CP eyeball method

Discussions about techniques for rocket building. Leave your tips and tricks to making the perfect rocket.

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Rex Fynns
Parachute Packer
Parachute Packer
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CP eyeball method

Postby Rex Fynns » Tue Jul 04, 2017 1:02 pm

G'day all,

Following a discussion in the intro section I would like to expand on a method for an eyeball redneck CP checking method to hear what others may think.

Expanding on the idea of a plan form silhouette cut out method which appears a good eyeball starting point but also ignores the subtlety of differences in lift and drag coefficients between a round body and fins I offer the following idea, likely done before I suspect.

Attach the rocket (this is for small rockets) clamped with zip ties between 2 pieces of half round aluminum tubing one of which one is attached to the end of broom handle at 90 degrees (screwed and glued into a half round recess at the end of the handle), the half round tube piece being around ~70mm in length.

The line of the broom handle is through the already established CG point.

A country quiet road and a ute is desirable at this point but not essential, leave your copy of the Koran that maybe lying around in the cab at home for this trip and don't pass police cars. With the window down and the rocket far enough out of the window to be in clean air simply drive down the road.

There are a couple of advantages with this method in that the torque effect of the weather cocking moment can be directly felt through the broom handle and its magnitude gauged at various speeds. Hopefully the rocket will show positive stability and fly horizontally. By progressively moving the attach point further aft you will come to a point where neutral stability occurs and the rocket remains perpendicular to the relative airflow.

This is an expansion on the swirling string method however it removes the centrifugal force influence allowing attachment at other than the CG point so that the actual CP can be determined.

Another advantage might be that with the rocket attached through the CG point along the line of the broom handle you can rotate the rockets AoA attack and get a feel for flow separation over the fins, this could be helpful with fins designed for extreme velocities sporting razor sharp leading edges which tend to create early flow separation over the fin. I suspect that through the very short moment arm of grasping the end of the broom handle you will feel the point of flow separation (stall).

One significant down side to this which could be mitigated through design refinement is that the turbulent airflow from the broom handle could induce early buffeting at low AoA until the fins moved with increasing AoA back into clean air.

All just a thought bubble at this stage but a method that appears quick and simple to set up and may produce something meaningful.

Of course many will offer that the various simulators do all this for you and more, which is true. However there is nothing quite like actually tangibly feeling the forces at play to gain a good appreciation of the science. And frankly the less time I sit in front of a computer and the more time I spend roaring down a country road in a ute with a rocket out the window the richer my life becomes.


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