Cyclics, Semantics and Teetering Rotors ~ A question [Archive]

NickLappos

18th Jul 2003, 15:42

Dave,
You are working toward a question of the way the phase angle changes for the rotor as a time variable value, which is very astute.

The rotor has two basic components of response to change, the short term and the long term.

Long term is the steady state response of the rotor and is the typical mixing value (often called "precession" as if it were).

Short term is usually a function of the rate of change of the cyclic pitch, where rapid cyclic movements get strong, transient phase responses that disappear when the rolling or pitching stops. This is especially true of very stiff rotors (high offset). The sharp roll response of a Boelkow when the longitudinal cyclic is moved rapidly is an example. IIRC, a sharp left roll will result in a sharp back tilt of the rotor, where a soft, slow left roll will produce almost no longitudinal upset. Pilots learn to actually change their phase compensation to reflect the cyclic rate they use (sharp roll, put in some longitudinal cyclic, slow roll, put in less). The computer controls of a fly-by-wire system have many terms to help untangle these responses. A secondary artifact of this kind of transient action is the torque response to cyclic changes, which is very pronounced on teetering rotors.

These kinds of transient dynamic responses are some of the reason why professionals smile at the rigid belief others hold in "gyroscopic precession" and the like. The rotor is a very complex dynamic animal, very much more complex than a dime store gyroscope.

sprocket

18th Jul 2003, 18:43

Dave: I guess the correct answer for your question is "A".

Lu Zuckerman

18th Jul 2003, 20:28

To: Dave Jackson

In response to the original question we must look at it in two different ways.

1) The helicopter is on the ground with the rotors turning with the disc aligned with the local horizon (Flat). The pilot pushes pure forward cyclic pitch. With the blades aligned along the horizontal axis the pitch horn is leading by 90-degrees and therefor the blades have the maximum pitch change. Greater low pitch input on the advancing blade and greater high pitch on the retreating blade.

The blade disc must translate from the flat disc to a tilted disc and this is not instantaneous. During this transition phase the disc will roll to the right until it arrives at the tilted position as commanded by the pilots application of forward cyclic. So yes the disc will tilt to the right but this may only take maybe at the most two-to-three seconds and then forward motion would take place but then again, the helicopter is on the ground and does not translate.

2) The helicopter is in a no wind hover. Once off the ground the pilot must adjust his point of balance compensating for CG and tail rotor translation so the disc is no longer flat. The pilot will have introduced left cyclic and either forward cyclic or aft cyclic which ever was necessary to compensate for CG. Now, when he introduces forward cyclic he may encounter the disc rolling motion but it may be suppressed by the tilt of the disc while in a hover. (I think). But even then the rolling motion will only last a short time until the disc is fully tilted and forward motion takes place.

To: Sprocket

Quote: ”It sounds like a condensed description of the dreaded wee WA phenomenon, it only occurs when the rotor disc is being moved by the cyclic … the reason the Robby’s have an 18-deg. delta three”.

Quote: “BTW, wasn't the original delta three angle calculated to be 19-Deg”?

It seems that what you are saying was taken from Frank Robinson’s’ response to my posts on a long ago thread. From what I understand Frank Robinson got a patent on a three hinge rotorhead, which required a great deal of calculations to arrive at the final design but I doubt seriously if he was able to calculate the aerodynamic responses of the blades under maneuvering conditions. I believe that the delta angle came out to 18-degrees simply because of the offset of the cone hinges. In his response he alluded to having considered a 90-degree pitch horn but rejected that and came up with a 72-degree pitch horn. He had to select the 72-degree pitch horn because of the positioning of the cone hinges relative to the teeter hinge.

Now let’s get back to Dave’s questions and my responses.

1) I do not know if you would be able to perform the same demonstration on an R-22 with flat pitch because to do so would place the rotorhead under high stress but I may be wrong. I would think that it would be necessary to introduce some collective pitch to get the blade tusks off of their stops. Cyclic input (swashplate movement) on the Robinson is the same as that on a Bell. However the pitch horn leads the blade on a Robinson by 72-degrees so the blade angle is not at its'maximum pitch input until the blade has advanced 18-degrees past the horizontal axis. Even so the blade is decreasing pitch because it is following the swashplate tilt it just does not get its’ maximum input until it has moved 18-degrees past the horizontal axis and then it will flap down. Here is where it gets hairy. If you believe in gyroscopic or aerodynamic precession you would think the blade responds 90-degrees or thereabouts after maximum pitch input then the disc will tilt down left of the nose by 18-degrees. But if you subscribe to the phase angle theory then the blade will respond 72-degrees later in direction of rotation in other words flap down over the nose and flap up over the tail. I won’t push it or try to convince you either way. To get back to the question I really don’t know if the disc will tilt to the right during the translation from a flat to a tilted disc.

2) I believe in a hover with the compensation for CG and tail rotor translation any tendency for the disc to roll would be suppressed due to the mechanics of the rotor system. I believe that is why Frank Robinson said the R-22 did not Wee "nor", did it Wa.

:rolleyes:

Dave_Jackson

19th Jul 2003, 12:45

Lu,

You and Sprocket have definitely uncovered the direction of this thread. It is an attempt at understanding a specific rotor activity by braking it down into small sequential steps. Hopefully, it may lead to a clearer understanding of the two problems mentioned by Frank Robinson; and then an evaluation of his solutions.

However, there is more to the thread than just the Robinson's rotorhead. It also includes an attempt to get a better understanding on how to apply opposed lateral cyclic on a helicopter with twin laterally mounted main rotors.

Because of the complexities involved, it should be easier to discuss the subject by using a series of small incremental steps. I.e. by using Lu's 'Marquis of Titan Rules' ~ ".. is fine as long as it is germane to the [specific] subject being discussed."

