Monday, January 10, 2011

Review: El Cheapo Pyramid

This is a simple mid-powered pyramid that flies on either one 29mm motor and/or three 24mm motors. The finishing matches the construction materials--'el cheapo'.

The parts list:

* four carrugated cardboard triangles, 12" base and 13.75" sides
* plywoods square, 4" x 4"
* one 3.25" 29mm tube
* three 3.25" 24mm tubes
* 1/4" lug
* lead shot
* duct tape

I started by attaching the cardboard triangles to form a pyramid. These were attached with epoxy and the joints were covered (hidden) with duct tape. The motor support plate was a scrap piece of 1/4" plywood. Before this was attached, I located the motor tube locations and drilled a hole for the lug. As the plate was epoxied into place, I measured the distance to each corner to make sure it was even. To find the location of the launch lug hole in the top of the pyramid, I dry-fit a 1/4" lug and used a rod to start a hole. I finished the hole with a hobby knife. After the lug was attached, I cut it flush to match the angle of the pyramid's side.

The theoretical CP of a pyramid is 1/3 of its height up from the base. I added lead shot to get the CG ahead of this point (as best as I could estimate). I now know from Bruce Levison's analyses that the CP is probably behind the bottom due to base drag effects.

There was no real finishing. To keep with the pyramid theme I found on on-line hieroglyphic translator and drew the various characters with felt-tipped markers (very high class). The 4 sides say:

1. Beware all who enter here
2. El Cheapo Pyramid of Death
3. No BATFE (shown in the first photo)
4. Miscellaneous character

The English translation is written on the inside on opposing sides.

This rocket has flown once on a G38, once on three E9s, and once on a G75 Metalstorm sparky. The G38 flight was nice and smoky with the low pressure area at its base clearly visible. The E9s made for a slow boost and the rocket seemed to stop abruptly at burn-out. The G75 was nice in the pyramid because all the sparks were clearly visible (ie at a nice low altitude).  In all cases, the pyramid slowly fell nose first to the ground.

This was a simple, inexpensive, and fast build. Everyone who likes saucers, pyramids, etc. should build one. Not necessarily exactly like this one. You can even paint it :)