Wednesday, January 05, 2011

Review: Grizzly Behr monocopter

In preparation for a line of monocopter kits, Art Applewhite published the plans for a monocopter made from Popsicle sticks. I didn't have enough Popsicle sticks, but I did have a few paint stirrers. They advertised Behr paints, hence the name. The Grizzly Behr flies on 18mm motors.


The parts list:
* 3 wooden paint stirrers
* 4 Popsicle sticks
* 18mm motor tube
* String
* Carpenter's glue
* Cardstock
* 1/8" Estes launch lug

One of the stirrers is used as the wing, one as the balance beam, and one is cut up for the motor standoff. I started by gluing the Popsicle sticks onto one of the stirrers. They are separated by the width of the stirrer, are centered on the stirrer, and are attached on opposing sides. This forms the balance beam and central hub of the monocopter.

The motor mount is conceptually borrowed from Art's design. It consists of a 1.5" piece cut from the third stirrer. I notched the hub-end and glued the other to the BT-20. I then wrapped the mount over the tube with a generous amount of string. The string was soaked with carpenter's glue and, where the string raised at the tube/standoff joint, I filled it with more glue. In retrospect, I'd recommend the twine Art specified. But, as usual, I substituted with what I had on hand.

To provide lift, the wing needs to be tilted in one direction and the motor mount in the other. I planned to mount the motor assembly and the wing between the opposing Popsicle sticks with one edge under one stick and over the other. However, I found this wouldn't work with the 4 sticks protruding from each side of the balance beam. This was easily fixed-–I merely lopped off two from each side. To give the center more support, I notched the wing so part of it overlapped the center section. I then glued the wing and motor mount to the hub, added thick fillets, wrapped each with more string, and the covered the string with more glue. This wrapping was likely overkill. The finishing touch on the wing is an overlapping piece of cardstock that provides a semi-airfoiled shape.

I drilled a 1/8” hole in the center of the hub and inserted a section of an Estes launch lug. Well, a monocopter should balance at the center of the hub and I hadn't accounted for the paper wrap so it was a bit off with a C6 loaded. Cutting about an inch off of the wing tip did the trick. I'm not sure this was required.

Flight and Recovery:
I added a 1/2" tape thrust ring to a C6-3 motor and friction fit it in the mount.

The Grizzly Behr flew off of my standard home-made pad using an old 1/8" drill bit as the "rod". The cutting end was clamped in the pad and I added a tape ring to leave a 1" section of the shank exposed.

The monocopter flew great. Most small monocopters merely flutter to the ground after burn out, but this one continued to rotate until it landed. Its spin profile made one observer state that it looked like a boomerang in flight (except it didn't return on its own). It did look pretty cool if I do say so myself.