The first cab off the rank with this build is the stabs. Both the stabs and wings are built-up balsa and ply construction and covered with film. This results in a very light and strong structure and the quality of the covering is fantastic.
The hinge slots are all pre-cut for you. Supplied in the kit are CA type hinges but for this build, my customer wanted to use Dubro pin hinges. These can be glued into the same slots cut by the factory. The glue to be used will be Pacer Hinge glue. This glue seems to etch into the hinge material creating an awesome bond.
The servos for the first model will be JR DS3421HV. The second model will use MKS HV6130H servos.
The servos are mounted within the stabs. I believe this results in better geometry compared to mounting with the top third of the Servo hanging out the bottom of the stab which is normal practice. The factory pre-installs a plywood servo Mount. The DS3421 fitted in easily with no adjustment of the hole needed. The Servo was wedged to the bottom side of the hole with a piece of folded cardboard. The logic here was to move the Servo horn further out. With the Servo neatly wedged into position, a sharp pencil is used to make the screw positions. This also dents the ply a fraction which stops the drill bit wandering off. Remove the Servo and drill the four holes. I used a 1mm drill bit in a pin vice and made the holes by hand. A screw was screwed into each hole to make a thread and then removed and hit with thin CA to harden things up.
Because the Servo is mounted within the stab, a slot must be cut for the horn and linkage to actuate. You can see in the pictures above the trusty masking tape is in use again... The position of the Servo Mount is marked out first and then the centre line of the Servo arm can be marked. This will be used to work out the linkage slot too. These marks will also come in handy when the control horn is mounted in the elevator later.
Don’t you love pictures? They tell a thousand words. Above is another gallery showing the Servo arm slots and the elevators dry mounted to the stabs. I’m very happy with how BJ Craft has done the elevator bevels.
It’s at this point you really need to decide what ball ends you’re going to use and the associated linkage. This information is required to determine the width and position of the Servo arm slot. In this case, we’ll be using the 2.5mm Dual Axis Ball Ends in conjunction with 2.5mm titanium rod.
The width of the Servo arm slot was calculated first taking into account the screw and locknut too. In this case the width was 10mm. The actual slot will be offset to the centre line if the Servo arm. I made the slot length 15 and 20mm either side of the Servo screw. The slot will probably need to be opened up from here later but better to require opening out than being too big in the first place. The ends of each slot have been rounded. I firstly cut around the marked slot with a scalpel. Put a new blade in the scalpel and it will cut beautifully. Then pry the masking tape and film off. The straight lines can be cut with the scalpel and ruler. The round ends were made with a small sanding drum in the Dremel.
With the slot cut, the Servo can now be fitted to check how good your measurements were... ;-)
The elevators were dry fitted to the stabs to check the fit. All fitted well and BJ Craft needs to be commended for this! The elevators won’t be glued on just yet. We’ll get the control horns mounted first. Before we do that I’ll have to cut out the template sheet from Lite ply. A number of Elements were supplied without this sheet. I was able to get the drawings from BJ and will cut the templates on my CNC router.
Ok, in the template sheet is a rectangular part with a slot in it. I thought this was the template for cutting the control horn slots. Maybe not for the elevators... Anyway, a little more masking tape on the elevators in the hardpoint area and a bit more measuring and marking resulted in the control horn slots being marked out correctly. The slot was cut with a sharp scalpel and the balsa was picked out with a Dubro hinging hook (for want of a better description). The horn slot was cut so the horn was at 90° to the surface. Looking at the pictures above, you’ll note the fibreglass control horns are not slotted or drilled on the tab to be glued. This is one area that had to be improved by drilling some “keying” holes. This makes for a much stronger glue joint. The tabs should also be roughed up a bit with coarse sand paper to help epoxy adhesion. The horns are glued in with 30 minute epoxy and I add some glass rovings to the mix for a bit more body to the adhesive. Make sure the whole tab is covered in glue and the holes are also full of epoxy. Push some epoxy into the slot on the elevator too. As you push the control horn into the slot any excess epoxy will be squeezed out. Wipe any excess off. I find cotton buds soaked in methylated spirits great for this. Again check the horn is at 90° to the elevator surface.it should be perfect if the horn was a neat fit into the slot. Set the assembly aside to cure and repeat for the second stab. You should be an expert now.... :-)
Now we’ll tackle the linkages. Supplied in the kit you’ll find some mild steel linkages that are threaded one end and have a Z bend on the other end. Whilst you could use them, I think they’re better suited to the round filing cabinet under your workbench. In other words, I’d bin them. I really don’t think they are suited to precision F3A with regards to slop. So instead I used dual axis ball ends along with 2.5mm titanium rod. This eliminates 99% of the slop. In amongst the kit you find a template or jig for the stab neutral. Separate this from the panel and slip it onto the stab gently. It doesn’t need to be super tight as we don’t want to dent the underlying balsa sheeting. This jig is needed because the Element doesn’t have counterbalances on the stab tips for determining a neutral point. The dual axis ball ends are fitted to the Servo arm and control horn with M2 x 8mm screws and M2 locknuts. Attach the Servo arm to the Servo in a position that will make the angle between Servo horn screw, dual axis ball end screw and linkage 90º. The last picture in the above gallery illustrates this dimension. I find using the end of a steel ruler helps setting this. I should mention the Servo should be at neutral 1.5ms. I use a Jeti BOX for this. Once you have the Servo arm in the right position, you should then be able to measure the required length of linkage. I measure the distance between the plastic threaded sections of the ball ends and then add the depth of the holes in the ball ends. In this case the depth is 8mm so 16mm get added. So this meant a total length of 105mm with each end having 10mm of 2.5mm thread. To thread each end of the link I use a HSS M2.5 Button die. Titanium is not the easiest material to work. I use a Dremel cutting wheel to cut the links to length and thread each end in a lathe on the slowest setting. You’ll also need a good thread cutting compound too.
With the linkage threaded on both ends we can now test fit it. On one end I place an M2.5 nut which acts as a locking nut. This completes the linkage.
As previously mentioned, this build will use Dubro pin type hinges. A small bevel was cut into the stab hinge slots. See photo above. The bevel allows for the knuckle of the hinge and results in a flush fit of the elevator. Dry fit the hinges and elevator to confirm a good fit. If all fits well, remove the elevator and all hinges. To prevent the hinges being glued up, I apply a small amount of Vaseline to each hinge knuckle. A cotton bud makes this easy. The glue of choice is Pacer Hinge Glue. This is the same company that makes Zap CA. The supplied nozzle is used to squeeze the hinge glue into each slot. Ensure you get good glue penetration into the hinge slots. Just do one stab at a time. Then insert the hinges into the stab. Take your elevator and then squeeze hinge glue into the hinge slots. The elevator can now be carefully fitted to the stab. Any excess hinge glue that squeezes out can be wiped up with a damp tissue or paper towel. Set things aside and allow the glue to cure. The hinge glue actually etches into the plastic hinge material. In other words, it sticks good!
The hinge glue is supposed to be touch dry in 30 minutes but I prefer to leave it overnight. If all went well you should have a nice free moving elevator. The Servo linkage can now be reconnected to the control horn.
I'm building two Elements at the moment. The second one will be contra driven and will sport MKS servos. Below is a gallery showing the MKS servos being fitted.
The stabs on the Element are not adjustable. They are fitted with two carbon tubes that are 8mm diameter. The aft shaft is approximately 30cm long and the front one about 15cm long. In one end of each tube you should find a balsa plug. This balsa plug needs to be glued into the tube. I used 30 minute epoxy but you could use CA. The stab retention screws will pass through this balsa plug.
If the carbon tubes seem sloppy, consider replacing them with quality 8mm carbon tubes. This will reduce stab slop when assembled to the fuse. Slop is your enemy.
IMPORTANT - If you look closely on the bottom of the right hand stab, you’ll see a small disc about 10mm diameter. This is the hard point for the stab retention screws. In the middle of these hard points is a hole. I’ve stuck pins in the holes to highlight this in the photos above.
Now the reason the above is important is that the carbon tubes must be glued into the LEFT HAND stab. Again, I used 30 minute epoxy. Make sure you wipe off the excess epoxy that is squeezed out when inserted into the stabs. Any build up of adhesive will prevent the stab from butting up against the fuse.
Stay tuned, more to come!