USAF DATCOM: Lesson #1 (3)

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Datcom_1-3.
Right now we are ready to run our very first DATCOM analysis. Make sure your file look exactly like the following:

kode

Save the file as for005.dat, in the same folder with Datcom.exe. Close the file, then run the datcom.exe (by double clicking it). In the same folder, datcom will create three other file, which will be for013.dat, for014.dat, and datcom.out. Right now Im gonna explain just the latest. Datcom.out is where the entire datcom output placed. Open it using notepad. If there nothing wrong in for005 file you created, the datcom.out should look like the folowing:

datcom.out

Congratulations. You've done your very first datcom analysis. Lets now observe what the datcom.out contains.
The structure of datcom.out is usually as follows:

figure

It contains (1) Datcom blah-blah (2) What you inputted in for005 (3) What Datcom read from your input (4) lifting surfaces cross section analysis (5) Static Aerodynamic analysis on flight condition you want (usually, this part is what you really want from Datcom).
By reading the output, Im sure you can guest each of its meaning. For a complete datcom.out interpretation, click here.
Right now, lets close the datcom.out, and go back to our for005.dat. As you may notice in datcom.out. The analysis is just carried out for one angle of attack condition. In fact, datcom could accomodate up until 20 angle of attack condition in one run. The same also true for the speed and air density (or height).
Now, change NALPHA value to 20.0, and ALSCHD to a range of angle of attack from -18.0 to 20.0, in the FLTCON namelist group, as shown as follows:

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Notice we also change the altitude (NALT, ALT) and speed (NMACH, MACH) for three condition each. Save your file (remember, as for005.dat). Close it, and run DATCOM.exe again. Your analysis in Datcom.out file will look as follows:

kode

Observe the analysis result. The aerodynamic characteristics is calculated for all angle of attack we inputted. Scroll down, and you will see more result for each speed and altitude combination. The information of speed, altitude, and other references, are described in the beginning of each results, as follows:

result

Now im going to explain one more variable in this first course. The LOOP variable, used in the FLTCON namelist. As you already notice, the DATCOM calculation before were done for each combination of height and speed. What if we want our analysis being conducted for each speed in each altitude? For this purpose, we need the LOOP variable. Write the variable in the FLTCON namelist in our for005.dat file, as follows:

kode

Save it again, close it, then run DATCOM again. Open the result. Now you get 9 condition analysis instead of 6. Try to change the LOOP value to 3.0. See what hapen in the result. For a complete explanation about the LOOP variable, click here.
Thats all for this very first lesson. Thank you.
And youre comments are always welcome. Download Source.
Next Lesson.

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USAF DATCOM: Lesson #1 (2)

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Datcom_1-2
If you forget to put a decimal, your datcom file would not run.

figure

another rule, always put a comma after the value, except maybe for the last variables value in a namelist, which you could simply end the value with dollar sign ($). Along with this explanation, many rules will emerge. For the complete writing rules, see here.
So now we have finished our wing input. What next? We just input the wing planform geometry. With only that, a wing cannot produce enough lift. It requires its cross section to be defined, the Airfoil! Lets use the most simple way to input Wing-1 airfoil using a control command, NACA. As explained before, our Wing-1 airfoil is NACA2412, a simple yet good airfoil for low speed aircraft. DATCOM, with the use of NACA control command, have a large collection of NACA airfoil, especially those that been created in the early years. You can see Datcom collection of airfoils here. We define our airfoil simply as follows.

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This sintax represented parameters as follows:

picture

The W after the control command NACA defined the naca controler to be used by Wing. You will find later that we can change this with H or V, which refers to the horizontal tail and vertical tail.
And now, our wing dimension is ready to be analyzed.
What we need to do next is to define the reference point for our calculation, then input our wing position from it. Usually we use the aircraft nose point for the reference point. However, we only have the wing, no fuselage. So lets just use our wing's root chord leading edge, as shown in the following picture:

figure

Then, we use the chord line as the reference line. From the defined reference point and line, we define the position of our center of gravity, lets just put it at 25% point of root chord and exactly on the chord line. We also need to define the wing root chord leading edge, which in this case, is zero horizontally and vertically. To input those positions, we use the namelist 'SYNTH', which mean synthesis. The code of this namelist are as follows. I assume you can guest each variable name meaning.

