RCRCM Sunbird – Build Report – Wings

Last Change: 19.05.2018

[⇐ Bill Of Materials] | [⇑ Overview] | [Fuselage ⇒]


Starting Point

In terms of shape, aerodynamics, control surfaces and colouring, the Sunbird kit is complete right out of the box. We quickly mount the wings to the fuselage for a test, using the connector provided. Fits! And looks good … from the outside. Inside, concerning components and control, there is utter emptiness. RCRCM’s kits address the advanced modeller.

We can tell by the holes in the wings where the servos are expected to go. How this can be turned into a precise, low-friction control mechanism generating sufficient deflection of the control surfaces is left to the modeller. A real challenge for the engineer. I love it!

Tools

For the linkage of the control surfaces you will have to do things, that the air plane modeller usually does rather reluctantly. We have to cut the rear spar in order to make way from the servo bay to the moving parts of the wing. Not just drill a hole into it but completely remove a small section of it.

This can be done without jeopardising the strength of the wing. The load-bearing structure of the Sunbird’s wing is the skin, some kind of exoskeleton made of carbon fibre. The wooden spars inside the wing are nothing more than spacers. The also guide the wing connectors, but they do not act as wing spars in the terms of aeronautical engineering. They are not supposed to carry flight loads or wing weight.

Now that we know we are allowed to do it, we have to figure out how it can be done. The wooden strips are firmly glued into the wing and hardly accessible. We purchased a flexible extension shaft and a set of small milling cutters for our Dremel. The warding files will be of great use as well.

Still, it is quite a fiddly task. With a wing span of just one and a half metres, the Sunbird is a small bird. There is little space to work in and to see what you are doing.(Readers of our German blog know our attitude towards work like this. See Baubericht F-86 von Great Planes.)

Clevises

As a little intermezzo, we make up our mind whether or not we are going to use the clevises supplied by RCRCM. They are crooked, twisted and even partly rusty (no. 1 on the picture below). They won’t fit and clutch the servo arms and the rudder horns precisely enough. And the seat of the threaded sleeves on the M2 rods range from very loose to too tight. Sorry RCRCM, this is scrap. We will do away with them.

Instead we use Kavan M2 clevises from our spares box (no. 3 on the picture). These are of much better quality. Unfortunately, they are slightly too long for the small offsets we have to deal with in the wing. We shorten the threaded sleeves by about 1.5 mm. This length will fit and leave enough thread for a secure link.

With the Dremel already in action on the clevises, we make ourselves a handy little tool. We need a device for the prototyping of the correct linkage geometry. We are going to mount and remove the clevises several times to and from their respective arms and horns.

To save us repeated trouble and to avoid the risk of damaging the Sunbird, we build a linkage rod with open clevises at both ends (no. 2 of the picture). One half of the fork is cut away. The pins of the clevises can easily be plugged in the bores this way.

Template for Deflection Angles

We cut a template for measuring and adjusting the control surface angles from a 1 mm sheet of white cardboard. It matches the wing’s profile between the aileron and the flap. The front of the profile is open, so that it can be slid onto the wing from the trailing edge. The area where the control surfaces move is cut out with the appropriate radius. A scale indicates the positions and deflections to be set.

We keep the template flat by some glued on strips of wood. The clamp hanging from lower edge is just weight to enforce its upright position. There is no other means for retaining it vertical.

This is ten minutes of work that will pay off soon. Not only during the build but also later when the settings are to be optimised. As an alternative, one could buy a generic tool for this, but we would not do without our purpose-built templates.

Ailerons

We start with the rudder horn. Using the warding files, we cut a slot into the aileron’s inner structure where the horn will be mounted. Its position is to be central under the linkage bulge of the upper wing skin. Do not make it too tight as the final position will be defined when the horn is glued in.

You only get one shot at gluing in or rather cast in the horn with UHU Endfest epoxy. In order to enforce the correct offsets and the alignment of the steering linkage, we attach the clevis and the push rod to the horn. This will allow us to check and adjust the position of the horn not only from the aileron’s side but also trough the servo bay and the linkage tunnel. Then we add an abundant amount of epoxy to the horn’s slot in the aileron, and we mean „abundant“. The resin is supposed to flow into the aileron towards the trailing edge and sideways alongside the front of the aileron. The rudder horn is then pushed from the servo bay trough the linkage tunnel into the slot.

For the final definition of the horn’s position, we fasten the aileron in its normal position with some masking tape. Two tooth picks are used to

  1. fix the clevis as high as possible under the linkage bulge and
  2. make sure that the pin of the clevis is exactly in line with the joint of the wing and the aileron.

Do not forget to check the position of the push rod in the servo bay and double-check symmetry of both wings!

While the epoxy is curing for about 24 hours, the wing must stand upright, leading edge up, aileron down. This will keep surplus resin from penetrating the hinge area, which would make our project come to a sudden end.

We use our template to move the aileron to the specified maximum throw of  +7 mm and -5 mm. To be sure, a generous millimetre is added in both directions. The end positions of the push rod are tagged with insulating tape. On the picture, the end positions are represented by the inner edges of the tape. This means that the clevis at the servo end needs to make 4 mm of travel between the respective full upper and full lower deflection of the aileron.

The length of the travel is what we want to know. No decision about the position of the clevis or the servo is taken at this point. To allow for the servo to develop maximum torque, our 4 mm of travel are to be achieved by the servo’s maximum rotation angle of about 60 degrees.

We do not need trigonometry for this. The naked eye can judge easily that the clevis must be hooked to the innermost bore of the servo arm. The arm is shortened accordingly and the bore is drilled out to 1.6 mm to accommodate the pin of the clevis. The connection between the clevis and the servo arm must be smooth-running but free of play.

