Tuesday, June 09, 2009

Whirlygig 38 progress and doubts

Well, I've charged ahead with the simple part of the project, that is, cutting tube, gluing fins, etc. The attached photo shows the fins attached and, for no particular reason, a chunk of 38mm tubing dry fit. The fins were epoxied in place and then I added a thick fillet of Gorilla Glue on the opposing inside edge. Neither glue adheres to the tint tube that well, so I thought I'd use both. As added insurance, I screwed the fins to the tube. You can see the screws on the higher fin in the photo. This was an benefit of using the relatively thick 1/4" plywood.

That's the progress, and now for the doubt.

This sucker already weighs 1 lb 10 oz. 26 oz. (no motor case). That seems very heavy for a gyro recovery rocket. As a data point, my Art Applewhite 18" saucer weighs in at 21 oz (also no motor). Like all saucers, this one has a lot of drag and comes down fairly slowly. It still thumps pretty hard. But then, so does any really heavy HPR bird. Slow moving doesn't imply no damage if it hits something. It just means that mobile objects like people can avoid them.

So, the suitability of gyro recovery will hinge a lot on whether this thing will reliably transition from a random tumble to a stable spin. My smaller Whirlygig seems to transition given enough altitude. If the big one goes straight up, then it may be OK. If it weather cocks, then who knows. My big saucer did well on a J228. On a J110, it was still under power when it headed towards terra firma. It still survived just fine, but provided a very high pucker factor. The other downside of weather cocking it that it could take the 'Gig over the crowd. Wind direction will definitely have to me monitored.

The 6 oz 'Gig flies well on with an average thrust of 9 N. At 26 oz, the 49 N-s motor should still provide a larger thrust-to-weight ratio, additional drag not considered. I'm wondering if I can find a 100' bridge with grass at its base and just chunk the 'Gig off and see what happens.

I'm also still thinking about adding a recovery system. The AeroTech EFT would work. It's not too expensive, but then I'm not sure I want to spend money on this project, which was intended to be inexpensive. Then, adding room for the recovery system and possibly the electronics will require that a good length of tubing sit above the body tube. I'm now worried that a mass significantly above the plane of rotation will destabilize the spin.

Yeah, FUD has set in big time.