Bell 206: JetRanger and LongRanger
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Bell 206: JetRanger and LongRanger
Why does the B206 'Boomerang' (transmission) mounts allow lateral movement but not fwd / aft movement ?
And just for a laugh, if the mast rocks lateraly in flight will the effect be felt as a pitching moment?
And just for a laugh, if the mast rocks lateraly in flight will the effect be felt as a pitching moment?
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To: Rotorque
Hopefully I don’t come off as an idiot in what I am about to say, as you probably already know.
The Nodamatic suspension is designed to eliminate the two per rev vertical beat that is common to Bell single rotor helicopters. When the Bell blades are aligned with the longitudinal centerline they lose lift and as such the fuselage drops. The Nodamatic allows the transmission to move in relation to the fuselage and the suspension system has calibrated weights that respond to the vertical bouncing of the transmission in relation to the fuselage. The moving of the weights causes a counter force to resist the movement and it cancels out the vertical beat in the fuselage. The way the Nodamatic is hinged I would assume it would allow lateral movement of the transmission when not in operation but not fore and aft. The primary movement is up and down during flight but I would think that if the trannie can be displaced laterally when static that some lateral movement could take place when maneuvering. I think.
Bell uses a similar concept with the bob weights on the 412 rotorhead. These weights are used to cancel out the traveling wave on the blades. Similar weights are used on BK helicopters and Sikorsky uses a system called BI Filar on the rotorhead.
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The Cat
[This message has been edited by Lu Zuckerman (edited 12 March 2001).]
Hopefully I don’t come off as an idiot in what I am about to say, as you probably already know.
The Nodamatic suspension is designed to eliminate the two per rev vertical beat that is common to Bell single rotor helicopters. When the Bell blades are aligned with the longitudinal centerline they lose lift and as such the fuselage drops. The Nodamatic allows the transmission to move in relation to the fuselage and the suspension system has calibrated weights that respond to the vertical bouncing of the transmission in relation to the fuselage. The moving of the weights causes a counter force to resist the movement and it cancels out the vertical beat in the fuselage. The way the Nodamatic is hinged I would assume it would allow lateral movement of the transmission when not in operation but not fore and aft. The primary movement is up and down during flight but I would think that if the trannie can be displaced laterally when static that some lateral movement could take place when maneuvering. I think.
Bell uses a similar concept with the bob weights on the 412 rotorhead. These weights are used to cancel out the traveling wave on the blades. Similar weights are used on BK helicopters and Sikorsky uses a system called BI Filar on the rotorhead.
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The Cat
[This message has been edited by Lu Zuckerman (edited 12 March 2001).]
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G'day Lu,
Thanks for that. The bell 206L uses the nodal type system that you described, although it looks complicated when you first open the cowls, it's not that hard to follow the reasons behind the engineering. Your description is pretty good.
I probably answered your question a bit too quick. The boomerang mounts that I was really talking about are the ones on the smaller basic 206. They don't have the 'nodal' system as such. There is literally two mounts that run longtitudinaly supporting the transmission and therefore mast, head etc. They are shaped as a 'boomerang' or a triangle I guess, with the apex attached to the MRGB and the two bases of each triangle attached to the roof of the cabin. The MRGB attachment bearing and lower attachment bearings allow movement from side to side, but not for and aft. The actual movement of the MRG is a form of 'paralelogram' in as far as the MRG will not tilt - just shift position lateraly, so the last question that I asked was just for fun (in a way).
But my question on why there is freedom to move lateraly and not longtitudinaly still stands.
Why is it so?
Thanks for that. The bell 206L uses the nodal type system that you described, although it looks complicated when you first open the cowls, it's not that hard to follow the reasons behind the engineering. Your description is pretty good.
I probably answered your question a bit too quick. The boomerang mounts that I was really talking about are the ones on the smaller basic 206. They don't have the 'nodal' system as such. There is literally two mounts that run longtitudinaly supporting the transmission and therefore mast, head etc. They are shaped as a 'boomerang' or a triangle I guess, with the apex attached to the MRGB and the two bases of each triangle attached to the roof of the cabin. The MRGB attachment bearing and lower attachment bearings allow movement from side to side, but not for and aft. The actual movement of the MRG is a form of 'paralelogram' in as far as the MRG will not tilt - just shift position lateraly, so the last question that I asked was just for fun (in a way).
