Dynaflight BOT repairs and alteration

[jcpatrick]

My Dynaflite BoT needs reworking after a long series of standard beginner crashes, and one unfortunate accident. (Unfortunate because my father-in-law was watching and while showing off, I miscalculated the height of the plane relative to an electric fence. One wing slammed the fencepost and the other wing ended up with the electric-wire embedded in it.)

The structure is repaired now, though it wasn’t easy because the 2mm (0.08) thick balsa skins aren’t compatible with 1/16” (0.063) thick balsa available in the US. There’s a lot of shims in the wings now!

While I had it stripped . . . Some guy named Ollie has repeatedly said sharp trailing edges are desirable. The wings currently have 3/32”-1/8” (0.09-0.13) thick trailing edge. Maybe I could use some lightweight glass over glued on foam or balsa. What’s the (expected) gain from sharpening or adding sharp trailing edges?


While its apart, I need to fix the tail. The balsa rudder takes landing weight and is skinned off. The glass rudder fin has poorly captured balsa rear closure, so it needs replacing too.

I know Drela pooh-poohed the idea that rudder rake causes elevator effect; but that was for a 10º rake. The BoT is raked at 40º —enough angle to misalign the control rod ends— and it’s enough to visibly elevate when the rudder’s sharply used.

While I put glass or kevlar on the bottom rudder edge (and glass the rudder fin) should I change the rudder rake?

[ejett]

Well, you do need to taper the trailing edge down or add a piece of basswood or hard balsa to the TE and sand it down to a sharp edge. You could use balsa and add thin strip of CF or even very thin ply and taper it down. The TE now is really too thick.

The rake fin does indeed introduce some elevator effect, but it is not an issue for this sailplane and actually gives needed "elevator mixing" in a tight thermal turn. If you are going to fly it primarily on the slope then you might consider taking some of the rake out of it. But frankly, I'd leave it like it is if it were mine.

I presume with the 0.80 balsa skins on the D-tube and the glass fin, you have the ARF version rather than a kit built plane, but in any case the fin is not raked 40 degrees on either of the planes I have owned. I have two more fuses and neither of them are raked that much either. Were you seeing issues with "elevator effect" on your plane?

EJ

[jcpatrick]

“the fin is not raked 40 degrees”
You are right, the rudder rake is closer to 30º. Even so, when it’s flying you can see elevator effect with hard rudder. Sometimes! :confused: It’s not consistent to this beginner’s eyes.

As you can see, the rudder fin has to be fixed anyway, and I was toying with the idea of ‘fixing’ other things as well. The hinge I put in allows the rudder to be removed easily, and I want to keep that feature. It might be that ‘good’ balsa (hard) replacing the old stuff is all I need.