With the servo still under control of the transmitter, the servo arm screwed on the servo drive’s cogwheel. This makes sure that the servo arm is set to its normal position and is forced to remain there during the following steps.

The arm is not supposed to be nice and perpendicular to the servo. Instead, it should be tilted one or to cogs backwards. This helps to achieve a better angle between the servo arm an the linkage rods, which run through the wing in a sloped way. In addition, this keeps the clevis from being blocked too early by hitting the hub of the servo arm when negative deflections are performed. This a a drawback of our very short servo arms.

Then, still under control of the transmitter and with the clevis and the frame in place, the servo is inserted into the bay. It is moved until the correct position is found. Correct position means in this case:

  • The servo can use its full rotation angle without being obstructed by any parts of the wing.
  • The servo arm is below the bulge of the servo bay cover.
  • The servo arm does not hit the servo frame in any position of its operating range.
  • There is enough room around the servo frame, so that it can be glued in with epoxy later.
  • The servo wires can be routed away from the servo and connected to the wiring harness in the wing.

We tag the position and remove the assembly from the bay again. The frame, this time without the servo in it, is now fixed in the bay with some drops of superglue and then cast in with a generous amount of epoxy resin (UHU Endfest) around the outer edges. Rock-solid.

Do not be upset if the servo does not fit the frame anymore after the epoxy is cured. The upper edges tend to tilt a little bit inwards when the frame is glued onto the concave inner surface of the wing. Chamfering the upper edges of the frame with the warding file will solve the issue. Or in our case, removing the QS-sticker from the servo did the trick as well.

Of course, if you are of the less patient variety and do not want to live up to Flying Tom’s building standards, you can do it in one step and fasten the whole assembly to the wing. Chances are however, that – when doing this – you are going to glue the servo to the wing skin by accident. This could seriously spoil the fun later, in case the servo needed to be replaced.  Even during the build, you are to remove the servo several times for adjustments and inspections. That is why we choose to do this the cumbersome way.

Flaps

The procedure for the flaps is about the same as for the ailerons. But there is one important difference: Whereas with the ailerons we strove for a more or less symmetrical deflection upwards and downwards, the flaps only need minimal upwards throw but have to go at least 80 angular degrees downwards for the butterfly. This makes higher demands to the geometry of the linkage. In addition, much higher actuating power ist needed to move the flap to such angles. Higher torque is provided by the servo and, in turn, the linkage must be able to cope with it.

We want to use the servo’s full rotation angle. So, with the servo arm in its normal position, the flap must already be half way down. Therefore we tape the flap to about 40º and adjust the servo arm to its middle position. Then the rudder horn is positioned and glued in with

  1. about 2 mm (diameter of the tooth pick) of air between the clevis and the wing surface above it to provide enough headroom
  2. and the pivot of pin which links the clevis to the rudder horn excatly in line with the rear edge of the wing.

The rudder horns of the flaps will protrude deeper into the wing as the ones of the ailerons do. So either the servos need to go closer to the leading edge of the wing or the linkages have to be shortened. We choose the latter because we want the servo arm to be exactly under the bulge of the servo bay lid. Unfortunately, our linkage is already clevis to clevis with no spare threaded rod in between. So we have to cut about 1.5 mm from each of the sleeves of the clevises to get the linkage short enough.

But this does only half the trick. The clevis collides with shaft of the servo arm and gets stuck about 10° short of its intended extreme position. This is remedied by grinding off some material from the clamp.

Voilà: 5º up, 80º down.

Wiring Harness and Fuselage Adapter

Der Flügelverbinder passt perfekt.

Die Öffnung für den MPX-Stecker des Kabelbaums wird leicht vergrössert. Wenn der Stecker auf der fest installierten Buchse am Rumpf steckt, soll sich der Flügel gut darüber stülpen lassen. Die Position des Flügels am Rumpf ist durch den Flügelverbinder und die beiden zusätzlichen Metallstifte gegeben, nicht durch die Stecker des Kabelbaums.

Auch darf das Herausziehen des Flügels nicht den Stecker aus der Buchse reissen. Die Kabelverbindung soll gezielt durch manuelles Lösen der Steckverbindung getrennt werden, nachdem der Flügel wenige Zentimeter vom Rumpf weg gezogen wurde.

Der RCRCM-Kabelbaum hat erfreulicherweise die nötigen Überlängen, damit die Servoanschlüsse nicht auf Nimmerwiedersehen im Flügel verschwinden. Die Kabel lassen sich soweit aus dem Flügel ziehen, bis die Stecker sicht- und greifbar werden.

Das hat allerdings den Nachteil, dass einige Schleifen der ziemlich dicken und sperrigen Kabel so im Flügel versorgt werden müssen, dass sie nicht in den Arbeitsbereich der Servos geraten. Wir klemmen (nicht kleben!) sie einfach mit ein paar Schaumgummistücken, welche wir von den Servoschächten her in den Flügel quetschen, ausserhalb der Gefahrenbereiche fest.

Servo Bay Lids

We are almost there. But now look at this. According to our taste, the lids are far too bulgy. The sleek appearance of the wings is completely spoiled. The slim X08 servos underneath the lids do not need this extra room. They could do with simple flat ones, except for the servo arm, that is. The plan is to add another two working hours or so to our project’s delay by making a set of new lids by ourselves.

OLYMPUS DIGITAL CAMERA

The lids are made of 0.3 mm aluminium sheet with the bulges saved from the original parts.

OLYMPUS DIGITAL CAMERA


[⇐ Bill Of Materials] | [⇑ Overview] | [Fuselage ⇒]