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Bobby,
My guess is that weight probably was not the reason for the "no metal prop" restriction. Metal props have the potential to develop destructive harmonic vibrations since they have little damping capability compared to a wood or
composite prop. Carbon fiber props are so stiff that they may not provide much damping of torsional resonance but I have no proof of that. The rubber dampers should provide some isolation between the engine and the rest of the drive train making it less
likely to excite those harmonics.
When I replaced the test stand rubber dampers with solid metal ones, I was trying to get a baseline for the engine side of the system without disassembling the plane. There was about 0.020" of clearance between the test stand damper
plate and the ID of the flex plate. Without the reduction drive input shaft, the damper plate could become off center until constrained by the flex plate. The damper plate apparently did not become off center to the same location from one run to the next,
thus preventing achieving a balance solution. The solid metal parts fixed the position of the damper plate on the flex plate allowing the assembly to be balanced. This was completely separate from torsional and prop harmonic considerations.
I would be interested in purchasing a supply of rubber dampers.
Steve Boese
Steve,
Tracy had a “no metal prop” exception for the rigid coupled drives. Was this due to weight or some other concern? My MT prop weights about 39#’s. I personally like the rubber dampers but I believe they should
be considered a consumable item. Flat spots on the 45 durometer appear with less than 100 hours. May be as few as 50 hours. They are cheap but the minimum order is 600 units. I have a quote and plan to try and get a group buy together. If a group buy is possible
I plan to buy 40 units and change every 50- 100 hours. A firmer material may last longer. Thinking about a 60 – 65 durometer. I’ll put the group buy information out to the group when I have time.
Bobby
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net]
Sent: Tuesday, January 15, 2019 2:56 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Rubber Damper
When I was performing a dynamic balance of the turbo Renesis on the test stand with only the flex plate and damper plate installed, I was unable to achieve a solution due to the angular position of the heavy spot changing from one run to
the next without attaching weights. Solid aluminum replacements for the dampers were made and installed. A dynamic balance solution was then achieved. It may be that the solid damper replacements would result in a setup very similar to the rigid coupling
described as used in Tracy's latest drives. This was only done on the test stand and I have not installed the solid parts in the plane.
Steve Boese
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Tracy,
Service Rubber located the part number. It was actually a 45 durometer. They are providing a quote with minimum quantity.
Thanks,
Bobby
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net]
Sent: Tuesday, January 15, 2019 12:29 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Rubber Damper
Tracy,
I’m exploring the possibility of have a batch of dampers made. Measuring the worn units is difficult. I found some information in the archives but need to confirm the measurements.
Supplier was Service Rubber in Texas. They have changed accounting systems and can’t located the part number. If you have the part number that would be most helpful.
OD 1.374”
ID 0.75”
Height --?
Durometer of 50. Looking at increasing to 65. Service Rubber recommended neoprene.
Depending on minimum quantity and price it will likely take a group buy. My 50 durometer develop flat spots very fast. I have been rotating them to move the flat spot to the opposite side. I think they need to be change every 50 – 100 hours.
Harder material may survive longer.
Thanks,
Bobby
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