Edson AB crash, VRS?
@Robbie: I only gave you the history of LTE, not an argument against or for anything. If you have lazy feet in almost anything that flies, except probably for the original Ercoupe, you will get in trouble. Even with a headwind.
Can I refer you to Nick Lappos amazing thread about Helicopter Urban Myth? .
Can I refer you to Nick Lappos amazing thread about Helicopter Urban Myth? .
The following 2 users liked this post by [email protected]:
Originally Posted by [email protected]
That one is definitely SWP and not VRS
Originally Posted by [email protected]
That one is definitely SWP and not VRS
i am sure a report will be forthcoming.
”If I’m getting smaller it means I’m leaving.”
SWP can lead to VRS but the fact they survived the impact would lead me to believe the RoD was not that of fully developed VRS.
I m with Crab here, having experienced full vortex ring once ( a real brown trouser moment ) what you see from inside is nothing like I saw. The ground comes up at an alarming rate, what you see is a slightly hire than normal rate of descent with not enough power to stop, basically crap energy mangement !
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Originally Posted by [email protected]
I'd be interested in your reasons for disagreeing albatross - for me the low Nr horn is a significant indicator of overpitching and asking for a collective pitch position that the engine can't provide enough power for.
SWP can lead to VRS but the fact they survived the impact would lead me to believe the RoD was not that of fully developed VRS.
SWP can lead to VRS but the fact they survived the impact would lead me to believe the RoD was not that of fully developed VRS.
Also overpitching the collective to a position in your armpit is often a reaction to sudden, low level, VRS especially if the planet is fast approaching. This massive over-torque will lead to the low rotor horn and an accelerated rate of descent.
albatross - I believe we are talking at cross-purposes here.
My comments about SWP are not to do with the Edson crash at all - that was VRS in my opinion.
The two other 350 links, one to a cockpit video and the other to the N6094s accident report are both SWP I believe but may well be separate accidents.
My comments about SWP are not to do with the Edson crash at all - that was VRS in my opinion.
The two other 350 links, one to a cockpit video and the other to the N6094s accident report are both SWP I believe but may well be separate accidents.
Thanks 212man and Nubian for looking for reports of the accident seen in the video linked in my post #78. The NTSB report found by Nubian certainly fits the bill as the portion of registration seen in video is consistent with N6094S.
This case doesn't turn up in ASN database no matter how I try to search for it. I also couldn't find details of that helicopter (c/n 2722) on helis.com database. Even though it occurred on 24 Aug 1999, the video wasn't posted to YouTube until 2012. NTSB report indicates: "...the probable cause(s) of this accident to be: The pilot's initial misjudgment of the wind speed and his decision to continue a downwind landing approach when his speed and altitude profile was inadequate, and his failure to maintain main rotor rpm, resulting in settling with power and a hard landing."
I haven't followed the distinctions being made in the more recent comments here. From what I had picked up from earlier discussions including on other threads, I thought SWP and VRS are just different expressions for the same situation? Sorry in advance if I am opening an old can of worms.
This case doesn't turn up in ASN database no matter how I try to search for it. I also couldn't find details of that helicopter (c/n 2722) on helis.com database. Even though it occurred on 24 Aug 1999, the video wasn't posted to YouTube until 2012. NTSB report indicates: "...the probable cause(s) of this accident to be: The pilot's initial misjudgment of the wind speed and his decision to continue a downwind landing approach when his speed and altitude profile was inadequate, and his failure to maintain main rotor rpm, resulting in settling with power and a hard landing."
I haven't followed the distinctions being made in the more recent comments here. From what I had picked up from earlier discussions including on other threads, I thought SWP and VRS are just different expressions for the same situation? Sorry in advance if I am opening an old can of worms.
I thought SWP and VRS are just different expressions for the same situation? Sorry in advance if I am opening an old can of worms.
Settling with Power effectively means running out of power because the aircraft doesn't have the performance to do what you are asking of it - usually in the low speed environment and associated with the loss of ETL ie trying to establish an OGE hover or making a steep approach into a landing site without the performance to do so. Low Nr is a classic symptom because the pilot keeps pulling the collective up as he/she realises they can't stop the descent.
