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  #1  
Old 07-13-2013, 11:57 AM
NASA515 NASA515 is offline
 
Join Date: Nov 2011
Location: Hansville, Washington
Posts: 536
Default ADAHRS Vibration and ADAHRS Mounting Brackets

When I first started my airplane's engine, and did initial ground runs, up until the present, I have had periodic verbal Caution messages, with an annunciation CAUTION: ACTV ADAHRS VIBRATION.

I posted on the Dynon Forum, and receiving no response, I called Dynon, to find out what it meant - especially the ACTV part - since that's not called out in the Installation Manual. Initially, they couldn't tell me - but later we figured it out as meaning ACTIVE - related to systems where there are two ADAHRS installed. and they recommended replacing the ADAHRS. I elected not to do so until I could track down the source of the vibration. I figured it was pointless to go through the contortions needed to crawl back into "that hole" - made even narrower now with the fuel tank installed, if I could even do it, that is, without pulling the tank - only to find that the reading was accurate and not related to the ADAHRS.

During my Phase 1 flight testing, the Caution came and went. I tried turning that Dynon Lady off, but couldn't, and have grown accustomed to her talking to me throughout most flights. She even started saying "G's", which took me a month to figure out. The vibration would sometimes trip the G-Meter, which I reset to 3.0

To gauge my vibration, I made flights with Tony T. in N577RV. I also had made multiple flights in the Vans demonstrator. My vibration - meaning mostly engine vibration - seemed more than his, but not out of line. He had done a prop dynamic balance. In any event, I went through a long process of trying to check and eliminate any vibration, including checking the engine mounts, the prop spinner run out, and doing multiple blade pitch resets. I even had Jim Scott of Aircore Aviation at Arlington do a vibration check and prop reset, including a careful setting of the blades to ensure there was no difference between them in pitch. I am also setting up a prop dynamic balance - which I figured was my last chance.

I downloaded all my data after each flight and analyzed the vertical and horizontal acceleration. Horizontal seemed OK, but there was definitely a lot of Vertical G. I did several RPM vs Vibration surveys, running from about 3500 rpm to 5200 rpm in 200 rpm increments and plotted the results to find some correlation between the two. The correlation, if any, seemed tenuous at best - the engine does seem to vibrate more around 4800 rpm (considered typical for 912s by many.) I also did similar airspeed checks and did a big Internet search. I thought I could trigger it sometimes doing stalls.

The vibration started to affect the PFD - with the horizon line and slip/skid ball jittering - sometimes very badly. Dynon sent me a new ADAHRS.

To make a long story short, I am now on my third ADAHRS box. Changing them is NO FUN. The third one I installed with thick rubber spacers between the ADAHRS feet and the Vans ADAHRS brackets - still no joy. I still have the vibration. I've had sit down meetings with Dynon about this.

A few days ago, as I was standing alongside my airplane, I pushed the area where the ADAHRS mounting brackets are located - I was amazed. The crown skin in that area was VERY flexible - almost like a trampoline. If you put your thumb on one row of support bracket rivets, and your other fingers on the other rivet line and rock back and forth, you will find - or at least - I found - there was tremendous flexibility.

I thought I had found my problem.

Doing the same at the F-1208 frame shows it to be very rigid -as to be expected. The crown skins between the frames are VERY flexible. Using the template, I re-confirmed the brackets were correctly located.

On Thursday, I flew my airplane into the Arlington Airshow. At Van's request, I parked my airplane on their show line next to their RV-7A and RV-14A demo airplanes - between the Vans and Dynon display tents. Steve has been my Dynon Tech Support contact - he was manning their tent. I showed him my findings - he was amazed and felt sure that was the source of my problem. Joe Blank was manning the Vans tent - same reaction. The second Vans guy (whose name eludes me) said they had been having the same problem on at least one factory built S-LSA airplane during flight testing.

Everyone felt this area would be subjecting the ADAHRS to a high level of vibration susceptibility - either due to engine/airframe induced vibrations, or simply due to aerodynamic forces at different airspeeds and attitudes, as the airstream (perhaps) burbled off the aft canopy/crown area (especially during high nose attitude maneuvers.)

Everyone agreed I needed to beef up the area, by stiffening the crown skin in the transverse direction (athwartship is the Boeing term). Various schemes were discussed - from using a part of a new F-1208 frame, fabricating from scratch stiffeners, and gluing or riveting them to the crown skin, to bonding lead/rubber pads (as I have used on boats) to the inside skin. Even attaching an external transverse doubler. I've also thought of duct-taping some sort of doubler to the outside and see if that eliminates the vibration problem.

