Blade Sailing

[WaspAir]

Coning is a slightly misleading term. It suggests a straight sided cone (from revolving a straight line) as if all bending happens at the root. A bowl-like shape is more likely, especially given the distribution of lift along the span and the flexibility of the blade.

Whether fully articulated with a flapping hinge, teetering and equipped with a Robinson coning hinge, or teetering while fixed to a hub bar, the blade will take on a new shape under load, just as the tips will droop when everything is stopped. The difference is only where the bending starts along the span.

 

[Jean Claude]

Jean Claude, thank you for your wise explanation. However… I can’t believe that centrifugal force is not playing a role in the rotor stability.

First of all, we use teetering rotors which don’t have the ability to get a correct conning angle because the two blades are rigidly joined between them. Only articulate rotor heads (and hinge less…) have this ability. The question, then, is what happens when a teetering rotor head with low rpm is forced to develop full lift.

What I have learned from incidents and accidents that has been shown in this forum is that both blades are bended “up” in gyrocopters with a good HS.

But what is going to happen in takeoff, when the air speed is low but to high for actual rotor rpm? The lift involved is not enough to bend the blades up, consequently the rotor hits the stops vigorously… I think that the cause is the lack of enough centrifugal force that would restrain the coning angle. But the teetering is not able to cone enough. Because of that the flapping is unstable and its range is increased.

We need to see that conning angle is different depending on rotor rpm and developed lift. Usually there is a mechanical conning angle made by the manufacturer. This conning angle is good for normal flying conditions, and blades flexibility works to adapt all normal situations. But this rotor is unable to cope with a rotor producing lift at too low rpm.

Of course, the "coning" varies as long as the Rrpm is not yet steady. But changes in coning have no effect on the margin left by our flapping stops.

With a no diverging flapping angle (i.e <4 degrees), even a fully unloaded rotor is not attacking the propeller or the vertical stabilizer (It is the job of the manufacturer)

This can only happen if the conditions of load, speed, and Rrpm give a diverging flapping (by stall of retreating blade) because the stops will then reached, then perhaps exceeded by the bending of the blade.

 

[Doug Riley]

In an obvious PPO accident that I investigated some years ago, the main spindle bolt (vertical 1/2" diameter) was bent. The bend may have been caused by repeated hard contact between the rotor hub and the teeter stops, as a result of retreating-blade stall.

As we all know from Chuck Beaty's analyses, a pitchover driven by a high thrustline may be sudden and violent enough to stall the retreating blade, even ay full RPM. The rapid pitching of the frame, if the pilot holds the stick still, increases the AOA of the retreating blade, possibly past its stalling AOA.

A bend aft would allow the blades to descend lower in the back of the aircraft than designed, possibly striking the tail or prop.

 

[ferranrosello]

In my opinion the blades bend up in most gyroplane accidents because the bottom of the blade is what hits the ground first.

Maybe, Vance. However, I know three accidents that happened because of unloading the rotor in which the rotor blades were bended up before hitting the ground. The only reason was the lack of centrifugal force when the rotors were reloaded.

 

[ferranrosello]

Of course, the "coning" varies as long as the Rrpm is not yet steady. But changes in coning have no effect on the margin left by our flapping stops.

Ok, Jean. You are totally right and it was my mistake to think that the centrifugal force had a role in restricting the flapping amplitude.

The question is that the flapping amplitude increases with airspeed (flap back) and with lower rotor rpm. In takeoff with too low rotor rpm the flapping amplitude is bigger than the allowed mechanical travel and the rotor hit the stops hardly and show a turn over tendency because is unable to equalize the lift dissymmetry. I really don’t know if this is retreating blade stall. The retreating blade stall is a question of angle of attack.

Therefore, may point is that the problem in this case is that rotor flapping fails in equalizing lift rather than retreating blade stall.

 

[Abid]

Ok, Jean. You are totally right and it was my mistake to think that the centrifugal force had a role in restricting the flapping amplitude.

The question is that the flapping amplitude increases with airspeed (flap back) and with lower rotor rpm. In takeoff with too low rotor rpm the flapping amplitude is bigger than the allowed mechanical travel and the rotor hit the stops hardly and show a turn over tendency because is unable to equalize the lift dissymmetry. I really don’t know if this is retreating blade stall. The retreating blade stall is a question of angle of attack.

Therefore, may point is that the problem in this case is that rotor flapping fails in equalizing lift rather than retreating blade stall.

It is definitely retreating blade stall. In GWS measurements with flapping angle measurement setup, I never saw flapping angle exceed 5 degrees. In normal flying it really was 2 degrees or less. Mike Goodrich has had that setup for longer on his ELA and perhaps he can also give his data.

 

[ferranrosello]

Ok, Abid. Maybe, but I don´t know. If the rotor hits the stops, How do you know that it is blade stall or lack of flapping angle to equalize lift?

 

[Mayfield]

Hello Colonel,

I'm having difficulty understanding your comment above. It would appear to me that there are two primary ways to change the lift of an airfoil, at a given weight and configuration. We can change AOA or we can change the speed of the airflow. Changing one requires a change in the other if we want to maintain a steady state. It sems to me that Doug and Abid have described what happens when too much airspeed is forced through the disc at too low RRPM. The rotor tries to increase lift by increasing blade AOA. Since there is not enough blade speed (RRPM), the system attempts to increase lift by increasing blade AOA. At some point the retreating blade reaches critical AOA and stalls.

I would appreciate your help in understanding why the increasing AOA at low RRPM would not result in retreating blade stall and the subsequent exceeding of flapping limits.

Regards,

Jim

 

[Tyger]

I think his point is that it is possible for the retreating blade to be hitting its stop before that blade is actually stalling...

 

[Mayfield]

Thank you Tyler. I guess I am having trouble visualizing hammering the stops without retreating blade stall.

Jim

 

[Aerofoam]

I think it may be an accelerated stall happening on the retreating blade. The blade has enough airspeed, but the sudden "apparent" increase of the retreating blade AOA, (when the advancing blade "sails") causes the stall that would not have otherwise happened if the blade had not had the sudden AOA increase.
Another way to visualize it is that when the advancing blade "sails", it has more lift, overpowers the retreating blade and pushes it downward due to the teeter. the sudden downward motion causes the separation bubble, because it is a change in relative wind, rather than an actual pitch change.
Once that stall starts, there is nothing to support the retreating blade and the advancing blade is still lifting and has upward momentum. so I can easily see the retreating blade move downward (It's also carrying downward momentum) way past it's normal flight path if there is not enough centrifugal force to maintain the normal operating plane of the disk.
This is sort of a cascade failure, this seems to be the Achilles heel of autogyros..

 

[Tyger]

I think most of us agree that the sudden "sailing" of the advancing blade is a result, not a cause, of the retreating blade stalling.

 

[WaspAir]

This is sort of a cascade failure, this seems to be the Achilles heel of autogyros..

That's part of the price of the simplicity of a teetering fixed collective rotor, not afflicting all gyros in general. Articulated systems don't suffer from this and have no requirement for rotor management.

 

[Aerofoam]

I think most of us agree that the sudden "sailing" of the advancing blade is a result, not a cause, of the retreating blade stalling.

That's a very general statement, I am digging in on the comment of a flap without a fully stalled blade.
It probably is fully stalled, but the initiation of the stall may be precipitated by the advancing blade, lack of centrifugal force and
the retreating blade may not be initially stalled.
There could be enough dis-symmetry of lift that the retreating blade although still flying, is overpowered by the advancing blade while at lower rpm's.
This sudden lift of the advancing blade causes the downward movement of the retreating blade which is a perfect prescription for starting an "Accelerated Stall".
Once a little lift is lost, it has no resistance to the downward path. So it may be possible that the advancing blade could initiate the retreating blade stall.
It's sort of a "Chicken or the egg" sort of thing, but would be easily tested with pressure sensors on a rotor to see how the exact sequence plays out.
Another interesting test would be to compare 2 similar sized rotors, one a 2 blade teeter, the other a 3 blade fixed pitch with flapping hinges.
My bet would be the teeter rotor would have a higher rate of this type of event because the advancing blade on the flapping hinge rotor would not contribute
to an accelerated stall on the retreating blade and the blade would be out of phase with that movement.
I suppose a 2 bladed flapping hub could be tested for an "Apples to Apples" comparison, but it would probably have vibration issues.