But my question on why there is freedom to move lateraly and not longtitudinaly still stands.
Why is it so?
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The 206B has a focal pylon to minimize 2/rev vibration due to inplane rotor forces. The focal pylon does allow movement both fore-and-aft and laterally. The f/a pivot is at the top of the "boomerang" and the lateral is at the bottom. Since the pylon mount(spring) is a the bottom, it is relatively easy to move the pylon by hand in the lateral direction. It is much harder to move the pylon in pitch due to the moment arm.
The Sultan
The Sultan
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To: Rotorque
Think about blade stall and what causes it. On blade stall there is a large portion of the blade where there is neutral airflow over the retreating blade and from that point inward the airflow is reversed in an increasing manner the closer you get to the center of the rotor disc. In the case of losing lift when the blades are aligned with the longitudinal centerline the airflow is passing down the span of the blade. Although the blade is still lifting it is lifting less when aligned with the centerline of the helicopter. In this case the decreased lift causes the helicopter to drop. As the blades move from that position the lift starts to increase until the blades are aligned with the lateral axis of the helicopter. During the transition from longitudinal alignment to lateral alignment the blade lift increases and the helicopter starts to rise. The Nodamatic suspension isolates the fuselage from the dynamic system and the linkage and bob weights on the Nodamatic system isolate the helicopter from the up and down movement of the dynamic system and absorb the transmitted energy thus providing a very smooth ride.
However the up and down movement of the dynamic system in relation to the fixed engines can result in problems. This was a significant problem on the original 214s and hopefully by now the problem has been solved. On the 214 the relative movement between the gearbox and the engine caused a two per rev misalignment on the short shaft causing the couplings to pump the lubricating grease out of the coupling.
This grease was picked up in the inlet airflow to the engine and the grease plated out on the engine inlet bell. The grease attracted sand and other small FOD and this disrupted the airflow into the engine. The disrupted airflow caused engine surging and in rare occasions it caused compressor stalls. When compressor stalls were noted the entire driveline and dynamic system had to be replaced and the removed elements had to be inspected and in most cases, discarded. It also increased the maintenance on both the engine and the short shaft as the grease had to be replenished and seals replaced and the engine had to be washed to remove the trapped contaminants from the engine inlet bell.
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The Cat
[This message has been edited by Lu Zuckerman (edited 12 March 2001).]
Think about blade stall and what causes it. On blade stall there is a large portion of the blade where there is neutral airflow over the retreating blade and from that point inward the airflow is reversed in an increasing manner the closer you get to the center of the rotor disc. In the case of losing lift when the blades are aligned with the longitudinal centerline the airflow is passing down the span of the blade. Although the blade is still lifting it is lifting less when aligned with the centerline of the helicopter. In this case the decreased lift causes the helicopter to drop. As the blades move from that position the lift starts to increase until the blades are aligned with the lateral axis of the helicopter. During the transition from longitudinal alignment to lateral alignment the blade lift increases and the helicopter starts to rise. The Nodamatic suspension isolates the fuselage from the dynamic system and the linkage and bob weights on the Nodamatic system isolate the helicopter from the up and down movement of the dynamic system and absorb the transmitted energy thus providing a very smooth ride.
However the up and down movement of the dynamic system in relation to the fixed engines can result in problems. This was a significant problem on the original 214s and hopefully by now the problem has been solved. On the 214 the relative movement between the gearbox and the engine caused a two per rev misalignment on the short shaft causing the couplings to pump the lubricating grease out of the coupling.
This grease was picked up in the inlet airflow to the engine and the grease plated out on the engine inlet bell. The grease attracted sand and other small FOD and this disrupted the airflow into the engine. The disrupted airflow caused engine surging and in rare occasions it caused compressor stalls. When compressor stalls were noted the entire driveline and dynamic system had to be replaced and the removed elements had to be inspected and in most cases, discarded. It also increased the maintenance on both the engine and the short shaft as the grease had to be replenished and seals replaced and the engine had to be washed to remove the trapped contaminants from the engine inlet bell.
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The Cat
[This message has been edited by Lu Zuckerman (edited 12 March 2001).]
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Thanks Lu,
It all makes sense now. I had a lot of trouble getting around the fact that there is a loss of lift when the blades are aligned for and aft. I assumed that the total lift would remain constant throughout each revolution.
