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Pumpkin drop event

Hobby King SunBird Motorized Glider Build Thread

Joker 53150

Mmmmmmm, balsa.
Time for another build thread!

The Hobby King SunBird, a blatant ripoff of the Bird of Time, but smaller. And that works great for me, as the original BoT is too large for my local field. I've wanted to build this plane for a while now, but other planes keep getting in the way including building a pair of the Hobby King Red Swans - one is stock and the other which is almost done is a lightweight version just waiting for the covering to arrive. The build thread for Red Swan #2 is HERE.

The SunBird:

An example of the Bird of Time:

So I've had the SunBird sitting here for a few months, and now that the weather is usually bad I'm catching up on all of the stuff that has been on hold. General maintenance on a few planes is done, repairs is accomplished on others, so time to build!

To start, I'm going into this build with my eyes wide open. I've built a couple HK balsa kits already and know what to expect: crappy instructions, lack of information on the process, substandard laser-cutting quality, and a plethora of other questions. It's like assembling a car motor based on blog posts by a drunk providing instructions in gibberish.

But that feeling of accomplishment once it's done is addicting! Hell, anybody can assemble an ARF kit, and building a high-quality kit isn't as hard as most would imagine, but these HK kits have their own appeal.

Starting things off, I laid out all the parts to inspect for damage. None visible at all, a good way to begin (and unlike a true HK kit). The quality of the laser cutting was typical and many parts will need tracing with a #11 blade to get the parts loose. Compared to the Red Swan kits the person who did the laser programming did a good job. The areas you need to cut to get the pieces out makes a lot more sense, which is hard to explain unless you build both kits. But take my word for it, this isn't done too bad. I'm not too sure about the wood selection as some of the pieces seem pretty hard/heavy.


As mentioned, the instructions for these kits isn't the best. Actually the instructions suck, which is being kind. Parts aren't labeled and the instructions really don't show how the parts all go together. Typical. There is currently what appears to be a good and comprehensive instruction manual by a fellow builder uploaded on the HK website - hopefully it's accurate as I plan to follow it. It doesn't cover the wing but hopefully will eventually.





As with the Red Swan builds I start the build by cutting all parts out whether I need them or not. Granted this could lead to losing parts but I'll take that chance. Considering the instructions don't help the build process I want to be able to pick any part quickly to test-fit things.

As time permits I'll update this build log to hopefully help others planning to try this kit. I'm sure I'll make mistakes along the way, but that's life.
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Joker 53150

Mmmmmmm, balsa.
If building the RS doesn't crush your confidence as a builder you're ready for the SunBird! :) I'm starting on one of the wings and it is as frustrating as the RS was for me the first time I built it. The spar webbing is all made too wide throwing off all of the rib spacing, so I need to go through and shorten each of them. Just like the RS.

It really is a shame, as it wouldn't take much tweaking to turn either of these into a really nice kit.

Joker 53150

Mmmmmmm, balsa.
Let's get this party started! I'm doing wings first, because I really like how they look, it is what attracted me to this kit and the original Bird of Time. Plus, wings can be a real pain in the ass so I wanted to get it finished first so I could move on to the fuselage.

First step was cutting out all of the ribs. Some came out easy while others needed a bit of persuasion with a #11 blade. But the process wasn't hard to too time consuming. I didn't realize at first that the top and bottom spar caps were plywood, an interesting change from the other kits. I assume this will provide good strength while allowing the kit to use thinner wood than would be possible with balsa. Look at the wing tip rib and how little material is between the top/bottom caps - had the kit included standard balsa caps I doubt the rib would be one piece as it is. Just a guess though...

So once all ribs were cut out I matched them up and made piles of each type. These piles were then divided in half - half for the left wing and half for the right. That told me what I had to work with on each wing. The next step was trying to figure out which ribs went with the outer panel and which were for the inner panel. I used the "instructions" as a rough guide for this. In case I forgot to mention it, the instructions are crap, although the print quality is quite nice and the paper is heavy stock and glossy. Kind of like if Playboy Magazine had a magazine filled with 60 year old women from Siberia. Quality material except for the content. My apologies to any 60 year old women from Siberia who happen to read this. Back to the build. So everything is laid out in a way that makes sense, the only question area is the rib layout on the inner panel at the fuselage, but I'll figure that out when I get there. First thing I'm working on is the outer panel.


