Table of Contents
FRICTION AIM Overview
FRICTION provides an estimate of laminar and turbulent skin friction and form drag suitable for use in aircraft preliminary design [1]. Taken from the FRICTION manual: "The program has its roots in a program by Ron Hendrickson at Grumman. It runs on any computer. The input requires geometric information and either the Mach and altitude combination, or the Mach and Reynolds number at which the results are desired. It uses standard flat plate skin friction formulas. The compressibility effects on skin friction are found using the Eckert Reference Temperature method for laminar flow and the van Driest II formula for turbulent flow. The basic formulas are valid from subsonic to hypersonic speeds, but the implementation makes assumptions that limit the validity to moderate supersonic speeds (about Mach 3). The key assumption is that the vehicle surface is at the adiabatic wall temperature (the user can easily modify this assumption). Form factors are used to estimate the effect of thickness on drag, and a composite formula is used to include the effect of a partial run of laminar flow."
An outline of the AIM's inputs, outputs and attributes are provided in AIM Inputs, AIM Outputs, and AIM Attributes, respectively.
The AIM requires dimensional units which are described in AIM Units.
Friction drag is estimated based on laminar (White, Viscous Fluid Flow, 1974 ed., pg 589-590.) and turbulent (an Driest II Method, NASA TN D-6945) flat plate skin friction. The reference length for lifting surfaces is estimated as the average segment chord length between two airfoil sections, and the total drag is an wetted area weighted sum of the segments. For bodies of revolution, the references length is the total length of body (nose-to-tail). The form factor for lifting surfaces is based on the average thickness to chord ratio for each segment, and the maximum diameter (estimated as 
FRICTION Modifications
While FRICTION is available from, FRICTION download, the AIM has re-implemented the calculations in memory. This has enabled the additional inputs not available in the original Fortran software.
The AIM previously used FRICTION source is still provided as part of CAPS, and contains a few modifications from the original source code. These modificiations allows for longer input and output file name lengths, as well as other I/O modifications. This modified version of FRICTION, friction_eja_mod.f, is supplied and built with the AIM. During the compilation the source code is compiled into an executable with the name friction (Linux and OSX) or friction.exe (Windows).
Examples
An example problem using the FRICTION AIM may be found at FRICTION AIM Examples.
