jsharkey
Well Known Member
Thanks for that one too!
Van's Air Force
Don't miss anything! Register now for full access to the definitive RV support community.
Prop clocking?
- Thread starter Paul Eastham
- Start date
Thanks for that one too!
More opinions
I'll try to address a few questions people have asked me directly:
I can't speak for Lycoming regarding approval of specific prop/crank indexes, but I feel they should be consulted before anybody changes something that has been approved or at least been operated over a very long period of time with good service history.
To know if a specific indexing has been approved or has been used before, you can consult the propeller Type Certificate Data Sheet under Note 9 for specific engine/propeller approvals. Our (Hartzell) prop indexing is determined by the model suffix letters. For example, an "HC-" prop has the blades in-line with the index bushing for a C2YR-1 configuration; we have several different indexes, "PHC-", "DHC-", etc. configurations that have different indexes. You will have to add the prop indexing and crank indexing to determing where the blade will eventually end up relative to the crankpins. For example, Continental has two crank indexes depending on the engine model, similar to the Lycoming -A1B6 vs -A3B6. An HC- prop would put the blades in different position depending on the crank indexing. As someone here found out, indexing on some propellers can locate the blades between the bushing locations.
You can also consult the propeller company application guide to see what has been used before. I think on Mike's website he references the Lycoming drive bushing Service Instruction that defines where the index bushing is relative to the crankpin. When in doubt, call the propeller manufacturer and ask.
The illustration that Dan Horton posted on 9/18 shows the prop/crankpin relationship that I tried to explain in a previous post. I'm going to use that illustration to explain the relationship when someone asks me again. That illustration should help explain why the propeller would resist the engine combustion dynamics in the horizontal plane when mounted 90 deg to the crankpins.
Les Doud
Hartzell Propeller
I'll try to address a few questions people have asked me directly:
I can't speak for Lycoming regarding approval of specific prop/crank indexes, but I feel they should be consulted before anybody changes something that has been approved or at least been operated over a very long period of time with good service history.
To know if a specific indexing has been approved or has been used before, you can consult the propeller Type Certificate Data Sheet under Note 9 for specific engine/propeller approvals. Our (Hartzell) prop indexing is determined by the model suffix letters. For example, an "HC-" prop has the blades in-line with the index bushing for a C2YR-1 configuration; we have several different indexes, "PHC-", "DHC-", etc. configurations that have different indexes. You will have to add the prop indexing and crank indexing to determing where the blade will eventually end up relative to the crankpins. For example, Continental has two crank indexes depending on the engine model, similar to the Lycoming -A1B6 vs -A3B6. An HC- prop would put the blades in different position depending on the crank indexing. As someone here found out, indexing on some propellers can locate the blades between the bushing locations.
You can also consult the propeller company application guide to see what has been used before. I think on Mike's website he references the Lycoming drive bushing Service Instruction that defines where the index bushing is relative to the crankpin. When in doubt, call the propeller manufacturer and ask.
The illustration that Dan Horton posted on 9/18 shows the prop/crankpin relationship that I tried to explain in a previous post. I'm going to use that illustration to explain the relationship when someone asks me again. That illustration should help explain why the propeller would resist the engine combustion dynamics in the horizontal plane when mounted 90 deg to the crankpins.
Les Doud
Hartzell Propeller
Anybody love a good mystery?
Mike got three sets of data with a vertical sensor suitable for direct comparison. They were 1-7 clocking on 4/12, 3-9 clocking on 4/16, and 3-9 clocking on 4/25 following a dynamic balance.
As Mike previously wrote, 3-9 clocking reduced cabin vibration level at the 1/2 order frequency.
However, 3-9 clocking increased cabin vibration level at around 1 HZ.
Here's a table, values taken from the above referenced data sets.
Interesting that the IPS values appearing in the 1 HZ range with 3-9 clocking are [I]higher[/I] than the "objectionable" 1/2 order frequency values with 1-7 clocking. If you look at Mike's data you'll also see these values are not an integer of rotation speed.
May be a lesson in human perception here. Mike, can you feel this 1 hz oscillation? Translated to G it would be very small.
I've already kicked this around with Les. He tells me there are no propeller [I]blade [/I] modes (emphasis mine) way down in the 1 HZ range. This is a good thing. I have some personal experience with a [I]whirl[/I] mode at around 1 hz (which you could definitely feel in the cockpit), so I remain very curious about this low frequency IPS increase.
Of course there may be many other possible explanations. I have a few, some of which are entirely benign. I won't tint the pool with them; better to just throw it out here for the group. Anybody have an idea?
BTW, I don't wish to be a wet blanket about Mike's experiment. I do think it is important to realize its experimental nature. Can't collect data and only focus on the good result you wanted to see. You gotta look at all of it. Sometimes the mystery is where you find the real gold in the experiment..or the tiger. Either way you always learn something.
Mike got three sets of data with a vertical sensor suitable for direct comparison. They were 1-7 clocking on 4/12, 3-9 clocking on 4/16, and 3-9 clocking on 4/25 following a dynamic balance.
As Mike previously wrote, 3-9 clocking reduced cabin vibration level at the 1/2 order frequency.
However, 3-9 clocking increased cabin vibration level at around 1 HZ.
Here's a table, values taken from the above referenced data sets.
Interesting that the IPS values appearing in the 1 HZ range with 3-9 clocking are [I]higher[/I] than the "objectionable" 1/2 order frequency values with 1-7 clocking. If you look at Mike's data you'll also see these values are not an integer of rotation speed.
May be a lesson in human perception here. Mike, can you feel this 1 hz oscillation? Translated to G it would be very small.
I've already kicked this around with Les. He tells me there are no propeller [I]blade [/I] modes (emphasis mine) way down in the 1 HZ range. This is a good thing. I have some personal experience with a [I]whirl[/I] mode at around 1 hz (which you could definitely feel in the cockpit), so I remain very curious about this low frequency IPS increase.
Of course there may be many other possible explanations. I have a few, some of which are entirely benign. I won't tint the pool with them; better to just throw it out here for the group. Anybody have an idea?
BTW, I don't wish to be a wet blanket about Mike's experiment. I do think it is important to realize its experimental nature. Can't collect data and only focus on the good result you wanted to see. You gotta look at all of it. Sometimes the mystery is where you find the real gold in the experiment..or the tiger. Either way you always learn something.
Last edited:
Kahuna
Moderatoring
Nope. Sure cant. I have not calibrated my butt for 1hz.
McCauley Prop Clocking
Well I finally got the correct bushing I needed to reclock the McCauley Mooney 201 prop I have on the Lycoming 0-360 I have in my RV6 and it revealed a totally new twist in harmonics that I didn't quite expect. Previously when the prop was in the 12-6 o'clock position (since the McCauley has the blades located 30 degrees in between the drive bushings it can't be set in the conventional 1-7 o'clock position like most Hartzell's) the #1 blade was 90 degrees from the #1 crank pin and I was experiencing extremely rough running between 1400-1750 RPM when below 10 inches of manifold pressure. This of course was making it very difficult to set up a good approach for landing since I either had to run above 1800 RPM (which resulted in way too much power for a good approach) or backed off to virtually idle (which basically required a good dead stick landing technique!).
The new position that I clocked the prop to is in the 2:30-8:30 o'clock position which is where this prop is supposed to be clocked at from the factory on Mooney 201's (this sets #1 blade 30 degrees ahead of the #1 crank pin...that's as close as I can get to lining up the blades with the crank pins), and at this position the rough running has now shifted up about 300 RPM from the 1400-1750 RPM range to a 1700-1950 RPM range and the overall roughness in that zone has dropped slightly. This actually makes it easy for me to set up a decent landing approach now since I can throttle back to around 1500 RPM at 10 inches of manifold pressure (which is just right for landing the RV6).
Kahuna & Dan, have you experienced such a strange change in prop harmonics as I did when reclocking your props?
I think I'm going to have a new prop extension made by Saber Manufacturing Inc that will allow me to clock my prop to the 3-9 o'clock position and see if that improves even more on the roughness when transitioning thru that bad RPM zone.
Well I finally got the correct bushing I needed to reclock the McCauley Mooney 201 prop I have on the Lycoming 0-360 I have in my RV6 and it revealed a totally new twist in harmonics that I didn't quite expect. Previously when the prop was in the 12-6 o'clock position (since the McCauley has the blades located 30 degrees in between the drive bushings it can't be set in the conventional 1-7 o'clock position like most Hartzell's) the #1 blade was 90 degrees from the #1 crank pin and I was experiencing extremely rough running between 1400-1750 RPM when below 10 inches of manifold pressure. This of course was making it very difficult to set up a good approach for landing since I either had to run above 1800 RPM (which resulted in way too much power for a good approach) or backed off to virtually idle (which basically required a good dead stick landing technique!).
The new position that I clocked the prop to is in the 2:30-8:30 o'clock position which is where this prop is supposed to be clocked at from the factory on Mooney 201's (this sets #1 blade 30 degrees ahead of the #1 crank pin...that's as close as I can get to lining up the blades with the crank pins), and at this position the rough running has now shifted up about 300 RPM from the 1400-1750 RPM range to a 1700-1950 RPM range and the overall roughness in that zone has dropped slightly. This actually makes it easy for me to set up a decent landing approach now since I can throttle back to around 1500 RPM at 10 inches of manifold pressure (which is just right for landing the RV6).
Kahuna & Dan, have you experienced such a strange change in prop harmonics as I did when reclocking your props?
I think I'm going to have a new prop extension made by Saber Manufacturing Inc that will allow me to clock my prop to the 3-9 o'clock position and see if that improves even more on the roughness when transitioning thru that bad RPM zone.
garnt.piper
Well Known Member
Prop Vibes
Hi guys, great stuff, I just finished reading the Sac Sky Ranch book and noted the recommended 3-9 clocking position. I too have a vibe problem and searching and finding this post has given me lots to mull over.
Sounds like I have a similar installation to tloof's - a Whirlwind 200C built on a McCauley hub and presently mounted the only way possible, approximately vertical with #1 TDC. Engine is a TMX-390 in a Giles G-200. Dynamic balance's have improved things, but still not good.
tloof - have you any update on your progress? Did you re-balance after changing the clocking position?
I was about to go through the re-clocking procedure and re-balance to try and imptove things, so any update on your progress may prove useful to me. Thanks
Hi guys, great stuff, I just finished reading the Sac Sky Ranch book and noted the recommended 3-9 clocking position. I too have a vibe problem and searching and finding this post has given me lots to mull over.
Sounds like I have a similar installation to tloof's - a Whirlwind 200C built on a McCauley hub and presently mounted the only way possible, approximately vertical with #1 TDC. Engine is a TMX-390 in a Giles G-200. Dynamic balance's have improved things, but still not good.
tloof - have you any update on your progress? Did you re-balance after changing the clocking position?
I was about to go through the re-clocking procedure and re-balance to try and imptove things, so any update on your progress may prove useful to me. Thanks
No, I never did have it dynamically balanced, though I do eventually plan to. I have contacted Saber Manufacturing and they are going to make me a new prop spacer that will allow me to shift the clocking position to the 3-9 o'clock position on my McCauley prop, so hopefully that improves it even more in the vibrations I am experiencing.
I thought the composite blades used on the Whirlwind propellers would have eliminated all the resonant vibrations that I'm experiencing on my McCauley metal prop, is that NOT the case? (I was thinking of having Whirlwind install a set of their 200C blades in my McCauley hup a while back, so let me know if your 200C is experiencing rough running zones!).
I thought the composite blades used on the Whirlwind propellers would have eliminated all the resonant vibrations that I'm experiencing on my McCauley metal prop, is that NOT the case? (I was thinking of having Whirlwind install a set of their 200C blades in my McCauley hup a while back, so let me know if your 200C is experiencing rough running zones!).
garnt.piper
Well Known Member
Whirlwind Prop balance
The carbon blades on the WW200C wouldn't damp much, I'd expect, as carbon is much stiffer than aluminium or steel. Thus the natural frequency should be higher than for an aluminium prop. I've suspected the MT props, being a compressed beech core with composite covering, would damp better. I have rpm restriction 2050-2300, 2600-2700 (2700 allowed - but don't go over it!). But it vibes everywhere, hard to find a sweet spot. I'm using the AS 'homebuilder special - hard' mount rubbers, which would not help matters. I think the combo of a 390, small hard mount rubbers, light prop on a carbon airframe is possibly the worse you could get!
I'll work on re-clocking the prop to the 3-9 by some method, do another dynamic balance, and see what happens. May change to the softer mounts also. I'll keep you informed of progress, or not. The Whirlwind does pull like a train though.
The carbon blades on the WW200C wouldn't damp much, I'd expect, as carbon is much stiffer than aluminium or steel. Thus the natural frequency should be higher than for an aluminium prop. I've suspected the MT props, being a compressed beech core with composite covering, would damp better. I have rpm restriction 2050-2300, 2600-2700 (2700 allowed - but don't go over it!). But it vibes everywhere, hard to find a sweet spot. I'm using the AS 'homebuilder special - hard' mount rubbers, which would not help matters. I think the combo of a 390, small hard mount rubbers, light prop on a carbon airframe is possibly the worse you could get!
I'll work on re-clocking the prop to the 3-9 by some method, do another dynamic balance, and see what happens. May change to the softer mounts also. I'll keep you informed of progress, or not. The Whirlwind does pull like a train though.
The 390/BA combination got a full survey by Hartzell, and has no operating restrictions. I'll be running mine in the surveyed position.
GrayHawk
Well Known Member
This is a very interesting thread.
What about changing the clocking on a Sensenich 70CM series on a Lycoming O320. This prop is known to have a tenth order, third mode vibration when operated above 2600 RPM?
It would seem that least vibration would only help in this case too???
What about changing the clocking on a Sensenich 70CM series on a Lycoming O320. This prop is known to have a tenth order, third mode vibration when operated above 2600 RPM?
It would seem that least vibration would only help in this case too???
RudiGreyling
Well Known Member
My head is spinning with all the theory, and it looks like the measured data is for a 6 cylinder.
So what is the recommendation for a 4 cylinder fixed pitch 2 blade Sensinich prop?
Thanks a lot,
Regards
Rudi
So what is the recommendation for a 4 cylinder fixed pitch 2 blade Sensinich prop?
Thanks a lot,
Regards
Rudi
garnt.piper
Well Known Member
Re-clock results
Well, I finally got the job done. Took the WW200C off and re-arranged the prop flange bushes (had to buy another two, the #1 bush is a larger diameter than the others) so I could clock the prop to 30deg of the crankshaft throws (rather than 90deg as originally fitted).
Initial balance run was near 2IPS, but we got it down to 0.1IPS with ~76 grams of weight on the backing plate, split in two locations. Couldn't get it under 0.1, we chased it around there for a while and then decided that was as good as it gets.
No measurements were taken in flight before or after, so only subjective opinions here. I think it is smoother, my right foot doesn't go to sleep in cruise now! Whether this is due to a better balance job or the re-clocking, I don't know. I haven't noticed any vibes down through the RPM range, but will look closer around the 2000rpm area. 2.5hrs like this now, and hope to fly this comp season with no cracked spinners or other maladies.
Well, I finally got the job done. Took the WW200C off and re-arranged the prop flange bushes (had to buy another two, the #1 bush is a larger diameter than the others) so I could clock the prop to 30deg of the crankshaft throws (rather than 90deg as originally fitted).
Initial balance run was near 2IPS, but we got it down to 0.1IPS with ~76 grams of weight on the backing plate, split in two locations. Couldn't get it under 0.1, we chased it around there for a while and then decided that was as good as it gets.
No measurements were taken in flight before or after, so only subjective opinions here. I think it is smoother, my right foot doesn't go to sleep in cruise now! Whether this is due to a better balance job or the re-clocking, I don't know. I haven't noticed any vibes down through the RPM range, but will look closer around the 2000rpm area. 2.5hrs like this now, and hope to fly this comp season with no cracked spinners or other maladies.
Kahuna
Moderatoring
Thats unfortunate. Actual data is the only way to determine vibe info, and the dymanic IPS reading on the case is really no help at all for vibes on your butt given the other systems inbetween.
garnt.piper
Well Known Member
Clocking
Too true Kahuna, I still may try and get some inflight measurements later. Logistics beat me this time, I'm in the sticks with no balancer nearby.
Too true Kahuna, I still may try and get some inflight measurements later. Logistics beat me this time, I'm in the sticks with no balancer nearby.
Re-clocking prop documents
John & Listers,
If anyone is interested, I can supply Lycoming SI1452, which is a reprint of Husky's SB3 on re-clocking the prop to 3 and 9 o'clock. I also have Mooney's SB M20-206, which is also on the same subject. If I knew how to post file attachments onto this list, I'd do that.
Charlie Kuss
PS I'm really late on this thread. I just stumbled on it today. I've really enjoyed reading everyone's comments and info on this subject.
