The stabs will be the first cab off the rank with this build. The MDF building boards have been cut to suit the stabs and are nice and flat. As each end of the jig is 90 degrees on the trailing edge, corresponding marks were made on the MDF with a Sharpie marker. This aids alignment prior to gluing with CA. Before gluing with CA, place some wax paper between the balsa and the MDF. We don't want the jig being glued to the MDF! Once you are happy with the alignment hit each of the five jig joints with CA and kicker. Then the wax paper can be pulled away from the jig.
As you can see in the photos, I've cut out the balsa stab ribs and done a dry fit to the jig base. Looking good!
The most important thing at this stage is to ensure you have a left and right stab! We don't want to end up with two lefts or two rights.... :-)
As this is the first kit, there are bound to be some minor issues with fit. One such issue is the stab socket hole in the root rib and inboard ribs is undersized. The hole wants to be 0.510" or just under 13mm. Production kits will have this issue addressed. In this case, a 0.500" reamer was used to open the hole out and a 13mm drill bit was used to finish it off to a neat fit with the socket.
One of the nice little features that Mark Hunt has designed into the ribs is a semi-circle cut-out under where the spars are glued into place. It basically prevents you gluing the rib to the jig. This can be seen in the stab root rib pictured above.
Another nice feature is the slotted spars which self-align with each rib. See picture in the gallery below.
I found that the stab socket holes needed a fraction more material removed. It just so happened that I had a 1/2" sanding drum which cut a perfect hole when spun up in the Dremel. At RPM the diameter must be just a fraction bigger than 13mm.
You'll need to attach the rib doublers for the stab socket and also the adjusters. I used the stab socket to help align the doublers. Remember to remove the socket before hitting it with the Zap. We don't want the socket glued in prematurely.... After gluing the doublers, I ran the 1/2" drum through the socket hole again just to clear any CA that may have foamed up.
Next, cut out the stab spars SS1, SS2, SS3 & SS4. Before fitting SS4, I sanded a 45 degree chamfer on the root end. The spars can then be slotted into the ribs as per the positions indicated on the drawing.
The first stab frame has now been glued and separated from the jig. The first thing to do was place some wax paper under the whole frame and then tape the jig base to the MDF with masking tape. Then ensure that all ribs and spars are fully seated in their positions. You can then start hitting each rib with CA fixing it to the base frame. I did this a few ribs at a time and then hit it with kicker. You can then go over each of the spar joints with CA and kicker.
Cut the stab socket to length (approx 110mm) and then fix into place with 30 minute epoxy. I kept all the waste material removed from the ply parts. There was a couple of round pieces of ply that suited capping the socket end perfectly. This was just glued with CA.
Once the epoxy on the socket has cured, you can then remove the stab frame from the jig. I used a sharp scalpel to gently cut each tab on the ribs. Once separated, give the places where the tabs were a light sand to remove any bumps. Also give the spars a light sand to ensure they are flush with the ribs.
Total weight so far is 29 grams.
We are now ready to make the control horn hard points. The hard points are simply made from three pieces of 1/4" balsa laminated and then sanded to the elevator profile. I used white glue for this as I know it's easy to sand.
The skins were the next item to be made. The skins are made from 2.5mm balsa and each sheet was glued together with white PVA. On the wings I'll probably use Aliphatic Resin wood glue by Titebond. Each sheet was taped together with masking tape and then folded over the table edge to open up the join for glue. Then place on a flat surface and wipe away the excess glue with a wet cloth or paper towel. Weights were used to keep things flat. You'll also need that wax paper again too.
Once the skins are dried, they can then be attacked with the sandpaper to remove any high spots around the joins. To attach the skins to the stab frames Polyurethane glue was used. On the first stab the glue was applied directly to each rib top and bottom. The inside of each skin was misted with water and then the whole assembly was compressed between the two jigs and weighed down with house bricks. This method of applying the PU glue turned out to be quite heavy. The excess glue created excess foam and 10 grams more weight than need be. On the second stab I applied the PU glue direct to the frame but then rolled the ribs with a foam roller to remove the excess. This saved 10 grams over the first stab. We learn from our mistakes....
Next the stab edges were sanded flush to the ribs all round. The leading edge was sanded to the point whereby the rib will present a flat surface to the LE capping. When you glue the TE and LE on make sure you allow enough overhang at the root rib to allow sanding flush. See photos. Aliphatic Resin wood glue was used to attach the LE and TE.
The next task is to sand the trailing edge flush with the surface and shape the leading edge. See gallery below. To aid sanding, a pen mark was made in the middle of the leading edge.
The elevator can now be separated from the stabs, sanded flush and capped. First step is to mark out where to make your cuts top and bottom with a pen. Your marks want to be between the two spars as shown in the picture below. Then using a straight edge, cut from both sides with a sharp scalpel. You don't need to cut right through as the elevator will snap off with very little effort once cuts from both sides are made. Sand the stab TE and Elevator LE flush to the spars and then cap with the laser cut parts SS5 and SS6. These parts are the same size to it doesn't matter which one is fitted to the stab or elevator. The only word of caution is to allow enough overhang at the root of the elevator for sanding flush with the bevel. Aliphatic resin was the glue of choice for this task. As a bonus, some of the excess glue has been removed from the first stab which has reduced the weight difference between the two stabs to within 5 grams. I'm a little happier now. :-)
The tips are laminated from 1/4" balsa which has been laser cut for you. Again, aliphatic resin was used to glue the four parts together with the aid of a couple of clamps. The tips will probably be hollowed out later to save a little weight.
