on the contrary. At least I think it was VRS.
The helicopter aerodynamics book I have right here states that the actual critical rate of descent to achive VRS is dependant on GW, HD, and "other pertinent factors" but in any case you should consider anything of more than 300 FT/MIN, and nearly 0 airspeed a probable VRS area, I used to think this was excessive even for say 8,500 FT 25 degrees celsius we commonly operate here, now I really don't think this is excessive except when I have lots air below me.
I will definately like to have more info on this subject since I obviously have much to learn, I only have about 2,500 hrs in helicopters, but I have only flown little below say 7,500 FT, this has probably led me to be extra careful with collective application, and helicopter operation, up here the things just tend to drop to ground much faster!
Thanks for your input.
Nick,
Do you recall the crash of a hovering Canadian H-3 which occurred at an airshow....the video has been shown all over several TV programs. The incident occurred when the aircraft was being repositioned from one parking spot to another and appeared to start from a relatively stable hover.....at least in the vertical plane. The aircraft was being turned .....dropped like a stone, impacted terra firma and rolled over. Interviews of the crew as shown on the video were consistent in confirming no mechanical malfunction occurred and the crew stated they had encountered vortex ring state or settling with power....key point being no descent or very slow rate of descent involved at the onset of the problem.
The question in my mind is if it matters how one begins to recirculate the downwash....be it while in descent...or at a hover....it would seem so long as you find yourself in a downward moving column of air and application of additional collective (power) only makes the situation worse....then you have arrived at the point of settling with power/vortex ring state or some very similar phenomenon however you want to name it.
In an ideal test, while in VRS, would not rate of descent vary with application of collective pitch? Higher rate of descent for increase in collective pitch applied and reduced rate of descent with decrease in collective pitch applied. Which is directly opposite of the reaction in normal flight conditions?
Do you recall the crash of a hovering Canadian H-3 which occurred at an airshow....the video has been shown all over several TV programs. The incident occurred when the aircraft was being repositioned from one parking spot to another and appeared to start from a relatively stable hover.....at least in the vertical plane. The aircraft was being turned .....dropped like a stone, impacted terra firma and rolled over. Interviews of the crew as shown on the video were consistent in confirming no mechanical malfunction occurred and the crew stated they had encountered vortex ring state or settling with power....key point being no descent or very slow rate of descent involved at the onset of the problem.
The question in my mind is if it matters how one begins to recirculate the downwash....be it while in descent...or at a hover....it would seem so long as you find yourself in a downward moving column of air and application of additional collective (power) only makes the situation worse....then you have arrived at the point of settling with power/vortex ring state or some very similar phenomenon however you want to name it.
In an ideal test, while in VRS, would not rate of descent vary with application of collective pitch? Higher rate of descent for increase in collective pitch applied and reduced rate of descent with decrease in collective pitch applied. Which is directly opposite of the reaction in normal flight conditions?
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BlenderPilot,
The stuff you were taught about the factors that cause VRS are correct, because they all affect the downwash speed, which directly affects the VRS condition.
Power required to hover is prime. As the weight grows, you need more power (and more downwash) to support the machine. Remember that Bernoulli is interesting, but it is the stream of air you are blowing downward that makes the lift to hold you up. More weight means more downwash velocity, so the VRS rate of descent actually increases slightly.
In your case with the 206, what was your approximate weight? How much maximum torque did you have available (as compared to the 80% you needed to hover)? What was your maximum available vertical rate of climb?
I published a web page that explains this, and I used a 3,500 lb Bell 206 at 8,000 feet for the example. See this pointer:
http://mywebpages.comcast.net/llappos/
I hope this helps explain things.
SASless I added some discussion of what probably happens in many cases when a pilot falls through on approach. I believe that this is what happened to that legendary Sea King. I recall that the machine was spinning right as it descended, probably because the pilot had drooped the rotor to try to hold the OGE hover, and he ran out of pedal. This is appears to be a classic case of "Settling with too little power" and not settling with power (as in VRS).
Regarding the old legend that in VRS more collective makes you go down faster, I don't think that is correct. More likely, if you are in VRS, the collective doesn't do much, and that is disconcerting enough. If you increase collective, you could droop the rotor rpm, and that would be a bad thing, as well.
