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Old 01-31-2019, 11:01 AM
NinerBikes NinerBikes is offline
Join Date: Dec 2018
Location: Granada Hills
Posts: 73

I know it wouldn't apply to a Rotax motor, but in car engines, they put in heavier flywheel's or dual mass flywheels, to smooth the pulses out of the compression and combustion strokes, especially on turbo diesel motors. VW, Audi, Porsche do this, a lot.

Those pulses are probably what tear up the dogs.

Also when you are descending when landing and you throttle back, the propeller indeed gets feathered and sloppy, being driven by the wind, not the motor, with not much or no thrust being created by the prop. Hence the slop, if your dogs are worn.

Last edited by NinerBikes : 02-13-2019 at 11:47 PM.
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Old 01-31-2019, 12:04 PM
Piper J3's Avatar
Piper J3 Piper J3 is offline
Join Date: Nov 2014
Location: Hinckley, Ohio
Posts: 1,258

I don’t agree with your assertion that prop gets “feathered” when engine is throttled back for landing.

A wind milling fixed-pitch prop creates a lot of drag. When the engine is throttled back the prop back-drives the engine, and in doing so, holds the dogs nicely against each other -so not chattering. This is why engine is running faster in glide than idle speed when stationary on the ground.

The key to gearbox longevity is engine minimum operating speed. Rolling elements – bearings, gears, etc. can take a lot of abuse under dynamic load but suffer badly if abused statically. The reason for the 1800 RPM idle speed is Rotax deems this to be the minimum speed where engine power pulses don’t hurt the gearbox.

That being said, I have my idle speed set at ~ 1700 RPM. This allows better aerodynamic drag and helps reduce float on landing. I have to manually operate minimum throttle to stay above 1800 RPM on the ground. Works very nice…
Jim Stricker
Hinckley, Ohio
EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N68203
1130 hrs Flying 46 Piper J-3 Cub N6841H
RV-12 E-LSA #120058 AWC July 2012 N633CM
RV-12 Bought Flying Oct 2015 with 48TT - Hobbs now 409

LSRM-A Certificate 2016
Special Thanks to EJ Trucks

Last edited by Piper J3 : 01-31-2019 at 12:21 PM.
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Old 01-31-2019, 02:39 PM
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randylervold randylervold is offline
Join Date: Dec 2004
Location: Mill Creek, WA
Posts: 594

Anyone know if the 912iS idle is even adjustable being controlled by the computer and all?
Randy Lervold
RV-12iS, empennage/tailcone complete, wings currently, fuse on order
RV-3B, first flight 2007 - sold
RV-8, first flight 2001 - sold
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Old 01-31-2019, 04:15 PM
rvbuilder2002 rvbuilder2002 is offline
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 7,999

Originally Posted by randylervold View Post
Anyone know if the 912iS idle is even adjustable being controlled by the computer and all?
Yes it is.

There is an idle stop screw on the throttle body.
Van's recommends the same idle speed stop adjustment for the ULS and the iS engine (1650 RPM), with and then follow the standard operating procedure of manually keeping the engine speed above 1800 RPM at all times.
Any opinions expressed in this message are my own and not necessarily those of my employer.

Scott McDaniels
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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