Digital Datcom
Encyclopedia
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 fixed-wing 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
.
. 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
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.
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.
, 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.
, elevators
, and ailerons can be defined. Digital DATCOM allows a multitude of flap types including plain, single-slotted, and fowler flaps. Up to 9 flap deflections can be analyzed at each Mach-altitude 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.
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).
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:
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.
Dynamic derivatives are not output for aircraft that have wings that are not straight-tapered or have leading edge extensions. This problem can be overcome by using experimental data for the wing-body (using non-straight tapered wing).
There is no method to input twin vertical tails mounted on the fuselage, although there is a method for H-Tails
. 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 canard-wing-horizontal tail configuration. This problem can be addressed by superposition of lifting surfaces through the experimental input option.
, as well as data table output in XML
for the JSBSim
and FlightGear
projects, as well as a free-format 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.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
Air Force
United States Air Force
The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...
Stability and Control Digital DATCOM is a computer program that implements the methods contained in the USAF Stability and Control DATCOM
USAF Stability and Control DATCOM
The United States Air Force Stability and Control DATCOM is a collection, correlation, codification, and recording of best knowledge, opinion, and judgment in the area of aerodynamic stability and control prediction methods...
to calculate the static stability, control and dynamic derivative characteristics of fixed-wing aircraft
Fixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...
. 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
Flight dynamics
Flight dynamics is the science of air vehicle orientation and control in three dimensions. The three critical flight dynamics parameters are the angles of rotation in three dimensions about the vehicle's center of mass, known as pitch, roll and yaw .Aerospace engineers develop control systems for...
.
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 Wright-Patterson Air Force BaseWright-Patterson Air Force Base
Wright-Patterson Air Force Base is a United States Air Force base in Greene and Montgomery counties in the state of Ohio. It includes both Wright and Patterson Fields, which were originally Wilbur Wright Field and Fairfield Aviation General Supply Depot. Patterson Field is located approximately...
. 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 Mach-altitude 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 numberMach number
Mach number is the speed of an object moving through air, or any other fluid substance, divided by the speed of sound as it is in that substance for its particular physical conditions, including those of temperature and pressure...
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 gravityCenter of gravity
In physics, a center of gravity of a material body is a point that may be used for a summary description of gravitational interactions. In a uniform gravitational field, the center of mass serves as the center of gravity...
and apexes of the wings. The X- and Z- coordinates are needed for the wing
Wing
A wing is an appendage with a surface that produces lift for flight or propulsion through the atmosphere, or through another gaseous or liquid fluid...
, horizontal tail
Tailplane
A tailplane, also known as horizontal stabilizer , is a small lifting surface located on the tail behind the main lifting surfaces of a fixed-wing aircraft as well as other non-fixed wing aircraft such as helicopters and gyroplanes...
, 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
Angle of incidence
Angle of incidence is a measure of deviation of something from "straight on", for example:* in the approach of a ray to a surface, or* the angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to the axis of the fuselage.-Optics:In geometric...
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 half-width, 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, half-span, twistWing twist
Wing twist is an aerodynamic feature added to aircraft wings to adjust lift distribution along the wing.Often, the purpose of lift redistribution is to ensure that the wing tip is the last part of the wing surface to stall, for example when executing a roll or steep climb; it involves twisting the...
, dihedral and sweep
Swept wing
A swept wing is a wing planform favored for high subsonic jet speeds first investigated by Germany during the Second World War. Since the introduction of the MiG-15 and North American F-86 which demonstrated a decisive superiority over the slower first generation of straight-wing jet fighters...
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
Canard
Canard may refer to:*Nicolas-François Canard , French mathematician and economist*Canard , a small wing mounted on the front of some aircraft, which can be beneficial or detrimental depending on the design of the plane and its intended use*Canard , a phenomenon in some slow-fast dynamical systems...
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, flapsFlap (aircraft)
Flaps are normally hinged surfaces mounted on the trailing edges of the wings of a fixed-wing aircraft to reduce the speed an aircraft can be safely flown at and to increase the angle of descent for landing without increasing air speed. They shorten takeoff and landing distances as well as...
, elevators
Elevator (aircraft)
Elevators are flight control surfaces, usually at the rear of an aircraft, which control the aircraft's orientation by changing the pitch of the aircraft, and so also the angle of attack of the wing. In simplified terms, they make the aircraft nose-up or nose-down...
, and ailerons can be defined. Digital DATCOM allows a multitude of flap types including plain, single-slotted, and fowler flaps. Up to 9 flap deflections can be analyzed at each Mach-altitude combination. Unfortunately, the rudder
Rudder
A rudder is a device used to steer a ship, boat, submarine, hovercraft, aircraft or other conveyance that moves through a medium . On an aircraft the rudder is used primarily to counter adverse yaw and p-factor and is not the primary control used to turn the airplane...