Nick

There can be little disagreement with what you say. The complexities of the rotor must be mind boggling. Then add to this the inability of the swashplate to fully satisfy the ever-increasing complexities of modern rotors. This is the reason for initially considering the simplest of rotors, in its most primitive environment.

Both the rotor and the aerodynamic environment described in the initial posting were by Frank Robinson, and this rotor must be as simple as it gets. It consists of a rudimentary teetering rotor, situated in a self-created downward flow of air. It also assumes that; the downward velocity of the air at each blade element, the in-plane velocity of the air at each blade element, and the rotational inertia of the rotor are initially constants.

If forward cyclic is rapidly applied, I think that;
1/ The amount of blade pitch removed at the various azimuths on the advancing side will equal the amount of blade pitch added at the mirrored positions on the retreating side.
2/ Therefore, the amount of blade teeter removed at the various azimuths at the front will equal the amount of blade teeter added at the mirrored positions at the back.

The rotational inertia should have no effect on this action, since we are looking at a 90-degree phase lag situation. In addition, the slight nonlinear changes in the thrust and drag at the blade elements will probably have an insignificant effect. Therefore, I am inclined to believe that the blades are flying to position and that there is nothing to cause a tipping to the right.

Additional support for this position comes from a plot of lateral cyclic to forward speed in Prouty's Even More Helicopter Aerodynamics. He shows zero lateral cyclic at very low forward velocities.

A more advanced rotor and/or horizontal motion will change the above argument, of course.

Sprocket,

Maybe so or maybe not. ;)

vorticey

19th Jul 2003, 20:35

the blades fly or force the disc to position, as the disc is tilting, gyroscopic precession is taking place (but is restrained by the pitch links adjusting to the desired attitude), when the disc stops at the disired attitude so does gyroscopic precession. the slight difference in the direction of travel would be compensated unoticably.
a computer could control the lateral effects of the twin rotor thing, with some sort of lateral pressure sensors able to adjust individual cyclic inputs slightly.
:confused: why not??

Lu Zuckerman

19th Jul 2003, 23:17

To: Dave Jackson

1/ The amount of blade pitch removed at the various azimuths on the advancing side will equal the amount of blade pitch added at the mirrored positions on the retreating side.

This is true for the Bell but not the Robinson. Try to follow me on this one. On the Bell what you say is true. The amount of negative and positive pitch applied to the blades is mirrored in the fact that if you added 3-degrees of pitch to the retreating side you would subtract the same amount from the advancing side.

Now, let’s look at the Robinson. The movement of the swashplate on the Robinson is the same as that on the Bell. The Robinson is rigged starting at the rigged neutral position. The fixed stops regulate the movement of the cyclic in any direction. You would assume with this set up if you moved the cyclic from the rigged neutral position to the forward stop you would get a specific pitch change in the blades when the basic pitch settings are being adjusted. You would also think that when the cyclic is moved to the rear stop from the neutral position the same degree of pitch would be reflected at the blade over the rear of the helicopter.

From a pure mechanical setup that is what would happen due to the similarities between the Bell and the Robinson control systems. However on the Robinson when adjusting the forward cyclic range the blade is set to a specific angle. After setting the forward range the cyclic is moved to the rear stop and the blade angle is set. However the setting on the blade for aft input is different from the forward cyclic setting. Both systems are mechanically the same and control inputs to the maximum range are the same but the blade angles are not. When the mechanic sets the aft range he will change the setting on the pitch link that was set for the forward range. The mechanic is told to check if the forward setting has been changed (and, it will have changed) but the mechanic is not told what to do. A similar condition exists for the lateral settings. Each time the pitch link is adjusted to set a certain range the preceeding setting(s) will also change. Each pitch setting also has a +/- range so in the end you will never really know what the pitch settings are.

When you factor in the collective range setting there is a chance that at full collective there will be too much pitch resulting in blade stall. This will be exceptionally dangerous if you have to put the helicopter into autorotation when you have and engine failure.

Then, there is the tail rotor rigging procedure. But don't get me going on this.

Robbie drivers just think about this the next time you fly.

:sad:

Lu Zuckerman

20th Jul 2003, 03:21

The mechanics of the Bell and Robinson are the same as far as pitch input (swashplate movement) is the same however the actions of the blade pitch change are different. When the blade of a Bell is over the nose and the pitch horn is over the lateral axis the blade is in a point of neutrality. In other words it is at the collective pitch setting. The same is true for the blade over the tail.

When the Robinson blade is over the nose the pitch horn must travel another 18-degrees until it reaches a point of neutrality (collective setting). That means that the blade pitch is increasing during this point in rotation. In other words the blade is 18-degrees past the longitudinal centerline when it is at the collective position and the aft blade is 18-degrees past the longitudinal centerline when it is at the collective setting.

To my knowledge there is only one other helicopter that has a similar setup and that is the Lynx which is rigged with the blades offset from the respective axes by 15-degrees as opposed to the Robinson’s’ 18-degrees. I have been told that the Lynx has an electronic system that senses disc movement in relation to cyclic input. If this system is disabled or, turned off the helicopter will roll 15-degrees to the left with the input of forward cyclic. But then again the individuals that subscribe to the phase angle theory state that the Robinson disc will tip down over the nose with the input of forward cyclic because the phase angle is 72-degrees where on the Lynx the uncorrected phase angle is 90-degrees.

I really don’t know what this means. You guys with the engineering degrees figure it out.

;)

Dave_Jackson

20th Jul 2003, 03:34

vorticey,

Is there a " slight difference in the direction of travel"? :confused: :confused:
That is the question. I suspect that Gyroscopic Precession and Aerodynamic Precession are virtually identical in a basic teetering rotorhead.

Lu,

"This is true for the Bell but not the Robinson."

