Vortex Ring Autorotation
Sokol - while blade shapes have changed significantly, the disc loading and therefore the downwash has changed enormously, mostly due to the gas turbine engine with its far superior power to weight ratio.
You can either take a large area of air (big rotor disc) and accelerate it a small amount (uses less power) or have a smaller area of air (smaller rotor) and accelerate it a greater amount (uses more power) to achieve the same rotor thrust to balance the mass of the aircraft. Modern rotor discs can be smaller because the engines are more powerful - hence faster downwash speeds.
Light pistons such as the R22 still have relatively low disc loading, hence they are more vulnerable to VRS because their downwash speeds are slower and it is easier to catch the rotor wake up in a descent.
The biggest change in helicopter blades design is not the lack of taper but the change in aerofoil sections, moving away from symmetrical to cambered shapes, especially with composite blades where that section can be altered along the length of the blade. Additionally, the lift is varied along the blade using twist rather than taper so the root is at a higher pitch angle than the tip.
You can either take a large area of air (big rotor disc) and accelerate it a small amount (uses less power) or have a smaller area of air (smaller rotor) and accelerate it a greater amount (uses more power) to achieve the same rotor thrust to balance the mass of the aircraft. Modern rotor discs can be smaller because the engines are more powerful - hence faster downwash speeds.
Light pistons such as the R22 still have relatively low disc loading, hence they are more vulnerable to VRS because their downwash speeds are slower and it is easier to catch the rotor wake up in a descent.
The biggest change in helicopter blades design is not the lack of taper but the change in aerofoil sections, moving away from symmetrical to cambered shapes, especially with composite blades where that section can be altered along the length of the blade. Additionally, the lift is varied along the blade using twist rather than taper so the root is at a higher pitch angle than the tip.
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Originally Posted by ShyTorque
The Whirlwind, which I flew very early in my career, was reputedly able to enter VRS at a rate of descent not much over 300ft/min. The blades were made from a simple, hollow metal spar, with metal pockets bonded on to form the constant blade profile along its length.
Originally Posted by [email protected]
The biggest change in helicopter blades design is not the lack of taper but the change in aerofoil sections, moving away from symmetrical to cambered shapes, especially with composite blades where that section can be altered along the length of the blade. Additionally, the lift is varied along the blade using twist rather than taper so the root is at a higher pitch angle than the tip.
Plus: Composite(plastics) blades have a tendency to higher stiffness, so the AOA in the center of the disc is not the same as with an Steel/Wooden Blade.
Disc load: Same helicopter, MTOW and empty(pilot and fuel are abord...) which gets earlier into VR?
Greetings,
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https://www.youtube.com/watch?v=pAwxHGBA2_I
Unfortunately height is not always available!
The guy who gets thrown out of the forward personnel door is a good friend of mine. Spent a fair bit of time in plaster! The crewie, who jumps from the cargo door to pull the guys out, has a broken spine!
The cause was attributed to 'settling with power' over a hot concrete surface with light winds and strong thermals.
Not a nice place to be.
Doesn't help the debate but shows the unintended consequences!
Unfortunately height is not always available!
The guy who gets thrown out of the forward personnel door is a good friend of mine. Spent a fair bit of time in plaster! The crewie, who jumps from the cargo door to pull the guys out, has a broken spine!
The cause was attributed to 'settling with power' over a hot concrete surface with light winds and strong thermals.
Not a nice place to be.
Doesn't help the debate but shows the unintended consequences!
Avoid imitations
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Lower stiffness of the Blades, higher tendency to twist.
But what point are you trying to make by the above statement? You said that older designs of MR blades changed in profile along their length. I pointed out that that wasn't always the case.
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[...]But what point are you trying to make by the above statement?[...]
Note: The question was originally why the helicopters in the document get into VR this early. I answered this question as questioned.
Greetings,
If anyone's interested (probably not 
), this is what happened to me:
Puma HC1, 6000' at night for an incipient VRS demo. The only way my friendly QHI could get her in was by going backwards and obviously down, I watched the VSI like a hawk and it was as we passed >800' rod and 0kts on the asi that we properly entered. I think it caught him out judging by the swearing coming from the RHS and the immediate and instant reaction was the aircraft went from approx 10 degrees nose up to what felt like 90 degrees nose down, but must have been at least >45 degrees. Simultaneously the aircraft rolled 180 degrees and this was definite: the town that was on the rhs was now on my side. I honestly thought I was about to die and the recovery took a surprisingly long time. We actually came out of sorts at 1500', with a full balls width of slip and 30kts airspeed. I guess the facts are: we lost 4500' through the full situation, we pitched extreme nose down as we entered at 800' rod and we rolled 180 degrees.