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 build-up.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
- Wing-Body Configuration
- Body-Horizontal Tail Configuration
- Body-Vertical Tail Configuration
- Wing-Body-Horizontal Tail Configuration
- Wing-Body-Vertical Tail Configuration
- Wing-Body-Horizontal Tail-Vertical 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:
- CL - Lift CoefficientLift coefficientThe lift coefficient is a dimensionless coefficient that relates the lift generated by a lifting body, the dynamic pressure of the fluid flow around the body, and a reference area associated with the body...
- CD - Drag CoefficientDrag coefficientIn fluid dynamics, the drag coefficient is a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment such as air or water. It is used in the drag equation, where a lower drag coefficient indicates the object will have less aerodynamic or...
- Cm - Pitching Moment CoefficientPitching momentIn aerodynamics, the pitching moment on an airfoil is the moment produced by the aerodynamic force on the airfoil if that aerodynamic force is considered to be applied, not at the center of pressure, but at the aerodynamic center of the airfoil...
- CN - Normal Force Coefficient
- CA - Axial Force Coefficient
- CLα - Lift Curve Slope (Derivative of Lift Coefficient with respect to angle of attack)
- Cmα - Pitching Moment Curve Slope (derivative of Pitching Moment Coefficient with respect to angle of attack)
- CYβ - Derivative of side-force coefficient with respect to sideslip angleSideslip angleSideslip angle, also called angle of sideslip , is a term used in fluid dynamics and aerodynamics and aviation. It relates to the rotation of the aircraft centerline from the relative wind...
- Cnβ - Derivative of yawing-moment coefficient with respect to sideslip angle
- Clβ - Derivative of rolling-moment coefficient with respect to sideslip angle
For complete aircraft configurations, downwash
Downwash
In aeronautics downwash is the air forced down by the aerodynamic action of a wing or helicopter rotor blade in motion, as part of the process of producing lift....
data is also included.
When compared with modern methods of computational fluid dynamics
Computational fluid dynamics
Computational fluid dynamics, usually abbreviated as CFD, is a branch of fluid mechanics that uses numerical methods and algorithms to solve and analyze problems that involve fluid flows. Computers are used to perform the calculations required to simulate the interaction of liquids and gases with...
, 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
Public domain
Works are in the public domain if the intellectual property rights have expired, if the intellectual property rights are forfeited, or if they are not covered by intellectual property rights at all...
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 straight-tapered or have leading edge extensions. This problem can be overcome by using experimental data for the wing-body (using non-straight tapered wing).
There is no method to input twin vertical tails mounted on the fuselage, although there is a method for H-Tails
Twin tail
A twin tail is a specific type of vertical stabilizer arrangement found on the empennage of some aircraft. Two vertical stabilizers — often smaller on their own than a single conventional tail would be — are mounted at the outside of the aircraft's horizontal stabilizer...
. 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 canard-wing-horizontal 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 front-end 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 AC3DAC3D
AC3D is a 3D design program which has been available since 1994. The software is used by designers for modeling 3D graphics for games and simulations - most notably it is used by the scenery creators at Laminar Research on the X-Plane...
, as well as data table output in XML
XML
Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
for the JSBSim
JSBSim
JSBSim is an open source Flight Dynamics Model software library that models the flight dynamics of an aerospace vehicle. The library has been incorporated into the flight simulation packages FlightGear and OpenEaagles and a commercial . It can also be called from a small standalone program to...
and FlightGear
FlightGear
FlightGear Flight Simulator is a free, open-source multi-platform flight simulator developed by the FlightGear project since 1997....
projects, as well as a free-format 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 open-source GUI for the Digital DATCOM created and hosted by the OpenAE http://openae.org/software community. OpenDatcom incorporates all the basic (non-experimental) functionality supported by the Digital DATCOM while providing real-time 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.External links
- Holy Cows Datcom
- OpenDatcom
- Digital DATCOM at Embry-Riddle Aeronautical University
- Yahoo Digital_Datcom Group
- Air Force Research Laboratory Control Theory Optimization Branch
- Aerospace Toolbox for MATLAB
- JSBSim
- FlightGear
- 1965-Datcom-Sections1-5.pdf
- 1965-Datcom-Sections6-9.pdf
- AFRL-TR-79-3032 USAF DATCOM User's Manual, Volume 1
- AFRL-TR-79-3032 USAF DATCOM User's Manual, Volume 2
- AFRL-TR-79-3032 USAF DATCOM User's Manual, Volume 3
- AFWAL-TR083-3048 McDonnell Douglas Corporation Final Report for DATCOM, April 1978