Remember the 'Marquis of Titan Rules'. You're getting a head of the thread. :uhoh:

At this point in the thread, the Robinson hub does not exist. All we have is a simple basic teetering rotor. Later a gentleman by the name of Frank Robinson might come along, and he will feel that this basic rotor undergoes a slight lateral tilt as it is in the process of following instructions and tilting forward.

Remember that there is no forward velocity, in this first of two questions.

Lets make damn sure that there is a problem. Then if there is, we can look into potential solution(s).

Save these good comments of yours for future posts.

sprocket

20th Jul 2003, 20:36

Dave:

1. In the scenario you have stated and with leading PCL's, the blade, anywhere in the front half of the disk will be increasing its pitch angle. Therefore if in a hover the blade AOA will be increasing, at least until it is at its forward position. (Answer ‘A’).

2. The resultant downward direction of the rotor disk at the front will tend to increase the AOA of the blade at the front of the disk further. The blade is not ‘flying’ to position, it is ‘fighting’ and losing to gyroscopic precession.

Dave_Jackson

21st Jul 2003, 03:21

Sprocket,

Sorry for the ignorance but I do not know what 'PCL' is. Therefore, the following may or may not answer your question.

As you have mentioned, the scenario in the initial post was put forth by Frank Robinson. In this situation, the minimum blade pitch is at 90-degrees azimuth and the lowest teeter (flap) location is at 180-degrees azimuth. Assume that the cyclic stick is advanced slowly, but not so slow that the forward motion is allowed to started. In this situation, The gyroscopic precession and the aerodynamic precession will both have a 90-degree phase lag. Therefore, there will be no conflict between the two.

Nick has initially mentioned another very different scenario. I assume that he is primarily talking about 1/ a rigid rotor, 2/ the craft having forward velocity, and 3/ the pilot imparting a fast cyclic change. In this situation, all of the factors that effect the tilting of the disk, with the exception of gyroscopic precession, are going to have phase angles that differ from 90-degrees. This is where the 'washed-out coupling effect' (Wee-wa) comes into play. Nick specifically mentions the changes that take place when only the speed of advancing the stick is changed.

I think that there is a crosscoupling called Wee-wa and that it is in addition to Acceleration (Control?) cross-coupling and Rate cross-coupling. I think that the 'washed-out coupling' varies from virtually nothing in Frank Robinson's scenario, to being relatively large in Nick's primary scenario. Based on this, I do not thing that Mr. Robinson should have used the word 'hover' in his statement. In addition. Lu is probably incorrect when he considers Wee-wa as being the same as Transverse Flow Effect.

This may, hopefully :), promote an argument.
Even if the above is correct, it does not imply that delta-3 is a viable means of overcoming Wee-wa.

Lu Zuckerman

21st Jul 2003, 03:57

To: Dave Jackson

As you have mentioned, the scenario in the initial post was put forth by Frank Robinson. In this situation, the minimum blade pitch is at 90-degrees azimuth and the lowest teeter (flap) location is at 180-degrees azimuth. Assume that the cyclic stick is advanced slowly, but not so slow that the forward motion is allowed to started. In this situation, The gyroscopic precession and the aerodynamic precession will both have a 90-degree phase lag. Therefore, there will be no conflict between the two.

Did Frank Robinson really make the above statement. If he did he was addressing a Bell head with 90-degree pitch horns.

On a Robinson the blade is advanced 18-degrees ahead of the lateral axis (where on a Bell head the blade will be laterally disposed on the lateral axis). When the Robinson blade is at its’ lowest pitch it is in the position described above (18-degrees advanced). According to Frank Robinson the blade phase angle is 72-degrees not 90-degrees.

:ok: :confused: :confused:

sprocket

21st Jul 2003, 04:28

Dave: The PCL is the pitch change link/pitch link, referring to them as situated at the LE of the blade and not the trailing edge.

Aerodynamic precession is a term I'm not familiar with. Does it have another name?

Dave_Jackson

21st Jul 2003, 06:44

Lu,

Yes, Frank Robinson was discussing the problems of a basic teetering head (Bell 47). He mentioned two problems with this design of head, and then went on to discuss his two solutions to these two problems.

This is the full text of Frank Robinson's posting (http://www.unicopter.com/B185.html#Frank) which was, in part, instigated by you. :cool:

All I am trying to do is dissect his post and look at it; one step at a time.
I.e.
~ 1/ Is there a washed-out coupling effect and is it a problem
~ 2/ Is there a transverse flow effect (of course there is) and is it a problem (of course it is).
~ then ~
~ 3/ Is his uses of delta-3 and change of phase angle a valid solution to item 1.
~ 4/ Is his uses of delta-3 and change of phase angle a valid solution to item 2.

As previously mentioned, my interest is Opposed Lateral Cyclic. It just happens to be related to your interest in the Robinson head.

If answers can be reached on 1/ & 2/ then and only then will it will be possible to consider 3/ & 4/. IMHO

Sprocket,

Aerodynamic precession is basically the same as gyroscopic precession. With gyroscopic precession an upward 'force' at one location will result in a maximum elevation 90-degress later.
With aerodynamic precession, a maximum 'angle of attack' at one location will result in a maximum elevation 90-degress later.

Both achieve the same results on a basic teetering rotor. Its just that they use different algorithms. On a rotor with delta-3 (or flapping hinge offset) GP will still have a phase lag of 90-degrees but the AP will have a phase lag of less than 90-degrees.

If you have the time and inclination, this pile of verbiage may be of help. (http://www.unicopter.com/0940.html) :O

PS.~ Edited to remove BS and add PS.

