Category Archives: Tips & Techniques

Tips & Techniques for building.

New Project – FT Viggen

It’s been a while since I last posted. Summer has been busy but I have found time to get out flying when I can. Like last summer, I got sidetracked with a scratch-build foamy EDF. This time I, went with an established design and put together an FT Viggen. I didn’t bother with documenting the build process at all since there is a great build video on the Flite Test site, and I just followed it along step by step.My only deviation from the FT build was to add a 1×2 mm carbon fiber wing spar. I re-used the fan and all of the electronics from my original (failed) Fan Trainer project, and I must say that I am pretty happy with end results.

I really enjoyed the build process. I transferred the downloaded plans onto the Dollar Tree foam boards by taping the plans down to the board, then pushing a pin through the plans at each corner, or intersection point. I then used a pencil and a straight edge to lightly “connect the dots”. Working with hot glue and foam may not be quite a s rewarding as a full on balsa build, but it fast, cheap, and surprisingly strong. Overall, I can’t argue with the results.

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I maidened it a few weeks back, and I must say that I was quite impressed with how stable she was. The guys at Flite Test did a great job with the design! Just a gentle underhand toss with the throttle at about 75% and she flew away and got right on step. She is very sensitive to aileron input and I kept looking for rudder while (i.e.; I’d move the rudder stick, but of course, since there is no moveable rudder so nothing would happen – duh). I may try to add a rudder (or just build another with one).

I did run into one problem and learned a good lesson about battery charging. In particular, that I learned that I need to be mindful of the power supply power/current capacity when charging.

I am using a 4s 3700 mAh batteries (slightly larger than the recommendation of 3300 mAh, but what I had available already). The last time I used these batteries was in my original Hanger 9 Alpha Trainer a few years back. Since then, I bought a new battery charger that allow supports much higher charging rates. While field charging the batteries using my Duracell Powerpack, the charger died about half way through the cycle. At first I was quite certain that my charger had crapped out (I have a Hyperion 1420i charger and they have a reputation for early failure). Fortunately, when I got home, I discovered that the 12V cigarette lighter pulg-to-banana plug adapter that I was using had a blown its 10 amp fuse (I didn’t even know it had a fuse!).

A little math shows the problem:

  1. Battery Voltage (3.7 per cell empty, but  4+ volts per cell charged):  4 x 4v = 16v
  2. Out power to battery:  16 x 7.4 = 118.4 watts
  3. Input current required at 12v (assuming 100% charger efficiency):  118.4 / 12 = 9.9 amp

Knowing that the charger is not 100% efficient, it is pretty easy to see why the 10 am fuse blew. Overall, a cheap lesson. But, given the reputation of the Hyperion 1420i (now discontinued), I may want to think about picking up a new charger before next year’s flying season.

Stella Wing Reinforcement

I started sanding the aileron assemblies today an noticed that they twisted quite easily and were more flexible than I was comfortable with. I know that the covering will help with this (quite a bit actually), but I had trouble with the ailerons in my RV-4 twisting as I shrunk the covering. Since I really like building things anyway, I decided not to leave things well enough alone and added some reinforcing braces. I actually saw this technique used in a old Stella build thread on one of the RC forums (not sure where right now).

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I started by cutting some balsa strips from scraps using my handy dandy Master Airscrew Balsa Stripper. This is great tool and makes quick work out of cutting any size stringer out of whatever scraps you happen to have lying around.

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Next I added the braces running diagonally in each open bay just under the profile of the rib. I kept all of the braces on the top side running in one direction, then flipped the aileron over an ran braces on the other side in the opposite direction. I was so amazed at the increase in stiffness, that I went ahead an applied the same treatment to the wing. The total increase in weight for both ailerons was about 3 grams. I did not get a before and after weight on the wing, but it would probably not be much more (maybe 4-5 grams) as it has the same number of braces, just slightly longer. All in a dramatic increase in stiffness for a weight penalty of well under 10 grams, not bad.

I did run into on snag when adding the wing braces though. I did the first half of the wing without weighting it down to the building board. I thought that since I was being careful fit each piece without adding any initial tension, that I would not introduce any initial twist and I wanted to be able to flip back and forth as I glued each brace in. Unfortunately, I did end up with a bit of twist locked into the wing, so I had to carefully cut out the braces and start again. This time, I did weight down the wing to my building board and only worked from the top side (working on one half of the wing at a time, of course). This was a little trickier, but resulted in a flat, true, and stiff wing.

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I also added another round of balsa filler to the seems on the fuselage and sanded it down with 220 grit followed by 320 grit sandpaper.

Edge Joining Balsa Sheeting

I tried out a the technique for edge joining balsa sheeting described on the Airfield Models website on my TF P-47 build. Note: If you have not checked the Airfield Models website, I highly recommend it. A wealth of information and some truly impressive work.

The technique is easy and quick and produces a strong joint. I will summarize the process, but suggest you read through the full description here. The key is the use of standard model airplane cement (such as Ambroid) in place of CA (which is hard to sand) or aliphatic resin (which is slow to dry).

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Step 1: Make certain that your edges are clean and straight. I used a single edged razor and a steel straight edge to cut pieces to the desired shape and 150 grit sandpaper on a hardwood sanding block to lightly clean up the edges before joining. In order to prevent rounding of the edges, hold the balsa flat on your work surface and gently slide the sanding block (with sandpaper on the edge perpendicular to the bottom) against the balsa edge.

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Step 2: Use masking tape applied perpendicular to the joint one one side to hold the two pieces together. The pieces should be pushed snuggly together.

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Step 3: Flip the piece over and apply a length masking tape along the seem.

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Step 4: Remove the tape applied in Step 2 and hinge the pieces back along the tape applied in Step 3.

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Step 5: Apply a liberal film of cement along both edges, then flatten the pieces against your work surface. Using a cloth damp with solvent (e.g.; acetone), wipe away the excess glue that oozes out and re-tape the joint with tape perpendicular to the seem. Be careful not to use too much solvent. My first attempt, I used too much and basically washed away all the glue! At this time, remove the tape applied along the seem and replace with tape perpendicular as well (this allows the glue to dry more quickly and minimizes the chances of solvent melting the tape glue and leaving goo on the surface).

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After the glue dries, remove the tape, lightly sand the joint and your done!