Dynamic rollover

Let's grab the blade tips of a teetering rotor and angle the disk at 15* and pull hard in opposite directions. The airframe will still hang straight down.

Now let's grab the blade tips of a rigid (or articulated) rotor and angle the disk at 15* and pull hard in opposite directions. The airframe will also go to an angle of 15*.

Because a rigid head disk can control the airframe, you can stop dynamic rollover with cyclic correction.


Hello Ed,

I could have a completely rigid rotor head and not be able to tip the Predator off its tires by pulling on the ends of the blades because the blades would flex.

Now I am even more confused.

How rigid is your rigid rotor?

It appears to me that you have now lumped rigid rotors heads and articulated rotor heads together.

Do I understand this correctly?

Do you feel that only teeter rotors have dynamic rollovers?

The rigid rotor systems I have seen still have flex in them.

They still fly the blades to the desired location.

Thank you, Vance
 
Vance,
teetering, fully articulated, rigid, they will all roll over.

Here's a picture of the Rotor head on a Bo 105. there are no flapping hinges or lead lag hinges, the flapping and lead lag of the blades occur on a relatively flexible portion of the blades themselves, just outboard of the head.

The advantage of the rigid rotor system its that it has excellent Control Power, that is the measure of how fast fuselage attitude changes with a change in rotor disc attitude. This allows the helicopter rotor system to have an effect on fuselage attitude in low G conditions.

The disadvantages are the occupants in the fuselage can get a rough ride because the disc movements are fed almost directly back into the fuselage.
also the rotor head is pretty expensive to manufacture.
And it is poor at slope landings because landing on any slope will put a strain on the main rotor mast when lowering the downslope skid on to the ground

Hope this helps remove some confusion

Paddy
 

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Vance,
teetering, fully articulated, rigid, they will all roll over.

Hope this helps remove some confusion

Paddy


Thank you Padraic,

That is what I thought but that is not how Ed sees it.

I am trying to understand the divergence of opinion.

I am seeking confusion on a higher level.

Thank you, Vance
 
Thank you for the kind words Stan.

You do just fine at explaining things.

I concur with your understanding of how the location of the hinge affects the force required to tilt the disk if you were simply tilting the mast.

I feel a swash plate pretty much manages these differences and it is why there is not much feedback with a swash plate.

I feel a rigid rotor head has some wonderful attributes as far as control response and dealing with negative Gs. I like its apparent mechanical simplicity.

My understanding of the mechanism of a dynamic rollover suggests that any helicopter with a controllable rotor disk can have a dynamic rollover, Padraic seems to concur.
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This is not a poll so that doesn’t help.

Ed wrote:

On a rigid head there is no teetering of the head in relation to the mast….the head and mast are one. If the blades are pulling out against one another at say 8000 lbs, the airframe weight is puny in comparison. If a rigid head was on the helicopter in Stan's video, left cyclic would have ripped off the skid (if the right skid was caught in asphalt) before the airframe would tip over. Nor would tip over occur with a gyro while landing in sidewind gusts, due to the power of that rotor with cyclic actuation. Of course...this is my opinion.

This does not align my opinion about how a rigid rotor head works or a dynamic rollover occurs so I am trying to reconcile these differences in our opinions.

I am trying to see through Ed’s eyes so I can understand how a rigid rotor head can make a dynamic rollover impossible.

Thank you, Vance
 
Rigid rotors have far more cyclic authority than a teetering one so it's easier to avoid reaching that critical bank angle in the first place (disregarding the mast moment indicator & mast stresses to keep things simple) - but once you reach it you're going over unless you either reduce collective or the pivot point with the ground goes away.
 
Exactly - except there's no leverage arm at all with a teetering rotor, just gravity.
 
Apparently I haven't made myself clear on dynamic roll-over. If you allow rotor rpm to decrease, the rotors centrifugal force and blade lifting power will not be able to prevent roll-over, regardless of the amount of cyclic deflection used. Also even if the rotor rpm is in the green, corrective cyclic used too late and/or not adding more collective stick for more lift too late, will allow tip-over. Stan's post #26 pretty much says what I mean…..

A rigid rotor and an articulated rotor act similarly. A rigid rotor reacts faster to control input (moving the airframe around) and its construction can be much simpler and lighter in weight.
 
Apparently I haven't made myself clear on dynamic roll-over. If you allow rotor rpm to decrease, the rotors centrifugal force and blade lifting power will not be able to prevent roll-over, regardless of the amount of cyclic deflection used. Also even if the rotor rpm is in the green, corrective cyclic used too late and/or not adding more collective stick for more lift too late, will allow tip-over. Stan's post #26 pretty much says what I mean…..

A rigid rotor and an articulated rotor act similarly. A rigid rotor reacts faster to control input (moving the airframe around) and its construction can be much simpler and lighter in weight.

I am not picking on you Ed; I am trying to discover where our fantasies are misaligned.

My understanding of a dynamic rollover does not involve decreased rotor rpm.

In my opinion a dynamic rollover involves rotor thrust that equals the helicopter weight, a skid in contact with the ground or a solid object and an inappropriate cyclic input that begins the event.

The video is a good example of these things. It appears that the skid caught on the ground and became the pivot point. He had been hovering so the lift was close to the weight of the helicopter. It appears that the pilot did not drop the collective and no matter what he did with the cyclic over she went.

