Vortex Ring / Settling with power (Merged)
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Nick, i was thinking more along the lines of over pitching. The "natural" panic reaction when the ground is coming up is to pull more collective. Agreed it wouldn't help with CFIT, but then ABS doesn't stop tail gating...
Thanks Lama - interesting discussion.
Thanks Lama - interesting discussion.
Nick, if you are still out in cyberspace while you sort Bell out - do you have updated links to all the excellent info you posted much earlier on this thread?(the others don't work any more)
I have been spreading the gospel regarding the VRS myths but could use some diagrams and data to convince some Doubting Thomases.
I have been spreading the gospel regarding the VRS myths but could use some diagrams and data to convince some Doubting Thomases.
Mighty Gem - yes thanks, I saw that the first time round but phrases like -
- are a bit beyond my O level maths and make me feel quite sleepy
I know Nick's web pages had some more 'pilot-friendly' info but none of the links work any more - I guess he has moved to another provider.
'rotor wake(s) being tracked in a Lagrangian sense using the free-vortex method of Bhagwat & Leishman'
I know Nick's web pages had some more 'pilot-friendly' info but none of the links work any more - I guess he has moved to another provider.
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Marvellous! Thanks very much Nick, I trust the new job is going well.
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Have been reviewing this thread from the beginning and finding it useful. I am trying to locate the website(s) frequently referred to in Nick Lappos's posts, but none of these links now work. Alternatively, looking for the formula mentioned for calculating downwash velocity. Any pointers?
From the momentum theory the induced flow can be calculated using the formula
Vi = √[ T / 2 ρ A ]
where
Vi Induced Flow Velocity [m s^-1]
T Thrust (approximately equal to weight in a hover). [N]
ρ Air Density [kg m^-3]
A Disc Area [m^2]
This is only an approximation, but is a good starting point for calculations involving the rotor system.
If you want any more PM me.
Vi = √[ T / 2 ρ A ]
where
Vi Induced Flow Velocity [m s^-1]
T Thrust (approximately equal to weight in a hover). [N]
ρ Air Density [kg m^-3]
A Disc Area [m^2]
This is only an approximation, but is a good starting point for calculations involving the rotor system.
If you want any more PM me.
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Vany:
Thanks. Trying to estimate downwash velocity for my Enstrom 480. After struggling with unit conversions for a bit, finally got it!
Mighty Gem:
Thanks, but this links to different info than the one Nick gave in earlier posts.
Thanks. Trying to estimate downwash velocity for my Enstrom 480. After struggling with unit conversions for a bit, finally got it!
Mighty Gem:
Thanks, but this links to different info than the one Nick gave in earlier posts.
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VRC
Hi to all VRS fans out there. So what's a nice guy like me doing here on a warm Easter Sunday morning ... I logged on to the thread at 08.00 hrs and two hours later seem to have covered more than 280 or so posts.
That's something I love about pprune ... being able to read the studies and experiences of the likes of Shawn, Nick, and the other highly experienced and knowledgable guys so there's no question of any egg sucking lessons coming from me. But as a 14,000 hr rotary guy - can I put in my two pennorth and relate some adjacent VRS experiences.
First I'm a trifle concerned to know I have been teaching what apparently is an incorrect VRS recovery technique in advocating zero use of collective lever ... my reasoning being that whilst it may assist recovery due to the reduction in downflow, the associated loss of height is unacceptable in a real situation.
So at grass roots level in training for the VRS exercise, I demonstrate the well established, One-Two-Three technique.
Having completed the pre-exercise checks, 'Number One' is to select a suitable LS picture albeit from altitude with power set to provide a suitable ROD (circa 60 knots.) Once settled in a steady descent ...the establishment of the 'Number One' condition for VR is emphasised.
VR requirement 'Number Two' is introduced with aft cyclic to progressively reduce speed at some 5 knots per second to below translational lift speed.
I like to point out the change in airframe attitude and increase in ROD as translational lift is lost. As has been said here, it is important that aircraft attitude remains steady if we are to find that elusive VR tube of air! With the VSI indication showing an increase, I invite my pilot to increase ROD with a further lowering of lever.
Now we need to wait for say 5 to 10 seconds as the airframe settles in a steady but increased ROD. The establishment of the 'Number Two' condition is emphasised.
