The Allure Bipe stabs are identical to the Alchemy/Alchemy Pro. As we will potentially be pushed for weight, MKS HV69 servos will be used in both stabs.
The HV69 comes in at 21 grams and when one compares this to a Futaba BLS173 at 28 grams a total saving of 42 grams is possible. That can’t be ignored!
The stabs don’t come with the Servo holes pre-cut in them. This allows the end user a few options with regards to mounting the servos. I’ll be mounting the HV69 servos on the bottom skin as done in my previous CK Aero builds. The factory supplies a plywood doubler that suits BLS173 sized servos for the stabs. I made new ones to suit the HV69 out of 2mm light ply. The HV69 is mounted with two wood screw only. Rubber grommets are supplied but they allow too much Servo movement in the mounting for my liking. I turned up some small stepped bushes on the lathe to hard mount the servos. Hard mounting is fine for EP applications. GP is another story!
You have some freedom on where the Servo is located in the stab skin. One constraint though is the location of the hard point in the elevator. You will also need to decide upon the control horn to be employed. In this case, Precision Aero carbon horns were used. We manufacture these control horns in house on our CNC router. As can be seen in the pictures below, masking tape was used to mark out the horn and servo locations. You will also need to decide on the Servo arms as this will also help determine the Servo position in the skin. We use MKS 6mm Alloy hubs and Precision Aero 17mm Carbon arms to suit. With all the components identified, the Servo centreline can be marked out. This was made 20mm out from the stab root. Another line parallel to the Servo centreline can be marked 17mm further out towards the tip. This line represents the centre of the linkage. But wait, we missed one critical part... That is the linkage and ball ends to be used. I’ve been using these 2.5mm dual axis ball ends for many years. They are simply the best we have found for zero slop. The critical dimension needed off them is the distance between the flats on the balls. This can be measured with a vernier caliper and for the 2.5mm variety is 4mm. Half of this 4mm dimension (2mm) is needed to mark out the elevator control horn location. I mounted the elevator horn so the tab to be glued in is on the edge of the elevator spar. If you fold open the elevator it’s easy to measure the distance back from the edge and transfer this to your Masking tape. Remember to allow for the ball joints on you linkages (2mm) when marking the slot in the root to tip direction. Once I was happy that both elevators were marked out correctly, I used a sharp scalpel to cut both of the slots. Take your time here and you’ll end up with a very neat slot. A small needle file can be used to tidy up or fine tune the slots. The control horn should sit flat on the elevator skin. If it doesn’t, you need to make the slot deeper until it does. A Dubro hinge slotting tool that fits into an Xacto handle is great fo picking out the slots. The horn wants to be a neat fit into the hard point and I set them so they are square to the skin. The end of a steel ruler is perfect for checking this. Each elevator horn was roughed up in the area to be glued with 120 grit sand paper. This helps the epoxy bind to the carbon. Each horn was then glued in with 30 minute epoxy mixed with glass powder. Ensure the slot is filled with epoxy and the tab of the horn is coated with epoxy. This includes the slot milled into the tab. The slot acts as a key once the epoxy is cured. Any excess epoxy can be wiped off with a cotton bud soaked in IPA or metho. I leave just a small fillet of epoxy around the horn. If needed, tape the horn into position so it doesn’t move whilst curing. Remember to cut a small relief into the stab skin so the control horn is cleared with down elevator deflection.
The next task was marking out the Servo location. The Servo centreline has already been marked. We know the distance between centres is 35mm on the HV69 Servo. The light ply Servo trays I made have pilot holes milled into them too. Marking out the screw holes is as simple as using the light ply tray as a template by lining up the pilot holes. The ply tray is aligned along the centreline to ensure any internal structure is cleared. A centre punch is then used to create a dimple where the Servo screws go. The dimple helps guide the 1.5mm drill bit when making holes in the skin. The ply servo trays can now be glued onto the inner stab skin and held in place with the Servo screws whilst curing. Once the epoxy has cured, the Servo holes can be cut into the skins using the Servo trays as a guide. A sharp scalpel and small sanding bar make light work of this job.
The Servo can now be fitted to the stab. Using a JETIBOX or Servo tester, set the Servo to neutral position or 1.5mS or 1500uS. With the MKS 6mm Hubs it’s possible to get almost perfect alignment of the Servo arm. Simply rotate the hub on the Servo spline to obtain the optimum alignment. Ball ends are fitted to the control horn and Servo arm with M2 x 10mm stainless steel screws and M2 Nyloc nuts. The length of the linkage can now be measured. If you’ve done an accurate job both links will be the same length. I made the links from 2.5mm titanium rod and run a 2.5mm thread on each end of the link.
The stabs are done.