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11-03-2014, 10:29 AM
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Join Date: Apr 2011
Location: SF East Bay
Posts: 852
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Quote:
Originally Posted by Captain_John
All of the guys on this forum compose an incredible resource!
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You can say that again.
Fantastic data provided. Thanks to all who had input. Great stuff.
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Sam
RV-8 with the Showplanes Fastback conversion
Emp completed except for glass work
Wings completed except for bottom skin and glass work
Fuselage underway
N18451 reserved
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11-03-2014, 03:02 PM
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Join Date: Oct 2011
Location: Atlanta
Posts: 878
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FWIW the FAA claims that rapid opening and closing of the throttle can detune a counterweighted crank. How that applies to formation flying I'm not sure. I never had an issue with it.
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RV-8 IO-360 (Bought)
RV-6 O-360 C/S (Sold)
Walkman aka Flame Out
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11-03-2014, 05:24 PM
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Join Date: Dec 2008
Location: Big Sandy, WY
Posts: 2,567
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GO300's have multiple weights and will detune easily on rapid throttle reduction. More than a few skylark owners have thought there was somethin wrong when there wasn't. They can be super smooth in cruise though.
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Actual repeat offender.
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11-04-2014, 11:16 AM
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Join Date: Apr 2008
Location: Lake Havasu City AZ
Posts: 2,393
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Counterweight
Quote:
Originally Posted by walkman
FWIW the FAA claims that rapid opening and closing of the throttle can detune a counterweighted crank. How that applies to formation flying I'm not sure. I never had an issue with it.
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The geared Lycoming engines were very prone to "detuning", mostly caused by pulling the power to idle at too high an airspeed on final. I think the guideline was not to pull the power to idle above 120 statute. Also the geared Lycomings were turning 3400 r/m or so on takeoff.
The counterweighted engines in the Pitts survive for 1400 hours of throttle abuse, no problems.
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11-04-2014, 11:45 AM
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Join Date: Apr 2006
Location: Fountain Hills, arizona
Posts: 117
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Spectrum Analysis
My 2 cents,
When having your engine/propeller balanced ask the service if they can perform a Spectrum Analysis, it will provide a report of each mode/node by ips and RPM.
The report should show each ? node amplitude. Example I have a o-540 with a 5th and 6th order counter weight, it at one time had the counter weights changed to heaver weights per SB which will accommodate a compact hub propeller design, similar to what Vans sells. It might detect other lurking issues.
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11-04-2014, 12:09 PM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Quote:
Originally Posted by rvator10
My 2 cents,
When having your engine/propeller balanced ask the service if they can perform a Spectrum Analysis, it will provide a report of each mode/node by ips and RPM.
The report should show each ? node amplitude. Example I have a o-540 with a 5th and 6th order counter weight, it at one time had the counter weights changed to heaver weights per SB which will accommodate a compact hub propeller design, similar to what Vans sells. It might detect other lurking issues.
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Let's not confuse X-Y axis vibration with torsional vibration.
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Dan Horton
RV-8 SS
Barrett IO-390
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11-04-2014, 12:55 PM
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Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 5,686
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Quote:
Originally Posted by rvator10
My 2 cents,
When having your engine/propeller balanced ask the service if they can perform a Spectrum Analysis, it will provide a report of each mode/node by ips and RPM.
The report should show each ? node amplitude. Example I have a o-540 with a 5th and 6th order counter weight, it at one time had the counter weights changed to heaver weights per SB which will accommodate a compact hub propeller design, similar to what Vans sells. It might detect other lurking issues.
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Here's an IO390 Spectrum Analysis

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Walt Aronow, DFW, TX (52F)
EXP Aircraft Services LLC
Specializing in RV Condition Inspections, Maintenance, Avionics Upgrades
Dynamic Prop Balancing, Pitot-Static Altmeter/Transponder Certification
FAA Certified Repair Station, AP/IA/FCC GROL, EAA Technical Counselor
Authorized Garmin G3X Dealer/Installer
RV7A built 2004, 1700+ hrs, New Titan IO-370, Bendix Mags
Website: ExpAircraft.com, Email: walt@expaircraft.com, Cell: 972-746-5154
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11-05-2014, 09:14 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Lycoming 4-cyl crank with pendulums, pins, and bushings. They're between the throws for cyls 3 and 4, and positioned 90 degrees to those throws. As such, they cannot counter recip forces. Some other engines do use torsional pendulums in a dual role, recip counterweight and torsional absorber.
From ECI's notes on dynamic crankshaft balancing:
A dynamic propeller balancer like Walt's uses a single sensor (here in green) mounted to a case bolt just behind the ring gear. It detects acceleration in two axis (X and Y). If the sensor could be mounted precisely on the crankshaft rotational axis, that is all it would detect, just the linear up-and-down and side-to-side of rotating imbalance (cyan arrows).
However, such a mounting is physically impossible; best we can do is a case split line bolt. That means there is a distance (an arm) between the crank axis and the sensor (here in orange).
The crankshaft/propeller system also vibrates torsionally. Here the system is simplified as two flywheels, one representing the prop and one representing the crank. The propeller mass moment of inertia is many times the inertia of the crankshaft assembly, thus the relative size difference.
Torsional vibration is a twisting of the shaft (the connecting stiffness) between the inertias. The two inertias oscillate in opposite directions, twisting the shaft one way, then reversing and twisting it the other way. We would not even notice torsional vibration in our engines except for one detail; the oscillation of the crank is transmitted to the engine block by the piston sidewalls. The engine block thus vibrates (rocks) around the crankshaft axis as well as in the X and Y axis.
Here's the important part. If the sensor could be magically mounted on the crank center, it would not detect any of the block movements with a torsional source. However, since it is mounted with a arm between the crank center and mount point, its X axis detects both linear and torsional movements, i.e. prop imbalance and torsional block reaction. That is why the balancer can show orders other than the 1st....even though the 1st is the only one that can be balanced with weights on the ring gear or spinner. The detection of other orders is useful, and they may change as rotating balance is adjusted with weights, but they're not a rotating imbalance. They are ghosts of the torsional vibration superimposed on the X axis detection.
Postscript 11/6: Order sensing description is subject to modification; consulting in progress.

__________________
Dan Horton
RV-8 SS
Barrett IO-390
Last edited by DanH : 11-06-2014 at 06:49 AM.
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11-10-2014, 07:52 AM
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Join Date: Oct 2005
Location: 08A
Posts: 9,500
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Follow up on the above.
Previously I wrote:
Quote:
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A dynamic propeller balancer like Walt's uses a single sensor (here in green) mounted to a case bolt just behind the ring gear. It detects acceleration in two axis (X and Y).
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I've since learned that the DSS MicroVib system (used by Walt) senses in a single axis only...the vertical, or Y axis in the above sketch. As such it cannot sense block rocking in response to torsional vibration or cylinder firing.
Turns out the series of half order multiples in the spectrum plot (0.5, 1.5, 2, 2.5, etc) are mostly harmonics of the firing order, and are transmitted to the sensor by conduction, not any actual movement of the engine. Think about a ringing bell, a sonar pulse in water, or placing you ear to the railroad track, and you have the right idea.
Bottom line, in the context of this discussion, is that the prop balance system tells nothing useful about torsional vibration, not even a ghost, including the effect of pendulums.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
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11-18-2014, 07:09 PM
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Join Date: Jun 2005
Posts: 846
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prize
Well guess I win the prize for the only RV to sling a prop! I'm going to have to change the name of my plane (six gun) I was taking some acro from Patty after it happened and she said to call it (prop slinger) but I think I will call it 6 gun still. Anyway I went flying on a Thursday for a short flight and returned to land pulled power back in pattern to land and the plane sounded like it was running really rough landed done a run up everything checked good shut down and done a few things at the hangar.Another RV showed up and wanted to fly around a bit so off we went another run up all fine so off we go for a few minutes and we part company back to each others airport 3 miles out I pull power back to land I'm at 1200 agl and bam there goes my prop I think because I can't see out the windshield call my buddy and let him know I'm going to try to make it back to airport.Long story short landed with out a problem and shut down and still didn't believe prop was gone got out and looked and it wasn't there got down and kissed the ground and said thanks Lord for letting me live. 6 Gun had a io-360 angle valve with a Hartzell BA and a Sabra 2.5 spacer all of which was no longer there and the ringgear also gone.The engine we built did not have counter weights so when I had it yellow tagged I had new weights installed and crank balanced. 51 hours on the plane when it happened so far I have not been able to find the prop so I'm not a 100% sure what happened only that I have six broken bolts left of the assembly in the crank lugs.I'm pleased with the outcome because I lived to fly again and one day I'll get it put back together so 6 Gun will fly again.
Bob
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