Sunday, September 24, 2017

RV-12 Nose Gear Fork Upgrade/Modification

Now that the DOG Aviation RV-12 is out of service for the annual condition inspection felt it would be a good time to change the nose wheel fork to the new WD-1230-1 RV-12 gear fork Van’s Aircraft has changed over to. The reason behind this change stems from a few RV-12 builders experiencing cracking of the nose forks adjacent to the welds and in a couple of cases, a complete collapse of the nose fork. It is felt that extensive flying from sod fields was a major contributing factor to the stress cracks forming.

To Van’s credit, the nose fork was redesigned and made from much thicker materials and the nose wheel mounting hardware was also changed in such a fashion to allow for full torqueing of the wheel axle without pressing on the bearings. (With the old assembly, the wheel nut was tightened until the wheel bearing was compressed creating a drag on the wheel then the nut was backed off 1/4 turn… the new hardware allows the axle nut to be tightened to full torque specs for the hardware without compressing the bearing to the point of placing a drag on the wheel).

Because the DOG Aviation RV-12 will be flown almost exclusively from paved runways, I was not going to change the nose wheel fork to the new model. What tipped the scale in favor of making the switch now was discovering wheel pants (still sitting on the shelves) installed on the old nose wheel fork can be reinstalled on the new nose wheel fork assembly … but it requires drilling six new  mounting holes. So, should I switch to the new nose fork at a later date after installing the wheel pants on the old style fork, it means there will be six vacated holes that will need to be filled, sanded and of course the wheel pants will need repainting. After hearing that, decided to just switch over to the new wheel fork now and that way there won’t be any issues when attaching the wheel pants when I get around to it. So the new nose fork was ordered along with the hardware changes necessary to install the wheel fork and wheel pants.
Photo of all the parts that come with the new wheel fork assembly and the additional hardware for the wheel pant attachment. The bag contains wave washers, new wheel spacers, longer bolts for the tow bar mounting point and a longer axle bolt with lock nut.

The above parts were separated where necessary, edges smoothed then the parts were prepared for primer then sprayed. One major difference that is quickly apparent is how much thicker the material is on the new nose fork which is very evident when looking at how the wheel pants will mount. As can be seen in the photo below, the old nose wheel fork was bent forming a flange that the wheel pants attach onto with nutplates … because the new wheel fork material is so much thicker, aluminum brackets need to be riveted onto the new nose wheel fork using long AN470AD4-9 rivets so there is a mounting surface for the wheel pants. One tip here, if priming, try not to get any primer inside the bushing the axle passes through or the two bushings that are on either side of the wheel because it will need to be removed to allow the axle bolt to pass through the tight fitting bushings … ask me how I know?

Frequent readers of the DOG Aviation Blog know I embrace modifications that make sense to me … so prior to installing the new nose fork, decided to add a grease fitting so the two bushings inside the nose fork can be kept well lubricated. The modification was quite easy ... just drilled a hole in the front of the new nose fork and tapped it for a standard grease fitting thread.
As one can see in this photo, the new WD-1230-1 nose wheel fork for the RV-12 on the left is much beefier than the old nose wheel fork on the right which was thin enough to bend creating a mounting flange for the wheel pants. The new wheel fork is too thick to bend like that, so there are rivet holes along the upper edge what will allow for mounting a bracket onto the side of the wheel fork for later use to mount the wheel pants. Also of note, one can see the grease fitting modification that was added to the nose fork.

Decided to begin the condition inspection by placing the RV-12 on a sawhorse so the new nose wheel fork could be installed. The new nose wheel fork comes with longer AN5 bolts for the toe bar attachment points (all the old washers are reused). Because I plan on installing the wheel pants in the near future, also riveted the mounting brackets in place. With the nose fork now ready to install, the swap was easy … but I did run into a small confusing hiccup by not looking at the new drawings. At first, I incorrectly “assumed” that the two Belleville washers and flat washer that were under the castle nut of the old nose wheel fork would be reused … but the hole in the gear leg for the cotter pin was not visible. I quickly figured out if the flat washer was removed, the hole for the cotter pin would be visible … when all else fails read the instructions. After referring to the new drawing, sure enough, it was discovered the large flat washer is not used with the new nose wheel fork … the castle nut now rides directly on the Belleville washers.

Prior to installing the cotter pin, one thing that needs to be checked is the preload tension … which should be between 18-20 pounds. A piece of wire was placed through the holes for the axle bolt and attached to a digital fish scale. The fish scale is pulled on until the fork swings and the castle nut is progressively tightened to compress the Belleville washers until the nose wheel fork swings at the desired 18-20 pound pull. The assembly ended up around just slightly over 20 foot pounds because that is where the existing hole for the cotter pin aligned with a slot in the castle nut.
The flat washer in my fingers is not used with the new WD-1230-1 RV-12 nose gear fork assembly.. only the two Belleville washers and the castle nut are reused to mount the nose gear fork onto the nose gear leg. The mounting brackets for the wheel pants can be seen here riveted onto the nose fork with AN470AD4-9 rivets.
Completed instillation of the new WD-1230-1 nose gear fork with grease fitting modification on the DOG Aviation RV-12.

With the nose wheel fork upgrades now complete, decided to direct my attention to the main gear legs which were overlooked during the priming session for the finish kit parts. More about that in the next posting.