With a solid vocal warning, condition 'Number Three' is introduced with a firm,(possible panic mode) raising of lever. In the case of the Enstrom/SH300/MD500 series, the usual indications are: airframe buffet, random yaw and roll, disc flapping and RRPM variation invariably accompanied by an increase ROD. The right yaw can be quite severe but recovers .. see later.
To achieve the condition, which for the first 20 or so seconds is incipient, it is important the pilot uses minimum control inputs, especially with cyclic. Recovery is the usual forward cyclic to obtain a positive ASI reading before applying power.
The highly experienced FE who does my client's skills tests prefers the combined cyclic AND collective lever recovery, so this method is also shown, while I emphasise the extra loss of height that may not be adviseable in a real situation.
Now here is where I need help.
I explain to my pilots that the random yaw, (always right in the above mentioned types) is due to the loss of T/R effectiveness in the disturbed air, but the self-recovery is due to the T/R blades finding smoother air at the edges of the disc. On one occasion I experienced a rapid 90 degree disc roll to the right which I decided was due to the M/R blade tips doing a ditto. Not a good explanation perhaps so the whiz kids invited to explain please.
Following the rapid roll experience, I do recall my gentlemanly Irish 500 pilot saying at the time ... 'Dennis - please can we not practice that particular exercise again.'
Just a closing note and as I have written elsewhere - being intrigued by the off-quoted airflow reversal as the upgoing air punches through the centre of the disc - I decided to investigate by super-glueing a couple of dozen wool tufts to the fixed mast on a SH 300 and then running through the VR exercise. The 300 has excellent overhead vision so I was delighted to find my tufts flapping nicely in the turbulent airflow with their tails now obediently pointing skywards! A Eureka moment for me. I'd found the airflow reversal!
As has been said, let's keep this subject going for ever for all to read.
Take care all ppruners. Dennis K.
That's something I love about pprune ... being able to read the studies and experiences of the likes of Shawn, Nick, and the other highly experienced and knowledgable guys so there's no question of any egg sucking lessons coming from me. But as a 14,000 hr rotary guy - can I put in my two pennorth and relate some adjacent VRS experiences.
First I'm a trifle concerned to know I have been teaching what apparently is an incorrect VRS recovery technique in advocating zero use of collective lever ... my reasoning being that whilst it may assist recovery due to the reduction in downflow, the associated loss of height is unacceptable in a real situation.
So at grass roots level in training for the VRS exercise, I demonstrate the well established, One-Two-Three technique.
Having completed the pre-exercise checks, 'Number One' is to select a suitable LS picture albeit from altitude with power set to provide a suitable ROD (circa 60 knots.) Once settled in a steady descent ...the establishment of the 'Number One' condition for VR is emphasised.
VR requirement 'Number Two' is introduced with aft cyclic to progressively reduce speed at some 5 knots per second to below translational lift speed.
I like to point out the change in airframe attitude and increase in ROD as translational lift is lost. As has been said here, it is important that aircraft attitude remains steady if we are to find that elusive VR tube of air! With the VSI indication showing an increase, I invite my pilot to increase ROD with a further lowering of lever.
Now we need to wait for say 5 to 10 seconds as the airframe settles in a steady but increased ROD. The establishment of the 'Number Two' condition is emphasised.
With a solid vocal warning, condition 'Number Three' is introduced with a firm,(possible panic mode) raising of lever. In the case of the Enstrom/SH300/MD500 series, the usual indications are: airframe buffet, random yaw and roll, disc flapping and RRPM variation invariably accompanied by an increase ROD. The right yaw can be quite severe but recovers .. see later.
To achieve the condition, which for the first 20 or so seconds is incipient, it is important the pilot uses minimum control inputs, especially with cyclic. Recovery is the usual forward cyclic to obtain a positive ASI reading before applying power.
The highly experienced FE who does my client's skills tests prefers the combined cyclic AND collective lever recovery, so this method is also shown, while I emphasise the extra loss of height that may not be adviseable in a real situation.
Now here is where I need help.
