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250 gram plank

#1
I used iforce2d's plans with the following mods:
  • Wings stretched from 24" to 32" span
  • Wings given a lot of washout
  • Fins hotwired insulation foam instead of DTFB
  • No spar; just the leading/trailing edge fiberglass reinforcement.
  • Wings/battery cover attached with nylon screws to 1/4" ply mounting points.
  • More square fuselage cross section.
I flew it yesterday.
  • The CG was too far forward.
  • During throw tests I added 24g of ballast to the tail; it still required about 3/8" of reflex on the ailerons.
  • Supposedly this has a special plank airfoil with built-in reflex so it should be stable with... none?
  • However I'm not really sure if the CG is exactly right for that.
  • Also the hot wire cutting accuracy wasn't great.
  • It had plenty of power and was more docile than I expected.
  • But I should have practiced in the sim more. Pilot error caused some hard landings.
  • The fins do a good job protecting the prop... but broke. In hindsight it was pretty obvious that they'd break right where they both did.
  • The nose broke off. It had a rectangle of fiberglass on the bottom, which took the landing impacts well, but it broke cleanly behind that.
  • The wingtip broke off in one of the two places I'd expect it to.
  • The fiberglass remains in good condition.
  • I was flying in a field they cut/bale hay and the grass was long and forgiving.

I've glued it back together for now but it'll weigh even more. I need to put some thought into lightweight fiberglass.
 

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#4
I reinforced the fins with barbecue skewer, fiberglass, epoxy, and tape. I then proceeded to crash it hard enough nose first to dislodge the motor. I think it's time for the $100 of electronics to migrate to a new $1 of foam.

I built a fixture for cutting replacement wings. Between crashes and fiberglass practice I think it will be worth it. It makes wings 16" long, tapered from 6" to 4", with an MH60 (related to the MH64 iforce2d uses but thicker) airfoil and 3 degrees of washout. The wings have an area of 80 square inches and weigh 11 grams for just the foam.

I used 6oz glass on the first one. I'm thinking 1-2oz is probably better.
 

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#5
Doing some research on efficient propulsion options for this plane. Not surprisingly, for static thrust at least, between the two motors and three props, bigger is better. The 1407 is 15g and the 1806 is only 20g so that seems like an obvious improvement. Prop is a bit more complicated -- I really like how the prop sits right in the middle of the box created by the tail fins and I don't want to make that much bigger.

I have some shitty old 2212/1000kvs and 1045 props from a trashed quad. I'm wondering if it's possible to build a sort of wind tunnel for this, to find the airspeed at which the smaller props start being more efficient. Of course, designing a good wind tunnel is basically as hard as designing a good airplane... so...

The stand itself uses a 5kg load cell and an hx711 amplifier -- super easy to connect to an arduino-type thing.
 

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