I asked this question before and got some mixed answers - mostly
relating to the wind because it definitely affects stability at the end of
the rod. More wind means less stable at launch.

Short Technical Answer:

You must be certain that the actual air velocity (vector quantity - speed
and direction) at launch rod exit is such that you do not stall the fins or
cause the CP to drift farther forward than a margin to CG. This usually
means that the angle of attack is less than 10 or 15 degrees (for stall), or
that your stability is calculated based on the center of lateral area (CLA),
ie., the cardboard cutout method. The CLA method takes into account the
force generated by the tube, where the Barrowman type equations do not.

Rule of Thumb Answer:

I found a section on www.Info-Central.org that recommends 30 mph or about 44
fps.

Practical Answer:

Get a longer launch rod or use a higher average thrust motor. (This is the
way I "solved" the problem of my Estes Phoenix. I moved to 1/4" lugs and
launch off of the 5' rod of a Mantis. The Aerotech D15's and E18's also get
it moving better than the Estes D12.)

By the way 30 fps might not be too slow, depending upon the rocket. You
might however, want to defer that launch until the wind is very low.

Quick calc - 30 fps up rocket velocity, 6 fps wind velocity.
sin AOA = V horizontal / V vertical = 6/30 = 0.2
AOA = arcsin (0.2)
AOA = 11.5 degrees (marginal?)
Airspeed = sqrt[(V horizontal)^2 + (V vertical)^2)]
Airspeed = sqrt[(6^2) + (30^2)] = [36+900]
Airspeed = sqrt[936]
Airspeed = 30.6 fps
(quick lesson on vector math, heh heh)

At 44fps (rule of thumb) 11.5 degrees AOA is at 8.8 fps. Anecdotal evidence
will suggest that we can go much higher than 11.5 degrees, even in my own
experience. Maybe 15 to 20 degrees is more suitable a rule, since 8.8 fps is
only a 6 mph wind.

20 degrees AOA for a 30 fps launch rod exit is calculated like this:
sin AOA = V horizontal

* / V vertical
sin 20 = V horizontal / *30
0.342 = V horizontal / 30
V horizontal = 10.3 fps

and for 44 fps vertical = 15 fps or 10 mph horizontal.

Hmmmmmmmm.

I guess the real question is "how do you calculate the CP position at a
given angle of attack?"

--

Tom Koszuta

Western New York Sailplane and Electric Flyers

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