Dynamics ~ 2 Blade Rotor w/ Offset Flapping Hinges [Archive]

Lu Zuckerman

7th Dec 2002, 20:02

What you sketch is physically equivilent to a normal articulated rotor head, with the added stress riser (weight increase) of the bend in the hub, which sees awesome CF loads (yes Lu, CF!).

Thank you Nick for the mention of CF. However I do not believe that the sketch is similar to an articulated rotor system, as it is underslung like a Bell. Most flex rotors that I know of are not underslung.

On conventional underslung rotor head the drive axis and rotating axis are nearly coincident with each other which minimizes leading and lagging. On a conventional flex rotor when forward cyclic is introduced the disc rises at the rear and drops at the front. Assuming optimal conditions, when this rotor system is in a hover the drive axis and rotating axis are the same with minimal lead and lag. But when the disc is tilted the two axes separate with the driving axis remaining coincident with the mast and the driven or rotating axis is forward of the mast centerline. It is this difference that causes leading and lagging. The greater the difference the greater the amount of lead and lag.

If Dave’s sketch were to be put into practice the in plane bending loads resulting from the lead and lag would eventually result in failure or very high maintenance. The Robinson head even though it can teeter has very high lead lag loads which manifests itself in very high wear on the cone/flap bearings.

Ducking for incoming.

:)

Dave Jackson

7th Dec 2002, 21:44

crab,

This simplistic idea is intended for the recreational (homebuilt) helicopter. The objective is to provide low hour pilots with a slightly more responsive cyclic control, then they now get from a teetering rotor.

Just trying to reduce the 'safe ~ homebuilt' oxymoron. :)

Nick,

Thanks for the technical critique. To clarify the idea, sketch C/ is just a slight elaboration of the previous sketch B/

http://www.unicopter.com/Temporary/1127-2.gif

The 'undersling' on sketch C/ is what hopefully differentiates this hub from that of a normally articulated rotor head. I believe that the teetering rotor works because the two blades' centers (of mass, lift, drag percussion etc. etc.) are basically inline with the underslung teetering hinge, during normal flight conditions. This condition also exists in sketch C/.

In addition, if one blade on sketch C/ flaps up 5-degrees and the other blade flaps down 5-degrees, this centerline still passes through the 'Virtual center'. Therefor synchronous flapping on rotor C/ should be the same as the teetering on rotor A/. Many moons ago you mentioned that the mast absorbs the small amount of lead-lag in a conventional teetering rotor. Theoretically, this should apply to sketch C/, as well.

I wonder if there might be a vibrational problem. The two-blade configuration may result in a 2P vibration when the rotor disk is not normal to the mast, whereas three or more blades should not.

helmet fire

7th Dec 2002, 22:20

Irlandes:
You are correct in that teetering can also be a form of flapping, but the point I was making is that flapping can, and does, occur without any teeter, and occurs offset from the teeter point even in some two bladed systems. The point you make about conning hinges is well made, however the Heuy has a preset cone in the hub and no hinges for this purpose.

Dave:

I think you are continuing to ignore the flapping issue. As above, the blade can flap independantly of teeter, thus when one blade goes up, the other doesn't just go down the same amount as per your discussion. This means that the centre of lift, etc, will not as you theorise, stay aligned. An easy way to see this is the conning angle increase , which is not flapping but it shows that both blades can come up at the same time and demonstrates that the blades can move up and down independantly of teeter. Thus you enter the lead lag issues mentioned by Nick courtesy of Hooke's, etc. I did not know that the rotor mast absorbs lead lag, and when I think of that small Huey mast absorbing all that, I shudder. Luckily on the Huey they do have drag braces to help absorb the forces.

Lu Zuckerman

7th Dec 2002, 23:31

To: helmet fire:

I did not know that the rotor mast absorbs lead lag, and when I think of that small Huey mast absorbing all that, I shudder. Luckily on the Huey they do have drag braces to help absorb the forces.

If you remember in my past discussions on the Robbie head I mentioned that the lead lag loads were reacted first by the cone hinges and transmitted into the rotorhead and then to the fuselage via the mast. The mast on any type of a semi rigid rotor that has a tendency to lead and lag is one of the reaction and transmittal points in the dynamic system and the drive train. All loads are zeroed out in the fuselage and must pass from the point of load origination through whatever mechanical device is in line with the load dissipation including the transmission. These cyclical loads could even be reacted by the engine.

