Main Gear Cover

Landing Gear Cover

(Accomplished differently than plans.)

The Problems

Step 4 innocently discusses how to build the landing gear cover.  But as I see it, there are three major problems with the plans process.  First, the joggles, which are carved in Chapter 7 before glassing the bottom and sides of the fuselage, seem to cause more problems than they solve.  Not only do you have to accurately carve the foam, you must also cut away the edges of the landing gear bulkheads themselves.  Builders usually wind up gouging out the foam adjacent to the bulkheads.  Second, everyone has trouble getting the glass to conform and stay attached to the joggles.  The FAQs and email archives are chock full of Rube Goldberg methods to forcibly hold the glass down during cure to avoid air bubbles.  Third, the aluminum slugs and the installation method in to the landing gear bulkheads leaves lot to be desired.  The aluminum slugs are thin and there's no room for error. Builders have reported drilling the holes AND MISSING the slugs altogether!  The threads in the slugs are easily damaged, and should you ever strip one of the attachment screws, you'll be tearing into your bulkheads to fix/replace the slugs.


My Process

My cover is built to be between the gear bulkheads.  The cover is attached to joggles within the bulkheads and is secured in place with screws and nutplates.  It doesn't require any joggles to be carved into the fuselage bottom, and it doesn't require any aluminum slugs.  If you strip a screw or nutplate, you simply replace the nutplate.  Replacing a nutplate is far simpler than replacing the aluminum slug. 

Now, a warning.  Warning!  The following procedure is far more difficult to explain and illustrate than it actually is to do.  The process is fairly easy and straight-forward!  It just LOOKS and SOUNDS complicated!

Step 1 -- The process starts with the fuselage upside down.  Take a foam block, cut it to the width of the landing gear well, and stuff it in there.  It is simpler if you can use one large block, but several smaller pieces will work just as well.  Once it is in place, sand the foam block flush with the fuselage exterior surface, NACA scoop walls, and NACA scoop ramp.  For the NACA scoop, make sure you leave a radius where the walls and the ramp (floor) intersect.  The radius should match the existing radii of the forward and aft sections of the scoop.  Vacuum the foam thoroughly, then apply strips of box sealant tape overlapping the foam and fiberglass.  The tape should overlap one inch onto the fore and aft edges of the foam.  You will later peel up this tape so that you can form a glass to glass edge between the exterior skin and the interior skin.


Step 2 -- Glass the external surfaces with 2-BID.  Start with the NACA scoop first.  Slurry the exposed foam for the NACA scoop sides and ramp.  Glass the NACA foam with 2-BID.  The BID should overlap just a tad onto the box sealant tape protecting the fuselage so you have material to trim away after cure.  Once cured, sand the walls flush with the top of the foam.  Make a flox corner on each wall, slurry the remaining foam, then apply 2-BID from the NACA scoop to the leg openings.  Let cure. 

Step 3 -- Once cured, reach under the fuselage and very carefully push the foam block out of the landing gear well.  It helps to first run a hacksaw blade between the BID and the release tape to unstick it.  Once free, trim the forward and aft edges of the external skin flush with the foam block.  Bondo two boards over the cover from left to right.  The boards will hold the shape of the cover and will keep it from warping when the interior skin cures.

Step 4 -- Carve the foam as shown in the "aft view".  All you're really doing is carving away 99% of the foam and leaving about 3/8th of an inch to ensure the cover is rigid.  At this point, I placed the main gear bow back into the gear well so I could trial fit the cover to the gear before committing to glassing the inside of the cover.  I found that I needed to carve a depression into the foam to clear the gear bow.  The M-drawing will give you a good idea of how much clearance is needed initially, but it's always good to check against the real thing.  While I was at it, I also trimmed the leg openings to match the main gear legs.  Once the foam was shaped, I cut away one inch of foam on the fore and aft edges and peeled up the box sealant tape applied in step 1.  I also removed one inch of foam at the gear leg openings.  This is done to provide for glass the glass edges.  I then rounded off the edges of the foam to make a good transition from the foam onto the fiberglass.  I slurried the foam, then glassed the inside of the cover with 2-BID.  I applied two additional plies of BID (one inch wide) along the fore and aft edges of the cover to build up some thickness.  The thickness is needed later to allow for countersink holes for the screws.  Once the cover is cured, trim the edges and recheck the fit of the cover to the fuselage and the main gear bow.  Remove the main gear bow and set it off to the side.


