Dynamics ~ 2 Blade Rotor w/ Offset Flapping Hinges

[Dave Jackson]

Rotors with offset flapping hinges are said to have better cyclic response and less chance of tailboom incursion then teetering rotors.

The following sketch shows;
A/ a conventional teetering hub, and
B/ an idea for a 2-blade rotor with offset flapping hinges.



Any comments as to the pros and cons of such a hub (sketch B) will be much appreciated.
In addition, does anyone know if this idea has ever been tried before?


Thanks
Dave J.

[heedm]

Not my field, Dave, but it seems to me that the elbow below the flapping hinge would undergo extreme stresses.

What are you hoping to accomplish by having this elbow rather than a straight blade root with the same offset?

[Dave Jackson]

Hi heedm,

In a teetering rotor, the undersling causes the CofG of each blade and the center of the teetering hinge to be inline, when hovering at GW. The reasoning for the elbows in sketch B/ is to create the same situation with the flapping rotor, and thereby eliminate the need for lead-lag hinges. [hopefully]

Dave J.

[helmet fire]

Good idea on the face it Dave, but I think you are confusing teetering wityh flapping. The Bell UH1 two bladed system, for example, teeters like the first sketch, ie the same as a "see-saw" might. However, it flaps like the second sketch (minus the elbows), th4e flapping occuring from the end of the hub tangs and along the blade itself. Even two bladed systems (though I know Lu disagrees here) allow individual blades to flap independantly of the teeter.

...................least I think so!

[Irlandés]

Helmet fire,
I'm not sure if I agree with that. Teetering is a form of flapping. Very often the coning hinges in teetering systems (if they are present) are mistaken for offset flapping hinges. I don't know too much about the Huey, but on the R22, the offset hinges take care of the coning and the teetering hinge takes care of the flapping.

Irlandés

[[email protected]]

DJ, I think both designs are rather old fashioned compared to what is available nowadays. Modern composite technology allows you to build a very efficient rotor head without mechanical hinges. You can have as much hinge offset (within reason) as you want and still keep the design simple, light and relatively maintenance free.

[Nick Lappos]

DJ,
What you sketch is physically equivilent to a normal articulated rotor head, with the added stress riser (weight increase) of the bend in the hub, which sees awesome CF loads (yes Lu, CF!).

The lag hinges are needed for the hook's joint issue as well as the need to reduce vibrations due to unequal blade drag during the blade's trip around the mast. Because of the hinge offset, the shear forces and moments in the lag axis are amplified and must be relieved.

[Lu Zuckerman]

Here we go again.

What you sketch is physically equivilent to a normal articulated rotor head, with the added stress riser (weight increase) of the bend in the hub, which sees awesome CF loads (yes Lu, CF!).

Thank you Nick for the mention of CF. However I do not believe that the sketch is similar to an articulated rotor system, as it is underslung like a Bell. Most flex rotors that I know of are not underslung.

On conventional underslung rotor head the drive axis and rotating axis are nearly coincident with each other which minimizes leading and lagging. On a conventional flex rotor when forward cyclic is introduced the disc rises at the rear and drops at the front. Assuming optimal conditions, when this rotor system is in a hover the drive axis and rotating axis are the same with minimal lead and lag. But when the disc is tilted the two axes separate with the driving axis remaining coincident with the mast and the driven or rotating axis is forward of the mast centerline. It is this difference that causes leading and lagging. The greater the difference the greater the amount of lead and lag.

If Dave’s sketch were to be put into practice the in plane bending loads resulting from the lead and lag would eventually result in failure or very high maintenance. The Robinson head even though it can teeter has very high lead lag loads which manifests itself in very high wear on the cone/flap bearings.


Ducking for incoming.

[Dave Jackson]

crab,

This simplistic idea is intended for the recreational (homebuilt) helicopter. The objective is to provide low hour pilots with a slightly more responsive cyclic control, then they now get from a teetering rotor.

Just trying to reduce the 'safe ~ homebuilt' oxymoron.


Nick,

Thanks for the technical critique. To clarify the idea, sketch C/ is just a slight elaboration of the previous sketch B/



The 'undersling' on sketch C/ is what hopefully differentiates this hub from that of a normally articulated rotor head. I believe that the teetering rotor works because the two blades' centers (of mass, lift, drag percussion etc. etc.) are basically inline with the underslung teetering hinge, during normal flight conditions. This condition also exists in sketch C/.