Lu Zuckerman

21st Jul 2003, 07:17

To: Dave Jackson

A delta three angle of 18 degrees was selected as the best compromise angle to reduce or eliminate the two undesirable characteristics described above, which would have been present in the R22 had a 90-degree pitch link design been used. Subsequent instrumented flight test data confirmed the choice of the 18-degree delta-three angle. Hopefully, this will help clarify a few of the misconceptions concerning the design of the R22.

I direct your attention to the following excerpt from the above quote: (which would have been present in the R-22 had a 90-degree pitch link (pitch horn) been used).

Frank Robinson makes a comparison between the Bell system and alludes to the fact that he had considered a 90-degree pitch link (pitch horn) but he rejected it in favor of the 72-degree pitch horn. At least that is what I read into it. If my interpretation is correct Frank wasn’t being truthful because his rotorhead could never employ a 90-degree pitch link/horn.

:ok:

Dave_Jackson

21st Jul 2003, 09:01

Lu

'Wee-wa' is very likely short for 'Washed-out Coupling Effect. I have only seen the expression 'Washed-out Coupling Effect' once, and that was in a single paragraph in Padfield's book 'Helicopter Flight Dynamics'. This paragraph referenced two reports, so I have just ordered the American Helicopter Society's report.

delta-3 is something that the rotorhub does to the blades. Phase angle is something that the control system does to the blades. The modified phase angle is there because the delta-3 is there. This is somewhat similar to articulated rotor, where the modified phase angle is there because the flapping offset is there.

I think that Frank Robinson is saying that there are two things that he does not like about the 'basic' teetering hinge. [For interest only ~ The Bell 47 is not quite a 'basic' teetering hub. It's head has two hinges, which are at located at 90-degrees to each other, similar to a universal joint. God knows what the second hinge does. Perhaps it's a holdover from the earlier use of a stabilizer bar.]

Again, I feel that we should become aware of Frank Robinson's perceived problems with the 'basic' teetering hub before starting to question his solutions to the problems.

The primary question is still unanswered. ~ Does the application of forward cyclic on a hovering helicopter with a 'basic' teetering rotor cause an immediate tilt of the disk to the right? :confused: :confused: :confused:

[email protected]

21st Jul 2003, 16:33

You chaps just don't take the hint about using the term precession when talking rotor blade movement do you? Read Nicks' post again, if the professionals don't believe it happens why do you keep talking about it and worse, muddying the waters by making up terms like 'aerodynamic precession'?

DJ I know you want to build the worlds best and cheapest helicopter but I do think you should get out more!!

sprocket

21st Jul 2003, 19:40

Does the application of forward cyclic on a hovering helicopter with a 'basic' teetering rotor cause an immediate tilt of the disk to the right?

Dave, picture this for a moment; a spinning solid disk in an ideal setting used to demonstrate precession will have a force at one particular point, applied to make the disk/wheel etc tilt. … now back to your rotor……

Still moving the cyclic forward in a steady state hover ……the difference here to the solid disk demo., is that in your helicopter rotor, the force/couple from the blades changing pitch acts in a relatively gradual manner!
The blade pitch angle, which determines the amount of cyclic force, takes 180 degrees before it gets to its maximum or minimum value. During that gradual change the force/couple has already started to tilt the rotor disk to an angle that precedes the desired disk tilt.
In this instance, the overall tilt down is to the front but a with a slight right hand component, which was instigated by the lighter but increasing force/couple prior to the blade reaching its max or min pitch angle.

Is that what you mean by “washed out coupling”?

The tilt down at the front becomes a tilt to the right by a few degrees. Therefore, my answer to your last question is “yes”.

Heli-Ice

21st Jul 2003, 23:51

Me, in my childish belief thought it was very simple to fly helicopters but now after reading this I don't know what to think??? :{

Dave_Jackson

22nd Jul 2003, 02:14

crab,

This thread is an attempt, by a few interested people, to acquire a better understanding of some complex activities. I don't think anyone is questioning the accuracy of Nick's remarks. They are contributory and appreciated.

The phrase 'Aerodynamic Precession' is used by Gareth Padfield &/or Gordon Leishman, two very respected helicopter dynamists.

Sprocket,

I think that the results from gyroscopic precession and from aerodynamic precession are identical, when one is considering a 'basic' teetering rotor.

" a spinning solid disk in an ideal setting used to demonstrate precession will have a force at one particular point, applied to make the disk/wheel etc tilt."

OK, but, remember that this is a 'solid disk" [structurally rigid disk] and therefor "a force at one particular point" will be transmitted to all points on the disk. The amount transmitted to the various points is a trigonometric function of the azimuths and the moment arm. The amount of this force that is transmitted to each radii-azimuth is identical to the force of a blade element at the same radii-azimuth.

When the consideration turns to a more complex rotor, Gyroscopic precession start to 'fly out the window'. In fact, IMHO, is better to use Aerodynamic precession in all circumstances; with the possible exception of when considering the hub portion of an articulated rotor with a large flapping hinge offset.
__________________

There is Rate cross-coupling, Acceleration cross-coupling and apparently a much less known coupling called Washed-out cross-coupling. I am guessing that Washed-out cross-coupling is associated with time and therefore the speed with which the cyclic stick is pushed forward etc. This seem to be inline with Nick's remarks and some information he provided in an earlier thread, where he mentioned that the flight controls on the Comanche could tailored to the specific flight situation.

If this is correct, then the relevance of Washed-out cross-coupling [Wee-wa] may or may not be important in a 'basic' teetering rotor

NickLappos

22nd Jul 2003, 08:19

The phrase aerodynamic precession is most apt, and is intended to draw us away from any idea that gyroscopes are involved. In truth, the varable rotor phase angle (varies with time, with speed, with density altitude, with rate of input, with rotor blade inertia, with hinge offset, with blade chord,......) is a complex motion that tells us how the blade's flapping oscillation compares with the 1 per revolution frequency of the aircraft.

