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VNE of light helicopter, altitude reduction ?

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VNE of light helicopter, altitude reduction ?

Old 24th Jun 2017, 17:01
  #21 (permalink)  
 
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VNE

Aerodynamics guys have been questioning this for years and like a cure for cancer it is elusive. We know about retreating blade stall and it is common when encountering strong wind shifts while flying at max power/speed. That's the shudder you identify with the need to reduce power/speed as in turbulence penetration airspeed. So how do we make a rotor go faster? Think about the number of blades compared to the rotor RPM. A MD 500 with skinny blades compared to a 206 with two wide blades. Making all things equal except the rotor system, more blades turning at the same speed will make more lift/airspeed with less pitch thus reducing retreating blade stall. Without getting into symmetrical and asymmetrical airfoil sections. I think the VNE demon lives somewhere in the rotor RPM/Rotor blade quantity region. Just mt theory. I don't stay awake nights over it.
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Old 25th Jun 2017, 10:39
  #22 (permalink)  
 
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Power off descent from LAMA altitude world record

Originally Posted by Thomas coupling View Post
That world record always amazes me. No-one has even come close since have they? If the ASI was off the clock how did he auto in IMC safely? I understand he was iced up on the windscreen for up to 6 minutes duration???
Fantastic feat of aviation. 1972 I believe?
Please enlighten me. Getting up, yes. But getting down: I seem to be missing what the big problem is?

I don't know the details, but the previous poster seem to simply have said that (presumably and in-line with the topic of this thread) the IAS due to VNE limitations at that ALT would have been so low that it didn't register on the ASI. It was not said that the ASI was u/s.

Either way, in an autorotation, what do you need the ASI for?? You can safely autorotate at any speed between a little backward, and power-off forward VNE. In fact, the lower the speed, the easier and more stable the auto is . Forward speed only becomes useful a few seconds before the landing, so that you can flare and arrest your rate of decent at touch-down.

So enter autorotation, keep the ship level, set for low or zero speed, and control RRPM with collective. In any case, stay below VNE, which as other posters have pointed out, would announce itself with vibrations from the onset of RBS. As mentioned above, the sweet spot around zero IAS should easily identifiable by its stable ride.

I'd also venture to say that all VNE and RBS-related issues here are linked to high AoA (needed of powered forward flight to push speed): Conversely, in an auto where your AoA is almost zero, VNE limitations should be much less critical.

I would be more concerned if he didn't have an AH (seeing that at 42,000 ft you are deemed to be in IMC, plus he had his windows iced up). Not sure how those work in a Lama, but even if electric suction pump driven, he should have had battery power for the 20 to 30 min he was descending.

Having said this, I wasn't up there at 42,000 ft. It must have been scary in the best of circumstances. I am afraid of heights, that's why I fly chopper.
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Old 26th Jun 2017, 02:51
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As purely an enthusiast, now I understand why I see helicopters appear to fly slower when they are flying at quite high altitudes, not just a trick of the eye due to the distance.
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Old 26th Jun 2017, 19:34
  #24 (permalink)  
 
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Vne is set during the design and test phase, and is the result of a mix of possible things. There is no reason why a manufacturer has to go to a limit to make a Vne, it could be just where they stopped testing and therefore where the public cannot exceed.


The FAA requires many points to be tested at 1.11 times Vne, so if a particular trait is going bad (vibration, stick stability, strong stresses on the rotors) the manufacturer will stop at a value of .9 times the trouble spot and declare that Vne.


It is not possible to tell what makes the Vne of any aircraft, unless you talk to the test team who did the work at the time. In fact, stopping at a given speed is sometimes done because the fight time and cost of diving to higher speeds, and then climbing back up to the next point, is so high that there isn't a reason to make a big hairy chested maximum speed when the customer can get everywhere he needs to go at a lower speed.
One thing is that once Vne is set, many calculations are made using it, and the stresses at that Vne help determine the component lives and overhaul times, so it is almost always true that Vne drives component lives. It is obvious, if you use Vne stresses for the calculated life, then Vne sets the life.


