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  #1  
Old 09-14-2022, 11:17 AM
HansLab HansLab is offline
 
Join Date: Jan 2015
Location: Netherlands
Posts: 173
Default shake it, baby!

I have an interesting experience, but let me share some figures first:
TT70 hours
new aluminium Cummins Spinner since 6 hrs. Beautifully made, nearly-perfect mounted per manual: even less movement of the pitot than with the original spinner.
New spinner is not balanced, yet.

The spinner-info is added because I never had this issue before I mounted that beauty..
Engine runs fine, temps where they should be. Taxiing for some minutes, run-up flawless. Takeoff nice as ever. Then the prop begins 'shaking' as if it isn't centered. changing RPM always solves it, and the flight is effortless.
Changing RPM's afterwards never ever got me shaking again...

This happens at some 100ft height, and no (read my lips: NO) shaking whatsoever at run-up.
Checked of course:
No loose motor mounts, engine rubbers - nothing.
No wear or tear on the engine-axle (as far as I can feel/see without deep examination).

Today I noticed a little gas stain in the drip pan of one of the carbs.
I thought it was my turn for sunk floats, but one Rotax guru (Roger Lee) said to be careful with quick actions, for sometimes '..it makes the carbs shake so bad the floats and needle valve can't control the fuel level and it vents out the tube'.

That shake is definitely what I got - but how?
Might the (slightly heavier) spinner be the culprit?
And how do I check that?

Any ideas highly appreciated!
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build & sold project Kitfox IV
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  #2  
Old 09-14-2022, 11:21 AM
larryMar larryMar is offline
 
Join Date: Nov 2018
Location: C77, Il
Posts: 113
Default

Did you remove the prop bolts to install the spinner? I would check the tracking of the propeller to insure that it is within 1/8" or less. With the spinner being on center, I would not expect it to affect the balance, but perhaps it isn't on straight or the bolts are not torqued equally....
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  #3  
Old 09-14-2022, 12:26 PM
HansLab HansLab is offline
 
Join Date: Jan 2015
Location: Netherlands
Posts: 173
Default tracking

thank you - will check.
Bacplate had to be swapped for a Cummins (more sturdy!) one, so yes, prop has been dismantled.
Bolts are toqued equally I'm sure - but even so I have this issue, so: I'll check.
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build & sold project Kitfox IV
Project bought & flying RV12 #120742
Donation 2021 paid

Last edited by HansLab : 09-14-2022 at 12:29 PM.
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  #4  
Old 09-14-2022, 01:41 PM
Piper J3's Avatar
Piper J3 Piper J3 is offline
 
Join Date: Nov 2014
Location: Hinckley, Ohio
Posts: 2,820
Default

Quote:
Originally Posted by HansLab View Post
This happens at some 100ft height, and no (read my lips: NO) shaking whatsoever at run-up.
This sounds like nose wheel coasting down. Imbalance of wheel at certain speeds will cause entire engine to jump around on rubber mounts...
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Jim Stricker - EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 830

LSRM-A Certificate 2016 for RV-12 N633CM
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
MJ Stricker (Father - CFI) - USAAF 1st Lt. Captain B-17H
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  #5  
Old 09-14-2022, 02:06 PM
seagull seagull is offline
 
Join Date: Dec 2019
Location: Highland, CA
Posts: 537
Default

What I do; tip the plane so the nose wheel is off the ground, easiest way is to put weight on the tail anchor. Remove the rear nose wheel faring. Spin the wheel to 1200 rpm and let it coast to zero. What you see will surprise you. If the wheel is out of balance the whole plane will shake at several different rpm’s.

To spin the wheel I put a 8” rubber backing disk on the electric drill and run the edge of the disk against the tire tread.
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  #6  
Old 09-14-2022, 05:22 PM
bobg56 bobg56 is offline
 
Join Date: Nov 2018
Location: Peachtree City, GA
Posts: 272
Default

Do you tap your brakes during climb out? I suspect this since you say your at 100'....could be wheels..
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  #7  
Old 09-14-2022, 08:56 PM
Cyclone Cyclone is offline
 
Join Date: Dec 2014
Location: Grinnell,IA
Posts: 66
Default Prop Shake

When I swapped out the nose fork per the SB; liftoff and 100’ altitude and a significant spinner/prop vibration relative to the cowl. I static balanced the nose wheel (same assembly as prior) and the vibration is gone.
I concluded the new reinforced fork introduced different harmonics.
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  #8  
Old 09-14-2022, 09:09 PM
WJaviation WJaviation is offline
 
Join Date: Jul 2020
Location: Pensacola
Posts: 72
Default

I have a similar vibration and I'm 99% sure it's the nose wheel out of balance. The degree of shake, well it's more like a heavy wobble, actually changed whether I have the nose fairing on or off. I plan to static balance the nose wheel at the next annual or next time I put the plane down for MX.

Try short field takeoff technique and see if it affects the magnitude and duration of the shake. Get the nose off the ground early, like as soon as 25-30kts indicated and wheely down the runway until liftoff. Compare that to rotating at 55-60 kts with the nose wheel spun all the way up.
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  #9  
Old 09-15-2022, 11:41 AM
HansLab HansLab is offline
 
Join Date: Jan 2015
Location: Netherlands
Posts: 173
Default

great suggestions, thank you all. The shake is quite severe to be originated by the frontwheel, but I've listened and will try the 1200RPM trick.
I do hope it is this simple, because anything else I could think of is try a TestFlight without spinner (but with backingplate), but that means without pitot as well. Didn't figure out how to solve that one - although I think I can fly the thing without pitot as well.
I'll keep you posted. DO keep sending ideas!!
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Hans Labruyere
build & sold project Kitfox IV
Project bought & flying RV12 #120742
Donation 2021 paid
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  #10  
Old 09-15-2022, 11:44 AM
Piper J3's Avatar
Piper J3 Piper J3 is offline
 
Join Date: Nov 2014
Location: Hinckley, Ohio
Posts: 2,820
Default

Quote:
Originally Posted by HansLab View Post
The shake is quite severe to be originated by the frontwheel
The imbalance of the front wheel can shake front of engine / spinner ~ 3/8". It's very noticeable...
__________________
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Jim Stricker - EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 830

LSRM-A Certificate 2016 for RV-12 N633CM
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
MJ Stricker (Father - CFI) - USAAF 1st Lt. Captain B-17H
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