In truth, real VRS is hard to get into, and hard to stay in. In a powerful helicopter, you can actually muscle your way out by just increasing collective (I was told that the test pilots in the Sky Crane when flying with no load could just climb up out of VRS). This is not true of any working helicopter, so it is not recommended for recovering.
Note from the chart that you can clean up the VRS flow entirely by DESCENDING faster until you get out of VRS, and into autorotation. I have done this countless times in an S-76 during testing. It is a bad idea if the ground is within 2000 feet or so, since the rate of descent gets to 4000 to 6000 feet per minute, a truly sporty fall!
The best cure for VRS is to nudge yourself forward to gain the forward velocity to clean things up, in a jet ranger, by 12 knots you are entirely out of it.
There are two big reasons for making a point out of this distinction:
1) If you try to hover with too little power, that is a chargable foul and indicates poor planning and headwork. This is too common, and is too easy to prevent. To call it VRS allows too easy of an excuse for the pilot.
2) The VRS accident for the V-22 was classic, full blown VRS, and was increased by simply back tilting the nacelles. In tests after the accident, crews were able to induce VRS in LEVEL flight, no descent, while back tilting the nacelles (so much for pilot error, huh?) If we in the helicopter world toss about the definition of VRS improperly, someone in tilt rotor land will say that VRS is common, and such incidents in a tilt rotor are "just like helicopters" so why all the fuss.
Nick
The stuff you were taught about the factors that cause VRS are correct, because they all affect the downwash speed, which directly affects the VRS condition.
Power required to hover is prime. As the weight grows, you need more power (and more downwash) to support the machine. Remember that Bernoulli is interesting, but it is the stream of air you are blowing downward that makes the lift to hold you up. More weight means more downwash velocity, so the VRS rate of descent actually increases slightly.
In your case with the 206, what was your approximate weight? How much maximum torque did you have available (as compared to the 80% you needed to hover)? What was your maximum available vertical rate of climb?
I published a web page that explains this, and I used a 3,500 lb Bell 206 at 8,000 feet for the example. See this pointer:
http://mywebpages.comcast.net/llappos/
I hope this helps explain things.
SASless I added some discussion of what probably happens in many cases when a pilot falls through on approach. I believe that this is what happened to that legendary Sea King. I recall that the machine was spinning right as it descended, probably because the pilot had drooped the rotor to try to hold the OGE hover, and he ran out of pedal. This is appears to be a classic case of "Settling with too little power" and not settling with power (as in VRS).
Regarding the old legend that in VRS more collective makes you go down faster, I don't think that is correct. More likely, if you are in VRS, the collective doesn't do much, and that is disconcerting enough. If you increase collective, you could droop the rotor rpm, and that would be a bad thing, as well.
In truth, real VRS is hard to get into, and hard to stay in. In a powerful helicopter, you can actually muscle your way out by just increasing collective (I was told that the test pilots in the Sky Crane when flying with no load could just climb up out of VRS). This is not true of any working helicopter, so it is not recommended for recovering.
Note from the chart that you can clean up the VRS flow entirely by DESCENDING faster until you get out of VRS, and into autorotation. I have done this countless times in an S-76 during testing. It is a bad idea if the ground is within 2000 feet or so, since the rate of descent gets to 4000 to 6000 feet per minute, a truly sporty fall!
The best cure for VRS is to nudge yourself forward to gain the forward velocity to clean things up, in a jet ranger, by 12 knots you are entirely out of it.
There are two big reasons for making a point out of this distinction:
1) If you try to hover with too little power, that is a chargable foul and indicates poor planning and headwork. This is too common, and is too easy to prevent. To call it VRS allows too easy of an excuse for the pilot.
2) The VRS accident for the V-22 was classic, full blown VRS, and was increased by simply back tilting the nacelles. In tests after the accident, crews were able to induce VRS in LEVEL flight, no descent, while back tilting the nacelles (so much for pilot error, huh?) If we in the helicopter world toss about the definition of VRS improperly, someone in tilt rotor land will say that VRS is common, and such incidents in a tilt rotor are "just like helicopters" so why all the fuss.
