Tuesday, May 14, 2019

Installing Main Gear Wheel Pants On The DOG Aviation RV-12

Van’s Aircraft offers a wheel pants kit for the RV-12 which was purchased long ago … but not installed prior to the first flight. Last Fall, decided it was necessary to get serious about preparing the wheel pants so they would be completed prior to sending the RV-12 to the paint shop … which should be any day now. So work was performed on and off throughout the Fall and Winter months and finished up last week. I truly underestimated the amount of work/time necessary to properly fit the nose wheel and main gear fairings (work on the nose wheel fairing will be covered in a separate post). Nothing is really hard, but if one wants to have nice tight fitting joints it takes a lot of cycles of fitting the fairing halves together, taking stock of the fit, remove, sand, reassemble the halves, retest the fit … repeat.

Van’s assembly plans begin by instructing the builder to begin working on the main gear pants first. The fiberglass components for each of the main wheel fairings are comprised of front and aft sections that are molded larger than the final size. Work begins by cutting the excess fiberglass on each of the U-00001A front and U-00001B aft fairings close to scribe lines marked on the fiberglass then hand sanding or filing down to the scribe lines. The long Dura-Block AF4406 sanding block that uses stick-on sandpaper really helps create straight flat edges for a nice fit.
The Dura-Block AF4406 sanding block was very helpful for creating long straight edges on the wheel pants.

Once filed to the scribe lines and a nice fit is established think you are done? NOPE, the fun is just beginning. At this point the wheel pant fairing halves are taped together and alignment marks are placed on the tape to insure proper alignment when the fairing halves are mated together after sanding.  Once the fairing halves are temporarily taped together, a measurement is made by comparing the position of eight dimples pre-molded into the  sides of the fairing halves with mounting holes on the U-00002 wheel fairing brackets. This measurement is accomplished by placing a light source inside the assembled fairing halves and comparing the spacing of the dimples in the fiberglass to the mounting screw holes in the U-00002 wheel fairing brackets.
The main gear U-00001A&B fairing halves are taped together and a light source is placed inside the fairing halves.
Getting ready to make the initial alignment check. The U-00002 wheel fairing bracket in my hand has four mounting holes along the outer edges. The U-00002 wheel fairing bracket is held inside the fairings and the light will shine through the mounting holes in the bracket onto the dimples in the fairings. If the spacing is to long, you have a lot more sanding to do … which I did.

I found placing a red dot in the center of the dimples aided nicely when trying to check alignment with the holes in the U-00002 wheel fairing brackets. In my case, the spacing was to long, so quite a bit of sanding was required on the both the U-00001A front wheel fairings to make them shorter so the dimples aligned with the holes in the U-00002 bracket. Oh, forgot to mention, it is also  necessary to make the same measurement on the other side of the fairing … so care must be taken when sanding to make sure while shorting up one side of the U-00001A front wheel fairing you are not creating a gap on the other side. Patience is a virtue, because it requires quite a few cycles of fit, measure, remove, sand, reassemble and measure again.
Looking very closely at this photo, one can see the four red dots I placed in the center of the dimples. There are two dots on the front U-00001A fairing and two dots on the aft U-00001B fairing slightly below the center line of the photo. The U-00002 bracket is held inside the fairing and material is gradually removed from the front fairing until the red dots are centered with the existing holes in the U-00002 bracket shown in the previous photo. There are also four dimples on the other side of the wheel pants that requite attention as well.

After the U-00001A front wheel fairings have been trimmed down so the spacing between the dimples in the U-00001A front fairing and the U-00001B aft fairing match the holes in the U-00002 wheel fairing bracket, the fairing halves are taped together again and drilled. The aft edge of the U-00001A front fairing has nine dimples. The dimples will be drilled to #40 beginning at the top and alternating from side to side inserting Clecos in the holes to secure the fairing halves as the remaining holes are drilled. At this time, the dimples in the sides of the front and aft fairing halves that were used to verify spacing with the holes in the U-00002 wheel fairing bracket are drilled to #30. This will allow the fairing halves to be Clecoed to the U-00002 wheel fairing brackets later during initial test fitting.

Of course, when you finish up with the first main gear fairing pair, the fun continues! …. You have another pair to mate for the other main gear wheel.

Due to the cold winter weather at the time, I deviated from the plans a bit and took a gamble and just went ahead and final drilled the #40 holes to #27 so I could work on installing the nutplates. Normally, this step would wait until after an epoxy resin/flox mixture is applied to the inside of the fairing halves to create a bed of flox for the U-00002 wheel fairing bracket to set in. It was way too cold at the hangar for epoxy resin to cure, so I skipped over the flox step and installed the nutplates.

After final drilling #27 all the #40 holes in the fairing halves, work began on installing the nutplates. A screw was used to hold the nutplates in position so the rivet holes could be drilled from the inside out. Next the rivet holes were machine countersunk. As can be seen in the photo below, the joggle on the U-00001B aft fairing prevents a countersink cage from being used so it was necessary to remove the countersink cage and carefully use the countersink bit without the aid of the cage. Also of note, if one looks closely, while using the countersink bit the nutplates were left in place …. doing this prevents the hole from being elongated because the tip of the countersink bit is being held by the rivet hole in the nutplate.
Countersinking the rivet holes that will be used to rivet the K-1000-06 nutplates onto the U-00001B aft wheel fairing. The countersink bit is being used without the typical countersink cage because the joggle in the fiberglass prevents the countersink cage from sitting flat. Also note, the nutplates are left in place during the countersinking process so the nutplate captures the tip of the countersink bit preventing it from drifting and subsequently elongating the rivet holes.

Before the fairings can be test fit on the airplane the U-00001A front and U-00001B aft fairings require a cutout for the landing gear leg. The day this work was performed I forgot my camera so can’t show the markings used to cut the fairing halves but below is a copy of the drawing so the reader can see what is involved. Van’s measurements shown in the drawing are just about right and only required minor filing/sanding to get the fairings to fit over the gear legs.
Here is a portion of the instructions showing how to create the slots necessary to allow the forward and aft fairings to slip over the gear legs.

Because the custom braided brake lines from Aircraft Specialty that are installed on the DOG Aviation RV-12 are a larger diameter, the lower aft portion of slot in the aft fairing needed to be made larger to accommodate the larger diameter brake lines. While doing this write-up I just realized a photo was never taken of the finished cutout which is unfortunate because they turned out perfect.
Trial fit of the U-00001B aft wheel fairing on the right landing gear. Note the #30 Clecos are used for the initial trial fit. The mounting holes will be enlarged later to #19 just prior to the step involving mixing up a batch of epoxy resin and flox.
Trial fit of both the U-00001A and U-00001B front and aft wheel fairings on the right gear leg. Truly ended up being a nice fit as one can see from the photo.
Trial fit of both the U-00001A and U-00001B front and aft wheel fairings on the left gear leg. Another nice fit.

Even though both the left and right wheel pants are now in place, there is still a lot of fiberglass removal required. Looking closely at the two previous photos, one can see a small metal tab with a hole in it laying on the hangar floor. It is necessary to remove the excess fiberglass material from around the tire to create approximately a half inch of clearance between the tire and the fiberglass all the way around the tire. That small metal tab is ½”wide and was used as a measuring gauge to remove the excess fiberglass in order to obtain the necessary ½” clearance all around the tire. Both the wheel fairings for the left and right landing gear required quite a bit of trimming … so there was a lot of on and off cycles necessary to obtain the ½” clearance. Once the necessary clearance was obtained, each wheel pant was mated off the aircraft and the inside edges of the opening for the tire were tweaked so the fiberglass opening was even from front to back.

As previously mentioned, the step involving setting the fairings in a thin bed of epoxy resin and flox was skipped over due to cold temperatures when that step should have been performed. Recently we had a day where the high was 81 and the nighttime low was forecasted to be in the high 50’s, so decided it would be the perfect day to perform the step requiring the flox mixture.

Preparation for the flox step involves placing tape over the U-00002 wheel fairing bracket in the areas that will contact the flox and placing wax over the tape. I elected not to use masking tape and used clear packaging tape covered in a very thin film of Vaseline as the release agent rather than using wax. I have had good luck in the past using packaging tape and a VERY thin film of Vaseline and know it works. Prior to mixing up the flox, have everything ready to go by first making sure the mounting screws are covered with wax or Vaseline, the area inside all the wheel pants where the flox mixture will be applied are scuffed up really well, all the U-00002 brackets are taped and coated with wax or Vaseline, a drill with a #19 drill bit is handy and a screwdriver is on hand.
Looking closely at this slightly out of focus photo, one can see clear packaging tape has been placed over the forward portion (right side) of the U-00002 bracket and folded over the edges. The aft portion (left side) has the packaging tape on the U-00002 bracket but it is not folded over yet so it shows up better in the photo. After covering the brackets with the tape, a VERY thin film of Vaseline was applied onto the packaging tape so the flox mixture won’t stick to the bracket.

Next a small batch of epoxy resin and cotton flox is mixed to the consistency of peanut butter and spread on the inside of the U-00001B aft fairing about 1/16” thick around the general area where the two U-00002 brackets will contact the aft fairing. The fairing is then loosely set in place and secured with screws … HOWEVER, the screws are NOT tightened tight … leave about 1/16” between the head of the screw and the fairing so the flox mixture is not squeezed out. Once all four mounting screws are securing the aft fairing, the U-00001A front fairing receives the flox mixture and is mounted with screws … here again, making sure not to tighten the screws all the way. At this point, the screws are removed one at a time and a #19 drill bit is used to fish out excess epoxy resin from the screw hole, return the screw and tighten enough to obtain the 1/16”gap. After all the screw holes have been cleaned, leave the fairing alone until the epoxy cures. ( Note: Here is where Van’s would have the builder drill #27 the holes that attach the front and aft sections of the fairing halves. As mentioned previously, I already performed this step when I skipped ahead due to the cold weather). This process was repeated for the other gear leg.
As can be seen in this photo the idea is to let the flox create a bed for the U-00002 bracket to set in.

Now for full disclosure …  not all of the flox areas turned out as well as pictured above. There were a couple of places where the process of blindly sliding the fairings in place scraped some of the epoxy resin/flox mixture away. It is very difficult installing the fairings evenly when you are working alone. Not to worry, wait for the epoxy/flox to cure, clean and sand the bad spot again keeping as much as possible of the hardened flox that is in the correct area … then reapply the flox. Carefully keep the fairing flexed while installing. Correct the bad spots one at a time and all will be well.

At this point the main gear fairings are complete and ready for paint. But wait! There is more! See the next post.