I have stuck this one in my little black book of silly questions. If you're wondering this little book is huge.
Cheers.
It all makes sense now. I had a lot of trouble getting around the fact that there is a loss of lift when the blades are aligned for and aft. I assumed that the total lift would remain constant throughout each revolution.
I have stuck this one in my little black book of silly questions. If you're wondering this little book is huge.
Cheers.
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To: Vfrpilotb
This is off the top of my head but I would think that the Hiller helicopters would be subject to the same phenomena because of similar blade speeds to that of the Bell. However I don’t think the Robinson would be susceptible to the two per rev bounce because of the high rotational speed of the blades but I could be wrong.
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The Cat
This is off the top of my head but I would think that the Hiller helicopters would be subject to the same phenomena because of similar blade speeds to that of the Bell. However I don’t think the Robinson would be susceptible to the two per rev bounce because of the high rotational speed of the blades but I could be wrong.
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The Cat
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Lu,
Your wrong!
All two bladers have 2/rev, all 3 bladers have 3/rev, all 4 bladers have 4/rev, etc, etc, etc. Its called N/rev.
Helicopters of all types require vibration attenuation of some sort to combat N/rev. Generally, this type of treatment equals between 1 to 2% of the aircraft's weight regardless of the number of blades to achieve an acceptable "g" level in the occupied spaces.
The Sultan
Your wrong!
All two bladers have 2/rev, all 3 bladers have 3/rev, all 4 bladers have 4/rev, etc, etc, etc. Its called N/rev.
Helicopters of all types require vibration attenuation of some sort to combat N/rev. Generally, this type of treatment equals between 1 to 2% of the aircraft's weight regardless of the number of blades to achieve an acceptable "g" level in the occupied spaces.
The Sultan
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To: The Sultan
The elements that you alluded to being employed to dampen out what you call N-rev do exactly that but not for the reasons you gave. These systems are dynamic in nature and are installed on the rotor heads of some helicopters to dampen or eliminate the vibrations caused by the blades and their reaction to pitch and load changes. These cyclical loads cause a traveling wave that if not stopped would cause a cyclical load to be applied to the rotorhead and reflected in the airframe and felt by the pilots and crew/passengers.
Sikorsky uses the Bifilar system that has very heavy bob weights that are disposed between the blades. With centrifugal force the apparent weight of the bobweight is significantly increased. The traveling wave in order to get to the rotorhead must displace the weight from its’ radial position. To move the weight causes energy to be absorbed and the wave is cancelled. The MBB-105 and the BK-117, Bell 412 and some Hughes helicopters including the early version of the Apache have bob weights that are connected in most cases to the blades.
Some helicopters have two types of bob weights that are sensitive to different frequencies of the wave. Some Bell helicopters employ a static weight that is embedded in the blade at the nodal point for the wave in order to cancel it out by absorbing the energy of the wave. The Aerospatial helicopters have a heavy weight suspended on top of a stiff spring with the assembly being mounted on top of the rotorhead. Boeing Vertol has a mechanism mounted under the pilots’ seats to perform the same function.
It is my understanding that Bell developed an electromechanical system that worked in the same manner as a noise-canceling headset. The device sensed the vibration level and the cyclical frequency and generated an opposite sine wave and equal in magnitude which eliminated the vibration. This device was going to be installed in B-205s and HU1s but I don’t know if it was ever placed in wide use.
I do not believe that the incorporation of any of these energy absorption systems was incorporated to compensate for the loss of lift on a two blade or, a multi blade helicopter when the blade are disposed over the longitudinal axis of the helicopter. To eliminate the vibration problem Bell came up with the Nodamatic suspension system which isolates the dynamic system and the transmission from the airframe. When they went to a four-blade system on the 412 they incorporated the bob weights. Some Bell four blade heads do not incorporate the bob weights so I would assume that they went back to the embedded weights.
It is my opinion that the N-rev is in effect the reason these systems were developed with the N-rev being caused by the induced vibration resulting from the traveling wave..
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The Cat
[This message has been edited by Lu Zuckerman (edited 14 March 2001).]
