last update: 10.03.2018
On this page we are going to
- add the rudder horns to the control surfaces
- assemble the fuselage and the tail section
- install the steering linkage for pitch and yaw control
1. Rudder Control Horns
Had I followed the building instructions, we would be looking at this from the other side now. PCM’s proposal is to access the rudders from the upper side and to implement a string and spring concept to control them. These guys certainly know how to build a lightweight model plane. I decide to go for a less fiddly and longer-living conventional steel rod linkage at the price of a little weight punishment. Running these rods on the upper side of the fuselage through the inner angle of the v-tail occurred to me as too crammed a solution. The horns have to go to the lower side.
2. Fuselage / Tail Assembly
There is an issue with the assembly of the tail boom of the fuselage and the v-tail section with the control surfaces. The dress rehearsal reveals that the boom’s diameter is slightly too large for the opening in the tail. Or the tail is too tight, depending on which side you are looking from. I got a perfect match on the Taser UP built in parallel for the Dr Jekyll And Mr Hyde project, but the F5J here needs a little help. The first idea is to sand the parts to fit. However, the thickness and delicacy of the carbon walls is such that I don’t dare to remove even these few excessive hundredths. Messing with the seating here would also mean jeopardising the preset longitudinal dihedral of 1.8 degrees. (The picture shows the UP Taser.)
So I resort to plan B. The parts are joined flush. There is enough mating surface to glue them firmly to each other. But I prefer to apply epoxy (UHU 300 endfest) rather than CA (superglue) as proposed in the manual. This will add some extra strength. A wooden plug will make sufficient interlock to retain the parts in the correct position while the resin is curing.
Please note my high-tech trestle with matching colours, extra weight for safe footing and special padding to avoid damage to the leading edge of the wings. The model is to stand upright in the ski boot all by itself. Its wingtips do touch the workbench, but it is not leaning against it nor against the red box. Once the perfect position is found, the wing tips are taped to the workbench to secure it.
The purpose of this setup is to measure and define the relative positions of the wings and the tail, with the front edge of the workbench being the reference line for the airplane’s lateral axis. Any force applied here may slightly bend or twist something temporarily, thus spoiling the symmetry of the persistent result.
„You just couldn’t do it the way it is proposed in the manual, could you?!“ you will ask. Erhh, no. PCM tells you to peer over the wings from the front and to adjust the tail section until optical symmetry is achieved. I don’t like that keen eye approach. There is too much hazard.
The second proposal is to lay the airframe upside down, with the wings and the two tail fins touching the ground. This looks sensible, at least from the geometric point of view. The flat surface is a perfect reference for symmetry. But you will notice that the wings bend down no less than 25 mm as virtually all the weight of the airframe is put onto the tips. They will never have to take such a load in real flight operation. This method makes you rely on the perfectly symmetrical elasticity of the wings. In our application here, this could very well be done with viable results thanks to the outstanding quality of the PCM kit. Flying Tom, however, chose to apply the ski boot method for more inherent accuracy. Once you’ve adopted that popular science approach to model engineering, you just can’t help ending up in things like these.
Then it’s the point of no return: With the model still on the trestle, remove the tail, add the epoxy, put it on again, adjust painstakingly and leave alone for curing (24h). The wooden plug will hold the position of the tail’s rear end. The front end is pressed to its seat by a toothpick taped to the boom.
When done, the longitudinal dihedral must be checked. Just in case you’re worrying about how to measure a v-tail setup, I recommend Eckart Müller’s exhaustively comprehensive blog entry at rc-network.de in German. Based on PCM’s specifications we were aiming for 1.8 degrees. The actual reading is 1.6. That is close enough for stable flight characteristics in interaction with the centre of gravity yet to assess.
As mentioned above, I am going to to replace the original string/spring linkage of the kit by conventional steel rods. The selected 0.8 mm rods are more like wires. They tend to evade sidewards at high loads if not guided properly. That’s why I’m also using white polystyrene tubing with an inner diameter of 0.8 mm – of course – and 2 mm on the outside. Rods and tubes together weigh slighty more than 5 grams per metre. About 1.2 metres are required but the tube doesn’t have to cover the full lenght of the rod. The weight of the string solution is 1 gram or less. For me, considering the longevity gained and the hassle saved, this extra weight is an investment in a worthy cause.