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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.
Instructions here.

Name of Rocket

 Date & Time


NL = Nose Length

Nose Form





BD = Body Diameter

BL = Body Length





FES1 = Fin1 Exposed Semi Span

FL1 = Fin1 Length

FP1 = Fin1 Position

Number of Fins1

3 4





FES2 = Fin2 Exposed Semi Span

FL2 = Fin2 Length

FP2 = Fin2 Position

Number of Fins2

3 4

All dimensions in  mm

CLA position:



  1. 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).
  2. 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.
  3. Important: do not tolerate leading zeroes in your entered numbers. The calculator may give out weird results.
  4. Set all unused values to zero.
  5. 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.
  6. 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 mm.
  7. Not much input checking is done. Don't enter any undue characters in the number fields.

Tubular fins

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 handle that.
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.
More 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.

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.
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 as well.
With kindest regards, Klaas Smith, 1st Nov 2004


Gyro Gearloose: Where's that CLA formula??



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Stand / Last Revision:   08.11.04  (02.11.02 )

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