Construction Tips From and Old Fighter Pilot 2

Anything related to Space Derby racing and event coordination.
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Colonel
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Construction Tips From and Old Fighter Pilot 2

Post by Colonel » Wed Apr 18, 2012 9:42 am

OK here are the construction tips and techniques we used to make a winning rocket:

You are not building a flying machine. You are bulding the most efficient string riding vehicle you can. The most important things to make this happen are: stable and balanced flight, light weight, ballistic shape, efficient transfer of power from propeller.
a. Think about the olympic bobsled announcer who says "that'll cost them a couple of tenths of a second" every time the sled hits the side of the track. The same thing happens if your rocket moves around during flight. Every gyration causes the hangar slots to bang against the fishing line track. This causes added friction which slows down the rocket. Because the rubber band engine loses power and the propeller loses thrust as the rubber bands unwind, the slowing effect of friction increases toward the end of the race. You can solve a lot of this problem by building a balanced rocket. Balance your rocket as your build it. Measure and mark the wood so you build a symmetrical vehicle. Put all the pieces of the rocket together (rubber bands too) and balance it side-to-side and fore and aft. We just set the rocket on the edge of a metal ruler and made a mark at the balance points. You don't have to glue the fins in the slots yet, just stick them in there. Mount the rocket hangar so the middle of the hangar sets right on the fore and aft (longitudinal) balance point and right over the side-to-side balance point.
b. Aerodynamics is not as important as people think in this application. However, a shape that promotes good airflow and reduces drag helps making sure the rocket flies level as it moves down the line. Iron bombs are designed to fly a stable ballistic path. This is a good shape for the rocket. We used a NACA 0030 airfoil based on an 8.5" chord as the basis for our shape. NACA 0030 is just a way to describe a symmetrical airfoil based on a longitudinal length of 8.5". The wood in the kit is 7.5" long, so our airfoil shaped rocket has a blunt end 1" from where it would taper to a point. To build this shape, measure from the nose of the rocket and make marks at these points (1/8", 3/16", 7/16", 5/8", 7/16", 1 1/8", 1 11/16", 2 9/16", 3 3/8", 4 1/4", 5 1/8", 5 15/16"). Draw a longitudinal line (the line where the two halves of wood join works well) and draw lines centered 90 degrees to the marks you made before to define the width of the body. The final drawing will look like a fish-bone. Use the following to define the thickness of the shape (at 1/8" from nose 3/8" wide, 3/16":9/16" wide, 7/16":3/4" wide, 5/8":7/8" wide, 7/8":1" wide, 1 1/4":1 1/8" wide, 1 11/16": 1 1/4" wide (same for 2 9/16" and 3 3/8"), 4 1/4": 1 1/8", 5 1/8": 1", and 5 5/16": 3/4"). Once you have your fish-bone diagram, draw straight lines to connect the ends of the "bones" and you'll have a nice symmetrical airfoil shape. We did not glue the halves of wood together before carving because we wanted to mount the hangar before gluing. We just held the pieces together with two rubber bands and carved. We started shaping by marking an octagon on each end of the wood and connecting the points longitudinally with lines. From there, we just cut the points off the wood with a sharp utility knife. We kept measuring, drawing lines and cutting until we reached our airfoil shape.



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