Dynamics ~ 2 Blade Rotor w/ Offset Flapping Hinges
Join Date: Jun 2001
Location: Australia
Posts: 251
Likes: 0
Received 0 Likes
on
0 Posts
dave
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.
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.
Thread Starter
Join Date: Nov 2000
Location: Vancouver, BC, Canada
Posts: 452
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jun 2001
Location: Australia
Posts: 251
Likes: 0
Received 0 Likes
on
0 Posts
dave
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.
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.
Iconoclast
Join Date: Sep 2000
Location: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Posts: 2,132
Likes: 0
Received 0 Likes
on
0 Posts
Two cents worth.
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.
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.
Join Date: Dec 2002
Location: canada
Posts: 170
Likes: 0
Received 0 Likes
on
0 Posts
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.
Thread Starter
Join Date: Nov 2000
Location: Vancouver, BC, Canada
Posts: 452
Likes: 0
Received 0 Likes
on
0 Posts
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
___________________
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.
___________________
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.
joke. 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.
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.
"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
___________________
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.
___________________
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.
joke. 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.
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.
Last edited by Dave Jackson; 11th Dec 2002 at 21:59.
Join Date: Jul 2002
Location: Texas
Posts: 512
Likes: 0
Received 0 Likes
on
0 Posts
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.
I'm not completely sure which side I'm on here, but I do think some consideration is worthwhile.
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:
[
You said it. Please read my previous posts on this thread - perhaps you too Dave 
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
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.
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
Thread Starter
Join Date: Nov 2000
Location: Vancouver, BC, Canada
Posts: 452
Likes: 0
Received 0 Likes
on
0 Posts
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
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.
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
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.
Join Date: Jun 2001
Location: Australia
Posts: 251
Likes: 0
Received 0 Likes
on
0 Posts
dave
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.
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.
Join Date: Dec 2002
Location: canada
Posts: 170
Likes: 0
Received 0 Likes
on
0 Posts
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.
Last edited by rwm; 12th Dec 2002 at 11:46.
Thread Starter
Join Date: Nov 2000
Location: Vancouver, BC, Canada
Posts: 452
Likes: 0
Received 0 Likes
on
0 Posts
vorticey,
"how can you say there's no head plane?
It's easy to say.
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. 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
"how can you say there's no head plane?
It's easy to say.
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. 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
Join Date: Dec 2002
Location: canada
Posts: 170
Likes: 0
Received 0 Likes
on
0 Posts
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?
Join Date: Jun 2001
Location: Australia
Posts: 251
Likes: 0
Received 0 Likes
on
0 Posts
dave
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.
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.
Thread Starter
Join Date: Nov 2000
Location: Vancouver, BC, Canada
Posts: 452
Likes: 0
Received 0 Likes
on
0 Posts
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
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
__________________
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.
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
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
__________________
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.
Join Date: Jun 2001
Location: Australia
Posts: 251
Likes: 0
Received 0 Likes
on
0 Posts
dave
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.
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.
Thread Starter
Join Date: Nov 2000
Location: Vancouver, BC, Canada
Posts: 452
Likes: 0
Received 0 Likes
on
0 Posts
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.
The proposed delta-3 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'.
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.
The proposed delta-3 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'.
Join Date: Jun 2001
Location: Australia
Posts: 251
Likes: 0
Received 0 Likes
on
0 Posts
dave
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?
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?
Iconoclast
Join Date: Sep 2000
Location: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Posts: 2,132
Likes: 0
Received 0 Likes
on
0 Posts
The rotorhead doesn't know any better.
what about the weight, that is thrown to one side when a blade leads, how is it balanced on multi blade helicopters?
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.
Thread Starter
Join Date: Nov 2000
Location: Vancouver, BC, Canada
Posts: 452
Likes: 0
Received 0 Likes
on
0 Posts
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 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.
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 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.