kode

Are we done yet? Nope. We need one more namelist for our simplest wing before we could analyze it. We need to define the wind/air properties, in which our wing-1 operates. For this we use the the FLTCON namelist, which mean flight condition. The required air property for datcom could vary, the simplest one is as follows:

kode

The code above includes the angle of attack, mach number, and reynolds number (you certainly could guest which one which). Notice there are variable NALPHA and NMACH, which is the number of angle of attack, and the number of the mach number respectively, in which we want to analyze our wing-1. Right now we input 1.0 for each of them, because we just want to analyze our wing at 0 degree angle, and at 0.2 mach. DATCOM could accomodate up until 20 angle of attack and 20 mach number on one runtime. I'll show you how to do this on our next analysis.

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USAF DATCOM: Lesson #1 (1)

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Datcom_1-1
so, lets begin the very first datcom lesson, by analyzing a wing planform. Just wing, without fuselage and tails. Let the wing, say, Wing-1, be as described in the figure below (click figure to enlarge):


The wing, as shown above, is a simple straight tapered wing, swept back, with some dihedral. The wing section is simply a naca 2412 airfoil.

Those geometrical parameters should be inputted to datcom using a dat file named specifically as for005.dat. This dat file should be in the same folder as the datcom.exe console. It is actually a good idea to make a specific folder for each analysis, maybe like the figure on the left.

So lets begin our work by creating a text file using notepad (wordpad or even ms word is also ok). Open a notepad file (obviously the name will be untitled first). Then lets use the first line by typing our analysis title, beginning with a control command CASEID, as follows (you may use your own title, but should always use the CASEID). The title in the example is "MyFirstDATCOM_Wing_1", as follows:



Next line, we define the dimension we want to use in our analysis, using the control command DIM, followed by the chosen dimension. In this case, lets use metric dimension, represented by M, as shown in the figure before. FYI, the DIM control command could define other dimension units like FT, CM, or IN. More information about DIM, click here.

Now, before we start some more complicated stuff, lets save our file first for005.dat file. We can easily do this by writing the .dat extension in the file name column, as shown below.


Next we move on for our first namelist, WGPLNF, the wing planform. A namelist defined as a group of variables that contain values of a aircraft part parameters. So, the WGPLNF namelist is a group of variable that define a geometrical values of a wing planform. Wing planform here refer to the top projection of a wing. Observe again our Wing-1 above.

What values do we need to declare to define the geometrical shape of wing-1? Right, we should declare the span, the root chord, the tip chord, sweep angle, twist angle, and dihedral angle. In datcom, we should also declare the wing type, which for our wing-1, its a simple 'straight tapered' wing planform, with a dihedral and lets assume the twist angle is zero.

So to declare the wing planform, we use the WGPLNF namelist, followed with the variables, enclosed with two dollar sign ($), as shown in the figure on the left.

I am sure you can figure out what the variables meant. However, I'll give you hints as follows:

TYPE = type of the wing (1 - Straight Tapered Wing, 2 -; Cranked Wing, 3- Double Delta Wing), explained later as the lesson goes
CHRDR = Root Chord Length
CHRDTP = Tip Chord Length
SSPN = Semi Span Length (Semi!)
SSPNE = Exposed (not covered by the fuselage) Semi Span Length (here, its the same as SSPN, explained later)
CHSTAT = Chord station where we measure our swept angle, here, we take the leading edge = 0. Commonly one use the 0.25, quarter of the chords
SAVSI = Swept Angle (positive backward)
DHDADI = Dihedral Angle
TWISTA = Twist Angle

The variables name is mandatory, you cannot name them other way. The variables I use in the example is the standard for a wing. There are few other variables existed in the WGPLNF namelist, which you could find here.

Notice in the previous figure, I wrote the namelist and variables, not in the first column of the window. It is on purpose. The fact is, Datcom requires the control command (CASEID, DIM) being written starting from the first column, the namelist (WGPLNF, preceeded by $) start from the second column, and the variables start from the third and beyond. Another rule is that you must input the values of each variables using at least one decimal. If you forget to put a decimal, (format *.*), your datcom file would not run. And one more thing, another rule, always put a comma after the value, except maybe for the last variables value in a namelist, which you could simply end the value with dollar sign ($). Along with this explanation, many rules in writing the for005.dat file will emerge. For the complete writing rules, see here.

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