Building the Hawk

Before You Start: 

You will need a 3’ straight edge, a nice sized drafting triangle, and clamping devices such as straight pins, weights, etc. You will need a jackplane and a sanding block at least 12” long that is flat and HARD ... no foam or rubber surfaces to back the paper. You will also need a straight, flat place to build. It is impossible to build this wing straight and flat on a crooked table. You may also want to follow my practice of going to Kinko’s and making a working copy of your plans, so the originals can be saved and copied again, should the need arise. You will need slow curing epoxy, fumed silica such as Cabosil, Titebond aliphatic resin glue, thin CA, and mid-viscosity CA glues. I also like to keep some 99% isopropyl alcohol handy for cleanup of epoxy.

 It’s always best to begin where the most work is, so I always start with the wings. Please remember that some of these materials are hazardous, and use caution when cutting carbon, and handling glues.

 Wing:

 Cut the plans so that each individual piece of the model is a separate piece of the plan. For the wing, you may want to cut the plan into the different panels so they can be built on a different board. Place either main panel plan on your board and cover with Monokote backing, or Saran wrap. Don’t use waxed paper for this first step. Find the spar caps and cut them to length as shown on the plan. Use a Dremel with a cutoff wheel for this so the cf doesn’t splinter. Taper the plywood box webs and round the outer corners. Figure out where the shear webs and the outer ply webs go. Cut the last W-1R rib at he front of the spar cap notch. (Note that the SW1 webs are 1/4 thick, and that you’ll have to use 2 of them glued together to make each shear web.) Assemble this back half of rib with a shear web on both sides. Cut another shear web in half, and make a full depth, half length web and add it to the inboard end of this assembly. Note that the assembled spar is very slightly taller at it’s front edge. A couple of passes over the sanding block with the shear webs, holding more pressure to the aft edge, will slightly angle the tops of the webs and angle the top spar cap down at the back edge.  Mix some slow epoxy and thicken it with cabosil ... NOT microbaloons. Assemble the spar out to the end of the plywood webs, being careful to make a perfect box of the spar butt that will house the joiner tube. This assembly will have only 1 aft W-1R rib section and the spar out to the end of the plywood. Wrap this assembly with Kevlar tow, making sure the tow stays flat, like a ribbon, and not round like a rope. Use thin CA only on the plywood webs to hold it in place, as the top and bottom will be saturated in sheeting the wing. There should be a hollow area about 5” deep in the root of the spar. The spar caps and the top and bottom of the plywood outer webs should be even. The aft plywood web should have made the same passes over the sanding block to compensate for the airfoil shape. Build both spars and make sure to make a right and a left. Make sure to use the thicker spar cap on top.

Starting again, make sure the plan is clear and covered with waxed paper or whatever you want to keep from gluing the wing to the plan. Selections for sheeting are important. Use firm sheets for the inboard panels, and lighter sheeting for the outer panels. Lay out the bottom wing sheet, including the root section bottom sheet covering to the trailing edge. Cut, notch, and lay out the  trailing edge, with everything exactly matching the plan. This should be glued to the center section bottom sheeting, which has been fitted exactly to the plan, and to the trailing edge. Always pin these pieces firmly and ensure straightness with the straight edge. Sand a slight trough in the sheeting area where the Kevlar wrap will lay so the spar area will remain aligned and the sheeting will not bulge too much over the spar. Carefully glue the bottom spar cap down to the bottom leading edge sheet with thick CA, making sure everything lines up exactly along the aft edge of the spar line. Cut the W-1R ribs fore and aft 1/8” behind and in front of the spar notches, to clear the plywood webs. Glue the W 1R ribs to the bottom center sheeting and the spar, using triangle braces or thickened epoxy to bond the ribs to the spar. If you are correct, the forward sheet will cover the front rib drawings, and you’ll be able to see just the faint edge of the back of the spar line on the plans. You will have a partially assembled spar with the top cap in place but drooping down as it goes out, and all center section ribs in place. Be sure to make the 1/8” W-1R rib at the root lean in 3.5 degrees.  Now start with the ribs and shear webs, The shear webs should be an exact fit, but be ready to correct them if they aren’t. Again, they should be slightly taller in front, and they should exactly match the rib spar slots. Generously butter the web and spar with thickened epoxy. Lift the top cap slightly and slide the web in, followed by the next rib, with a bit of epoxy on it where it touches the caps and webs. Wipe off the excess with a good stick, and repeat the process to the end of the wing panel. Be careful that the ribs exactly match the plan placement. If not, add or subtract as necessary from the ends of the webs for correct spacing. Note that the last web space doesn’t get a web, and set the 1/8 joint rib aside.  It is extremely important to have NO gaps between the spar caps and the webs and ribs. Be sure the leading edges of the ribs are even and the trailing edge of the ribs are glued back into the trailing edge notches. Using a piece of scrap trailing edge stock,   gently wedge the front of the sheeting up to contact the bottoms of the ribs. Ca the ribs to the sheet. Remember that anywhere 2 pieces touch, you will need glue. Thin CA works best for this. Add the 1/4 square sub-spar using thin CA. Now add the 1/4 sq. leading edge piece. It should come to the edge of the sheet, but if it doesn’t exactly, it’s OK. Be sure the leading edge contacts each rib all the way from the top to the bottom. Also be careful not to push the sheet down off the ribs. If you are correct, the bottom sheet will be conforming to the ribs up to where they meet the leading edge, and then will bend down slightly to conform to the right angle between the ribs and the leading edge. Be sure to CA the leading edge to the sheeting between the ribs. When this assembly has cured for a day, it can be lifted from the plan and put out of the way until ready to join to the outer panel. Build both center panels before beginning the outer panels, unless you have lots of room and can do both at the same time.

 Build the outer panel by first placing the notched trailing edge piece and leading edge sheet over the waxed paper and plan. Again check for straightness. Taper all 4 of the spar caps at once with a small jackplane. Stack them and do them as 1 piece. They should taper from 1/8 X 1/2 to 1/8 X 1/4. Block sand as necessary to have good, clean gluing surfaces. Once tapered, check the fit by laying the spars out on the plan and slipping the ribs onto it at their positions. Glue the bottom balsa spar to the bottom sheet with medium CA, using the straight to ensure straightness. Beginning at W 3, glue the ribs and webs in place with Titebond, all the way to the end. Check the position of the front and rear of each rib as you go to ensure straightness and fit into the trailing edge piece. The end rib of this panel is 1/8 balsa and needs to tilt 15 degrees, so trim the web accordingly. Apply Titebond to the tops of the webs and ribs, and press the spruce top spar cap into place. If you have a clamping arrangement or some way to evenly weight it down, do that now. Clean off the excess glue that oozes out, to save weight. Again using the scrap trailing edge, wedge the sheeting up and CA the ribs to it. Add the leading edge piece just as you did for the center panels. When dry, about 4 hours, remove from the board and build the other outer panel. Lay the polyhedral joint ribs aside until ready to join the panels.

 Build the winglets in the same way remembering that where it joins the wing it needs that rib to tilt 15 degrees, so the winglet can tip up 30 degrees total The only difference is there are no spars. The sheet should be webbed at the aft edge with 1/32 shear webs.  Add the tips and sand them to shape. There is no joiner for this junction so after the top sheeting is applied and the joint is sanded straight on the panel and winglet, the winglet is glued on with Titebond or medium CA.

Plane and sand the leading edges to the proper angle to allow the sheet to bend over the ribs and smoothly attach to the leading edge. Remember to use firmer sheets for the inboard panels. Cut the sheeting to length, allowing a touch extra for curvature in the joints. Add a strip 3/8” wide to the top sheet for the inboards to give full coverage from the spoiler bay to the front of the leading edge piece. Lightly sand the top, front portion of the ribs using 220 grit paper on your hard sanding block. This should even the tops of the ribs with each other, and with laser cutting, it should be very minimal. Apply a bead of Titebond to the length of the front of each rib, stopping 1/8” before getting to the leading edge or the spar. Apply medium CA to the spar using a touch extra where the Kevlar wraps the spar. Press the sheeting to the spar and ribs and hold until the CA grabs. Now apply a bead of medium CA to the leading edge piece and press the sheet in place, beginning in the middle and working toward the ends. Be sure the sheet is stuck down to the leading edge, spar, and all ribs. The sheeting should extend beyond the spar by 1/4”. This will give some clearance for spoiler linkage. Apply a few phone books and leave for 2 hours minimum. When dry, add the spoiler tubes and complete the center section sheeting. Sheet the outer panels the same way, though the sheeting will not have to have the 3/8 strip added.

When all the sheeting is complete, trim the polyhedral joint sheeting close enough to sand perfectly straight. Take your time and make this joint perfect. Prop it up at the prescribed angle and sand it using the edge of your building board as a guide. Make the plywood joiners by gluing 8 of the plywood pieces together with epoxy, with a slight bend in each to match the spar sweep. These will need to align perfectly with each other, or they won’t fit in the wing. This piece will need to be fit into each panel carefully by sanding and trimming. Once satisfied that all pieces fit and the wing panels fit together correctly at 7 degrees, add the forward sections of the polyhedral ribs, and epoxy the joiner in place. Epoxy the rest of the polyhedral joint, with everything propped up and weighted so it is sure not to move. Be careful to remove the excess epoxy with an alcohol rag. This cleanup step will make sanding much easier, so don’t omit it. Once the joint is cured, add the aft ribs and sheet the small section between the sub-spar and the edge of the d-tube sheeting. Add the radius gusset to the outer panel.

It’s easier to add the tip block and sand it to shape before the winglet is added, so do that before adding the winglet to the wing. Sand the winglet joint to fit and either CA it in place or use Titebond. Make the spoilers from 1/4 X 1 trailing edge stock. Work out your favorite linkage or make servo platforms as you wish. Do NOT put the spoiler servo in the leading edge D-tube, as wing strength WILL be compromised. The wing is ready for sanding at this point. Put it aside until needed for making the fuselage/wing connection.

Sand the root to it’s 3 1/2 degree angle. Sand the outside of the 15/32  X 12” brass tubing, and plug both ends with scrap balsa to keep the epoxy out. The best way to do this is to sharpen the tube edge and use it as a cutter to cut a plug from scrap 3/32 sheet. Secure these plugs with a drop of thin CA. DO NOT CUT THE JOINER TUBE! Line up the wings together as if you were going to make a 1-piece wing. They should fit perfectly together with both propped up as the plans show. Once this fit is completed, it’s time to install the joiner tube. Lay one wing flat on the board. Mix a batch of epoxy and thicken it to the consistency of toothpaste, enough to fill the cavity around the tube in one wing. Start by filling the cavity about half full and slowly push the tube in, clearing the epoxy that oozes out. Slip both of the 1/8” ply root ribs on and glue the first one on to the wing you’re working with. Slip the tube through the ply rib of the other wing and slip the wing over the tube. Line the wings up flat and straight, and allow the epoxy to harden. Fill the cavity of the second wing. Slip this wing into place, cleaning the epoxy off that oozes out. Glue the rib in place, and weight the whole thing down straight. If you are correct, your wings will be flat on the table, perfectly aligned with the root ribs in place, and about 2” of the tube in the middle. This allows the tubes to be exactly aligned from one wing to the other. Allow this to cure for a day, and cut the wings apart by cutting the tube with a zona saw. Add the alignment pins when the joiner and fuselage are mated. This completes the assembly of the wing. They are ready for sanding and covering.

 Stabilizer:

The stab is built with sticks, is then sanded, and the leading edge is sheeted with 1/16” sheet. Build both halves at the same time and saw them apart with a jewelers saw, or zona saw. It’s easier to keep things aligned by keeping the trailing edge in 1 piece until sanding is complete. Begin by cutting the 3/16 X 3/4  trailing edge stock to size and adding the 1/16 X 1/4 spruce to the back edge. Pin it down. This will require shimming the aft edge up 3/32”, and the front edge 1/32”. Next add the 1/4 sq. leading edge piece. Add the tip pieces, and the root pieces, including all gussets and laying out the tubing. This should have a rough outline of the entire stab when finished. Add the diagonals. Be absolutely sure to make them fit as shown, for maximum strength. Use epoxy when installing the tubes, but not too much. When all glues are hard, lift the stabs and mark the center of the leading edge down the entire span. Sand them to the shape shown, in preparation for sheeting. Cut 1/16 X 1/4 spruce for the top and bottom spar caps. They must be placed where shown on the plan. Glue the bottom cap to the stab ribs. Apply 1/16 sheet shear webs out 2/3 of the stab span, and add the top cap. In the tube area, add the shear webs to the front and back of the caps, filling the voids around the tubes with thickened epoxy. Sheet the leading edges of the stabs. Add the root area sheeting and sand to shape. Cut the stabs apart only after all shape sanding is done. This should leave a nice symmetrical airfoil for the entire stab.

 Rudder:

The rudder is constructed without ribs. It uses external strips, placing the weight where the strength is needed. Cut the 1/2 sq. fin post and rudder post to length and taper them to 3/8 X 1/2 at the top as shown. Mark them and drill them for hinge points together. Pin the rudder post to the plan. Cut the other outline pieces and gussets to shape and fit them carefully. Use a sanding block to perfect the joints so there won’t be any gaps in them when you pick it up. Make the trailing edge by laminating 1/16 sheets with 1/64 plywood in the middle. Use a thin spread of Titebond for this, and clamp it for an hour. Make sure the ply protrudes from the front of the laminate 1/4” to index the strip ribs. Use a piece if 1/16” sheet to shim a piece of 1/8 X 3/8 stick and attach it to the back edge of the rudder post. It will need to taper as the rudder does. This will index the strip ribs to the post. Add the laminated trailing edge to the rest of the framed rudder outline shimmed up 3/16” to center it with the plywood spline forward. Cut the diagonal strip ribs to shape and glue them to the rudder post and trailing edge spline and sheet. When dry, flip the rudder and add the strip ribs to the other side. Sand the rudder post to a perfect radius to fit slightly into the fin, and to hinge at the center of the radius. Fit the fin post to the fuselage and glue the hinge points in place with the hinge line  9/32“ behind the post. Glue these only to the fin post at this time. Slot the rudder post to allow the rudder to swing 30 degrees each way, while barely missing the fin post. Clamp the assembly into the fiberglass fin and check for movement. The fin post should be recessed about 3/16” into the fin. Trim and sand until all the pieces fit and the rudder swings properly. Put aside until the fuselage is ready.

 Fuselage:

The fuselage is a fiberglass Condor fuselage, and you should have acquired one by now. Mark and drill the joiner holes, index pinholes, and the holes for the spoiler tubes, or wires, as the case may be. The stab axle area should be reinforced with 1/16” plywood, and the axle holes drilled. Use a Dremel with a router attachment for the stab actuator arc. Once this is done, add the belcrank for the stab, and the pushrods for the rudder and elevator. The fin post may be added once the wing and joiner are aligned, and the stab belcrank and pushrods are in place. I like to use 2 parallel surfaces about 3” apart for aligning this wing and fuselage. A couple of short 2 X 4s will be fine. Put them on your workbench, and mark their locations. Mark your joiner so you will know it’s centered in the fuselage. It has a 7-degree bend in it and will skew your wings if not properly aligned. Roughen the joiner in the area inside the fuselage and slip the joiner and incidence pin in place. Slip the wings on and set the whole assembly on the boards. It’s important that the boards not move, and are perfectly parallel. Move the model to be exactly in the middle with the centerline of the fuselage parallel to the boards. MEASURE...don’t guess.  Check that the fin is vertical. Measure from the aft edge of the polyhedral joint to the fin centerline. It should be the same if the wings are perpendicular to the fuselage. If not, you will need to adjust some of the holes in the fuselage. This should be done slowly with a fine round file, with as little material removed as possible. The bottoms of the wings should be parallel and  the wings perpendicular to the fuselage, with the fin perfectly vertical. This can be frustrating, so take your time, and think about what you’re doing. You can now tack glue the joiner and incidence pin in place. Use enough thickened epoxy to be able to remove the wings without disturbing the locations of the joiner and incidence pins. Let this cure for a day. Remove the wings and apply thickened epoxy to the entire joiner area to prevent the joiner from twisting in the fuselage. Place the stabs on their 1/8” music wire joiner. The belcrank will require a piece of 1/8” brass tubing, just long enough to fit through the fin. This tube will be bonded into the fin with 1/16 ply backing plates. Locate and drill the hole for the joiner and rout the arc for the actuator pin. Assemble the whole model and check the stab for alignment. When viewed from the rear, the stab should be perpendicular to the fin and level with the wings. Again, take your time, as these alignment procedures are the most important steps for a straight flying model. Once this is aligned, glue the tube to the plywood backing plates with the belcrank and pushrod on it. Don’t glue the belcrank to anything. It must move freely. Now epoxy the backing plates into the fin taking care not to get epoxy on any moving parts. Assemble the model again and make final alignment adjustments to the stabs. Allow to cure for a day. Remove the wings and stabs and epoxy the fin post in place, checking with the rudder for movement, and proper placement. Be careful to keep epoxy off the fuselage exterior...keep it neat. The fuselage is ready for final assembly.

 Final Assembly:

 Work out the details for your radio installation. I like using a 1500 mil nicad pack and 1/8” ply for a servo tray. Rig your spoilers and ensure operation with thread and masking tape, or, if you opted for servos in the wings, check the routing of your cables. Sand the wings carefully. In the area where the spars are wrapped, the sheeting may bulge slightly. Sand this area to conform to the airfoil, as the sheeting isn’t too important for strength above the carbon spars. If you have built it correctly, it won’t get down to the wraps. Sand the leading edges correctly, using the sanding guides provided with the ribs. This is the most important portion or the sanding exercise. Once satisfied, sand the entire wing with 220-grit paper on your hard block. Work slowly using only the weight of the block for pressure. The wing is ready to cover. Sand the rudder and stabs as you did the wings. Cover in your favorite colors of your favorite covering material. Install the radio, rig the spoilers, and hinge the rudder. Add the tow hook. You are now ready to balance the model. Because of the sweep of the tips, the model will balance further back than you might think. Start with the incidence set at 1 1/2 degrees positive, and 1 1/2 degrees washout from the poly joint to the tip. Balance the model at 4 1/2 inches behind the leading edge. This setup will give good performance right off and can be changed by adjusting the CG and incidence. My suggestion for maximum performance is to continue to move the CG back, while bringing the incidence to as close to 0 as possible. I don’t like dive tests much and I sure don’t recommend inverted flight as a measure of CG accuracy...but you’ll know when all is right. It will groove in cruise and show thermals well because it will be pretty easy to upset.

 

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