Horizontal Stabilizer and Elevator

Horizontal Stabilizer and Elevator

The Horizontal Stabilizer and Elevator will be built first. This is because most of the skills required to build the rest of the composite structures in the aircraft will be learned here. The horizontal stab will be built in three sections and if any errors are made during construction of it or its parts, it is relatively inexpensive to scrap them and try again. This section of the manual will contain quite a bit more detail and hand holding than the rest. This is to lead the first time builder through the steps and to foster skill and confidence. The technique used to build the horizontal stabilizer is different that that used to build the KR stab. Instead of skinning a solid block of foam, a structure consisting of ribs, a spar and a leading edge will be built. This frame will then be sheeted with foam that is fiberglassed on both sides. This results in a structure that is quite a bit stronger than the traditional method. This is done not only because the Super2 has a larger horizontal stabilizer, this will be the technique used to construct the wing. It will be beneficial to learn these techniques on a smaller flying surface. The vertical fin will be constructed in a manner similar to the KR’s.

First, all of the materials and tools should be collected. A complete list of materials for this step follows. In some cases, as noted, the amount of material may support the build of more than just the horizontal stab. Once the raw material is on hand, we will measure out our glass and prepare our raw stock—the fiberglass and foam sandwich material our parts are fabricated from. We will then proceed to fabricate individual parts. We should be completely satisfied with the quality of each part we create before moving on to the assembly of the horizontal stab. If a part does not meet your quality standard, throw it out.

Required Material List

Bid Fiberglass cloth – 3 yards (ACS 01-00642 38" RA7725)
UNI Fiberglass cloth (ACS 01-00641 RA7715/38W F16)
Carbon fibre cloth – 4 ¼ yards (ACS 01-28260 Graphite 282 5.6 oz.)
Epoxy (ACS 01-42150 PR2032/PH3660)
Flox (ACS 01-14800)
CA glue and Accelerator
20 min epoxy
1 each 24” X 48” sheet of 4.5 lb. ¼” thick last-a-foam (ACS 01-13550)
2 each 24” X 48” sheet of 4.5 lb. 3/8 thick last-a-foam (ACS 01-13600)
1 each 24” X 48” sheet of 2 lb. 2” thick polyurethane foam sheet (ACS 01-11900)
sheet of masonite
3/32” Aircraft plywood.
6 each hinge rod end (MM-4 Aurora Bearing)
6 each eye bolt (AN44-13A Eyebolt Undrilled)
6 each large washers ( AN970-5)
1/16" aluminum sheet
6 each nutplates (K1000-4)
6 each nutplates (K1000- 5)
2 each weldments
bell crank and inserts
counter weight arm
counter weight support
lead

Raw material preparation.

This is a photo of our desired result. A 24" by 48" sheet of ¼” last-a-foam with a single layer of BID (Bi-Directional) fiberglass on both sides. This piece will be enough to cut all of our horizontal ribs, our horizontal stabilizer spar, elevator spar, vertical fin spar and rudder spar. We will then repeat the process and make two 24" by 48" sheets of 3/8" last-a-foam with a single layer of BID (Bi-Directional) fiberglass on only one side each for sheeting both sides of the horizontal stab.

Finished Panel Stock

Step 1: It is a good idea to prepare all the cloth for a job before hand and fold it up and put it in marked zip lock bags. That way when, when your resin is mixed you are ready to go. So first we cut all the glass for the first step of preparing our three sheets of foam for the horizontal stab--that is we want to have four sides of glass bagged and tagged..

Clear your work table and roll out your BID. The first thing you need to do when starting a new roll of BID is to cut a large triangle of cloth off the end to give you an edge on the 45 degree Bias. Do this now. There are two ways to insure you are cutting on the 45 degree bias. One is to use a 45 degree slot in your table that can be used as a guide. You must pull the glass straight to use this method. My favorite method is to observe the pattern in the glass cloth--you can see straight lines formed by the weave of the BID where the horizontal and vertical fibers intersect. Cut along this interface.

