Autorotation
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The rotor RPM in a steady autorotation won't change much from zero to Vne (autorotation), after all the transient effects die down, and the collective is fully down (or in the same position).
At least that's my experience from doing lots of autos in the Bell 206 series.
Above Vne (autorotation), the rotor RPM can decay pretty quickly, which is why it's the Vne(autorotation).
The rate of descent increases any time you go faster or slower than the Velocity for Minimum Rate of Descent (VsubY) - that should be obvious when you think what a graph of Rate of Descent vs. Airspeed is all about.
But the rotor RPM doesn't change.
The reason the Rate of Descent increases away from VsubY is that going has more fuselage drag (pretty obvious) When you go more slowly, the reasons become too complicated for me to remember or care about, so I give up and just show that the Rate of Descent increases....
At least that's my experience from doing lots of autos in the Bell 206 series.
Above Vne (autorotation), the rotor RPM can decay pretty quickly, which is why it's the Vne(autorotation).
The rate of descent increases any time you go faster or slower than the Velocity for Minimum Rate of Descent (VsubY) - that should be obvious when you think what a graph of Rate of Descent vs. Airspeed is all about.
But the rotor RPM doesn't change.
The reason the Rate of Descent increases away from VsubY is that going has more fuselage drag (pretty obvious) When you go more slowly, the reasons become too complicated for me to remember or care about, so I give up and just show that the Rate of Descent increases....
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i am confused , just for the record
in a r22 and most other small helis any zero airspeed and vertical decent with no engine to the ground above say 6 ft you are going to bend something
if your talking about going vertical / backwards / s-turns / 360 /180 or just plain straight in in the auto , remember you must have the indicated airspeed to be able to flare , and reduce the rate of vertical decent ,at the bottom , this is done at approximately anywhere from 100ft - 200ft . depends on where you want to end up
there is no vertical straight down with no indicated airspeed and safetly pull of a touch down auto , from height .
If someone wants to argue this please bring your video camera
in a r22 and most other small helis any zero airspeed and vertical decent with no engine to the ground above say 6 ft you are going to bend something
if your talking about going vertical / backwards / s-turns / 360 /180 or just plain straight in in the auto , remember you must have the indicated airspeed to be able to flare , and reduce the rate of vertical decent ,at the bottom , this is done at approximately anywhere from 100ft - 200ft . depends on where you want to end up
there is no vertical straight down with no indicated airspeed and safetly pull of a touch down auto , from height .
If someone wants to argue this please bring your video camera
Nemo Me Impune Lacessit
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Had the dubious privilege of being the first Bristow pilot from the Duri/Kotapinang operation in Sumatra to experience an engine failure during flight in a B206, (PK-HBC), in December 1968.
Certainly found plenty of energy in the blades, had got very lucky as I had an open space to go for rather than the usual 150'-200' trees and needed to pull a bit to clear the last line of trees but RPM returned almost immediately when I dumped the lever.
Seem to remember the IAS was about 50kts in descent, flared off at the bottom, big tug on the collective when the grass became obviously grass and cushion on - only problem was that the grass was 5' elephant grass and there was three foot of water! A bit of damage but it flew again a month later when the spares arrived.
Certainly would never have considered a vertical descent in the hope of recovering it all at the bottom!
Certainly found plenty of energy in the blades, had got very lucky as I had an open space to go for rather than the usual 150'-200' trees and needed to pull a bit to clear the last line of trees but RPM returned almost immediately when I dumped the lever.
Seem to remember the IAS was about 50kts in descent, flared off at the bottom, big tug on the collective when the grass became obviously grass and cushion on - only problem was that the grass was 5' elephant grass and there was three foot of water! A bit of damage but it flew again a month later when the spares arrived.
Certainly would never have considered a vertical descent in the hope of recovering it all at the bottom!
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something like this A.Agincourt,
http://www.youtube.com/watch?v=SKC8aAN1UoM
or this
http://www.youtube.com/watch?v=AQwJUz7LVBk
http://www.youtube.com/watch?v=SKC8aAN1UoM
or this
http://www.youtube.com/watch?v=AQwJUz7LVBk
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A.agincourt - the use of b**llocks might have been a bit harsh but your posts are somewhat random in their content and you are the one who took us in this direction in the first place by criticising me (when I was just trying to help someone else), not the other way round.