Regarding the drag braces on the Huey, they absorb the loads and transmit them from the blade to the rotorhead and then to the mast and ultimately the fuselage as indicated above. The Huey rotorhead has minimal lead and lag because it is underslung so any transmitted loads are of a low magnitude.

:cool:

Dave Jackson

7th Dec 2002, 23:58

Lu,

Agreed; this rotor will have to be 'beefed up' at the cost of some additional weigh.

helmut fire,

The situation is probably not all that bad.

The amount of undersling and the static pre-cone angle are selected to suit the mean coning angle of the rotor. A change in the active coning angle on both sketch A/ and on sketch C/ will cause the CG of the two blades to come out of alignment with their (virtual) teetering hinges. In other words, this is a problem for the conventional teetering rotor as well as 'idea C/'.

Assume a hypothetical 2-blade articulated rotor head, which is flying with a coning angle or +4 degrees on both blades. If blade #1 flaps up 3 degrees and blade #2 flaps down 3 degrees then blade #1 has a total angle of +7 degrees and blade #2 has a total angle of +1 degree, from the normal plane of the disk. This means that the 'centers' of blade #1 are closer to the disk's axis of rotation then are the 'centers' of blade #2. Hence, the need for lead-lag hinges. With an underslung teetering rotor, this problem of the blade 'centers' moving toward and away from the mast is much reduced.

The Hooke's Joint effect is not a major problem because the two blades are accelerating and decelerating in unison. Of course, a rotor with more than two blades will have a problem with the Hooke's Joint effect and must have a provision for lead-lag.

Nick Lappos

8th Dec 2002, 17:46

Lu and DJ,
The idea that the structural arrangement with an elbow somehow makes it behave differently (underslung) is not true. If you have two points connected by a speghetti strand of loops, whirls and spirals, these things make the structural analysis of that part complex, but the rotor head behaves as if all that stuff were just a connection. The head DJ sketches will have some precone in it, because when it spins up, the blades will flap up slightly to allow the centroid of the blade to align with the support hinge. That is it. Not much magic there.

The underslung head is somewhat unstable, but the very small divergent tendency, where a slight flapping will lead the mast to create more flapping, is not significant for such small angular contributions.

DJ also writes "Rotors with offset flapping hinges are said to have better cyclic response" No, it is not just said, it is a fact.

That is one interesting aspect of posting facts and opinions, there tends to be a concept that everything is an opinion. Teetering rotors have relatively weak cyclic control, articulated and bearingless rotors have very much better, faster and more powerful control. This is said, but it is also measured and factual.

Dave Jackson

8th Dec 2002, 20:36

Thanks guys, and particularly Nick.

You're right, the idea is flawed. Thank God it was submitted as an idea, for discussion, and not as an irrefutable fact. ;)

Perhaps somewhere out there, is the means to provide better cyclic control without have to incur the complexities of an articulated rotor or the weight of an absolutely rigid rotor. Perhaps it's the hub spring that Nick mentioned about a year ago. :confused: :confused:

Dave Jackson

9th Dec 2002, 04:49

All bets are off. http://www.unicopter.com/7up.gif

You guys mentioned, correctly, that in sketch [B] and [C] the line between the blade centroids will be above the virtual hinge when the helicopter is operating at mean thrust.

Thanks again for finding fault and instigating a revision.
Please (do not ;)) find fault with sketch [D]

http://www.unicopter.com/temporary/1127-3.gif

This configuration is identical to [B] and [C], except that there is now a Tie bar, which holds the two yokes at the pre-cone angle.

Should one blade flap (teeter) up then the other blade will flap (teeter) down. This is now operationally identical to the conventional teetering rotor [A]. In other words, under mean thrust, the line between the blade centroids will pass through the teetering center on [A] and through the virtual center on [D]. Even when disks [A] and [D] are tipped, this line will still pass through the center of both configurations, under mean thrust.

