Robinson R22 Corner [Archive copy]

[Dave Jackson]

Lu

If you and I are reading SouthXross' remarks correctly, then we all three agree that an increase in the coning/flapping angle of the R-44 will result in an increase in pitch. We must assume though, that Frank Robinson knew what he was doing when he located the pitch link's upper joint inboard of the coning/flapping hinge. This is particularly so, since he has gained much prior knowledge from the R-22.
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>"With the application of collective, the pitch will increase and the connect point is no longer coincident with the cone /flap axis ..."<

Agree.
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>"... the blades flap up or down from this position there will be a pitch change which is known as pitch coupling."<

Disagree. It is known as 'pitch-flap' coupling and is one of many couplings.
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A minor thought re the R-22 and the R-44:

The outboard hinges are referred to as flapping hinges and are also referred to as coning hinges. I suggest that under normal conditions they only function as coning hinges. This is because the strong centrifugal forces will pull any flap out of these hinges and place it in the teetering hinge. Sound reasonable?

[Kyrilian]

SouthXross,
For most intents and purposes, as I understand, the primary dynamics occur around the primary (center) hinge, while the coning hinges provide slight unloading of what would otherwise be a beam in bending (such as the JetRanger). The coning hinges do not play into the flapping dynamics (except perhaps a tiny bit). What this means is that you can neglect the two coning hinges when you model the dynamics, and think of it as a JetRanger. Lu says that you want the pitch link to be in line with the teetering hinge (in this case the center hinge) but that is only if you do not want delta-3. This is how the JetRanger is rigged.

When you preflight an R22 or R44 you can push up on the blade to make the whole thing teeter about the teetering hinge (and not the coning hinges, right? If you are very careful, you will note that as you tip one blade up, it should pitch down very slightly because the horn is 'outboard' of the teetering hinge (regardless of where it is with respect to the coning hinges). The same thing will happen in flight. Now, if you fixed the teetering hinge and went out flying to pretend you were flying a two-bladed articulating head, you'd be in trouble! In this case, the effective hinges would be the coning hinges, and the pitch link would be attached inboard. Of course, if you switched sides so that the pitch horn was at the trailing edge of the blade, then you'd want it inboard for stability, but that's just semantics. Make sense?

[Lu Zuckerman]

To: Dave Jackson

“If you and I are reading SouthXross' remarks correctly, then we all three agree that an increase in the coning/flapping angle of the R-44 will result in an increase in pitch. We must assume though, that Frank Robinson knew what he was doing when he located the pitch link's upper joint inboard of the coning/flapping hinge. This is particularly so, since he has gained much prior knowledge from the R-22”.

Response:

As I stated previously if the pitch horn / pitch link connection was inboard of the cone hinge then it was by design. I have a picture before me of an R-44 rotor head and it appears that SouthXross was correct. It also appears that if the blade cones up pitch will be added but I can’t see why he would do it that way as it is counter to basic rotor system design. But, then again his whole rotor system goes against the grain of conventional design.

“The outboard hinges are referred to as flapping hinges and are also referred to as coning hinges. I suggest that under normal conditions they only function as coning hinges. This is because the strong centrifugal forces will pull any flap out of these hinges and place it in the teetering hinge. Sound reasonable”?

Response:

Please read the R-22 / R-44 POHs as they indicate that certain flight maneuvers or the incorrect response to zero G can lead to excessive flapping and the blades could hit their droop restraints. The blades do flap under normal maneuvering conditions and this is born out by the unusual wear patterns on the cone hinges and the teeter hinge to a lesser extent. The wear patterns are the result of leading and lagging which occur when blades flap.


To: Kyrillian

“For most intents and purposes, as I understand, the primary dynamics occur around the primary (center) hinge, while the coning hinges provide slight unloading of what would otherwise be a beam in bending (such as the JetRanger). The coning hinges do not play into the flapping dynamics (except perhaps a tiny bit). What this means is that you can neglect the two coning hinges when you model the dynamics, and think of it as a JetRanger. Lu says that you want the pitch link to be in line with the teetering hinge (in this case the center hinge) but that is only if you do not want delta-3. This is how the JetRanger is rigged”.