540 and the original post on this thread were talking about very low speed or even vertical autorotations to an EOL yet you have started on about zero speed touchdowns which is not the same thing at all. You have your ideas about P of F (your hint at Boscombe implies a TP course) which are at odds with the mainstream view - if you really are so knowledgeable then please pass on that wisdom so we might all learn, but please do include some references.
Your theory (I call it that because you were the one arguing the case) is that cyclic pitch change causes a permanent decay in Nr - Shawn (who is a TP) says not.
I know that the P of F we teach is not the whole truth, it is a convenient explanation for what we know happens in the real case. However, it serves its purpose well since the alternative is many pages of greek flute music and other hard sums that make your head hurt.
As for the rather childish reference to other threads then I suppose you must be a SAR expert as well
540 - you are correct, if you try a vertical autorotation all the way to the ground and therefore have only the energy left in the head to save you, you will bend the aircraft rather badly.
540 and the original post on this thread were talking about very low speed or even vertical autorotations to an EOL yet you have started on about zero speed touchdowns which is not the same thing at all. You have your ideas about P of F (your hint at Boscombe implies a TP course) which are at odds with the mainstream view - if you really are so knowledgeable then please pass on that wisdom so we might all learn, but please do include some references.
Your theory (I call it that because you were the one arguing the case) is that cyclic pitch change causes a permanent decay in Nr - Shawn (who is a TP) says not.
I know that the P of F we teach is not the whole truth, it is a convenient explanation for what we know happens in the real case. However, it serves its purpose well since the alternative is many pages of greek flute music and other hard sums that make your head hurt.
As for the rather childish reference to other threads then I suppose you must be a SAR expert as well
540 - you are correct, if you try a vertical autorotation all the way to the ground and therefore have only the energy left in the head to save you, you will bend the aircraft rather badly.
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Agree Crab - having gone to the same "school" I know I was on a QHI course and not trying to graduate as a Bach. Air Eng.
When an auto rev check is conducted as part of a MTF a KIAS to be flown is specified (as well as noting AUW/PA/OAT) - this is because different KIAS produce different auto RPM with flat pitch.
GAGS
E86
When an auto rev check is conducted as part of a MTF a KIAS to be flown is specified (as well as noting AUW/PA/OAT) - this is because different KIAS produce different auto RPM with flat pitch.
GAGS
E86
Eagle - that is true but it is because differing speeds give differing RoD and therefore affect Nr (so in that respect AA is right but increasing speed will give higher RoD and Nr) but that would also be the case if the speed was reduced below normal auto speed, the RoD would increase and so would the Nr at flat pitch.
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While flying an R44 with my instructor he had me do an auto in a strong headwind which resulted in us actually flying backwards over the ground. It is the airspeed which is important not the ground speed so in effect you can still come down vertically but with 40kt airspeed and hence still flare.
In addition to the limited amount of energy available to arrest the ROD at the bottom if a no IAS auto is taken to the ground, is there not also the risk of vortex ring state? i.e As pitch is applied at the bottom an induced flow will oppose the inflow from below; No airspeed; high ROD; pitch applied: sounds like a recipe for VRS, or at least "falling through" at the bottom.
G0lfer: not sure whether a 40kt flare is going to do much for you; not enough energy to reduce the ROD or rate of closure. That is more a constant attitude speed.
TT
G0lfer: not sure whether a 40kt flare is going to do much for you; not enough energy to reduce the ROD or rate of closure. That is more a constant attitude speed.
TT
Last edited by Torquetalk; 10th Dec 2007 at 13:16.
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Assuming you're still in autorotation at the bottom, surely no vortex ring? I seem to remember one of the possible (not necessarily advisable) escapes from vortex ring is to go straight into autorotation, as rotors being driven is one of the required factors for VRS?
Rotors being driven is not a requirement for VRS; pitch applied is. The rotors don't care whether the drag force is overcome by the energy from the engine (as in powered flight) or rate of descent flow (as in autorotation).