The Vne chart is what the pilot uses, so it uses Calibrated Airspeed and thus will drop as altitude increases. Of course, the density drops and true airspeed achieved uses less CAS. I say CAS and not IAS because the CAS uses no instrument error. Use the error placard to adjust your IAS to CAS if you decide to bounce near Vne regularly. The typical drop in Vne with altitude is also a bit more that just density because the manufacturer wants the lives calculated to be based on more or less constant stresses, so as retreating blade stall is peeking out at higher altitudes, the Vne drops a bit more than just CAS to keep the stresses and lives where they were at lower altitude.


For some helicopters, Vne is set by handling where the "longitudinal stick stability" becomes a problem, for others it is vibration, for others it is rotor speed performance, and for some it is rotor stall.
Basic rule: Don't exceed Vne, unless you are drawing Test Pilot paychecks.
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Old 26th Jun 2017, 23:10
  #25 (permalink)  
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Originally Posted by NickLappos View Post
Vne is set during the design and test phase, and is the result of a mix of possible things. There is no reason why a manufacturer has to go to a limit to make a Vne, it could be just where they stopped testing and therefore where the public cannot exceed.


The FAA requires many points to be tested at 1.11 times Vne, so if a particular trait is going bad (vibration, stick stability, strong stresses on the rotors) the manufacturer will stop at a value of .9 times the trouble spot and declare that Vne.


It is not possible to tell what makes the Vne of any aircraft, unless you talk to the test team who did the work at the time. In fact, stopping at a given speed is sometimes done because the fight time and cost of diving to higher speeds, and then climbing back up to the next point, is so high that there isn't a reason to make a big hairy chested maximum speed when the customer can get everywhere he needs to go at a lower speed.
One thing is that once Vne is set, many calculations are made using it, and the stresses at that Vne help determine the component lives and overhaul times, so it is almost always true that Vne drives component lives. It is obvious, if you use Vne stresses for the calculated life, then Vne sets the life.


The Vne chart is what the pilot uses, so it uses Calibrated Airspeed and thus will drop as altitude increases. Of course, the density drops and true airspeed achieved uses less CAS. I say CAS and not IAS because the CAS uses no instrument error. Use the error placard to adjust your IAS to CAS if you decide to bounce near Vne regularly. The typical drop in Vne with altitude is also a bit more that just density because the manufacturer wants the lives calculated to be based on more or less constant stresses, so as retreating blade stall is peeking out at higher altitudes, the Vne drops a bit more than just CAS to keep the stresses and lives where they were at lower altitude.


For some helicopters, Vne is set by handling where the "longitudinal stick stability" becomes a problem, for others it is vibration, for others it is rotor speed performance, and for some it is rotor stall.
Basic rule: Don't exceed Vne, unless you are drawing Test Pilot paychecks.

Excellent, thank you.
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Old 27th Jun 2017, 00:23
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Hi LRP. Nick was just kidding.

For the question: Vne of Light Helicopter*, Altitude Reduction?
*decades ago-any helicopter

The answer really is 6 kts/1000'

Nick will amplify.

Best,
John
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Old 27th Jun 2017, 01:13
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It is not possible to tell what makes the Vne of any aircraft
Started out in the TH13M and they said the Vne was dictated by the bubble. The B-17 bomber it was the engine cowling securing system.
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Old 27th Jun 2017, 04:25
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Originally Posted by JohnDixson View Post
Hi LRP. Nick was just kidding.

For the question: Vne of Light Helicopter*, Altitude Reduction?
*decades ago-any helicopter

The answer really is 6 kts/1000'

Nick will amplify.

Best,
John
OK. Must be something wrong with this chart.
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Old 27th Jun 2017, 07:05
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Hot'n'hi said
Conversely, in an auto where your AoA is almost zero, VNE limitations should be much less critical.
BONG! WRONG! Take off all your clothes.

There is of course an angle of attack, or you wouldn't have any lift, or autorotative force on the blade, and as a result you would drop like a rock at terminal velocity. Go back to your basic books to see the resultant between rotational flow and RoD airflow, now from below, which replaces induced airflow from above.