The elements that you alluded to being employed to dampen out what you call N-rev do exactly that but not for the reasons you gave. These systems are dynamic in nature and are installed on the rotor heads of some helicopters to dampen or eliminate the vibrations caused by the blades and their reaction to pitch and load changes. These cyclical loads cause a traveling wave that if not stopped would cause a cyclical load to be applied to the rotorhead and reflected in the airframe and felt by the pilots and crew/passengers.
Sikorsky uses the Bifilar system that has very heavy bob weights that are disposed between the blades. With centrifugal force the apparent weight of the bobweight is significantly increased. The traveling wave in order to get to the rotorhead must displace the weight from its’ radial position. To move the weight causes energy to be absorbed and the wave is cancelled. The MBB-105 and the BK-117, Bell 412 and some Hughes helicopters including the early version of the Apache have bob weights that are connected in most cases to the blades.
Some helicopters have two types of bob weights that are sensitive to different frequencies of the wave. Some Bell helicopters employ a static weight that is embedded in the blade at the nodal point for the wave in order to cancel it out by absorbing the energy of the wave. The Aerospatial helicopters have a heavy weight suspended on top of a stiff spring with the assembly being mounted on top of the rotorhead. Boeing Vertol has a mechanism mounted under the pilots’ seats to perform the same function.
It is my understanding that Bell developed an electromechanical system that worked in the same manner as a noise-canceling headset. The device sensed the vibration level and the cyclical frequency and generated an opposite sine wave and equal in magnitude which eliminated the vibration. This device was going to be installed in B-205s and HU1s but I don’t know if it was ever placed in wide use.
I do not believe that the incorporation of any of these energy absorption systems was incorporated to compensate for the loss of lift on a two blade or, a multi blade helicopter when the blade are disposed over the longitudinal axis of the helicopter. To eliminate the vibration problem Bell came up with the Nodamatic suspension system which isolates the dynamic system and the transmission from the airframe. When they went to a four-blade system on the 412 they incorporated the bob weights. Some Bell four blade heads do not incorporate the bob weights so I would assume that they went back to the embedded weights.
It is my opinion that the N-rev is in effect the reason these systems were developed with the N-rev being caused by the induced vibration resulting from the traveling wave..
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The Cat
[This message has been edited by Lu Zuckerman (edited 14 March 2001).]
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Lu,
Not all of the N per rev. systems are dynamic. On the 206B the isolation mount reduces the lateral 2 per rev generated by the 2 bladed m/r system. The isolation mount is nothing more than a rubber shock absorber.
As the Sultan pointed out 2 bladed rotors have a 2 per rev and 3 bladed 3 per etc. I don't know if the reason is due to the traveling wave or not but this same phenomena can be measured in ground based fan systems. This is also known as the "Blade Passage Frequency"
In addition to the BOB weights as you called them the (Bell calls then Pendulum Absorbers), the 222 employs a Nodel Beam System to reduce the vertical 2 per rev's and a nose Frahm damper to reduce the lateral 2 per rev's. All three are dynamic but only one is mounted on the rotor. Which one reduces the traveling wave?
In addition to the SPA's (Simple Pendulum Absorber) you mentioned on the Bell 412 they also employ a nose Frahm dampner for lateral 4 per rev. reduction. The 412's you saw without SPA's were probably Agusta made as they have some customers that would rather live with the payload gain than having a smoother ride and the associated payload loss when the SPA's are installed. Weights haven't been added to m/r. The 407 doesn't have SPA's but does have the Frahm damper installed on top of the rotorhead.
Sikorsky uses the Bifilar on the 76 to reduce the 3 and 5 per rev's and a mass absorber similiar to the Frahm to reduce cabin 4 per rev's on later models. On the S76A model they use the VTA (Vertical Trim Amp.)to reduce the vertical 4 per rev. This unit mounted in the nose uses a combination of hydraulics and electronics to perform the magic feat.
The Aerospatial (ECF) AS350/355 employ the spring mass on top of the head to reduce lateral 3 per rev's and then hammers under the pilot's seats to reduce the vertical 3 per rev. The hammers are just a weight mass mounted on a flat spring and tuned to reduce the verical N per rev. of the m/r.
I don't know if I agree with your statement regarding loss of lift as the blade passes over the longitudinal axis. If this happens as you say then on a two bladed rotor one would think you would feel a 1 per rev. and on a 4 bladed rotor you would also experience a 1 per rev. In flight a 1 per rev. that is not a result of mass imbalance is usually caused by the unequal lift of one m/r blade.
