TILTROTOR PRECESSION

[rotorque]

All the talk about the R22 and gyroscopic forces etc, has made me wonder about the forces acting on the rotor systems of the tiltrotors.

Can anyone shed light on what happens when the rotorsystems tilt forward. Due to gyroscopic precession, does the reaction put stress on the attachment points of the engine pods?. Going by pictures it looks as if the forces would be acting to tilt the disc down towards the fuselage (in line with the wing spar)as the engines rotate.

Are the rotor systems controled cyclicly? Shorely if you were to force the rotors to tilt when you rotate the engines you are going to introduce some incredible forces. Is there some form of coupling that does both at the same time ie: as engines rotate there is a cyclic change as well?

I don't mind if there are different opinions about it - I would like to hear them all.

Cheers.

[Lu Zuckerman]

To: Rotorque:

You have asked several questions that require different answers for each question.

First of all the Proprotors on the tilt rotor have two different functions. One is as a helicopter rotor, which is fully articulated, and second, as a propeller which from a design standpoint is not articulated. This is not to say that it is not free to move while in a propeller mode due to precessional or flapping loads but any movement is countered by sensors in the Proprotor system which introduce changes in the swashplate position via a computer controlled hydraulic actuator system.

In the helicopter mode, the Proprotor is a fully articulated rotor, which is totally elastomeric with all rotational forces being transmitted via an elastomeric constant velocity joint. With that in mind, you have to go back to what you learned in basic helicopter rotor dynamics.

There are several forces acting on a rotor system two of which are flapping which is aerodynamic and the other is lead and lag, which are induced by physical forces called conservation of angular momentum. The flapping is introduced by changing the force balance across the rotor by the introduction of cyclic control resulting in the tilting of the rotor in the direction of cyclic movement. This was discussed on the R22 forum. The only time a rotor system will lead and lag on a conventional articulated rotor system is when the rotor disc is tilted and the center of rotation is shifted from the center of drive. The greater the shift the more pronounced the leading and lagging. All of these movements are accommodated by the elastomeric elements in the Proprotor. Do you remember what I said about the elastomeric constant velocity joint in the Proprotor drive system? Well, on a constant velocity joint, when there is an angular shift in the drive axis, and the driven axis, there in effect is no change between the driving axis and the rotational axis,(that's why they call it a constant velocity joint) essentially minimizing if not totally eliminating the laws of conservation of angular momentum, keeping leading and lagging to a minimum.

Here is where my understanding falters. I know that when the nacelles are tilted forward the helicopter control mode is locked out but I don’t know at what angle of tilt this occurs. In any case I believe your question about Precessional (Read gyroscopic) forces do cause the rotor to change thrust vectors. The same thing is true when in an airplane mode. The thrust vectors will change when the Tiltrotor is banked or maneuvered in any way. But, there are the flapping sensors I mentioned above which detect any movement from the normal rotational plane. These sensors cause the swash plates to move countering the precessional/gyroscopic forces keeping the Proprotors in the desired plane of rotation.

As far as mechanical loads being applied to the engine pod during tilting or, for that matter, when the Proprotors are in the helicopter or airplane mode, these loads are reacted by the Proprotor drive shaft in the same way as that by a helicopter rotor shaft.

Regarding the cyclic input during transition and in the aircraft mode the answer is yes. But it is introduced by the computer and not the pilot and this action is initiated by the flapping sensors.

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The Cat

[This message has been edited by Lu Zuckerman (edited 16 October 2000).]

[This message has been edited by Lu Zuckerman (edited 16 October 2000).]

[This message has been edited by Lu Zuckerman (edited 16 October 2000).]

[rotorque]

Thanks mate, great response.