I explain to my pilots that the random yaw, (always right in the above mentioned types) is due to the loss of T/R effectiveness in the disturbed air, but the self-recovery is due to the T/R blades finding smoother air at the edges of the disc. On one occasion I experienced a rapid 90 degree disc roll to the right which I decided was due to the M/R blade tips doing a ditto. Not a good explanation perhaps so the whiz kids invited to explain please.
Following the rapid roll experience, I do recall my gentlemanly Irish 500 pilot saying at the time ... 'Dennis - please can we not practice that particular exercise again.'
Just a closing note and as I have written elsewhere - being intrigued by the off-quoted airflow reversal as the upgoing air punches through the centre of the disc - I decided to investigate by super-glueing a couple of dozen wool tufts to the fixed mast on a SH 300 and then running through the VR exercise. The 300 has excellent overhead vision so I was delighted to find my tufts flapping nicely in the turbulent airflow with their tails now obediently pointing skywards! A Eureka moment for me. I'd found the airflow reversal!
As has been said, let's keep this subject going for ever for all to read.
Take care all ppruners. Dennis K.
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Dennis:
Sadly won't be coming to UK for 20th May - some other time perhaps?
My preferred method of demonstration for VRS incipient stage is - start in level flt at 60 knots, downwind (and this only works downwind). Reduce power by about 20% of power for level flight (not % torque, % of power being used) and decelerate maintaining altitude.
The point is to show the symptoms - and they start as the airspeed reduces through about 15-10 knots - low frequency airframe buffeting, followed by uncommanded pitch, roll and yaw, and if power is increased, a possible increase in rate of descent.
The aim is to make the student aware of the symptoms they'll see and feel and hear in the cockpit and like stalls in a FW airplane, stay away from them, or recover at the first sign of them.
Sadly won't be coming to UK for 20th May - some other time perhaps?
My preferred method of demonstration for VRS incipient stage is - start in level flt at 60 knots, downwind (and this only works downwind). Reduce power by about 20% of power for level flight (not % torque, % of power being used) and decelerate maintaining altitude.
The point is to show the symptoms - and they start as the airspeed reduces through about 15-10 knots - low frequency airframe buffeting, followed by uncommanded pitch, roll and yaw, and if power is increased, a possible increase in rate of descent.
The aim is to make the student aware of the symptoms they'll see and feel and hear in the cockpit and like stalls in a FW airplane, stay away from them, or recover at the first sign of them.
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As has been said, let's keep this subject going for ever for all to read.
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Regardless of different type characteristics, the main sympton that must be recognised is always, the beginnings of a very unpalatable sinking feeling in the pit of the stomach.
The student that grasps the mettle at that point and demonstrates effective recovery is on a winner.
Those who gravitate, (ahem) to the feelings, being a tightening of a lower muscle in the body form will inevitably be sh@t on from a great height.
The student that grasps the mettle at that point and demonstrates effective recovery is on a winner.
Those who gravitate, (ahem) to the feelings, being a tightening of a lower muscle in the body form will inevitably be sh@t on from a great height.
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Light Vs Heavy-Risking Vortex Ring
I am amidst a discussion with some of our fellas.
.... Need some expert opinions
For a RW pilot flying (rather descending from a peak to the valley bottom) with feel, visual perspective,sensory cues and probably with some cursory reference to instruments- ASI, VSI and Alti ("seat of the pants".. as most of us: ..pilot's..; would tend to, during such routine shuttles of short flight-legs in the mountains) is it more probable to enter/ risk entry into Vortex Ring state (Main Rotor) in...
(a) Lightly loaded (AUW) heptr (Say empty with minimal fuel)
OR
(b) Same ac Loaded to Max Capacity (MAUW)
Some brief explanation for the same...perhaps with specific cases of effect of winds
.I believe
, contrary to popular opinion
, its the lighter heptr that would catch the pilot early into vortex ring than the heavier one..??!!!
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Vortex ring state happens when the downwash velocity from the main rotor equals the speed of the up-flowing air.
It's impossible to say whether lighter or heavier weight will make a difference - it depends on the amount of power being used in the descent (lower power setting equals less downwash velocity which means you can get vortex ring state at a lower rate of descent)
It's impossible to say whether lighter or heavier weight will make a difference - it depends on the amount of power being used in the descent (lower power setting equals less downwash velocity which means you can get vortex ring state at a lower rate of descent)