Sunday, February 21, 2010

5th Day of Wing Flights

Over the past week I cut pieces of foam and glued them into the wing, filling in the battery compartments I had originally formed in the wing. I replaced it with a strip of velcro along the wing centerline and a velcro hold-down strap. This would allow me to move the batteries fore and aft to adjust the centerline. However, with the 1050 mAh batteries, even with the battery all the way forward the CG was just barely ahead of the neutral point. (NP) According to guidelines I've seen the CG should be slightly ahead of the NP.

Saturday John and I flew off the lake, and we both flew the wing a few times. All flights were with the smaller 1050 mAh 3s batteries- and all flights were white-knuckle, barely in control flights. All flights were with the battery as far forward as I could manage and still have the battery captured with the velcro strap. The wing was very sensitive in pitch, and several times the wing unexpectedly went into a unrecoverable spin into the ground. Luckily the wing has proved to be very durable and did not suffer any apparent damage. The foot or two of soft powdery snow makes an ideal crash-pad.

Sunday I tried a couple more flights. The first few flights were with the 1050's. However, on one flight the velcro loosened and the battery ejected back into the prop, slicing the battery open:

Unfortunately the slice must have penetrated one of the cells, because one cell now reads zero volts. Luckily the battery didn't go into melt-down and start on fire. The battery is currently on a plate in a stainless steel utility sink until I figure out what to do with the battery.

But, since the plane was still flyable, I strapped on a 2200 mAh cell. With the heavier, longer battery I was able to cantilever the battery an inch or two over the front edge of the wing. I was able to move the CG about an inch farther forward than I've tried before. Wow, what a difference that made. The wing was instantly much more stable- I could even fly it hands-off. It still had pretty wild stall characteristics, but I was able to actually recover out of the stall and prevent a crash. With the bigger battery I was able to get in a 10+ minute long flight. The landing looked great- a higher speed "greased" landing. But, the prop caught a chunk of ice when it was bellying in and ripped the motor right off the wing. At least I know how to get the wing to fly well!

Another interesting observation was the freewheeling of the prop acted like a speed brake. I'm using a 9x6 prop which looks pretty large on this wing. With the ESC brake turned off, on cut-throttle gliding you could hear the slipstream spinning the prop and wasting kinetic energy.

I'm going to try and start construction of the "real" speedwing this week. There are a few key design changes I'm planning on making to ensure the new wing is more robust and controllable than the prototype:

  • Extend the spar components across the wing centerline
  • Minimize large cavities within the wing core. Perhaps create a short fuselage extending ahead of the wing to house the batteries
  • Allow battery fore-aft movement to get CG approximately 1" ahead of NP
  • Lots of control surface travel isn't required, so don't use most aggressive control horn placements, this will improve control surface accuracy
  • Perhaps shorten wingspan slightly, and don't taper wingtips as much- attempt to tame stall behavior. Stalls should start from wing root, not wing tip. The prototype had a very large chord difference between tip and root.
  • Use thinner material and an improved technique for winglets- reduce drag and reduce likely hood of tips breaking off on rough landing.Cover foam with econocote or otherwise improve surface finish. Prototype was very rough.
  • In addition to using contrasting colors between top and bottom, also use some sort of contrasting pattern to differentiate top and bottom for difficult lighting situations

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