Gareth Padfield (a friend) and Gordon Leishman (an acquaintance) are very respected experts in our field, they would have no problem discussing how rotors behave without once using gyroscopes in the explanation.

heedm

23rd Jul 2003, 13:18

Didn't we all agree long ago that the gyroscope thing was just a demonstration tool and an easy way to dumb down a complex topic?

In truth, gyroscopic precession is a crazy thing that tops and the like do when they lean from the vertical. The explanation for the GP goes something like the explanation for what a helicopter's rotors do.

Okay, Regis, FINAL ANSWER.....(from a physicist and helicopter guy) not only does gyroscopic precession have nothing to do with helicopters, the rotors don't even PRECESS!!!! (unless you stir the cyclic)

Matthew.

Lu Zuckerman

23rd Jul 2003, 22:25

QUOTE: The phrase 'Aerodynamic Precession' is used by Gareth Padfield &/or Gordon Leishman, two very respected helicopter dynamists.

When Messrs. Padfield and Leishman were attending physics and aerodynamics courses at university level what theory was taught to them? Was it gyroscopic precession or was it aerodynamic precession? Did the term gyroscopic precession evolve into aerodynamic precession by way expansion of engineering knowledge where computer analyses and complex testing proved the gyro theory to be incorrect? Or, was the new aerodynamic precession “invented” in order for these two gentlemen and other writers of engineering texts to sell more textbooks. Granted, I don’t have the engineering knowledge to fully understand all of what is in these textbooks but I have read several where the author(s) changed their theories over a series of textbooks.
In a case like this if the student were exposed to the first textbook and not all of them he /she would enter the world of engineering with a limited knowledge of the subject.

If in fact aerodynamic precession is the case there are a lot of pilots and mechanics in this world that were never exposed to this theory and they fully believe in gyroscopic precession. This does not make them stupid or place them in a time warp.

I have the training texts from different helicopter firms and although this material is dated these firms are still teaching that the rotor has two characteristics of a gyro. Rigidity in space and precession. The present FAA Rotorcraft Flying Handbook slightly downplays the importance of gyroscopic theory but they do not offer a replacement theory. Who is right and who is wrong. It depends on how and WHEN you were taught while in university or flight school and by extension mechanics school. Although some of you might object, both theories work.

Here is a question to ponder. On the Cheyenne helicopter control inputs to the rotor pitch horns are input from a “gyroscope rotor” suspended above the main rotor. When the main rotor responds to the input is it being displaced by gyroscopic precession or, aerodynamic precession?

QUOTE: Okay, Regis, FINAL ANSWER.....(from a physicist and helicopter guy) not only does gyroscopic precession have nothing to do with helicopters, the rotors don't even PRECESS!!!! (unless you stir the cyclic)

You can’t have it both ways. If rotors do not precess then both theories (gyroscopic precession and aerodynamic precession) go out the window.

Be gentle Nick’

:confused:

Dave_Jackson

24th Jul 2003, 03:48

Lu,

"Did the term gyroscopic precession evolve into aerodynamic precession by way expansion of engineering knowledge where computer analyses and complex testing proved the gyro theory to be incorrect? " ~ :ok:

Time and knowledge progress (well ~ sometimes ;) ). Would you not agree that back in the beginning, its doubtful that Leonardo da Vinci considered gyroscopic precession when contemplating his air-screw.

If this thread does move on into; phase angle, delta-3, and possibly flapping hinge offset, You will have to put gyroscopic precession totally aside, or there will be absolutely no way of grasping the interrelationship of the above three activities.
______________________________

Meanwhile, back to some of heedm's earlier posts, to get reacquainted with ~ [/]"the rotors don't even PRECESS!!!!"[/i] :uhoh:

heedm,

I tried to look up a previous point you had made, but the eyes started going. I think you previously commented on the precession of the azimuth of greatest tip. In addition, you mentioned that the azimuth of greatest tip advanced extremely slowly, until the force of the primary rotation had significantly decreased visa vie the perturbing force, i.e a wobble. Is this somewhat similar to what you're referring to by "stir the cyclic"?

Edited to add note to heedm

Dave_Jackson

26th Jul 2003, 10:00

Padfield's book and a referenced report say "... washed-out coupling, which can occur in helicopters with feedback control systems". Delta-3 must be considered a mechanical 'feedback control systems', since it pull out pitch in a relationship to the amount of flap.

This raises the circular question; was delta-3 incorporated to minimize washed-out coupling, or did the incorporation of delta-3 cause washed-out coupling?

The following two quotations are from separate posts by Chuck Beaty, a gentleman with considerable technical knowledge about rotorcraft. They provide additional information.

1/
" Delta-3 coupling binds the rotor more tightly to the mast position, perhaps a good thing for tail rotors but a bad thing for main rotors."

2/
"Subject: Wee-wa

Bramwell in "Helicopter Aerodynamics" after tossing about some fancy math, derives some fairly simple expressions for cross coupling.

The rotor tilt in the commanded direction is proportional to: 16 * (tilt rate)/(angular velocity) * (1/y)
y is the mass constant of the rotorblade (Lock #), the ratio of aerodynamic force to mass. High inertia blades have lower Lock # than the other way around. Angular velocity is that of the rotor, everything being in radians/sec.

The tilt in the crosswise direction is: (tilt rate)/(angular velocity)

What this says is that cross coupling might not be noticed in a Bell or Hiller but might be a problem with a Robinson.

Irrelevant sentence snipped.

I've met Frank Robinson and remember having a discussion with him about his use of delta-3 coupling back when he was still working on the R-22 prototype. He's a very competent engineer and wrote a number of papers about tail rotors while he was a project engineer at Bell-Textron but I suspect tail rotors gave him his fixation about delta-3 coupling where suppression of cyclic flapping is considered desirable.