I guess some of the exact detail may be lost to time but I wrote it all down afterwards and remember it clearly to this day. I've been told by several Puma pilots of old and bold that you can't get a Puma into full VRS, but I promise that is what happened. After my 'incident' all incipient VRS was done in the wonderful MSHATF.
Certainly don't want to ever get into that again
), this is what happened to me:Puma HC1, 6000' at night for an incipient VRS demo. The only way my friendly QHI could get her in was by going backwards and obviously down, I watched the VSI like a hawk and it was as we passed >800' rod and 0kts on the asi that we properly entered. I think it caught him out judging by the swearing coming from the RHS and the immediate and instant reaction was the aircraft went from approx 10 degrees nose up to what felt like 90 degrees nose down, but must have been at least >45 degrees. Simultaneously the aircraft rolled 180 degrees and this was definite: the town that was on the rhs was now on my side. I honestly thought I was about to die and the recovery took a surprisingly long time. We actually came out of sorts at 1500', with a full balls width of slip and 30kts airspeed. I guess the facts are: we lost 4500' through the full situation, we pitched extreme nose down as we entered at 800' rod and we rolled 180 degrees.
I guess some of the exact detail may be lost to time but I wrote it all down afterwards and remember it clearly to this day. I've been told by several Puma pilots of old and bold that you can't get a Puma into full VRS, but I promise that is what happened. After my 'incident' all incipient VRS was done in the wonderful MSHATF.
Certainly don't want to ever get into that again
Sokol
The empty one - rotor thrust must equal mass so the heavier helo has to accelerate the air down quicker using more AoA and giving a faster downwash speed - you point is?
Disc load: Same helicopter, MTOW and empty(pilot and fuel are abord...) which gets earlier into VR?
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SimonK, I'm not sure why your Puma QHI was trying to demo that, it certainly wasn't in the OCU syllabus!
Having said that, from personal experience of a similar nature, I can confirm that it certainly IS possible to get a Puma into FDVRS, even when you're not trying. I wasn't flying when the aircraft entered; but although I was in the LHS (but not acting as QHI), I did eventually "assist with diagnosis and recovery action" before we finally hit the ground with the VSI pegged hard on the bottom stop, in hostile territory too. We went from around 10,000 feet at max continuous power (job required a very high OGE hover) to ground rush very quickly indeed. It was twenty five years ago but I still recall it quite easily. Something I wouldn't wish to repeat in a hurry.
Having said that, from personal experience of a similar nature, I can confirm that it certainly IS possible to get a Puma into FDVRS, even when you're not trying. I wasn't flying when the aircraft entered; but although I was in the LHS (but not acting as QHI), I did eventually "assist with diagnosis and recovery action" before we finally hit the ground with the VSI pegged hard on the bottom stop, in hostile territory too. We went from around 10,000 feet at max continuous power (job required a very high OGE hover) to ground rush very quickly indeed. It was twenty five years ago but I still recall it quite easily. Something I wouldn't wish to repeat in a hurry.
Sokol - greetings
that wholly depends on what forces are acting upon it - another pointless question???
If you mean 'will a heavier aircraft take more power to arrest a descent than a light one?' then the answer is yes but that is't pertinent to the discussion about VRS - more pertinent to SWP.
Which helo (full or empty) decelerates faster in descent?
If you mean 'will a heavier aircraft take more power to arrest a descent than a light one?' then the answer is yes but that is't pertinent to the discussion about VRS - more pertinent to SWP.
Hi ShyTorque,
No it wasn't in the OCU syllabus but it was for the same thing you were doing! Think it was a Fooked up demo basically but there you go, did a few corkers myself when I was a QHI aswell....but not quite that bad
Cheers!
No it wasn't in the OCU syllabus but it was for the same thing you were doing! Think it was a Fooked up demo basically but there you go, did a few corkers myself when I was a QHI aswell....but not quite that bad
Cheers!
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The "old and bold ones" who tried to tell you it wasn't possible possibly never actually got a low enough IAS, just less than 40 kts when the ASI went to sleep.
Backwards and down trying to hover up there was always bad for the grey hair count; that's also what happened to us.
I used to aim to achieve the high hover from a gently climbing straight quickstop, far more predictable and quieter on the ground, too.
Backwards and down trying to hover up there was always bad for the grey hair count; that's also what happened to us.
I used to aim to achieve the high hover from a gently climbing straight quickstop, far more predictable and quieter on the ground, too.