Along with the ribs and spar caps I also placed all the spar webbing in place, keeping an eye on the order in which they are installed. The webbing is smallest at the tip and gets larger towards the center. In theory, the webbing should also PROPERLY space the ribs out so all the pieces line up for assembly. At least that is the way it would happen in a good quality design. Surprisingly, that isn't what happens here. So in the picture you can see the webbing all held in place with spring clamps ($5.00 at Harbor Freight for a bunch of them, a GREAT deal!). The problem is that almost all of these webbing pieces are too long which keeps the ribs from lining up properly at the leading and trailing edges. Look closely at the thin piece of balsa at the trailing edge of the aileron ribs, how there is a gap at the left and right ends. That gap shouldn't be there and that piece should touch the wingtip rib and the large rib at the left of the picture. The rib webbing needs to be trimmed.


Here is a good example of the webbing problem. The web should be the same width at the wing spar as it would be if it were at the leading or trailing edge of the wing. This web is shown at the trailing edge where it is the width of a rib too long. So I simply mark it and cut that excess off. That process is repeated across the length of the wing. It isn't a big deal, but it's irritating that the kit manufacturer didn't bother to do this right. This is the same thing I found on the Red Swan kit.


Measuring, marking, and cutting, one pair of webs at a time. Once these are all done I'll cut the webbing for the other wing to make that assembly go faster. I'll also probably modify the webbing a bit by installing it between the top/bottom spar caps instead of in front/behind them. This will remove a bit of extra material and should still leave me with a good strong spar. I'm not 100% positive it's a good idea, but this wing isn't much larger than that of the Red Swan and I think what I've got in mind will be stronger than what I used on that build.


By the time I get to the wing tip you can easily see how much material I've removed from the webbing. The aileron will need to be trimmed slightly once the wing is glued up. The wing tip is made from two layers of balsa glued together. Kind of big and blocky, so I may cut a couple holes in it to lighten it up a bit. The dial caliper I'm using to measure the ribs isn't necessary, but it does work nicely for this project. You could get the same results carefully using a ruler or scrap pieces of balsa and a pen.


One item of note that again isn't really covered in the manual, there is some dihedral between the inner and outer wing panels. This angle is set by plywood sections which slide in between the spar caps. A couple notes on this... First, you'll want the angle of the rib to match the dihedral angle so when you slide the inner and outer wing sections together the ribs sit flush with each other. Also, to get this plywood joiner in place you need to remove the balsa on the rib between the spar caps. It appears that this only needs to be done on the two end ribs, and that the plywood will end up by the third rib. I'll know for sure as I get a little further in the build. But keep this in mind as you build because trying to remove that balsa from the ribs after everything is glued together so you can slide the joiner in is a pain in the butt (as I found out on the Red Swan). I'll probably glue some of the webbing in place at this part of the wing, then cut the ribs as needed for the joiner to fit, and then finish the webbing. Or something like that. It isn't a huge deal, but figuring it out now is better than waiting.


Joker 53150

Mmmmmmm, balsa.
As I'm assembling the wing and looking at the rib webbing I'm hating it more and more. Not because it's heavy or wouldn't work properly, but because most of the pieces aren't quite tall enough. They're all too wide as mentioned in the previous post, meaning I had to shorten them, but most are also too short meaning they don't go all the way from the top of the top spar to the bottom of the bottom spar. This isn't a huge issue for strength, but it irritates me. On my Red Swan I had the same issue and it's somewhat visible thru the lightweight covering, so this time I looked at some options.

The idea I settled on was moving the webbing from outside the spars to between them, as shown in the first picture. This involved cutting each piece down a bit. I also cut a third for each area so the webbing would be a solid block between each rib. This may have been overkill, but by doing this I'm keeping about the same amount of balsa between each pair of ribs as would otherwise be there. Plus I think it's just plain cleaner looking this way.


