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Skookum Robotics Skookum Robotics SK-360 SK-540 & SK-720 Digital Flybar |
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04-13-2014, 09:44 PM | #1 (permalink) |
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Join Date: Dec 2012
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Working a Tail Vib.
Hey guys,
Just checking back in now that I have finished building my replacement 770. I'll try to keep this one out of the trees. It fly's very well for a new build and my SK720BE is telling me all is good for the motor (slight vibe but no biggie) and the main rotor is smooth as well. The tail rotor is needing some attention though. My main rotor RPM's are 1500 and 1800 which translates to around 8,000 and 10,000 on the tail. At 8,000 RPM the tail has a pretty calm magnitude of 2.5 but step up to Idle 1 and 10,000 RPM and the magnitude jumps to 16. It's all fore and aft so I would think a good balance is needed. That's a pretty big jump in vibration level so I would think it will be easy to improve on that one. Looking forward to smoothening it out. It's funny, before the SK was installed I would have never known of the vibration. My tail shaft bearings would have given up early and I would have blamed them as being poor quality. My old Goblin had a very quiet tail from a vibration standpoint and at 421 flights until it's death hitting the tree, the tail bearings were still tight. Last edited by A VIKING; 04-15-2014 at 04:13 AM.. |
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04-14-2014, 01:31 AM | #2 (permalink) |
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Join Date: Sep 2011
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Try switching the tail blades between the grips. You might be surprised
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2 X X7 | Scorpion 4525 Ultimate 4+6T YY / Xnove 4530 525 | scorpion Tribunus 200II ESC | Edge 713 | Logo 690SX | Pyro 440 7+8T YY | Castle 120HV | X3 | 4S | HW 50A V2 ESC | Scorpion 2221 2050KV 9T D | SK540 / Brain2 | Mks Last edited by omerco; 04-14-2014 at 02:42 AM.. |
04-14-2014, 01:47 PM | #3 (permalink) |
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Also, if it is a tail balance issue. You will see higher up/down vibes than fore aft. Can you post up that log?
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04-14-2014, 03:15 PM | #4 (permalink) |
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That is true. Could be something oval in the gear somewhere and tight mesh? That might cause it on only one axis
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2 X X7 | Scorpion 4525 Ultimate 4+6T YY / Xnove 4530 525 | scorpion Tribunus 200II ESC | Edge 713 | Logo 690SX | Pyro 440 7+8T YY | Castle 120HV | X3 | 4S | HW 50A V2 ESC | Scorpion 2221 2050KV 9T D | SK540 / Brain2 | Mks |
04-14-2014, 03:51 PM | #5 (permalink) |
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I have deleted all my logs to start fresh, so I have none to share at this point.
Its been super windy today so I haven't been to keen on taking it out. I did check on tail blade COG to see if they matched which they did so I swapped their position and on installation in the grips I let them hang vertical by their own weight in the grips which were held perfectly vertical and tightened the bade bolts down. I did this procedure on my old 770 as well and the tail blades hang with the same forward sweep to them in relation to the grip center line. Your would think that they would hang perfectly vertical, straight down following the centerline of the grips but they do not. This forward swept position is their normal hanging position due to the weight distribution in the blades and should also be their natural position when spun. If I didn't tighten the grips to hard they would find this position on their own but I tighten up the blade bolts pretty tight and the blades don't get the chance to find balanced center when spun. On my first runs I had the blades tightened down and in a straight position, maybe they just didn't like it and gave me a buzz. We'll see on my next run, I'll post the log if I don't see any improvement. |
04-14-2014, 08:34 PM | #6 (permalink) |
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I did have the original log!
OK, I see that I did keep the last log which shows a +16 magnitude tail vib, see the screen shot.
After adjusting the tail blades as described in my last post above the tail vib dropped to a magnitude of 2! See the second screen shot. You can also see in both screen shots my ever present motor vib. I can reduce it to magnitude 1 very easily with a 1/2 inch square of clear packing tape but of course the tape doesn't last long at 21,000 RPM. So from a magnitude +16 to a magnitude 2 with a little blade manipulation, I like it! So what do you guys see in the screen shots, I didn't include the whole flight because it all looks the same. |
04-14-2014, 08:48 PM | #7 (permalink) |
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What I do with the logs is roll them back and forth. Zero'd in on the widened vibe. This allows me to try and visualize what the assembly is doing and why it is moving the way it is.
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04-15-2014, 04:26 AM | #8 (permalink) | |
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Quote:
That's exactly what I do also LMH! Rapidly scrolling back and forth through the log gives you a motion picture of the vibs throughout the flight and what their reaction is to fight loading and RPM changes. In this case scrolling back and forth only produced the same image so that's why I only posted a "still" picture. The rest of the flight is the same image. Pretty cool stuff. Does anyone know what we are measuring when looking at "Magnitude" on the log? With full scale aircraft we measure "IPS" or "Inches per Second" as amplitude. So lets say in my case with a magnitude 2 on the vib log, that would be a 2 IP vibration. That's a very big vib so I hardly think their is any correlation between "Magnitude" and "IPS". We always work at getting full scale vibs below .2 IPS (that's point 2 IPS) and in most cases we can get that number down to .05 IPS. |
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04-15-2014, 12:56 PM | #9 (permalink) |
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Location: Smyrna, GA
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If I remember correctly, the actual numbers don't really mean anything in particular. It's just a visual scale where low is good and higher is worse. I can't remember if this pertains to the values seen on the scale, or the overall vibe score given by the software or both.
The question has been asked before though, and I believe there is no particular mathematical formula being represented here, but Georgi will correct me if I'm wrong.
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Tony Synergy 516, Gaui R-5 Speed (RIP), Cypher Vtol Jet (RIP), Spirit FBL, Hobbywing and Scorpion Tribunus ESC, Xnova motors. The girl in my DX9 tells them all what to do |
04-15-2014, 01:24 PM | #10 (permalink) | |
Join Date: Mar 2007
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Quote:
as a base to work from a red vibe light is caused by vibration levels of around 10G's seen at the unit, low frequency or high frequency triggered. so scale it from that. there is a research lab not far here that test systems to failure points, one of their demo's is to show how the blades must be free to lead/lag, in forwards flight your tail blades will vibrate fore/aft if tightened as you say, allow the blades freedom to work with physics, please let them move!
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04-15-2014, 02:09 PM | #11 (permalink) |
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Since I love to experiment I'll try freeing up the tail blades a little to let them move.
My only fear is because the Goblin has no washer between the tail blade and grip the fore and aft movement caused by natural tendencies of the blades in flight may wear a spot in the blade from rubbing into the grip. I would hate to lose a tail blade one day. I agree that blades have to lead / lag and flap in their normal flight path but these blades are so small and light with a very high RPM that I would think those forces would not be minimal at best. Does this research company do work on full scale or model helicopters? I guess to prove it out I could loosen the grip bolts and let them move. If lead lag does occur I should see evidence of that movement in the form of a small rub or wear mark from the grips. I noticed in a photo of LMH's ships he had a couple that appeared to have lose tail blades. Since I am the only one I fly around, are lose tail blades the norm or do people snug them down a little but not enough to hold them motionless in lead lag? Thanks Georgi! |
04-15-2014, 02:56 PM | #12 (permalink) |
Join Date: Mar 2007
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=] some of mine fall under their own weight, the lead/lag movement is evident but never gives a problem, nor a vibration because of that. stress free rotation = longevity.
the research centre deals in full size mostly and uses models for demo's, they have a wind tunnel which was my involvement, for track use (high speed upside down aerofoils!)
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04-15-2014, 03:50 PM | #13 (permalink) | |
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Quote:
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04-15-2014, 04:05 PM | #14 (permalink) |
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Thanks Guys!
I will comply. Takes all the challenge out of locking them down exactly where they need to be. I did have 421 flights on the old Goblin 770 with locked tail blades and minimal tail vibs....just saying. |
04-15-2014, 05:51 PM | #15 (permalink) |
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Isn't it funny how two almost identical setup machines by a competent builder can vary here?
