Wednesday, October 22, 2008

Multiplex FunJet!

I recently purchased the Multiplex FunJet :) It literally does live up to its name! It's an easy build - requiring all of 2-3 hours for someone with experience, and ~maybe~ a little longer for someone with less building experience.

There isn't much to glue together. I spent the most time deliberating over the motor/battery/components combination for a decent COG. Once I figured out where everything was going I then sealed her up. I had to extend the length of the wire going from the ESC to the battery and rx. I forgot to get more servo extensions when I bought the plane so I ended up cutting the leads off of old and trashed servos (I keep them around for gears and their connectors). A little soldering and heat shrink and the servo leads and throttle lead were run to their final destination - the rx. The rx was velcro'd down right where the cockpit is permanently covered. This gives me "wiggle room" to adjust the COG (center-of-gravity) when I change from 2S packs to 3S packs.

Since I power helis and planes with A123 cells, I'm used to flying a little heavy and doing the math to get good power+flight times (i.e. higher kvm, re-gearing, props/pitch, etc.). In this case, I'm flying about 7-8 minutes on 3S1P 1100mAh and I'm actually *lighter* than the specifications - a nice change of pace for me.

Now that I'm thinking about it, I can actually swap the current motor for the inrunner I have sitting around and go with 3S2P (2x3S1P in parallel) for about 15 minute run-times and come in slightly heavier than spec.

Btw, here's my setup:

  • 3000 kv hextronic outrunner (hobbycity ($8.95 ea) - click here for the link)
  • 6x4e prop
  • 40A Suppo ESC (lightflightrc ($19.95)- click here for the link). This ESC defaults to NIMH lvc so A123 batts can be run without worrying about early lvc cutoff if the ESC defaulted to LiPo
  • 3S1P A123 batts (custom-built by me - I make lot's of these, contact me for pricing) good for 7-8 minutes with the current config. 2S1P is about 6 minutes with the current config.
  • HS-65HB servos (ran out of HXT900 servos ($3.49 ea) - just ordered a bunch. Good centering, strong and fast servos - never stripped one) Hobbycity - click here for the link
  • AR6100e rx

This setup draws 23A max. 2S1P draws 12A max. I'm going to swap in a 2500Kvm motor this weekend. I've seen a significant increase flight times on some of my wings - but not much reduction in power.

As a side note, I maidened her during lunch the other day (top painted yellow, bottom = plain white). What a BIG mistake - I couldn't tell top from bottom when I was doing a low and fast pass and a quick roll. That got me a nose-in to the ground. I snapped the fuselge in half exactly in the cockpit halfway point (like the Twister that I mentioned previously). I put 4 carbon-fiber (4" length) pieces between both sections, used some CA+kicker and had her back up in the air that evening (not to mention the bottom is now painted solid blue for good contrast).

In case you're wondering, here are the battery weights:

  • 2S1p A123 1100mAh: 90g
  • 3S1p A123 1100mAh: 130g
A quick technical note: I only call these A123 because that's who manufactures them. Technically, they are LiFePo4 (lithium iron phosphate - which is a chemistry that is not exclusive to A123 Systems, Inc.).

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