In addition, if one blade on sketch C/ flaps up 5-degrees and the other blade flaps down 5-degrees, this centerline still passes through the 'Virtual center'. Therefor synchronous flapping on rotor C/ should be the same as the teetering on rotor A/. Many moons ago you mentioned that the mast absorbs the small amount of lead-lag in a conventional teetering rotor. Theoretically, this should apply to sketch C/, as well.


I wonder if there might be a vibrational problem. The two-blade configuration may result in a 2P vibration when the rotor disk is not normal to the mast, whereas three or more blades should not.

[helmet fire]

Irlandes:
You are correct in that teetering can also be a form of flapping, but the point I was making is that flapping can, and does, occur without any teeter, and occurs offset from the teeter point even in some two bladed systems. The point you make about conning hinges is well made, however the Heuy has a preset cone in the hub and no hinges for this purpose.

Dave:

I think you are continuing to ignore the flapping issue. As above, the blade can flap independantly of teeter, thus when one blade goes up, the other doesn't just go down the same amount as per your discussion. This means that the centre of lift, etc, will not as you theorise, stay aligned. An easy way to see this is the conning angle increase , which is not flapping but it shows that both blades can come up at the same time and demonstrates that the blades can move up and down independantly of teeter. Thus you enter the lead lag issues mentioned by Nick courtesy of Hooke's, etc. I did not know that the rotor mast absorbs lead lag, and when I think of that small Huey mast absorbing all that, I shudder. Luckily on the Huey they do have drag braces to help absorb the forces.

[Lu Zuckerman]

Once again, into the breach.

To: helmet fire:

I did not know that the rotor mast absorbs lead lag, and when I think of that small Huey mast absorbing all that, I shudder. Luckily on the Huey they do have drag braces to help absorb the forces.

If you remember in my past discussions on the Robbie head I mentioned that the lead lag loads were reacted first by the cone hinges and transmitted into the rotorhead and then to the fuselage via the mast. The mast on any type of a semi rigid rotor that has a tendency to lead and lag is one of the reaction and transmittal points in the dynamic system and the drive train. All loads are zeroed out in the fuselage and must pass from the point of load origination through whatever mechanical device is in line with the load dissipation including the transmission. These cyclical loads could even be reacted by the engine.

Regarding the drag braces on the Huey, they absorb the loads and transmit them from the blade to the rotorhead and then to the mast and ultimately the fuselage as indicated above. The Huey rotorhead has minimal lead and lag because it is underslung so any transmitted loads are of a low magnitude.

[Dave Jackson]

Lu,

Agreed; this rotor will have to be 'beefed up' at the cost of some additional weigh.


helmut fire,

The situation is probably not all that bad.

The amount of undersling and the static pre-cone angle are selected to suit the mean coning angle of the rotor. A change in the active coning angle on both sketch A/ and on sketch C/ will cause the CG of the two blades to come out of alignment with their (virtual) teetering hinges. In other words, this is a problem for the conventional teetering rotor as well as 'idea C/'.

Assume a hypothetical 2-blade articulated rotor head, which is flying with a coning angle or +4 degrees on both blades. If blade #1 flaps up 3 degrees and blade #2 flaps down 3 degrees then blade #1 has a total angle of +7 degrees and blade #2 has a total angle of +1 degree, from the normal plane of the disk. This means that the 'centers' of blade #1 are closer to the disk's axis of rotation then are the 'centers' of blade #2. Hence, the need for lead-lag hinges. With an underslung teetering rotor, this problem of the blade 'centers' moving toward and away from the mast is much reduced.

The Hooke's Joint effect is not a major problem because the two blades are accelerating and decelerating in unison. Of course, a rotor with more than two blades will have a problem with the Hooke's Joint effect and must have a provision for lead-lag.

[Nick Lappos]

Lu and DJ,
The idea that the structural arrangement with an elbow somehow makes it behave differently (underslung) is not true. If you have two points connected by a speghetti strand of loops, whirls and spirals, these things make the structural analysis of that part complex, but the rotor head behaves as if all that stuff were just a connection. The head DJ sketches will have some precone in it, because when it spins up, the blades will flap up slightly to allow the centroid of the blade to align with the support hinge. That is it. Not much magic there.

The underslung head is somewhat unstable, but the very small divergent tendency, where a slight flapping will lead the mast to create more flapping, is not significant for such small angular contributions.

DJ also writes "Rotors with offset flapping hinges are said to have better cyclic response" No, it is not just said, it is a fact.