Response:

Regarding your comment on cone hinges please read my response to Dave Jackson on this matter. The Bell has the pitch horn / pitch link connection approximately coincident with the teeter hinge with low pitch and neutral teeter. However when the pilot pulls collective the two points are no longer coincident with each other and when the blade system teeters you get “pitch flap coupling”. On the Robinson R-22 or any other rotor system that has flapping capability the pitch horn /pitch link should be approximately on the center of the cone or flapping hinge. It seems that the R-44 has deviated from this design norm. On helicopters that have no flapping hinge but are able to flap then the pitch horn pitch link connection should be near or at the theoretical flapping point. On these types of helicopter s you get “pitch flap coupling when the connection is no longer coincident with the theoretical flapping point. During maneuvering this theoretical hinge point can change and there is a change +/- in the pitch flap coupling. The only time you would not get pitch flap coupling on any helicopter is when the connection point is coincident with the flapping / coning axis. This can only happen when the rotor system is static or possibly at speed with no pitch. However helicopters do not fly in this condition and therefore, are subject to pitch change +/- when the blades flap above or below the commanded orbit (tip path).


"When you preflight an R22 or R44 you can push up on the blade to make the whole thing teeter about the teetering hinge (and not the coning hinges, right? If you are very careful, you will note that as you tip one blade up, it should pitch down very slightly because the horn is 'outboard' of the teetering hinge (regardless of where it is with respect to the coning hinges). The same thing will happen in flight. Now, if you fixed the teetering hinge and went out flying to pretend you were flying a two-bladed articulating head, you'd be in trouble! In this case, the effective hinges would be the coning hinges, and the pitch link would be attached inboard. Of course, if you switched sides so that the pitch horn was at the trailing edge of the blade, then you'd want it inboard for stability, but that's just semantics. Make sense"?

Response:

There is an excellent web site for the Robinson helicopter and I believe it was once hosted by MIT. On this web site they show exactly what you are describing in that when the blade system is teetered down over the nose pitch is removed. If you looked at the rear blade then pitch would be added. This is a static demonstration of pitch flap coupling. In performing this demonstration it would be exactly the same if you did it on a 206. If on an R-22 you immobilized the head and moved a blade on its cone axis the pitch change would be minimal because the connection point and the cone hinge are nearly coincident with each other. If you pulled collective and did the same thing there would be a change in pitch. If you did it on the forward blade the pitch would decrease. If you rotated the blade an additional 18-degrees the pitch change would be even greater. If you did the same on an R-44 according to the picture, the blade would increase in pitch and if you increased collective the pitch change would be even greater. Again if the blades were rotated an additional 18-degrees the change would be greater. Once again I repeat this is counter to sound design. If Frank Robinson designed it this way he had his reasons. Maybe someone on this thread can contact him and ask why.

[Kyrilian]

Lu,
'Round and round we go

"Please read the R-22 / R-44 POHs as they indicate that certain flight maneuvers or the incorrect response to zero G can lead to excessive flapping and the blades could hit their droop restraints."

And this is different from any other teetering rotor how??

"The Bell has the pitch horn / pitch link connection approximately coincident with the teeter hinge with low pitch and neutral teeter. However when the pilot pulls collective the two points are no longer coincident with each other and when the blade system teeters you get
“pitch flap coupling”."

This is insignificant. Try it out and you'll see that this is on the order of sine(theta)*cosine(beta), where theta is the blade angle (zero if the link point is in the same plane as the feathering hinge) and beta is the flapping angle. For 5° of flapping and 10° blade angle you get (0.173)*(1-0.996)=0.000692. Take the arcsine of this and you get a change in feathering due to flapping of 0.038°, which is .76% of the flapping angle. Compare this to something like 30% for an 18° delta-3 hinge. The point I'm trying to make is that the cosine term is negligible for small angles and the 206 head has no significant coupling. Thus, the effect that you speak of (found on all helicopters with positive pitch pulled in, you say) is not worth worrying about. So please stop getting tripped up by it. This is not flap-pitch coupling!