I feel it very quickly exceeds the control range of the helicopter and the only remedy once the pivot around the object has begun is to drop the collective.

I was taught that increasing the collective will hasten the rollover.

In my opinion the increased control response of a rigid rotor is nice but not of much help once the pilot makes the inappropriate control input.

What am I missing?

It is a little windy, the sun is out and I am done with work.

I am off to the airport where I am going to work on the Predator for a bit and hopefully go flying. I need to find a better way to manage the cooling cowl fairings.

Thank you, Vance
 
Left skid against a post stop, so that this skid is the tip radius.
With collective down, and using cyclic stick only.

You can do this with full control using a rigid head.
You cannot do this with a teeter head.

Vance - In my opinion the increased control response of a rigid rotor is nice but not of much help once the pilot makes the inappropriate control input.
I agree 100%.
 
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When I get into a sticky area where I'm voicing my opinion to smart people like in this thread, and the voices of Doug, Chuck, AL, Pete, Chris, and many other knowledgeable people are silent….I get really concerned. I do not want to say anything that is misleading. So if my statements are incorrect, I hope one of them will speak up. Until then….I'll just assume I’m right on ;).

By the way...in the drawing above, the rotor blade bending is much exaggerated….you really wouldn't see much, especially with the blade tip path blocking the view.
 
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Once you exceed the critical bank angle the type of rotor system is irrelevant - the only thing stopping a rollover is reducing lift immediately or the pivot point disappearing, it's too late for opposite cyclic.

It's just a matter of where the rotor thrustline is vs the aircraft cg. Past that critical angle all the opposite cyclic in the world rotor thrust is still pulling you the wrong way.

You can balance a teetering rotor ship on one skid, you'll just need to get it light on the gear & have sufficient gear friction (just like the rigid rotor) - that rollover video demonstrates it perfectly in fact! The only difference is the technique, you can't just use brute force cyclic input. Can't do much of that without overstressing a rigid rotor ship anyways...

In both cases going too far will mean wadding up a perfectly good helicopter.
 
rollover will occur with any type of rotor head.

Its the moment around the arm between the point of contact and the rotor head, caused by a misaligned rotor thrust vector (Improper application of cyclic control or allowing the aircraft to drift into something like a runway light.)

once you start goin over ( around 12-15 degrees from level) you are probably too late.
the correct method of recovery is to lower the collective to reduce the rotor thrust vector, thus reducing the rolling moment.

Application of cyclic in the opposite direction to the roll will not work, regardless of rotor head type.

Ive never flown a rigid head (The Gazelle is sort of rigid but not quite), but I've flown teetering (R22, R44, B206 and B222A and B), fully articulated (TH55, S300, H500,A109C, A109E, A109S, S76B, SA342L), Starflex (AS350BA and B2) Spheriflex (EC120).
The same dynamic rollover recovery techniques are used in all of them.
 
I think everybody is correct , we happen to be discussing many different aspects regarding dynamic rollover with different rotor systems , which can make for a lot of variations.

A two blade teetering rotor has the smallest C of G range ..... so cyclic input will have the smallest effect when trying to (use cyclic) to stop a rollover.

The articulated head would have a bit more cyclic authority (at the start) of a rollover because the blade hinge points are farther out on the head. (wider C of G)

The rigid head would have the most (cyclic) advantage at the start of a rollover. There are no natural hinge points , and C of G range depends on blade stiffness (by design) and blade stiffness (caused by centrifugal force)

Another way to look at it , picture an absolutely rigid rotor (which does not exist) that is 30' in diameter. . The leverage (cyclic authority) would extend 15' and right to the tip of each blade and could have a huge C of G range. Such a system does not exist but you get the picture.

Here is another (poor) picture to add to the confusion.
 

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I think you are right Arnie….we are all correct and I agree with what you said. Your picture shows graphically what you're saying. I changed the picture in post #33 and some wording to try and get across what I wanted to point out. Place the two noted copter skids against a stop and using the cyclic only, lift the R/H skid. The rigid head can do it….the teetering head cannot do it. In my opinion, the rigid head is therefore more likely to be able to stop a rollover, whether helicopter or gyro. To bed I go....
 
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With a teetering rotor it's all related to load - no load on the rotor, no leverage on the airframe.

The mast diameter doesn't change where the rotor pivots, there is zero flapping hinge arm no matter how large or small it is.

That's why low g flight is so deadly with them, you can't control what the airframe is doing when the rotor is unloaded.
 
With a teetering rotor it's all related to load - no load on the rotor, no leverage on the airframe.

The mast diameter doesn't change where the rotor pivots, there is zero flapping hinge arm no matter how large or small it is.

That's why low g flight is so deadly with them, you can't control what the airframe is doing when the rotor is unloaded.

That's the way I see it too Brett....whether in positive or negative G's, with a rigid head and enough rotor rpm, you have control of both the disk and the airframe.....
 
Look at it like this - with no weight there's nothing to apply leverage to with a teetering rotor, since it pivots at center.

It's one of these things that would be easier to see with a model, I'm not the best in the world at describing things.
 
Very good read Stan.....I wish he would have delved further into articulated and rigid rotor actions involving low G loading and their correct control input. Although he did say that these problems only involved 2-bladed teeter rotors.
 
In Nam the choppers did a lot of low level contour flying and many came back to base with bump marks and dents on the mast. This was a very good bump prevention video....thanks Kai. I like rigid ;).
 
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