Monday, September 10, 2012

Schwing 88: Assembly log #1

I started few days ago the assembly of the Schwing 88 from, everything went well for the moment. No surprise, only smooth step by step building.

The fuselage:

The main work consists in gluing the ballast tueb and the servo tray in the fuselage. Before this operation, I prepared the ballast tube. I used some  3mm plywood as tube end stopper. After cutting it at the approximative diameter, I made a hole in the center, then fixed it on my mini-drill and sanded it to obtain a perfectly round piece, at the exact diameter. I glued it in place with some expoxy, then installed a 2mm carbon rod in front of the stopper, piercing the tube side to side.A small piece of hard foam is inserted in the tube to act as a shock absorber.

After opening the front of the tube to allow the ballast slug to enter/exit, I installed a small wood block with a screw nut inside. The wood block is rounded a bit to go in the bottom of the fusleage. The ballast tube is glued in the fuselage using SikaFlex 11FC+ Adhesive and Joint Sealant. I added a piece of hard foam in the fuselage pushed at the rear end of the ballast tube to secure the ballast tube against vertical shocks at landing.

I then glued the servo tray in place after sanding the inner side if the fuselage to improve fixing. I used 10 mn rapide epoxy first. Then I applied some fiber class going from the tray to the fuselage side in order to perfect the fixing. I always do this on my plane and have never been disapointed. This is very robust !

The next operation is the most tricky. As the fuselage is thin, and well occupied with the ballast tube, the routing of the wires from the roots to the receiver is a bit challenging. I finally proceed by starting from the receiver side, routing 2 cables on each side of the ballast tube till the connector location.

As there is no thickness at that place to improve the fixing of the green connectors, I did a small wood plate going from one side to the other side, inside the fuselage, and glued with rapid epoxy. I proceeded to the soldering of the wires on the green connectors. when secured with some hot glue, I epoxied the connectors in place, on the inner wood plate. The wire are then pushed in the fuselage along the ballast tube, and to finish, a piece of hard foam is inserted behind the joiner holes to avoid the wires to move. I forgot to say that I had to enlarge a bit the connector holes. The height was fine, but they weren't large enough. Not a big deal.

After evaluating the 2 approaches (elevator servo in the fin or in the front, on the servo tray), I finally choose to install the servo on the servo tray for accessibilty and maintenance reason. It is also true that my exemplar arrived with the bell crank already installed and without elevator servo bay in the fin. Commands are made from carbon rod, so I used MPjet brass coupler and MPjet plastic clevise for the rudder. I will use the same on the servo side.

Let switch to the wings:

After locating the exact position of the servo arm to be aligned with the control horn, I glued the modified MKS 6125-mini wood servo frames in place with rapid epoxy. I use the 6125-H instead, but the servo frame aims to block the servos horizontally. Then, I glued the servos in place with the servo arm at the right position to have an asymetrical movement for the flaps to obtain the correct deflection in each direction up/down for both flaps/ailerons.

Control rods are made from M2.5 threaded rod and 2.5 metal clevises. On the control surface side, I have to grind the top of the clevises because they were touching the command exit skin. On the servos side, I also grinded the metal clevises to connect them of the first hole of the arm and still allow some movement. The clevises are then immobillized on the threaded Rod, at the exact length to suppress any slop. Once connected to the servo, it give very tight command with no slop.

I then prepared the servos harmness for the wings using MPX green plugs and universal connector, crimped. After soldering the 6 wires (2x3) to the MPX connector, I used hot glued to immobillized everything.

What is next ?

Not much to do to complete the assembly.  On the wings, I just need to glued the green connector in place in the root, then close the servo bay with covers.  on the fuselage, I will finish the control command on the servo side in the fuselage, complete the radio installation with the battery and the receiver and proceed to the balancing (at the very end).

Stay tuned !

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