VRS is all aerodynamic and involves recirculation at the blade tips and stalling at the blade roots leaving only the middle part of the blade producing useful lift - it is also encountered in the low speed environment which often leads to the confusion. You have caught up with your own downwash and the rotors really don't like it.
In full VRS the outcome is usually fatal due to the high rate of descent - in SWP often the aircraft gets bent with minimal or minor injuries because the rate of descent is lower.
In both conditions, continuing to raise the collective just makes things worse - in SWP you just decay the Nr further - in VRS you worsen the recirculation and the root stall.
Both conditions are easily avoided and encountering either is usually the result of poor performance planning, poor wind awareness or simply poor piloting.
Originally Posted by [email protected]
albatross - I believe we are talking at cross-purposes here.
My comments about SWP are not to do with the Edson crash at all - that was VRS in my opinion.
The two other 350 links, one to a cockpit video and the other to the N6094s accident report are both SWP I believe but may well be separate accidents.
My comments about SWP are not to do with the Edson crash at all - that was VRS in my opinion.
The two other 350 links, one to a cockpit video and the other to the N6094s accident report are both SWP I believe but may well be separate accidents.
My failure to take into account “Thread Drift”.
Silly me!
Cheers
Albatross.
PS When I typed Crab..Spell Check changed it to Crap. Funny! Glad I caught that error.
Thanks 212man and Nubian for looking for reports of the accident seen in the video linked in my post #78. The NTSB report found by Nubian certainly fits the bill as the portion of registration seen in video is consistent with N6094S.
This case doesn't turn up in ASN database no matter how I try to search for it. I also couldn't find details of that helicopter (c/n 2722) on helis.com database. Even though it occurred on 24 Aug 1999, the video wasn't posted to YouTube until 2012. NTSB report indicates: "...the probable cause(s) of this accident to be: The pilot's initial misjudgment of the wind speed and his decision to continue a downwind landing approach when his speed and altitude profile was inadequate, and his failure to maintain main rotor rpm, resulting in settling with power and a hard landing."
I haven't followed the distinctions being made in the more recent comments here. From what I had picked up from earlier discussions including on other threads, I thought SWP and VRS are just different expressions for the same situation? Sorry in advance if I am opening an old can of worms.
This case doesn't turn up in ASN database no matter how I try to search for it. I also couldn't find details of that helicopter (c/n 2722) on helis.com database. Even though it occurred on 24 Aug 1999, the video wasn't posted to YouTube until 2012. NTSB report indicates: "...the probable cause(s) of this accident to be: The pilot's initial misjudgment of the wind speed and his decision to continue a downwind landing approach when his speed and altitude profile was inadequate, and his failure to maintain main rotor rpm, resulting in settling with power and a hard landing."
I haven't followed the distinctions being made in the more recent comments here. From what I had picked up from earlier discussions including on other threads, I thought SWP and VRS are just different expressions for the same situation? Sorry in advance if I am opening an old can of worms.
Report_LAX99LA282_47145_5_20_2023 9_41_50 AM.pdf
The ship must not have been damaged too badly, because the ROTORSPOT database still shows it as still owned by Sunshine, still active and not withdrawn from use or written off or whatever.
It's interesting (to me, at least) that the PIC reported a total time of 11,871 hours, with 8,347 in that actual make/model! So much for experience being helpful in avoiding accidents, eh? Hah. I've said it before and I'll say it again: Helicopters are VERY EASY to crash. And having a ****load of time gives you no immunity or even protection from doing so, despite what you might egotistically believe about yourself and your mad skillz as a pilot. And it gets back to what I've said about landing/taking-off into the wind: After we get our PPL and get out in the world, we get to thinking that it's not *that* important anymore...that we can handle a downwind landing...easy-peasy!...done it a million times....OOPS! Another one bites the dust, hey-hey!
Originally Posted by [email protected]
No, that is what was taught and promulgated in some regions - particularly the US but they all seem to understand the difference now.
Settling with Power effectively means running out of power because the aircraft doesn't have the performance to do what you are asking of it - usually in the low speed environment and associated with the loss of ETL ie trying to establish an OGE hover or making a steep approach into a landing site without the performance to do so. Low Nr is a classic symptom because the pilot keeps pulling the collective up as he/she realises they can't stop the descent.