Step 5 --  Prep sand the interior faces of the forward and aft landing gear bulkheads to accept the BID joggles.  You'll need to prep-sand a swatch of about 2 inches from the edges.  Apply strips of release tape onto the inside fore and aft edges of the landing gear cover.   Bondo a few mixing sticks onto the outside of the cover.  Place the cover into position over the gear well and bondo the other half of the mixing sticks to the outside of the fuselage.  MAKE SURE the cover is FLUSH with the fuselage bottom and the NACA scoop. 

Flip the fuselage right-side up.  Make up a batch of 2-BID tapes that are about 2 inches wide.  What you'll be doing is applying the 2-BID tapes between the landing gear bulkhead and the gear cover so that 1 inch overlaps onto the inside of the gear cover and the rest overlaps onto the inside face of the bulkheads.  This is fairly easy to do when the joggle is straight, like from the leg openings to the start of the NACA scoop.  But the BID tapes will not want to wrap 90 degrees AND bend around the four corners of the NACA scoop AT THE SAME TIME! 

So what you must do is apply the BID tapes in short lengths.  Measure from the leg opening to the start of the NACA scoop (the corner where the fuselage bottom and the side of the NACA scoop meet).  Add one inch and cut off a segment of BID tape to that length.  Fold the tape and apply it to the cover and joggle.  I make BID tapes by wetting out the BID on plastic or saran wrap, covering with a top sheet of plastic or saran wrap, then cutting the tapes to width and length.  To facilitate handling, I only remove one sheet of plastic.  Using a brush, I stipple the BID tape in place, starting at the leg opening and progressing toward the NACA scoop.  As the BID tape sticks into place, I then and only then begin removing the second sheet of plastic.  When you get to that first corner, the BID tape will distort as it gets pressed into the corner.  That is okay for now.  Believe me, no one will see it.  Now measure the length of the NACA side.  Add TWO inches and cut that length of BID tape.  Press the first inch into the corner you just did, overlapping the BID tape you just applied.  Press the rest of the BID tape into place, then try your best to form the remaining inch around the second NACA corner, the one formed between the NACA side and the ramp.  This is the basic procedure followed along all lengths and all corners.   Believe me!  This is harder to explain than it is to do.  It's quite intuitive once you see what's going on.  The truth be told, the overlap is only necessary on the first 2 plies.  After that, you can simply butt the tapes together at the corners.

You keep making BID tapes and applying them until the joggles are thick enough for nutplates and rivets.  I found three passes (6 plies) to be more than adequate.  Four plies would have probably worked just as well.

Step 6 -- Once the joggles are cured, it's time to drill the holes for the attachment screws and nutplates.  First, remove the cover and trim the joggles to be one inch wide as measured from the face of the bulkhead.  Trial-fit a nutplate under the joggle.  Try to fit it such that it is in the middle of the available space between the vertical part of the joggle (6 plies can get pretty thick!) and the end of the joggle.  Measure this distance from the bulkhead.  For me, that distance was approximately 5/8th of an inch.  Replace the cover, space out the location of your holes, and then drill through the cover and through the joggles.  Once the holes are drilled, remove the cover and install the nutplates.  Go back and countersink the holes in the cover.  I've attempted to show these details in this figure.


Step 7 -- There's one last detail to discuss, but it's very important.  As it turns out, the joggles make the gear well a little too narrow.  Thus you will need to notch out small sections of the joggles for the landing gear tabs to slip through.  This is very easily accommodated.  You can hopefully see this in the pictures below.  The notches are directly above the tabs. 


Again, I found it more difficult to explain and illustrate this process than actually doing it.  For me, it seemed straight-forward and much easier than the plans method.  And it doesn't have all the pratfalls associated with the plans method.  I once asked Nat why he didn't do it this way.  He said that he figured it would be too hard for the average builder to do.  Plus, he couldn't figure out the clearance issues with the landing gear tabs.