As with [B] and [C] the offsets in [D] will result in a more powerful cyclic control then that of [A].

vorticey

9th Dec 2002, 12:41

just a theory on the tetering hinge. i dont think there are any lead and lag forces on a teetering head because the spining axis (head) is ALWAYS parallel with the tip path plane. one blade just doesnt go up and one comes down, THE CONE TILTS. the forces you say are absorbed by the mast would be hunting motion of a universal joint. also the head would be under slung to counter the weight shift from the cone tilting to one side.
:D

Dave Jackson

9th Dec 2002, 17:46

vorticey,

When the tip path plane is not parallel to the mast plane, there will be a 2 per revolution speed variation between the two planes. This is because of the Hookes Joint Effect. This may be envisioned by considering that the tip path plane is attached to one end of the Hookes joint and the mast plane is attached to the other end. In addition, profile drag will vary as the rotor turns. The mast absorbs this rotational oscillation in most helicopters with teetering rotors.

There is no strong reason for lead-lag hinges in a 2-blade rotor because the two blades accelerate and decelerate together.

An exception to the above is the Kaman helicopter. It has two mechanically interconnected teetering rotors, for a total of four blades. Because of this interconnection, the helicopter has lead-lag and delta-3.

You are correct in regard to the weight shift and therefor the need for undersling.

Shawn Coyle

10th Dec 2002, 00:47

Dave:
a) Quick - get a patent on it!
b) what about the R-22 hub- it has flapping hinges as well as a teetering hinge, doesn't it?

heedm

10th Dec 2002, 02:41

Dave, one concern that I don't think has been mentioned is that with a hinge offset you generate a mast moment. I believe this is one of the reasons the response is more crisp, but it comes with a penalty...the mast must be stronger.

Must admit though, plan D does seem to be worth further investigation.

Dave Jackson

10th Dec 2002, 05:26

Hi Shawn,

a) ~ A 2-blade articulated rotor is probably going to experiences 2P vibration at advanced cyclic positions. The idea in this thread has a similar objective as Bell's 'hub spring', and this may be the reason why Bell filed a couple of patents on possible means to reduce the vibration.

This idea may be OK for the intermeshing SynchroLite since this craft has two rotors with two blades each, for a total of 4 blades. This means that the vibration will have a rate more than twice that of a comparable single-roter craft and an amplitude less than half. Plus, the SynchroLIte is intended for the US Ultralight category, and by regulation, it is therefor limited to a benign velocity of 55 knots. Want to share the patent's cost and royalities? :)

b) ~ Yes, but this is Lu's department :D

heedm, thanks for the input.

You're right. The rotor hub and mast etc. must be stronger. isn't Bell experiencing a similar situation right now, with a modification to existing helicopters?

Another penalty is the highly probable 2P vibration, as mentioned.
The boring reason for this is; If the cyclic was to be pushed forward it will cause the tip path plane of the disk to tip down at the front, in respect to the mast plane. This must mean that when the blades are at 0º & 180º azimuth, the centrifugal force and hub offsets are attempting to 'torque' the mast plane into alignment with the tip path plane. This is in addition to the thrust from the rotor, which is attempting to 'drag' it into alignment. When the blades are at 90º & 270º azimuth, only the previously mentioned 'dragging' activity is taking place. This 'torque ~ no torque' will probably create a 2 per rev. vibration.

vorticey

10th Dec 2002, 10:56

the design you are working with, would increase coning angle simply because of the elbow length. you will need drag and lead hinges because it doesnt matter that the cofg and centre of mast is in line. what matters is the angle from the mast of each blade.
teetering heads are different because the head is tilting on the mast causing the moments you talked about earlier between mast and head. the blades DONT change there angle from the spinning axis on teeter heads theorfore no lead and lag can be present. i think the wear marks on the robinsons lu talks about are from start up (shuddering belts) and maybe hooks joint moments absorbed at the hub.

p.s. if some body can explain that the head and blade tips are not always parrallel on a teetering head, my coments are not true.:cool:

Dave Jackson

11th Dec 2002, 00:46

vorticey,

I don't fully understand your concerns.

If the mean coning angle is greater, then the blades' center of mass (etc.) will be higher. This means that the pre-cone angle should be greater and the teetering hinge should be higher.

