Rotor Blade Stall

[Will Crisp]

Greetings!

It takes a lot of effort to spin rotor blades up for take off and even longer to stop them after landing. Would someone explain how a rotor blade can stall so suddenly in flight? Any chance of spinning them back up before the final curtain call?

Thanks in advance!

[Screw]

Screw-In

Other than making contact with an object in flight the only other thing that can stop rotors in flight would be the introduction of airflow through the top of the rotors. This is usually achieved by a bad attitude from Pilot Induced Occilation. Once the blades slow to a certain point, which is usually fast, you lose attitude control, A high thrustline machine will Push the aircraft over therefore recovery is not possible.

Usually when people speak of blade stall, they are talking about the retreating blade. Forward speed of any rotorcraft is limited to the stall speed of the retreating blade.

Hope that helped.

Screw-Out

[Vance]

I have experanced a dramatic blade slow down at the edge of an updraft. I was trying to maintain straight and level flight in the afternoon in the desert when I suddenly rose 300 feet and then watched the rotor tach read 195 rpm as we lost a couple of hundred feet of altitude. My instructor cut the power and everything turned out fine. There was no divergent pitching before this and we had been flying straight and level for over a mile. This has puzzled me. In a Robinson Helicopter, a rotor blade stall is considerd to be a non recoverable event. I am trying to learn more about this conundrum on another thread. Thank you, Vance

[Doug Riley]

From the viewpoint of the rotor, suddenly exiting an updraft is no different than suddenly entering a downdraft. In either case, the rotor's angle of attack is reduced abruptly. This, of course, reduces the quantity of drive power reaching the rotor. Cutting off the rotor's driving air in a gyro is the equivalent of suddenly de-clutching the rotor in a helo: in both cases, you've disconnected the power supply, but the rotor is still generating some thrust, using up its flywheel energy very quickly in the process.

How much of this RPM loss the rotor can tolerate and still be able to recover its RPM when the AOA is restored will depend on various characteristics of the rotor as well the gyro's airspeed. The rotor WILL recover upon re-establishment of proper AOA as long as you don't get catastrophic retreating-blade stall (same thing as ground flapping only it happens in the air and the baldes hit the tail-prop).

The stabilty of the airframe plays a huge part in preventing problems in turbulence. If the thrustline and HS are set up properly, the airframe will actually swing so as to tend to preserve a constant AOA as you fly into and out of vertical turbulence. It's safer to set up the airframe to do this automatically than to try to out-guess Mother Nature by doing it with the stick and/or throttle.

[Will Crisp]

Thanks John :) Your post was helpful, but I am still a bit cloudy on exactly what is happening during the reatreating cycle of the blade. :confused: I need to study more.

Vance :) Thanks for sharing your experience. It's always reassuring to hear real world encounters, first hand.

Doug :) Thanks for confirming that rotor blade stall is not an instantaneous event. From what I've read, it seemed to be described that way and I could not make sense of it. Is this a fair sumation? "Rotor blade stall is a process that may happen quickly, but not instantaneously".

[Doug Riley]

Will, a rotor blade is neither more nor less than a very ordinary wing, with a pretty ordinary airfoil (except that a slight turned-up trailing edge substitutes for the separate tail cone and HS you'd use on FW plane).

As an ordinary wing, a blade can stall in part or all over. It can stall suddenly or gradually. Stalled areas can spread out or shrink. All of this depends on the pattern and speed of airflow over our wing/blade as it flies around.

The rotor disk is an imaginary thing, so of course it, as such, can't stall (abruptly or otherwise). When we talk about stalls in rotors or other wings that travel in circles, we're talking about stalls of individual blades.

In normal flight, each blade has stalled regions that expand and contract as the blade completes one circuit. These regions are near the center of the rotor. In normal flight, these stalled regions aren't of any concern, other than as drag-makers that waste some power. If the stalled region expands out to cover the whole blade, more-or-less bad things are going to occur. This CAN happen in a rev or two, which is to say in a fraction of a second.

The kind of retreating-blade stall that limits a gyro's top speed does NOT come on very suddenly. It's a gradual spreading-out of the stalled region of the retreating blade, to the point where the craft increasingly pitches up. The pitch-up naturally slows the gyro down, making airspeed somewhat self-limiting.

The kind of retreating-blade stall that can occur with a large drop in RRPM is different color of horse, however. If it happens at all, it's going to happen right when the rotor's angle of attack is again increased after a period of very low/zero AOA. This increase of AOA can be the result of the aircraft's flight path as well as a pilot input. Though I haven't tried it (and don't plan to!), I'd expect this type of stall to be very sudden and catastrophic, as the retreating blades slam the rear teeter stops and seesaw into the tail or prop.

