N200LZ: Chapter 14 (part two)

[TMann]

It is interesting to read the latest posts by AirNico & Bob Setzer regarding the way a perceived substandard part can gnaw on you.

 

I was not happy with either the exterior shape or weight of my center section spar. I also was very dissatisfied with the technique to construct it. After several nights of staring at the ceiling, I decided to rebuild it....... from the outside in.

 

After drawing out the 'jig' in AutoCad, it became clear that a mold could be constructed in much the same way as the jig is fabricated by finishing out the last side (that creates the bottom) in 3/4 MDF and lining it with Vacuum Bag film followed by peel ply. The slight twist in the top exterior leg was formed in the mold using plaster and the 1/2 inch x 1 inch wedge was added to simulate the cutouts for the leading top and bottom tapers.

 

I started my layups 8/30 and should have a finished product and weight this week.

 

I started with the outside skin, followed by the top and bottom spar cap. Then I installed the shear web. All of these layups were vacuum bagged.

The 1 inch foam was installed in two stages. I ripped a piece to cover the spar cap area (2 3/4 inch) back buttered with micro and installed and vacuum bagged the part.

 

The other piece of 1 inch foam was ripped to 2 3/4 inches and then the 1/2 inch x 1 inch wedge was cut out and again it was fitted and bagged until the micro cured. After the cure, I used the Fein to cut the lower foam flush with the upper (forward) foam. The offset is due to the thickness of the spar cap.

 

For the infinity retracts there is a mod that comes into play here. The shear web is left exposed (outboard 19 inches) and the foam is tapered in a gentle slope to the shear web. Later in the process a layup of 13 plies of UND will cover the three internal faces (bottom, forward and top). NOTE: This is the technique used as a retrofit vs. the new construction technique. It ends up with a much cleaner finished product and it is sure a lot easier to shape the internal foam while it's open like this.

 

I was trying to install the inside BID layup with 1/8 inch hard points and all the additional associated layups last Friday after work but I could see I was running out of time. In order to make the best use of my remaining time, I made all of the Multi UND and various BID layups on vacuum bag film and stored them in the freezer for the night. Saturday AM, I started the internal layup (layup #2 per plans) and then pulled out my previous nights work, cut to the proper dimensions and fit into place. The layup was in the bag by noon.

 

Sunday I fabricated the 1/4 inch forward face foam. Because I am using the infinity retracts, I incorporated the crush plate (16 plies of BID) that I had previously fabricated into the foam and installed the BID on both sides of the outboard pieces. The inboard piece and the interior bulkheads were fabricated and skinned last night.

 

As long as I had everythig was still open, I fabricated some 1/2 inch PVC Conduit to run along what will be the bottom aft corner from about BL 8.0 and extending about 10 inches outboard of the bulkheads. I made a layup of two plies of BID on a piece of Vacuum Bag film & Peel Ply and installed it over the conduit. It went back into the vacuum bag for the last time to compress the layup over the conduit shape and into the corner. When I remove this part tonight, (and remove the PVC conduit). I'll be cutting this into two pieces to strattle the bulkhead and flox in place tonight.

 

To this point, all layups have been vacuum bagged. I substituted West 703 Carbon Fiber tapes for the standard glass tapes.

 

My target weight is 30 lbs or less. The first attempt came in at 42 lbs. I attribute the weight to lack of experience, the construction technique and the glass roving type tapes.

 

I should have a final product as well as a final weight by Thursday.

[Hennie Engelbrecht]

 

I decided to rebuild it....... from the outside in.

 

TMann

 

How about some photos???

 

Hennie

[TMann]

How about some photos???

 

I'll upload some pics tonight

[TMann]

Pic 1: The end view of the MDF mold. You can see the 1/2 inch x 1 inch wedges that you would normally trim at the end of step 5. There is an extesion on the ends of about an inch. This allows me to run the layps long and then trim to size. The left side extention is canted to reflect the transition from perpendicular to a .32 inch narrower floor. The area is filled with plaster. This is what is accomplished in theory with the spar cap templates.

Pic 2: Top View. The side previously (plans method) used sticks to hold the foam in place. This can be done with three pieces of MDF. The center section on a stock EZ would be 18 inches wide.

Pic 3: The mold is lined with Vacuum Bag film and the topped off with peel ply. An extra piece of 3/4 x 1 1/4 material was added to the top of the mold which the layups overlap. Later I remove them and trim back to the plans dimensions.

Pic 4: Outside skin (first layup) in the vacuum bag.

 

Pic 5: First Spar Cap layup. These are West 703 CF Tapes.

[TMann]

Pic 6: This shows the process through the shear web. At this point I have trimmed most of the shear web to match the edges of the spar cap.

 

Pic 7: This is the Interior foam. The outboard 19 inches slope to the spar cap/ shear web per the mods required for the Infinity Retracts. Not shown here is an additional .5 x .75 foam extension to allow the interior layup to be oversized and then trimmed back to plan dimensions.

 

Pic 8: Interior layups complete. The extra oversized trim is removed and the part is trimmed back to size.

 

Pic 9: The exterior CS4 pieces which have been modified to incorporate the 16 ply BID Crush Plate. This makes for a better part in my opinion.

 

Pic 10: Here it is all trimmed and ready for the bulkheads and CS4 sections.

[Hennie Engelbrecht]

Great explanation

 

Thanks

 

hennie

[TMann]

Here is how I fabricated the conduit for electrical.

 

Pic 1: I cut a piece of 1/2 inch conduit at 4.5 degrees (to match the 9 degree bend in the strake) then taped them to the corner that makes up the bottom/forward portion of the spar. I did a two ply BID layup that was insulated from everything via peel ply and vacuum bag film. I slipped the spar into the vacuum bag to hold the plies tight in the corner around the PVC until they had cured.

 

Pic 2: This shows the finished BID conduit. I cut it to allow for the bulkhead which I installed last night. The BID conduit will be floxed & taped into place tonight. I'm hoping to close out the spar later in the evening so I can get it out of the mold in the next day or two and get a weight.

[TMann]

33 lbs (vs 42 lbs)

That was a hard fought 9 lbs but well worth it. The final shape was much better than before.

[airwrench]

Excellent method of triumph over the "heavy" spar. The weight you saved is only gonna shine when you start your weight/balance. You cast off a bunch...

[TMann]

I still have the outside hardpoint layups which will add another pound or so.

 

Initially I was a little disappointed but after kicking it around, I think it was worth it.

 

Things that saved me weight:

• Everything could be vacuum bagged
• CF spar caps
• More precision

Things that added weight:

• I used the PVC foam which I suspect added some weight over the plans foam.
• The Crush plate and UND plies for the Infinity gear.

[airwrench]

CF spar caps.....the bend-to-yeild charactaristics of these over fiberglass

are different. I am a big fan of a light bird though, and I would personally like to use carbon in a few other areas but......it's a precious material to experiment with,

[TMann]

it's a precious material to experiment with,

I'm using the Berkut layup schedule as outlined in the plans for CS Spar, Wing Spar & Canard Spar as well as the CF layup schedule for the wing skins as well as the Berkut templates.

 

I'm not really cutting new territory here.