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Datcom+

Downloads

July 4, 2010

datcom_Windows_2.7.exe

datcom_linux_2.7.tar (datcom-modeler does not run automatically)

1. Fixed AC3D format for vertical fins. Fixed Citation.dcm as well.

2. Added datcom-modeler executable. Output is in name_1.ac, but it is not automatically displayed. Datcom-modeler does fuselages better than Datcom+ does. Datcom-modeler is open source. The source for datcom-modeler is available at http://gitorious.net/datcom-modeler. Thanks to Ron Jensen and Anders Gidenstam.

3. Fixed PreDat.exe to ignore comments that don’t have “*” in column 1, but in some other column.

4. Fixed some missing tables for spoilers in the .XML output.

5. Probably fixed some other stuff that I lost track of.

An exciting new feature was uncovered, which allows you to process your input file while in Notepad++ (Sorry, Windows only).

1. To set up Notepad++ to process your Datcom file directly from Notepad++:

Open up a .DCM file with Notepad++.
Select Plugins -> Plugin Manager -> Show Plugin Manager
Scroll down to the RunMe plugin, select the check box, and press INSTALL. After it installs, Notepad++ will have to be restarted.

Now, we need to make a minor modification to the RunMe plugin.

Click Plugins -> RunMe -> Options
In the upper left corner, select "Save only current file". That way, if you forget to save it before running it, it saves your file for you.

Now, in order to run this file through the Datcom processor, just hit SHIFT-F5, or select Plugins -> RunMe -< Shell excute current file

After the first time that you run your input through the Datcom processor, you can open up the .OUT file. Then, any time that you process your input file from Notepad++, it will automatically be updated.

Also, if you would like to automatically start up AC3DView when you process your file (or some other application that you ALWAYS do), modify the file:

c:\Program Files\Datcom\bin\Datcom.bat (or wherever Datcom is installed on your computer)

Near the end of this script, there is a command to automatically start up AC3DView. Uncomment this line (remove "REM").

For people with previous installations of Datcom+ and Notepad++

1. I fixed the problem where Notepad++ showed a shortcut key of Ctrl+Alt+D to process the current .DCM file. I've removed this. You do NOT have to perform this step, but if you want to remove that command from Notepad++, this is what you have to do:

· Uninstall Notepad++. You do not have to uninstall Datcom.

· Delete the C;\Program Files\Notepad++ directory

· Reinstalling Notepad++

· Reinstall Datcom+, at least version 2.6a

Digital Datcom User's Manual Digital_Datcom_Users_Manual_1.2.pdf Updated 2/28/08 Now with Bookmarks, thanks to Andreas Gäb

Predicted_Aero_Model.pdf - This was a write paper I wrote a while ago, comparing the results from DATCOM with actual simulators. I was very impressed with the results.

Known Problems

1. Defining airfoils manually (with upper and lower surface points), rather than using NACA numbers, does not provide any output for the AC3D picture only.

2. Fuselages are not drawn correctly if defined other than a circular cross-section.

What is Datcom+?

A long time ago, on a planet very much like this one, the United States Air Force had a method for determining the aerodynamic coefficients of an aircraft before it was built. It was contained in a series of notebooks with over 3100 pages. The Air Force built this knowledge into a computer program, called the Digital DATCOM.

Basically, what it allows you to do is to estimate the design aerodynamic coefficients of an aircraft either from a design, or for an existing aircraft,

·       Lift coefficients

·       Drag Coefficients

·       Side force coefficients

·       Roll moment coefficients

·       Pitch moment coefficients

·       Yaw moment coefficients

·       Changes in coefficients due to Power effects

·       Changes in coefficients due to Ground Effects

DATCOM+ is the Digial DATCOM program, with some front-end and back-end added for user convenience.

Front End - The input file format for DATCOM is not very user friendly.

$WGPLNF TWISTA=-3.0$

By adding a file pre-processor, the data file can contain comments and previous values, such as

* TWISTA Twist angle, negative leading edge rotated down (from
* exposed root to tip)
#T34 $WGPLNF TWISTA=-3.0$
$WGPLNF TWISTA=-3.0$

Back End -The original Digital DATCOM is rather prolific. With a short 125 line input file, it can generate 21,000 lines of output. Making sense of this data can take weeks. By adding a different format output section to the original program, the data is output in two new formats:

· Free-format LFI tables, for plotting with LFIPLOT.

· XML format, compatible with JSBSim

· AC3D Model

The AC3Dview program will display a view of the aircraft, at least what the DATCOM model says that it looks like. The left picture is of a Citation, the right pictures shows something wrong!!!

The LFIPLOT program will easily generate a data plot, such as this one for Lift Coefficient

The DATCOM+ program also generates output compatible with the JSBSim program. For example, the basic lift coefficient table from above looks like:

<function name="aero/coefficient/CLwbh">
    <description>
        Lift due to alpha
        Increase in CL decreases Period and damping,Dutch Roll damping
        CL is low for landing
    </description>
    <product>
        <property>aero/function/ground-effect-factor-lift</property>
        <property>aero/qbar-area</property>
        <table>
            <independentVar>aero/alpha-deg</independentVar>
            <tableData>
                -16.00     -1.286
                -8.000     -.6159
                -6.000     -.4200
                -4.000     -.2235
                -2.000     -.3126E-01
                   .000     .1609
                 2.000     .3557
                 4.000     .5533
                 8.000     .9495
                 9.000     1.038
                10.00     1.117
                12.00     1.254
                14.00     1.363
                16.00     1.328
                18.00     1.218
                19.00     1.167
                20.00     1.136
                21.00     1.115
                22.00     1.092
                24.00     1.062
            </tableData>
        </table>
    </product>
</function>

The JSBSim output is broekn into two pieces, which was just recently added. DATCOM+ tries to output the main file (e.g. citation.xml), which includes structure for propulsion, gear, autopilot, etc, but there is generally little to nothing in this file. The other file contains the aerodynamic coefficients (e.g. citation_aero.xml), DATCOM+'s strong suit. The main output cannot be plugged into the JSBSim program directly, as the XML file format used by JSBSim contains additional information such as propulsion and controls. The aerodynamic file should be a drop-in, though. So, once you build a JSBSim core model for your aircraft once, you can play with DATCOM+ and have it generate new coefficients easily.

In a future life, maybe DATCOM+ will be modified to provide simplistic propulsion and control models to allow the generated XML file to be dropped into JSBSim without modification.

I highly recommend that you investigate the JSBSim project at their web page http://www.jsbsim.org/

You can plot out the table sin a JSBSim configuration file using a great program by Agostino De Marco, called JIFF. It only makes sense to generate plots for the aerodynamic tables, so select that file (e.g. citation_aero.xml). An example of that output is shown here:

Yahoo Digital_Datcom Group

I recommend that you sign up for the Yahoo Digital_Datcom user's group at http://tech.groups.yahoo.com/group/digital_datcom/. I will post information about future updates there, and if you have problems or questions, you might find help there.

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