Chapter 14 - Section 4

Install the Forward Face


Micro In CS4

I prepared the CS4 pieces per plan with minor repairs. I also used a hot glue gun and tagged a few spots along CS2 & the back wall (part D) so that it stayed straight and flush while CS4 cures. 


Cassandra was home during Christmas break and wanted to lend a helping hand. So Susann and Cassandra put on their space suits, sleeve protectors, nitrile & vinyl gloves and dove right in. As for me, I decided to take it easy and took some pictures...not! With all that help, we microed the CS4 in place and weighed them down to cure in less than 2 hours -- DONE!


Notice the thermometer showed 62 degrees? We ended up building a small heat tent (draping plastic over the whole center spar, plus a small electric heater) to help the micro to cure.


Determining the Spar Trough Dimensions

I spent quite a bit of time to figure out the trough dimensions because I wanted to be able to visualize it before I started sanding. Dimensions for the lower trough are pretty straight forward, so is the trailing edge of the upper trough. The top inner trough is a bit complicated especially at the top outboard ends. I searched through the archives and looked at many builders web sites with no clear picture of the inner trough. My eventual interpretation came from P.14-2 Fig 7, P.14-9 Section C-C & D-D and templates from M-20.


Most builders hinted that the floor of the inner step of the upper trough gets lower (relative to WL22) from the center line towards the outboard ends of the spar. My interpretation was the opposite. Here's how I came to my understanding:

First, using the M-20 BL52 and BL67.5 templates, I projected a horizontal line from the top corner (as indicated 'hook over CS4') all the way across the templates. This horizontal line happens to be WL22 (Refer to Section C-C & D-D). Then I projected a vertical line up the inner step to intersect WL22 (I'll call it Point A). Then I measure the distance between the floor of the inner step and Point A. This distance (H) is the height of the inner step from the CS4 surface (i.e.WL22). As it turned out, distance H at BL52 is greater than H at BL67.5 - meaning, the trough is shallower as I move outboard. I repeated the same process, using Section C-C and D-D with the same result.


I also picked up from other builder's sites that the center section spar (per plan) tends to be a bit too thick when matching up to the wings in Chapter 19. General consensus is to remove "a bit" more foam. Only Marc Zeitlin's site mentioned "a bit" as about 1 to 2 mm. I sent a couple private e-mail to Marc and Wayne Hicks on the subject. Marc suggested adding 1mm (.04") to the depth and Wayne recommended to get the 'should be' dimension from the wing root template. I decided to pursue both.  


M-20 and Wing Root Templates

I made the M-20 templates (total of 8) per plan except I added .04" additional depth for the spar cap trough for all templates. Note the large templates to the left - they are the wing root templates per Wayne Hicks recommendations.


Here's how I arrived at my two (2) wing root templates:

The BL31 wing root template is from M-3 and M-24 (wing foam cut template for BL31). I traced out the forward part of the template AND projected forward, the spar cap troughs (both top and bottom). This gives me the total thickness of the wing root that will match up to the trailing edge of our center section spar at BL31. Note the upper and lower spar cap troughs has different depths. I also traced out the WL17.4 on the template (explained later).


The BL67.5 wing root template comes from M-23 wing cut foam template at BL67.5. I traced out the aft part of the template all the way up to the 'shear web' cut line, including the top and bottom corners. That's the thickness of the wing root that will match up to the trailing edge of our spar cap at BL67.5.


Once we know the thickness measurements of the matching wing root, all we need to watch out for (down the road) is to make sure the matching surfaces of our completed center section spar is slightly below or equal to (but not higher) the wing root surfaces.         


[Hindsight] I don't know about other builders, but when I tried to fit my template against the lower spar cap trough after I sanded it down, I found out that the width of the template could not fit over the center section spar. Puzzled, I measured the distance between the template inside corners. For example, the width of the inner corners for lower template at M-20 BL0 is 5.3". After I sanded down CS1 Plus CS2 to 5.12", I then added CS4 (.25"), that should give me a total of 5.37". That means the template is pretty close. However, I was 5.5" - almost 0.2" off. Don't know where I messed up. I was not able to find anything in the archives, I decided to keep the center section spar trough at 3" and widen the inner corner edges (mainly at CS4) to get the 5.5" width. Hope it won't come back and bite know where!      


Marking the Foam Before Sanding

I removed the cured spar off the fixture without much difficulty - the packing tape kept it from sticking to the fixture. I took all the dimensions I gathered (above) and transferred them onto the foam to guide the subsequent sanding effort. I also marked out the BL17.4 line along the entire length of the spar. Then I butted the wing root template against this WL17.4 at BL31 (shown left). Note the thickness of the wing root barely protrudes above (and below) the total height of the center section spar. If you look closer, at the top, there is a double line drawn on foam surface - the upper line is the plan depth at BL31, which is .53". The lower line indicates the new depth - recall I added .04" depth making the new depth of 0.57".


[Hindsight] This turned out to be the most used templates during the spar cap glassing step - Thanks Wayne! 


Now, per Chapter 14, Page 3, Fig. 11, the approx. thickness of the spar cap at BL0 should be about .584". Subtracting a couple of layers at BL31, the thickness is estimated to be .534" thick (.584" - minus a couple layers(.025*2)). With my new trough depth of .57", I should have .036" + the little bit of space between the wing root template that is protruding above the top of the spar. 





Similarly, at the bottom, my new trough depth at BL31 is set for .42 and the spar cap thickness is estimated to be .378 (.424 - .023*2) leaving .042 + the little bit of space between the wing root template that is protruding above the top of the spar.


I think there is a good correlation between the wing root template and the spar cap trough for the center section spar. Actually, they look kind of close, so I decided to remove .06" of foam to make sure I got a bit more play room. 


[Hindsight] I may not have to remove extra foam after all because after I was done with the spar cap lay-up, I ended up .06" too low. As a result, I add 4 more spar cap layers to fill up the trough. (refer to step 6).  




Here's another picture of my marking on the foam at BL67.5. The upper marking is the trough depth at the inner step and the lower marking is the trailing edge trough depth. As other fellow builders, I also made a 90 degree sanding block for the occasion.










Carving Out the Foam

My plan was to carve out the foam down to the inner depth over the entire spar FIRST. Then I will carve the slope from the inner step to the trailing depth in the SECOND step.


I used a Rotozip and a 1/2" diameter sanding disc to remove the 1/4" foam and micro. The sanding disc generated so much heat causing the disc to delaminate. Leaving a few good size gouges on my blue foam. Since then, I slowed down a bit and was able to remove the rest of the edges without incident.


Removing the blue foam with the angled sanding block was easy. Since it was so easy, I got a bit careless and put another big gouge in the blue foam ...again, this time, on the opposite side!!! Once I finished the FIRST step, the SECOND step (carving the curvature) was quite simple - its almost anti-climatic. I planned, planned, measured and measured - carving the curvature was all over in 10 minutes.


Here's a picture after I finished carving the foam with the templates along the designated BL locations. Note that there is a small gap between the foam and the lower trough template. It's because I took .06" more foam but did not correct the templates for it. I did correct the templates for the upper trough, that's why they fit better.









Cutting the 1/2" x 1" Angle

I delayed this step until Step 7 because I was concerned that the shaped corners might get dinged up during the spar cap lay-up steps - I am glad I did. 


After I completed the spar cap lay-ups, I marked out the 1/2" x 1" dimension with a fine Sharpie along the entire length of the spar. Between a jig saw and sanding stick I was able to complete this task in a reasonably short time.