Based on your theory of the traveling wave, what would cause a 2 per rev. imbalance on a Bell 412?
With respect to your 2 per rev in engine to transmission driveshaft, most any driveshaft that is misaligned will show a 2 per rev of shaft speed. I doubt the 2 per rev caused the grease loss but more likely was caused by burping the grease from the seals due to misalignment.
Chuckolamofola
Not all of the N per rev. systems are dynamic. On the 206B the isolation mount reduces the lateral 2 per rev generated by the 2 bladed m/r system. The isolation mount is nothing more than a rubber shock absorber.
As the Sultan pointed out 2 bladed rotors have a 2 per rev and 3 bladed 3 per etc. I don't know if the reason is due to the traveling wave or not but this same phenomena can be measured in ground based fan systems. This is also known as the "Blade Passage Frequency"
In addition to the BOB weights as you called them the (Bell calls then Pendulum Absorbers), the 222 employs a Nodel Beam System to reduce the vertical 2 per rev's and a nose Frahm damper to reduce the lateral 2 per rev's. All three are dynamic but only one is mounted on the rotor. Which one reduces the traveling wave?
In addition to the SPA's (Simple Pendulum Absorber) you mentioned on the Bell 412 they also employ a nose Frahm dampner for lateral 4 per rev. reduction. The 412's you saw without SPA's were probably Agusta made as they have some customers that would rather live with the payload gain than having a smoother ride and the associated payload loss when the SPA's are installed. Weights haven't been added to m/r. The 407 doesn't have SPA's but does have the Frahm damper installed on top of the rotorhead.
Sikorsky uses the Bifilar on the 76 to reduce the 3 and 5 per rev's and a mass absorber similiar to the Frahm to reduce cabin 4 per rev's on later models. On the S76A model they use the VTA (Vertical Trim Amp.)to reduce the vertical 4 per rev. This unit mounted in the nose uses a combination of hydraulics and electronics to perform the magic feat.
The Aerospatial (ECF) AS350/355 employ the spring mass on top of the head to reduce lateral 3 per rev's and then hammers under the pilot's seats to reduce the vertical 3 per rev. The hammers are just a weight mass mounted on a flat spring and tuned to reduce the verical N per rev. of the m/r.
I don't know if I agree with your statement regarding loss of lift as the blade passes over the longitudinal axis. If this happens as you say then on a two bladed rotor one would think you would feel a 1 per rev. and on a 4 bladed rotor you would also experience a 1 per rev. In flight a 1 per rev. that is not a result of mass imbalance is usually caused by the unequal lift of one m/r blade.
Based on your theory of the traveling wave, what would cause a 2 per rev. imbalance on a Bell 412?
With respect to your 2 per rev in engine to transmission driveshaft, most any driveshaft that is misaligned will show a 2 per rev of shaft speed. I doubt the 2 per rev caused the grease loss but more likely was caused by burping the grease from the seals due to misalignment.
Chuckolamofola
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To: Chuckolamofola
First of all, I would like to congratulate you for knowing the technical names of the dynamic and electrical elements used to decrease or eliminate unwanted vibrations on helicopters. Some of these units were created after I left the respective programs and I have never worked on some of the programs so my information comes from Aviation Leak and Space Technology, Rotor & Wind and other trade journals. My responses are keyed to the numbered paragraphs in your post (Below)
1) Not all of the N per rev. systems are dynamic. On the 206B the isolation mount reduces the lateral 2 per rev generated by the 2 bladed m/r system. The isolation mount is nothing more than a rubber shock absorber.
2) As the Sultan pointed out 2 bladed rotors have a 2 per rev and 3 bladed 3 per etc. I don't know if the reason is due to the traveling wave or not but these same phenomena can be measured in ground based fan systems. This is also known as the "Blade Passage Frequency"
3) In addition to the BOB weights as you called them the (Bell calls then Pendulum Absorbers), the 222 employs a Nodal Beam System to reduce the vertical 2 per rev's and a nose Frahm damper to reduce the lateral 2 per rev's. All three are dynamic but only one is mounted on the rotor. Which one reduces the traveling wave?