[Will Crisp]

Will, a rotor blade is neither more nor less than a very ordinary wing, with a pretty ordinary airfoil (except that a slight turned-up trailing edge substitutes for the separate tail cone and HS you'd use on FW plane).
I'm with you so far, Doug, although I have to admit I've never noticed the turned up trailing edge on rotor blades.
As an ordinary wing, a blade can stall in part or all over. It can stall suddenly or gradually. Stalled areas can spread out or shrink. All of this depends on the pattern and speed of airflow over our wing/blade as it flies around.
I have good grasp of this concept from my fixed wing studies.
The rotor disk is an imaginary thing, so of course it, as such, can't stall (abruptly or otherwise). When we talk about stalls in rotors or other wings that travel in circles, we're talking about stalls of individual blades.
I don't quite follow how each blade can do it's own thing without the next blade following suit, but I'm hanging in there. Is it that each rotor blade acts the same way during each segment of a rev cycle?
In normal flight, each blade has stalled regions that expand and contract as the blade completes one circuit. These regions are near the center of the rotor. In normal flight, these stalled regions aren't of any concern, other than as drag-makers that waste some power. If the stalled region expands out to cover the whole blade, more-or-less bad things are going to occur. This CAN happen in a rev or two, which is to say in a fraction of a second.
YIKES!
The kind of retreating-blade stall that limits a gyro's top speed does NOT come on very suddenly. It's a gradual spreading-out of the stalled region of the retreating blade, to the point where the craft increasingly pitches up. The pitch-up naturally slows the gyro down, making airspeed somewhat self-limiting.
That makes sense...a gradual increase of speed produces a gradual increase of rotor stall...on the retreating cycle of blade revolution.
The kind of retreating-blade stall that can occur with a large drop in RRPM is different color of horse, however. If it happens at all, it's going to happen right when the rotor's angle of attack is again increased after a period of very low/zero AOA. This increase of AOA can be the result of the aircraft's flight path as well as a pilot input. Though I haven't tried it (and don't plan to!), I'd expect this type of stall to be very sudden and catastrophic, as the retreating blades slam the rear teeter stops and seesaw into the tail or prop.

Let's see if I understand you well enough to rephrase: Rotor blades flap (on the ground, or in the air) whenever air passes through them, prior to the blades having a chance to spin up fast enough to establish the imaginary, rigid rotor disc.
Ground example: If a gust of wind comes up to strike the blades of a craft sitting on the tarmac, the blades will simply flap up and down longitudinally, and perhaps, start to rotate.
Ground example #2: During taxi, rotors blades will flap when trying to increase ground speed before rotors have a chance to spin up and form their rigid disc (and support themselves).
In-flight example: RRPM has decayed because of insufficient air flow through them (decreased AOA). Flapping occurs when the air flow is reintroduced through the rotors and there is insufficient RRPM to maintain the imaginary, rigid rotor disc (only the problem is compounded because the weight of the aircraft is now on the rotor blades, causing extreme flapping).
Am I on the right track?

[Doug Riley]

The terminology is a problem here. Normal "flapping" refers simply to the rotor disk's adoption of a plane of rotation that's not square to the spindle. It's also sometimes called "rotor blowback" and is normal and desirable. The rotor disk's axis of rotation leans farther aft than the mechanical axis of the spindle. If you can picture how the teeter bolt travels as a result, you'll grasp the genius of the teeter hinge:

Because the advancing blade has more airspeed than the retreating one, it tends to have more lift. It therefore tries to fly upward. Because of gyrsocopic lag, the blade only arrives at its maximum upward excursion 90 degrees later, i.e. when it reaches its most forward position (12 o'clock from above). The teeter hinge allows this to happen without any futher tilting of the spindle. The result? The rotor disk ends up tilted back relative to the spindle.

This, in turn, means that when the next blade hits the 3 o'clock (advancing) position, THE TEETER BOLT DE-PITCHES IT. How come? The teeter bolt is square to the spindle, but the rotor disk isn't. Relative to the plane of the rotor disk, the teeter bolt applies a cyclic pitch change at each half-revolution. The advancing blade gets de-pitched and the retreating balde gets up-pitched. IOW, the teeter hinge compensates for the airspeed difference between the advancing and retreating blades by applying an increase of AOA to each blade as it hits the retreating position.

