Fitting a Firewall

So we have finally far enough to try fitting the firewall to the front.  I had already clecoed the firewall together (not shown), so fitting it up is pretty straight forward.

I began by clamping the firewall in place with wood blocks an C-clamps.  It took a lot of adjusting to get it straight.

Here is the firewall clecoed in position.  If I wasn’t working in a crowded shop, I would probably have to make airplane noises at this point!

Here is a closer view of the c-clamps with the wood blocks – no clecos yet.

Now we have clecos!  Note the board across the frame.  This was used to ensure the fuselage is straight, because I still am missing some bracing in the front of the nose, and the whole thing can be twisted pretty easily.

Anyhow, this is a boat makers trick.  I used the board on the front of the fuselage, and clocked it against the horizontal stabilizer (on a boat, you use two boards) to ensure there is no twist.  Note the prop blocks under the right corner, which were used to ensure very near alignment.  Call it a very primitive jig.   At this point, I have managed to push all the existing holes on the motor mounts into the firewall and re-usable framing and clecoed them in place.

Here you can see the angle brace on the left, and it is already drilled – as it is being re-used.  On the far side, I am more or less duplicating the pattern, using a rivet fan.  The fan won’t fit into the space in the normal manner, as the structure would get in the way – but it isn’t a problem.  I’m just skipping every other rivet spot on the fan, and that makes it all fit.

Here’s one of the new undrilled weldaments.  At this point, it is pretty well set all except on the bottom.  That will come after I remover the firewall.

Speaking of Wedaments, lets re-visit the ones that I need to replace on the top.

Here is the damaged one removed, and you can see the weld starting to crack.  I’m definitely happy to be replacing this.  All of the holes on this one have been duplicated in the structure and firewall, so it is pretty easy to fit the new one in its place.

And here is the new one next to the old one.  Note how much more heavy duty the new one is.  Its made of three pieces instead of two, and the plate is about doubly thick by comparison.

New motor mount going in.  I used C-clamps to pull it tightly into the corner of the longeron.  Then on to drilling and riveting to the longeron, followed up with drilling and clecoing to the firewall.  At this point, I am starting to run low on clecos.

Holes pushed through the firewall and structure.  Other side is similar.

While I’m at it, lets get that left front skin done.  This is pretty easy, as the old one straightens well enough to make a good template.  All the good quality holes can be drilled in to the new skin, and any that is wallered out will be drilled into the skin from the inside after the skin is clecoed onto the aircraft.

Cutting on the lines around the edges of the skin.  I actually snapped this picture to illustrate why I like offset snips – note that I can cut in a straight line without slicing my hand on the freshly cut edges.  Also, look closely at this photo, as you can see that I can only count to 4 on this hand.

Here is the unadjusted skin, clecoed onto the front.  I’ll play with it a bit to get it to fit smoother.  Note that there are no holes on the front where this would rivet to the firewall.  There is no reason to match any rivets where the other parts don’t fit against pre-drilled parts.  I’ll drill both sides of this seam at the same time.

Also – a brief rant.  Note that half of the plastic is missing on the side.  Wicks aircraft sold me this skin, and I didn’t notice it was missing almost half the plastic on one side until I got it home.  Bleh!

RV4 – Out with the Old, In with the New

Its been quite a while since I have made an update, and I have a long way to go on this post.  While going over the aircraft, I noticed a small skin ripple on the right side near the back of the passenger seat.  I also had a pretty good ripple in the same skin forward of this portion.  So, I determined that that skin had to come off.

I stripped the aircraft fuselage, and got it set up on a table in the sheet metal area so I could begin serious work.  the first order of business was setting it on its side and drilling off  the belly – which extends all the way to the rear of the passenger area.

I’m finally starting to get a good enough idea as to what needs done to start ordering parts.  I plan to splice the formers in F404 – the spar mount area – simply because there is so much drilling and realigning that would have to be done to replace them.  (NOTE  – I changed my mind on this later – but I get ahead of myself.)  Speaking of parts, I was able to find a used engine mount and landing gear legs for $250 plus shipping.  They are shown below.  They will need stripped and a couple of hairline cracks re-welded.  This will call for a trip to my welder – cause I don’t do airplane structural welding.

After drilling off the belly, my first order of business was to build the new one.  That way, I could cleco it on to test fit and keep the project aligned.

It is 0.040 2024-T3, and I got a 4×6 sheet in person from Wick’s aircraft.  It was cheaper than shipping it in from anywhere else.  I was able to straighten up the old belly good enough to use it as a template, and the whole project thing came together in only a few hours and clecoed into place beautifully.  Any areas where it was too damaged to locate any parts were intentionally left large.  It’s easy to trim a piece shorter.

 

Any footballed holes on the old belly were ignored while drilling the new belly.  Once the new belly was clecoed in place, I was able to get those holes by drilling through the holes in the structure back outward through the belly.  I am very happy with how the belly came out.

Once the belly was re-built, it was time to take it off again for access to the work.  I swear I have had this thing off and on at least a half a dozen times already, and I have a LOT more times to go.

With the belly removed again, I was able to splice the crushed former under the pilot seat.  I used a new former from Vans, and it went together rather well.

Here is the new bulkhead portion with its fresh zinc chromate resting against the old one.  Note the overlap.

I used an L-shaped portion from the unused part of the new bulkhead to double up the back of the flange.  It had to be drilled to fit the new belly, of course.

AC 43-13-1B says that it takes 66 #3 rivets to make this splice.  You can count if you want!

This is why we disassemble after drilling.  Not only are their burs, but even when drilling and clecoing, a LOT of little chips get between the layers.  Not good in a permanently riveted structure.

All the rivets driven – except the ones that go through the bottom of the flange into the belly.  I am pretty happy about this repair – and if the rest of the project goes like this, I’ll be done in no time.  (Ha ha ha.)

After forming the belly, I decided to splice the F404 side pieces (F-404A-L x 2 and F-404A-R x 2).  I formed nice patch plates from annealed material and heat treated them to give me extra strength.

Damage reminder.  Other side similar in scope.

Splice plates being made of annealed aluminum.  You can see the blanks and my home-made flanging dies.

Bending the  lip.

Lip formed.

Four of the plates – one for each of the F-404 side formers.

Bake at 930 F for 30 minutes, then quench in cold water.  Metal goes from 2024-O to 2024-T42.

And the obligatory zinc chromate…

Fitting over the damaged formers.

Picking up existing holes.

Clecoed in place.

Riveted in.

I spent several days of work on this process, but was not happy with the result.  I didn’t feel that the covers could be fit in a structural manner, and I wasn’t happy with the alignment on the right side former.  So I decided to drill them out, and replace them.  Several more days of drilling ensued, than a lot of time waiting for new parts from Vans, which did not have them in stock.  In the meantime, I had also noticed some cracking on the lower left engine mount, so I decided that had to be replaced as well.

While waiting on parts, I decided it was time to go wild on the forward parts of the plane.  I removed everything but the structure I intended to keep.

Parts to be removed

While waiting for the new parts to come in, I built the right rear skin from another sheet of aluminum I purchased in person at Wicks.  It also clecoed into place nicely.

Then I started assembling the bulkheads that go in front of the spar together, carefully measuring each piece and only clecoing, so final assembly could be made into the aircraft and would guarantee fit.

Location of parts on forward bulkheads before removal and transfer.

 

I also used this time to begin assembling the firewall and bringing parts across from the old firewall.  I am really running out of things to do waiting for the bulkheads and weldament on back-order from Vans.

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Good news – I have my parts.  Now it’s time to begin assembling the center of the airplane. Matching existing spacing and holes is critical.  Luckily, I have had several months to plan this stage.

Inserting the new former begins by locating the six spar web 3/16 holes that cross the former.  They are shown bolted into place with short bolts in this view.  This was done by aligning the locating holes in the new bulkheads with the locating holes in the old bulkheads, then pushing the the spar web holes through.  Now the piece can be bolted up as shown, and the rest of the rivet holes and spar holes can be match drilled into the former.

Match drilling as described above.  This technique allowed the rear formers to be done.  The forward formers were done in a similar manner, but since the forward spar carry-through bulkhead was drilled out of the plane, it was easier.

As the picture notes, this is the left front corner of former F404.  Everything has been match drilled and back-riveted in place.  You can see that the locating holes on the two parts don’t quite line up.  I’m not sure if the plans want this to be this way, but I am re-assembling an aircraft, and all my parts have to be how the original was – and this is how it was.  The holes in the lower left are for the trim cable and the rudder cable.  I’ll have to re-chromate the areas around the rivets on the floor support angles.  The only way to remove these rivets was to chew off the shop head with a micro-shave, and it has left some scarring.

While I wait for the Zinc Chromate to dry, I am going to jump across to installing the lower left weldament.

Luckily, I have enough drilled holes to perfectly align the new part.  I began by nestling the weldment into position against the special angle (bottom right of the photo, bottom left of the plane) and clamping it into place.  I was able to drill the existing bolt holes from the bottom of this angle into the weldment.  Then, I was able to push the side #4 rivet holes into the weldment – again from the same angle.  This allowed the end cleco to reach a hole in the semicircular brace, locating it precisely.  The rest of the holes in the semicircular brace could then be drilled into the weldament from the inside.  Now, the diagonal brace could be clecoed into place, and the remaining holes could be drilled into the weldment from the outside.  Everything precise and elegant.  I love this kind of puzzle.

As I go to cleco the frame together, I notice that the top weldament is not well aligned.  I can pull it in place with several clecos, but it appears to be sprung.  I start inspecting it closely, and find that it is, in fact, sprung.   Look closely at the photo above, and you can see how the two welded layers of 4130 steel are no longer lying next to each other – but have started to pull away.  That isn’t good.

Here is a view from inside on the same mount.  You can see the impression of the nut on the inside, and that indicates that there had to be some bending.  Looking much closer than you can see without blowing up the picture, I can see two areas of possible cracking.  And, as we all know, crack kills.  This is a $37 dollar part – so it’s going to be replaced.  That is a LOT of peace of mind for $37.

While I’m at it, I’m just going to replace both uppers.  That gets rid of a lot of potential problems, and it upgrades all the weldaments to the more modern and presumably stouter design.  Whats another eighty bucks compared to dying in a plane crash – or perhaps worse yet – (partially) surviving one?

Jason – helping me rivet in the new F404 side formers on the rear half of F-404.

Working the front half of F-404 into place over the control stick.  Note those beautiful new side formers…

After clecoing in the front half of F-404, it was on to fitting the other forward bulkheads.

Bulkheads setting in place.  They need lots of tiny adjustments to fit perfetly.

View looking forward from F-404.  You can see the missing skin panels and firewall – but all in good time.  First we fit the parts so they can be clecoed in place, then we disassemble, zinc chromate,