With picture 2 you can see that most of the webbing is now hidden between the top/bottom spars. It feels very strong, stronger than the Red Swan wing I'm going to cover in the next few days. I didn't move the webbing inside on the innermost part of this section of wing. You can see a couple pieces in this picture on the left side of the wing. This is because the plywood which forms the dihedral runs through the space between the spars. I could have skipped using full webs here but I had them cut and wanted that part of the wing to be nice and strong. I also had to get a little creative on the two ribs at the left, as the dihedral plywood runs right through them. I'm cutting them into two pieces and will re-install them as the wing sections go together. I'm not sure if it's the BEST way to do this, but I'm confident it will work regardless of what the instructions do or don't say (mainly don't).


Joker 53150

Mmmmmmm, balsa.
Now that the Red Swan #2 is done and waiting for a maiden flight I can turn my attention back to the SunBird. The starboard wing's main structure is nearing completion, and it has been trickier than the wing of the Red Swan for sure, but it isn't too bad. The trickiest part has been getting the dihedral plywood installed. I think most people would simply build the center and outer wing sections and then join them together, and that's what I did for this wing half. Now that I've got it mostly done I think it would be better to build the spar with the dihedral first and then add all the pieces to finish it up.

By the way, the instructions are crap so I'm still looking at the far left edge of this wing half (the eventual center of the wing) to see how I want to finish it. I know the center of the wing is going to be sheeted and the ribs are sized to make this work, but a little head-scratching is needed to make sure I don't build it incorrectly.


To do this I glued the two dihedral pieces together and let it dry. Then I came back and glued the top & bottom spars to it, and then clamped it in place. Everything was pushed flat against the bench to make sure it is flat. The ends of the spar will be a little long but are easily trimmed to final length once the wing is completed.


Here you can see the dihedral brace and spar caps secured in place waiting for the glue to dry. I had to cut four ribs to make room for the dihedral brace, and will glue the front ends back in place as progress continues. Gluing them right now would be a waste of time as there isn't any additional structure yet to help secure them. I'll run a length of carbon fiber for the leading edge which will certainly help, as will some webbing used to cover the spar at the dihedral.


Joker 53150

Mmmmmmm, balsa.
I ran across the picture below of a plane similar in design to the SunBird (I don't think it's a real BoT) and I really like the look. I've been waiting for a design that really grabbed me to use some transparent covering and this may be it. The original orange/cream design of the BoT was my first plan for this bird, but a design similar to this has me re-thinking that plan.


Joker 53150

Mmmmmmm, balsa.
It'll take me another month to figure out which transparent color to use. Yellow, Blue, Red, and Green are available from SoLite. I like blue but I'm a little worried that blue would be hard to see at altitude. Green and Yellow aren't high on my list, so I may end up with Red. Or Blue. I may have to just buy both colors and do a test piece to see which looks best.


Dedicated foam bender
Blue should be fine as long as you have some other contrasting color to make it visible at height. You'll want to have some kind of contrast or pattern on the bottom for visibility anyway. It won't take much, maybe a wide stripe or two chordwise in white...

All my dad's gliders were done in black and white, Black on the bottom of the wings and stab, and white everywhere else...seemed to work well but I like more variety on mine...
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I'm suffering from artistic block on my ASW-15 Sailplane. I think I'm going with opaque white and red trim. Maybe some black stripes on the bottom of the wings for visibility and orientation. My craftsmanship is definitely up to a transparent film, yet!

Joker 53150

Mmmmmmm, balsa.
Last of the progress for the day. The four wing sections are all connected and the final pieces are going together. I put the wing on top of the Red Swan to compare the size difference - it's certainly got more area.