In VRS an induced flow from applied pitch is opposed by an upflow of air from a high rate of descent: the lack of airspeed localising this confused airflow to the disk.
With pitch down, you won't have VRS: but if you apply pitch with a high rate of descent and no airpseed, what then?
TT
In VRS an induced flow from applied pitch is opposed by an upflow of air from a high rate of descent: the lack of airspeed localising this confused airflow to the disk.
With pitch down, you won't have VRS: but if you apply pitch with a high rate of descent and no airpseed, what then?
TT
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autorotation
OK Lets run through this situation again. Machine R22. In hover at ten feet. No wind in any direction. Sudden engine failure. Will it get down intact. At four feet its a bit scary. Anybody done ten feet? Bug.
There are some high time FEs who do high hover engine failures; inc R22. Know of one who has demonstrated 12ft. But I think they lower the lever initially to preserve the RRPM; raising it again nearer the ground. That takes a lot of experience and presence of mind...
The aircraft is not in autorotation in a hover engine faillure practice, merely accelerating towards the ground. It takes 2-300 feet for the aircraft to fully establish in autorotation (watch the VSI accelerate then stabilise at a given ROD in an auto done from say 1500ft)
Doubt very much if there would be enough energy in the disk to cope with an engine failure at 12ft if the lever were left in the same position. And the odds of Joe pilot lowering the lever faced with a sudden acceleration to the ground seem slim.
TT
The aircraft is not in autorotation in a hover engine faillure practice, merely accelerating towards the ground. It takes 2-300 feet for the aircraft to fully establish in autorotation (watch the VSI accelerate then stabilise at a given ROD in an auto done from say 1500ft)
Doubt very much if there would be enough energy in the disk to cope with an engine failure at 12ft if the lever were left in the same position. And the odds of Joe pilot lowering the lever faced with a sudden acceleration to the ground seem slim.
TT
Last edited by Torquetalk; 10th Dec 2007 at 14:33.
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autorotation
Was in a 22 a few weeks ago where my friend was practising engine chops at four feet, pulling collective at the last minute. Quite exciting, but lets you know how quickly the rotor rpm decays. Definate case here for some special high enertia system for light helicopters
I don't think VRS is an issue at the end of a vertical auto to EOL - as you pull pitch you are not inducing a flow down through the rotor, you are simply slowing the rotor down by increasing AoA, producing a burst of rotor thrust and lots of rotor drag - the Relative air flow is still form underneath the disc until you hit the ground. If you had power applied to the rotor then it would be a different matter.
In a steady state vertical auto, rotor thrust equal to aircraft weight is produced (otherwise you would keep accelerating towards the ground) as well as sufficient torque to drive the rotor. At the bottom, that surplus of energy driving the rotors is used instead to augment the rotor thrust (but it doesn't last for long) and reduce RoD (hopefully) for touchdown.
As for hover EOLs I have had the lever raised on me from a 5' hover in a Gazelle as I retarded the throttle and suddenly we were at 8-10 feet with the Nr decaying - I don't think I have ever said/shouted "I have control" so forcefully
In a steady state vertical auto, rotor thrust equal to aircraft weight is produced (otherwise you would keep accelerating towards the ground) as well as sufficient torque to drive the rotor. At the bottom, that surplus of energy driving the rotors is used instead to augment the rotor thrust (but it doesn't last for long) and reduce RoD (hopefully) for touchdown.
As for hover EOLs I have had the lever raised on me from a 5' hover in a Gazelle as I retarded the throttle and suddenly we were at 8-10 feet with the Nr decaying - I don't think I have ever said/shouted "I have control" so forcefully
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In R22's, for a hover EOL demo even from 8 ft, I lower the lever first before hoisting it. However, the chances of someone actually doing this in a real situation are pretty remote. I'm also very sceptical about successful EOL's from a true vertical auto in nil wind, having fallen straight through in spite of full collective when a student instructor got a bit slow on a constant attitude EOL. I suspect that the people who say it can be done may not be doing it on flat calm days and in helicopters that the ASI under reads in low speed autos. Just a thought!