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rainier wrote: ↑Tue Jan 19, 2021 7:06 am
These kinds of issues are typically caused by a sloppy mount of the AHRS. It is allowed to move relative to the airframe. This typically affects the device on take-off (acceleration and bumps from runway) or during turbulence.
These kinds of movements can dramatically exceed the measurement range of the gyros - this directly results in the AHRS loosing track of your attitude. During take off this tends to affect pitch mostly but bank can also be affected during normal flight.
Other favorites causing similar issues are shelves holding the AHRS or mounting the AHRS on the actual skin of the aircraft - like under a seat etc (this tends to beat like a drum from the prop wash and can severely affect the AHRS).
The very best mounts for an AHRS are typically on a sturdy airframe member like a wing spar box and mounted using velcro tape - this works extremely well filtering out high frequency vibrations while being firm enough to prevent the AHRS from moving independent of the airframe.
Having some similar issues with my SP-7... want to troubleshoot this - so, to make sure I'm clear.... do you glue the velcro tape to the mount point and then strap the AHRS in? any role for some damping material (I'm thinking something like blanket material between the AHRS and the velcro straps?
Was considering upgrading my -7 to the SP-9 but it may well be a simple mounting issue....
Yes - I tend to use just two strips of self adhesive velcro to mount SP7 or SP9.
Make sure you choose a good surface - if must be solid. Be VERY aware of any shelves (no matter how well made they look), brackets etc. NEVER mount on the skin of the aircraft (that is just like a drum due to prop wash).
In my Sling 2 I get excellent results if mounting under the seat on top of the wing spar box which is a very strong structure (The Sling is a low wing aircraft).
The aim is: The SP-7/9 must never shake independent from the aircraft's frame (no matter how tiny the movement may be - it's bad and will severely degrade the measurement results). On the other hand - the mounting must not be so firm that high frequency vibrations get into the unit - the velcro tends to take care of that nicely.
The SP-9 has ring gyros which are not much affected by linear vibrations. The SP-7 can be affected but the vibrations (or harmonics thereof) must be in the 20-30Khz range to create issues.
Shaking of the platform (i.e. the SP-7/9) is bad - this contains rotational movement that easily exceeds what the gyros can measure so the maths that does the complete attitude update 100 times per second gets fed false information and that can VERY quickly topple the attitude.
Lastly shaking, depending on the exact specifics can also make it difficult to get an accurate estimation of the linear acceleration forces acting on the gyros so any aiding becomes imprecise.
Both the SP-7 and SP-9 are designed with fairly low aiding constants - so they rely on gyro measurements more than normal. This gives them an advantage in sustained maneuvering where acceleration forces make estimates unreliable. However the downside is that they do not tolerate sloppy mounting. Since that is in the builders/installers control however the bottom line is that this is the better way of doing it provided installation is of a high standard. If you ever get the chance, have a look at an avionics bay in an airliner and have a look at the mounting arrangements used for the on board inertial sensors. Despite the airliners having huge mass which helps dampen any sudden movement, great care gets taken here.
With out sensors you don't need to go to this kind of length - but some basics go a long way.
I fixed my SP9 problem by removing the rubber shock mounts and mounting the SP9 on Velcro directly to the shelf then putting a small nylon tie strap for double backup over the SP9 using 2 holes through the mounting shelf.
The mounting shelf was rigid as designed and didn't need any changes.
Only new issue I've seen was on my 3rd flight after fixing the mount after about 45 minutes of doing left hand turns over my airfield (reason for this is another story) a AHRS fault "Attitude Suspect" code started flashing. Attitude did start to wander a little when compared to my G5 backup, so I reset the iEfis attitude and did some right hand turns and error went away? After landing I reset the SP9 gyro bias , haven't flown it again to see if this fixed it, but also don't want to do 45 minutes of left hand turns either!!
hpmicrowave wrote: ↑Sun May 09, 2021 4:32 pm
I fixed my SP9 problem by removing the rubber shock mounts and mounting the SP9 on Velcro directly to the shelf then putting a small nylon tie strap for double backup over the SP9 using 2 holes through the mounting shelf.
The mounting shelf was rigid as designed and didn't need any changes.
Only new issue I've seen was on my 3rd flight after fixing the mount after about 45 minutes of doing left hand turns over my airfield (reason for this is another story) a AHRS fault "Attitude Suspect" code started flashing. Attitude did start to wander a little when compared to my G5 backup, so I reset the iEfis attitude and did some right hand turns and error went away? After landing I reset the SP9 gyro bias , haven't flown it again to see if this fixed it, but also don't want to do 45 minutes of left hand turns either!!
Anyway no more SP9 Gyro crashes on takeoff.
Cecil
Wondering if you fully fixed this issue, having the same issue with mine gonna try the velcro tomorrow on the shelf and see if that fixs it if not i guess i'll be moving it forward somewhere
We have two Voyager EFIS's in our fiberglass Jabiru 430, with nearly 1000 odd hours on it. No aerobatics here and not as nimble as RV's.
Our 4 x Sp's units have all been attached with Velcro since the plane was new. We have owned the plane for the last two years and during this time I have never seen either of my EFIS having any type of AH or Compass issues. So this part of my EFIS system has been rock solid with Velcro mountings.
Our cabin is lined with a felt/wool type material glued straight to the fiberglass skin, it's great for velcroing placards in place around the cabin as well as our 2 x SP-4 and 2 x SP-2 which are the older models without cases.
Small fiberglass shrouds where made for the 2 x SP-4 and Velcroed to this roof with everyday type J hook Velcro. The SP-2 are mount with the same Velcro in a cavity under the front seats.
The Velcro certainly secure the SP-4's shroud to the roof without question, but they are only a few grams in weight.
If you are not sure about the strength of normal Velcro, consider getting T barb Velcro.
Here in Australia, Velcro makes different type of Velcro hook tape, there is the J type barb which is the most common type of Velcro seen everywhere.
Then there is the stuff I use in my business which is Velcro MVA 8, a T barb type, it's pretty rugged stuff and I only use it for more permanent fixtures as it certainly gives you much more holding strength.
When you go to pull this T barb Velcro apart from the self-adhesive woolen tape, you will often pull the woolen tape adhesive away from it's surface before breaking the T barb Velcro joint. So to help stop this, we found using a thin stainless steel ruler between the two Velcro sides with a bit of a sawing motion while pulling apart the Velcroed parts will help leave the self-adhesive tapes in place.