The stab capping has now been sanded flush with the stabs and elevators. The bevel has not been put on the elevator yet as the elevator will be taped back onto the stab for aligning and shaping the tips. Weights of the stabs so far are 80 grams and 72 grams. The heavier one is obviously the one first skinned with the second being significantly better. For once I'd like to see the first effort turn out better than the second. Perhaps that's a dream...
I also cut a piece of ancient covering 1" bigger than the whole stab to get a projected finished weight. My estimated finished weight will be 95 grams and 87 grams. I can live with that.
The stab tips have now been glued on with Aliphatic resin. I decided not to hollow out the stab tips as it would have reduced the gluing surface area for a very minimal weight saving. The gluing area is literally 1/2" X 2" and a large amount of the tip block will become dust anyway. Also, the last thing anyone wants to do is knock a tip off the stab...
The elevators were reattached to the stab with masking tape. A strip of masking tape was also placed along the chord of the tip to act as a sanding aid i.e. just sand down to the masking tape. The tips were sanded to shape using 120 grit paper. Then I removed the masking tape from the tip chord and final sanded with a finer grit. The masking tape helps prevent accidental gouging of the stabs with the coarse paper.
Stab Alignment - A separate article has been written on this aspect of the build here: Wing And Stab Alignment.
The Stab servos Im using are MKS DS9660A+. They are a coreless metal geared servo in a mini servo format. The only thing left to do here was drill the screw holes for the servos in the Ply plate. A piece of cardboard either side of the servo helps centralise it prior to marking the hole positions (see pic below) with a pencil. A 1/16" drill bit was then used to drill the holes. The screws and rubber grommets supplied with the servos were used to secure the servos in place.
The Stab Adjusters were attached to their respective ribs earlier but now that sheeting has been fitted, we need to make some holes to access the adjusters. If the adjuster is removed from the Stab and then pull the grub screws out you can then re-fit them and accurately mark out where the holes in the skin should be. I used a bent pin for this. When you make the hole start small and then work your way up to the final hole size. This allows minor adjustment of the hole centre if you're slightly out. I opened my access holes out to 4mm.
The bevels on the elevators are somewhat easier than the Ailerons in that they are not tapered. To aid setting the correct bevel, I made up a small paper template with +/-15 degree lines marked on it. This template was stuck to the inside face of the stab tip with masking tape doubled over. The centre line of the elevator is lined up on these template marks to ensure we get at least 15 degrees of throw with the bevels.
First step is to mark the elevator centre line (hinge point). Simply measure and mark the centre at both ends of the elevator and then join the dots. Then you can mark out the bevel on the top and bottom of the elevator. I ended up measuring 4mm back from the elevator LE capping on both sides. You can also join the lines to the centre line at the elevator root and tip. This helps when sandin g. A straight hard and long sanding bar is used for sanding bevels. I think my bar is 1' long and has 240 grit anti-clog paper glued to it. Then sand the bevel along the grain until you get down to the previously made marks. Check the elevator throw against your template and if all is well you'll have 15+ degrees of throw.
Make sure you put the stab retention screws on the bottom side of each stab.
Holding the Stabs in place is very important. :-) In this case nylon M4 screws will be used. During stab construction two 6mm balsa doublers were placed at each end of the stab tube. This not only supports the stab tube it also hardens up the area for a retention screw. The first thing to do is place some masking tape on the stab skin in the approximate place of the socket. Then insert the stab tube and mark out the outline of the socket. Placing the stab in the jig and using a square against the tube helps to accurately mark this out. Then using the stab tube again, determine where the end of the tube is and mark it. From here we know there are two 6mm balsa doublers so the hole position can be placed appropriately. I placed mine 6mm from the end of the tube. Now we have the hole centre marked, a small indentation with a centre punch/pop will help stopping the drill wander off when we get to that stage.
Now when you cut the stab tube to its correct length, there should have been approximately 1" of tube left over. I hope you kept it as it will come in handy for the next task. With a sharp knife, sharpen the inside edge of the left over tube. We are going to use this to cut some balsa tube bungs with it. Search through your balsa stock/scrap for some hard, thick balsa. I found some hard 12.5mm balsa to do the job. Then it's just a matter of placing the scrap stab tube in the drill press and cut out the balsa bungs. The sharpened tube will cut through the balsa like butter and after each cut you'll need to pull the tube cutter out of the chuck to push the balsa bung out. Make two bungs of course... The bungs can then be epoxied into both ends of the stab tube.
Next I placed the Stab half back into the jig with the stab tube pushed home. A 3.3mm hole was then drilled through the stab, socket and tube. You want to go right through the tube and socket but NOT right through the stab. Most drill presses have an end stop so use this if you like. Now take an M4 tap and run it through the stab, socket and tube. A drop or two of CA in the threads afterwards helps harden up this area further. Repeat this for the other stab half but DON'T insert the stab tube and DON'T tap the hole yet. We'll drill and tap this end of the tube afterwards. The M4 nylon screw can now be fitted to the stab which was tapped.
Fit both stabs to the fuse and adjust the stab incidence so both are the same. The actual setting is unimportant at this point but they both must be the same incidence. Turn the model upside down.
Control horn slots.