RCRCM have done their job. There is hardly any rework needed.
However, when inspecting the fuselage, the seam between the two halves of the hull catches the eye. The black carbon material shows off through an unpainted gap of a width of about half a millimetre. This is quite an unsightly contrast to the otherwise immaculate white surfaces. Maybe later, once the Sunbird has reached flying status and cosmetic procedures are worth the effort, we will tackle the issue with a fine paint brush.
In addition, some minor flash consisting of excess colour and resin is seen and felt alongside the seam. But this is neither a risk of injury, nor does it affect the flight characteristics. Sanding it down would make the black seam even wider. Therefore, we decided to leave the flash as is.
Tow Hook for High Start and Bungee
Considering the targeted flight weight of approx. 700 grams without ballast, the Sunbird still qualifies as a HLG, a hand-launched-glider. But we know that even these flyweights quickly grow heavy when a lot of re-launches are required due to low winds or missing thermal updraft. First you might be happy that – this time – you managed to throw it without dislocating your shoulder. Then you hurry to grab the controls on the transmitter to initiate the final approach and land again safely.
„Flying is landing“ says the smart pilot. Right! But a test flight is no use if it is not long enough for finding the centre of gravity or for judging the behaviour of the controls. And, of course, airplane models do not age on the basis of accumulated flight hours – as the big ones do – but on the sheer number and quality of landings. „Less landings is more flying“.
To cut a long story short: We need a tow hook. Not for daily business but for exceptional situations once in a while.
The hook is to be mounted approx. 15 – 20 angular degrees in front of the centre of gravity (cg). Be aware that the cg’s position is not on the lower surface of the airfoil where we check it before the flight. It is in the fuselage substantially below the wing. Where exactly, we don’t know at this stage. And perhaps we will not bother to find out when then model is completed. Our hook is supposed to be multi-purpose. So an educated guess about the cg will do nicely.
For installation we sand down the above mentioned flash to allow for a smooth fit of the hook.The Simprop hook we use (see bill of materials) matches the curvature of the fuselage’s cross section perfectly.
The internal layout of the fuselage is dominated by the ballast tube. It is perfectly clear where it has to go: The cg of the tube – empty or fully loaded – must meet the cg of the fuselage to avoid cg creeping when using ballast. So the tube will be squeezed in between the wing connector and the tow hook fastener with its front end protruding some 10 millimetres into the cockpit area.
To allow for adding and removing ballast, approxiately 40 millimetres of room in front of the tube must remain accessible through the cockpit opening. That means that all other components will sit either very close to the bottom of the fuselage leaving the top half open for access, or they need to go to the front half of the cockpit area or to the nose.
Moving the components towards the nose is no threat to the cg’s position. According to current estimates, we will have to add no less than 200 grams of ballast to the nose anyway. This is quite a lump for a small plane like this.
The only purpose of the ballast tube is to provide a means for adding weight to the model in a cg-neutral way. Such need may arise in competitive conditions where high speed goes before good gliding properties or when very strong winds are to be taken. The tube’s position is in the middle of the fuselage, more or less alongside the longitudinal axis. There is nothing to fasten it to. We make a wooden bulkhead which is insert into the fuselage right after the cockpit. It will keep the front end of the tube in its position.
Do not forget that the weight of the ballast tube may be as high as 360 grams. Sufficient and precisely cut contact surfaces are required for a sturdy mount. However, we also need some recesses: For the canopy closing mechanism at the top, for the rudder and elevator linkages at both sides and for the wires and antennas at the bottom.
The bulkhead ist glued into – or rather cast into – the fuselage with epoxy. We are lucky enough that the tube does not need any glue to fasten firmly. Its rear end is supported by the fuselage floor. Due to that and the final position of the bulkhead, the tube is canted a little bit in the bulkhead’s bore and kept under tension. We are not able to move it even a fraction of a millimetre now.
We are not taking about adding more weight on purpose here, as we did with the ballast tube. This is about pushing the centre of gravity (cg) to where it is supposed to be for a start. Our first attempts to get the cg into a flyable range are kind of frustrating. It is more than obvious that the nose is far too light. The plastic bag on the picture below contains a complete set of balancing weights for car wheels. Its 240 grams just manage to move the cg to the foremost position of 60 mm. With the smallest battery we are evaluating for the Sunbird under the canopy, that is. So things can only get better from here, using a heavier battery and/or configuring the cg a little bit less defensive.
But anyway, there is just not enough room for theses dozens of square pieces of lead. We need a more compact solution for this. The plan is to cast all or at least most of the lead bits required into a single piece which matches the inner shape of the Sunbird’s nose.
The shape is copied with a positive model made of red Plasticine. The bolt helps to push it into the nose, and its thread keeps it from slipping out of the Plasticine when the model is pulled out. The negative model we use as a casting mould for the lead is made of modelling plaster.
We are not going to melt the expensive wheel balancing lead. Much cheaper lead pellets of 3 mm diameter are used instead.
Always keep in mind that lead is heavily poisonous. Do the casting outside to avoid any lead vapours in your workshop and be sure to wash hands, tools and surfaces afterwards, before doing anything else. And, this is very important, let your mould dry thoroughly before the cast. Otherwise you are likely to get a splash of molten lead into your face, because any remaining moisture will explode.
We choose to scale our fixed nose ballast to 150 grams. The rest will be added later in a more conventional way to retain some flexibility. The eye bolt helps to position the ballast properly in the nose. It is removed once the lump is glued in.
Again, we use epoxy to glue it in. Any remaining gaps between the ballast und the fuselage hull are also filled with epoxy resin. One of the less brilliant ideas of mine is to add 1.5 mm lead granules to the filler resin for more weight. Unfortunately, the weight gain of this method is negligible, whereas the mess caused is not. The removable ballast pack with additional 30 to 50 grams of trim weight will cover it up.