Then lay out a sheet of 2’ X 4’ foam on the glass with one long edge lining up with your fresh 45 degree bias edge leaving about an inch exposed. Then mark, or just go ahead and cut along the other long edge. This will result in enough glass to cover one side of your sheet. You will have to cut the triangular end off one side of the strip and lay it over the other end to completely cover the sheet. See the photo below. Go ahead and cut the triangle off. Cut a total of four strips from your roll of BID and cut the triangle off the end of each, bag and tag these. Two will be used for the ¼” sheet and one each will be used for the two 3/8” sheets. Put your glass away and spread a drop cloth on your work table. It is time to lay down some fiberglass.

Cutting Cloth on the 45 Bias

Step 2: Our next job is to slurry one side of the foam. Refer back to the beginning of the manual and mix up a batch of slurry using about 100 grams of resin (this would be 100 grams of resin, 27 grams of hardener and about an equal amount by volume of glass bubble microspheres).

Slurry Consistency

Pour the mixture onto the sheet and using a squeegee, spread the mixture across the foam. You want to get as thin a coating as you can get away with, we just want to seal the foam. Spread it with the squeegee, the squeegee it off leaving the foam wet. Use moderate pressure on the squeegee, but do not damage the foam.

Slurry the Foam

Spread Slurry

You may have to mix up a smaller batch of micro to finish the first panel. When you are satisfied with your slurry job, layout the fiberglass on the panel.

Layout Glass BID on Slurried Panel

Fiberglass does not like being pulled on the 45 degree bias, it will bunch up in one axis. Stretch the glass out along its original axis of the direction of the glass threads by pulling it in the direction of the selvage. Lay the section you cut off one end to completely cover the sheet. The slurry will hold the cloth in place and make it easier to get it oriented.

Step 3: Mix up a batch of epoxy, again about 100 grams of resin is a good starting point. Pour it on the cloth and using a brush and the squeegee, wet the glass out. Start in the middle and force the air bubble toward the edges. You will get the feel for the correct amount of resin. This takes a little practice, the brush can be used to add resin or remove resin. Use the squeegee to smoothly distribute the resin across the sheet.

It is a good idea to use a bit too much resin and work it into the glass and insure there are no trapped air bubbles. Once you are satisfied that the glass is well wet out, take sheets of paper towels and lay across the surface and lightly soak up the exess resin. Don't wipe the surface, just spread the paper towel out over the fiberglass surface, rub it down gently, then peel it off. You should see the surface going from a glossy look (too much resin) to a flat appearance (just right). Using this technique you will be able to obtain a panel that is optimized for its strength to weigh, fully wet out, but not too much excess resin.

When one sheet of fiberglass is not enough to cover the part, you can overlap multiple sheets. The photo above is a good example of two sheets of fiberglass cloth used on a panel. Insure that you have about 1.5" of overlap where the two sheets of BID meet. Wet out the first sheet first, then lay down the second sheet and insure you have not trapped any air bubbles where the overlap occurs.

When you are satisfied with your fiberglass job, set the ¼” sheet aside to cure on a flat surface.

Step 4:You can now move onto the two sheets of 3/8” last-a-foam. Slurry and glass only one side of each of two sheets of 3/8” last-a-foam. Set these aside to cure with the long edge of the foam supported an inch or two off the ground, the glass side up. This will put a slight curve in the sheets which will help you later when you sheet the horizontal stab.

Step 5: Wait 24 hours and repeat the glassing process on the other side of the ¼” last-a-foam. Leave the 3/8” sheets only glassed on one side. Congratulations we are now ready to start cutting out parts.

Cutting out the parts.

