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Old Jul 01, 2009, 03:05 PM
Master of the Wind
G Norsworthy's Avatar
United States, CA, San Jose
Joined Sep 2008
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XC racer #8 in a series

This build log will cover the 8th XC racer in a series built over the last 20 years. It includes influence from the XCBD, Supra, Wiley, and assorted experience collected over 20 years. I will try not to repeat what Anker has shown recently in the XCBD log, only what is different. I will show the entire process through flight. The design is targeted at the west coast races, but would be ideal for the Level V tasks as well.
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Old Jul 02, 2009, 03:08 PM
Master of the Wind
G Norsworthy's Avatar
United States, CA, San Jose
Joined Sep 2008
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Cutting Cores

Templates were printed on paper and cut from plastic laminate. Cores were cut using a Feathercut. For the longer sections a homemade bow with lots of tension gave better results. There was a bit of a learning curve at the beginning, so there is more spackle than I would like, but the next one will be very nice. The wing and stab tips, and LE were hand sanded with cardboard templates.
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Old Jul 03, 2009, 05:22 PM
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Subscribed! I'm interested to see this one come together :-)

Dan
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Old Jul 03, 2009, 10:04 PM
Master of the Wind
G Norsworthy's Avatar
United States, CA, San Jose
Joined Sep 2008
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How much does one of these cost?

Here is a breakdown for enough stuff to build two of these. The only major item lacking is the glass for the wing which was left over from the last one. Two rolls from Thayer runs about $100 and is enough to build at least three, maybe more.

item quantity each total
formica 1 16 16
foam 3 26.66 79.98
resin,tape 96.81
ACP mylar, bags, carbon 1 97.88 97.88
US Comp kevlar, crowfoot 1 139.45 139.45
plywood 1 19.66 19.66
double stick 1 9 9


total
458.78
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Old Jul 03, 2009, 10:28 PM
Master of the Wind
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United States, CA, San Jose
Joined Sep 2008
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True up beds and cores

Extruded foam always has internal stresses that relax when cut. No matter how careful you are in the blocking step, the angles at the transitions will never match. That is why I cut them very slightly over and plan to match them up later. We will be building on the bottom. I cut the templates as tall as possible and went through the top in a few places near the root. Consequently there was more spackle and sand time than I would like but we must press on. The templates will be adjusted down 1/8" before the next cutting session. These templates were drawn with the top point constant and the bottom rising towards the tip as the airfoils get thinner. This adds a slight dihedral effect and looks better to the eye.

Lay out some masking tape in case you don't want to use a sharpie on the landlady's granite countertop and draw the sweep. Use the LE because that is where we lined up the templates when we cut the cores. Sand the bottom beds until they fit over the lines, glue and weigh. Once the beds are finished you can follow with the cores.
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Old Jul 03, 2009, 10:51 PM
Master of the Wind
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United States, CA, San Jose
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Install root and half ribs

I must admit I neglected to account for the short center section and designed in 3/8" Al rods for joiners. In 20 years I've never bent or broken one except in contact with dry lake beds, hillsides, etc but those were all 7-8 ft center sections. It's already in so we go ahead with it but in the future there is room for 1/2" rods.

I drew out the geometry for the (now 1/2") joiner rods plus brass tube to verify that it fits in the space at 9" width. The dimensions are not on the PDF because it will mess up the scaling on the big drawing but you get the idea.

Next I plot the ribs and half ribs on paper with two concentric circles for drilling the hole for the tube, one for the pilot hole and one for the big drill. The paper is stuck on with glue stick and the ribs are cut out with band saw and belt sander. Next the holes are drilled as carefully as possible.
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Old Jul 03, 2009, 11:44 PM
Master of the Wind
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United States, CA, San Jose
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Install root and half ribs cont

Slide the root and half ribs onto brass tube and sand as needed to make sure the bottoms are all even. We will be referencing the bottom for all the glueing and from here on it is more important that the bottoms match than the ribs fit the foam perfectly.



Assign pairs to each side of the poly joint for cutting and fitting. Adjust the holes in the root/half pair so that the tube will fit at an angle. Match the foam at the joint and hand sand the core roots to approximately 4 degrees. The exact amount is not critical because we will sand again after the ribs are bagged on.

Cut slots for the half ribs so that the total distance from end to end is 9" through the joint, including both sides. Use the master drawing to position the ribs correctly. The rod should go through the high point of whatever airfoil is in use. Part of the reason I deviated from my usual straight trailing edge is the rods are much easier to place and install if the high point of the wing is in a straight line from tip to tip. Check the fit of the pieces to make sure the bottom matches the core, and grind off any area away from the holes that sticks out the bottom. Do not cut near the holes as this is where we are referencing.

Glue the root and half ribs with 30 minute epoxy. This gives a little extra time, especially if it is hot. Use a minimum of glue as this is just for placement. Later steps up to and including bagging will fill and anchor permanently. Do this in the bottom beds with the cores weighed down well and the ribs pushed down against the bottom bed so all the bottoms are lined up.
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Old Jul 03, 2009, 11:55 PM
Master of the Wind
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Install tubes and check alignment

Sharpen one of the brass tubes and cut through the foam to the half rib. You can start with a Dremel and a 1/8" drill bit to get halfway first. Cut as needed to get the tube into the hole in the half rib. Go a little past the rib and mark the location where to cut the tube. We want the brass tube all the way through the rib and jammed against the foam so there is no path for laminating resin to get inside when we bag.

Install all the tubes and check the angle of the rod against a flat table with a ruler. If all the bottoms lined up all four sides will be exactly the same. Rough up the tubes and glue in.

Grind down the ribs to match the cores. Open a 1/8" channel to fill with epoxy and microballoons to fill any gaps around the tube and tie the tubes to the skin. See Anker's thread for details.

http://www.rcgroups.com/forums/showt...=building+xcbd
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Old Jul 05, 2009, 03:04 AM
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Went to Greg's place to take a look of this project

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Old Jul 05, 2009, 07:18 PM
Master of the Wind
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Fuse layup

Let's switch gears here and do the fuse layup. There are many ways of going about this, and all make a good product with good technique and experience. This method evolved over a period of about 10 years of off and on possession of the GJ fuse mold.

The fuse layup will be 3 oz glass for the skin and 5 oz kevlar for the inside layer. It will deviate from previous versions in that the scrap kevlar will be cut up to build up the interior where the solid wood cross pieces go for the wing bolts. In previous editions the area was reinforced with 1/8 ply but the curvature of the fuse requires lots of microballoons and clamps, and it never bonds across the whole length anyway. This way will utilize all the kevlar scrap. There are four strips of 3.5 carbon uni in the tail boom to prevent flex.

The first step is to use a paper pattern of the inside to trace cut lines on the glass and kevlar. The glass will overlap the flange by about 1/2", and the kevlar is cut carefully to remain entirely within the flange. If successful we do not have to cut any kevlar at all after the layup.
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Old Jul 05, 2009, 07:57 PM
Master of the Wind
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United States, CA, San Jose
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Fuse layup, cont.

At this point the photographer had to leave so you will have to take my word for it. The next step is to lay in peel ply over the wet cloth. Then a layer of paper towels over the peel ply. Use masking tape if needed to keep things in place. Now slide the whole thing into a tubular bag and seal the ends with clips. Pull down to about 8" of Hg and leave it overnight.

Yes, we are going to join this with some of the 3 oz glass pinched between the flanges. It is not very thick, and it protects the flange. This is the best way to bag the halves and be sure there is coverage all the way out. We will wet sand off the flashing later. Not very elegant but then if you saw how dusty it is in Cal Valley you would see it doesn't matter. The other way to do it is a lap seam. That way works well too but the glass/resin is not as good as a bagged fuse.

Once the peel ply is out and the flange is cleaned up we are ready to seam the halves together.
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Old Jul 05, 2009, 08:03 PM
Master of the Wind
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Forgot to mention the 5 oz kevlar is crowfoot, and is a double layer from the nose to about 5" behind the TE of the wing.
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Old Jul 06, 2009, 01:55 AM
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Went to Greg's workshop again this afternoon where Greg was working putting the two half-fuselage together.
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Old Jul 06, 2009, 01:33 PM
yyz
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USA, CA, Paso Robles
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Nice!

Right on, Greg. Thanks for doing this build thread. I know a lot of people will be watching and learning from this.

Mike
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Old Jul 06, 2009, 01:57 PM
Who needs a pilot??
danstrider's Avatar
Alexandria, VA
Joined Jul 2002
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Can you explain what I'm looking at in the last picture? I can tell it's down the tube of the tail with the vertical fin going up, but what's the flap? Is that the joining tape before it's smoothed out?

Thanks and it's great to see so many pictures! Keep them coming please :-)
Dan
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