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#1
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Rotor Balancing
I just had a strange one. I have been balancing my rotor system using a DSS
balancer where I would get the ips and phase angle measurements, turn the rotor to the angle with respect to the photo cell described by the phase angle information, then sight over the velocimeter and add weight to the rotor system on the Opposite side of the mast. This Add Opposite has worked time and time again on the main rotor. Recently I did my annual condition inspection and went completely thru the controls, re-adjusting every thing and carefully zeroed out the lead/lag, checked the head shift and then did a hover check. Results 0.96 ips @ 270. Well with the arrangement I have with the velocimeter, the photo cell and the reflective tape on the swash plate this mean that most of the inbalance was chord wise. I added weight to the side Opposite and flew again. Results: 1.96 ips @ 274!!! I couldn't believe the datat. For giggles I reversed the weight to an Add Same condition and got the following results: 0.26 ips @ 184. What little inbalance that was left appeared to be span-wise. Question: What could cause the phase angle information to flip 180 degrees from what I was used to seeing? The only thing that I'm familiar with that causes the 180 flip with all things else being equal is to be operating above a critical speed and the rotation rate hasn't changed and the mass in the rotor mast and head hasn't changed. Further I added some washers to the blade indicated in the 0.26 ips measurement using the Add Opposite technique and it appears that I should have used the Add Same again. So there seems to be some strong consistancy in the mysterious 180 degree phase flip. I expect the Add Same on the tail rotor as I have checked it out and found a critical speed below the operational speed which will produce the velocimeter being on the side opposite of the inbalance weight. |
#2
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Rotor Balancing
"Stuart & Kathryn Fields" wrote in message .. . I just had a strange one. I have been balancing my rotor system using a DSS balancer where I would get the ips and phase angle measurements, turn the rotor to the angle with respect to the photo cell described by the phase angle information, then sight over the velocimeter and add weight to the rotor system on the Opposite side of the mast. This Add Opposite has worked time and time again on the main rotor. Recently I did my annual condition inspection and went completely thru the controls, re-adjusting every thing and carefully zeroed out the lead/lag, checked the head shift and then did a hover check. Results 0.96 ips @ 270. Well with the arrangement I have with the velocimeter, the photo cell and the reflective tape on the swash plate this mean that most of the inbalance was chord wise. I added weight to the side Opposite and flew again. Results: 1.96 ips @ 274!!! I couldn't believe the datat. For giggles I reversed the weight to an Add Same condition and got the following results: 0.26 ips @ 184. What little inbalance that was left appeared to be span-wise. Question: What could cause the phase angle information to flip 180 degrees from what I was used to seeing? The only thing that I'm familiar with that causes the 180 flip with all things else being equal is to be operating above a critical speed and the rotation rate hasn't changed and the mass in the rotor mast and head hasn't changed. Further I added some washers to the blade indicated in the 0.26 ips measurement using the Add Opposite technique and it appears that I should have used the Add Same again. So there seems to be some strong consistancy in the mysterious 180 degree phase flip. I expect the Add Same on the tail rotor as I have checked it out and found a critical speed below the operational speed which will produce the velocimeter being on the side opposite of the inbalance weight. If anyone out there is interested in rotor balancing other than me, this is a follow up to the previous post. The next day I ran another check and this time I used "Add Opposite" and have the balance down to 0.1ips. I talked to Murray Sweet of Canadian Home Rotors and he told me to disconnect the battery of the balancer and re connect it and it often cures the 180 degree phase flip. On the DSS balancer, I'm going to add a battery disconnect switch. Hopefully this will eliminate the funny phase data and save me one run up. |
#3
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Rotor Balancing
