Helicopter aerodynamics question

[RotaryWingBob]

In my fixed wing training there was discussion that as altitude increases (and air density decreases) that true airspeed increases with respect to indicated, but that the V speeds remain the same for indicated airspeed.

Why then does Vne in KIAS for our R22 decrease with altitude? Is this because the onset of retreating blade stall depends on true airspeed rather than indicated airspeed? And if so, why?

 

[Bonanza]

Good question.
This is one of those things that I have always accepted as "because it does"
In the NOTAR there is quite a complex set of plates for calculating the Vne for a given set of conditions. I have used it lots of times but never asked the question.
Now, where is John Lancaster when you really need him?
Stephen.

 

[maximus]

Can you roll a helicopter?

 

[gismo]

Can you roll a helicopter?

I can't, but it's been done.

 

[Tom-D]

I can't, but it's been done.

Any one can, but few can recover.

 

[gismo]

In my fixed wing training there was discussion that as altitude increases (and air density decreases) that true airspeed increases with respect to indicated, but that the V speeds remain the same for indicated airspeed.

Why then does Vne in KIAS for our R22 decrease with altitude? Is this because the onset of retreating blade stall depends on true airspeed rather than indicated airspeed? And if so, why?

I don't really know the answer for sure but here's my guess:

The "indicated airspeed" experienced by the retreating blade is equal to the difference between the forward speed of the helicopter ("TAS" / unaffected by air density) and the rearward speed of the rotor blade ("IAS" / affected by air density). So as you decrease the air density, you get a double whammy, the retreating blade "IAS" is reduced for the same rotor RPM and the indicated forward speed decreases for the same true forward speed. As a result I'd expect the Vne to decrease faster than the IAS/TAS rate with altitude.

 

[Ed Guthrie]

In my fixed wing training there was discussion that as altitude increases (and air density decreases) that true airspeed increases with respect to indicated, but that the V speeds remain the same for indicated airspeed.

Why then does Vne in KIAS for our R22 decrease with altitude? Is this because the onset of retreating blade stall depends on true airspeed rather than indicated airspeed? And if so, why?

Too late in the evening and too much jet lag for my brain to function properly, but first principles tells me that stall speed is pegged on indicated speed, so retreating blade stall should be, too. IOW, no, it isn't a blade stall issue.

The problem my adled brain is having is figuring how blade speed compares to (fixed) wing speed.

 

[RotaryWingBob]

Too late in the evening and too much jet lag for my brain to function properly, but first principles tells me that stall speed is pegged on indicated speed, so retreating blade stall should be, too. IOW, no, it isn't a blade stall issue.

The problem my adled brain is having is figuring how blade speed compares to (fixed) wing speed.

My problem also, Ed. It should be simple -- the advancing blade has its rotational speed plus headwind (aircraft airspeed), the retreating blade has its rotational speed minus the airspeed of the helo. But I can't figure it beyond there.

If the DPE asks about this I'm road kill...

 

[Bonanza]

If the DPE asks about this I'm road kill...

I'd be very surprised if the DPE knows the answer himself.
Stephen.

 

[Ed Guthrie]

My problem also, Ed. It should be simple -- the advancing blade has its rotational speed plus headwind (aircraft airspeed), the retreating blade has its rotational speed minus the airspeed of the helo. But I can't figure it beyond there.

If the DPE asks about this I'm road kill...

In my ancient copy of AC61-13B, "Basic Helicopter Handbook", the discussion of decreasing Vne with altitude is in Chapter 9, within the section "Retreating blade stall", which I take as a subtle hint that the two are related. How is still beyond my sleep deprived brain but I suspect Lance F. is theoretically near perfect as usual.

 

[RogerT]

In my fixed wing training there was discussion that as altitude increases (and air density decreases) that true airspeed increases with respect to indicated, but that the V speeds remain the same for indicated airspeed.

Why then does Vne in KIAS for our R22 decrease with altitude? Is this because the onset of retreating blade stall depends on true airspeed rather than indicated airspeed? And if so, why?[/QUOTE

I'd think in terms of the increased angle of attack required on both
blades with the less dense air at higher altitudes.

 

[RogerT]

My problem also, Ed. It should be simple -- the advancing blade has its rotational speed plus headwind (aircraft airspeed), the retreating blade has its rotational speed minus the airspeed of the helo. But I can't figure it beyond there.

If the DPE asks about this I'm road kill...

Bob .. email me on the side about this.

rtracy@alltel.net

 

[Ryan Ferguson]

In my fixed wing training there was discussion that as altitude increases (and air density decreases) that true airspeed increases with respect to indicated, but that the V speeds remain the same for indicated airspeed.

Why then does Vne in KIAS for our R22 decrease with altitude? Is this because the onset of retreating blade stall depends on true airspeed rather than indicated airspeed? And if so, why?

Man... I miss chatting on the boards! I love these sorts of questions. Good one, Bob.

Keep in mind that as your altitude increases in a helicopter, so must blade pitch angle. Cruising @ 70KTAS at S.L. (15 C) might require a relatively small blade pitch angle and associated AOA. Cruising @ 70KTAS at 8,000 feet, however, probably requires almost all of your available power; you'll be pulling close to maximum pitch + full throttle (governed in an R22, twisted on in 269/300). You're correct to think of retreating blade stall - that is indeed the driving factor for the reduced Vne. The reason for this, of course, is that as the retreating blade flaps down to increase AOA (compensating for uneven rotor thrust production, aka dissymetry of lift) it will be much closer to exceeding its critical AOA if the blade is already at a high pitch angle. Thus, Vne decreases with altitude because the retreating blade will stall at lower true/indicated airspeeds due to its higher AOA.

Put another way, it would probably be possible to safely exceed high altitude Vne if you were in a steep descent - because your collective would be lowered and the retreating blade's AOA would be much lower as a result. This is explained for concept only, and is NOT recommended!

 

[RotaryWingBob]

Thanks, Ryan, that was helpful as always. I reached the same conclusion after a brief flight with my CFI yesterday in hot weather. I remarked that it was taking 23" MP to get into a hover. That's when it dawned on me -- the MP gauge was telling me I was using more collective than I would have on a cooler day and that therefore I had a higher rotor blade AOA, and that the higher AOA would lower the airspeed at which the retreating blade would start to stall.