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Thursday, May 22, 2014

Nose and nose gear - part 7

Attaching the nose frame to the fuselage (28.7 hrs)

The nose gear structure is now one solid piece, and its ruggedness stands in sharp contrast to the somewhat flexible F-22 center post it is supposed to be mounted to. 

Just as I was pondering this very thing, the latest CSA newsletter brought news of a landing incident in which a Cozy landed hard enough to damage F22 (among other things). Unwilling to rebuild it, the seemingly uninjured pilot was now selling his pane as scrap.

CSA's newsletter #114, sad news on page 5.

For those who are not familiar with the Cozy, it’s basically a Long EZ stretched sideways to accommodate 4 people, and shares a lot of Long EZ parts and hardware.

Long EZ (left) and Cozy (right) at Rough River

One can never really make an unbreakable structure (as it can always get smashed harder), but I was approaching a critical juncture in my build, and I needed to make a decision about F-22's constitution before it would get covered up. I could either add some strength to it right now with a minimal weight penalty, or leave it “as is” forever, since it would be much harder, and probably heavier to do it later.

I decided to make a small addition of one ply of Kevlar underneath one of BID, and F-22’s center post did become stiffer, even though still a bit flexible.

To start the mod, I dusted off my enlarged F-22 drawing, and used it to dimension BID and Kevlar properly.

Old tracing of the F-22 bulkhead getting reused

My Kevlar addition would basically resemble a Times New Roman capital T.

Proposed stiffener

Double checking dimensions

One issue I had never faced before, was how to do a vertical layup. I could try doing it as usual, but the epoxy would tend to run down under gravity, and the whole layup might end up curing on the shop floor overnight. 

Enter Cab-O-Sil, a really magical substance as far I am concerned. Cab-O-Sil does not change the properties of the fluid it is mixed in, it just increases the fluid viscosity. 

Aircraft Spruce says: "Cab-O-Sil is a fumed light weight silica thickener used to reduce the flow of epoxies on vertical surfaces, as well as filling pinholes with its smooth texture".

Cab-o-sil is essentially powdered glass and works great at thickening liquids (such as epoxy resin). It is also a good  alternative to a fiberglass gelcoat when mixed with resin, creating a "mud" that has the same properties of commercial gelcoats but at a much lower cost (see Chris using Cab-O-Sil for finishing in Chapter 25).

Special attention must be paid to safety when using Cab-O-Sil as the powder can easily coat your lungs and cause serious health problems.

Cab-O-Sil by the gallon

Having never used it before, I followed my friend Chris’ directions... “add just enough for a soft butter consistency, so that the resin won’t sag”. 

Adding Cab-O-Sil to pure epoxy

Like "soft buddah"

Coating F-22

One thing to keep in mind is that the Cab-O-Sil/epoxy mix will not wet the cloth as well as pure epoxy, so the sequence I used was:

  1. pre-preg the cloth with pure epoxy 
  2. add the Cab-O-Sil/resin mix to the structure
  3. apply the wet cloth to the structure
  4. squeegee/stipple out all air bubbles
  5. remove the plastic backing
  6. peel-ply as necessary
  7. scratch test the cup after cure

F-22 wet with epoxy/Cab-O-Sil

Wet Kevlar/BID sandwich applied to F-22

Plastic backing being removed (peel-ply to be added next).

The epoxy/Cab-O-Sil mix dried up very hard (excellent!)

This sequence worked out very well, and no epoxy was found on the floor the next morning.

F-22 lightly sanded after peel-ply removal

Moving on... 

An issue I discovered and talked about a couple of months ago, was the fact that the nose gear assembly ended up being slightly wider than the structure it was supposed to attach to (F-22 center post).

Nose gear assembly is wider than what it attaches to

Rear view of the problem area

You might recall that my solution to this conundrum was basically a modified flox-corner, with flox also penetrating the two structures perhaps ½" (1.3 cm) (not shown in the next picture).

Now the time had come to execute this plan, so I started by removing foam and micro from F-22 center post, and the rear of both NG-30s...

Removing the foam

Sanding off the micro adjacent to the fiberglass

Perma-Grit flat tool sanding action shot

Foam and micro removed from F-22

Foam and micro removed from both NG-30s

... next I painted pure epoxy in the voids...

Wetting the slots with pure epoxy

Same thing, different part.

... filled the gaps with flox, and mated the parts.

Abundant flox addition

Other half of the flox corner

Putting it all together

At this point, I applied BID tapes to all intersections, except where I couldn’t get to because of the brackets. The aluminum foil helped me lower them in position, and had no spring back whatsoever, unlike my usual plastic sheet which always tends to unroll to a flat state.

Slightly big flox fillets

BID tape pre-preg using foil and plastic

BID tape application

One side done (aluminum foil removed), other side getting the BID.

Mega-BID tape stretching from the left side...

... to the right side.

Mega-BID tape wrapped around the center post

This was the right time to ensure that the nose structure was perfectly aligned with the longitudinal axis. 

Laser beam hitting all the centerline marks

Satisfied that all seemed straight, I peel-plied everything and went to bed.

Blue strap slightly pulling the structure to the right

Center post peel-plied

Same thing, different angle.

These are the same threaded screws Wade gave me to hold the fuselage together in Chapter 6

The following morning, after removing the hardware that was keeping the nose together, the laser remained right on target, and measuring distances from the tip of NG-30s to the corners of F-22 revealed no detectable differences using my decimal scaled tape measure. This meant that the nose alignment was at least within 0.05” (1.3 mm) from the aircraft centerline

Simply amazing!

Incredible accuracy for a fiberglass structure.

Left view

Right view

After scuffing up the surface a little...

Ready for secondary bonding

... I prepared the remainder of the BID tapes...

More BID tapes getting readied

... and finished up the structural connections.

Tying in the top and bottom of the nose structure to F-22

Top, bottom, and lower corner taped.

Right side

Moving on, again... 

Because my alodine was 2 years old, and very weak, I had put off treating the aluminum pieces I had made for the nose structure, as well as those for the landing brake, but my new order was in, and I got on with this task at last.

Note: Does anyone know how to recycle old alodine (DuPont 225S and 226S)?

Parts to be "alodined"

Air drying after the process

The tanned "three musketeers" 

The NG-8s were floxed in position per plans, and other items took their final position as well.

Floxing NG-8

Right side cleaned up

Left NG-8 attached permanently

The actuator spacers got "alodined" as well

Nose to fuselage connection after peel-ply removal

Some clean up necessary. Longer bolts already ordered.

With the nose structure solidly attached to the rest of the fuselage, I concentrated on the nose leg once again.

Nose leg bolted to the nose structure

Fortunately for me, NG-1L fit very well within the two NG-30s, and lined up exactly with the notch in F-22.

Nose leg folded back to check for proper alignment with F-22's bottom notch

After marking a channel on the outer skin of the plane, I carefully cut through it, and carved out the foam to reveal the place the gear leg will retract into while airborne, or parked on the ground.

Highlighting the removal plan

I hate cutting though my work, but it is necessary sometime.

Working upside down while foam and fiberglass falls on your face is not cool. A pouncing Long EZ is!

The nose gear leg seemed to enjoy its new home.

Flush fit

"Aim High"

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