Settling with Power effectively means running out of power because the aircraft doesn't have the performance to do what you are asking of it - usually in the low speed environment and associated with the loss of ETL ie trying to establish an OGE hover or making a steep approach into a landing site without the performance to do so. Low Nr is a classic symptom because the pilot keeps pulling the collective up as he/she realises they can't stop the descent.
If you just said, "overpitching", or "he's just too heavy to go that slow", there wouldn't be any confusion.
Yeah, I admit I've been confused as to what you actually mean everytime you guys use this antiquated term. In fact, I actually thought you were referring to the Canadian definition, where you just wait too long to start slowing down.
If you just said, "overpitching", or "he's just too heavy to go that slow", there wouldn't be any confusion.
If you just said, "overpitching", or "he's just too heavy to go that slow", there wouldn't be any confusion.
I’ ll will stick to my antiquated terminology for 2 completely separate conditions of flight.
Vortex Ring State and Settling With Power were always referred to and taught separately when i got my licence here in Canada.
Yes, one can lead to the other but they are not the same.
Sorry if you are confused.
Overpitching? You can be overpitching and climbing like a homesick angel. Of course this can lead to Over torque, over temp, N1 topping, Loss of RPM, loss of tail rotor authority ect. ect. including meetings with the Chief Pilot where no tea and cookies will be offered and severely PO’d maintenance staff
I’ ll will stick to my antiquated terminology for 2 completely separate conditions of flight.
Vortex Ring State and Settling With Power were always referred to and taught separately when i got my licence here in Canada.
Yes, one can lead to the other but they are not the same.
Sorry if you are confused.
I’ ll will stick to my antiquated terminology for 2 completely separate conditions of flight.
Vortex Ring State and Settling With Power were always referred to and taught separately when i got my licence here in Canada.
Yes, one can lead to the other but they are not the same.
Sorry if you are confused.
Anyway, I'm only confused because the term has at least three different definitions depending on where the pilot using it was trained.
How about this, if you're so dedicated to using this term, then could you at least specify which country you are from, so there's no confusion (and/or resulting arguments) over what you actually mean?
The outcome almost always is.
If you get into one of those situations where the helicopter goes down faster as you pull up more on the pole, then you'll do one of two things: 1) If you're high enough, recover using Vuichard or some other technique and then come on PPRUNE and swear to us that you got SWP or VRS and tell us why; or 2) Crash. If you select Option #2, it won't matter whether it was SWP or VRS.
If you get into one of those situations where the helicopter goes down faster as you pull up more on the pole, then you'll do one of two things: 1) If you're high enough, recover using Vuichard or some other technique and then come on PPRUNE and swear to us that you got SWP or VRS and tell us why; or 2) Crash. If you select Option #2, it won't matter whether it was SWP or VRS.
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crab and albatross: thanks for the SWP vs VRS distinction. Out of interest, I looked through BASI Aviation Safety Digest issues to see what terminology they had used. ASD 68 from 1970 has an article discussing a Bell 47G-3B-1 crash in New Zealand attributed to Vortex Ring State or Power Settling as if the two are interchangeable terms:https://www.atsb.gov.au/sites/defaul..._68_may_70.pdf
ASD 51 of 1967 discusses accident with a Bell 47-G2 in Western Australia, but in that case it is attributed simply to overpitching:
https://www.atsb.gov.au/sites/defaul..._51_jul_67.pdf
FH1100 Pilot: Thanks for looking up the fate of N6094S. It would have been an expensive repair. The pdf document link you provided didn't work for me though.
ASD 51 of 1967 discusses accident with a Bell 47-G2 in Western Australia, but in that case it is attributed simply to overpitching:
https://www.atsb.gov.au/sites/defaul..._51_jul_67.pdf
FH1100 Pilot: Thanks for looking up the fate of N6094S. It would have been an expensive repair. The pdf document link you provided didn't work for me though.
Sorry it's a long post, but for the benefit of Robbiee and his mates, here is a gathering of Nick Lappos's Urban Myths, plus some notes from the late Shawn Coyle, both these gents being highly qualified test pilots:Helicopter Urban Myths
These Urban Myths pervade our understanding of helicopters and how they operate. Each is fundamentally incorrect, but most are generally held as gospel, because training, lore and reference documents have repeated them long enough that they are simply accepted.