Mr. Robinson is not only president and chief engineer of Robinson Helicopter but is also chief promoter and head salesman. When forging dies for blade grips and pitch arms are bought and paid for, it's sometimes expedient to make theory fit practice.

With a model rotor at least, if one wants to see some really nasty "wee-wa," skew the teeter hinge and observe the nutating behavior. It will go crazy with a 45 degree skew angle. Model rotors don't exhibit any detectable cross coupling because the ratio of rotor rpm to tilt rate is so high.'

Confusion runs wild. It's all very interesting.

Lu,

Why not give Chuck Beaty a call. Hell, why not give Frank Robinson a call. :O

heedm

26th Jul 2003, 11:38

A precession is a motion that spinning masses do where the axis of the spin traces a cone. Stick a flashlight pointing upwards on a moving turntable that is mounted horizontally. If the flashlight makes a dot on the ceiling then there is no precession. If you then pick up the turntable and keep tilting it so that the flashlight traces a circle on the ceiling, then you have precession.

Helicopter rotors, in the simple case, do not exhibit any such behaviour. In the full analysis, even a simple spinning top has at least two frequencies/amplitudes of precession. So I imagine a helicopter's rotors would show many of these.

Gyroscopic precession occurs when a force acts continuously on a spinning object at an angle to the spinning axis. Like when a spinning top starts to lean over, gravity pulls down on the object causing the top to precess. This can be shown quite easily in the classroom. It can then be broken down in the classroom to show that it's caused by a force acting on a rotating object apparently being realized 90 degrees later in the rotation.

Because it's easy to show in the classroom, it's also an easy way of convincing prospective helicopter pilots that a control system that appears to be rigged 90 degrees out of sync will actually work.

If you agree with me to this point, we'll all have to agree that gyroscopic precession is a big lie told to make life easy.

--------

I haven't read Padfield's or Leishman's books yet, so I still don't know what aerodynamic precession really is. If it's an attempt to explain why the controls are approximately 90 degrees out of sync, then I'd question if they even used the word precession accurately.

However, if they are referring to some higher order response of the rotor system, then they may in fact be using the term accurately.

---------

Dave, "stirring the cyclic" is a term for overcontrolling. Lots of students do it. They get task saturated trying to hover, end up moving the cyclic all over the place which means more effort required in controlling yaw & power. Also bad when operating close to power limits since it results in your total thrust vector leaning from the vertical and time averaging to a shorter but vertical vector.

Matthew.

Lu Zuckerman

26th Jul 2003, 11:41

To: Dave Jackson

You say I’ll love this referring to the text that followed. Hell I not only do not love it I don’t understand it. I understand diagrams not words. Regarding my calling Frank Robinson I did just that about seven years ago. I told him that I could show him how to greatly simplify his rigging procedure, which could be performed in about 30 minutes with one man, and you didn’t have to level the helicopter. You could even rig the R-22/44 on a pitching and rolling tuna boat. All it took was two very simple tools that could be manufactured in their own shops with the tools being offered as special tools and sold to the operators. A few days later Franks’ son called me and stated that Robinson helicopters was not in the business of making special tools. As a result the Robinson helicopters are a bear to rig and the rigging instructions are very vague and misleading for the mechanic.

The author of the text you quoted said if I understand that there is no pitch coupling on a high speed rotor system simply because the blades can’t react fast enough. If that is the case, how can a Robinson rotor system respond so quickly with a 72-degree lead angle. If this same reasoning was applied to the tail rotor which rotates considerably faster than the main rotor and there were no pitch flap coupling the tail rotor would eventually fatigue and it and possibly the tail rotor gearbox would would break away from the tail boom.

When collective is pulled and cyclic input is made the pitch link connect point on the horn is above the flapping axis and any flapping up will subtract pitch and when the blade flaps down pitch will be added just like on a tail rotor. Now maybe the blades can’t react due to their rotational speed but the pitch flap coupling is still there.

:confused:

Dave_Jackson

26th Jul 2003, 12:51

heedm,

Got it, Thanks.

PS. Don't tell Lu, but Frank Robinson used the 'G' word when he said; "an aerodynamic moment on the rotor disc is required to overcome the gyroscopic inertia of the rotor." :D

Lu,

Help! Forget the math. Just reread Chuck Beaty's conclusions. You will find that he is somewhat in agreement with you.

I believe that most tail rotors have a delta-3 of 45-degrees. This means that any flapping is instantly pulled out. No 90-degrees. No discernible delay. No sh1t.

Lu Zuckerman

26th Jul 2003, 22:30

To: Dave Jackson

I believe that most tail rotors have a delta-3 of 45-degrees. This means that any flapping is instantly pulled out. No 90-degrees. No discernible delay. .

Edited to remove the words no ****.

On two blade tail rotors the delta hinge is offset by 60 degrees. You say that any flapping is instantly pulled out. Please explain. When the tail rotor flaps one blade will decrease in pitch and the opposite blade will increase in pitch thus equalizing the lift across the tail rotor disc. On multi blade tail rotors that are free to flap the blades act like those on the main rotor. When the blade flaps in either direction the pitch flap coupling will tend to return the blade to the radial position. This is a form of relieving stress on the blades and that’s why they are free to flap. However with flapping you get lead an lag and this is catered to by the design of the blade (S-58 , S-61) or by the incorporation of a lead lag hinge and a damper (CH-37).

:ok:

Dave_Jackson

27th Jul 2003, 00:51

Lu,

I should have removed the tail rotor paragraph from Beaty's post, 'cause I know very little about them, and do not want to. The tail rotor is about to become a historical relic. :eek: Should have probably removed his math paragraph, as well. :rolleyes:

My limited understanding is that collective pitch change is wanted in a tail rotor but teetering or flapping is an undesirable. They use delta-3 to minimize the flapping. 45-degrees will eliminate any pitch change caused by flapping. 60-degrees will put in an opposing pitch, which will drive the blade back to the "home position'. You are probably correct, and my guestimate of 45-degrees was wrong.