4) In addition to the SPAs (Simple Pendulum Absorber) you mentioned on the Bell 412 they also employ a nose Frahm damper for lateral 4 per rev. reduction. The 412's you saw without Spa’s were probably Agusta made as they have some customers that would rather live with the payload gain than having a smoother ride and the associated payload loss when the SPA's are installed. Weights haven't been added to m/r. The 407 doesn't have SPA's but does have the Frahm damper installed on top of the rotorhead.
Sikorsky uses the Bifilar on the 76 to reduce the 3 and 5 per rev's and a mass absorber similar to the Frahm to reduce cabin 4 per revs' on later models. On the S76A model they use the VTA (Vertical Trim Amp.)to reduce the vertical 4 per rev. This unit mounted in the nose uses a combination of hydraulics and electronics to perform the magic feat.
The Aerospatial (ECF) AS350/355 employ the spring mass on top of the head to reduce lateral 3 per rev's and then hammers under the pilot's seats to reduce the vertical 3 per rev. The hammers are just a weight mass mounted on a flat spring and tuned to reduce the vertical N per rev. of the m/r.
5) I don't know if I agree with your statement regarding loss of lift as the blade passes over the longitudinal axis. If this happens as you say then on a two bladed rotor, one would think you would feel a 1 per rev. and on a 4 bladed rotor you would also experience a 1 per rev. In flight a 1 per rev. that is not a result of mass imbalance is usually caused by the unequal lift of one m/r blade.
Based on your theory of the traveling wave, what would cause a 2 per rev. imbalance on a Bell 412?
6) With respect to your 2 per rev in engine to transmission driveshaft, most any driveshaft that is misaligned will show a 2 per rev of shaft speed. I doubt the 2 per rev caused the grease loss but more likely was caused by burping the grease from the seals due to misalignment.
General response:
Sikorsky first discovered the traveling wave phenomenon when they installed a movie camera on top of an S-51 rotorhead that was mounted on a whirl stand. Sikorsky was looking for indications of leading and lagging and they found two things that they had not anticipated. Up until that time it was thought that the advancing blade lagged due to increased airloads and the retreating blade led due to decreased air resistance. The opposite was found to be true. What they also discovered was that the blades as viewed spanwise looked like a sinusoidal wave used in describing AC electrical flow. It is my understanding that when this movie was shown to helicopter pilots some of them stopped flying.
Responses:
1) The N per rev I was addressing on the Bell has to do with the alignment of the blades along the longitudinal axis and the loss of lift. This causes a two-per-rev vertical beat and has nothing to do with a lateral beat. I would think that if you have a lateral beat it relates to an imbalance condition.
2) I agree with what The Sultan stated about the vibrations on helicopter blades in general. As I indicated in (1) above the vibrations can be traced to one of several phenomena. On two bladed rotor system the vertical beat can be traced to the alignment of the blade as on a Bell. The other vibrations can be traced to an imbalance in the rotor system or, the presence of an uncompensated for traveling wave.
3) If the Bell philosophy carried through from other designs none of the described elements counters the traveling wave. As I had indicated in my post, Bell embedded a weight in the blade at the nodal point of the traveling wave and it is this weight that stops the wave from reaching the rotorhead.
4) I had indicated that the 412 rotor system had the pendular weights but that other Bell four blade systems did not. I also alluded to the fact that they may have reverted to the embedded nodal weights in the blades. It was my understanding that the Bifilar system was developed to counter the traveling wave generated vibrations. Whether they were three per rev or five per rev I can’t say. The system you described by Sikorsky that employs hydraulics and electronics was I believe developed by Hughes and was never used by them. The system has sensors that detect the vertical beat and sends a signal to the hydraulic servos to extend or retract to counter the vertical beat by introducing a countering pitch change. On all of the other elements we agree. I just didn’t know the correct names.
5) On a two-blade system you feel a two per rev vertical beat because the blades comes into alignment with the longitudinal centerline twice in a 360-degree rotation. The one per rev you described on a four-blade system could relate to an out of track on one of the blades. Using my theory to explain why you don’t get the vertical beat on a four-blade system due to the alignment of two blades with the centerline of the helicopter is that you still have two blades generating lift. I don’t know if this is true or not but, you don’t get this type of a beat on a four blade system unless something is amiss.
6) As described in my post this grease pumping phenomenon was first discovered on the 214 because of the movement of the transmission relative to the fixed engine. Assuming a rotational speed of 250 RPM on the rotor, the engine to transmission misalignment occurred 500 time per minute. The misalignment could be several inches depending on the maneuvering of the helicopter. This is what I stated in my post and you agreed with it. The two per rev caused the problem and the Nodamatic suspension compensated for it.