John & Listers,
If anyone is interested, I can supply Lycoming SI1452, which is a reprint of Husky's SB3 on re-clocking the prop to 3 and 9 o'clock. I also have Mooney's SB M20-206, which is also on the same subject. If I knew how to post file attachments onto this list, I'd do that.
Charlie Kuss
PS I'm really late on this thread. I just stumbled on it today. I've really enjoyed reading everyone's comments and info on this subject.
garnt.piper
Well Known Member
hi Dave,
I think it was 30 behind the crank throw, which correlated with the prop being in the standard hand-swing position when stopped (10-4 o'clock). I balanced at 2500rpm, and was quite happy after that with the balance for comp aeros. In cruise it was still vibie, so I cruised at 2500 also. Then I fitted one P-mag and the difference was remarkable, the biggest improvement in smoothness of all my changes. A good dynamic balance is important, but for whatever reason elec ignition made the largest difference. I now cruise at 2300rpm/18"MAP LOP and it is good (I don't carry much fuel, 63L only). take care and enjoy.
I think it was 30 behind the crank throw, which correlated with the prop being in the standard hand-swing position when stopped (10-4 o'clock). I balanced at 2500rpm, and was quite happy after that with the balance for comp aeros. In cruise it was still vibie, so I cruised at 2500 also. Then I fitted one P-mag and the difference was remarkable, the biggest improvement in smoothness of all my changes. A good dynamic balance is important, but for whatever reason elec ignition made the largest difference. I now cruise at 2300rpm/18"MAP LOP and it is good (I don't carry much fuel, 63L only). take care and enjoy.
Hi Garnt,
Thanks for the info! Interesting effect of the P-Mag, I wouldn't have expected that...
In going through some of the McCauley documentation it appears that not all their hubs were setup for the same clocking. I wonder if WhirlWind took that into consideration?
WhirlWind's manual says to clock the prop at 10-4 o'clock, which in my case is impossible without changing the crank flange bushings. After seeing Lycoming's S.I. #1098H I thought someone had changed the bushing locations. After pulling the prop I see they're right where they're suppose to be...
Thanks,
Dave
Ps. I don't carry much more fuel than you
Laser 230
Thanks for the info! Interesting effect of the P-Mag, I wouldn't have expected that...
In going through some of the McCauley documentation it appears that not all their hubs were setup for the same clocking. I wonder if WhirlWind took that into consideration?
WhirlWind's manual says to clock the prop at 10-4 o'clock, which in my case is impossible without changing the crank flange bushings. After seeing Lycoming's S.I. #1098H I thought someone had changed the bushing locations. After pulling the prop I see they're right where they're suppose to be...
Thanks,
Dave
Ps. I don't carry much more fuel than you
Laser 230
Same dilemma here. My Whirlwind clocks at 3 & 9. Bushings in crank seem correct. Talked to Whirlwind and they suggest counter boring the hub at the time it comes in for service next to allow for 10 & 4 clocking.Hi Garnt,
Thanks for the info! Interesting effect of the P-Mag, I wouldn't have expected that...
In going through some of the McCauley documentation it appears that not all their hubs were setup for the same clocking. I wonder if WhirlWind took that into consideration?
WhirlWind's manual says to clock the prop at 10-4 o'clock, which in my case is impossible without changing the crank flange bushings. After seeing Lycoming's S.I. #1098H I thought someone had changed the bushing locations. After pulling the prop I see they're right where they're suppose to be...
Thanks,
Dave
Ps. I don't carry much more fuel than you
Second, to anyone who can help, how does clocking have any effect whatsoever on the dynamics of the engine/prop system?? I'm all ears, using my ME degree and lots of industrial rotating machinery experience to try to understand.
Last edited:
Hmm. Good question. Exercising my memory, it's one "hole" off what I thought it should have been. Need to re-research.
As for the clocking orientation affecting the engine system vibration, I just see no physical reason for it to matter. The crankshaft, pistons, and other whirling parts do not know that the blades exist. They only know of a rotating mass that's pulling the shaft forward. The point is only academic really.
Last edited:
garnt.piper
Well Known Member
Yes, I don't understand exactly why a P-mag wouldmake thedifference, but I'll take it. The Lyc. eng. numbers -A1A, A3A, A4A etc indicate the bush positioning and thus clocking of prop as I understand it. The '1' is a different bush position to the '3' and the '4'. An -A1B6 is the same as an -A3B6 apart from the prop flange bush locations, which per the Lyc SI can be changed.
garnt.piper
Well Known Member
Schwamer's Sky Ranch manual explains the vibration modes well - I can't attempt an explanation here. I couldn't get the prop aligned with the crank throws, it was always 30deg off due to prop hub holes and prop flange bush locations. Maybe some props have different drillings to McAuley.
pyolet
Active Member
Low Pass....It is academic and it is based on established laws of mechanics...and it does matter. It's the "right hand rule".
I never would have believed it could make a difference, but was having a persistent "throb" at cruise RPM that only got worse after a very precise dynamic balance job. Reading the Rocket and F1 guys search for a solution to the same symptoms convinced me to try this on my io-540/Hartzell CS pair and it has completely eliminated my vibration/throb issue.
Clock prop so it's at 3-9 when #1 is TDC. Here's the diagram.
Your link's not working for me. Please check.Low Pass....It is academic and it is based on established laws of mechanics...and it does matter. It's the "right hand rule".
I never would have believed it could make a difference, but was having a persistent "throb" at cruise RPM that only got worse after a very precise dynamic balance job. Reading the Rocket and F1 guys search for a solution to the same symptoms convinced me to try this on my io-540/Hartzell CS pair and it has completely eliminated my vibration/throb issue.
Clock prop so it's at 3-9 when #1 is TDC. Here's the diagram.
"Established laws of mechanics, and it does work?" Come on, now! Let me in on the secret (with some facts and physics). Right hand rule, I presume you're referring to to gyroscopic precession. OK. That effect happens the same whether the blade is clocked at 3 or 12. And the crank shaft still only knows there's a spinning mass with essentially symmetric axial thrust load (take out P factor). Or is it related to P factor? How could that matter? I cannot imagine the radial pulses from power stroke speeding the prop enough to cause thrust load variation between revs.
Last edited:
Lycosaurus
Well Known Member
Posting Pyolet's image:
Clock prop so it's at 3-9 when #1 is TDC. Here's the diagram.
Clock prop so it's at 3-9 when #1 is TDC. Here's the diagram.
Last edited:
Walt
Well Known Member
Posting Pyolet's image:
Clock prop so it's at 3-9 when #1 is TDC. Here's the diagram.
That's actually one of our famous Dan H. images.
The original is found in post 39 of this thread, with text. It's an illustration of probable mechanics, not settled fact.
Lots of good stuff early on, so recent readers may want to start at the beginning. Pay particular attention to Les Doud's posts (28 and 52).
How did I miss this good discussion in 2008? That was the era of the divorce. Oh yea.
Last edited:
I moved the bushings around and moved the prop 120? clockwise looking from the front. Blade #1 is now 30? off from being in alignment with the #1 crank throw. The blades start to come up on compression about the 11 o'clock position and the engine can now be hand propped.
The dynamic balancing took less weight and got the balance to .03 ips where it was .05 ips before.
I didn't notice any improvement in the air... So now I'm looking elsewhere for the culprit. I've just went through the cowl to engine interface and improved the fit where things might have been touching too hard. Weather hasn't cooperated for another test flight.
Beyond that I'm going to look at the airframe. Tail wires have been suggested, but I think their frequency would be higher than what I'm feeling.
The engine itself seems to be running smooth, but it could be an uneven combustion between the cylinders.
WhirlWind suggested balancing the prop without the spinner and then add the spinner to the mix and add additional weight for the spinner in its front bulkhead.
Mine has been balanced with an Aces 2020. One of these may do a more thorough job http://www.rpxtech.com/DynavibeGX2.html
Dave
The dynamic balancing took less weight and got the balance to .03 ips where it was .05 ips before.
I didn't notice any improvement in the air... So now I'm looking elsewhere for the culprit. I've just went through the cowl to engine interface and improved the fit where things might have been touching too hard. Weather hasn't cooperated for another test flight.
Beyond that I'm going to look at the airframe. Tail wires have been suggested, but I think their frequency would be higher than what I'm feeling.
The engine itself seems to be running smooth, but it could be an uneven combustion between the cylinders.
WhirlWind suggested balancing the prop without the spinner and then add the spinner to the mix and add additional weight for the spinner in its front bulkhead.
Mine has been balanced with an Aces 2020. One of these may do a more thorough job http://www.rpxtech.com/DynavibeGX2.html
Dave