1) Vortex Ring State (VRS) can happen at as little as 300 foot per minute descent, it does not have to be a higher descent rate
2) VRS is more likely at high altitude and high gross weight
3) Hovering with the nose off wind consumes much more power
4) Blade stall is always preceded by vibration
5) Winds affect the power we require when we are in forward flight
6) Downwind takeoffs are absolutely forbidden
7) The Height Velocity curve is a precise guide to the engine failure danger zone
8) Engine failure is the most common accident cause, so full CAT A is the most cost effective safety enhancement we can incorporate into new helicopters.
9) The legal definition of VFR is sufficient to assure flight control and safety using outside references
10) "They" sometimes hide things from us. We should not trust them, the only reliable information we can trust is our own wits.
11) The helicopter is perched on a ball of high pressure air when close to the ground, and "falls off" this ground cushion when it moves forward.
12) Phase lag is cause by gyroscopic precession, and is always exactly 90 degrees
13) LTE is when you run out of power pedal and can be experienced by any single rotor helicopter.
14) NVG are dangerous and should only be used by gifted military pilots.
15) You have to first learn to fly fixed wing before you take helicopter training
16) Torque limits, overspeed limits, temperature limits, hours and airframe limits have huge safety factors built into them by the engineers, so it is OK to bust them every now and then.
Davy07,
The books say so, but they are quite wrong. VRS can only be induced by descending at least as fast as 75% of the downwash velocity of the helicopter and at a forward speed of not more than about 8 knots. For a Robbie this is at least -750fpm rod. For an S76 it is about -1500 fpm.
Why is 300 taught? Because one can start a VRS event by entering a hover with too little power, slip into an overpitching event and in short order, enter VRS. In a helo with scads of OGE hover performance, the 300 fpm is truly a myth, in one without, it is misleading as a VRS cue, but good word as an overpitching warning.
Manfrom Uncle, what you are correctly saying is that going faster uses more power. The wind has nothing to do with it. 80Kts airspeed uses the same power both up and down wind.
For the record, there is no pressure gradient below the rotor, in fact, that lame conventional explanation doesn't even hold water for a millisecond when you realize that ground effect only works on the induced power. How does a "pressure bubble" single out induced power as the only recipient of its wonder?
In fact, pressure bubblers have some difficulty explaining how ground effect works for an airplane at 250 knots, when the "pressure bubble" is about 1/4 mile behind the wing.
Maybe you should get a hold of Shawn Coyle's "Cyclic and Collective":
http://i159.photobucket.com/albums/t...g?t=1195957331
Dear Shawn, Mr. Publisher, dear mods: if this should violate any copyrights and isn't appreciated to be posted, please remove it immediately or have me remove it - I just can't explain it better )
delta 3,
OK, your work is truly great, and I can see the reduction on angle of attack and power. It is truly a desktop universe, and I mean that in a good way.
The problem is that without careful explanation, one could actually think you show how the "pressure bubble" under the rotor is what ground effect is, and that would be untrue. Ground effect is when the ground plane flattens out the flow, causing the rotor to behave as if the blades were much longer. As a result, the tip losses for the blades are sharply reduced, causing a sharp reduction in the induced power requirements. The reason why ground effect does not show very well at 100 knots is that induced power is very low there, but ground effect is still present at 100 knots.
For an airplane, the case is easier to picture. As the wing comes down into ground effect, the tip wash reduces sharply, and the wing behaves as if it were infinitely long. This requires less angle of attack (which had caused a large drag rise prior to entering the ground effect). The induced drag wastes power, so ground effect shows itself as a reduction in angle of attack and a reduction in the power required.
Ground effect is not caused by a pressure rise, nor is the advantage to the aircraft due to any pressure that pushes the aircraft up (or any other such pressure bubble nonsense.)
Ground effect explained
Misinformed Instructors who propel the pressure bubble myth should be asked to do a quick Yahoo search using "induced drag ground effect" so that they can learn how to tell their students the truth, and not convenient myths. We would be all the better for it. Pressure bubblers, please note:
Induced drag explained:
http://galileo.phys.virginia.edu/cla...ero/node5.html
http://www.grc.nasa.gov/WWW/K-12/airplane/induced.html
Ground effect explained:
http://www.faatest.com/books/FLT/Cha...oundEffect.htm
http://www.pilotfriend.com/training/...g/aft_perf.htm
"Many pilots think that ground effect is caused by air being compressed between the wing and the ground. This is not so. Ground effect is caused by the reduction of induced drag when an airplane is flown at slow speed very near the surface."
The quiz for today, for the bright student:
Define induced drag
define aspect ratio
explain how aspect ratio affects induced drag
explain how proximity to the ground affects aspect ratio
I love this block, we can go round and round it forever!
Some points:
1) I never said that ground effect was a myth, I said that the "pressure bubble" was a myth! Ground effect is very real, and amounts to a typical 15% power savings for most helicopters (where the rotor cannot get closer to the ground that about .3 radius.
2) Those who love the pressure bubble theory are welcome to it, believing myths is not against the law! However, ask yourself why ground effect disappears when you hover over long grass - and don't tell me that grass absorbs pressure! The grass slows the outflow, which is the mechanism that changes the blade angle of attack and reduces the hover power.
3) The pressure bubble theory cannot explain why ground effect only reduces the induced drag of the blades, if that theory is correct, more velocity makes more pressure, and thus saves more power. If the air velocity gets banged against the ground, and pressure builds, then a higher velocity should make even more pressure, and more ground effect, right? No, wrong! Maximum ground effect is about +15% power, regardless of how fast the downwash velocity is. In fact, ground effect is no different for highly loaded rotors, with faster downwash, than it is for low disk loading rotors with gentle downwash.
4) If pressure bubbles push the aircraft up, then high speed airplanes, with the wake hitting the ground hundreds of meters behind the aircraft, should see no ground effect. But they do! That is because the effect is on the wing, where the angle of the flow around the wing is changed by the presence of the ground.
5) Ground effect makes the wings or blades act as if they are longer, and this cuts the tip losses that make the induced drag. They have nothing to do with the pressure under the blade or wing, they have everything to do with the reduction in outflow, and the reduction in the tip vortex pattern due to that outflow.
With my respect for Shawn unabated, I must respectfully disagree with him. There is a meaningful difference between the two cases, light vs heavy. But first we must clarify what we mean by Vortex Ring State (VRS) - and what is meant by a different problem called "Settling with Power (SWP)" or "over pitching" or "insufficient power to Hover OGE"
Most accidents where a hovering helicopter falls and crashes and VRS is blamed are actually cases of SWP or over pitching, where the hover performance is marginal, and insufficient reserve power (power margin) is available to allow moderate climbs and descents while OGE. The aircraft "falls through" the hover, hits hard (usually with just a bent helicopter and bruised ego) and then someone says "It was VRS." Sometimes the mistaken person is an official accident investigator!
In a helicopter at high MGW, with only slight or no margin between the power needed to HOGE and the power available from the engines, "over pitching" is more likely than in a lightly loaded helicopter where lots of power above hover power is available. When lightly loaded, there is much extra power available above the hover power, so the lightly loaded helo is much less likely to experience "over pitching" and thus the lightly loaded helo is much less likely to be mistakenly labeled as a VRS accident.
Now the truth: Since true VRS involves the descent of the helicopter into its own downwash, and since in a light helicopter the downwash velocity is quite a bit less than in that same helicopter when heavy, a lightly loaded helicopter needs much less rate of descent to experience true VRS.
Thus, heavy helicopters require more descent rate to get true VRS, and so are less likely to enter that state, but heavily loaded helicopters have more over pitching power control accidents that are too often labeled "VRS", so the mistaken pilot lore says heavy helicopters are more likely to experience VRS.
For the record, no helicopter can experience true VRS unless it is descending nearly vertically at about 800 to 1000 feet per minute.
Also for the record, most helicopters can experience SWP or overpitching at rates of descent near zero if they have little hover power margin.
Also, heavily loaded helicopters have less propensity to enter VRS because they need more vertical descent rate than lightly loaded helicopters, which need less descent rate to get into VRS.
These Urban Myths pervade our understanding of helicopters and how they operate. Each is fundamentally incorrect, but most are generally held as gospel, because training, lore and reference documents have repeated them long enough that they are simply accepted.
