(This post was originally published on flyingtom.blog on 01.07.2017.)
It’s always the same story with used cars: As good as new. First owner. No accidents! A real bargain! – But what is the actual mileage? Are there any hidden damages? – Once you forked over the money and got out of the door, you picture the seller chuckle to himself. He knows and you do not. But you will learn – the hard way, of course.
The same applies to model airplanes. There is an obvious mismatch between the statistics of mishaps and disasters I witness or I hear of at the airfield or at the slope and the sheer number of used models in so-called mint condition for sale on the web auction platforms. „Only a handful of flights, premium building standards, no crashes nor hard landings, but all guarantees excluded.“ Flying Tom’s Topsky 3.0 Disser DLG happens to be one of these „sold as seen bargains“, with the „seen“ part being reduced to some low-resolution photographs in the internet.
We’ve been having issues with the Topsky right from the start: Besides some minor repairs on the control surfaces mentioned in the add, the right wing was broken somewhere in the past and showed traces of heavy repair work leading to unbalanced weight distribution. Then, after about 10 minutes of ground testing, the elevator servo burnt due to what was suspected to be a short circuit. It also destroyed its neighbouring rudder servo. (see Hangsegeln auf der Grossen Scheidegg, German only). Two new servos had to be bought before the first flight.
And now that we finally seemed to be ready for some serious flying, the aileron servos, both, spoil the fun. Everything looks fine from a distance. But as soon as there is some load on the ailerons, they stop moving. Instead, their respective servos move inside the wing, driven bei the steering linkage. The servo bays turn out to be far too large and the covering glass fibre tape does no longer hold the servos in place. The hard foam material filling the wing is worn out. Form closure is long gone and the tiny balsa chocks squeezed in do not really help.
One could resort to desperate measures and simply glue the servos in. An often used and fast, yet short-sighted solution. This would not comply with Flying Tom’s building standards. So we decide to use servo frames supplied by RCsolutions. These are rather expensive compared to other frames for the Dymond D60 servo, but they fit the „holes“ in our wing perfectly.
We also evaluate the wooden frames supplied by Horizon Hobby (Artikel-Nr. 058HALD60). Their advantage is that the servo remains lying on the upper wing skin with no frame parts in between, whereas the RCsolution frames add some 0.5 mm design height. On the other hand, they are rather beefy and would require much larger wing openings and a lot of rework. We stick with the ones from RCsolutions.
We test fit the servo and the frame. They are a perfect match. Even without the screwed-on fastener, the servo is kept firmly in the frame. Be careful when doing this as the frames are very delicate while not glued to some surface.
The wing openings need to be enlarged to the size of the frame to allow for an easy installation and to remove the frayed wing skin material around the opening.
Surprise, surprise! – The wing connector rods protrude into the servo bay. We have to remove some material from the frame to make it fit between the upper wing skin and the rods.
After having double-checked their position and with the servos still plugged in the frame, we glue the frames to their positions using epoxy resin. Do not be too generous with the glue at this time. You want to avoid glueing the servo to the frame or to the wing skin. A drop of superglue will help to keep the assembly in position while the epoxy cures.
Then we remove the servos from the frames. Now it is the time to be generous with the glue. The empty spaces outside the frame are cast with epoxy to finally fix the frames in their positions and to add some strength to the wing again. The large hole we cut into the skin may have weakened it a little bit. However, do not overdo it. To much resin will add weight and you do not want rock-hard overflows inside the frames or on their top surfaces where the servos and the fasteners will go.
The servos fit tightly into the frames. They would hardly ever fall out but we will use the screw-on fasteners anyway. The screws are a little bit too long. Their points would hurt the upper wing skin if not shortened by approximately 1 mm. Then we cover up the holes again with glass fibre tape. But this time, the tape is not used to keep the servos in place but only for aerodynamics and to keep out dust and moisture.
Back in business, up where we belong …