Here is an after thought:
The traveling wave has always been present on helicopter blades from the very beginning but the resultant vibrations were determined to be acceptable to pilots and passengers. When the problems began to surface was when helicopter manufacturers began developing and installing high lift blades which unlike previous designs, which were symmetrical, the new high lift blades were unsymmetrical. The symmetrical designs had highly predictable aerodynamic characteristics where the unsymmetrical blades did not. Unsymmetrical blades want to climb and dive and as such must be restrained by fixed points on the rotorhead and in the flight control systems. This locks the forces in the blades and external devices were incorporated to eliminate or at least minimize the resultant fuselage vibration.
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The Cat
[This message has been edited by Lu Zuckerman (edited 14 March 2001).]
First of all, I would like to congratulate you for knowing the technical names of the dynamic and electrical elements used to decrease or eliminate unwanted vibrations on helicopters. Some of these units were created after I left the respective programs and I have never worked on some of the programs so my information comes from Aviation Leak and Space Technology, Rotor & Wind and other trade journals. My responses are keyed to the numbered paragraphs in your post (Below)
1) Not all of the N per rev. systems are dynamic. On the 206B the isolation mount reduces the lateral 2 per rev generated by the 2 bladed m/r system. The isolation mount is nothing more than a rubber shock absorber.
2) As the Sultan pointed out 2 bladed rotors have a 2 per rev and 3 bladed 3 per etc. I don't know if the reason is due to the traveling wave or not but these same phenomena can be measured in ground based fan systems. This is also known as the "Blade Passage Frequency"
3) In addition to the BOB weights as you called them the (Bell calls then Pendulum Absorbers), the 222 employs a Nodal Beam System to reduce the vertical 2 per rev's and a nose Frahm damper to reduce the lateral 2 per rev's. All three are dynamic but only one is mounted on the rotor. Which one reduces the traveling wave?
4) In addition to the SPAs (Simple Pendulum Absorber) you mentioned on the Bell 412 they also employ a nose Frahm damper for lateral 4 per rev. reduction. The 412's you saw without Spa’s were probably Agusta made as they have some customers that would rather live with the payload gain than having a smoother ride and the associated payload loss when the SPA's are installed. Weights haven't been added to m/r. The 407 doesn't have SPA's but does have the Frahm damper installed on top of the rotorhead.
Sikorsky uses the Bifilar on the 76 to reduce the 3 and 5 per rev's and a mass absorber similar to the Frahm to reduce cabin 4 per revs' on later models. On the S76A model they use the VTA (Vertical Trim Amp.)to reduce the vertical 4 per rev. This unit mounted in the nose uses a combination of hydraulics and electronics to perform the magic feat.
The Aerospatial (ECF) AS350/355 employ the spring mass on top of the head to reduce lateral 3 per rev's and then hammers under the pilot's seats to reduce the vertical 3 per rev. The hammers are just a weight mass mounted on a flat spring and tuned to reduce the vertical N per rev. of the m/r.
5) I don't know if I agree with your statement regarding loss of lift as the blade passes over the longitudinal axis. If this happens as you say then on a two bladed rotor, one would think you would feel a 1 per rev. and on a 4 bladed rotor you would also experience a 1 per rev. In flight a 1 per rev. that is not a result of mass imbalance is usually caused by the unequal lift of one m/r blade.
Based on your theory of the traveling wave, what would cause a 2 per rev. imbalance on a Bell 412?
6) With respect to your 2 per rev in engine to transmission driveshaft, most any driveshaft that is misaligned will show a 2 per rev of shaft speed. I doubt the 2 per rev caused the grease loss but more likely was caused by burping the grease from the seals due to misalignment.
General response:
Sikorsky first discovered the traveling wave phenomenon when they installed a movie camera on top of an S-51 rotorhead that was mounted on a whirl stand. Sikorsky was looking for indications of leading and lagging and they found two things that they had not anticipated. Up until that time it was thought that the advancing blade lagged due to increased airloads and the retreating blade led due to decreased air resistance. The opposite was found to be true. What they also discovered was that the blades as viewed spanwise looked like a sinusoidal wave used in describing AC electrical flow. It is my understanding that when this movie was shown to helicopter pilots some of them stopped flying.
Responses:
1) The N per rev I was addressing on the Bell has to do with the alignment of the blades along the longitudinal axis and the loss of lift. This causes a two-per-rev vertical beat and has nothing to do with a lateral beat. I would think that if you have a lateral beat it relates to an imbalance condition.
2) I agree with what The Sultan stated about the vibrations on helicopter blades in general. As I indicated in (1) above the vibrations can be traced to one of several phenomena. On two bladed rotor system the vertical beat can be traced to the alignment of the blade as on a Bell. The other vibrations can be traced to an imbalance in the rotor system or, the presence of an uncompensated for traveling wave.
3) If the Bell philosophy carried through from other designs none of the described elements counters the traveling wave. As I had indicated in my post, Bell embedded a weight in the blade at the nodal point of the traveling wave and it is this weight that stops the wave from reaching the rotorhead.
4) I had indicated that the 412 rotor system had the pendular weights but that other Bell four blade systems did not. I also alluded to the fact that they may have reverted to the embedded nodal weights in the blades. It was my understanding that the Bifilar system was developed to counter the traveling wave generated vibrations. Whether they were three per rev or five per rev I can’t say. The system you described by Sikorsky that employs hydraulics and electronics was I believe developed by Hughes and was never used by them. The system has sensors that detect the vertical beat and sends a signal to the hydraulic servos to extend or retract to counter the vertical beat by introducing a countering pitch change. On all of the other elements we agree. I just didn’t know the correct names.
5) On a two-blade system you feel a two per rev vertical beat because the blades comes into alignment with the longitudinal centerline twice in a 360-degree rotation. The one per rev you described on a four-blade system could relate to an out of track on one of the blades. Using my theory to explain why you don’t get the vertical beat on a four-blade system due to the alignment of two blades with the centerline of the helicopter is that you still have two blades generating lift. I don’t know if this is true or not but, you don’t get this type of a beat on a four blade system unless something is amiss.
6) As described in my post this grease pumping phenomenon was first discovered on the 214 because of the movement of the transmission relative to the fixed engine. Assuming a rotational speed of 250 RPM on the rotor, the engine to transmission misalignment occurred 500 time per minute. The misalignment could be several inches depending on the maneuvering of the helicopter. This is what I stated in my post and you agreed with it. The two per rev caused the problem and the Nodamatic suspension compensated for it.
Here is an after thought:
The traveling wave has always been present on helicopter blades from the very beginning but the resultant vibrations were determined to be acceptable to pilots and passengers. When the problems began to surface was when helicopter manufacturers began developing and installing high lift blades which unlike previous designs, which were symmetrical, the new high lift blades were unsymmetrical. The symmetrical designs had highly predictable aerodynamic characteristics where the unsymmetrical blades did not. Unsymmetrical blades want to climb and dive and as such must be restrained by fixed points on the rotorhead and in the flight control systems. This locks the forces in the blades and external devices were incorporated to eliminate or at least minimize the resultant fuselage vibration.
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The Cat
[This message has been edited by Lu Zuckerman (edited 14 March 2001).]
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Useful website for B206
Just found a useful (well I think it is!) website with an on-line manual for the TH-67 (JetRanger).
It's the site of a company which conducts "Initial Entry Rotary Wing Flight Training" for US Army Aviation Center in Fort Rucker, Alabama, and provides the US Army supplement to the FAA approved Flight Manual.
Click here:
B206 Manual
Does anyone know of any similar sites?
[ 20 October 2001: Message edited by: Heliport ]
It's the site of a company which conducts "Initial Entry Rotary Wing Flight Training" for US Army Aviation Center in Fort Rucker, Alabama, and provides the US Army supplement to the FAA approved Flight Manual.
Click here:
B206 Manual
Does anyone know of any similar sites?
[ 20 October 2001: Message edited by: Heliport ]
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206L fins
Does anyone know the official Bell explanation for the horizontal stabilizer on a Longranger extending out further on one side than the other and for the fins on the end of the stab being offset opposite to the vertical fin orientation. My guess is to help direct airflow onto the vertical fin to increase it's efficiency. I'm sure all the aerodynamicists I read here will have theories as well and whilst I will be interested to hearing them I would like to know the Bell story in particular.