While discussing delta-3, there are two different ways of implementing it.
See the two sketches at the top of this web page. (http://www.unicopter.com/0941.html#delta3)
[A/ By flap hinge geometry] has the lead/lag component in it, which you refer to.
[B/ By control system geometry] is the arraignment that Robinson uses in its main rotors.
____________________

Meanwhile, back on track.

The initial post on this thread was an attempt to see if lateral cyclic is required, when the stick is being advanced on hovering craft, which has a basic teetering rotor. No practical answer has yet been give, but Prouty seems to imply that lateral cyclic is not required at this point in time.

If this is the case, then what heck is "Wee-wa' and where is this number one problem that Frank Robinson apparently sees?

Lu Zuckerman

27th Jul 2003, 03:38

To: Dave Jackson

QUOTE: While discussing delta-3, there are two different ways of implementing it.
See the two sketches at the top of this web page.
[A/ By flap hinge geometry] has the lead/lag component in it, which you refer to.
[B/ By control system geometry] is the arraignment that Robinson uses in its main rotors.

In B above the only time there is no pitch flap coupling is when there is no collective input. At this time the pitch link / pitch horn interface is in alignment with the cone hinge. When pitch is added the pitch horn / pitch link interface will rise above the cone hinge and in this condition when the blades flap about the cone hinge you will get pitch flap coupling. The same is true when the blades flap about the teeter hinge this will also result in pitch flap coupling. So, I ask the question what is the control system geometry espoused by Frank Robinson?

:confused:

heedm

27th Jul 2003, 04:00

PS. Don't tell Lu, but Frank Robinson used the 'G' word when he said; "an aerodynamic moment on the rotor disc is required to overcome the gyroscopic inertia of the rotor."

pssst, Dave. I think he knows. :(

Dave, reference your initial post, the downwards flapping of the forward blade and upwards flapping of the aft blade will result in angle of attack changes that cause the disk to roll right (ccw system). Unfortunately, I think it is inaccurate to state that "...if no lateral cyclic is applied, the rotor disc will have some lateral tilt while the rotor disc is tilting forward" because that indicates this effect (don't know if it's weewa) has an end result of a laterally rolled disk. It seems clear this effect puts in an input, but there are many other effects going on.

I think the best way to deal with this is to flight test a simple system.

Matthew.

Dave_Jackson

27th Jul 2003, 06:19

Lu,

I agree with what you are saying, and more. Actually, a variation of the collective will change the coning angle and thereby slightly affect the pitch-flap coupling in both A/ and in B/. You are also correct in that Robinson's flapping/coning hinges could affect the pitch-flap coupling.

But, this thread is not specifically about the Robinson. In addition, bring in Robinson's flapping/coning hinges will make a complex subject even more complex. Lets just discuss pitch-roll cross-coupling, and include Frank Robinson's related 'Wee-wa'.

In fact, it looks like Mr. Robinson may have created the word, and if so, he is the only person who can define it.

Heedm,

I do not fully understand your comments, but like you, I am beginning to disagree with the quoted statement by Frank Robinson.

_____________

To try to keep this thread on topic, I am going to start a separate thread on delta-3 and phase-lag. It will be the definitive answer to both subjects :rolleyes: :yuk: and it should answer Lu's running question about 'what happened to the remaining 17-degrees of teetering'. :D

Lu Zuckerman

27th Jul 2003, 09:18

To: Dave Jackson

I do not fully understand your comments, but like you, I am beginning to disagree with the quoted statement by Frank Robinson.

That's strange. I seem to remember a participant on this forum once saying that it was a bunch of crap.

:hmm:

heedm

27th Jul 2003, 11:55

Thought through this a little more and came up with something that I think is worth presenting here.

When you apply forward cyclic, you reduce pitch on the right and increase it on the left. This means that while tilting the disk, you generate a dissymetry of lift. However, since the forward blade experiences increased alpha and thus generates more lift, the disk tilts to the right.

So you decrease lift on the right but at the same time tilt the disk to the right. Wouldn't that just be crazy if those two effects cancelled each other out with the result that no lateral cyclic results in no roll during abrupt manoeuvring?

What I was trying to say before is not quite that the quote was a load of crap, just that it wasn't complete. It didn't consider other transient effects on the disk, and it didn't consider whether the (weewa?) effect was desirable or not.

Matthew.

Dave_Jackson

27th Jul 2003, 12:38

heedm,

You said; "However, since the forward blade experiences increased alpha and thus generates more lift, the disk tilts to the right." This is probably true when the craft has forward velocity. When the craft is stationary, I believe that the blade only flies to position and the coning angle will not result in a change in alpha between the front and the back

Frank Robinson stated; "In a steady no-wind hover, when forward cyclic pitch is applied ...". His intent was not to write a legal document, so perhaps he did not really mean hover.

Lu Zuckerman

27th Jul 2003, 21:18

To: Dave Jackson

In a steady state no wind condition you cannot hover without having input some degree of cyclic pitch. So it is purely theoretical to say different. When the pilot lifts off to hover he makes adjustments in cyclic to compensate for tail rotor translation and for CG location. In this case he is not starting with a level disc when he adds forward cyclic. As the helicopter moves forward the tail rotor translation is not as strong so the pilot can adjust his cyclic and just about then he is into translational lift and is into Induced flow and Transverse Flow Effect and the pilot is making all sort of cyclic adjustments. So, if Wee Wa exists it is most likely mixed up in all of the other things and is most likely a theory that can only be proved on paper or in closely controlled testing.

IMHO as a non pilot.

:confused:

Dave_Jackson

28th Jul 2003, 02:33

Lu,

You said; "So, if Wee Wa exists it is most likely mixed up in all of the other things and is most likely a theory that can only be proved on paper or in closely controlled testing."

I agree that a number of things are taking place. But, if 'Wee-wa' does not have a noticeable effect, why did Frank Robinson want to address it when he designed his rotorhead and later when he explained his rotorhead on this forum?

It would be nice to know what 'Wee-wa' is an acronym for, but it appears that he has defines it as ".. the rotor disk will have some lateral tilt while the rotor disk is tilting forward ..". Perhaps it stands for 'small washed-out coupling'. The following is from the NASA Annual Report for 1993 "It is speculated that the washed-out coupling configurations were a higher frequency and lower amplitude coupling than the control and rate coupling."

He also says; " .. the forward blade has a downward velocity and the aft blade has an upward velocity. This increases the angle-of-attack of the forward blade causing it to climb, and reduces the angle-of-attack of the aft blade causing it to dive."

I can accept this if there is significant forward velocity, but I question it when he specifically says " In a steady no-wind hover".

It may or may not be detectable in actual hovering flight, but 17º is not insignificant. I suspect (from a purely theoretical perspective) that if a Bell 47 pilot was to snap the cyclic forward he would only see a forward tilt of the rotor disk. If a Robinson pilot was to snap the cyclic forward he would see a forward tilt of the rotor disk, plus an amount of left tilt.

Disclaimer: :D In no way does this speak to the pros or cons of using delta-3 in a main rotor. It is only an attempt to better understand rotor characteristics.

Lu Zuckerman

28th Jul 2003, 03:13

To: Dave Jackson

I'm surprised our friends from OZ never mentioned it. I went on the internet to find something about Wee Wa and came up with Wee Waa which is located in NSW Australia.

Strange!!!!

:rolleyes:

sprocket

28th Jul 2003, 04:24

Named thus because you just dont want to go there, lest you get trampled!

Dave_Jackson

29th Jul 2003, 11:08

Nick, on the related thread thinks that I mixed Cause and Effect. :{
I think that Frank Robinson may have mixed Cause and Effect. :ooh: [list=1]
Frank Robinson's description of 'Wee-wa' appears to be the same as 'washed-out coupling effect'.
I agree with Nick's remark that "Delta three ..... acts like a stability augmentation system."
The AHS report says that 'washed-out coupling effect' can occur in helicopters with stability augmentation systems'
[/list=1] This creates an enigma.

How can Frank Robinson use a 'stability augmentation system' (delta-3) to overcome the 'washed-out coupling effect' (Wee-wa) when it is the 'stability augmentation system' that caused the 'washed-out coupling effect'? :confused: :confused: :confused:

Lu Zuckerman

30th Jul 2003, 02:54

To: Dave Jackson

THe Stability Augmentation System or on some helicopters the Stability Control Augmentation System (SCAS) monitors fuselage displacement both from rate and magnitude and does not respond to the movement of the blades relative to gusting. In this case it is an auto pilot that will maintain the last commanded position of the fuselage. The Delta 3 will cause the blade to reposition itself to the in track condition with this movement being the result of pitch flap coupling (Delta 3).

The SCAS also monitors rate and magnitude of cyclic input and through the built in control laws it will limit the amount of pilot valve input to the servo in order to protect the airframe from overstress. This occurs so fast that in some cases the blades have not responded to the cyclic input. Apache has this system.

IMHO:rolleyes:

heedm

30th Jul 2003, 11:48

Dave,

I hope I'm not droning on about something you completely understand. I got the feeling you didn't see one important part of the coupling we're discussing.

You mentioned, "When the craft is stationary, I believe that the blade only flies to position and the coning angle will not result in a change in alpha between the front and the back." Consider the angle of attack of the blade as it cycles around. The craft is stationary in zero wind, no cyclic inputs, so the angle of attack is pretty close to constant.

You suddenly apply forward cyclic, causing blade pitch and thus angle of attack to decrease at the 3 o'clock, and increase at the 9 o'clock. At 12 and 6, the blade pitch doesn't change from the steady state, but now that the blade is flapping down at 12 and up at 6 the relative airflow changes. It comes more from below the blade as the blade moves down (12 o'clock) which cause angle of attack to increase even though blade pitch is the same. Of course angle of attack increased means greater lift, which means the disk will tilt to the right.

I think Frank Robinson probably did mean hover when he wrote "In a steady no-wind hover..."

Dave_Jackson

30th Jul 2003, 13:35

heedm,

You're raising a good point and I certainly don't understand it all.

Nick Lappos may be the only person on the forum who can give a definitive answer, but here is my take on it.

On one side, we must assume that Frank Robinson knows what he is talking about and is therefore correct.

On the other side, I think what you are describing may be acceleration (control) cross-coupling. In addition, at hover I question if there even is an acceleration cross-coupling. The reason for this thinking is that blade is consistently adding pitch between 3:00 and 12:00 and this could be considered aerodynamic precession. This aerodynamic precession would be functionally identical to gyroscopic precession (please excuses the use of the word 'precession' :)) on this basic teetering rotor.

Your argument may be more appropriate for forward flight, were there is 'transverse flow effect' and the effect of the coning angle. To have an increased angle of attack at the front during hover and have it be higher on the left than the right, something will have to be pulling down on the front of the disk ~ I think.

The final reason for questioning Frank Robinson's remark was mentioned in my last post where "'Wee-wa' appears to be the same as 'washed-out coupling effect'.", and 'washed-out coupling' is a different type of cross-coupling from 'acceleration cross-coupling' and 'rate cross-coupling'

Then again, you may very well be correct. :confused: :confused: