The latest BTC-4 variant (a.k.a. Mark’s Most Recent Folly)

Since this was my first try at designing a twin, I decided I wasn’t going to say anything about it until I had made the maiden flight successfully.  That way, if it was a dismal failure I could just quietly scrap it for parts and no one would be the wiser, saving my sterling reputation.  Luckily for me, it worked out fine.

As it’s a twin, I kept debating whether to call it a BTC-4 variant, or start with the BTC-5 line.  But, I decided it’s a BTC-4 since, like the previous BTC-4s, it has:
a) ¼” balsa fuselage
b) Brodak engine mounts for the engines
c) 4” airfoil shaped ¼” balsa from SIG for the wing
d) And minimal finish other than a couple of coats of flat clear LustreKote.

I only took a couple of pics while building, as I was more wrapped up in building/thinking/building/re-thinking/building/second-guessing/building than I was in documenting.  Besides, it’s pretty obvious how it’s put together just by the finished photos.  It uses two Bee engines with 8cc stunt tanks.  One engine has a TD piston and cylinder, the other has a SPI P/C set from Bernie.  The outboard engine has washers under the left side of the engine to give it a couple more degrees of right thrust than the inboard engine has.   Finished weight is 10.15 ounces.  Plenty light for twin .049s.  








And here’s video of the test run of the engines.  I love those yellow safety props.  Really easy to see and keep your fingers out of.  I’ll have to get the fuel burn under control on the inboard engine after the proof of flight tests.




And here’s the maiden flight.  Flying on 42’ 20# Spectra lines.  It surprised me a little at the speed on take-off, but after looking at the video it seems that the speed was probably more of a perception in my mind than an incredibly fast flight.  Even after the inboard engine quits, it stays tight on the lines and very controllable.  I just did straight and level for this first flight, with a little up and down to test control authority.  I wanted to bring her home in one piece today.




I think it might need a few grams of tail weight, but maybe not.  It flies nice and easy as it is.  And is very controllable, as I said.  The glide is just not as flat as the previous two BTC-4 planes.  For this first flight, I kept the pushrod on the inner hole of the bellcrank to keep her tame.  Next flight will be with the pushrod in the outer hole of the bellcrank.  I’ll get the inboard engine to run longer and increase the throws, and then we’ll see about weight shifting.  I’m also going to pull the props and do a better job of balancing them.

The Under Wraps Mark


edit: I took a stopwatch to the video. Dual engine lap times average 3.29 seconds, and single engine lap times average 3.88 seconds. Still pretty spry on one engine.

The Spry Mark