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Quite right TOURIST
You wouldn't even need to do as much as 30degrees and 30kts, before you re-applied power and climbed away.
and you are right you can make vortex ring from autorotation, but it's a fairly extreme scenario. If you auto vertically at zero airspeed (ie zero wind day and vertical) and you just pull hard at the bottom there is a strong chance of just 'falling through' as you make a vortex rather than significantly arrest your descent. If you are just off zero speed at say 10kts airspeed its still a pretty hairy arrival anyway, but worth staying composed, unlikely to kill you if you stay flat. Depends on the helicopter. (rotor energy / power required gives useful 'hover time')
You wouldn't even need to do as much as 30degrees and 30kts, before you re-applied power and climbed away.
and you are right you can make vortex ring from autorotation, but it's a fairly extreme scenario. If you auto vertically at zero airspeed (ie zero wind day and vertical) and you just pull hard at the bottom there is a strong chance of just 'falling through' as you make a vortex rather than significantly arrest your descent. If you are just off zero speed at say 10kts airspeed its still a pretty hairy arrival anyway, but worth staying composed, unlikely to kill you if you stay flat. Depends on the helicopter. (rotor energy / power required gives useful 'hover time')
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There is something odd going on here:
ShyTorque: "from around 10,000 feet " "before we finally hit the ground with the VSI pegged hard on the bottom stop"
and
SimonK: "6000' at night for an incipient VRS demo." "we lost 4500' through the full situation, we pitched extreme nose down as we entered at 800' rod and we rolled 180 degrees."
and
Democritus (I think): '5000ft lost unable to get out of the condition' with a test pilot
It seems something is going on because these stories don't square with the many people who do fully developed VR demonstration regularly, without any big deal.
Is it possible that there are helicopters with these terrible characteristics?
If so we need to find out.
Can anyone shed some light on it?
ShyTorque: "from around 10,000 feet " "before we finally hit the ground with the VSI pegged hard on the bottom stop"
and
SimonK: "6000' at night for an incipient VRS demo." "we lost 4500' through the full situation, we pitched extreme nose down as we entered at 800' rod and we rolled 180 degrees."
and
Democritus (I think): '5000ft lost unable to get out of the condition' with a test pilot
It seems something is going on because these stories don't square with the many people who do fully developed VR demonstration regularly, without any big deal.
Is it possible that there are helicopters with these terrible characteristics?
If so we need to find out.
Can anyone shed some light on it?
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that wholly depends on what forces are acting upon it - another pointless question???[...]
[...]If you mean 'will a heavier aircraft take more power to arrest a descent than a light one?' then the answer is yes[.][...]
The empty one - rotor thrust must equal mass so the heavier helo has to accelerate the air down quicker using more AoA and giving a faster downwash speed[.]
Greetings,
Sokol - greetings
No.
If the heavy helo has to displace more air (same size disc) quicker to balance the mass (in the hover) then the speed of the downwash must be higher. That is where F = MA comes in.
If you require a rate of descent of at least half your downwash speed to get into VRS, then the heavy helicopter must have to descend at a faster rate than the lighter one.
If your Vi is higher (as it is in the heavy helo) then your Vd has to be higher to reach the required ratio for VRS.
The lighter helo has to decend faster to get in the same state as the heavier helo, right?
If the heavy helo has to displace more air (same size disc) quicker to balance the mass (in the hover) then the speed of the downwash must be higher. That is where F = MA comes in.
If you require a rate of descent of at least half your downwash speed to get into VRS, then the heavy helicopter must have to descend at a faster rate than the lighter one.
If your Vi is higher (as it is in the heavy helo) then your Vd has to be higher to reach the required ratio for VRS.
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If the heavy helo has to displace more air (same size disc) quicker to balance the mass (in the hover) then the speed of the downwash must be higher.
(You can refer this question right to the which helo decelerates faster question)
We are up to the point, explaining why there are so raw windtunnel tests.
Greetings,
So you are saying that the lighter helo wouldŽnt have a downwash speed as high as the heavy helo?
wouldn't
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Sorry, I haveŽnt specified my question enough.
I meant the initial downwash(when you start pitching up), both Helicopters are in the same pitch, refered to the helicopter and not to the environment.
(the environment then is the key factor to get to the point)
Greetings,
I meant the initial downwash(when you start pitching up), both Helicopters are in the same pitch, refered to the helicopter and not to the environment.
(the environment then is the key factor to get to the point)
Greetings,
Sokol - sorry, I don't understand what you mean.