27 February, 2006

Days 4 & 5 with the Tern: Fuselage Part 3

Today I have come very close to finishing the fuse. It is covered, and I am in the process of attaching the control linkages. I forgot how hard it was to cover tail feathers once they are installed on the fuse, I won't make that mistake again.

Not many pictures, I don't want to give too much about the color scheme away before it is done, but here is a tease. These are the covered ruddervators.

25 February, 2006

Day 3 with the Tern: Fuselage Part 2 - Control problems.

The day started a bit strangely. I was blown out of the water by Mrs. Tillman in match play this morning. She had me down 2 on the front nine, and I managed to squeak back to even by the turn with a par and a birdie on 8 and 9. I think that the hot dog did me in, because straight out of the gate on the back I was down 1, then 2, then 3 by the 12th hole! Every time I would nail a great shot, she would sink an impossible putt to push the hole. I just couldn't catch a break, and she closed me out 3 and 2 after 16.

Perhaps that was just the beginning of the control problems. When I got back to working on the Tern, I started tinkering with my pushrod arrangements. I had planned to put the servos in sideways and stacked in between bulkheads 3 and 4 (see yesterday's post for the cutaway). I quickly realized that this wasn't going to work.

With some quick (OK it was an hour and a half of tinkering) thinking, I decided to move one servo aft of the towhook block, and keep one forward. I could then run both pushrods out over the aft deck and bring them out with the tail boom.

After getting that all sorted out, I closed up the top of the fuse and got B1 ready for the nose block.

To help me keep track of all the stuff I am trying to get done this winter, I made a to-do list.

24 February, 2006

Day 2 with the Tern: Fuselage Part 1

Today was my day off, and I was much more productive than normal - mostly because the weather was not nice enough to golf.

I began with a quick finish of the tail feathers. I drilled the mount plate and rounded the forward and aft edges, and then took some care to mount the stabs at the same height on the plate and the same decalage. After that was done, I moved on to the fuse.

Nothing makes me revise my thinking like actually building a fuse. I had planned to use a thin spruce plate for the towhook, and run it through a mousehole in two bulkheads. I decided that this wasn't the best plan, so I eliminated one of the bulkheads and made the towhook plate out of basswood. Much as is done on the Spirit, I used 6-32 blind nuts in three locations to give some Cg flexibility for the tow point.

The only other change that I made was to use basswood for B4, which is the main loadbearing bulkhead. It takes most of the load from the hookplate, and is the tie to the wing bolt. The aft deck went together nicely, and the Magic Magnet system made the height offset a breeze.

The aft wing bolt point is just between the full fuse bulkhead and the first bulkhead on the aft deck. While the double triangle stock at the aft deck attachment point has stiffened that up quite a bit, I am counting on the wing box to carry some of the tail load for big positive moment maneuvers. Given the stiffness of the tailboom, this shouldn't be a problem.

Once everything was test fit, the assembly of the fuse sides with the Magic Magnet system was the easiest that I have ever seen. I used the vertical stands to provide pressure at each bulkhead (replacing my fingers), and was able to get it perfect in two shots (one for each side). As you can see, I used some presses to hold the fuse down in the center so that it wouldn't twist when I had only one side on. I have to say, that worked even better than I had hoped, and this seems a truly great tool for making slab sided fuselages.

Overall, I call this a pretty good second day. The only problem was a lack of planning that caused me to glue some bulkheads in place before I had made cutouts for the pushrods. That may come back to bite me, as I couldn't get the pushrod holes cut at the angle that I wanted, but I don't think it will be anything more than cosmetic.

23 February, 2006

Day 1 with the Tern: Tail Feathers

I started the build of the Tern today, chosing to begin with the tail feathers. The assembly of the stabs and ruddervators went fine, but I suppose that I would feel pretty bad about myself if I couldn't assemble some balsa stick parts over a plan. This was the first time that I used my GP Magic Magnet system, and it worked really well. It isn't tremendously better than T-pins for this type of build, but I do look forward to using it on the fuse. Here is a picture of the stab in work.

You can see the magnetic hold downs that I was using around the frame.

The part of today's work that I was the happiest with was the fabrication of the stab mount block. I planned this as a trapezoidal piece of balsa that would act as both a mount for the stabs (they are offset from horizontal at 39 degrees), and as a receiver for the composite tail boom. I cut a piece of balsa to the right thickness (5/8), and then cut the bevels in the sides with my bandsaw. It turned out nearly perfectly on the first try.

I just couldn't resist taking the stabs and taping the rudders to them and then pinning both assemblies to the mount just to see how it would look. As I hoped, it came out very well. Not bad for a night's work.

22 February, 2006

Design of Spars

In working on the Tern 2VS, I have come across a lot of information on the internet on how to size a spar for bending moment. Sadly, a lot of it seems to be generally wrong, and relies upon the underprediction of material properties to avoid disaster. I decided to evaluate how wrong the common method was, so I derived the correct equations. I put the writeup together here.

20 February, 2006

Preparing the Spirit...

This morning I set out with one task: finish the Spirit's ailerons. I finished the right aileron, got it hinged with no problem, and then assembled the wing for a radio trial of everything.

I immediately realized that the servo arm on the flap servo was too long, it was hitting the ballast box. So I took the wing apart and changed the servo arm. It was at this point, that I noticed (to my horror) that the flap servo had no position control. This could only mean one thing, and a dissection of the servo revealed that one of the gears had indeed shed its teeth.

