geometric imbalance in semi rigid rotor?

[helipilotnz]

my final cpl exam is soon (P o F) and was asked, what is used to overcome the geometric imbalance in a semi rigid rotor system?
the answer was given as an underslung yoke. i would like to know how it does this, if it doesn't have a lead/lag hinge and the blade is ment to flex.
thanks for any help
helipilotnz

[vorticey]

when a blade flaps up it's weight gets closer to the spinning centerline (mast) and it has less centrefugil force, this is fine when the blades cone together, but if they're tipped forward with cyclic the rear blade will be higher and closer to the mast centre line than the front one. so every time one blade goes up while the other goes down, there would be an unbalance tug. an underslung head transfers the head centre (c of g) away from the unbalance tug to help the situation.
others will say that this geometric imbalance is the hooks joint effect and the under slinging balances this out, i cant see that being right. hope this helps, you'll see heaps of threads on this stuff if you search.:ok:

[Winnie]

I think the answer you are looking for is this:
With the underslung system, there will be no imbalance, because the underslinging actually makes on blade flap up, one down, and the center of mass for both blades stay at the same location, thus preventing any coriolis effect to take place. On the underslung system, the mast attachment is on top (jesus nut) while the blades are actually attached below that spot.

Hope it helps somewhat.:)

[PPRUNE FAN#1]

To Helipilotnz:

Vorticey got it right, but Winnie's response kind of confused me. The subject of underslung rotors is admittedly hard to understand. Or maybe it's just hard to explain. But even some very experienced pilots I've met pretended to know the "why's" and "how's" whilst they really didn't. Let's see if I can clear it up a little.

Preface the discussion with the realization that we are talking about the blades with respect to the mast only, not anything else. To help visualize this, get a piece of paper. Now draw a vertical mast and a horizontal rotor with the teetering hinge at their intersection. Now draw an arc above the helicopter, from blade tip to blade tip, centered on the teetering hinge.

If the blades did not cone, there would be no problem, but they do. In flight, the blades will be coned. That is, they will be "bent" upward from that horizontal axis that is perpendicular to the mast. You can't avoid it. As both blades are coned, they are "shorter" than if they were not coned. Look at your picture of the rotor in profile to see what I mean. Unconed, the blades may have a diameter of 12 meters. Coned, they rotor diameter may only be, say, 11.9m. A small but meaningful difference.

Remember, there is no undersling at all in our imaginary system. Now, tilt the coned rotor disk with respect to the mast. See what happens? One blade effectively becomes longer as it flaps down toward true perpendicular. The other blade becomes even shorter as it flaps further upward! This creates an imbalance in weight toward the longer side. How do we accommodate this? Simple!

If we locate the flapping hinge so that it is above the blades themselves, then the whole rotor system will move back and forth as it flaps/teeters, much like a pendulum. Now, as our coned rotor system tilts with cyclic inputs, one blade actually does get a little shorter whilst the other blade physically gets a little longer.

Stand up on the roof of a Bell 206, pull the droop stops down, and look right at the hub as you flap the rotor from one extreme to the other. You'll be amazed at how much the rotor moves laterally.

So!

If the center of mass of the individual blades does not change cyclically, then there is no need for a lead/lag hinge. That should be evident, because in a two-blade system, if one blade leads, the other must lead too. But in a multi-blade system, each blade flaps independently of the others, and the blade has no undersling. As it flaps, it speeds up and slows down, requiring that extra hinge, whether it be a mechanical one (A109), or a flexible hub (AS350), or limber blades (BO105).

Actually, all of the above is fairly useless rot. It won't help you fly the helicopter any better to know any of it. But it's interesting, I suppose, since we pilots are usually the type to wonder how things work. (Well, that and the fact that they ask us these silly and irrelevant test questions about it.) I know I personally have all sorts of disassembled mechanical bits littering my past. I can take a transmission apart like a pro! But...umm...putting it all back together again? Well, that's another story. I'll stick to flying the bl**dy things, thank you.

[helipilotnz]

thanks for the information but i dont think that is what i am after, i probably am loosing it but a previous question was, which hinge compensates for geometric imbalance? the answer was a dragging hinge. so then was thinking that geometric imbalance would be like looking down on a three bladed system and not all the angles being 120 deg and so moved about the drag hinge. the next question was asked(as above) and as a semi rigid rotor system could in theory have three blades i wondered how the blades in an underslung system would move back to the 120 deg position if it does not have the facility to drag? i might be reading to much into it so i will crawl under a rock now and stay quiet. thanks for your help though.
helipilotnz

[Lu Zuckerman]

The explanations given above are basically correct relative to the underslinging of the rotor system to minimize the tendency to lead and lag. The blades do lead and lag but only slightly because the design is not perfect. On some Bell blades you will note a drag link which reacts the tendency to lead and lag and transmits these loads into the rotorhead.

When you get to the Robinson rotor system all of the explanations relative to underslinging go all to hell. The Robinson head is underslung but the blades are free to cone or flap independently which results in leading and lagging. The blades flex in plane and the loads are reacted by the coning hinges. When a high time Robinson head is removed for overhaul or sooner the cone hinges are worn elliptically.

Regarding a three-blade semi rigid head you don’t have to look any further than the V-22. The Proprotors are mounted to the drive shaft on a rubber bearing and this bearing works like a constant velocity joint and as such the blades do not lead and lag but they are free to flap.

:cool:

[Dave_Jackson]

Lu,

Three blade semi rigid rotor !!! (http://www.unicopter.com/temporary/Dragonfly_Rotor_Pics.html) :rolleyes:


Well --- hopefully, in the near future. :8

[BlenderPilot]

Can any pilots out there tell me if knowing this stuff has ever been useful while flying around in a 206 for example?

I used to know all this stuff (and more) and now I just want to forget all of it, knowing this stuff just confused my learing process and distracted me from the more important stuff.

[vorticey]

the questions are the same:
what is used to overcome the geometric imbalance in a semi rigid rotor system? underslung yoke overcomes or compensates for geometric imbalance.

which hinge compensates for geometric imbalance? a dragging hinge?? i would have got this one wrong, a dragging hinge just lets the blade move for and aft with coriolis effect, i dont think it compensates for anything. :confused:

tip:- answer the questions with the answers they want!

[Lu Zuckerman]

To: Dave Jackson

Not only is there a Santa Clause there is a three blade semi rigid rotorhead. In order to get lead and lag the two significant axes must deviate from each other. The first axis is the driving axis. When an articulated rotor system is in the neutral or non-flight condition the driven axis is coincident with the driving axis. With the input of cyclic in any direction the driven axis will deviate from the driving axis and the so called Hookes joint effect takes place and the blades speed up and slow down in each rotation (lead and lag).

On the Proprotor used on the V-22 the rotor head is supported by a rubber bearing that acts like a constant velocity joint. In this type of joint the two axes remain coincident with each other resulting in no lead and lag. The blades are free to flap but since the two axes remain coincident with each other the flapping does not result in lead and lag. If lead and lag takes place the blades are restrained.

:cool:

[Dave_Jackson]

BlenderPilot,

You're probably right. The only time that this lead/lag stuff would be of interest to the pilot is when there's a thump, and the blade is leading or lagging the helicopter by a few hundred feet. :uhoh:

Lu,

Thanks for your remarks about the gimballed rotor and lead/lag, on the V-22. A search on this subject came up with a paper that was presented in an AHS forum last month. LOAD ALLEVIATION IN TILT ROTOR AIRCRAFT THROUGH ACTIVE CONTROL; (http://pcwww.liv.ac.uk/eweb/fst/AHSSLA_feb11.pdf) It discusses the in-plane loads, which the authors feel is one of the top four concerns with the tilt rotor aircraft.

It should make for interesting reading, particularly when trying to envision of act of blade flapping on a teetering hub and an articulated hub, versus a gimballed hub. :hmm:

[Lu Zuckerman]

To: Dave Jackson

I didn’t get past the first paragraph mainly because I would most likely not understand the following paragraphs but my main reason for not going any further in the text was that the research that went into the paper dealt with the XV-15. Probably every problem listed in the text of the paper has most likely been overcome in the design of the V-22 Proprotor. The Proprotor has a load alleviation system built in that senses blade flapping and compensates with a computer controlled cyclic input to counter the flapping. The way I understand it the flapping is not like that of a helicopter in that the blades flap up and down. The Proprotor as I understand it flaps resulting in the tilting of the disc. If one disc tilts in relation to the other there is a difference in the thrust vector of the two Proprotors while in the helicopter mode. This is especially bad when in the airplane mode and maneuvering causes precession of the two Proprotors with the precessing being in opposition to each other thus altering the thrust vectors of the props.

:ooh:

[Dave_Jackson]

Lu,

The answer is so simple, it's a wonder that the gurus have not discovered it. Virtually all the unacceptable in-plane forces are the result of blade flapping. The solution is just ~ stop the flapping. Mind you, that brings up the heretical subject of Absolutely Rigid Rotors. http://WWW.UNICOPTER.COM/HILARIOUS.GIF


For what it's worth, which is absolutely nothing, my 3-blade offset teetering rotor has lead/lag hinges at each blade. One novel feature is that the three blades are linked so that each blade's lead/lag angle is interdependent upon the lead/lag angle of the other two blades. The sum of the three lead/lag angles always equal zero. Trigonometrically, it looks ideal for the function of rotor flapping.

Time will tell if the rotor moves slowly upward, or, its bits and pieces move quickly sideways, all over the field. :{

[vorticey]

a constant velocity joint at the end of the mast would remove all the need for lead lag hinges. ;)

[Dave_Jackson]

vorticey,

Both you and Lu are saying that the use of a gimbal in the center of the hub will eliminate, or significantly reduce, in-plane loads. In other words, the substitution of a CV joint for a knuckle (Hooke's) joint uncouples the flapping from the lead/lag. I guess that this is also the position of the V-22 designers.

But, it raises an interesting paradox. The previously mentioned report, in discussing the V-22, states; "For gimballed rotors it can be shown that the one-per-rev. rotor yoke in-plane, or chordwise, bending moments are directly related to the out-of-plane (flap) moments on the rotor.'

It is also interesting to consider that Doman used a constant velocity joint in his LZ-1A helicopter. (http://avia.russian.ee/vertigo/doman_lz-1a-r.html ) This development apparently went no further than the operational prototype.

Both the V-22 and the LZ-1A have pre-coning in their hubs. Perhaps the fact that the rotors' centers of mass and thrust are not concentric with the gimbal joint (ie. they are a hypothetical undersling distance above the gimbal) may have something to do with it. :confused: :confused: :confused:

[Lu Zuckerman]

Although the following deals with Cardan joints and CV joints the situation is the same for conventional articulated rotorheads (Universal joint) and a Constant Velocity Joint rotorhead. The text deals with the application of CV joints on front drive automobiles.

Regular single Cardan universal joints can't do the job because they have a troublesome characteristic: When operating at any appreciable angle, and with the input side turning at a constant rpm, the output side speeds up and slows down twice each revolution. Since the amount of this velocity change increases with the angle, the range of movement encountered during suspension action and steering in a FWD design would put plain joints far beyond their capacity, and result in a great deal of torsional vibration. So, more complicated and expensive CV joints are necessary -- they get their name from the fact that no matter what the angle, the input and output shafts always spin at identical velocities throughout each revolution.

The angular difference on the Cardan joint is the same as the deviation between the driven and driving axis on a rotor system. The greater the difference the greater the lead and lag.

:{ When will it end.

I posted this some time ago:

Explaining leading and lagging is probably the most difficult thing to do. I have read several books on helicopter flight theory that have you standing in space and looking down on the spinning rotor disc. In one example you are standing above the helicopter and looking down at the spinning disc when only collective pitch has been added and discounting any built in bias that counters the propeller effect of the tail rotor. In this position looking down you would see that the blades are equally spaced like a plus sign or cross. If you stayed in this position and forward cyclic (any cyclic input) was added you would see that the blades are no longer equally spaced. Instead of a plus sign or cross (equally spaced) the blades take the shape of a peace sign. The advancing blade is slightly ahead of the radial position and the retreating blade is slightly behind the radial position and the other two blades (assuming a four-blade rotor head) are in the radial position.

The authors of the book will then have you standing in space and looking straight into the tilted disc, which previously had the advancing and retreating blades, displaced from the radial position and they tell you that in this position the blades are equally spaced. That’s what I meant by confusing.

Another way is to try to visualize the hovering disc in picture form and superimposing the tilted disc over it. The hovering disc appears as a circle as viewed on paper. The tilted disc appears as an ellipse. Both the hovering disc and the tilted disc have a point of rotation. The tilted disc rotational point is slightly ahead of the hovering disc point of rotation. Now the confusion factor comes back into play. Draw the ellipse over the circle and displace the two points of rotation. Draw the four blades in the radial position on the hovering disc. Now, establish the centerline on the tilted disc and make a point or dot on the circumference of the ellipse at the point where the centerlines intersect. Now, draw a line from the center of the hovering disc to each of the points you made on the circumference of the tilted disc. Voila you have the peace sign.

The reason this happens is the law of conservation of angular momentum. The blades are being driven around the point of rotation of the rotor mast at a given speed. When you tilted the disc the blades wanted to not only rotate about the tilted disc centerline they want also to be driven about that same centerline. Since the blades are anchored at the rotor head the lead lag hinge allows the tips of the blade to move while still being the blade root remains anchored. The forces will cause the advancing blade to lead and on the other side the retreating blade will want to stay at that position and it lags until the mechanical forces exerted by the rotor head bring it back to the radial position and as the individual blades rotate about the disc the process is repeated at twice the speed of the rotation speed. That’s why when you have a bad damper you get a two per rev.