"The OTHER Kevin in San Diego" skiddz "AT" adelphia "DOT" net wrote in message ... On Wed, 27 Feb 2008 09:03:34 -0800, "Stuart & Kathryn Fields" wrote: I expect the Add Same on the tail rotor as I have checked it out and found a critical speed below the operational speed which will produce the velocimeter being on the side opposite of the inbalance weight. If anyone out there is interested in rotor balancing other than me, this is a follow up to the previous post. The next day I ran another check and this time I used "Add Opposite" and have the balance down to 0.1ips. I talked to Murray Sweet of Canadian Home Rotors and he told me to disconnect the battery of the balancer and re connect it and it often cures the 180 degree phase flip. On the DSS balancer, I'm going to add a battery disconnect switch. Hopefully this will eliminate the funny phase data and save me one run up. I'd be interested if I knew WTF "Add Opposite" etc was. Ok a little discussion of balancing. If you have a shaft rotating at a speed below it's "Critical speed" and it is out of balance, and you put a pencil up near the shaft, it will mark the heavy side and if you are going to correct for the out of balance condition, you need to put a weight on the "Oppisite" side of the shaft. Hence "Add Opposite". When running a balance check on my helicopter, the balancer reads rpm, ips and phase angle. I interpret the phase angle measurement to mean the position of the rotor with respect to a reflective tape where the max vibration point occurred. I position the rotor at that angle and look across the shaft from the velocimeter and Voila I "Add Opposite" and I have a balanced rotor after about three to 4 runs. However, the tail rotor is spinning at a speed that is above the "Critica Speed" and there is a 180 degree phase shift encountered. Back to the pencil it now marks a spot on the side opposite the heavy side so that corrective weights must be located by "Add Same". This techniqu has gotten me balancer readings down below 0.1 ips. (Military says anything below 0.4 is ok) Note if you run at the "Critical Speed" and it doesn't come apart, the pencil makes a mark 90 degrees behind the heavy spot. So if you are close to the "Critical Speed" your pencil is going to mark somewhere and if you try to "Add Opposite" or " Add Same" you are going to be chasing the little dots around your polar chart if you use one. Gawd I love helicopters they are an unending source of questions. Stu Fields |
#4
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Rotor Balancing
Kevin: I sent you an e-mail with an attachment of an article I wrote for
the March issue of our magazine titled Rotor Rigging that goes into more details and has a graphic showing the relationship between the photo cell, the velocimeter and the reflective tape. Picture is worth 10,000 of my words. "The OTHER Kevin in San Diego" skiddz "AT" adelphia "DOT" net wrote in message ... On Wed, 27 Feb 2008 19:14:08 -0800, "Stuart & Kathryn Fields" wrote: Ok a little discussion of balancing. If you have a shaft rotating at a speed below it's "Critical speed" and it is out of balance, and you put a pencil up near the shaft, it will mark the heavy side and if you are going to correct for the out of balance condition, you need to put a weight on the "Oppisite" side of the shaft. Hence "Add Opposite". OK, figured it was probably something like that. When running a balance check on my helicopter, the balancer reads rpm, ips and phase angle. I interpret the phase angle measurement to mean the position of the rotor with respect to a reflective tape where the max vibration point occurred. I always understood phase angle to be the relationship between the feathering axis of a blade and the point at which its control link attaches to the swash (Advance angle - we use "phasing" in the RC world to describe the same thing) How do you determine where to put the reflective tape? Based on the pencil mark at the heavy side? I position the rotor at that angle and look across the shaft from the velocimeter and Voila I "Add Opposite" and I have a balanced rotor after about three to 4 runs. Might be lack of coffee after a whopping 3.5 hours of sleep, but ya lost me there. However, the tail rotor is spinning at a speed that is above the "Critica Speed" and there is a 180 degree phase shift encountered. Define "critical speed" and why the 180 degree shift? Back to the pencil it now marks a spot on the side opposite the heavy side so that corrective weights must be located by "Add Same". This techniqu has gotten me balancer readings down below 0.1 ips. (Military says anything below 0.4 is ok) I think you need to redo it until it's 0.0 ips. Job's not worth doin' unless it's done right! Note if you run at the "Critical Speed" and it doesn't come apart, the pencil makes a mark 90 degrees behind the heavy spot. So if you are close to the "Critical Speed" your pencil is going to mark somewhere and if you try to "Add Opposite" or " Add Same" you are going to be chasing the little dots around your polar chart if you use one. Ok, I'm off to get coffee 'cuz ya lost me again.... Gawd I love helicopters they are an unending source of questions. I know we do a LOT of tweaking/balancing on our RC helis, but at least I understand that. hehehe |
#5
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Rotor Balancing
