CLA is the center of lateral area, described here.
The usual cardboard method for determining the CLA is time consuming
and tedious. Therefore I made a simple java-based calculator. With
this you can make a fast approximation for the CLA position. Even
for tubular fins.
- The CLA calculator works very well for cylindrical rocket shapes. This picture
shows how you have to estimate the required values from your real
world rocket shape. Basically you have to make guesses with your
calibrated eyeballs on how to transfer the real rocket (and fin)
shape into cylindrical (and rectangular) shapes, respectively (green
lines). OR you may use the conical shape approximation for
the nose cone; adapt the body length as shown (orange lines).
- The second set of fins is helpful for calculating rockets
with front fins, as they are used for boosters in multistage
applications. If you have only 2 front fins, use the 4 fin setting,
the result will be correct.
- Important: do not tolerate leading zeroes in your
entered numbers. The calculator may give out weird results.
- Set all unused values to zero.
- The rocket's name and the date/time have been added for
printing purposes: the printouts can be filed with your other
rocket design documents. Eventually, mark the Calculator, and
"print marked" only.
- You can even do calculations in the input cells. Neat: measure
the circumference CF of the rocket, enter CF/pi as the diameter.
The default values give you an idea, resulting in a CLA of 500
- Not much input checking is done. Don't enter any undue characters
in the number fields.
This CLA calculator can be used with tubular fins as well. Klaas
Smith (KS) did test it with several rockets. Here are some snippets
of our email conversation:
KS: I make actively use of your CLA calculator. I am wondering if, and how,
I could use the calculator to calculate the CLA with rockets
with (a) circulair fin(s) behind the body of the rocket. Do you
have any ideas, suggestions or experience with this?
UH: Hmm, tubular fins, as I call them, are not as simple as flat,
level fins. If it is just 1 tube fixed straight *behind* the rocket
( and coaxial), I would suggest to enter the tube fin data just
with *negative* FP1 values. I checked the CLA calculator, he can
KS: Herewith I would like to give you some feed-back on my experiments with
the "tubular" fin type rockets by using your CLA calculator.
As discussed I took over your suggestions and calculated with negative
FP1 values (fin height + distance inbetween fin and rocket). I took
over as well your suggestion to "shadow" the tubular fin
and approaches the calculation presuming 4 fins and using 0.5 times
the diameter works very well. I have built several rockets with
different lenghts and different distances and the stability and
reproducability is very high. With all the rockets the the rocket
and the tubular fin have the same diameter. Small inbetween distances
f.e. 150 mm and large distances f.e. 450 mm, everything works very
well after calculating CLA and adapting point of gravity 1 diameter
to the top of the rocket.
Example: your fin is 50 mm long and has 101 mm
distance to the rocket (distance between both), you should enter
FP1 = 151 mm and FL1 = 50 mm.
difficult is FES1. The shadow method applies mainly for flat
surfaces. Tubular ones (hollow or not) will behave somewhat different.
In our example, if the fin has 60mm diameter, try entering 4 fins
and FES1=30mm as a start. This approximates the same shadow projected.
Again thanks a lot for using your CLA
calculator and to my opinion our water rocket colleagues could use
your CLA calculator to calculate CLA for "tubular" fins
With kindest regards, Klaas Smith, 1st Nov 2004