"On the Robinson R-22 or any other rotor system that has flapping capability the pitch horn /pitch link should be approximately on the center of the cone or flapping hinge. It seems that the R-44 has deviated from this design norm. On helicopters that have no flapping hinge but are able to flap then the pitch horn pitch link connection should be near or at the theoretical flapping point. On these types of helicopter s you get “pitch flap coupling when the connection is no longer coincident with the theoretical flapping point. During maneuvering this theoretical hinge point can change and there is a change +/- in the pitch flap coupling. The only time you would not get pitch flap coupling on any helicopter is when the connection point is coincident with the flapping / coning axis. This can only happen when the rotor system is static or possibly at speed with no pitch. However helicopters do not fly in this condition and therefore, are subject to pitch change +/- when the blades flap above or below the commanded orbit (tip path)."

See above

"There is an excellent web site for the Robinson helicopter and I believe it was once hosted by MIT."

A very comprehensive sight indeed. It was actually written by Paul Cantrell who I believe has no connection to MIT other than that someone at MIT asked if they may host it. Other sites did and still do host it to my knowledge. I digress...

"On this web site they show exactly what you are describing in that when the blade system is teetered down over the nose pitch is removed. If you looked at the rear blade then pitch would be added. This is a static demonstration of pitch flap coupling. In performing this demonstration it would be exactly the same if you did it on a 206."

I beg to differ. See above.

"If on an R-22 you immobilized the head and moved a blade on its cone axis the pitch change would be minimal because the connection point and the cone hinge are nearly coincident with each other. If you pulled collective and did the same thing there would be a change in pitch."

As I said in my earlier post, if you immobilized the head (fixed it to the rotor shaft and kept the teetering hinge from moving) you would have disasterous consequences if the pitch link was inboard of the coning hinges (now flapping hinges) as this would be positive (unstable) coupling. The problem with your argument is that this is not how the rotor is rigged. The head is not immobilized, so pretending that it is makes no sense. There is no hinge spring on the teetering hinge but there are quite stiff springs on the coning hinges. Thus, any flapping motion will occur about the teetering hinge, and not the coning hinges. Coning angle does not vary dynamically in any significant way, and the flapping up of one blade coincides with the flapping down of the other one (for aerodynamic reasons, not because there is any hinge restraint). The coning hinges serve a pseudo steady-state requirement, not 1/rev loading.

"Once again I repeat this is counter to sound design. If Frank Robinson designed it this way he had his reasons. Maybe someone on this thread can contact him and ask why."

Last time he offered an explanation you dismissed it. I doubt he wants to waste his time defending against such attacks.

[Lu Zuckerman]

To: Kyrillian


“This is insignificant. Try it out and you'll see that this is on the order of sine(theta)*cosine(beta), where theta is the blade angle (zero if the link point is in the same plane as the feathering hinge) and beta is the flapping angle. For 5° of flapping and 10° blade angle you get (0.173)*(1-0.996)=0.000692. Take the arcsine of this and you get a change in feathering due to flapping of 0.038°, which is .76% of the flapping angle. Compare this to something like 30% for an 18° delta-3 hinge. The point I'm trying to make is that the cosine term is negligible for small angles and the 206 head has no significant coupling. Thus, the effect that you speak of (found on all helicopters with positive pitch pulled in, you say) is not worth worrying about. So please stop getting tripped up by it. This is not flap-pitch coupling”!

I’m impressed but I didn’t understand a word or number of it and I doubt if 1% of the participants on these threads understand it either.

On a Bell helicopter if the pitch horn / pitch link is coincident with the teeter axis and the static blade is tipped down over the nose there should be no change in pitch. This is because the teeter hinge and the pitch link connection are on a common rotational axis. If you performed the same action on the Robinson there will be an increase in pitch on the blade over the nose and a decrease in pitch in the blade over the tail. This is caused by the pitch link connection not being on the center of rotation (teeter hinge). If you immobilized the head and raised the forward blade on the cone hinge and the pitch was at a point where the pitch link connection and the cone hinge were coincident with each other there would be no pitch change. If you added pitch and the two points were no longer coincident with each other and raised the same blade there will be a pitch change with pitch being decreased. If you performed the two tests on the rear blade the results would be the same.

If you performed this same test on a Sikorsky head with the swashplate in the neutral position there would be no change if the blade were raised with the two points being coincident. If you increased pitch on the blades and raised the blade there would be a change in pitch because the two points are no longer coincident with each other. As I stated previously this is the same phenomenon exhibited on a tail rotor. It is called the delta hinge effect.

Now I don’t know how this is taught in MIT or any other engineering schools but in the 18-19 factory schools I have attended this is how it is taught.