Nick
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Nick,
With reference to your interesting last post and your second paragraph, would it also be the case that at height, as density altitude increased downwash velocity for a hover would need to increase, requiring an even higher RoD to get VRS??
With reference to your interesting last post and your second paragraph, would it also be the case that at height, as density altitude increased downwash velocity for a hover would need to increase, requiring an even higher RoD to get VRS??
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Helinut,
Yes, although the effect is not too large. The downwash velocity varies with the square root of the density, so a 25% reduction in density would result in a 15% increase in the downwash velocity. For a jet ranger, downwash velocity at sea level would be about 1600 ft/min, at 8,000 feet altitude, it would be 1800 ft/min.
On the other hand, the power the rotor needs to hover goes up by about 1% per thousand feet, so the aircraft needs 8% more power to hover. Also, engine power falls of sharply with altitude (not easy to predict in a rule of thumb) but if you lost 10% power on the engine, then a good hovering helo at sea level could have a 15 to 18% power deficit at 8,000 feet.
This would lead some folks to say that it is more prone to "settling with power" but in truth it is more prone to "settling without enough power"
Yes, although the effect is not too large. The downwash velocity varies with the square root of the density, so a 25% reduction in density would result in a 15% increase in the downwash velocity. For a jet ranger, downwash velocity at sea level would be about 1600 ft/min, at 8,000 feet altitude, it would be 1800 ft/min.
On the other hand, the power the rotor needs to hover goes up by about 1% per thousand feet, so the aircraft needs 8% more power to hover. Also, engine power falls of sharply with altitude (not easy to predict in a rule of thumb) but if you lost 10% power on the engine, then a good hovering helo at sea level could have a 15 to 18% power deficit at 8,000 feet.
This would lead some folks to say that it is more prone to "settling with power" but in truth it is more prone to "settling without enough power"
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I usually never disagree with what the esteemed Mr. Lappos says, but in this case, I think he might be slightly off-base.
We tend to think of helicopter rotors as being absolutely and repeatedly predictable in their behaviour, much like the plank of an airplane is. However, when it comes to what is going on within a rotor system in flight, things get less precise. Ray Prouty has published charts in Rotor&Wing Magazine which show the data points to be all over the place, with a line more or less arbitrarily drawn through them at an "average" to give us a baseline for predicting performance. But what this tells us is that there are many variables - perhaps an infinite amount - which affect the way a particular rotor (or indeed, individual blade) will perform at any given time.
When it comes to VRS or SWP, we tend to focus solely on rate-of-descent of the aircraft, as if it were independant of other factors. Well, the air moves too, eh what? And sometimes that air moves vertically, not just horizontally. Suppose a helicopter developed a small ROD just as it encountered a slight updraft. The net result would be an increase in effective ROD, which might or might not be noticeable unless we were studiously watching the VSI, a gauge not known to be in our instrument scan during hovering ops (mine wags up and down considerably in a OGE hover). Therefore, I personally believe that making declarative statements like: "A UH-60 needs a ROD of 2500 feet before it can ever get into VRS" is the height of irresponsibility. Bollocks! I propose that in addition to the "hard" numbers that we know will aggravate or excite VRS, there are other factors which are quite nebulous that figure into the equation.
Did that UH-60 crew on Mt. Hood get into VRS? Who can know for sure? Even if the charts said (which we usually translate into "prove") that the aircraft should have been capable of the job, perhaps the rotor was closer to the "edge" than we know. Perhaps the rotor's reserve of lift at that HOGE altitude was borderline, and perhaps they experienced just enough of an updraft [in the mountains? come on, mate!] to cause them to fall off the bubble and into incipient (maybe not full-blown) VRS.
Then again, it appeared that they were hovering nose-in toward the slope. Is this true? Maybe I didn't see the whole accident sequence. Why not turn ninety-degrees to the slope? Would that not have given them better visual cues? And would that not have given them an easier "out" of they needed to lower the collective and bail from the situation for any reason? I wasn't there, but I sure wouldn't want to try to hold a stationary hover with a wall of white filling my windscreen.
Contrary to popular belief, gradual engine failures in a twin-engine helicopter do not cause a similar yaw-snap to the right (Yank birds) as they do in a single. As one engine falls off, the other goes to full power or beyond to take up the slack. We do not see any evidence of a catastrophic engine failure from the video (smoke, flame, etc), so I would move away from that line of thinking. Although the extreme coning of the blades does indicate (to me, at least) a reduction in MR rpm - or perhaps a over-application of collective pitch. Whatever DID happen, I believe it to be aerodynamic in nature and not mechanical - we'll soon find out.
Finally, Nick surmises that the Canadian Sea King at that airshow did not get into a setting-with-power situation, but rather a settling-with-too-little-power, owing exlusively to the right-yaw of the nose as the ship came down. Well, old boy, you're entitled to your opinion. I propose that it was indeed SWP (VRS), judging by the incredible rate-of-descent that built up so quickly, and the extreme coning of the blades as the poor pilot yanked everything he had to stop it, to no avail. The right-hand yaw was simply either his failure to fully mash the left pedal all the way (focused as he was on other, more pressing things in that brief second of time he had between "stable hover" and "Crikey, that hurt!"), or the inability of the tail rotor to hold the nose straight with the collective lever on the up-stop.
It is a myth to think that VRS is "hard" to get into, or that it must be excited deliberately. Anyone who would make such foolish statements has obviously never gotten into it inadvertently in the real world. Those of us who have know that all the "hard" numbers in the world don't mean a thing when the bottom falls out when you least expect it. Anytime you're in a HOGE situation you are susceptible, chaps.
We tend to think of helicopter rotors as being absolutely and repeatedly predictable in their behaviour, much like the plank of an airplane is. However, when it comes to what is going on within a rotor system in flight, things get less precise. Ray Prouty has published charts in Rotor&Wing Magazine which show the data points to be all over the place, with a line more or less arbitrarily drawn through them at an "average" to give us a baseline for predicting performance. But what this tells us is that there are many variables - perhaps an infinite amount - which affect the way a particular rotor (or indeed, individual blade) will perform at any given time.
When it comes to VRS or SWP, we tend to focus solely on rate-of-descent of the aircraft, as if it were independant of other factors. Well, the air moves too, eh what? And sometimes that air moves vertically, not just horizontally. Suppose a helicopter developed a small ROD just as it encountered a slight updraft. The net result would be an increase in effective ROD, which might or might not be noticeable unless we were studiously watching the VSI, a gauge not known to be in our instrument scan during hovering ops (mine wags up and down considerably in a OGE hover). Therefore, I personally believe that making declarative statements like: "A UH-60 needs a ROD of 2500 feet before it can ever get into VRS" is the height of irresponsibility. Bollocks! I propose that in addition to the "hard" numbers that we know will aggravate or excite VRS, there are other factors which are quite nebulous that figure into the equation.
Did that UH-60 crew on Mt. Hood get into VRS? Who can know for sure? Even if the charts said (which we usually translate into "prove") that the aircraft should have been capable of the job, perhaps the rotor was closer to the "edge" than we know. Perhaps the rotor's reserve of lift at that HOGE altitude was borderline, and perhaps they experienced just enough of an updraft [in the mountains? come on, mate!] to cause them to fall off the bubble and into incipient (maybe not full-blown) VRS.
Then again, it appeared that they were hovering nose-in toward the slope. Is this true? Maybe I didn't see the whole accident sequence. Why not turn ninety-degrees to the slope? Would that not have given them better visual cues? And would that not have given them an easier "out" of they needed to lower the collective and bail from the situation for any reason? I wasn't there, but I sure wouldn't want to try to hold a stationary hover with a wall of white filling my windscreen.
Contrary to popular belief, gradual engine failures in a twin-engine helicopter do not cause a similar yaw-snap to the right (Yank birds) as they do in a single. As one engine falls off, the other goes to full power or beyond to take up the slack. We do not see any evidence of a catastrophic engine failure from the video (smoke, flame, etc), so I would move away from that line of thinking. Although the extreme coning of the blades does indicate (to me, at least) a reduction in MR rpm - or perhaps a over-application of collective pitch. Whatever DID happen, I believe it to be aerodynamic in nature and not mechanical - we'll soon find out.
