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VNE of light helicopter, altitude reduction ?

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VNE of light helicopter, altitude reduction ?

Old 13th Feb 2012, 09:29
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VNE of light helicopter, altitude reduction ?

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

Thx
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Old 13th Feb 2012, 09:50
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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.

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Old 13th Feb 2012, 10:19
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3kts/1000' for an S61.
What must have been the Vne for the Allouette that holds the worlds altitude record for helos at 42000'
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Old 13th Feb 2012, 10:41
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...

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.
The engine flamed out and it became the longest auto on record...

170'

Last edited by 170'; 13th Feb 2012 at 11:12.
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Old 13th Feb 2012, 13:18
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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.
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Old 13th Feb 2012, 15:21
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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????
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Old 13th Feb 2012, 18:25
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The IAS Vne reduction on the R22 is to take into account that the IAS at altitude will read lower that the TAS.
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Old 13th Feb 2012, 18:52
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...

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

Another record but hugely different equipment.

ps...It's Errrm.....Frédéric North
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Old 13th Feb 2012, 20:03
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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!

Last edited by Gemini Twin; 13th Feb 2012 at 20:21.
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Old 14th Feb 2012, 00:42
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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
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Old 14th Feb 2012, 06:11
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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?
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Old 14th Feb 2012, 14:58
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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.
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Old 14th Feb 2012, 16:16
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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.
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Old 14th Feb 2012, 20:10
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Thread drift I know - but has Fred North's altitude record been officially recognised?
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Old 15th Feb 2012, 06:46
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did you ever try an emergency evasive maneuver (avoid flock of birds)
Never had the pleasure. 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.
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Old 15th Feb 2012, 16:57
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TRC...

Apparently not ratified by FAI - Still quoting Jean B as current altitude record:
12 442 meters = 40 820.21 feet.
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Old 16th Feb 2012, 08:56
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FAR / CS 27

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.
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Old 15th Nov 2012, 16:30
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How do they get on with things like the EC X3?
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Old 15th Nov 2012, 19:52
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X3 is experimental, and has two engines.
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Old 15th Nov 2012, 21:11
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Are these aircraft actually Gyro Dynes?

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.
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