Tuesday, July 1, 2014

ADAHRS Stiffener Modification

As mentioned last week ,Van’s has finally begun shipping the long awaited for ADAHRS stiffener modification. The modification was developed because some builders were getting some jittery screens on the SkyView and in particular the “ball” on the slip indicator. The issue was isolated to the tail cone skin being quite flexible in the area of the mounting brackets for the ADAHRS unit. Apparently, prop wash is causing that area to flex and vibrate a bit … which is affecting the quite sensitive magnetometer within the ADAHRS unit. The Van’s fix is to add a small stiffener bulkhead just forward of the ADAHRS mounting brackets to remove the flexing of the tail cone’s skin in that area.

The part number for the kit is 12SVAHRS Rib Kit. The kit consists of one F-00062 stiffener bulkhead, 13 rivets, and an instruction sheet to go into the construction manual.
ADAHRS stiffener kit - basically it is only one stiffener bulkhead which is riveted onto the top skin on the tail cone.

Instillation of the kit is simple if the stiffener bulkhead is installed prior to mating the tail cone onto the fuselage. Those who already have the tail cone mated to the fuselage will have more of a challenge installing the stiffener. Because the tail cone has already been constructed, one rivet had to be removed so the F-00062 stiffener bulkhead could be attached to the tail cone’s upper skin. After removing the rivet and securing the stiffener to the tail cone’s upper skin with a Cleco, the remaining rivet holes in the stiffener were used to match drill the tail cones upper skin.

I asked Mike to come over and give me a hand which made things easier. Because the stiffener is quite flexible, I clamped a piece of 90 degree bent aluminum behind the web of the stiffener to keep it straight. After the center Cleco was installed a measurement was made from the center of the stiffener to the forward edge of the tail cone skin. With Mike’s help taking measurements, we worked out from the center keeping the rib in position while match drilling the rivet holes in the rib into the tail cone’s upper skin.
Match drilling the F-00062 ADAHRS stiffener bulkhead into the tail cone’s upper skin.
The F-00062 drilled and secured in position with Clecos. For what it is worth, the stiffener removed all flex in the area of the ADAHRS mounting brackets … so it looks like job well done Van’s.

After the match drilling was completed, the ADAHRS stiffener bulkhead was removed so all the holes could be deburred. Those building their RV-12’s as E-LSA are ready to prime and rivet the stiffener bulkhead onto the tail cone at this point.  However, because the DOG Aviation RV-12 is being built with flush rivets, one additional step is required … dimpling the newly drilled holes in the tail cone along with dimpling the rivet holes on the flanges of the F-00062 stiffener bulkhead for flush rivets. For dimpling the holes in the tail cone’s upper skin, the 120 degree dimple dies that have a hole in them for a nail were used with a hand rivet puller to hand squeeze the dimples into the upper skin. The stiffener bulkhead was dimpled with the pneumatic squeezer outfitted with 120 degree dimple dies.
Mike holding the nail and female die in position as I pull on the nail with a hand rivet puller which compress the male die into the female die to create the dimple.
Dimpling the rivet holes in the F-00062 ADAHRS stiffener bulkhead for 120 degree flush rivets.

Now that the rivet holes in the flanges of the ADAHRS stiffener are dimpled, the stiffener is ready to be included in the upcoming primer session and will get added to the pile of parts. The last few days have been spent deburring the pile of parts that were retrieved from the hangar. All the parts are deburred now, but a few require some dimples where nut plates will attach … so will try to do get all that out of the way prior to priming.