Wednesday, September 23, 2015

Kfm4 Flying Wing Build Log 05

According to the CG Calculator for my wing, for a 20% CG position, it's 11.8mm from the leading edge of the wing. Unfortunately, this requires a fairly heavy weight up front in order to balance the motor and prop! I knew there had to be a reason why the popular wings tended to be more swept back! The more swept back the wing, the easier it is to get the CG right as it is moved further away from the leading edge.

Several solutions, apart from adding weight, present themselves. A light motor would help alot, and I'll probably even drop the prop down to a 7in, no idea if it'll save weight at all. I tried putting my 3s 2200mah battery up front, that balances it up well enough, but is over double the weight of my 1000mah battery, so, not particularly ideal either.

I've also tried putting a 40A esc up front by the battery, and that helps too, but it's overkill as the motor is theoretically rated for up to 14A. In the end, I'm sure it will be a combination of all the above to get the wing just right.

Testing the CG at the 25% position _somewhat_ works. I'll cross that bridge once my esc arrives.

Ah, the things we learn :)

I've also quickly fashioned a pair of winglets. It seems like in some flying wings, it's possible to get yaw stability by adding in drag on the leading edge by chopping the corner off the leading edge. While this is a cool idea, I decided to go with the winglets instead as adding in random things I hardly know about is not a very good idea. Heck, I have no idea how to design this wing, just going by seat of the pants intuition here.

Other things I've prepped include the control horns for the elevons. After I've had control horns fly off in flight on my previous aircraft, I've decided to play it safe and use a proper dubro control horn. To spread the weight of the control horn onto the elevon better, I've cut small plywood sheets and foam-tac'd them on.

I'll need to figure out the servos as well!

No comments: