Cyclics, Semantics and Teetering Rotors ~ A question
 

Is the following statement correct? :confused:

"In a steady no-wind hover, when forward cyclic pitch is applied, the 90-degree rotor disc will end up tilted in the forward direction, but if no lateral cyclic is applied, the rotor disc will have some lateral tilt while the rotor disc is tilting forward."

Please note that this is before the start of forward motion. In other words, at this point in time, there is not yet any; Transverse Flow Effect, Inflow Roll, Rotor Upwash, Flap Back or Blow Back.

Thanks.

Dave, that is how I understand it. When you state "90- degree rotor disc", I presume you mean "with 90 degree pitch arms/horns"

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.

What did you say
 

To: Sprocket

Watch it. You are not allowed to use the number 18 in conjunction with the word degree on this forum.

:rolleyes:

Lu .....
 

I just thought I'd sneak that one in while the others were distracted with the "Clash" thread. :ooh:

BTW, wasn't the original delta three angle calculated to be 19 Deg.?

Sprocket,

Yes, you're correct. The question relates to a basic teetering rotor. The post should have stated "with 90 degree pitch arms/horns". This will exclude the Robinson.
______________________

The question relates to;

A/ Does the downward tipping of the front of the disk increase the angle of attack of a blade, which is in the front half of the disk.
~ or ~
B/ Does the blade 'fly to position' with no significant change in its angle of attack.


Thanks.

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.

Having said that ....
 

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

Once more with feeling.
 

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:

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. ;)

yes dave, i recon your right.
 

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??

T'aint necessarily so.
 

To: Dave Jackson

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:

Oh, and another thing.
 

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.


;)

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.

Reason for my answer.
 

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.

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.

I beg to disagree. Well, not exactly beg.
 

To: Dave Jackson

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:

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?

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 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. :O


PS.~ Edited to remove BS and add PS.

Is Wee Wa short for weaselword?
 

To: Dave Jackson

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:

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: