 USAF Digital DATCOM

The United States Air Force Stability and Control Digital DATCOM is a computer program that implements the methods contained in the USAF Stability and Control DATCOM to calculate the static stability, control and dynamic derivative characteristics of fixedwing aircraft. Digital DATCOM requires an input file containing a geometric description of an aircraft, and outputs its corresponding dimensionless stability derivatives according to the specified flight conditions. The values obtained can be used to calculate meaningful aspects of flight dynamics.
Contents
History
In February 1976, work commenced to automate the methods contained in the USAF Stability and Control DATCOM, specifically those contained in sections 4, 5, 6 and 7. The work was performed by the McDonnell Douglas Corporation under contract with the United States Air Force in conjunction with engineers at the Air Force Flight Dynamics Laboratory in WrightPatterson Air Force Base. Implementation of the Digital DATCOM concluded in November 1978.
The program is written in FORTRAN IV and has since been updated; however, the core of the program remains the same.
A report was published, separated into three volumes, which explains the use of Digital DATCOM. The report consists of
 Volume I, User's Manual
 Volume II, Implementation of DATCOM Methods
 Volume III, Plot Module
Inputs
Section 3 of the USAF Digital DATCOM Manual Volume I defines the inputs available for modeling an aircraft. The inputs are categorized by namelists to facilitate reading the file into FORTRAN.
Flight conditions and options
The FLTCON Namelist describes the flight conditions for the case. A maximum of 400 Machaltitude combinations can be run at once, with up to 20 angles of attack for each combination. The user can specify whether the Mach number and altitude varies together, the Mach number varies at a constant altitude, or the altitude varies at a constant Mach number. Both subsonic and supersonic analysis can be run in Digital DATCOM.
The OPTINS Namelist defines the reference parameters for the aircraft. The theoretical wing area, mean aerodynamic chord, and wing span are input along with a parameter defining the surface roughness of the aircraft.
Synthesis parameters
The SYNTHS Namelist allows the user to define the positions of the center of gravity and apexes of the wings. The X and Z coordinates are needed for the wing, horizontal tail, and vertical tail in order for the aircraft to be synthesized correctly. DATCOM does not require that the origin for the aircraft has to be the nose of the aircraft; any arbitrary point will do, but all of the dimensions need to be referenced from that point. Incidence angles can also be added to the wing and horizontal tail.
Body parameters
The BODY Namelist defines the shape of the body. Digital DATCOM assumes an axisymmetrical shape for the body. Up to 20 stations can be specified with the fuselage halfwidth, upper coordinate and lower coordinate being defined at each station. For supersonic analysis, additional parameters can be input.
Wing, Horizontal and Vertical Tail parameters
The WGPLNF, HTPLNF and VTPLNF Namelists define the wing, horizontal tail and vertical tail, respectively. The basic parameters such as root chord, tip chord, halfspan, twist, dihedral and sweep are input. Digital DATCOM also accepts wing planforms which change geometry along the span such as the F4 Phantom II which had 15 degrees of outboard dihedral.
Canards can also be analyzed in Digital DATCOM. The canard must be specified as the forward lifting surface (i.e. wing) and the wing as the aft lift surface.
For airfoil designations, most traditional NACA 4, 5, and 6 airfoils can be specified in Digital DATCOM. Additionally, custom airfoils can be input using the appropriate namelists. Also, twin vertical tails can be designated in Digital DATCOM, but not twin booms.
High Lift and Control Devices
Using the SYMFLP and ASYFLP Namelists, flaps, elevators, and ailerons can be defined. Digital DATCOM allows a multitude of flap types including plain, singleslotted, and fowler flaps. Up to 9 flap deflections can be analyzed at each Machaltitude combination. Unfortunately, the rudder is not implemented in Digital DATCOM.
Digital DATCOM also offers an automated aircraft TRIM function which calculates elevator deflections needed to trim the aircraft.
Other Inputs
Other Digital DATCOM inputs include power effects (propeller and jet), ground effects, trim tabs, and experimental data. The EXPRXX Namelist allows a user to use experimental data (such as coefficient of lift, coefficient of drag, etc.) in lieu of the data Digital DATCOM produces in the intermediate steps of its component buildup.
All dimensions are taken in feet and degrees unless specified otherwise. Digital DATCOM provides commands for outputting the dynamic derivatives (DAMP) as well as the stability coefficients of each components (BUILD).
Output
Digital DATCOM produces a copious amount of data for the relatively small amount of inputs it requires. By default, only the data for the aircraft is output, but additional configurations can be output:
 Body alone
 Wing alone
 Horizontal tail alone
 Vertical tail alone
 WingBody Configuration
 BodyHorizontal Tail Configuration
 BodyVertical Tail Configuration
 WingBodyHorizontal Tail Configuration
 WingBodyVertical Tail Configuration
 WingBodyHorizontal TailVertical Tail Configuration
For each configuration, stability coefficients and derivatives are output at each angle of attack specified. The details of this output are defined in Section 6 of the USAF Digital DATCOM Manual Volume I. The basic output includes:
 C_{L}  Lift Coefficient
 C_{D}  Drag Coefficient
 C_{m}  Pitching Moment Coefficient
 C_{N}  Normal Force Coefficient
 C_{A}  Axial Force Coefficient
 C_{Lα}  Lift Curve Slope (Derivative of Lift Coefficient with respect to angle of attack)
 C_{mα}  Pitching Moment Curve Slope (derivative of Pitching Moment Coefficient with respect to angle of attack)
 C_{Yβ}  Derivative of sideforce coefficient with respect to sideslip angle
 C_{nβ}  Derivative of yawingmoment coefficient with respect to sideslip angle
 C_{lβ}  Derivative of rollingmoment coefficient with respect to sideslip angle
For complete aircraft configurations, downwash data is also included.
When compared with modern methods of computational fluid dynamics, Digital DATCOM may seem antiquated. However, in its day, the program was an advanced estimation tool, and certainly much faster than plowing through pages and pages of engineering texts. Digital DATCOM is no longer supported by the USAF and is now public domain software.
Limitations
Inlets, external stores, and other protuberances cannot be input because Digital DATCOM analyzes the fuselage as a body of revolution. The simplification affects the coefficient of drag for the aircraft.
Dynamic derivatives are not output for aircraft that have wings that are not straighttapered or have leading edge extensions. This problem can be overcome by using experimental data for the wingbody (using nonstraight tapered wing).
There is no method to input twin vertical tails mounted on the fuselage, although there is a method for HTails. This problem can be addressed by approximating the twin vertical tails as a single equivalent vertical tail mounted to the fuselage.
Digital DATCOM cannot analyze three lifting surfaces at once, such as a canardwinghorizontal tail configuration. This problem can be addressed by superposition of lifting surfaces through the experimental input option.
Current Development
There are intentions among those that use this package to improve the overall package, through an easier user interface, as well as more comprehensive output data.
DATCOM+
Digital DATCOM has been resurrected by Bill Galbraith of Holy Cows, Inc. and improved. While the original DIGDAT program has been left relatively untouched, there has been a new frontend created that will allow the user to name the input file with something more significant than FOR005.DAT. The new input file format allows the user to place comments in the input file. There have also been hooks placed in the DIGDAT that allow for alternate outputs in addition to the original output format, which is 132 columns wide and slightly user abusive if you intend to import the data into another application. There is a graphical representation of the aircraft output in AC3D, as well as data table output in XML for the JSBSim and FlightGear projects, as well as a freeformat LFI (Linear Function Interpolation) data table file.
Holy Cows has packaged the Digital DATCOM program DIGDAT, with the described improvements, into a package called DATCOM+. Along with the DIGDAT program, there are viewers for the AC3D, XML, and LFI format output files. Data tables can easily be output to the screen or to PNG files for inclusion into reports. The source code is not available from Holy Cows, Inc., but they provide executables for Windows, Cygwin, and Linux.
Mathworks Aerospace Toolbox
Aerospace Toolbox includes a function for importing output files from Digital DATCOM into MATLAB. This function lets you collect aerodynamic coefficients from static and dynamic analyses and transfer them into MATLAB as a cell array of structures, with each structure containing information about a Digital DATCOM output file.
OpenDatcom
OpenDatcom is an opensource GUI for the Digital DATCOM created and hosted by the OpenAE [1] community. OpenDatcom incorporates all the basic (nonexperimental) functionality supported by the Digital DATCOM while providing realtime input error and bounds checking. An alpha version of the program was released November 1st, 2009 to the general public, and OpenDatcom remains under continuous development.
References
 Williams, John E., Vukelich, Steven R. "The USAF Stability and Control Digital DATCOM. Volume I. Users Manual." AFFDLTR793032 Volume I, Nov. 1979.
 Williams, John E., Vukelich, Steven R. "The USAF Stability and Control Digital DATCOM. Volume II. Implementation of Datcom Methods." AFFDLTR793032 Volume II, Nov. 1979.
 Williams, John E., Vukelich, Steven R. "The USAF Stability and Control Digital DATCOM. Volume III. Plot Module." AFFDLTR793032 Volume III, Nov. 1979.
 Blake, W. B. "Prediction of Fighter Aircraft Dynamic Derivatives Using Digital Datcom," AIAA 3rd Applied Aerodynamics Conference, AIAA19854070, Colorado Spring, CO, October 1985.
 The Mathworks, Inc. "ModelBased Design of a New Lightweight Aircraft," AIAA Modeling and Simulation Technologies Conference and Exhibit, AIAA20076371, Hilton Head, SC, August 2007.
See also
External links
 Holy Cows Datcom
 OpenDatcom
 Digital DATCOM at EmbryRiddle Aeronautical University
 Yahoo Digital_Datcom Group
 Air Force Research Laboratory Control Theory Optimization Branch
 Aerospace Toolbox for MATLAB
 JSBSim
 FlightGear
 1965DatcomSections15.pdf
 1965DatcomSections69.pdf
 AFRLTR793032 USAF DATCOM User's Manual, Volume 1
 AFRLTR793032 USAF DATCOM User's Manual, Volume 2
 AFRLTR793032 USAF DATCOM User's Manual, Volume 3
 AFWALTR0833048 McDonnell Douglas Corporation Final Report for DATCOM, April 1978
Categories: Aerospace engineering software
 Aerodynamics
 Aircraft controls
 Fortran software
 WrightPatterson Air Force Base
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