Speedwing Contest Design

Today I had the final crash of the prototype speedwing. The 2200 mAh battery cantilevered over the leading edge is now about 1" shorter.... Nothing like a high speed, full throttle perpendicular impact into ice!
So, I thought it was about time to start working on the "real" design. So far, here are the major changes from the prototype:

• Tip wing section is about 60% of the root wing section- hopefully this will avoid the nasty tip stalls of the prototype
• Reduced wingspan to 24" from 40". Even with this major decrease in wingspan, with the increase in wing tip chord length the wing area is still close to the prototype - about 302 square inches
• Removed winglets. They kept on breaking on the prototype. If this doesn't have enough directional stability I'll add them later.
• Added cockpit to cover batteries, and allow longitudinal orientation of the batteries.This will also reduce the need to core out the wing as much- which should make the wing much stronger.
• Extend spar across wing center joint. 

Estimated specs:

• Wingspan 24"
• Wing area 302 square inches
• Estimated all-up weight: 1.1+ pounds ~ 500+ grams (this seems to light, but maybe it's correct)
• Max power: 1100 watts

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

Hot Wire Cutter, Part II

The original hot wire cutter setup used an extension spring to tension the wire in a wood frame. This worked, but the spring simply was not stiff enough to fully tension the wire during cutting- which left scallop marks on the foam as the wire formed a curve as it cut. I removed the spring, then tried cutting again. This time it was very obvious how much the wire sags when it heats! Take a look at these photos and video:

Before heating wire: (at roughly the center of the wire)

 

After heating wire:

Major saggage! Even more compelling, check out this video:



So, before I make Rev2 of the speedwing I'll be adding a higher K factor spring to this hot wire cutter.

Speedwing prototype 3rd Flight

I flew the speedwing prototype again today after several small adjustments:

1. Moved the CG forward about 0.5" ahead of neutral point. This seemed to help quite a bit- the wing was a lot more stable and easier to control. It could probably stand to get moved forward slightly more.
2. Increased the aileron dual rate to 50%. Both aileron and elevator are currently at 70% expo.

The CG was moved by taping lead shot to the leading edge. Not pretty, but it works.

I managed to fly a couple laps of the field, then I was disoriented or something and hooked a wingtip on the snow. The landing, while spectacular, didn't do as much damage as I would have thought. The winglet that didn't break last flight came off, and a new crack opened up near the motor mount but that's it. Also, one of the set screws in the motor mount fell out so I'll have to find something to replace it with. Not too bad. I also flew it with the 1050 mAh 3s battery- which may have helped keep the speed down and things under control.



For the "real" wing I'll definitely be doing a couple things differently:
1) extend both spars across wing midline to strengthen that foam joint- it's just not strong enough now and breaks during every hard landing
2) add some kind of small fuselage extending forward of the wing leading edge, and have a long battery box to allow easy adjustment of the cg.

First Flights

The prototype wing flew for the first time today. I tried a sidearm vertical toss launch at 3/4 throttle which seemed to work well.

Quite a handful- even with the dialed-down power system it has total vertical performance. But, I didn't really have good control. The first flight (not captured on video) was very short and ended in a soft crash into nice cushy snow- no damage. The controls were much too sensitive- I dialed back both elevator and aileron to 30% travel.

The second flight was longer and slightly more successful.However, at the end it still got away from me, and I had a harder nose-first landing in the snow. This did a little more damage- tore the motor mount off and damaged one of the winglets. Both are easy fixes and the gorilla glue is drying right now.

Both flights were too short to see if the trim needed to be adjusted. It couldn't have been too far off! Also, I'm glad I chose red for the bottom, but a brighter topside color might work better. It'll take some getting used to picking up orientation with the different aircraft shape.

Lessons learned:

1. Increase aileron travel slightly
2. keep elevator travel at 30%
3. dial in more expo
4. Try flying with the 1050mAh battery next time, both flights today were done with the much heavier 2200 mAh battery
5. The center of gravity might be slightly too far back- for both flights it was right at the neutral point. I might move both battery locations forward 1/2 if possible.

Paint & Final Weigh-In

I used Krylon H20 to prime and paint the wing. On the top surface I used the primer, then two coats of white. On the bottom, two coats of red only- no primer. The primer didn't appear to help with coverage or tack, 3M painter's tape still pulled the white paint off of the reinforcement tape even after a day's dry time.

The All-up weight, not including batteries, is 330 grams. Paint, glue, hinges, tape and spruce spars added approximately 111 grams. If battery weight is included, with the 1050mAh 3s flying weight should be around 430 grams.

Then, plugging the numbers back in to webOcalc:

Flies Like: Backyard Flyer.

Power Level:
(with white highlighted prop) Amazing. Extremely 3D capable. (9x6 prop)
Thrust/weight about 2.2:1
Minimum Pilot Skill Needed: Beginner level.
Minimum Flying Field Size: 1000 x 730 feet.
Minimum Battery Capacity: 910 mAh, 20 C, lithium polymer.
Suggested ESC Rating: 20 A to 24 A.
Power Into / Out of Motor: 194.4 watts in / 145.8 watts out.
Power To Weight Ratio: 205.99 watts/pound.
Estimated Stall Speed: 12.4 mph.
Wing Loading: 6.43 oz/square foot.
Cubic Wing Loading: 3.96 oz/cubic foot.
Maximum Pitch speed: 50.9 mph