Changes in Italics: Hopefully they print out that way.

“On this web site they show exactly what you are describing in that when the blade system is teetered down over the nose pitch is added. If you looked at the rear blade then pitch would be removed. This is a static demonstration of pitch flap coupling. In performing this demonstration it would be exactly the same if you did it on a 206." On this point I’m in error. If the blade is over the nose on the Bell and you tipped the blade down over the nose and the two points were coincident with each other there would be no pitch change because the two points are actually on a common pivot point. This is due to the 90-degree pitch horn.

Regarding the Robinson head, if you performed the test in order to get maximum pitch change the head should be disposed 18-degrees past the longitudinal centerline. This is caused by the 72-degree pitch horn. You will get some pitch change if the blades are disposed over the longitudinal axis but not as much.

Here is another test you can perform on a Robinson. Do it first on a Bell and then on the Robbie.

On the Bell place the blade over the nose. Move the stick fore and aft. The blades will not move. Move the cyclic left and right. The blades will move. Now, place the blade over the lateral axis. Move the cyclic left and right. The blades will not move. Now, move the cyclic fore and aft. The blades will move.

Now, try the same thing with the Robinson. With the blades over the longitudinal centerline move the cyclic fore and aft. The blades will move. Now place the blades over the lateral centerline and move the cyclic left and right. The blades will move. The only way you can get the same responses on the Robbie as you got on the Bell is to dispose the blade 18-degrees ahead of the respective axes. In that case when you move the cyclic in the stated directions the blades will not move. This is caused by the 72-degree pitch horn on the Robbie as opposed to the 90-degree pitch horn on the Bell. Thus, the infamous 18-degree offset.

Regarding my comment about immobilizing the head I picked that up from an illustration made on a previous post. Regarding the method of rigging the Robinson the head is immobilized during the rigging procedure.

Regarding flapping on the Robinson. Discounting the extreme flapping caused by certain maneuvers or a bad recovery from zero G there is flapping and there is a tendency to lead and lag caused by the flapping. This is born out by the wear patterns on the cone hinges and the teeter hinge. Contrary to what you stated flapping goes on all the time. Someone else indicated that flapping was minimal because of the centrifugal loading on the blades which according to this individual would keep the blades aligned with the inplane axis of the rotor head. He obviously missed out that with all of this centrifugal loading the blades are coned and the centrifugal loading does not return the blades to the disc plane as long as the helicopter is airborne.


"Once again I repeat this is counter to sound design. If Frank Robinson designed it this way he had his reasons. Maybe someone on this thread can contact him and ask why."
“Last time he offered an explanation you dismissed it. I doubt he wants to waste his time defending against such attacks”.

Frank Robinson’s reply to me addressed pitch flap coupling and how it effected the phase angle of the rotor system. All of his comments were addressed to the R-22 and not the R-44 which is the subject of this discussion.

With your vast understanding of helicopter aerodynamics please tell me why when a helicopter hits a gusting condition you would have the blade increase pitch when the gust has raised the blade from the tip path. It seems to me that this is counter to normal design. This is why I made the comment that if he designed it that way he had his reasons. I just don’t know what that reason was. If you were designing a helicopter would you do it that way?

[Grey Area]

Lu,

It is not clever to rubbish aeronautical engineers (or test pilots ) because you don't understand them and just because you don't understand the math doesn't make it wrong. It works, try it (that's what test pilots do), don't just look at it and shout "I DON'T UNDERSTAND, IT CAN'T BE" it got the Luddites nowhere nor will it improve your standing.

Believe me it works and the math is correct. Granted it's a trade off but so are all your beloved Bells and Sikorskis.

[ 22 November 2001: Message edited by: Grey Area ]

[Balance!]

SouthX - So you're SouthX - I told you you'd open a can of worms......rgds balance

[Rotor Nut]

Here we go again - the main point has been said - it works, it flys, and like anything it can be abused.

I flew an R44 as suggested by LZ earlier this year to test his theories. None of what he forecast happened [whereever you push that stick the helicopter follows]. The rotor head works fine for me! I don't care what maths are involved.

[HeloTeacher]

I got a chance to drop in on PPrune and look what I found.

I haven't been abel to try any more specific tests as per this never-ending discussion but I have a couple observations in the last little while during some STC test flights in an RH44.