Once the ¼” sheet of foam has been glassed on both sides and is cured we are ready to cut out the parts for the tail. Refer to the diagram below on how to allocate the parts to the sheet to insure we can get all the parts we need out of the one sheet. If you mess up a part, don’t worry about it. You can always make up another ¼” fiberglass foam sandwich. We will be needing it for additional parts in the future anyway.

Parts List

2 ea. Hrib1,2,3,4
2 ea. Erib 1,1a,2
2 each horz stab spar
2 each elevator spar

Divide Panel to Minimize Waste

Step 6: First we mark up the sheet with a sharpie marker. Divide the sheet down the long axis, then divide one half along the short axis. Each of the smaller rectangle pieces will result in a set of ribs and the 48” long piece will give us our spars. Cut these sections on the marked lines with an industrial razor knife.

The Ribs

Step 7: Print out several copies of the rib templates. Cut out one set and arrange the ribs on the smaller rectangular panels as shown in the diagram below.

Cut out the Ribs

Step 8: Use a scroll saw, band saw, exacto knife or packing knife to cut the pieces out. Replace your blades if needed.

Smooth Edges

Cut the parts slightly oversize, then sand the edges down to the proper size. Use your second template that you printed out to measure parts against. Lay the parts on the template and continue sanding the edges until the part just barely fits within the black lines. Be careful, it is easy to get carried away with this sanding process and end up with parts that are too small.

Finished Set of Horizontal Stab Parts

Looks like we are making progress.

Adding flanges to the ribs

Step 9: One more feature has to be added to the ribs before these parts are complete. Flanges are created by using a custom made flanging tool. We will be adding flanges to both sides and both edges of HRIB1, HRIB2 and HRIB3 as well as both edges on the inboard side of HRIB4.

Cut scraps of ¼” last-a-foam 1” wide and about 12” long. Glue two of these to another scrap 2.25” wide and 12” long. Use bits of scrap ¼” foam to create the space between the smaller strips. Gorilla glue is the best glue for foam to foam. Apply gorilla glue to one side and spray the other side with water. We want our flanges about 5/16” from the edge, so we need to build up the SFT (special flanging tool) with several strips of duct tape. The reason for this is that we will be sheeting with 3/8” foam sheets shooting for a nominal foam thickness of 5/16”, not more than 3/8” and not less than ¼”.

Flange Construction

Step 10: The construction of the horizontal ribs are completed with the addition of flanges on both the top and bottom edges. Flanging tools can be made with strips of foam as shown above. The goal is to create a flange 5/16” below the edge of the rib. This is done by glueing ¼” foam strips to another strip of foam, then covering with three layers of duct tape. Use the rib to obtain the proper spacing of the smaller strips of foam. Gorilla glue or polyurethane glue is great for gluing the foam strips, just be carefull that the parts are well secured as the glue expands somewhat.

Flanges Held in Place

Step 11: Insure that the ribs are completely seated in the flanging tools. The flanges determine the skin thickness of the horizontal stabilizer. We are targeting a thickness between ¼” and 3/8”.

Cutting Glass for Flanges

Step 12: Cut 2” wide strips of fiberglass to create the flanges from. Be sure to cut the glass on the 45 degree bias. This should go without saying.

Flanges Curing

Step 13: Layup the flanges, two layer of BID on the 45 degree bias. The flanges should be about 1” on the rib and one inch overhang. Let cure.

Completed Ribs

Here are our completed ribs. Redrill the air holes if they were covered up by the fiberglassing of the flanges. Remember that the smallest two ribs are only flanged on one side. OK, we are ready to assemble the horizontal stabilizer.

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A technique that I like is the use of a very sharp industrial razor knife. Cut the patterns out with a pair of sissors, then trace the outline onto the panel. Use the razor knife to lightly score the fiberglass along the line, using a straight edge where practical.. Repeat the scoring until the fiberglass is cut all the way through. This process allows you to make very precise parts and does not generate the fibergalss dust that is so bad for your lungs. You have to be exceedingly careful not to cut yourself, always think of where the knife will go if it slips.