Now that you are thoroughly confused let’s talk about the constant velocity joint. Are you familiar with a Hookes Joint or a common universal joint? On this type of a joint the rotational speed of the input and the output are the same. If you displace the joint so that the points of rotation of the input and output are not coincident with each other the out put rotational speed will increase and decrease with each rotation. The greater the increase in deflection the greater the difference in rotational speed increase and decrease. This will increase to a point where the joint will lock up. On a constant velocity joint this does not happen. Although there are operational limitations on the amount of angular displacement there is no difference between input vs. output rotational speed.

Imagine, the normal articulated rotor head as a Hookes Joint, where there is a speed differential when a deflection takes place. The Proprotor, on the Tilt Rotor aircraft, has a constant velocity joint in the rotor drive system, and there is no difference in rotational speed, because the drive and rotation, are coincident with each other.

I’m starting to foam at the mouth so I had better end this now.


Here is the make-up of the Proprotor:

The dynamic elements of the Proprotor that allow and control movement are:

1) CENTRIFUGAL FORCE BEARING: Reacts the main centrifugal force of the proprotor hub/blade system axially. Also accommodates blade pitch motion torsionally. This is like the elastomeric bearing in a S76 or in a UH60.

2) INBOARD BEAM: Accommodates pitch motion of the blade torsionally, reacts in-plane (lead-lag) loads radially and out-of plane(flapping) loads/motion by cocking. It is part of the proprotor blade pitch change system.

3) FEATHERING BEARING: Used in conjunction with the inboard beam pitch change bearing to share the torsional pitch motions of the blade pitch change system.

4) OUTBOARD SPINDLE: Part of the blade pitch change system. Handles the majority of the blade pitch motion in the torsional direction, at the same time reacting in-plane (lead-lag) and out-of plane (Flapping loads.

5) LOWER PITCH LINK ROD END BEARING: Transmits motion from the swashplate to the pitch change system. It also reacts control loads radially and accommodates pitch change motions torsionally.

6) GIMBAL BEARING: Part of the swashplate system. This bearing reacts control and aerodynamic pitch link transmitted loads radially and axially, and accommodates the tilt of the swashplate torsionally.

7) LINK COUPLING: Transmits torque from the drive shaft to the hub/proprotor blades, while accommodating angular misalignment from the drive shaft to the hub.

8) HUB SPRING SET: Large spherical bearing which react the thrust (axial) load of the proprotor and accommodates angular misalignment of the hub resulting from proprotor disc tilting. THIS IS THE CONSTANT VELOCITY JOINT.

Now I'm really foaming at the mouth.


:uhoh:

[helipilotnz]

i sat the exam on wednesday and didnt get an inbalance/underslug question. the information is still relevant to me as i will be starting a C cat rating in a couple of weeks and need to know P of F really well. i wont know the results for a week but think i have done enough for a pass.
thanks again
helipilotnz

[Dave_Jackson]

$hit Lu. You didn't have to write a book. :oh:

The various reasons for lead/lag hinges were very concisely posted on Eng-Tips last year by Mr. airtabman. This gentleman occasionally posts on this forum, under the pseudonym of Shawn Coyle.

Few would argue the Constant Velocity Joint doesn't do what its name implies. The intriguing part, to me at least, is that since the Proprotor is mounted on a constant velocity joint, why does its flapping generate lead/lag?


"Now I'm really foaming at the mouth."

Lu; you've got to layoff the beer while typing. :D

[vorticey]

i read it last time lu! (im also a motor vehicle machanic) but daves write, why is there lead lag and out of plane flapping with this cv joint?

[Lu Zuckerman]

As I had indicated previously the flapping has to do with the rotor system tilting out of plane, which will change the thrust line of the rotor. There are sensors on the Proprotor which send a signal via the electronic control system to the servos, which move the swashplate to correct the tilting. Mechanical linkages in the Proprotor restrain flapping of individual blades.

If there are mechanical deficiencies in the constant velocity joint they would be minimal and mechanical linkages in the Proprotor restrain any tendency for lead and lag. For the same reason Bell places drag links on some of their blades to react to any tendency for the blades to lead and lag on an underslung rotor system.

When the V-22 was first placed on flight status the Marine pilots were afraid of the fly-by-wire control system so in order to move forward they would tilt the engine/Proprotor three degrees forward. In this case there is no deviation between the driving and driven axes so even with any perceived mechanical deficiencies there is no lead and lag. This is still done. In order to build up forward speed for transition to the airplane mode the engine/Proprotor is tilted even further so the same condition exists. It would appear that the only time the V-22 is flown like a helicopter is to move laterally near the ground. Based on past experience the V-22 behaves very badly when flown as a helicopter at altitude.

IMHO