1) Vortex Ring State (VRS) can happen at as little as 300 foot per minute descent, it does not have to be a higher descent rate
2) VRS is more likely at high altitude and high gross weight
3) Hovering with the nose off wind consumes much more power
4) Blade stall is always preceded by vibration
5) Winds affect the power we require when we are in forward flight
6) Downwind takeoffs are absolutely forbidden
7) The Height Velocity curve is a precise guide to the engine failure danger zone
8) Engine failure is the most common accident cause, so full CAT A is the most cost effective safety enhancement we can incorporate into new helicopters.
9) The legal definition of VFR is sufficient to assure flight control and safety using outside references
10) "They" sometimes hide things from us. We should not trust them, the only reliable information we can trust is our own wits.
11) The helicopter is perched on a ball of high pressure air when close to the ground, and "falls off" this ground cushion when it moves forward.
12) Phase lag is cause by gyroscopic precession, and is always exactly 90 degrees
13) LTE is when you run out of power pedal and can be experienced by any single rotor helicopter.
14) NVG are dangerous and should only be used by gifted military pilots.
15) You have to first learn to fly fixed wing before you take helicopter training
16) Torque limits, overspeed limits, temperature limits, hours and airframe limits have huge safety factors built into them by the engineers, so it is OK to bust them every now and then.
Davy07,
The books say so, but they are quite wrong. VRS can only be induced by descending at least as fast as 75% of the downwash velocity of the helicopter and at a forward speed of not more than about 8 knots. For a Robbie this is at least -750fpm rod. For an S76 it is about -1500 fpm.
Why is 300 taught? Because one can start a VRS event by entering a hover with too little power, slip into an overpitching event and in short order, enter VRS. In a helo with scads of OGE hover performance, the 300 fpm is truly a myth, in one without, it is misleading as a VRS cue, but good word as an overpitching warning.
Manfrom Uncle, what you are correctly saying is that going faster uses more power. The wind has nothing to do with it. 80Kts airspeed uses the same power both up and down wind.
For the record, there is no pressure gradient below the rotor, in fact, that lame conventional explanation doesn't even hold water for a millisecond when you realize that ground effect only works on the induced power. How does a "pressure bubble" single out induced power as the only recipient of its wonder?
In fact, pressure bubblers have some difficulty explaining how ground effect works for an airplane at 250 knots, when the "pressure bubble" is about 1/4 mile behind the wing.
Maybe you should get a hold of Shawn Coyle's "Cyclic and Collective":
http://i159.photobucket.com/albums/t...g?t=1195957331
Dear Shawn, Mr. Publisher, dear mods: if this should violate any copyrights and isn't appreciated to be posted, please remove it immediately or have me remove it - I just can't explain it better )
delta 3,
OK, your work is truly great, and I can see the reduction on angle of attack and power. It is truly a desktop universe, and I mean that in a good way.
The problem is that without careful explanation, one could actually think you show how the "pressure bubble" under the rotor is what ground effect is, and that would be untrue. Ground effect is when the ground plane flattens out the flow, causing the rotor to behave as if the blades were much longer. As a result, the tip losses for the blades are sharply reduced, causing a sharp reduction in the induced power requirements. The reason why ground effect does not show very well at 100 knots is that induced power is very low there, but ground effect is still present at 100 knots.
For an airplane, the case is easier to picture. As the wing comes down into ground effect, the tip wash reduces sharply, and the wing behaves as if it were infinitely long. This requires less angle of attack (which had caused a large drag rise prior to entering the ground effect). The induced drag wastes power, so ground effect shows itself as a reduction in angle of attack and a reduction in the power required.
Ground effect is not caused by a pressure rise, nor is the advantage to the aircraft due to any pressure that pushes the aircraft up (or any other such pressure bubble nonsense.)
Ground effect explained
Misinformed Instructors who propel the pressure bubble myth should be asked to do a quick Yahoo search using "induced drag ground effect" so that they can learn how to tell their students the truth, and not convenient myths. We would be all the better for it. Pressure bubblers, please note:
Induced drag explained:
http://galileo.phys.virginia.edu/cla...ero/node5.html
http://www.grc.nasa.gov/WWW/K-12/airplane/induced.html
Ground effect explained:
http://www.faatest.com/books/FLT/Cha...oundEffect.htm
http://www.pilotfriend.com/training/...g/aft_perf.htm
"Many pilots think that ground effect is caused by air being compressed between the wing and the ground. This is not so. Ground effect is caused by the reduction of induced drag when an airplane is flown at slow speed very near the surface."