It was off to the hobby store to find a replacement, and luckily they had one last Hitec (no box, no horns and no grommets). I snatched it up for the deal that they offered and ran back home.

Finally I completed the plane, and with all of the servo problems now fixed, the radio mixing is a dream.

As would be expected, it snowed last night, so now I have to wait some more for a trial flight.

That gives me time to start my next project: the Tern 2VS.

19 February, 2006

I hate hinges

I went to rehinge the Spirit ailerons today. Well, I really went to do it last night, but I couldn't find my CA hinges. I had used four in the big giant pack, and I think I threw the rest out.

So today I go and get some balsa and some hinges, and then I get the left aileron back on. I go to reconnect the servo, and I notice that I am still having this horrible problem with center point drift. I assumed that it was a problem with the radio, and that I was trying to do too much mixing. Well, I played with it for a while, and noticed that at the top of the aileron throw, the servo was bouncing, and after each bounce, it would lose a little ground against center.

Each time I would recenter it, and neutralize the trim electronically. Then the problem would continue, until it had slipped so much that it couldn't even be centered any more. Thats when I noticed that the horn was slipping against the servo itself. I pulled it apart, and sure enough, I was using a Futaba horn with a Tower servo. The shaft of the servo is the right size for the horn, but the teeth within the horn are the wrong shape.

I ended up going back to the hobby store and buying a pack of DuBro horns that are for Tower servos. Now I don't seem to be having any problem with it. Let this be a lesson (that I obviously needed to learn). Keep your servo horns segregated by manufacturer. That mistake could have cost me a glider!

After completing the left aileron and the flap servo horn changes, I just didn't have anything left to do the right aileron. Thank goodness tomorrow is President's Day. Thank goodness that I have such a great local hobby shop (Doug's Hobby ).

The moral of this story is that nothing can make you feel like an idiot quite like actually being an idiot.

18 February, 2006

Why we build in the winter...

At this point of the year, I can't decide whether I want to be more upset that I can't go golfing, or can't go flying.

It really doesn't matter, but at least my workshop stays warm.

Both of these were last Sunday, but it is doing the same thing today.

14 February, 2006

Finishing the Spirit 100

With the Dude out of the way, I set my sights on completing my Spirit 100. On my initial installation of the electronics, I had everything nicely in place, but with the ballast required to balance out the huge tail, I had little room left in the forward compartments for the stuff that needed to go there, most notably the switch.

I tried a lot of configurations without avail (receiver under wing, wires over and under servos). I eventually settled on getting the switch out of the way with a Du-Bro switch harness and charging jack behind the servos (mighty tight installation, but a good spot for it).

After this was done, I had little need to frequently remove the hatch, so I ran the wing servo connections (should have used a bundled cable) and the battery lines in between the polystyrene cockpit and the balsa frame. After I plugged in the Rx, I had just enough wiggle room to get the rubber band back in place that holds the whole thing down.

After a few flights, I will likely add a spruce bolt plate and drop a 10-32 wing bolt through the hatch where it attaches to the forward wing bulkhead. Tomorrow (with any luck), I'll rehinge those sloppy ailerons and be ready for business.

10 February, 2006

Sophisticated Dude

After pranging my Sophisticated Lady a month back, I decided that the fuse was unsalvagable, but the wing was undamaged. Naturally, I put everything else on hold and just built a new fuse. I finished it today, and I have dubbed it the Sophisticated Dude, because it has none of the elegant charm of the Lady.

I kept the band-on wing but used a laminated bulkhead (0.25 balsa with 0.0625 ply facings) to retain the dowels more securely than the original design. I also added a little more space up front for the battery, so that I could use a bigger pack if I wanted.

The biggest change was the use of a CFRP shaft for the tailboom. While folks have been doing this for years, Midwest now makes a very good line so you don't have to go and get an arrow shaft. They were kind enough to also provide stiffness test data to me upon request. For those of you that are interested, all of the hollow tubes can be approximated (in bending) as a material with a modulus of 1.125 Msi, and then a standard MOI calculation for the tube size.

I made every attempt to salvage the tail feathers from the original; I wanted some sense of continuity. Unfortunately, the stab required a conplete rebuild with new spar and new ribs, but I was able to use the original form. The fin was not so cooperative, and it came apart when I was removing it from the fuse. I built a new fin and rudder assembly, getting rid of that T-tail contraption that always made me a little nervous. You can see the size difference next to my Talon fuse.

After radio installation, but prior to balancing, the fuse weighed in at 16 oz, and the wing at 11 oz. With the 4.625 square feet of wing area on the Dude, it has an unballasted wing loading of 5.8 oz/sqft, not bad for a 2 meter bird. I think that I'll be able to get it in trim with less than 6 oz of lead, which would leave me about 7 oz/sqft. Might even be to light to fly down here this time of year.

Turns out I was able to balance it with 4 oz of lead in the nose. Embarrasingly enough, I also read the scale wrong the first time. The weights should have been 15.5 oz for the fuse and 10.5 oz for the wing. This gives a total loading of 6.5 oz/sqft, not too shabby. I think that the CFRP tailboom really helped, leading to less weight in the fuse, but also less weight needed to balance the plane.

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In fact, I just got off the treadmill...