Rotor blade stall
 

For a start.....view this video and notice all the flexing and other movements of the blade as it rotates around the Mast.

Your simple questions have a complex answer.

Stalling a blade usually only happens in high speed forward flight and at high weights.

The retreating blade has less relative airflow to work with so must operate at a higher AoA, so it will stall first, most dramatically at the tips. There will be roughness, vibration, then the nose will pitch up and it will roll to one side or the other, depending on the direction of rotation.

Remedy - lower collective, slow down, change underwear.

Please stop posting links to that video - it scares me every time I see it.... :ooh: (first time was forty years ago, during RAF helicopter training)!

Agreed Shy - you know that stuff is happening above your head but it's much better not to think about it:ok:

If it talks through the controls I would usually end up with a write up in the log. But nothing a little WD-40 couldn't fix.

The M/R collectively can also lose lift if it becomes unpowered and the RPM drops below a certain point. However, some rotor systems allow for the pilot to rebuild that unpowered RPM depending on altitude and skill level.

The main rotor spins at pretty much a constant rpm throughout flight. When you pull pitch the rotor slows down, so you spin it back up using the engine. If you continue to pull pitch once the engine has topped out eventually the rotor will slow down to its critical angle and stall.

Once the main rotor stalls you fall to the Earth like a brick! :8

I think in Ray Prouty's book he makes the points that, a helicopter blade stall is different from an airplane wing stall.

His point is that while an airfoil will invariably stall around 15 to 17 deg AOA, In a helicopter, however, the blade will reach maximum pitch for only a few millisecond.
that instant of time is not long enough for the stall to develop. As the result rotor wind tunnel have shown a rotor blade can reach 30 deg AOA and still not be stalled.

But it DOES stall. Quite dramatically, too.

The other interesting part is the recovery:

Aeroplane stall: forward stick, increase power, increase speed

Helicopter stall: back stick, decrease power, decrease speed.

That video reminds me of two old adages:

1. Helicopters don't fly, they only belt gravity into submission.
2. Helicopters are just flying fatigue test articles.

I takes me hat orf to you guys who can control that structural behaviour, at least most of the time. :D

Regards

Blakmax

Shaft109 - google BERP blades (British Experimental Rotor Programme) to see how some of the problems of high speed stall on the advancing side of the disc and low speed stall on the retreating side of the disc can be overcome using clever design.

The shape of the paddle creates a high lift tip capable of operating at 20 deg AoA due to the vortices it produces to limit spanwise flow from blade to tip and also to keep the flow energised over the tip itself.

For the advancing side, the tip is thin and swept - as you might expect for a jet fighter wing with a high MCrit.

Also aero-elastic tailoring changes the blade profile along its length reducing the pitching moments along the blade caused by high variations in AoA as it goes round and round.

AC
Not necessarily... You can stall the blades in the hover too...

So far people have been describing one kind of stall - retreating blade stall, where high speed flight causes excessive flapping and can stall the outer area of the blade when it is at the 9 o'clock position (in a CCW turning rotor). As someone mentioned, Ray Prouty talked in one of his books about how rotor blades can continue to generate high lift/drag in this situation because the AOA is not above the critical AOA long enough for flow to totally separate (I believe he said it separates, but then reattaches). I only had retreating blade stall once, a side effect of high speed flight at high altitude. The aircraft definitely let me know it was unhappy - the cyclic resisted being pushed forward, and the entire aircraft shuddered and vibrated each time I did (until it finally got through my thick skull what was going on). On the other hand, one very high time guy I know mentioned that some aircraft may try to roll inverted if you try to push it into deep retreating blade stall. I don't plan on finding out if he is correct.

I'm surprised that nobody has mentioned stalling the rotor system following an engine failure (at least, it would be one scenario where it could happen). If you allow the rotor RPM to decay, and allow a high sink rate to develop, the relative wind from below can drive the entire rotor disk into AOA above the critical AOA, and in this case the rotor will stall and stop rotating (and the aircraft will drop out of the sky). I believe it will happen in a fraction of a second, i.e. a human isn't fast enough to do anything about it once it starts, and there isn't a recovery. (to do an airplane style recovery of lowering the nose of the aircraft, we would need a collective pitch that could produce large negative pitches on the rotor system. Full size rotor systems simply aren't built or rigged with that capability (some have the ability to produce small negative AOA for special reasons).

So, basically we train helicopter pilots to never let the situation develop in the first place, since there isn't the possibility of recovery.

Older aircraft, H-19 (Whirlwind in British vernacular), H-34 (Wessex in British vernacular), Chinook CH-47A, most any helicopter in reality can be pushed hard and encounter Retreating Blade Stall.

The commonly accepted indications of that is pitch up and roll left (thinking American direction of rotation) with a high forward speed, high Density Altitude, and/or High Gross Weight.

That does not work for the Tandem Rotor design....depending upon which Rotorheads enters RBS first.....usually what is felt is large increase in airframe vibration and a loss of performance re acceleration, climb, or airspeed.

I have experienced RBS numerous times (for varying reasons) and never found it unsettling due to the amount of upset.

Perhaps I reacted upon feeling the increased vibrations and being aware the aircraft was approaching the calculated One thus I was aware of the possibility of RBS being encountered.

The normal reaction of a helicopter pilot, even if not awake enough to understand what is going on, is to reduce Collective and as a result....slow down slightly.

That seems to be a natural reaction for most unusual events and is just something that is ingrained after flying helicopters for some period of time.

That observation is derived from the days we hand flew the aircraft all the time as Autopilots were not installed on what we were flying.

As RBS is directly related to Airspeed....and generally (depending upon the aircraft type) gives warning in the form of vibrations and other easily observed reactions....the pitch up alone is enough to slow the airspeed and if followed by a reduction in Collective....normal flight is easily attained again.

Yes, RBS is pretty much self-correcting and you would have to force it to stay in the condition.

However, overreacting can make matters worse - if you offload too much collective during recovery, the pitch-up of the stall creates a very powerful flare effect due to the high speed and the Nr will rapidly increase. I know someone who oversped the head on a Wessex in exactly this way.

Take a look at Dynamic Stall somewhere on the NASA AMES website.

My basic rotary training course was in the RAF Whirlwind 10 (single turbine, floppy stick, quite a big aircraft for a basic trainer). As part of the course we were sent up to 10,000 feet so the QHI could demonstrate RBS. My instructor did his thing, which was a less than convincing demo because not much happened apart from a bit of vibration, then said:

"There, that was RBS, did you see it?"
"No..." said I, being honest.
"Well I'm not doing it again!" came the reply, and he didn't!

But my previous QHI (sadly killed in a motorcycling accident during the time of my course) had already inadvertently given me a far more effective demo. One morning he took control as I approached our relief landing ground. He lowered the nose and descended rapidly (for a Whirlwind) to about 150 feet, then pulled the aircraft into a very steep turn. The old Whirly didn't like that and all by itself it rapidly rolled about 90 degrees left and pitched hard up! Now THAT was RBS!

Thankfully, he had turned right, so the aircraft rolled upright. Had it been a left turn, I probably wouldn't be here because the aircraft would have rolled inverted.

He apologised to me later, debriefed me on what had happened and I think we both learned a lesson about flying from that.

Crab, One should treat the helicopter like a girlfriend and not a wife.....gently, softly, lovingly but firmly!:=