Vne in an auto is still hugely important, largely because of the huge increase in drag which both slows down the blades and increases RoD.
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Old 27th Jun 2017, 08:39
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LRP, decades ago all SA machines had symmetrical airfoils and the 6kts/1000 was a stock/comical answer to all Vne questions in the engr dept.
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Old 27th Jun 2017, 13:31
  #31 (permalink)  
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Originally Posted by JohnDixson View Post
LRP, decades ago all SA machines had symmetrical airfoils and the 6kts/1000 was a stock/comical answer to all Vne questions in the engr dept.
Got it. I started in OH-23's, don't recall what Vne was, pretty much flew as fast as it would go unless the vibration got intolerable.
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Old 27th Jun 2017, 16:18
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John Dixson is absolutely correct, in the Sikorsky Test Pilot Office, the stock answer to any question was either:


1) Six knots per thousand
2) Hot and Dusty
3) All we really did was dance....
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Old 27th Jun 2017, 19:22
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Reminds me of the most useless placarded table ever - the Vne in autorotation for the S76!

Originally Posted by NickLappos View Post
John Dixson is absolutely correct, in the Sikorsky Test Pilot Office, the stock answer to any question was either:


1) Six knots per thousand
2) Hot and Dusty
3) All we really did was dance....
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Old 27th Jun 2017, 20:11
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Hold on,212, the 76 Project pilot will be along with the whole story!
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Old 27th Jun 2017, 20:40
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Vne in an auto is still hugely important, largely because of the huge increase in drag which both slows down the blades and increases RoD.
I think the most important part of that is that the driving section of the blade in auto is reduced, the dragging section increases and at high speed, Nr cannot be maintained. Hence the Vne in auto for many helicopters.
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Old 27th Jun 2017, 21:29
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Originally Posted by [email protected] View Post
I think the most important part of that is that the driving section of the blade in auto is reduced, the dragging section increases and at high speed, Nr cannot be maintained. Hence the Vne in auto for many helicopters.
I think the most important thing is that you look out the window, whilst modulating the collective to maintain Nr. Checking your density altitude and working out he Vne is low on the list of priorities.....
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Old 27th Jun 2017, 21:31
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Originally Posted by JohnDixson View Post
Hold on,212, the 76 Project pilot will be along with the whole story!
Thanks John - look forward to it! Must be time for a few more cold ones!
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Old 27th Jun 2017, 22:04
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I wrote the Vne Auto chart for the S-76! It is a fine story in itself, and I think it illustrates the randomness of Vne determination.

The FAA requires that you must have Longitudinal Static Stability (LSS), where the stick position for any speed must be ahead of a lesser speed. This makes sense to any pilot, but the way it is tested makes little sense. The Normal pilot trims to a speed and notes the stick position, then flies to a new speed and sees that the stick position is a bit forward of the previous point and he is happy. The FAA (and their European friends) however do it by settling to a speed, and then advancing 20 knots without increasing power or collective pitch. They then check the stick position at the new faster speed (and fail to note that the new speed has created a 1500 fpm descent) and find that it is at the same place or even (heaven forbid) a little behind the slower point. They then declare that the aircraft has the dreaded negative static stability and must be banished. The fact that the screaming descent creates a large horizontal tail angle that pushes the nose down and forces the stick back completely escapes their sharp eyes.

Now back to the S-76. In the cookbook LSS test in a autorotation, you must trim to .9 times Autorotation Vne and then push to 1.1 times autorotation Vne and if the stick comes back, you have a problem. I was the guy who found out that 1) the FAA has no rules on what the auto Vne must be and 2) at 136 knots or slower, the LSS stick check worked well. As a result, a fine Sikorsky engineer (Dick McCucheon) and I wrote that Auto Vne chart on a piece of paper and it became the new placard. John Dixson was in the back with a cup of coffee, laughing too hard to help, as I recall.
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Old 27th Jun 2017, 22:13
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I thought the PBA sorted that out? Oh well, I spent many an hour in the cruise gazing at the placard thinking WTF?
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Old 28th Jun 2017, 02:05
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Good on you, 212man! The PBA did help that case, its contribution was included in the comment I made. The stroke of that Pitch Bias Actuator was not quite enough to fully cancel out the effects of the descent on the horizontal tail. This whole tale is told in 29.175 (bad pun intended).
The problem is that these rules were written when a typical helo Vne was 100 knots, so a .7 to 1.1 Vne stick check went from 70 knots to 110 knots. When Vne is 155 knots, the check goes from 108 Knots to 139 knots to 171 knots. The much bigger speed change brings on a much bigger descent, so the "level flight" check trimmed at 139 knots yielded a 1500 fpm climb at 108 and a 1800 fpm dive at 171!

Last edited by NickLappos; 28th Jun 2017 at 15:23.
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