Am I missing something? Your last opportunity to shut me up.

[Lu Zuckerman]

In my attempt to discuss the so called 18-degree offset I was shut down by the likes of NickLappos and every other member of this forum that had any detailed knowledge of helicopter aerodynamics especially regarding phase angle and or phase angle shift.

It was my contention that the Robbie had a 90-degree phase angle and because of the 18-degree control offset the disc would tilt down to the left when forward cyclic was pushed. I was corrected when told that even though there was an 18-degree offset the rotor system had a 72-degree phase angle. Here is where it gets a bit blurry. The Bell has a 90-degree phase angle +/- so when the cyclic was pushed forward the blade would drop down over the nose (+/-). The blade having the maximum decreased pitch over the right quadrant and the maximum reaction 90-degrees later caused this. So far so good.

The Robbie on the other hand has an 18-degree control offset which means that the lowest pitch angle happens after the blade has crossed the right quadrant and moved 18-degrees in the direction of rotation. With a 72-degree phase angle the blade will have maximum down flap over the nose. If this is the case can you please try to visualize what the tip path looks like if the blade drops in the remaining 72-degrees from its position in the rotating disc. Also the opposite is true for the retreating blade which flaps up from 18-degrees past the left quadrant.

I have good powers of visualization and imagery yet I can’t see this happening. What am I missing?


:confused: :E :E :confused:

[heedm]

Consider the blade flapping to be a pendulum. The natural frequency of the pendulum is the period of rotation for a bell 206 rotor. For some helicopters the natural frequency of the blade is greater than the period of rotation of the rotor. This means that if the blade were allowed to do whatever it wants, in the latter example, the blade would flap through more than one cycle for each revolution of the rotor.

Of course, the blade isn't allowed to do whatever it wants. This is not a free oscillation, but a forced oscillation. Since cyclic pitch controls have the precise frequency of the rotor and since the generated forces are greater than restoring forces of the blade (pendulum), the blade flaps with the frequency of the forced oscillations (cyclic pitch control).

Lu, I know your background would have covered that stuff, but I thought a recap would clear parts up.

The blade is flapping at the frequency of the rotor even though that is not the blades natural frequency. Thus, to have the tip path plane lowest in front of the helicopter, it will be highest aft, and in a median position at the remaining cardinals.

The phase angle is just how much lead time you have to give the blade for it to react to the driving force. i.e. for the blade to stop flapping downwards, apply an upwards force 72 degrees earlier for a Robinson, 90 degrees earlier for a 206. The upwards force peaks 18 degrees after lowest flapping angle, but stays as an upwards force until 72 degrees prior to the highest flapping angle.

Small caveat, this is just the physics answer of the simplified system. The full system is quite unlikely to always have precisely a 72 degree phase angle, rather that was the best compromise with all factors considered. The true phase angle will have some variance, I wouldn't be surprised to hear of +/- 5-10 degrees for most helicopters. I'm sure some on the forum will have more detail on this.

Hope this helps.

Matthew.

[Dave_Jackson]

Lu,

It all has to do with the linkages;

The toe bone is connected to the foot bone
The foot bone is connected to the ankle bone
The ankle bone is connected to the leg bone
The leg bone is connected to ................. ;)
________________________

Seriously; :uhoh:

The swashplate is an ideal control device for simple teetering rotors. This is because they both operate with a 1P cycle. However, as rotor head technology has advanced, the swashplate has becoming less able to fully satisfy the rotor requirements. Unfortunately, the only simple modification that can be applied to a swashplate (& spider) control system is to change the phase angle.

Offset flapping hinges and delta3 are two methods that have been used to 'improve' the rotor. These two methods are very different from each other. However, since the swashplate is the de facto control system, and since the only simple modification is to change the phase angle, this modification is used with both of the above DIFFERENT rotor heads.

IMHO, heedm, and Nick, and many other, are all correct when talking about phase lag in the context of a rotor with offset flapping hinges. However, I strongly believe that, the delta3 rotor behaves differently, and that phase lag does not fully satisfy the requirements of delta3, but it is the only available method.

It will be easiest to come to an understand of the relationship between phase lag and offset flapping. Only then, should the more complex subject of phase lag and delta3 be considered. After that, any remaining masochists could consider phase lag and delta4. :yuk:

[headsethair]

Lu: Don't fret. Just put on your suit and beard, pre-flight the sleigh and get going. You'll be late. (And watch out for the offset on Blitzen's rig......)

[RDRickster]

I'm not going to get involved in the engineering aspects of this technical debate... way over my head. However, I'm happy to stir the waters up a bit! :p Here is a post from Frank Robinson on the subject, from a few years ago, that has been reposted here for convenience and referral...

I have read some of the comments about the R22 helicopter printed in this forum. Most were favorable and I appreciated that. However, some were obviously misinformed, and I will comment on several of those.

SIDESLIP WITH THE R22
Concerning the caution against excessive sideslips in the R22 flight manual, this was in part due to a misunderstanding by the FAA. In the Army training film on mast bumping, it showed excessive side slipping as one cause of mast bumping. This was true for the Army Bell Cobras and Hueys, because both aircraft have high centers-of-gravity and relatively low side silhouettes due to their high-mounted powerplants and low-mounted tailcones. During a severe sideslip, the resultant fuselage drag could be below the center-of-gravity and cause the helicopter to roll out of a turn, instead of into the turn, i.e. a negative dihedral or adverse roll characteristic. Airplanes prevent this by having wings with positive dihedral.

The basic R22s and R44s have low-mounted engines, high tailcones, and aerodynamic mast fairings. Consequently, neither the basic R22 or R44 had any tendency toward adverse roll during FAA certification. However, all helicopters (including the R22 and R44) tend to have an adverse roll characteristic when they are equipped with inflated floats, because the floats move the side silhouette area down considerably. For that reason, I did not object to the caution in the R22 flight manual against extreme sideslips during forward flight.

R22 FAA TYPE CERTIFICATION
During the R22 certification, both the FAA test pilots and our own company test pilots flew the R22 through all required maneuvers and flight regimes, and it met all of the FAA regulations. No exemptions were issued for the R22 by the FAA during its certification. Also, I was not a DER (designated engineering representative) during the FAA certification of the R22. No DERs were used during its original certification. After it was certified, the FAA appointed me as a DER with limited authority, so I could approve some minor design changes which commonly occur during production of a new aircraft.

R22 ROTOR SYSTEM
I have read various explanations in this forum attempting to explain the dynamic and aerodynamic characteristics of the R22 rotor system, especially the 18-degree delta-three angle designed into the R22 swashplate and rotor hub. This is a highly technical subject which can only be fully explained using very technical engineering terms. However, since there appear to be a number of misconceptions and a great deal of interest by some pilots and mechanics, the following is a physical explanation of the reasons for the 18 degree delta-three phase angle.
First, keep in mind that the 18 degrees is only in the upper rotating half of the swashplate. The lower non-rotating swashplate is aligned with the aircraft centerline and always tilts in the same direction as the cyclic stick.

Many helicopter engineers have difficulty understanding how delta-three (pitch-flap coupling) affects the phase relationship between the rotor disc and the swashplate. Delta-three only affects the phasing when the rotor disc is not parallel to the swashplate and there is one-per-rev aerodynamic feathering of the blades. For instance, feathering occurs while the rotor disc is being tilted, because an aerodynamic moment on the rotor disc is required to overcome the gyroscopic inertia of the rotor. But once the rotor disc stops tilting, the rotor disc and swashplate again become parallel and the delta-three has no effect on the phasing. Aerodynamic feathering also occurs in forward flight, because it is necessary to compensate for the difference in airspeed between the advancing and retreating blades. Otherwise the advancing blade would climb, the retreating blade would dive, and the rotor disc would tilt aft.

The R22 rotor system was designed with 18 degrees of delta-three to eliminate two minor undesirable characteristics of rotor systems having 90-degree pitch links. 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, sometimes referred to as “wee-wa.” This occurs because while the rotor disc is tilting, 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. If no lateral cyclic was applied, this would result in a rotor disc tilt to the right while the rotor plane was tilting forward. Pilots subconsciously learn to compensate for this by applying some lateral cyclic as the cyclic is being moved forward. The amount of delta-three required to eliminate “wee-wa” in the R22 rotor system was calculated to be 19 degrees.

The other undesirable characteristic in rotor systems having 90-degree pitch links is the lateral stick travel required with airspeed changes during forward flight at higher airspeeds. The ideal rotor control system would require only longitudinal stick travel to increase or decrease the airspeed. This is not possible with a 90-degree pitch link system, because the rotor coning angle causes the rotor disc to roll right as the airspeed increases. This occurs because the up-coning angle of the forward blade increases that blade’s angle-of-attack with increased airspeed, while the up-coning angle of the aft blade reduces its angle-of-attack. Consequently, the forward blade then climbs while the aft blade dives, thus causing the rotor disc to roll right with increased airspeed. To compensate for this with a 90-degree pitch link rotor, the pilot must apply some left lateral cyclic as the airspeed increases. The amount of delta-three required to compensate for this effect in the R22 rotor system was calculated to be 17 degrees.

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.

Frank Robinson

[Lu Zuckerman]

With all of the above reasons most of which indicated that the phase angle is the measurement in degrees of rotation for the blade to flap to its’ maximum downward or upward point in response to control input depending if it is advancing or retreating.

My point is that you try to visualize the disc or tip path plane and if the blade takes 72-degrees to respond to the maximum input, what is the path of the blade in relation to the tip path plane. On the Bell or any other helicopter the tip path plane is one continuous line (discounting gusting and resultant flapping). On the Robinson the blade does not deviate from the tip path plane if it did you would encounter serious vibration. If this is the case at what point during rotation does the blade respond to the maximum control input and describe the path of the blade in relation to the tip path plane?

It’s going to be a long winter. 10” on the ground and more coming.

:E :E :confused:

[Dave_Jackson]

At Bell;
Pilot tells Control to go to 10º.
Control tells Pitch to go to 10º.
Pitch tells Flap to go to 10º.
Flap is very obedient and runs like hell to 10º, in 1/4 revolution.


At Robinson;
Pilot tells Control to go to 10º.
Control tells Pitch to go to 10º.
Pitch tells Flap to go to 10º.
Flap runs like hell toward 10º, but Delta comes along and says that Pitch has changed his mind and only wants 8º.
Flap stops at 8º in less time than 1/4 revolution because he was running so fast at the start.

Control tells Pitch that he wants 10º, not 8º
Pitch yells at Flap to go 2º more degrees.
Then that little **** Delta comes along again and reduces Flap's trip.
Every time Pitch give an instruction to Flap, Delta shows up and reduces it.
Final, with Pitch yelling at Flap for more, and Delta reducing Pitch's instruction, Flap finally makes it to 10º.

[TimS]

I'm just amazed the bl**dy things fly at all !!

[Helipolarbear]

Lu,
I'm glad ther are experts on the R22 like you...contibuting great aeromechanical questions and comments. It provokes very good thoughts. However, as I rarely disagree with your comments, I have a small query.

To quote you: 'My point is that you try to visualize the disc or tip path plane and if the blade takes 72-degrees to respond to the maximum input, what is the path of the blade in relation to the tip path plane'.

The tip path plane is the combined average sweep of 2,3,4,5,6,7 bladed rotors on their individual rotational plane, and thus produces a relevent line which aids the pilot to cue his path and gives him a valid cue also for the direction of the TAF produced.
The offset for gyroscopic precession is academic, but must include such considerations as bellcrank coupling, airframe torque twist, trans torque twist etc.........just where is the great Zukerman going with this?
Did I mention....Happy Xmas or Chanukah......Lu....you the R22
Man....by all accounts! :p ;) :cool:

[Lu Zuckerman]

To: Helipolarbear

I really don't know where I am going with this. What I am trying to do is reconcile myself to what is happening in the Robbie rotor system while it is turning in flight. Every thing that I have heard on this forum is contrary to what I was taught and what I have taught. This is not to say that the information offered on this forum is wrong. As they say you can’t teach an old dog new tricks and I am old (73 this month). When I ask a question I am bombarded with Newtonian Physics and other engineering speak that is beyond me and I would assume many of the other members of this forum. Another major problem that I have encountered is the difference in terminology and/or meaning relative to gyroscopic precession, centrifugal force and many other dynamic properties relative to rotor systems.to include phase angle shift.

Here is the problem as I see it. (When forward cyclic is pushed). On a Bell helicopter when the blades are aligned with the longitudinal centerline they are in a point of neutrality (at the collective pitch setting). On a four blade Sikorsky system (possibly not on the S-76) when the blades are aligned with the longitudinal centerline the blades are at a point of neutrality (at the collective pitch setting). On the Robinson the when blades are 18-degrees ahead of (in the direction of rotation) the longitudinal centerline they are in a point of neutrality (at the collective pitch setting).


With my limited understanding of helicopter aerodynamics I assumed that when the pilot pushed forward cyclic that due to “Gyroscopic precession” the disc would dip down to the left of the nose by 18-degrees. Everyone disagreed with me stating that the Robbie rotor system had a 72-degree phase angle. It was my contention that with the pilot making adjustments to the cyclic to counter CG offset, inflow roll, blowback and transverse flow effect that he in effect masked the 18-degree offset and was totally unaware of it.

In other posts I indicated that the 18-degree offset would effect the recovery from Zero-G if the pilot moved the cyclic back to load the rotor in that the 18-degree offset would exacerbate the right roll past the point of recovery. I indicated that to counter the offset the pilot must move the stick to the rear and to the left. Tim Tucker has stated this but not for the same reasons.

On repeated occasions I have asked the Robbie pilots on this forum to perform two very simple tests to either prove me right or wrong. On member of this forum did one of the tests and indicated with movement of the cyclic straight ahead from the rigged neutral position the helicopter did fly to the left.

I would hire a Robbie for an hour to perform the tests but the only R-22 in my area was destroyed in a dynamic roll over.

Hopefully this answers your question.

:E :confused: :E

[Gomer Pylot]

First, let me say that I've never been inside a Robinson of any type. I have no idea what happens in one, and have no strong desire to learn.

But Frank Robinson is entirely correct when he says that with most helicopters, left cyclic is needed when you initially start forward, and after ETL, as you accelerate, more left cyclic input is needed. If the R22 does indeed give some left tilt with forward control input, it could, in theory, eliminate the requirement for left input to prevent the right roll and drift.

So what's the problem? I don't give a tinker's damn exactly where the tip path plane goes, as long as the result is that the helicopter goes where I want it to. I've grown accustomed to putting in the left cyclic all the time, but it took some getting used to when I was young and stupid and learning to fly helicopters while wearing a green uniform. If 18 degrees of offset makes it easier to fly, then I'm all for it, regardless of the difficulty Lu has in visualizing what happens. The R22 obviously does fly.

[Lu Zuckerman]

To: Gomer Pylot

But Frank Robinson is entirely correct when he says that with most helicopters, left cyclic is needed when you initially start forward, and after ETL, as you accelerate, more left cyclic input is needed. If the R22 does indeed give some left tilt with forward control input, it could, in theory, eliminate the requirement for left input to prevent the right roll and drift.

In all of my posting and all of the responses I have never seen a post that indicated the need for the input of left cyclic in a Robbie. Mostly the pilots are indicating the need for right input to counter inflow roll, blowback, and transverse flow effect.

If what you say is true I do not believe that FR planned it that way. In fact if it does flap down over the left side then there is a distinct possibility or running out of right cyclic. It's possible

:E :E

[RDRickster]

I've never "run out of right cyclic" in a normal flight condition in the Robbie. In fact, I've never even come close (even when doing some fairly aggressive turns while practicing 180 autos). I hope I'm never in an attitude where I have to find out that you might be correct!

[Gomer Pylot]

Lu, perhaps I wasn't clear enough. I wasn't talking about Robinson helicopters needing left cyclic. I plainly stated I have never been in one. Every other model I've flown, and I've flown several, needs lateral cyclic to maintain a straight track when the cyclic is moved forward. If the Robinson needs none, than that indicates that Mr. Robinson's design works pretty well, doesn't it?

[Flight Safety]

Lu, a couple of quotes from Frank Robinson, thanks to RDRickster...

First, keep in mind that the 18 degrees is only in the upper rotating half of the swashplate. The lower non-rotating swashplate is aligned with the aircraft centerline and always tilts in the same direction as the cyclic stick.

Many helicopter engineers have difficulty understanding how delta-three (pitch-flap coupling) affects the phase relationship between the rotor disc and the swashplate. Delta-three only affects the phasing when the rotor disc is not parallel to the swashplate and there is one-per-rev aerodynamic feathering of the blades. For instance, feathering occurs while the rotor disc is being tilted, because an aerodynamic moment on the rotor disc is required to overcome the gyroscopic inertia of the rotor. But once the rotor disc stops tilting, the rotor disc and swashplate again become parallel and the delta-three has no effect on the phasing. Aerodynamic feathering also occurs in forward flight, because it is necessary to compensate for the difference in airspeed between the advancing and retreating blades. Otherwise the advancing blade would climb, the retreating blade would dive, and the rotor disc would tilt aft.

Frank is saying that Delta-3 is used to help level the rotor disk (because of the blade-flapping to blade-pitch-angle coupling), but when the disk is level, phasing is not affected because the lower swashplate tilts in the same direction as the cyclic. This should be fairly clear.

What's interesting however is how this pitch-flap coupling is implemented in the R-22 rotorhead. Keep in mind that the R22/R-44 two blade rotorhead is unique in that it has both a teetering hinge and 2 flapping hinges. No other rotorhead to my knowledge, has this hybrid arrangement of 3 flapping hinges, so that each rotor blade can flap on 2 hinges, instead of just one.

Now consider trying to build Delta-3 into a rotorhead where each rotor blade can flap on 2 different hinges (a fully articulated type flapping hinge and a teetering type hinge). Do you design the Delta-3 to respond to the teetering hinge, or do you design the Delta-3 to respond to the blade flapping hinge? A very Interesting question!

Frank appears to have decided to design the Delta-3 to respond to the movements of the teetering hinge only. Schematics of the rotorhead that I've seen, appear to show that no Delta-3 was included to respond to movement of the flapping hinges.

What do suppose the dynamics would be with a system like this?

Lu, I suggest that you look at this very closely, as it took me a year and half to understand the dynamics of this system, with Delta-3 present for the teetering hinge, but no Delta-3 for the flapping hinges.

I wonder if you'll come to the same interesting conclusions that I did.

Hint: In the patent application for this design, it is stated that centrifugal force from the rotor blades is required to help stabilize the rotorhead. I wonder why?

[Lu Zuckerman]

To: Flight Safety

Frank appears to have decided to design the Delta-3 to respond to the movements of the teetering hinge only. Schematics of the rotorhead that I've seen, appear to show that no Delta-3 was included to respond to movement of the flapping hinges.

No self respecting Robbie pilot would refer to the Cone Hinge as a flapping hinge in fact very few of them will even admit to the blade flapping on the cone hinge. (CJ Eliassen are you there?)

You refer to the drawing on the patent application as your frame of reference. If I remember correctly the drawing shows the blades at the low pitch setting. In this instance the pitch horn/pitch link connection is coincident with the cone/flapping hinge. In this case if the blade were to be moved up there is no pitch flap coupling. Now, try to visualize the blade in the high pitch position, which raises the pitch horn/pitch link connection above the cone/flap axis. This means that if the blade is raised there is pitch flap coupling and pitch would be removed from the blade just like on a fully articulated rotorhead.

Suffice it to say there is pitch flap coupling when the blades teeter and when they flap.

Frank Robinson’s' comment about the relative positioning of the swashplates really proves my point about the 18-degree offset.

The lower swashplate is just like a Bell in that it moves in the direction of cyclic stick movement. The Bell has a 90-degree lead on the pitch horn so the upper (rotating) swashplate is in the same position as the non-rotating swashplate. On the Robbie the same is true however since the pitch horn on the Bell leads 90-degrees and the Robbie pitch horn leads by 72-degrees the blade on the Robbie does not get maximum pitch input until it has moved 18-degrees past the lateral axis (With forward cyclic input).

So what does Frank Robinson’s comment about the positioning of the upper and lower swashplates really mean? If anything.

:E :E

[RDRickster]

Agreed. Flapping generally occurs, as a unit, about the main hinge bolt. Individual blade coning occurs about the coning hinge.

[Flight Safety]

Lu, how often we look at the same things but see them differently.

I agree in part that the coning hinges have no Delta-3 when the rotorhead is level on the teeter hinge (which places the conning axis coincident with the pitchhorn/pitch link connection). I also agree that when the rotorhead is not level on the teeter hinge, then some Delta-3 is present when the blades move on the conning hinge.

But to me this is a result of the Delta-3 present by design when the rotorhead tilts on the mast via the teeter hinge. Because Delta-3 is not present at all times on the conning hinges, I stated in my earlier post that the Delta-3 seems to have been designed primarily for the teetering hinge. I also admit that when the rotorhead is tilted on the mast, the Delta-3 that's brought into play at the conning hinges, does in fact change the pitch of the blades back towards a level disk.

My question to you and what I'd like for you to consider, is that bearing this facts in mind, what happens when a Robbie rotor blade hits either a conning stop or a droop stop while in flight?

[Lu Zuckerman]

To: Flight Safety

My question to you and what I'd like for you to consider, is that bearing this facts in mind, what happens when a Robbie rotor blade hits either a conning stop or a droop stop while in flight?

This as they say is another kettle of fish. I still do not know who they are.

If and when a Robbie blade hits the flapping range stop or a droop stop in flight there is one critical thing that must be taken into consideration and that is the intensity of the contact and the energy imparted. If the contact is mild you might get a bump in the same way that a pilot would get in a fully articulated rotorhead. In that case the pilot can back off from his cyclic input and the bumping goes away. If in the case of the fully articulated rotorhead with severe bumping and the pilot does not take the hint and back off you could get (possibly) structural deformation.

On the Robbie head if the pilot encounters severe contact of the droop stop (The blade tusk contacting the droop stops) you can get mast bumping and the same is true if the pilot encounters severe contact of the upper flap range stops.

In your post you indicated the position of the blades in respect to the rotorhead. The only time the blades are disposed radially from the head is with low pitch and the cyclic is in the rigged neutral position. When the pilot pulls pitch the blades will cone up until the centrifugal force and lift counter each other and the helicopter will rise and all things being equal the disc (tip path) will be level with the horizon. This is discounting translation and CG adjustment. In any case the pitch horn/pitch link connection will be above the cone hinges and any deflection of the head resulting from cyclic input or gusting (teetering) or (flapping) will result in pitch flap coupling.


:E :E

[RDRickster]

Lu,

You said...
If and when a Robbie blade hits the flapping range stop or a droop stop in flight....
I can't imagine, given the centrifugal forces involved, that could ever happen in a normal flight condition. Low-G, servere turbulance, or possibly low RPM may allow droop stop / blade tusk contact. Didn't you previously post some examples of this?

[Flight Safety]

Lu, an excellent post, but a couple of points...

You said,

If and when a Robbie blade hits the flapping range stop or a droop stop in flight there is one critical thing that must be taken into consideration and that is the intensity of the contact and the energy imparted. If the contact is mild you might get a bump in the same way that a pilot would get in a fully articulated rotorhead.

What will it "bump" into? On a fully articulated rotorhead, it will bump into a hub that is rigidly attached to the mast. That's not the case in this design.

You also said,

When the pilot pulls pitch the blades will cone up until the centrifugal force and lift counter each other and the helicopter will rise and all things being equal the disc (tip path) will be level with the horizon. This is discounting translation and CG adjustment. In any case the pitch horn/pitch link connection will be above the cone hinges and any deflection of the head resulting from cyclic input or gusting (teetering) or (flapping) will result in pitch flap coupling.

How will the pitchhorn/pitch link connection be above the conning hinges? I thought that in this design, when the disk is level, there is no Delta-3 available to the conning hinges because the pitchhorn/pitch link connection is coincident with the conning hinge axis. Assuming you lift off into a simple hover (no wind), there will be no Delta-3 imparted into the conned rotorblades.

BTW thanks Lu, this discussion is really getting fun. :D

[Dave_Jackson]

Lu,When the pilot pulls pitch the blades will cone up until the centrifugal force and lift counter each other and the helicopter will rise and all things being equal the disc (tip path) will be level with the horizon. This is discounting translation and CG adjustment. In any case the pitch horn/pitch link connection will be above the cone hinges and any deflection of the head resulting from cyclic input or gusting (teetering) or (flapping) will result in pitch flap coupling. It appears that, the undersling dimension on a teetering rotor is based on the conning angle, when the rotor is at some 'average' loading. For instance, when the craft is maneuvering while flying at its cruise velocity.

IMHO it would seem to make sense to have the pitch horn/pitch link connection level with the cone hinge when the rotor is under similar conditions to the above. This way, the geometric errors of the control linkages are very minimal during 'average' loading and they only have a small effect when the collective is at its extremes.

________________

P.S.
This thread started by questioning the need for a reduced phase angle in the control system of a rotor that incorporates delta3. Since you prefer non-technical descriptions, my previous post was written in a lighthearted manner, but it does explain why the phase angle is less than 90º.

To explain it slightly differently; Assume that the Bell blade and the Robinson blade both start off, at the same speed (rotational speed and flapping speed), towards a 10º flap. The Bell blade gets there 90º later. The Robinson's delta3 reduces the objective to 8º flap (for the first revolution only). Therefor the Robinson blade gets to its objective sooner. In other words, it gets there in fewer degrees of rotation because it did not have to flap as far as the Bell blade did.

[Lu Zuckerman]

To: Flight Safety and Dave Jackson who posted while I was typing my response.

On a fully articulated rotorhead (Sikorsky type that has offset hinges) if the blade flaps to its’ lower limit it will bottom out on the centrifugal droop stop. If it flaps to its’ upward limit it will hit an adjustable stop that limits the upward movement. The rotorhead reacts these bottoming loads and the load is ultimately reacted by the fuselage and the pilot feels it in the seat of his pants and in some cases there is an accompanying noise.

On the Robbie when the normal range of movement is exceeded in the upward movement on the teeter hinge the rotorhead will bottom out on the teeter stop. If the contact load is severe enough mast bumping will result with the ultimate fracture of the mast. If the upward flapping limit on the cone hinge is exceeded the blade spindle will contact the Up-coning stop. If the contact energy is severe enough the upward movement of the blade can cause a divergence of the rotorhead driving the opposite blade downward.

If the blade movement is downward the spindle tusk will contact the droop stops If the energy exchange is strong enough the spindle tusk can fracture and the result is blade incursion. If the energy does not result in the fracture of the spindle tusk the entire rotor systen can diverge and mast bumping will result.

The pitch horn/pitch link connect point is coincident with the cone hinge when there is no pitch in the blades. In this case if an individual blade reacts to gusting it can flap up with no pitch flap coupling. If the entire rotor system reacts to gusting in this situation then there is pitch flap coupling. If pitch is pulled the two points are no longer coincident with each other and any deflection of a blade due to gusting will result in pitch flap coupling. The same is true if the rotorhead teeters due to gusting.

When the pilot pulls collective pitch the swashplate moves upwards and this in turn raises the pitch link. As a result the blades increase pitch and the coincident points diverge from each other.

QUOTE: Assuming you lift off into a simple hover (no wind) and no compensation for tail rotor translation and CG shift), there will be no Delta-3 imparted into the coned rotorblades.

This is true because the blades are following the level swashplate and the distance between the pitch horn and the rotating swashplate remains constant. It is only when the blades deviate from this condition due to gusting that you get pitch flap coupling. Even when the swashplate is tilted the same is true. You only get pitch flap coupling when the blades move up or down in relation to a fixed swashplate (during flapping about the cone hinge). However if the blades are in the pure radial position and the head moves on the teeter hinge you will get pitch flap coupling.

:E :E

[RDRickster]

Okay, even the feeble minded can comprehend most of what you folks are getting at; however, there is a fair amount of disagreement. Lu, you said...
...If the upward flapping limit on the cone hinge is exceeded the blade spindle will contact the Up-coning stop....
We'll get to the other parts of your post, but I'd like to see a specific example of this. I'm unaware of any NTSB reports indicating this has happened on a Robbie. Perhaps you have a specific example?

[Lu Zuckerman]

To: RD Rickster

The up coning stop is obviously there for a purpose and that is to limit the upward flap of the blade. I do not know if any contact between the spindle and the Up-coning stop resulted in mast bumping. However in many cases where there was a loss of control accident there is a lot of evidence of the spindle tusk contacting the droop stop with such force to cause the spindle tusk to fracture. As I indicated previously when this happens there is nothing to limit the flapping and the blade hits the tail cone or the cabin.

This is the result of extreme flapping brought about by one of several things including heavy control inputs during sideslip and flying out of trim among others.

I got into a big discussion with CJ Eliassen on the Helicopter Forum and he stated I was wrong about blade flap on the cone hinges and he also indicated that a fractured tusk was the result of extreme teeter not extreme flap. If the blades only coned on the hinges no amount of teeter will result in contact of the tusk on the droop stop. THe spindle will contact theTeeter stop first and this as you know is mast bumping.