Finally, Nick surmises that the Canadian Sea King at that airshow did not get into a setting-with-power situation, but rather a settling-with-too-little-power, owing exlusively to the right-yaw of the nose as the ship came down. Well, old boy, you're entitled to your opinion. I propose that it was indeed SWP (VRS), judging by the incredible rate-of-descent that built up so quickly, and the extreme coning of the blades as the poor pilot yanked everything he had to stop it, to no avail. The right-hand yaw was simply either his failure to fully mash the left pedal all the way (focused as he was on other, more pressing things in that brief second of time he had between "stable hover" and "Crikey, that hurt!"), or the inability of the tail rotor to hold the nose straight with the collective lever on the up-stop.
It is a myth to think that VRS is "hard" to get into, or that it must be excited deliberately. Anyone who would make such foolish statements has obviously never gotten into it inadvertently in the real world. Those of us who have know that all the "hard" numbers in the world don't mean a thing when the bottom falls out when you least expect it. Anytime you're in a HOGE situation you are susceptible, chaps.
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to Flare Dammit:
Your disbelief is earned of experience, and there is nothing wrong with behaving at all times OGE as if the sky was going to fall at any minute, but that still does not mean it will. It also doesn't mean that you won't find trouble, because you are looking in the wrong place for it if you believe VRS is common. It is power deficit that is common, and that is what I contend we should stamp out. For you to blanket all effects as one, ignore the actual physics and press on can seem quite safe, but it is opening yourself up to problems because you really don't know what causes the problem, or how to prevent it. I suggest, Flare Dammit, that you open up a bit.
I should note for you that purely vertical rejected takeoffs in twins from heights up to 100 feet are routine, and the descents can result in 1000 feet per minute drops without VRS. I have done at least 1000 such maneuvers.
The fact that you can conjecture a reasonable down draft causing VRS, or that that wildly spinning Sea King doing so because the pilot was too busy are examples of such compartmentized thinking. As far as real VRS data are concerned, I will post some on my brand new web site, if you'll look at it!
Vertical maneuvers are easy, and safe when there is enough regular ordinary available power. One does not need magical rotor VRS bugaboos to get in trouble if there is not sufficient power reserve in the OGE hover.
The US Army specifies that the Hawk must have 500 feet per minute climb rate OGE at 4000 feet and 32 degrees C while at full mission gross weight. That helicopter can be maneuvered vertically with impunity, down to rates of descent as I described.
My suggestion is to try the dreaded maneuver in question at safe altitude, and see. Use a light aircraft with gobs of extra power. Begin to descend carefully, in stages, about 250 fpm at a jump and see what indicated rates of descent you get, and how the trimmed power changes. If things get flakey, note the ROD and nose out of the maneuver, then post the results here.
As for the Bollocks, tried them once and didn't like them! I started off "slightly off base" and ended up "foolish" but that is par for this course, I'm afraid!
Your disbelief is earned of experience, and there is nothing wrong with behaving at all times OGE as if the sky was going to fall at any minute, but that still does not mean it will. It also doesn't mean that you won't find trouble, because you are looking in the wrong place for it if you believe VRS is common. It is power deficit that is common, and that is what I contend we should stamp out. For you to blanket all effects as one, ignore the actual physics and press on can seem quite safe, but it is opening yourself up to problems because you really don't know what causes the problem, or how to prevent it. I suggest, Flare Dammit, that you open up a bit.
I should note for you that purely vertical rejected takeoffs in twins from heights up to 100 feet are routine, and the descents can result in 1000 feet per minute drops without VRS. I have done at least 1000 such maneuvers.
The fact that you can conjecture a reasonable down draft causing VRS, or that that wildly spinning Sea King doing so because the pilot was too busy are examples of such compartmentized thinking. As far as real VRS data are concerned, I will post some on my brand new web site, if you'll look at it!
Vertical maneuvers are easy, and safe when there is enough regular ordinary available power. One does not need magical rotor VRS bugaboos to get in trouble if there is not sufficient power reserve in the OGE hover.
The US Army specifies that the Hawk must have 500 feet per minute climb rate OGE at 4000 feet and 32 degrees C while at full mission gross weight. That helicopter can be maneuvered vertically with impunity, down to rates of descent as I described.