In a very general sense, the teetering rotor is no different from the articulated rotor. Both have tip path planes and mast planes. These planes are not parallel to each other during most flight conditions.
_______________

As an aside; on sketch D/, if the ctr-ctr distance on the Tie-bar was slightly longer than two-times the offset length, the pre-cone angle will automatically increase when the rotor teeters. This *might* be an advantage when maneuvering.

vorticey

11th Dec 2002, 10:52

coning angle reduces thrust. by pushing the inside of the blade down (the elbow) coning is increased.
you said > In a very general sense, the teetering rotor is no different from the articulated rotor. Both have tip path planes and mast planes. These planes are not parallel to each other during most flight conditions.
NO dave this is wrong. you missed out one thing, the rotor head plane, which is the same as the mast plane on a fully articulated or solid rotor systems aswell as your drawings. however on teeter systems the head is always parallel the tip plane (please explain if it is not). As the head (not the mast) is the spinning axis for the blades, the angles from the blades to the spinning axis (head) never change, the blades CAN'T flap seperatly so no lead and lag is needed.

Lu Zuckerman

11th Dec 2002, 12:12

This is an extremely difficult subject to discuss without diagrams.

On an articulated rotorhead you have two axes of rotation. One is the driven axis and the other is the rotating axis. Assuming that when in a hover the rotor tip path plane is parallel to the horizon or level in relation to the rotorhead then the two axes are coincident with each other. When cyclic is introduced the tip path plane will tilt in the direction of cyclic movement. When this occurs the rotating axis will deviate from the driven axis and result in leading and lagging. The amount of lead / lag is dependent upon the amount of separation of the two axes.

With the tilting of the tip path plane there is a condition called interlock which in simple terms is the “centrifugal or centripetal” energy will try to align the rotorhead and the helicopter with the tip path plane causing the helicopter to lean in the direction of cyclic movement. This is most noticeable when the cyclic is pushed forward. Most helicopters have some type of horizontal stabilizer to minimize this leaning action. On helicopters with a very high interlock level the horizontal stabilizer is controllable and usually is connected to the cyclic. On others the horizontal stabilizer is controlled by the autopilot or by electrical control actuated by the pilot. This is true on some gunships.

On the proposed design there is an offset hinge and underslinging of the blades. I believe that the underslinging may not have the desired effect because when cyclic is introduced you will deviate the driven from the rotating axes and introduce lead and lag. I don’t have the background to say that the underslinging will cancel out the lead / lag action so I won’t carry this argument any further.

Regarding the wear pattern on the cone hinges of the Robinson I believe it is caused by lead and lag of the blades. If it were caused by start-up and belt chatter I would think that Frank Robinson would have to go back to the drawing board.

:)

rwm

11th Dec 2002, 17:34

I'm glad to see some kind of tie bar to the other blade finaly. was wondering why there was no tt strap. But my question is if this is for a home built, why worry about how responsive it is. In a 206 it has an underslung teetering head, and it is very responsive compared to the similar head on a 204/205/212. So unless this this is one large homebuilt why not just keep it simple and proven. This may also mean safer to some.

Dave Jackson

11th Dec 2002, 21:37

vorticey

"coning angle reduces thrust. by pushing the inside of the blade down (the elbow) coning is increased. "

I don't think that the offset will change the coning angle much from that of a conventional teetering hinge. In fact, a change in a coning angle from 2 degrees to 3 degrees (cosine 1) results in a thrust change of only 0.05%. This cone is also fighting the closure of the streamtube, so the change might be even less.

An early Sikorsky had a coning angle of 10 degrees.

"NO dave this is wrong. you missed out one thing, the rotor head plane, which is the same as the mast plane on a fully articulated or solid rotor systems as well as your drawings. However on teeter systems the head is always parallel the tip plane (please explain if it is not)."

