Monday, October 05, 2015

Kfm4 Flying Wing Post Mortem

Yesterday, I took the wing out for a flight in the evening, but it barely flew for 5 seconds before the motor cut, and I had to do a deadstick landing for real. When I arrived at the wing, the ESC was beeping the overcurrent tones, and the motor was just jittering back and forth very strongly when throttle was applied.

The motor was also nigh impossible to turn, even with the connections to the esc removed. A short while after, it started to free up, but it made clicky noises, and would sometimes just jam up for no reason. Seems like it threw a magnet :(

Having no spare motor, the only other choice was to take the CF2812 1534Kv motor from my Teksumo. Same motor mount, but it should have less torque and much higher rpm! I didn't change the prop and the wing flew fine, just much faster! I also could not fly as slow compared to the1200Kv motor, wonder why? Maybe the torque is not as good at the lower rpms but comes in at higher rpms? No idea....

Anyways, I took her out for a spin this evening, and she flew totally swell, as in the video below:
 Also compiled a selection of crashes :P

Unfortunately, this is the last time I will be flying her. It seems like the mods I made to hold the battery, as well as the holes used to hold down the servos have a very detrimental effect on the airframe, and the cumulative crashes cause the front section to fold up, as well as the left wing to fold up along the weaker portion by the servo mount.

She still flies, but tends to tip to one side, as I think the damaged section of wing, which actually bends upward, may be causing a dihedral effect. There are also other issues with the wing. While the front leading edge of the wing has 3 layers of DTF, the trailing wing section only has one section, and in reality is very weak. The 1 layer wing has detached around the motor mount, and I _think_ this changed the sound of the prop, perhaps because the air entering the prop now becomes even more turbulent?

I think if I were to remake this wing, I would make the following changes:
- More sweep to move the CG back. Balancing this wing has been difficult.
- A lighter motor - both for ease of balancing the cg. I also don't need the speed... for now.
- A carbon fiber spar, that would help the area weakened by the servo.
- Consider switching the elevons from foam to balsa (maaaaybe?) may improve handling if there is less flex.
- Perhaps 2 pieces of DTF minimum would help longevity.
- Figure out a way to allow thrust angles to be set easily! Ripping out the motor mount is a really bad idea.
- Colours or something to differentiate from top/bottom! There were several times I've lost orientation, not fun!

My next design will have the following requirements:
- Ease of building: Probably going with a plank wing and Kfm3 airfoil
- Prop safety: Prop will be high and not impact ground. (What about folding prop?)
- Minimum parasitic drag on wing
- Battery to be mounted below wing - perhaps the lower CG will give some amount of self-leveling, and not weaken the wing.
- Carry a mobius-type camera
- Carry a fpv camera
- Possibly have a tail section? vtail or elevator/rudder.

Good luck, me!

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