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Hi Chuck and Ed could there be any relationship to the lack of lubrication to these housings? The Lubrication increase could dampen any resonant frequency causing chatter? What would your opinion be on that idea? Anyone?
From: "Ed Anderson" <eanderson@carolina.rr.com>
Date: 2006/03/16 Thu AM 12:21:11 EST
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: [FlyRotary] Re: Motor teardown results
MessageThe chatter marks are certainly apparent, Chuck. Whoa! 150
hours is certainly not much out of a brand new housing. It could be as you suggest that the two piece seal played a role. However, I have another suggestion. Measure the apex seal slot at the top (across the gap) and see what the distance is. Do it both on the good rotor and the damaged one if you can find an undamaged spot. If like my rotor, the slots are worn, I believe they may contribute to the chatter and breaking of the apex seal.
Here's the theory. As the slot "V"s there is less support for the seal near
its top, the bottom is still fairly well constrained. As a consequence, the top portion of the seal is more able to be swayed back and forth by combustion and friction forces. In other words, instead of just rubbing on the rotor housing wall with the smooth top of the seal, the edge of the seal starts to drag along the wall. The seal (with less support at the top) acts like a stiff spring (up to a limit) and "vibrates" as it slides along the housing wall leaving the chatter marks. At some point the forces become too much and the seal breaks.
It might be interesting to determine the vibration frequency of a seal
( given its dimensions and composition) and then measure the average distance between the peaks (or valleys) of the chatter. Then given the resonant frequency of the seal determine at what rotor rpm it would leave chatter marks of the measured distance. {:>). No, I don't have a clue about how to do this, but I'll bet some mechanical engineer could come up with something.
Just a theory, of course, but if your apex slot tops are within the
specifications called for by Mazda then that would discredit the theory. If they are worn then it might tend to give it support.
When my seal broke it also caught the next seal down in is slot and
smeared the top of the slot such that it could not come back up.
You and I are apparently the only ones (that I know of ) that seem to have
had this failure mode of an apex seal disintegrating for no apparent reason.
Thanks for sharing the photos and information.
Are you going to try for Sun & Fun?
Ed Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com
Ed
----- Original Message ----- From: Chuck Dunlap To: Rotary motors in aircraft Sent: Wednesday, March 15, 2006 10:44 PM
Subject: [FlyRotary] Motor teardown results
I finally got time to get my failed 13b apart. The rear rotor was missing
one apex seal, the other 2 were intact but jammed due to the fragments of the broken one. The rotor housing has the classic chatter marks all around. Doing a Google of "13B apex seal chatter" results in plenty of examples of this failure mode. Apparently the chatter is from the natural resonant frequency of the seal, it eventually fatigues the seal causing failure.
The front rotor was in good shape, but the housing had the same chatter marks, but not as severe as the rear rotor. The front rotor housing was brand new 150 hours ago when I rebuilt this motor after the Grand Canyon forced landing. At that time the good rotor housing had slight chatter marks but I reused it. The front rotor was destroyed at that time due to foreign object entering the rotor.
Both rotors were missing the corner seal rubber plugs on the side with the short apex seal segment. Perhaps the small segment vibrated more, disintegrating the rubber seals?
The first 260 hours of flying my rotary bird was with stock 3-piece seals, and I never saw any chatter marks. The last 250 hours has been flown with 2 piece seals and now I have chatter marks .
I now have Tracy's 2 piece apex seals and I wonder how they will perform, it appears that the heavier weight of the 2 piece seals puts the resonant frequency in the cruising range of 6000RPM?
Chuck Dunlap
RV6 13B
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Doug Mueller
RX-6 13BT
N900DM
Boulder City(61B),Nevada
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