In most conditions, that's exactly what we want and is a brilliantly simple solution to the problem of the difference in speed between advancing and retreating. Think back to FW flight, though -- we're compensating for low airspeed by increasing angle of attack! (And we're doing it with a mindless automatic device that doesn't know when to stop up-pitching that retreating blade.) The process has its limits. Overdo it by continuing to try to compensate when the retreating blade's airspeed is hopelessly inadequate and what happens: stall!

That's the "other" kind of flapping: the kind in which the action of the teeter hinge is so large that the retreating blade stalls. The stall is not a result of the rotor disk's being inadeqately rigid; it's a result of the difference in lift between the advancing and retreating blades being so large that it's beyond being equalized by upping the retreating blade's angle of attack. The retreating blade stalls in the attempt to equalize.

Rotor RPM plays into this in that the higher the RRPM, the smaller the % difference in lift between the advancing and retreating blades at a given airspeed. At any given airspeed above zero, low RRPM exaggerates the difference (as a percentage of total lift) to the point where the retreating blade can't handle any more AOA and stalls.

[Will Crisp]

Doug,

I hit the books, again, last night. Thanks to you, I am a man on a mission to fully understand the information you've shared.

Like the name given to a diverse group of academics who served as advisers to President Franklin D. Roosevelt...you are a "brain trust".

Your patience and diplomacy have not been overlooked!

Thank you.

[darrellwittke]

Thanks for your patience and diplomacy Doug R.

Anecdotally, Chuck B. (who Doug is rapidly replacing) mentioned his friend who flew a heavier two place machine doing daredevil maneuvers. One of the maneuvers was a high speed pass with a hard pull up at the end. Seems Charlie? got light on the end at the zero g/low angle of attack phase, two large bangs were heard by the folks on the ground and Charlie meekly landed, apparently managing to get the blades flying properly again after stalling. He landed safely but chastened.

Al Hammer or other keepers of the old threads/stories may be able to give further details. Point being, anecdotally at least (unmeasured), it is possible to get the blades flying again in such a situation. (as above)

[Victor Duarte]

Darrel, i also saw this behaviour once : the machine was making a tight turn and up and i saw the blades significantly bend and also slaps. The pilot also had to work to recover a smooth flight curve ( i was to buy this machine, i didnt buy it :D.).

I wonder if the thing people relate here is not simply a vortex state that occurs if the G is to high, doing so, the relative wind is too important from below and makes the blade stall and decreases autorotationnal vectors and creates a vortex from above that could be the source of bangs when the blades hit it. true false ?
Thanks

[PW_Plack]

Doug, would blades with a wider chord (lower aspect ratio) produce a more gradual onset of stall than narrow blades?

If rotor RPM decayed past critical with very low airspeed, would a vertical descent safely restart autorotation fairly quickly? (Assuming you could keep a CLT machine right-side up as it fell...)

[Doug Riley]

To paraphrase Sen. Lloyd Bentsen, "Mr. Riley, you're no Chuck Beaty." Chuck B. is the real deal. So, by the way, are several other trained engineers and scientists who hang out here. I'm just gyro hobbyist trying to figger this stuff out.

If you were already in a vertical descent when you somehow lost some RRPM (hit a bird? but how likely is that?), then, no, you wouldn't have the retreating-blade stall problem... because there IS no retreating blade in a vert. descent. However, in an autogyro operating at a given rotor AOA, the angle of attack of the individual blades automatically increases as RRPM goes down.* That's fine until that AOA gets up to the stall level over a large area of both/all blades. Then you'll encounter a runaway stall of the rest of the blade area, much as happens in a retreating-blade stall except it happens on both/all blades at once. You might be able to head off this catastrophe if you had collective pitch that could be reduced to negative quickly. It's the same sort of problem that helos encounter when transitioning to autorotation.

The exact point-of-no-return will depend on blade loading, blade pitch, blade airfoil section and other factors, but you can expect all gyro rotors to have a critical RRPM from which they can't recover in flight, even if they start out in a vertical descent. Keep in mind that, with low enough RRPM, the blades will get floppy and fold up even if they haven't yet stalled altogether.

* The blade AOA equals the pitch plus the angle of the vector sum of the air velocity caused by rotation and the air velocity caused by flow into the disk. As the magnitude of the rotational velocity becomes less, the inflow to the disk becomes more and more dominant in determining the angle of that vector sum. The inflow in a vertical descent is at 90 deg. to the disk, so you're obviously headed for a stall as RRPM goes down.

Even a "wide chord" rotor blade still has a very high aspect ratio. The "mush stall" effects that we hear about with low A/R wings kick in at A/R's of around 4 and below. So I don't believe that widening chord on a rotor is going to reduce the A/R by enough to make any difference. You'd have to have rotor blades that looked like fan blades to notice much difference.