The quiz for today, for the bright student:
Define induced drag
define aspect ratio
explain how aspect ratio affects induced drag
explain how proximity to the ground affects aspect ratio
I love this block, we can go round and round it forever!
Some points:
1) I never said that ground effect was a myth, I said that the "pressure bubble" was a myth! Ground effect is very real, and amounts to a typical 15% power savings for most helicopters (where the rotor cannot get closer to the ground that about .3 radius.
2) Those who love the pressure bubble theory are welcome to it, believing myths is not against the law! However, ask yourself why ground effect disappears when you hover over long grass - and don't tell me that grass absorbs pressure! The grass slows the outflow, which is the mechanism that changes the blade angle of attack and reduces the hover power.
3) The pressure bubble theory cannot explain why ground effect only reduces the induced drag of the blades, if that theory is correct, more velocity makes more pressure, and thus saves more power. If the air velocity gets banged against the ground, and pressure builds, then a higher velocity should make even more pressure, and more ground effect, right? No, wrong! Maximum ground effect is about +15% power, regardless of how fast the downwash velocity is. In fact, ground effect is no different for highly loaded rotors, with faster downwash, than it is for low disk loading rotors with gentle downwash.
4) If pressure bubbles push the aircraft up, then high speed airplanes, with the wake hitting the ground hundreds of meters behind the aircraft, should see no ground effect. But they do! That is because the effect is on the wing, where the angle of the flow around the wing is changed by the presence of the ground.
5) Ground effect makes the wings or blades act as if they are longer, and this cuts the tip losses that make the induced drag. They have nothing to do with the pressure under the blade or wing, they have everything to do with the reduction in outflow, and the reduction in the tip vortex pattern due to that outflow.
With my respect for Shawn unabated, I must respectfully disagree with him. There is a meaningful difference between the two cases, light vs heavy. But first we must clarify what we mean by Vortex Ring State (VRS) - and what is meant by a different problem called "Settling with Power (SWP)" or "over pitching" or "insufficient power to Hover OGE"
Most accidents where a hovering helicopter falls and crashes and VRS is blamed are actually cases of SWP or over pitching, where the hover performance is marginal, and insufficient reserve power (power margin) is available to allow moderate climbs and descents while OGE. The aircraft "falls through" the hover, hits hard (usually with just a bent helicopter and bruised ego) and then someone says "It was VRS." Sometimes the mistaken person is an official accident investigator!
In a helicopter at high MGW, with only slight or no margin between the power needed to HOGE and the power available from the engines, "over pitching" is more likely than in a lightly loaded helicopter where lots of power above hover power is available. When lightly loaded, there is much extra power available above the hover power, so the lightly loaded helo is much less likely to experience "over pitching" and thus the lightly loaded helo is much less likely to be mistakenly labeled as a VRS accident.
Now the truth: Since true VRS involves the descent of the helicopter into its own downwash, and since in a light helicopter the downwash velocity is quite a bit less than in that same helicopter when heavy, a lightly loaded helicopter needs much less rate of descent to experience true VRS.
Thus, heavy helicopters require more descent rate to get true VRS, and so are less likely to enter that state, but heavily loaded helicopters have more over pitching power control accidents that are too often labeled "VRS", so the mistaken pilot lore says heavy helicopters are more likely to experience VRS.
For the record, no helicopter can experience true VRS unless it is descending nearly vertically at about 800 to 1000 feet per minute.
Also for the record, most helicopters can experience SWP or overpitching at rates of descent near zero if they have little hover power margin.
Also, heavily loaded helicopters have less propensity to enter VRS because they need more vertical descent rate than lightly loaded helicopters, which need less descent rate to get into VRS.
The following 2 users liked this post by Ascend Charlie:
AC - most of those links don't work any more unfortunately.
I was lucky to find Nick Lappos and Shawn Coyle on these pages 25 years ago and have tried to pass on their wisdom where possible ever since.
I was lucky to find Nick Lappos and Shawn Coyle on these pages 25 years ago and have tried to pass on their wisdom where possible ever since.