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Dynon ADAHRS vibration

StephenC

Active Member
Hi all
I am having an issue with a vibration error message whenever I operate from a grass runway. I get an error message ADAHRS vibration when I am taxiing and on the T/O roll. The unit is mounted on two aluminium struts across the fuselage behind the baggage compartment bulkhead. This appears to be a common place for mounting them so I was wondering if anyone else has seen this problem and if anyone had taken steps to reduce the problem. My thoughts were to use some rubber mounts or sandwich some rubber washers between the unit and the struts.
Thanks for any suggestions
Steve
 
Same location

Steve, I just installed the ADAHRS in that same location.
I believe it to be a very common area to install it.
I never heard of vibration issues before.
Hope like you to hear from the hive minds!!!
 

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Yes Eric that is how mine is only I have it much further forwards. I have looked at isolation mounts but they all seem to use zinc plated steel which would be a problem for the compass.
 
Adahrs vibration

Hi Stephen
I have the same issue, if you want to call it an issue. It's very intermittent and seems to occur only at slightly higher taxi speeds across rough surfaces. I mounted my Dynon Adahrs unit on a shelf across two angles running between the longerons directly behind the baggage access bulkheads as it was a retrofit and a good friend mounted his unit the same way.
I get the error message sometimes on start up if the engine isn't firing on all cylinders for a few seconds..it goes away and also as described above on rough surfaces at say a 1200 rpm taxi speed. Slowing down helps. It has never affected the operation of the Adahrs, it has never occurred in the air, and since I have seen it on occasion, I carry on with my flight, and again the error message had never occurred in the air or caused issues with Skyview.
I have thought of remounting it on rubber grommets but since it's so intermittent and doesn't seem to affect the operation of the Adahrs, I have not.

cheers
Dave C
RV-7
C-GCPD
 
I have been having the same issue in my RV9A. I have dual Dynon ADHRS units mounted on top of each other (with the Dynon dual mount) in a shelf spanning the side walls behind the baggage compartment. I operate off a grass strip. My solution was to insert a 3/8 inch piece of Dynamat material between the ADHRS and the metal shelf and use (gasp!) zip ties to secure the mounting plate to the shelf. While this holds the ADHRS unit well in alignment is does provide a little vibration dampening and less transmission of vibration than metal screws, which so far has worked.
 
The length of the mounting struts is an important factor. The natural frequency, which you'd like to be higher, drops as the length increases. So if you move the assembly aft things will improve.

If there's a way to lower the weight of the ADAHRS unit(s) that'll be an improvement.

Adding bending stiffness to the struts by increasing their height will significantly improve things, too, even if their position doesn't change.

Dave
 
I just upgraded from an Advanced Flight 4500 to a 5400. The 4500 had an internally mounted ADAHRS with a magnetometer on a rear bulkhead mount, whereas the 5400 uses the same Dynon ADAHRS mentioned here. So...I'm in the process of switching it out now. I removed the now-unneeded magnetometer and noted that the bracket it was mounted to was very springy. Rob Hickman told me that it had to be very rigid for the SV-AHRS-200, so I removed that old bracket and went with one that Van's makes specifically for that Dynon ADAHRS unit for the RV-7 and -9, and a similar one for the RV-10. I haven't completed the ADAHARS install yet, have to run the wiring and plumb it, but that mount is extremely rigid. It's $20 and designed and drilled specifically for the Dynon ADAHRS unit. It will accommodate mounting two ADAHRS units.

https://shop.vansaircraft.com/cgi-bin/shop.cgi?browse=airframe&product=AHRS-mount

https://www.vansaircraft.com/service-information-and-revisions/op-60/
 

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I suspect the beam you have is bouncing a bit so a thin piece of Sheetmetal to stiffen up the beam should help. You could just tape it in initially and see if it makes a difference.

or

A simpler option might be the bottom reinforced if you have room.
 

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Thanks for the replies. I think the first stage would be to stiffen the cross struts. As I recall they are 20mmx20mmx1mm angles so I can thicken them up and perhaps go for something a bit wider or add a brace as suggested. I do suspect a bit of damping could help and I do have some sound insulation rubber so may try placing the unit on that. Someone has also suggested using zip ties to secure it rather than screws. I would like to use the Vans mount but not sure if it would fit my RV6 and the other issue is the wiring extension and new pitot and static lines. Whatever I have a few ideas to try.
 
Is there any reason not to remount the unit to the far right or left of the bracket as close to the longeron as possible. The high frequency vibration that is throughout the rest of the airframe would be there, but lower frequency that actually has some relative motion would not be as much. It would be an easy experiment to slide it to the side.
 
I fixed mine.

I had the same issue very rarely but I did have it when operating from a bumpy grass runway. What I did was added a thin sheet, like 20 thousands or so, from the top stiffener on the fastback down to the shelf. It's like a foot long and maybe 3 inches or so wide. I riveted it to the top skin stiffener right in the middle of the shelf, down to the shelf with a little 90 bent in the end to catch the shelf. I riveted it to the two angles making up the shelf. Problem solved. I have one ADHARS on each side of it.
 
Just a follow up after asking for assistance. I increased the size of the two rails going across the fuselage to 25 mm x 32mm x 1.6mm. I also rivited some struts between the two beams. I used the original screws to attach to the longerons. So far it seems the vibration issue has now disappeared.
Thanks for the help.
 
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