I can't recall ever seeing a reference to 'rotor head plane'. There are four rotor planes of interest. They are described on; Rotor Axes (http://www.UniCopter.com/B329.html#Axes_Systems_Rotor)
___________________

Lu,

"This is an extremely difficult subject to discuss without diagrams. "

This diagram, from Leishman, may help. Its acronyms are described on the above web page.

http://www.UniCopter.com/Temporary/Planes.gif
___________________

rwm

"In a 206 it has an underslung teetering head, and it is very responsive compared to the similar head on a 204/205/212. "

The following is theory and not a fact, until Nick says so. :Djoke:D. With forward velocity, a craft with a teetering rotor and a craft with an articulated rotor may have the same rate of response. This is because the craft with a teetering rotor could be rigged to have a greater rotor pitch change for a given cyclic movement. At or near hover, the articulated rotor is more responsive, for the reason given by Lu and Nick.

"my question is if this is for a home built, why worry about how responsive it is. "

The primary application for this idea would be the US Ultralight category. No licenses are required in this category, therefor it makes sense to have a helicopter that is as safe as possible for the low time pilots. A fast response, symmetrical intermeshing configuration and rotor governor will all contribute toward a safer helicopter.

http://www.UniCopter.com/Temporary/LightbulbIdeaNarrow.gif

All hinges on the rotor hub in sketch D/ will have delta-3, as does the Robinson's teetering hinge. Interestingly, this hub therefor offers another 'theoretical' advantage, that of a cone-pitch-coupling governor.

If the Tie-bar was to have an elastic property then as the rotor RPM slows, the cone of the disk will increase, by stretching the Tie-bar. This causes the cone-pitch-coupling (delta-3) to reduce the pitch of the blades and thereby help maintain rotor RPM. :cool:

GLSNightPilot

12th Dec 2002, 02:59

Dave, I'm not sure I agree with your contention that fast response equals safety in an amateur-built and -flown aircraft. The opposite may be closer to the truth. Combine a twitchy aircraft with an inexperienced pilot, & the result may very well be parts 'all over Hell & half of Texas'. Couldn't a slower, more deliberate control response in fact be safer in this case? 12k+ hours later, I prefer a responsive aircraft, but I still vividly remember my first flight in a TH55, & it's a good thing a nervous IP was in the left seat to correct my severe overcontrolling tendencies.

I'm not completely sure which side I'm on here, but I do think some consideration is worthwhile.

helmet fire

12th Dec 2002, 03:15

rwm:

the 205/212 head "feels" less responsive than the 206 due to the stabiliser bar and dampers fitted to the Huey series 2 bladed rotor head. These act gyroscopically to reduce sensitivity to gusts but this reduction in sensitivity also reduces the cyclic sensitivity - so obviously there is a balance to strike with the dampers.

vorticey:

[p.s. if some body can explain that the head and blade tips are not always parrallel on a teetering head, my coments are not true.

You said it. Please read my previous posts on this thread - perhaps you too Dave :rolleyes: ;) ;)

As I said earlier: Individual blades CAN AND DO flap on two bladed rotor systems independantly of teeter. That means Dave, that the part of your theory upon which you state: if one blade flaps up 3 degrees, the other flaps down " is actually not always correct and this aspect of your reasoning should therefore be discounted.

The different flapping (independant of the see-saw action of teeter) of each blade is a cause of the lead/lag in two bladed systems that some of you are suggesting does not exist.

Lu: thanks for the mention of CPF;) ;) :D

Dave Jackson

12th Dec 2002, 06:58

GLSNightPilot

You may well be correct, since I am only working with theory. Stability and control can be at opposite ends of the spectrum, but I am hoping that a 'rotor hub offset' will reduce the delay in the rotor's response to cyclic inputs, without being detrimental to stability. Does this sound feasible?
_________________________

helmet fire

There is no disagreement with your Statement " Individual blades CAN AND DO flap on two bladed rotor systems ". I was, perhaps over simplistically, trying to cover the basic activity of the teetering rotor. The amount of undersling on a teetering rotor is set for the mean operational condition. Flight activity such as changes in thrust will increase and decrease the coning angle. Maneuvers will cause blade flexing. Both will pull the rotor's actual center(s) away from the fixed teetering center, and vibration will occur.