"Hawkdoc" wrote in message ... Back to the pencil it now marks a spot on the side opposite the heavy side so that corrective weights must be located by "Add Same". This techniqu has gotten me balancer readings down below 0.1 ips. (Military says anything below 0.4 is ok) I think you need to redo it until it's 0.0 ips. Job's not worth doin' unless it's done right! Stu, not sure where you got the .4 standard from. On US Army rotorcraft the standard was for .2 IPS in all modes (ground, hover and flight Vne). Some birds where impossible to get all three so we always went with hover and flight. Some of the reasons could be structural, mechanical or even the flight hours on the individual blades. Kevin, 0.0 sounds nice but, is an impossible quest. To many outside factors in play to ever achieve (wind, temp, age, even sunlight). I've had birds down below .1 IPS and couldn't tell the differance with others at .3. Besides 1 to 1's aren't bad in comparisson to 2 to 1's (tail rotor). 2V1 are the ones that make a lousy flight on a clear smooth day, the ones that can make you feel sick and listless. 2V1 are the buzz in the controls that put your body to sleep. James Rule #1 Always remember your aircraft was made by the lowest bidder Oops. James. I was the builder and if problems show up I know exactly who to blame. The 0.4 ips was given to me so long ago I don't remember the source. But as I recall I thought it had to be gospel. I do all the main rotor balance jobs in hover and the lateral 1/rev stays good thru forward flight. The vertical only increases slightly at 90 mph. However, with fuel tank less than 1/2, I get a lateral 2/rev that is above 2 ips. This 2/rev is from the main rotor as the 2/rev with the tail rotor would be 92hz and very hard to feel. With full tank the main rotor 2/rev goes away. Haven't been able to get to the cause yet so just keep the fuel above 1/2. Believe that there is a resonance in the airframe somewhere that likes the 2/rev frequency of 17hz. I agree I couldn't tell the difference in feel from 0.1 ips to around 0.3. BTW you have brought up something that has been confusing to me. I got a ride in the Hummingbird which is a S-52 derivative and found myself after a short period in smooth air feeling nauseous. I thought at first that I had some food poisoning and aborted further flying. Later I thought that it might have come from the blade flicker coming thru the eybrow windows. I've never felt nausea in any helicopter before. I thought the Hummingbird was pretty smooth, but maybe you've hit the cause of my nausea. Stu Rule #1 with amateur built ships the source of your problem is very close. |
#6
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Rotor Balancing
-- Kathryn Fields Experimental Helo magazine P. O. Box 1585 Inyokern, CA 93527 (760) 377-4478 ph (760) 408-9747 publication cell "Hawkdoc" wrote in message ... Oops. James. I was the builder and if problems show up I know exactly who to blame. The 0.4 ips was given to me so long ago I don't remember the source. But as I recall I thought it had to be gospel. I do all the main rotor balance jobs in hover and the lateral 1/rev stays good thru forward flight. The vertical only increases slightly at 90 mph. However, with fuel tank less than 1/2, I get a lateral 2/rev that is above 2 ips. This 2/rev is from the main rotor as the 2/rev with the tail rotor would be 92hz and very hard to feel. With full tank the main rotor 2/rev goes away. Haven't been able to get to the cause yet so just keep the fuel above 1/2. Believe that there is a resonance in the airframe somewhere that likes the 2/rev frequency of 17hz. I agree I couldn't tell the difference in feel from 0.1 ips to around 0.3. BTW you have brought up something that has been confusing to me. I got a ride in the Hummingbird which is a S-52 derivative and found myself after a short period in smooth air feeling nauseous. I thought at first that I had some food poisoning and aborted further flying. Later I thought that it might have come from the blade flicker coming thru the eybrow windows. I've never felt nausea in any helicopter before. I thought the Hummingbird was pretty smooth, but maybe you've hit the cause of my nausea. Stu Rule #1 with amateur built ships the source of your problem is very close. Yup, forgot that one, about you being the builder LOL no aspirations intended. As to the increase in speed this would be probably controlled by your tabs if you have one. Not sure if you have access to a storbe but if you need a climbing blade would be the first suspect to look at. In regards to the 2/rev I'd doubt the M/R being the problem. As I was taught and experienced, the problem was always secondary drive related. You've got the Hz now you need to figure out what resonates at that frequency. I suspect a bearing could be your culprit based on your sysmptoms. Have you worked up the different CG's based on when it occurs versus full? A CG change could cause a bind in the air frame loading a bearing. Lastly, vibrations, fumes, air flow versus temp or as you suggest flicker vertigo from the eyebrow windows or you could have had an inner ear infection that day. One time I had an irate Lt. Colonel complain his aircraft was out of balance. I went for a test flight and found nothing wrong with it. This went on for a month with the LTC wanting my head because he insisted there was a problem. We finally figured out that they had installed the wrong window and it was really noticeable at his eye level. Good luck and let us know what you find out. James Rule # 7 A plan never survives the first 30 seconds of combat James: When I set up to measure the 2/rev, I put two reflective tapes to be read by the photo cell. This, thru the filter in the balancer guarantees that I'm reading 2/rev frequencies. Also I can show mathematically, and I think with a wet thumb, why any two bladed helicopter will generate a 2/rev vibration at the rotor head level. 