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04-16-2014, 06:31 PM | #16 (permalink) |
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Flew a couple flights today with my tail blades a little lose.
When I checked my tail vib numbers they were magnitude 4. With the tail blades locked down I had a vib of 2. Sooooo.....I'l experiment some more but will go with the method that produces the least tail vib for me. |
04-22-2014, 10:35 AM | #17 (permalink) |
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Experimenting with the tail vib on a G770.
After many, ,many flights I have concluded that tight tail blade mounting is smoother from a vib perspective than lose blade mounting in the grips.
At an RPM of 10,500 on the tail the lowest vib magnitude number I could get with the stock G770 tail rotor was 2. It did not matter if I added or subtracted weight in any amount, the vib always went up. If I had lose fitting blades in the grips the vibe level was 4. If I added weight to one or the other blades in an effort to correct this it only got worse. As you can see in the photo of my tail assembly the blades appear to be in a swept forward position in relation to the grips. This is the position the blades hang at when the bolts are lose and the tail hub is in a vertical position. With the hub and blades in this position I carefully tighten up the blade bolt to hold the blade firm in that position. Care must be taken when tightening the bolt or you will knock your best balance number off. The blades hang in that position because they are attaining their COG from the bolt hole and this should be the position the blade wants to fly at when spun. If I were to make the blades appear straight in relation to the hub I would have a vibration. Anyway, interesting stuff and as always these are my results, yours may very as all tail blades are not created equal...but centrifugal force affects them all the same way. So at this point I am super happy with what my SK720BE allowed me to correct. My tail vibes are 2 as is my 420 flight Quantum motor and my main rotor vibs are lost in the chatter at the bottom of the scale. Of course the main rotor will show itself in the vib logs flying 3D but no where near the spikes I was getting with my old 770. |
04-22-2014, 11:33 AM | #18 (permalink) |
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Nice work! I typically tighten mine to a moderate point where they can move and then spin up and let them go to where they want to be at RPM before tightening them down any further.
I really wished I could have solved the vibration issue with my Comp dampened tail design, but no amount of hours spent on balancing, parts changing or money thrown at it would get any better than 5, and it was almost as bad with no blades at all on it. Went back to the original version, and problem gone and back to .6! I did remove the stock brass links and switch over to the plastic links and balls and KDE dual slider, and its slop free now, so Im happy. Im just not sure if the new tail design actually makes vibes worse rather than better as the design claims to do. Ive got a nice $60 paper weight now either way
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Tony Synergy 516, Gaui R-5 Speed (RIP), Cypher Vtol Jet (RIP), Spirit FBL, Hobbywing and Scorpion Tribunus ESC, Xnova motors. The girl in my DX9 tells them all what to do |
04-22-2014, 12:23 PM | #19 (permalink) |
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I actually run my tail blades finger push tight. Move them close to straight before spool up. And on my Trex based heli I have to run them looser.
My Comp tail only ran vibe low a couple flights. Then started acting up. But the early grips all showed up with nasty vibes on mine. All five sets! Even after running abec 7 bearings in one set, but that knocked them down by half. My best results were using HeliOption triple bearing tail grips. Which I am still running on my 800. The problem with these is that the grip hub metal is soft, so almost anything bumping the tail messes them up. And the work to install them was a huge pia. I almost had Bob at RcTek design a set for our Sab's. His tri rotor tail on my Tri is the lowest vibe tail I have ever seen. I had much better results changing out my Sab tail blades to be honest. Like Mark on his thread we tried out quite a few different brands, including the Kbdd's. My lowest vibes were recorded with the NHP's and next in line were the Rail. The very worst were the stock Sab blades. Three sets I tried were all vibe high.
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04-22-2014, 12:35 PM | #20 (permalink) | |
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Quote:
I have a few of those paper weights also. I do have the KDE dual slider still on the tail assembly of my wrecked 770 I could use on this new 770. That would be interesting to see if it would reduce vibs further. At least I have good base line vib levels to monitor now, any increase from these levels means I better be looking. |
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