Sunday, December 19, 2010

Review - Whirlygig-24

When I first saw Matthew McFarland's Whirlygig in FlisKits' Deuce Bash Contest, I immediately knew I had to build one. This is a 24mm version of this odd, helicopter design. Since there was little theory to help determine stability (how do you swing test this anyway?), I built mine out of leftovers. As a result, the design is not optimized. It is heavier and chunkier than it probably has to be. I am providing this to show the overall dimensions that seem to result in a stable model. If you build one with another form factor, please launch safely! Note I used an oversize lug so I could fly from the high power pads. This has also become very popular with the folks at MDRA, NARHAMS, and NOVAAR!

The parts list:
* Mailing tube, 2" x 22"
* 24mm tube, 4.125"
* 24mm PNC, type unknown, 2.5"
* ¼" balsa fin stock (2 pieces, 3" x 7")
* ¼" launch lug, 2.25"

Building this rocket was easy. I first found the center of the tube and marked centerlines on the top and bottom. I then located the fins based on these centerlines. My fins are approximately 120 degrees apart and are inset one inch from the end of the tube. The lug was mounted in a notch next to the motor tube. I used epoxy for the cone, motor tube, and launch lug. Wood glue was used for the fins. The fins extend all the way across the tube, leaving a 5" x 3" surface exposed. The motor tube extends below the body tube by 3/8".

The tube I used was speckled with paint from a previous project and the nose cone was green. That's all the finishing it got for its maiden voyage. I later filled the balsa and painted the rocket half black and half fluorescent orange.  I have since replaced the cone with a fish-shaped fishing lure (with the hooks removed, of course). 

I have flown my 'Gig 15 times, once on the following motors: AT RMS F24-P, AT SU E15-4, Ellis SU F20-P, Est SU D12-P, Est SU E9-P.  I add a tape thrust ring and friction fit the motor lightly. The first flight was on the D12 and I didn't know what to expect. The boost was straight and had a slow rotation. It didn't go high enough to begin spinning on the way down. The E9 flights go much higher, and it attained enough altitude to begin spinning on the way down. The F20 flight, needless to say, was faster and even higher. Oddly enough, the 'Gigs do not seem to rotate while on the rod, so I don't know how much the spinning has to do with stability. This effect is not easy to see on the F20, but is clear on the smaller motors.

This is a fun rocket and is sure to get everyone's attention. I built another version that used Nike-style fin cans in place of the balsa. All I can say is make sure you have enough fin area. That version was not stable!

I have also built two more successful alternative versions as follows:

  1. Baseline version: status = flown; tube length = 22", fin root = 3", fin span = 5", fin angle =120 deg from top; 15 flights using D12, E9, EM F20, and F24 motors.
  2. 'Shortie': status = flown; tube length = 16", fin root = 3", fin span = 5", fin angle = 120 deg from top; two flights on E9s, it spun a lot faster than the baseline.
  3. 'Getting Sideways': status = flown; tube length = 16", fin root = 6", fin span = 4", fin angle = 105 deg from top; on its one E9 flight, it weather cocked more than the others and autorotated down with its nose still pointing up.