Stick sense was very good, any specifics would require more math than I've had time for. There was not an obvious angle.

Coning hinges: during the auto entries we encountered a minimum NR of 78-79%. A large vibration resulted and we landed post-haste. The cause was teflon wear goo (a technical AME term) that had been dragged down into the normal wear location on the coning bearing by our excessive coning angle (resulting from far too low NR). The bearings were cleaned and replaced with no further problems. With a set of these bearings on my desk and a good briefing from our maintenance people, I have good confidence in saying that the coning hinges are just that.

From what I've seen and read here, I'll offer these points. If a blade moves up or down a pilot will call it flapping. Flapping about the conign hinge (I call this coning) is restricted by the tusks. Flapping about the teeter hinge is restricted by teflon bumpers on the mast. Decide which flapping you are talking about.

[RW-1]

Hmmmm ..... well nothing more be said here.

RN sums up everything nicely.

[Hone22]

Yet another R22 Question

Hi all,

Anyone out there have fuel burn figures for the R22 Beta 2?

Is it too much to ask manufacturers to include a fuel burn graph?

The H300 has a very useful one that gives burn rates at different power settings but, I have yet to find any mention of Fuel burn in the robbie manual (am i looking in the wrong place).

Cheers & fly safe

Hone.

[baranfin]

I have been told to use 10gph for fuel computation. But I have never tried to figure out my own consumption average or anything.

[RW-1]

I was told it is 8 GPH, of course that would be brand new ship, brand new engine hehe ....

Baranfin, I plan with 10-11 as well.

[piloteddy]

The Beta II's clean aerodynamic design allows a high cruise speed up to 110 mph and an average fuel consumption of only 7 to 10 gallons per hour.

Taken from: http://www.robinsonheli.com/BetaII.htm

Hope it helps.

[Hone22]

R22 shut down procedure

Hi all,. .been chatting to a mustering pilot (r22) in Oz here and here told me standard practise for shutdowns was to turn off the mags. He had actually been told company policy was not to pull the mixture/cutout as cable breaks in the outback were too problematic(?).

Having thought this through a wee tad..................well a little fuel in the pots of a warm/hot engine will vaporise quick smart so shouldn't be a problem.

Initial reaction was "you what" (yrs of being told/shuting down meself the standard way) but really nothing wrong here...........or is there

Opinions Please (horror stories due to ......gratefully accepted)

Cheers & fly safe. .Hone

[baranfin]

Im not totally sure but I have often wondered the same thing, I mean isnt that how one shuts off his car? Although being young I have never dealt with a car with mixture control.

. .Although I would still stick to the factory recomended procedure. <img src="wink.gif" border="0">

[ 23 January 2002: Message edited by: baranfin ]</p>

[Lu Zuckerman]

This is based on what I learned over fifty years ago (52 to be exact). If you shut the engine down by turning the mags off and the twist grip is in the secured position the vacuum behind the butterfly will cause a very significant increase in the suction (I was told that there is no such word it is a differential of pressure). This will draw in a large amount of fuel from the idle jets and possibly the main jets depending on which side of the butterfly they are located. This raw fuel will be ingested into the hot cylinder and it may as stated above vaporize but it is still highly combustible. If there is a hot spot in the cylinder the fuel will ignite and the resultant firing of an individual cylinder could develop sufficient force to crack the crankshaft.

[vorticey]

later model cars are turned off by the computer(fuel and spark together) a lot of later carbyed cars used to have devices that turned the fuel off when the ignition was switched off(fuel cut off soliniods) to provent run on from a hot engine. older cars had less compression and diddnt suffer from running on so much.. .i dont know about cracking the crank lu and i dont see how there could be more vacuum when the engine slows down but the best way to shut an engine down in a plane would be to take away the fuel so a flick from the magneto wont start the prop when your having a bit of a look at it.. .helicopters its harder to accidently turn the engine and cause any unwanted spark so it probably doesnt matter so much.

if you dont use the mixture cutoff, how do you know its servicable? <img src="confused.gif" border="0">

[vorticey]

R22 maggie check 75%

i think the magie check should be done at 104% insted of 75%. its much easier to tell a crook plug. i wonder why they changed it?? <img src="mad.gif" border="0">

[Michael Jupp]

noise abatement ?