The only thing that I couldn't quite grasp was the constand velocity joint. How can you maintain the drive axis in line with the axis of rotation if you use any form of cyclic control. I would of thought the only way to do that would be to have the engine drive shaft directly in line with the rotor thrust at all times. Which I guess is right once the proprotors are locked from the cyclic input. ??

rotorque

[Lu Zuckerman]

To: Rotorque


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. The point of the drive center and rotational center are no longer coincident with each other. Since the blades are anchored at the rotor head the lead lag hinge allows the tips of the blade to move while 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 four times the speed of the rotation speed. That’s why when you have a bad damper you get a four 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 is slightly lower that the input speed. The greater the increase in deflection the greater the difference in rotational speed. This difference in speed 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. That's why on a front end drive car they have a constant velocity joint at each end of the drive shaft.

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 up to the mechanical limits of the joint.


I’m starting to foam at the mouth so I had better end this now. If this is still confusing, I can use another analogy but I'm afraid you might not be familiar with it unless you have been to a very large amusement park. Let me know.


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The Cat

[This message has been edited by Lu Zuckerman (edited 17 October 2000).]

[tiltrotor]

I haven't really had the time to thouroughly read through the postings, but lead and lag on a rotor is pretty simple to explain.

Think of a rotor blade, just like any other object it has a center of mass. Now spin it, like an ice-scater. The center of mass will stay at a certain distance from the center of rotation (rotormast) thus, the blades will rotate at a constant speed. However, as flapping is introduced to the blades, the up and down movement will cause the blades' center of masses to move closer/ further away from the center of rotation along its chordwise span. Caused by this, the blade speeds up or slowes down- lead and lag this is also known as coriolis effect.

Easy- like figure scaters- as they move their arms close to their body, they incredibly increase speed.

FOR ROB: NOPE IT WASN'T ME WHO STARTED!!!

Aerodynamic forces have to be quite similar on the nacelles of the tiltrotor, however, to what extent is a good question. i would guess that the forces are less pronounced then on a conventional rotor, basically just because of it's smaller size. I may be wrong in my guess.

[Lu Zuckerman]

That's another way of describing it and probably more easy to follow. However In my answer I was addressing a constant velocity joint vs. a Hookes Joint. In any case whether you use my explanation or yours it still comes down to the CONSERVATION OF ANGULAR MOMENTUM.

[This message has been edited by Lu Zuckerman (edited 19 October 2000).]

[FlyAny]

At about 45 degree nacelle angle the flapping controller inhibits flapping. Pitch change is done thereafter through elevator control. This because as you guessed, flapping at high speeds imparts high blade loads

[tiltrotor]

The Cat:

This place isn't about loosing or winning and I can certainly tell you that I am not out here for a "I have to win and be right" battle. I am sure you know stuff that many of us don't know just like the other way around.

But this being a public forum should give anybody the right to benefit from it, so I guess the simpler an explanation is, the more people will understand. Certainly not everybody does understand the technical apsects and highly important technical terms you throw in.

My apologies, this is not ment as a personal attack towards you, I am just trying to make it possible for everybody who comes here to understand stuff and learn from it.

[Lu Zuckerman]

Dear Tilt,

I have edited out that statement. It was not meant to offend or to demean anyone. I put it in there to indicate that your input was correct in describing lead lag and my input was (Hopefully correct in describing a constant velocity joint).

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The Cat

[This message has been edited by Lu Zuckerman (edited 19 October 2000).]

[rotorque]

Hey guys,
Just got back and have read the feedback.

The ice-skater analogy is the same one I was taught and the idea of lead and lag is not too daunting if explainded that way, although to go into it any deeper I guess we need to look at the vectors etc. Lu you probably have a unique ability to 'see' vectors etc as a normal process.

The question that keeps popping into my mind is How does a constant velocity joint work? You did well explaining the uni joint idea but I just can't grasp the fact that you can even get constant velocity once you have changed the axis (lamens terms)of drive. As you mentioned, output RPM decreases once the axis is no longer inline. How in the world do they do that?. Is it a fluid filled thing or is it a gearing system that allows constant velocity.
It intrigues me !!!

[Lu Zuckerman]

To: Rotorque,

I contacted several manufacturers of constant velocity joints and asked for the operating principles. I also have several drawings which I downloaded from the internet. These will be sent to you via email.