:E :E

[[email protected]]

Is this flapping problem why Frank used Delta 3 in his rotor design? In a tail rotor, delta 3 is used to reduce the amount of flapping that takes place by reducing pitch as the the blade flaps - therefore the use of delta 3 will do the same on a main rotor. The R22 blades are long and thin and not very heavy and as such will respond very quickly to a pitch input changing the AoA. For the same design reasons they will not experience the same level of aerodynamic damping as a wider blade and therefore will out pace the swash plate if nothing is done. On this rotor, the phase lag will not be 90 degrees due to the rapid blade response (the same sort of thing achieved with high hinge offset rotors) hence the need to reduce the blades flapping response by 18 degrees so that when the cyclic goes forward the low point of the disc is at the front.
Any tendency of the blades to flap too much or too quickly is countered by the delta 3 effect in the first 18 degrees of rotation. This would reduce the amount of cyclic control required to counter inflow roll and flapback.

[Flight Safety]

Lu, I was tired when I posted yesterday and I've slept since then, so I take back some of what I said.

I'm convinced that there is no Delta-3 at the conning hinges of Robbie rotorheads. It doesn't exist even when you raise the pitchhorn/pitch link connection to create lift. You're correct when you said that raising the pitchhorn/pitch link connection for lift, does in fact cause this connection point to diverge from the conning hinge axis, but the connection point diverges vertically and remains basically in the same plane as the conning hinge axis. This is really no different from the typical geometry in the flapping (conning) hinges and pitchlink connection points found in most fully articulated rotorheads that have no Delta-3. In this case, a small geometry change does takes place when the pitch link is raised or lowered, but this has nothing to do with Delta-3.

So in the Robbie design then, there is basically no Delta-3 leveling correction provided to the rotorblades, if they simply flap up and down on the conning hinges (assuming the teeter angle doesn't change). Centrifugal force provides the main force (plus lift) to recenter the blades in the conning hinge. It's only when the teeter angle changes that Delta-3 leveling correction is applied.

So back to the issue of a blade hitting a droop stop or conning stop. I've been thinking through the issue of what happens to the teeter angle when a stop is hit and what effect this has on the pitch of the rotorblades?

I personally think that hitting a stop would cause an abrupt change in the teeter angle of the hub, and an abrupt change in the pitch of both rotor blades. It appears that the pitch change would cause a correction back towards a level disk.

What's interesting is that an abrupt change in the teeter angle puts the opposite blade near the same stop that was hit by the first blade. Since the blade pitch angles are aburptly changed to correct for the teeter angle change, the thought has occured to me that with the opposite blade placed nearer to the same stop and the abrupt pitch change driving the blade in the direction of that same nearby stop, an oscillation could possibly be set up.

I'd love to hear your thoughts on this possibility.

[Dave_Jackson]

CrabIs this flapping problem why Frank used Delta 3 in his rotor design? It is said that smaller rotors react faster than larger rotors. Robinson probably incorporated delta3 to lengthen the flapping time, so that the rotor did not get too far ahead of the pilot's reaction time. I think that his intent was to achieve the same improvement as was provided by the gyrobar of the earlier Bells and the paddles of the Hiller's, and do it with a smaller weight penalty.

On this rotor [w/ delta3], the phase lag will not be 90 degrees due to the rapid blade response (the same sort of thing achieved with high hinge offset rotors) IMHO, the difficulty in understanding the relationship between 'Delta3 & Phase lag' is that most people equate this relationship with the 'Offset flapping hinge & Phase lag' relationship. The action of a rotor with offset flapping hinges is very different from the actions of a rotor with delta3. The offset flapping hinge causes the rotor to respond faster to a given cyclic flight control input. Delta3, temporarily, reduces a given cyclic flight control input (i.e. flapping pulls out a percentage of the control's pitch instruction), which causes the rotor to respond slower to a given cyclic flight control input.

Phase lag is a flight control feature, which is applied to two different rotor features.

[RDRickster]

Lu,

You said...
....in many cases where there was a loss of control accident there is a lot of evidence of the spindle tusk contacting the droop stop with such force to cause the spindle tusk to fracture...
Again, I find it unlikely this is the result of individual blades reaching the limits of travel on the Coning Hinge. I'll combine my thoughts with the response below, as well...

Flight Safety,

You said...
....if they simply flap up and down on the conning hinges (assuming the teeter angle doesn't change). Centrifugal force provides the main force (plus lift) to recenter the blades in the conning hinge. It's only when the teeter angle changes that Delta-3 leveling correction is applied...
At close to 2700 rpm, the change in coning angle about individual blades is absolutely minimal. I don't know this for fact, but what I've read leads me to believe that one of the reasons RHC designed a Coning Hinge for each blade is because they could not make the rotor blades flex-in-plane to a significant degree. Therefore, Coning Hinges were built into the rotor head to allow individual coning (as well as to compensate for other aerodynamic forces).

Don't get me wrong. Of course, the blades have some flexibility (the reason lead/lag dampers aren't present). Nevertheless, the centrifugal forcesforces along the complete length of both blades cause the entire blade segment to "flap" about the Main Hinge Bolt (Teeter Hinge). Flapping occurs along both blades (as a single unit) about the Main Hinge Bolt (Teeter Hinge). Coning occurs about individual blades about the Coning Hinge.

Some of this goes back to Lu, but I can't fathom how spindle tusks contacting the mast have anything to do with coning angles. If a blade were at that extreme limit, then wouldn't it be likely that the entire blade length were at extreme angles about the Main Hinge Bolt (Teeter Hinge)? Again, low-G, severe turbulance, and/or low RRPM are probably the only things that could create such a condition. Therefore, I submit that in the normal operation of a Robbie, this is unlikely to happen. Isn't it?

[Lu Zuckerman]

To: RD Rickster

Regarding the contacting of any stops during normal flight you are completely correct. It is the abnormal conditions when the limit stops are contacted. With low rotor RPM the first stops to be contacted are the Up-coning stops. This limits the upward travel of the blade, which is pivoting on the cone hinges. Under these conditions the blade spindles contact both Up-coning stops. If this happens the blades will most likely fold up.

Under certain conditions of low rotor RPM and especially during Zero G the blade spindles will contact the Teeter stop. Either one or both stops will be contacted if there is extreme flapping on the teeter hinge resulting in the failure of the mast.

If there is extreme flapping of the blades with the spindles pivoting at the cone hinges the spindle tusks will contact the droop stop cross bolt and if the force is great enough the spindle can fracture. If the force is great enough and the spindle tusk does not fracture the head can be moved from its’ plane of rotation and the opposite blade spindle will contact the teeter stop and possibly damage the mast.

If you understand the above you can see that the spindle has three points of contact with the stops. There is the Up-coning stop, which is the rotorhead, and this limits the up coning range. There is the teeter stop, which limits the amount of head teeter, and finally there is the extension of the spindle (the tusk) which limits the downward movement of the blade

To: Flight Safety

Back to pitch flap coupling: The coning capability of the Robinson head is just like the flapping capability on a Sikorsky fully articulated rotorhead. On both aircraft when the controls are in the neutral position and the head is turning the pitch horn / pitch link connection point is coincident with the hinge (cone/flapping). When the collective is raised the hinges are no longer coincident with the pitch horn / pitch link attaching point. The blades will cone/flap up and there is no pitch flap coupling.

Once the helicopters are in flight the helicopters blades are subject to aerodynamic forces. If we can address gusting then both helicopters can be stationary. Forward flight or stationary makes no difference. If the rotor system is subject to gusting the blades will be moved from their track.

When the blade flaps up it will try to move the pitch link up with it. The blades are mounted on a spindle that permits pitch change. Since the swashplate is rigid then any upward movement of the blade will result in the pitch horn moving downward in relation to the blade or upward if the blade flaps down. This change in pitch provides a restorative force to bring the blade back in track by increasing or decreasing the blade pitch.

Back to RD Rickster

You mentioned that the Robbie blades are flexible inplane so it is not necessary to have a lead lag damper. This is correct, however the lead lag forces which are generated by blade flapping on the cone hinges is strong enough to cause the cone hinges to elongate into an egg shape. This is not true on all Robbies but mainly on those that have been abused.


:E :E

[headsethair]

Lu: it's the 21st. If you don't get some translational and depart Lapland soon, you will be late. Mince pies and glasses of brandy waiting! Ho ho. (Be wary of the tendency for the reindeer rig to right roll on departure - that's because a couple of the reindeer have a right testicle larger than the left. But Rudolph appears to have a very large left one that helps to pull things straight.)

[Lu Zuckerman]

To: headsethair

When all else fails balls will win out. By the way the sub title of my book "Finger Trouble" is "Grey hair and balls the secret to airline safety".

Merry Christmas and a Happy New Year.

:E :E

[[email protected]]

Dave - hence my point about using delta 3 to modify the flapping response of the R22 rotor which has low inertia and low aerodynamic damping - it reponds too quickly to cyclic input, just like a rotor with high hinge offset.

[headsethair]

LU: peace and goodwill to you and your balls. And fullsome praise to my favourite flying instrument : the piece of string.

[Dave_Jackson]

Crab

I agree with your statement that delta3 is incorporated to reduce the response rate of small rotors. Conversely, high hinge offset is incorporated to increase the response rate of large rotors.

The control system's phase lag is used with both, to assure that the craft tilts in the requested direction.
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Related trivia;
Most helicopters have a cyclic stick travel of 1" to 2" for each degree of blade pitch. The Sikorsky ABC helicopter initially had maximum cyclic range of only +3º to -6.1º. This was done to give the pilot better control, since there was more cyclic stick movement for each degree of pitch. The phase angle was only 40º

The craft had a hard landing, because it ran out of (forward ~ I think) cyclic. On the second craft they increased the range of cyclic travel. They also made the phase angle flight adjustable, from 70º to 0º. (Lu ~ This is a long way from Bell's 90º. ;) )