I recall an assortment of other postings here that described verbal "G's" messages, and jittering horizons and slip/skid balls. All may be related and a result of this installation configuration.

I plan to call Vans first thing Monday morning and get their views and advice, before I engineer my own solution, and will keep the crew here advised.

Bob Bogash
N737G
Now at 75 Flights
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  #2  
Old 07-13-2013, 12:11 PM
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DonFromTX DonFromTX is offline
 
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Wow Bob, thanks. Like hundreds of other 12 owners, I rushed to the garage and pushed around on that area, you are right, it is spongy beyond belief! I will be paying attention to your fix, while I still have not put my tank or rear bulkhead in. Should be an easy fix for sure.
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  #3  
Old 07-13-2013, 01:08 PM
Dave12 Dave12 is online now
 
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You might want to consider the use of 3M Double Sided Tape. I think it comes in several strength ranges. I have addressed several oil cannings by using this tape and some light gauge aluminum bent into angle. It works great and is very strong. I believe it is the same product used to bond the Kruger Sun Shields to the canopy.
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  #4  
Old 07-13-2013, 01:25 PM
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DonFromTX DonFromTX is offline
 
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Since I still have full access to the area, I am going to rivet in some strength.
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  #5  
Old 07-13-2013, 01:57 PM
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bhassel bhassel is offline
 
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I haven't started the tailcone yet so I'm not sure where the ADAHRS actually mounts. Do you have a picture? It's between the F-1208 and...

Thanks,

Bob
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  #6  
Old 07-13-2013, 02:05 PM
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DonFromTX DonFromTX is offline
 
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This photo might help. You must ignore the reflection however. The strength in the center is nice, but you can see how it would "rock" on the mounting.
[IMG][/IMG]
Quote:
Originally Posted by bhassel View Post
I haven't started the tailcone yet so I'm not sure where the ADAHRS actually mounts. Do you have a picture? It's between the F-1208 and...

Thanks,

Bob
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  #7  
Old 07-13-2013, 02:57 PM
NASA515 NASA515 is offline
 
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It's on the top centerline between the Aft Cargo Bulkhead and the F-1208 frame.

Bob Bogash
N737G
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  #8  
Old 07-13-2013, 08:39 PM
E. D. Eliot E. D. Eliot is offline
 
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Default Good catch!

Thru actual flight experience, the Van's RV-12 is getting better and better. I appreciate all of the 'pioneering' of those of you who have completed and are flying 12s. Wonder how Van's will fix this? Should be pretty simple, I hope.

Thanks to all of you who are flying 12s - I hope to start construction soon!
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  #9  
Old 07-15-2013, 05:26 PM
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Bill_H Bill_H is offline
 
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That double sided attachment tape is really something - from Walmart Aviation, it is less than 1/8 inch thick. I used it to attach my rudder wedge, thinking "If this wedge happens to need shortening, I'll just peel it off and eventually final-mount with RTV." Well, the wedge was perfect, so I started to (try to) peel it off for the RTV. Hmmmm. You know, I think it is perfect as it is! I have no doubt it would easily solve oil canning if attached to aluminum angle.
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  #10  
Old 07-15-2013, 05:53 PM
NASA515 NASA515 is offline
 
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Talked to Gus at Vans first thing this morning. They've heard of a few, but don't think it's endemic. The S-LSA problem was solved by re-pitching the prop and ensuring equal angles between blades (which I've already done - 3X.)

His advice - add a stiffener, with glue, pro-seal, tape, rivets - whatever suits my fancy, and report back. It could be transverse - or even longitudinal, tying the brackets to the frame behind it. Funny - Dynon's very first suggestion was to beef up the brackets.......

I plan to get the prop dynamically balanced first to eliminate that avenue once and for all. If that doesn't work, I may scuba dive again into "that hole" and conjure up some sort of stiffening arrangement.

Meanwhile - back at the ranch - since we made this "Ah-Ha" discovery and amazed the Vans and Dynon folks at the Arlington Show - the system has felt threatened and to defend itself has gone cold-turkey on these warnings. Since then, I've flown KAWO - KPWT - KHQM - KPWT -- about 3-4 hours with the box performing just like good little boxes are supposed to perform. Not a peep out of it.

To be continued......

Bob Bogash
N737G
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