That is one interesting aspect of posting facts and opinions, there tends to be a concept that everything is an opinion. Teetering rotors have relatively weak cyclic control, articulated and bearingless rotors have very much better, faster and more powerful control. This is said, but it is also measured and factual.

[Dave Jackson]

Thanks guys, and particularly Nick.

You're right, the idea is flawed. Thank God it was submitted as an idea, for discussion, and not as an irrefutable fact.

Perhaps somewhere out there, is the means to provide better cyclic control without have to incur the complexities of an articulated rotor or the weight of an absolutely rigid rotor. Perhaps it's the hub spring that Nick mentioned about a year ago.

[Dave Jackson]

All bets are off.

You guys mentioned, correctly, that in sketch [B] and [C] the line between the blade centroids will be above the virtual hinge when the helicopter is operating at mean thrust.

Thanks again for finding fault and instigating a revision.
Please (do not ) find fault with sketch [D]



This configuration is identical to [B] and [C], except that there is now a Tie bar, which holds the two yokes at the pre-cone angle.

Should one blade flap (teeter) up then the other blade will flap (teeter) down. This is now operationally identical to the conventional teetering rotor [A]. In other words, under mean thrust, the line between the blade centroids will pass through the teetering center on [A] and through the virtual center on [D]. Even when disks [A] and [D] are tipped, this line will still pass through the center of both configurations, under mean thrust.

As with [B] and [C] the offsets in [D] will result in a more powerful cyclic control then that of [A].

[vorticey]

dave

just a theory on the tetering hinge. i dont think there are any lead and lag forces on a teetering head because the spining axis (head) is ALWAYS parallel with the tip path plane. one blade just doesnt go up and one comes down, THE CONE TILTS. the forces you say are absorbed by the mast would be hunting motion of a universal joint. also the head would be under slung to counter the weight shift from the cone tilting to one side.

[Dave Jackson]

vorticey,

When the tip path plane is not parallel to the mast plane, there will be a 2 per revolution speed variation between the two planes. This is because of the Hookes Joint Effect. This may be envisioned by considering that the tip path plane is attached to one end of the Hookes joint and the mast plane is attached to the other end. In addition, profile drag will vary as the rotor turns. The mast absorbs this rotational oscillation in most helicopters with teetering rotors.

There is no strong reason for lead-lag hinges in a 2-blade rotor because the two blades accelerate and decelerate together.

An exception to the above is the Kaman helicopter. It has two mechanically interconnected teetering rotors, for a total of four blades. Because of this interconnection, the helicopter has lead-lag and delta-3.

You are correct in regard to the weight shift and therefor the need for undersling.

[Shawn Coyle]

Dave:
a) Quick - get a patent on it!
b) what about the R-22 hub- it has flapping hinges as well as a teetering hinge, doesn't it?

[heedm]

Dave, one concern that I don't think has been mentioned is that with a hinge offset you generate a mast moment. I believe this is one of the reasons the response is more crisp, but it comes with a penalty...the mast must be stronger.

Must admit though, plan D does seem to be worth further investigation.

[Dave Jackson]

Hi Shawn,

a) ~ A 2-blade articulated rotor is probably going to experiences 2P vibration at advanced cyclic positions. The idea in this thread has a similar objective as Bell's 'hub spring', and this may be the reason why Bell filed a couple of patents on possible means to reduce the vibration.

This idea may be OK for the intermeshing SynchroLite since this craft has two rotors with two blades each, for a total of 4 blades. This means that the vibration will have a rate more than twice that of a comparable single-roter craft and an amplitude less than half. Plus, the SynchroLIte is intended for the US Ultralight category, and by regulation, it is therefor limited to a benign velocity of 55 knots. Want to share the patent's cost and royalities?

b) ~ Yes, but this is Lu's department


heedm, thanks for the input.

You're right. The rotor hub and mast etc. must be stronger. isn't Bell experiencing a similar situation right now, with a modification to existing helicopters?

Another penalty is the highly probable 2P vibration, as mentioned.
The boring reason for this is; If the cyclic was to be pushed forward it will cause the tip path plane of the disk to tip down at the front, in respect to the mast plane. This must mean that when the blades are at 0º & 180º azimuth, the centrifugal force and hub offsets are attempting to 'torque' the mast plane into alignment with the tip path plane. This is in addition to the thrust from the rotor, which is attempting to 'drag' it into alignment. When the blades are at 90º & 270º azimuth, only the previously mentioned 'dragging' activity is taking place. This 'torque ~ no torque' will probably create a 2 per rev. vibration.