VNE of light helicopter, altitude reduction ?

[daniel-fr06]

What is the factor to determine the VNE of a light helicopter?
Why the Robinson R22 VNE is reduced with altitude ?

Thx

[Peter3127]

Vne is due to either transonic conditions on the advancing blade tips, or more likely retreating blade stall. Both are exacerbated by increased density altitude.

The more likely case, retreating blade stall, will occur at lower forward speed as altitude increases. As the air density reduces as a larger blade angle of attack is necessary to produce the required total rotor thrust (that opposes weight and drag) required for straight and level flight. A larger retreating blade angle of attack is the limiting condition as the stall is reached.

:)

[Thomas coupling]

3kts/1000' for an S61.
What must have been the Vne for the Allouette that holds the worlds altitude record for helos at 42000' :eek:

[170']

Jean Boulet died a year ago-15th Feb... it was an early Lama and he would have had no indication on ASI at that altitude in a Lama. :eek:
The engine flamed out and it became the longest auto on record...

170'

[GeorgeMandes]

In the 407, I understand VNE is set by long term durability considerations related to mast bending. In the legacy 407, if your avionics had the ability to display or calculate true air speed, 140 knots true was a good wag on VNE. In the GX, the airspeed tape goes red, and the avionics ping at you like an overpeed in a jet, when you reach VNE. In rough air, at altitude, I noted airspeed fluctuations would trigger the overspeed warning.

[Thomas coupling]

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?

I have also seen a micro light alongside the summit on Everest, what was his Vne????:mad:

[the beater]

The IAS Vne reduction on the R22 is to take into account that the IAS at altitude will read lower that the TAS.

[170']

The only thing I can think of is that he might have taken it to the shudder prior to RBS and held it just under - It’s hugely pronounced in the Lama. The higher you go the worse the shudder gets. You’d have to deaf, dumb and blind to get to the pitch and roll.

But, as you know better than me? Losing airspeed in IMC is as dangerous as exceeding VNE, so I guess he would have kept it on the back edge of the shudder, which makes me shudder just thinking about it?...

Let’s see! IMC, No Starter motor (taken out after start to save weight)
Reputedly no fuel anyway: This could be anecdotal, but widely believed in France.
Auto in IMC with no ASI and a vacuum driven Attitude Indicator ?

But there again, Jean Boulet was no stranger to high altitude, and well prepared for the task by the standards of the day!

Fred North*|* World Altitude Record – 42,500FT (http://www.fred-north.com/?page_id=338)

Another record but hugely different equipment.

ps...It's Errrm.....Frédéric North :E

[Gemini Twin]

Just for interest during flight testing of the turbine Bell 47 our flight vibration test point at 20000 ft was 1.11% vne which equalled 45mph.
Yes it did seemed like we were standing still!

[fijdor]

VNE at 21,000 ft and -2 deg C on a Bell 205 with 205 blades was 55 kts indicated +- couple of kts, you would get into a retreating blade stall at the same moment.

Pretty close to the standard "decrease VNE 3kts/1000 ft "

JD

[rotorfan]

Expanding on the original question, what allows for some helicopters to have a much higher Vne than the example R22? I calculate the R22 rotor tip-speed at 458MPH, well below the speed of sound. (I understand that is a practical limit. I can't imagine a blade going supersonic, then subsonic, on each rev and staying together for long.) If a heli has a Vne of 150K, is it that high mainly because the rotor speed at the tip is much higher than the R22? I can see where spinning the rotor faster would give a lower AoA on the retreating blade, allowing for a higher airframe forward speed before reaching critical AoA. But, it also makes the advancing blade closer to going supersonic.

Further obfuscation: the advantage of the X2 (for example) is having a balance of lift on the two discs, so RBS on one disc is balanced by RBS on the other. (Perhaps a better way to say it is that lift losses on one disc due to RBS are compensated for by the other disc having max lift in that area.) So, at the X2's top speed, are the retreating blades stalled?

[Shawn Coyle]

rotorfan:
…it's complicated… as they say in Facebook.
Lots of reasons for Vne - rotor hub stresses, mast twisting moments, retreating blade stall (and margin for same) - for example, did you ever try an emergency evasive maneuver (avoid flock of birds) in the cruise in some popular machines? You might be surprised at how close to something unpleasant you are.
But nearly all helicopters have a reduction in Vne with increase in density altitude due to TAS effects - whether it's for the advancing or retreating side is not up to us to argue. We have only to obey the limits.

[[email protected]]

Rotorfan, I'm sure Shawn will correct me but I seem to remember that the limit in forward speed of conventional helicopters is to do with advance ratio which is the relationship between tip speed and forward speed.

As in everything to do with helicopters, what you gain on the retreating side you lose on the advancing side so slowing down the rotor helps on the advancing side but gives higher AoA on the retreating side and vice versa.

