Marko's bulb calculator

Marko Majic has developed a very useful bulb calculator, currently standing at version 1.3, and has given me permission to offer it for downloading. It is a 450 kB *.ZIP file, that turns into a 1 MB executable called "bulbcalc.exe" when unzipped. (So you know you'll need an unzipper utility...) These are some instructions for using it.

When you fire it up, you see a window showing a view of a bulb shape, and a set of numbers running down the left side. The numbers give various weights of the bulb shape for given bulb lengths. One row of the set is highlighted, being the length of the bulb that gives the specified target weight.

Opening window

To specify the bulb shape you want, first go to the "File > Select NACA foil..." item. A list pops out. Chose between the 00, 63, 63A, 64, 64A, 65, 65A, 66, and 67 series. All the shapes are symmetrical, and the actual thickness of the bulb depends upon the choice you make for the bulb parameters in the next step.

Menu to select NACA shape

Now it is time to set the bulb parameters. Go to the "File > Set bulb parameters" menu item, and enter the values you require for target weight, material density, height/length ratio (thickness to chord ratio), and width/height ratio (flatness, if width/height ratios is greater than 100%) into the dialogue box. You are offered lead as the default material density. Small values of height/length, around 10% or 12%, give long skinny bulbs. Larger values, around 20% or 22%, give dumpy bulbs. The bulb is symmetric if the width/height is 100, and becomes progressively flattened as you use values of 140%, 180%, and so on.

Bulb parameters dialogue box

When you are done, you can view the results, which is essentially a list of the sections of the bulb you have specified. Go to the "File > View bulb data" menu item. The "Set options" dialogue box pops up, where you select the number of sections you want, and the distribution of these sections. A "cosine" distribution provides more sections forward, and fewer aft. If you set 9 sections, you will get 11 stations of information.

Options dialogue box

When you click on OK, a window opens showing the bulb data (CG, volume, wetted area), and showing the section measurements you need to make your own mould. Change the units (metric or imperial) as you wish. Click on the button to change the options if you want.

Bulb and section data

So we can see that, for the 00xx NACA bulb shape and thickness to chord of 15% chosen (ie NACA 0015), a 2.4 kg bulb will be about 28 cm long, and have a wetted area of 252 sq.cm. That is a very useful result for not a lot of work. Now, let's see, what happens to wetted area if I use a dumpy bulb? What about a 65A-0xx NACA section? Wonderful. Thanks, Marko.

The "File" menu offers some options to download airfoil data, and import section co-ordinates if you want to design your own bulb. The "View bulb data" dialogue also allows you export the bulb data to a text file, so you can play with it elsewhere. Heck, you can use the program just to show you what the various NACA sections look like....

2005-12-18