 

[Tyger]

I am afraid I disagree with you. The rotor teeters because the advancing blade would otherwise have more lift. It teeters only to the point where the lift is equalized, unless the retreating blade either hits the stop (perhaps without stalling) or actually stalls – and then, of course, hits the stop hard – because its angle of attack has become too great in the "apparent wind" (which is the only "wind" that matters). It's already been shown by Jean-Claude that centrifugal force, or lack thereof, has nothing to do with when the stops are hit.
I am not sure what you mean when you speak of an "accelerated stall" of a rotor blade. In normal airplane parlance, those are stalls that happen when under increased G loading. I don't think this is at all the case in "blade sailing" events.

 

[Mayfield]

It teeters only to the point where the lift is equalized,

Elegantly put.

 

[Aerofoam]

I think you may have missed the point, or I failed to make it..
I agree with everything you are saying in general, BUT there may be a point in the forward movement of the aircraft without enough rotor RPM, that the precipitating event is the dis-symmetry of lift BEFORE the retreating blade actually stalls.
In the moment the advancing blade is generating much more lift, the retreating blade is on the verge of stall and maybe a larger inboard portion is stalling. That would allow the advancing side to rise without restraint, the upward acceleration causing a downward acceleration on the retreating blade which is the straw that broke the camels back, it put's it into full stall and then it hits the stops.
The accelerated stall is generally G related, but a sudden downward movement of a wing in flight generates a force in the opposite direction of lift and an apparent wind at higher AOA that tends to shear off the leading edge causing the stall which is essentially what happens in an accelerated stall because the G forces cause the plane to have greater loading, and or, movement in opposition to the direction of lift.
We studied this very thoroughly about 15 years ago when we were trying to determine why flying wing sailplanes were going wonky on winch launches. It turned out that the lack of tail moment and dampening from the tail moment arm was allowing the wing to rotate too quickly,
suddenly changing the AOA before necessary airspeed, so the airplane was sort of being pulled into a stalled flat spin parallel to the ground.
This is the same movement in opposition to the direction of lift and it causes a stall on a wing that had enough airspeed to be flying.
Holding some down input on the stick for about 2 seconds allowed enough airspeed for the wing to rotate more gradually into a steep climb with better airspeed during the transition from low AOA to high AOA.

 

[mark biddle]

Well i can honestly tell you that i have been a passenger in a 160 hp dominator piloted by a very experienced friend and in demonstrating the performance of the machine he threw the gyro into a hard sudden left bank at high speed and the teeter stops were definitely hit.It sounded like a piece of tin being rapidly whacked with a small hammer.He quickly straightened up and it disappeared instantly and his comment was "whoa we don't want that".So i know you can hit those stops by putting big pressure on the rotor.

 

[ferranrosello]

I think his point is that it is possible for the retreating blade to be hitting its stop before that blade is actually stalling...

Thank you for your help. This is what I wanted to say. I think that we have a general misconception. A rotating blade stall doesn’t behave like conventional fix wing. This is because the stall happens in the 90º azimuth first. However, it is stalled only a fraction of second and then is flying again. Only when the stalling condition spreads to other blade azimuths and the stall is established in a notable retreating side sector is noticeable.

 

[ferranrosello]

I don't know if in a bad flapping take off condition the retreating blade is stalled or not. What I know is that the entire required flapping movement cannot be achieved, and consequently the dissymmetry of lift is not cancelled.