The Lynx still (I think) holds the world speed record and it achieved it by use of the BERP blades - these not only allowed the tips to work at very high speed (thin swept tips with a high critical mach no) but also clever use of high lift blade sections allowing the retreating side to stay producing lift even at AoA of 20 degrees. The enormous variation of airspeed throughout the 360 degree rotation led to nasty pitching moments and required clever selection and blending of the aerofoil sections to balance out.

A symmetrical aerofoil section is not so cleverly tailored (but is much cheaper and easier to produce) and will be therefore more limited in its forward speed.

[TRC]

Thread drift I know - but has Fred North's altitude record been officially recognised?

[rotorfan]

did you ever try an emergency evasive maneuver (avoid flock of birds)

Never had the pleasure. :eek: Actually, that's the kind of thing I worry about while flying. I enjoy rotary flight much more than planks, but never feel as relaxed in the heli as seized-wing. Thanks Shawn and crab for the info. I need all I can get. Still a relative rookie, I'm always amazed at the complexity of rotary flight.

[170']

TRC...

Apparently not ratified by FAI - Still quoting Jean B as current altitude record:
12 442 meters = 40 820.21 feet.

[Grey Area]

For a single engined helicopter the VNE(Power On) can also be limited by the requirement to achieve (reduce speed) to VNE(Power Off) without "exceptional piloting skill, alertness or strength....." under any operating condition including "sudden, complete power failure..."
This is certainly the case for AS350.

[Pittsextra]

How do they get on with things like the EC X3?

[Shawn Coyle]

X3 is experimental, and has two engines.

[Jack Carson]

During forward, high speed flight, the main rotor of the X-3 is significantly unloaded when compared to a helicopter in cruise flight where both lift and forward thrust is provided by the main rotor. Some of the X-3’s weight is supported by wing lift generated by the wing structure that supports the propellers. The remaining lift is provided by the main rotor in an autorotative state. This is also evident in X-3 high speed forward flight videos where you can see the actual angle of the main rotor disk tilt. The X-3’s the rotor disk tilted slightly aft in forward flight. Similarly, Sikorky’s X-59 and X-2 helicopters torque to main rotor drops to near zero in high seep flight thus becoming a gyro dyne. Gyro dyne is probably the correct designation for these new aircraft.

[1stCavDiv]

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.

[Hot and Hi]

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.

[Tickle]

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.

[NickLappos]

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.

[LRP]

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.

[JohnDixson]

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

[megan]

It is not possible to tell what makes the Vne of any aircraftStarted 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.

[LRP]

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.

[Ascend Charlie]

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.

[JohnDixson]

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.

[LRP]

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.

[NickLappos]

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....

[212man]

Reminds me of the most useless placarded table ever - the Vne in autorotation for the S76!

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....

[JohnDixson]

Hold on,212, the 76 Project pilot will be along with the whole story!

[[email protected]]

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.

[212man]

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.....

[212man]

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!

[NickLappos]

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.

[212man]

I thought the PBA sorted that out? Oh well, I spent many an hour in the cruise gazing at the placard thinking WTF?

[NickLappos]

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!

[[email protected]]

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..... so in your pre-flight planning you have not considered your DA and hadn't thought what your max speed in auto might be - you are pushing forward for range to avoid landing in the shark-infested custard and with the lever fully down, your Nr is decaying and your landing point is sliding gently up the windscreen..........:)

[212man]

so in your pre-flight planning you have not considered your DA and hadn't thought what your max speed in auto might be - you are pushing forward for range to avoid landing in the shark-infested custard and with the lever fully down, your Nr is decaying and your landing point is sliding gently up the windscreen..........:)

That's why we do 'auto revs' checks for the operating environment we are in.....

[albatross]

I worked with a supposed genius in a hot environment. He could not understand that the VNE chart for a 212 was based on DA. Then he bitched about the 87kt vne I calculated. I asked him if he actualy new what the TAS was at 6000 feet at 38C at 87 kts IAS...101kts. TAS. He looked like a deer in the headlights..he just didn't understand. He just hated seeing IAS less than 100 KTS... some macho thing ...Those VNE numbers may not be accurate ..long time ago..but I guess my message is that VNE IAS goes down with DA but your TAS does not go down that much.
212 man ...you are using common sense again ...stop that!
Sometimes we would operate in a -20...-40C environment in the winter and then change to +30c for the summer. Checking outo rpm was a normal spring/fall check event amongst other checks. Especially if we changed from operating from bases at 1000ASL to a mountain environment where you were starting from 5000ASL.

[212man]

212 man ...you are using common sense again ...stop that

I try hard to shake it off, but it seems to stick......

[[email protected]]

Rather missing the point - of course the aircraft should be correctly rigged for the conditions but you don't do your autorev checks at Vne for auto do you?