Once I have that material I think I can better explain the constant velocity drive in the V22 Proprotor.

Here is a bit I found on the internet although it doesn't say much.

Constant Velocity Joints (CV Joints)

Front wheel drive cars need u-joints which not only allow up and down motion, but steering motion as well. the angle at which they turn requires a different design than the standard U-joint. Constant velocity, or CV joints are universal joints that are able to transfer torque at large angles efficiently. These joints transfer power very smoothly. They are comprised of four basic parts: 1. The outer section, which has grooves machined on its inner surface, 2. the bearings, which are usually in a "cage", 3. the inner ball, which has grooves on its outer surface for the bearings to ride in, and 4. a rubber boot to protect the unit from dirt and moisture.

A common cause of CV joint failure is cracks in the CV boot. As dirt enters the CV joint, its parts grind themselves until a clicking noise is heard when turning, or until they fail completely. The boots should be replaced as soon as cracking is visible in their rubber folds.

With this information and the drawings you will either learn what they are or, you will be even more confused.



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The Cat

[elpirata]

to Lu Zuckerman

I dont know how you find time to write your mega long posts, I feel tired out just reading them !



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elpirata

[FlyAny]

Lu

Where is it you believe there is a CV joint on a V-22 Proprotor? When I ask this it is with the picture in mind of the device that lets me drive my car through a pot hole without snapping the drive shaft, or, for the same reason not twisting the shaft from the transmission to the differential. If there is a widget (CV Joint) that allows the direction of torque of the output shaft of the proprotor to change I'll be pretty surprised.

[Lu Zuckerman]

To: Fly Any,

Dear Fly,

As Gomer Pile used to say, SURPRISE, SURPRISE,SURPRISE!

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 bearings 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.

ALTHOUGH THE STRUCTURAL ELEMENTS ARE MADE OF METAL THE HUB-SPRING BEARINGS ARE BUILT UP LAMINATIONS OF METAL AND RUBBER. JUST LIKE THE ELASTOMERIC BEARINGS ON A SIKORSKY HELICOPTER.

To be truthful, I still don't know how the damned thing works as it requires you to visualize in three or four planes at the same time.


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The Cat

[This message has been edited by Lu Zuckerman (edited 24 October 2000).]

[FlyAny]

Impressive parts listing Lu. Now, while keeping in mind that you are engaged in discussion whith someone who has been climbing up and down the side of helicopters for the purpose of pre-flight inspection over thirty-one years, and always enjoys learning, but never enjoys his even temperment being abused. WHERE IS THE CV JOINT IN A V-22 PROP-ROTOR SYSTEM!

If you have the slightest intention of carrying on a simi-oblique-to-rational discussion such as the one you have been draging around the block in the R-22/44-a-thon, please stop the thread here.

[This message has been edited by FlyAny (edited 24 October 2000).]

[Robbo Jock]

See point 8.

[FlyAny]

Robbo Jock

Thanks. I read right past it.

Lu

Some of what you say is correct. The name sure is.

"It" is named a Constant Velocity Joint as you said.
"It" is the drive trunnion sandwiched between the upper and lower hub springs.
"It" is a high wizz elastomeric flapping hinge.
"It" does not function to change the direction of the axis of rotation. (My concept of a constant velocity joint, as on my car)

There is no offset of the hub.

The point is put to rest.

Apologies for my earlier tone.

[Lu Zuckerman]

To: Fly Any

Dear Fly,

It is exactly like the CV joints on your car. The CV joints on you car transmit the drive torque and at the same time allows deflection either caused by the steering system or up-and-down movement of the suspension. In the case of the Proprotor maybe you can understand it if you visualized the movement of the swashplate. Only in this case it is restricted from axial movement like the swashplate but it can assume what ever position the gyroscopic forces caused it to move to. Due to its construction and design the input/output are 1:1 with no slow down or speed up.

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The Cat