My suggestion is to try the dreaded maneuver in question at safe altitude, and see. Use a light aircraft with gobs of extra power. Begin to descend carefully, in stages, about 250 fpm at a jump and see what indicated rates of descent you get, and how the trimmed power changes. If things get flakey, note the ROD and nose out of the maneuver, then post the results here.
As for the Bollocks, tried them once and didn't like them! I started off "slightly off base" and ended up "foolish" but that is par for this course, I'm afraid!
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Nick, old boy,
Such thin skin for a Yank! I do believe you were only slightly off-base. The "foolish" comment was directed at the other person who initially postulated that a UH-60 needs a vertical descent rate of 2500 fpm to get into VRS. What rot!
And speaking of rot, check out the video of your "wildly spinning" Sea King. It did no such thing, Nick, and I'm quite surprised at your efforts at hyperbole or perhaps faulty memory. The bird settled straight down with only a slight right yaw moment set up. It hit the ground, if you recall, at about ninety degrees to the right of its initial heading.
And I did not speculate that a *downdraft* causes or caused VRS, what I said was that an UPDRAFT could cause it! Below ETL, an updraft does not have the same effect as it would when the aircraft is above ETL.
It's lovely that the UH-60 has such good vertical maneuverability at a hover even OEI. You must consider it SuperCopter! But what you're assuming - where you go wrong, in fact - is that the performance of the rotor is as predictable as the performance of the engines. Silly boy.
And as for your suggestion to go out and try to excite VRS, my response would be the same as it's always been: I have done it. But what might excite it on one day might not on another. There are too many variables. It's not like an airplane wing, where we know that it will always stall at predictable speeds.
That is my only point.
Such thin skin for a Yank! I do believe you were only slightly off-base. The "foolish" comment was directed at the other person who initially postulated that a UH-60 needs a vertical descent rate of 2500 fpm to get into VRS. What rot!
And speaking of rot, check out the video of your "wildly spinning" Sea King. It did no such thing, Nick, and I'm quite surprised at your efforts at hyperbole or perhaps faulty memory. The bird settled straight down with only a slight right yaw moment set up. It hit the ground, if you recall, at about ninety degrees to the right of its initial heading.
And I did not speculate that a *downdraft* causes or caused VRS, what I said was that an UPDRAFT could cause it! Below ETL, an updraft does not have the same effect as it would when the aircraft is above ETL.
It's lovely that the UH-60 has such good vertical maneuverability at a hover even OEI. You must consider it SuperCopter! But what you're assuming - where you go wrong, in fact - is that the performance of the rotor is as predictable as the performance of the engines. Silly boy.
And as for your suggestion to go out and try to excite VRS, my response would be the same as it's always been: I have done it. But what might excite it on one day might not on another. There are too many variables. It's not like an airplane wing, where we know that it will always stall at predictable speeds.
That is my only point.
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I have never flown a blackhawk, or a bell, for that matter. But I do have a LOT of time in military helos in europe and an awful lot of that has been in the mountains. I don't know if the unfortunate crew suffered an engine failure/rundown, or whether they were hit by a dowdraft (Cold slopes make katabatic winds - I've seen them descend at 40 kts). What I do know, is that as soon as I watched the aircraft approach the slope (still under (apparently)full control), I wondered aloud what the pilot thought he was doing. Where was his escape route? He needed to be higher than he was from the survivors (His winch was long enough - I've checked), and his heading needed to be at least 30 deg left of where it was. Yes, I realise that I wasn't there, and that I couldn't possibly know all this for sure, but it was obvious to me. When I went to work (military sqn) one of my fellow drivers said 'Did ya see that blackhawk on the news...was that guy begging to crash or what'. Callous and unfeeling I know, but the sentiment is not lost on me, or others I suspect. I sincerely hope they are all ok, it looked at least like the fuselage stayed together, so fingers crossed.
I expect this will generate some negative comments, but if anyone can learn from this, then good. Mountains Bite, so watch out.
I expect this will generate some negative comments, but if anyone can learn from this, then good. Mountains Bite, so watch out.