Oh!! for an Absolutely Rigid Rotor :D:D

I agree that there is lead-lag in the blades but the blades on a teetering rotor will primarily lead-lag in unison. This puts a loading on the mast but not between the two blades. Correct me if I'm wrong, but I think that the Hookes joint effect (or call it cyclic Coriolis effect) is the primary reason for the lead-lag. Out of plane flexing of the blades splits the centers and causes vibration.

vorticey

12th Dec 2002, 08:28

exactly dave! the hooks joint only is what causes the hunting of the (teetering) head, which will transfer to the blades together. and its because of the difference in angle between the mast and the head, the blades have nothing to do with it.
how can you say there's no head plane?

so all im saying is it will have to be fully articulated to work.
maybe MORE coning IS better, try it.

rwm

12th Dec 2002, 11:35

i don't read much here about blade flex. i work on BK117, BO105, Bell412 right now and have some experiance with 206 and 212s and all the blades flex. The blades on the BO and BK do all the lead/ lag and flapping for the head because they flex. The 412 uses elastomers for the flapping and lead/lag. I have been told that the rotorway exec uses elastomers too, and it is a simple two blade head with about 35 plus years of service. You talk about intermeshing blades, and i don't seem to see many of them out there, could it be because they are too complex. You want simple and light for inexperianced pilots in an ultralight helicopter, then stay simple and reliable. Don't get me wrong, i like the idea, but the last thing the homebuilt market needs when the feds are willing to give us a bit of slack is a complex helicopter to be built and maintained by some bozo that probably doesn't even check the oil in his car yet alone some flying machine that might see 50 hrs a year. Oh and about that stabilizer bar on the bell mediums, it also assists with stick movements because you are now dealing with a large helicopter, and without that bar, and you lose hydraulics, you are no longer flying, as you would break the cyclic stick before you moved the head.

Dave Jackson

12th Dec 2002, 19:20

vorticey,

"how can you say there's no head plane?

It's easy to say. :eek: Just tell a white lie. :)
Seriously, I only said that "I can't recall ever seeing a reference to 'rotor head plane'. ". Perhaps it is another way of referring to the mast plane ~ shaft plane ~ hub plane.

This page should give some answers on Hookes joint effect. (http://www.unicopter.com/B274.html#Coriolis_and_Hooke) If you disagree with the information, please say so.

What do you mean by 'hunting'?

_______________________

rwm

Small point but, the elastomeric bearing on the Rotorway is on the teetering hinge.

"You talk about intermeshing blades, and i don't seem to see many of them out there, could it be because they are too complex. You want simple and light for inexperienced pilots in an ultralight helicopter,
"

We all have our favorite subjects and you found mine. :)
The intermeshing configuration offers much for the ultralight helicopter and low time pilot. A few reasons are; the high thrust to weight ratio, the benign yaw control, plus fewer components in the drive train and rotors. (the single rotor helicopter ain't really a single rotor).

More: Intermeshing Configuration - Concerns (http://www.unicopter.com/B280.html)

rwm

14th Dec 2002, 06:34

I agree that there are plenty of benifits to intermeshing blades, but i am a bit concerned about the interest of applying it to a small home built. I supose that if it were built professionaly, and maintained by someone who looks after things, it would be just fine, but the home built market doesn't have everyone with that mindset. I would hate to see a good idea lost to a couple of preventable accidents. Ever hear about the parasol wing?

vorticey

14th Dec 2002, 08:02

head plane = hub plane

hunting = Coriolis Acceleration/Deceleration Effect, or Knuckle Joint Effect. what ever makes you feel good.

do you see how the tip path plane on a teetering head never becomes unparrallel to the head plane, theorfore seperate flapping is not posible under normal operating contition, and is why they dont need drag and lead hinges.

Dave Jackson

14th Dec 2002, 20:07

rwm,

Everything that you said is extremely valid.

A possible, or partial, solution may be to combine the power train, rotor hubs and flight controls in a single compact package. This 'package' might be certified. The homebuilder then attaches to it a fuselage, landing gear, blades and engine. This concept is being considered at Dragonfly (http://www.drag-on-fly.com/dragonfly.html)

Coincidentally, Dennis Fetters, of Mini 500 fame, has reappeared and started posting on rec.aviation.hombuilt about a week ago. There are many posts and they get quite nasty.

No, I'd never heard of the parasol wing before. Here's one, but at Mach 4.5 it might not comply with the Ultralight regulations (http://www.arnold.af.mil/aedc/systems/78-1398.htm) :D
__________________

vorticey,

" head plane = hub plane"

hub plane = mast plane = shaft plane.
This plane is normal to the mast.

"the tip path plane on a teetering head never becomes unparrallel to the head plane"

You may be confusing the parts of a teetering rotor hub. A basic teetering hub is comprised of a mast head (hub) and a yoke. This head is firmly attached to the mast and therefor its only motion is that of rotation. The teetering hinge couples the head (hub) to the yoke. The yoke rotates with the hub and it also teeters with the blades.

When you refer to the 'head plane' you may be thinking in terms of the yoke and not the mast head.

vorticey

16th Dec 2002, 08:21

one more time:
sounds like you know what im saying anyway dave.
head plane = yoke plane in your mind. realy there's no yoke, its not realy a universal joint, but more like a hinge.

im just wondering if you'r understanding what im saying?

all the enginears here call the part that holds the blades on, the head. not that it matters what you call it, youll still need lead and lag hinges.:D

Dave Jackson

16th Dec 2002, 23:28

vorticey,

We may be going too far into a complex subject, but here is some more info.

"head plane = yoke plane in your mind"

Just the opposite. The 'head plane' is not the 'yoke plane'. To my knowledge, there is no such thing as a 'yoke plane'. This theoretical 'yoke plane' is the 'tip path plane', unless one wants to take into account out-of-plane blade flexing or possibly the Robinson tri-hinge rotor.

You're definitely correct in that the conventional teetering hinge is not a universal joint (Hookes joint), since it only has one hinge. The rotor hub on the Bell 47 is the only one that is a true universal joint, which I know of.

The use of the phrase 'Hookes joint' by some is either because it's a leftover from the past, or some may consider the feathering hinge as the second joint.

Basically, both blades led/lag at the same time (due to Knuckle Joint Effect :)), therefor there is little increase in moments between the two blades. There is a change in the moment between the complete rotor and the mast, when the rotor is tipped in respect to the mast.

This is going to really muddy the water. :D
The proposed delta-3 (http://www.unicopter.com/0941.html#delta3) is by flap hinge geometry and not by control system geometry, as is used on the Robinson. Delta-3 by flap hinge geometry has an in plane component. This means as the blade flaps up or down the tip advances and thereby absorbs some of the 'lead'. When the blade returns to the normal position the tip retreats and thereby absorbs the some of the 'lag'.

vorticey

17th Dec 2002, 11:36

you said > This theoretical 'yoke plane' is the 'tip path plane'
thats all i was trying to say, that the blades never flap seperately from the spinning axis (yoke plane or what ever) on a teetering system.

i know how the delta hinge works, if a blade flaps up it would normaly decrease pitch and maybe lead. but on the main rotor, pitch is there for a reason, so it will have to be rigged so it doesnt reduce pitch, just provide a leading hinge. so it might as well be a normal lead hinge.
you say its all ready used on some helicopters, i just cant work out why it is needed? doesnt cyclic control the flapping perfectly? or is it for the spring effect?

what about the weight, that is thrown to one side when a blade leads, how is it balanced on multi blade helicopters?
:rolleyes:

Lu Zuckerman

17th Dec 2002, 14:38

what about the weight, that is thrown to one side when a blade leads, how is it balanced on multi blade helicopters?

Once again we get into the realm of having to explain something without being able to show it in a diagram form.

I once read in an aerodynamics book (yes I did read an aerodynamics text) where the author placed the reader in the dangerous position of standing above a spinning rotor system in order to explain this very point.

First the reader had to accept the following: The spinning rotor system has two axes of rotation when cyclic pitch is introduced. There is the driving axis, which is the point of drive for the rotor system, and it is coincident with the mast centerline. Then, there is the driven axis, which is coincident with the center of the tilted disc, which is no longer coincident with the center of drive (the mast).

Because the two axes are not coincident with each other you have lead and lag. The reader is told to stand in a point directly over the drive axis and to observe the rotating disc. Looking down the observer will note that the spinning blades are not equally disposed. That is, they are not 90 degrees apart but in fact the leading blade is forward of the lateral position and the retreating blade is not at the ninety-degree position but is in fact behind that position. For the sake of argument and using the forward blade as the 0-degree position and the rear blade as the 180-degree position the advancing blade is at the 87-degree position and the retreating blade is at the 273-degree position. Thus forming what looks like the peace sign. Even though the blades appear to be out of balance there is no vibration. Please refer to the note below.

Now the observer is told to stand on the driven axis. Looking down on the rotating disc the observer will note that the blades are equally disposed at 90-degrees to each other.

You have to use the power of imagery or all is lost.

Note: When I learned about helicopter aerodynamics from the Sikorsky comic book I was told that the blades lag behind the radial axis due to drag and inertia and never come up to the radial point. And, that all leading and lagging takes place behind the radial axis, which is in conflict with what the noted author of the aerodynamics text is trying to prove. You as the reader can make up your own mind.

:D

Dave Jackson

17th Dec 2002, 20:18

vorticey,

Delta-3 on a main rotor, such as the Robinson's, only pulls out a small percentage of the pitch that was input from the cyclic stick. I think that it is designed this way to make light helicopters easier to fly, just as the earlier Bells had counterweights and the Hiller had paddles. It may also help minimize the effect of gusts.
____________________

Lu,

The analogy of a 4-blade articulated rotor and the peace sign is an interesting one. The following assumes that the helicopter is flying up the page and azimuth-0 is at the back of the helicopter (bottom of the page).

If only the Coriolis effect (http://www.unicopter.com/B329.html#Coriolis_Effect) was considered then the blades would be at azimuths of 0, 93, 180 & 273-degrees. When drag is also considered, the fact that this drag is greater on the advancing blade then on the retreating blade is probably responsible for changing the 93-degrees to 87-degrees.

I now join you in bending over. :( Kinky. :D

vorticey

18th Dec 2002, 11:25

you > Now the observer is told to stand on the driven axis. Looking down on the rotating disc the observer will note that the blades are equally disposed at 90-degrees to each other.

i find this very hard to agree with lu, but thanks for the expenation.

Lu Zuckerman

18th Dec 2002, 23:40

To: vorticey

These same descriptions can be found in several helicopter principles of flight text used in various US Universities. I am not saying they are true, just that this is how the subject was explained to the readers of the texts.

To: Dave Jackson

You are waging a war of words and their meaning. It is the same war I was involved in when I discussed Gy@#$%^&*c Pre^&**%#n. You will note that many of the corrective comments about your choice of words did not come from the US or Canada. They are coming from Oz, New Zealand and the UK. Accept the fact that neither of the participants in the discussion will ever change their views even though you are both discussing the same subject.

:D

vorticey

20th Dec 2002, 10:11

is this what you believe? (lu's post) i dont under stand how three of the 4 blades can be pointing forward but still be in balance?
:)

Dave Jackson

21st Dec 2002, 05:50

The currently designed rotor (sketch D) will offer better control but it will also exhibit vibration at large displacements of the cyclic stick, when the tip path plane is not parallel to the rotor mast plane. This is due to the 2P change in the tipping moment on the mast, as the 2-blade rotor rotates. [described in blue above.]

It looks like the following is a solution to this vibration problem. http://www.unicopter.com/7up.gif

The hub in sketch D is now to be constructed as a 3-blade rotor, instead of the previous 2-blade rotor. The former tie-bar between elbows of the two yokes is now replaced by a triangular tie-bar, which connects the elbows of all three yokes. This means that the roots of the three blades teeter (flap) in an interlocked unison.

Temporary Coriolis (Hookes Joint Effect), will now input a lead-lag between the three blades as they flap. But, delta-3 by flap hinge geometry also inputs a lead lag into the blades as they flap. Therefor the angle of the delta-3 can be set so that the amount of lead at a specific angle of teeter (flap), due to the Hookes joint effect, can be absorbed by the same amount of lead input by the in-plane component of the delta-3 angle.

In other words, as the blade flaps up, the delta-3 will absorb the Hookes joint's lead and as the blade flaps down, the delta-3 will absorb the Hookes joint's lag. Sketch of rotor hub. (http://www.unicopter.com/1140.html)

If anyone is still reading this thread and can find fault, please say so. Thanks.

Dave J.