Joining the wing halves isn't shown in the instructions, surprisingly... It appears people have had various ways of doing this, and I'm using what I learned on the Red Swan to try and make it work for me. I'm going with one big thick rib in the center of the wing like the Red Swan (although I deviated from this on the second Red Swan to save weight). This center section will be sheeted top & bottom and wing hold-down bolts will go through this rib block to secure the wing to the fuselage. This big block of ribs is made from a total of 10 pieces with the rib split between the spar. A plywood piece runs between the wing halves to hold the dihedral angle and also provide plenty of strength. Between the ply spar, mega-rib, sheeting, and the CF leading edge I expect this wing to be plenty strong. I'm not planning on any crazy aerobatics, but obviously want the strength if needed.


Joker 53150

Mmmmmmm, balsa.
I'm finally getting around to focusing on the fuselage. Wing work still continues in the background, but most of that is using epoxy to secure the carbon fiber leading edge (4mm thick wall tube instead of the leading edge stock that comes with the kit). I'll skip a couple of the early steps in the fuselage construction as it's almost identical to the Red Swan builds I've done, plus I forgot to take pictures as I was building it...

The trickiest part of doing the fuselage is getting the curve of the bottom sheet to match the curve of the sides at their bottoms. To make it work the kit includes some hardwood strips that get glued to the sides and the bottom sheet then glues up against these strips. It's hard to describe, but makes sense when you start assembling the fuselage. If done correctly the sheeting for the bottom will line up evenly with the fuselage sides and will require minimal sanding. It's a Hobby King kit, so good luck with that...!

There is also a plywood tray that mounts inside the fuselage, which the elevator and rudder servos mount to. This tray typically sits down on top of a pair of stringers mounted inside the fuselage, just like the Red Swan. However, at least on the RS kits that doesn't leave any clearance for the pushrods to clear the bulkhead at the rear of the plywood tray. Taking a cue from another build I saw information on I installed the tray UNDER the stringers. This will leave little room between the servos and bottom sheeting, but it appears that I'll have barely enough room for it to work. Worst case is I have to shim the servos up a little. This should lower the servos by about 3/16".

In general, this fuselage is very easy to build due to the low part count and box design. The key requirement is making sure the center section (with the servo try) is built square and the glue allowed to dry before moving on. I built this section last night, clamped some bracing in place to square it up, and let the adhesive dry all night before starting up again today.





Well-known member
Joker 53150
Coming alone nicely but 4mm thick wall carbon tube sounds a bit extreme (and heavy) for the leading edge.
Is it to provide impact resistance?

Joker 53150

Mmmmmmm, balsa.
I almost went with 3mm like I used on the Red Swan builds but felt the bigger 4mm just looked better on the larger wing. It is labeled as thick walked, but it certainly doesn't look that thick to me.

Joker 53150

Mmmmmmm, balsa.
The tail feathers were glued up last night and left to dry overnight. These are pretty straight-forward and easy to assemble, although it would be nice if the "ribs" used were marked somehow so you could tell which piece goes in each spot. A few of them are very close in size so there is a little trial & error in getting them set properly. Also, as with the Red Swan the ailerons are larger than necessary. I'm not sure why they make them this way but I'll trim the excess off as I finish the remaining details.

With the tail done I could do a quick mock-up to see how it all looks. This is a great feeling when you can see it look like an actual aircraft! All surfaces have some work to do before it's ready for covering, but it's getting there!



As mentioned, the tail was pretty easy. It is worth noting that the stringers on the vertical stabilizer aren't laser cut with the rest of the parts. You simply have to cut them to length from the leading edge material included with the kit. Cut, fit, sand, repeat. It isn't really visible in this picture, but it should be noted that there are two slots hidden under the horizontal stabilizer which are for elevator control. I guess you could consider one for the elevator and one for the rudder, but unless I'm missing something there is no way to use one of these slots for the rudder as it would interfere with the elevator control. I may end up cutting a simple slot in the top of the fuselage similar to the Red Swan but would rather find a more elegant solution.


Last (for now), there are a couple pieces of balsa included in the kit and it doesn't really show how/where they are to be used. It is pretty simple - you just glue them to the inside of the fuselage and they will make sure the canopy sits in the right spot when closed. I'll cover more of the actual canopy work soon.