1st. In a hover the air velocity that generates the is being generated only by the rotation of the blades. You have pretty much a constant lift and drag force no matter where the blade is with respect to the main axis of the helicopter. However once you begin forward flight, you now have a lift and drag force caused by the rotation plus both lift and drag forces caused by the velocity of the air caused by forward flight. It is not hard to see how the lift and drag forces due to forward flight would be higher on the advancing blade than on the retreating blade. The math does verfy that there would be a 2/rev force generated. Further if the vibration was being caused in the drive train it should be there also when hovering. My 2/rev only shows in forward flight. As soon as I enter the hover I can feel the ship smooth up and the data recorded in my balancer confirms this. Further I can't imagine a bearing making 16.7Hz vibrations. Especially at the energy level needed to shake the instruments. Another Safari owner reported 2/rev readings at reduced fuel levels of 3.7ips!! and vibrations that rendered his instruments unreadable. We recently hung the helicopter from the overhead beam and used a laser vibrometer to record frequency response readings on the helicopter. On the frame just below the fuel tank we recorded 20Hz. This was taken with low fuel and just by tapping on the frame with a rubber mallet. Now the true resonant frequency of just that frame tube without the rest of the frame should be significantly higher than 20Hz. It seems the thing that is responding to the 16.7Hz from the main rotor is more involved than just a couple of frame tubes. Witnesses of the test included several professional vibration whizzes. Their opinion is that it is going to require a more in depth look to identify possible fixes. |
#7
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Rotor Balancing
"Hawkdoc" wrote in message ... James: When I set up to measure the 2/rev, I put two reflective tapes to be read by the photo cell. This, thru the filter in the balancer guarantees that I'm reading 2/rev frequencies. Also I can show mathematically, and I think with a wet thumb, why any two bladed helicopter will generate a 2/rev vibration at the rotor head level. 1st. In a hover the air velocity that generates the is being generated only by the rotation of the blades. You have pretty much a constant lift and drag force no matter where the blade is with respect to the main axis of the helicopter. However once you begin forward flight, you now have a lift and drag force caused by the rotation plus both lift and drag forces caused by the velocity of the air caused by forward flight. It is not hard to see how the lift and drag forces due to forward flight would be higher on the advancing blade than on the retreating blade. The math does verfy that there would be a 2/rev force generated. Further if the vibration was being caused in the drive train it should be there also when hovering. My 2/rev only shows in forward flight. As soon as I enter the hover I can feel the ship smooth up and the data recorded in my balancer confirms this. Further I can't imagine a bearing making 16.7Hz vibrations. Especially at the energy level needed to shake the instruments. Another Safari owner reported 2/rev readings at reduced fuel levels of 3.7ips!! and vibrations that rendered his instruments unreadable. We recently hung the helicopter from the overhead beam and used a laser vibrometer to record frequency response readings on the helicopter. On the frame just below the fuel tank we recorded 20Hz. This was taken with low fuel and just by tapping on the frame with a rubber mallet. Now the true resonant frequency of just that frame tube without the rest of the frame should be significantly higher than 20Hz. It seems the thing that is responding to the 16.7Hz from the main rotor is more involved than just a couple of frame tubes. Witnesses of the test included several professional vibration whizzes. Their opinion is that it is going to require a more in depth look to identify possible fixes. Stu, It's been quite a while since I was taught most of this. (20 plus years, I probably needed to go through my boxes of junk and refresh my memory) so I'm a little confused. I've balanced most Army helicopters excluding the Chinook (thank god!). This has included the pole and tape method, to the Vibrex and the last that I can't remember. Maybe we're not talking the same thing. 1 /rev problems are decribed as shaking or bounce. 2 per/rev problems are hums or better described as buzzing feelings. Is this what your talking about? I'm not sure I'm understanding your comment about putting 2 pieces of tape to get the 2 per rev info. 2 pieces on one blade? a piece on each? How does the system know the master blade? This was the track portion of track and balance. Not alot to be done with this except to ensure both blade fly through the same space. If one blade was higher than the other we'd make a P/C link adjustment. If one blade climbed as our speed increased we'd make a trim tab adjustment. These were usually the last things done. On the newer system we'd put a piece of tape on one blade but only to give the computer a target 0 degree referance angle. All the vibration analysis was done by accelrometers. One on the swashplate and one in the cockpit. We'd then dial in the M/R speed and get a reout in ips at and angle coresponding to target 0. Again, I'd have to dig out the books but as I remember 1/rev such as the problem your friend describes are M/R balance issues. Shaking so violent you can't see the instruments is M/R. Think large mass and pendulum. We'd get that on some ships depending on load and configuration going in and out of ETL due to changes in the air flow over the M/R. Let me know if we're on the same page and I wish you all the best. Sorry to hear about your ruined flight the other day at least the gas was cheap. I just hope the repair didn't destroy that price advantage and great feeling of the day. James James: The 2/rev I'm referring to is the vibration incurred in any two bladed rotor system that is encountered only in forward flight. To get my balancer to read the 2/rev vibration levels, I have to fool the filter that is set with just one piece of reflective tape which reads the vibrations occurring at the rotor speed by putting two pieces of reflective tape, one 180 degrees from the other, and the filter will now be enabled to read vibrations at twice the normal rotore speed hence now 2/rev. The Master blade idea is only meaningful in the 1/rev tracking and balancing measurments. With the 2 pieces of reflective tape the Master blade is ambiguous and the phase angle is not of interest; only the amplitudes. I typically set the track at full chat with the skids just light on the ground using the pole and tape, or in our case a paper towel inner roll on the end of a pvc pipe and then tracking lights installed in the tips of the blades looking back at the pilot. The tracking is adjusted with the pitch links and if a climbing blade is encountered during forward flight, trim tab adjustment is used. The Safari helicopter has a resonance that I believe occurs in the airframe when the fuel level gets below 1/2 tank. This vibration frequency measures to be 16.6Hz whis is 2/rev. Similar measurements have been made on other two bladed helicopters. Some show no wiggle in the pilot's seat but have some above the transmission mount. Other's like the Safari show both vertical and later 2/rev vibrations in the cockpit. My ship exhibits very little vertical 2/rev but I have moved the battery out in front of the instrument panel ala the Bell 47. I still however can measure 2.2ips of lateral measuring just below the swashplate. Having to perform these measurments on a non-certified ship that has no instructions in a maintenance manual has forced me to dig in deeper than a lot of guys who had the advantage of a factory supplied manual discussing such things. It is both a plus and a minus. BTW just back from Bensen Days and Sun'n Fun. At Bensen days I hover taxiied a 254# helicopter. Kinda nervy checking your self out in a single seat, ultra lite helicopter. I expected it to have a strong tendency to travel to several different zip codes simultaneously. Not so. Pretty darned stable little bird. See the Mosquito Air. Stu |
#8
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Rotor Balancing
On Apr 17, 6:51 am, The OTHER Kevin in San Diego skiddz "AT" adelphia
"DOT" net wrote: On Tue, 15 Apr 2008 12:32:41 -0700, "Stuart & Kathryn Fields" wrote: BTW just back from Bensen Days and Sun'n Fun. At Bensen days I hover taxiied a 254# helicopter. Kinda nervy checking your self out in a single seat, ultra lite helicopter. I expected it to have a strong tendency to travel to several different zip codes simultaneously. Not so. Pretty darned stable little bird. See the Mosquito Air. 254lbs? Wow. I almost weigh as much as that... Was wathing video of the Mosquito XET last night. Cool little ship... Yes, much nicer than the Hirth powered option. That lawnmower engine sound drives me nuts! But these T62-32 units are getting harder and harder to find nowadays. |
#9
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Rotor Balancing
On Apr 17, 9:49 am, JohnO wrote:
On Apr 17, 6:51 am, The OTHER Kevin in San Diego skiddz "AT" adelphia "DOT" net wrote: On Tue, 15 Apr 2008 12:32:41 -0700, "Stuart & Kathryn Fields" wrote: BTW just back from Bensen Days and Sun'n Fun. At Bensen days I hover taxiied a 254# helicopter. Kinda nervy checking your self out in a single seat, ultra lite helicopter. I expected it to have a strong tendency to travel to several different zip codes simultaneously. Not so. Pretty darned stable little bird. See the Mosquito Air. 254lbs? Wow. I almost weigh as much as that... Was wathing video of the Mosquito XET last night. Cool little ship... Yes, much nicer than the Hirth powered option. That lawnmower engine sound drives me nuts! But these T62-32 units are getting harder and harder to find nowadays. Oh, I see it is actually a T62-2A1, not a -32. Is the price/ availability any better? |
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