Wingspan: 71 in
Wing area: 470 sq in
Length: 41.75 in
AUW: 26.6 oz
Wing loading: 8.15 oz/sq ft

Dave Thornburg's Doodler

Doodler as built in 70s w/repaired wing

The Doodler is a 2-channel glider that was designed and marketed by Dave Thornburg (Southwestern Sailplanes) in the early 70s. It is the first RC model I ever built. I built it sometime around 1972, and didn't build or fly another RC model until 2006. It's been following me from house to house for 37 years, lurking quietly in the basement rafters.

Cox 049 powered! Transmitter, brick (rx & servos), and battery
Tail group with sewn hinges LRB (Little Red Brick) in place

The Doodler has a solid balsa wing with an undercambered Jedelsky airfoil. The left wing was broken at one point, and simply epoxied back together. It does not have a towhook, and the fuselage floor was not reinforced to support one, so it could not be launched by winch or even hi-start. Instead, it used a Cox 049 glow engine to reach thermaling height. The engine had no throttle or cutoff, and would run at full throttle until the fuel was exhausted.

The radio was an EK Logictrol LRB (Little Red Brick) system. This consisted of a 3-channel transmitter and an all-in-one flight pack (the "brick") containing a receiver and two servos. The transmitter was powered by a large 9V dry cell, the flight pack by four AA batteries.

The airplane never flew very well, and (to my novice eyes) seemed very difficult to control. I don't think it ever stayed in the air long enough to run out of fuel. The whole experience was so frustrating that I just gave up, and didn't try RC flying again until 2007.


New servos
BP Hobbies BL-2212-10 motor
Rx and speed control
Magnetic battery hatch closure

In September of 2009, when I'd had a couple of years of RC flying under my belt, I noticed the Doodler hanging in the rafters and decided to get it flying again. My goal was to catch at least one thermal with it, something I never did back in the early 70s. I ripped out the old radio gear and glow engine and installed some low-cost modern equipment:

I also added a magnetic hold-down for the battery hatch. All-up weight in this configuration was 25.8 ounces, giving a wing loading of 7.9 oz/ft2.

The maiden flight of the newly-outfitted glider was on 2009-09-17. Because the BP Hobbies motor had not yet arrived, the first flight used a Lightflite 1100kv CD-ROM outrunner motor (taken from a 6-ounce foamie) and LightFlite 12A speed control instead. As expected, the LightFlite motor had barely enough power to keep the glider airborne, but the experiment was instructive anyway.

The glider was neither easy nor fun to fly. It responded sluggishly to rudder input and was almost impossible to pull out of a turn. If it ever reached thermalling altitude, it would not be maneuverable enough to stay in one. Because of its high wing loading (around 8 ounces per square foot), it flew fast for an alleged "beginner's" glider.

With the BP Hobbies outrunner installed, the Doodler climbed a little faster, but still took several minutes to climb 300 feet. A more powerful motor or better folding prop would be nice.

Here is a short video of the first flight (with the less-powerful LightFlite motor).


Not enough dihedral! Dihedral
Wing with polyhedral Wing with polyhedral

The Doodler's maneuvering problems were almost certainly caused by the wing's shallow dihedral. Dave Thornburg's instructions called for a 4" dihedral (or about 6.5°) on each wing. This wing appeared to have no more than 2.5" (4°) per side. It is unclear how the wing could have been built this way, since the kit came with the wings pre-beveled. In any event, the Doodler needed more dihedral.

I couldn't cut the wing at the center joint, because the joint had been reinforced with a thick, immovable layer of Celastic—reinforcing a new joint on top of that would have been impossible. Instead, I used a miter saw to cut off 14" from each wingtip, and sanded 3-1/2" of polyhedral into each tip. I reattached the wingtips with Titebond III glue, and re-covered the whole wing with Monokote.

New Doodler on bench

(It turns out to be hard to sand dihedral angles into an undercambered solid balsa wing. If I had been thinking, I would have used the miter saw to cut the proper angles into the wingtips.)

The wings now have 6-1/4" of effective dihedral at each tip. Between the saw kerf and the higher polyhedral angle, wingspan has been reduced from 71" to 69.5" and the effective wing area from 470 sq in to 460 sq in. However, all-up weight is down to 24.9 oz, probably because of a lighter covering job, so wing loading is down from 7.9 oz/ft2 to 7.8 oz/ft2.

The Doodler handles much better with the polyhedral wing. On its first real outing, on Sunday 2009-11-08, it rode several thermals to 'speck-out' altitude. Nothing will ever make this airplane a great performer, but finally, after thirty-seven years, it flies!

More pictures:

Guts EK Logictrol 3-channel transmitter Transmitter data plate New version Polyhedral panels cut Undercambered Jedelsky airfoil Gluing the polyhedral joints Gluing the polyhedral joints Covering stripped New polyhedral wing -- needs covering Wing repair from the dawn of time Bottom of repaired wing