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Copyright 2004, Holy Cows, Inc. All rights reserved.

DATCOM Predicted Aerodynamic Model
Bill Galbraith billg@holycows.net www.holycows.net

updated: July 22, 2004

1.0 Introduction The fundamental purpose of the USAF Stability and Control DATCOM is to provide a systematic summary of methods for estimating stability and control characteristics in preliminary design applications. DATCOM iscontained in a series of notebooks with over 3100 pages. The Air Force Digital DATCOM program incorporates the methods contained in the DATCOM notebooks into a computer program. 2.0 DATCOM and DATCOM+ Programs Okay, so what IS this DATCOM stuff? Well, a long time, back before the advent of a computer on everyone’s desk, engineering was done through calculations done by hand. Aircraft designerswould spend years deriving predicted aircraft performance parameters before an aircraft’s design was finalized and an actual airplane was built. Much of their work was compiled into a series of notebooks, a compendium of the data they knew. This series of notebooks, which filled a 6-foot bookshelf, made the design process easier. With the advent of digital computers, a program was written to makethis data compendium more user-friendly. Rather than poring over volumes of paper copies, the user could specify to the computer the defining physical characteristics of the design aircraft, and very quickly obtain design coefficients of the design aircraft. Iterations for design changes could be made much more quickly. So, why do we care? Well, if the design is frozen, as we find with an aircraft inproduction, we can use the DATCOM program to derive the defining characteristic coefficients. Those can be applied to a generic flight simulation program, thus yielding a representative simulation of the aircraft. The original DATCOM program is written is some rather messy FORTRAN IV, but the predictive capabilities are very impressive. The original program probably was loaded with punch cards(Look up punch cards at a museum if you don’t know what I’m talking about). The output generated is quite extensive, but rather burdensome to work with.


Copyright 2004, Holy Cows, Inc. All rights reserved.

DATCOM+ is an attempt to make DATCOM a little more user-friendly. The original code has been ported to a PC, minor execution errors have been corrected, and a new front-end and newoutput formats have been added. The original code remains intact as much as possible, to maintain it’s capabilities without the introduction of new errors. 3.0 The Front-End The input format of DATCOM is a series of NAMELIST statements, listing input variables which describe the aircraft under consideration and variables which control the execution and output of DATCOM. For example:
DIM FT $FLTCONNALPHA=20.0$ $FLTCON ALSCHD(1)= -16.0, -8.0, -6.0, -4.0, -2.0, 0.0, 2.0, 4.0, 8.0,9.0, 10.0, 12.0, 14.0, 16.0, 18.0, 19.0, 20.0, 21.0, 22.0, 24.0$ $FLTCON WT=7000.0$ $OPTINS SREF=320.8$ $OPTINS CBARR=6.75$ $OPTINS BLREF=51.7$ $SYNTHS XCG=21.9$ $SYNTHS ZCG=3.125$

Now, by looking at these variables, you can probably figure out what they are, but the third variable, ALSCHD isn’t readily apparent.It is the list of angles of attack that you want to investigate for your aircraft. In order to know that, you’d have to have the 300 page DATCOM Users Manual available, and you’d have to pour through it looking for all the right terms. Over the years, you’ll eventually learn what each of the parameters mean, and how best to use it, and you’ll have a collection of previous aircraft to review if youwant to refer back to you previous work. The new front end to DATCOM contained in DATCOM+ does all of this for you. Let’s look at one term, BLREF, from the new version of the input file:
* BLREF Lateral reference length value of wing span used by program #T-34 $OPTINS BLREF=33.396$ $OPTINS BLREF=51.7$

Okay, here we have a comment with an asterick in front of it, and the comment describes the...