Aircraft Aerodynamic Characteristics - ITLiMS

The Faculty of Power and Aeronautical Engineering of Warsaw University of Technology – Aircraft Design Department
Project 4 – Aircraft Aerodynamic Characteristics
In this project student has to calculate aerodynamic characteristics of the aircraft.
Methods utilized are mix of analytical and computational analysis. Part of the software can
make only inviscid analysis and it has to be completed by analytical methods and 2D viscous
analysis.
In the manual to the project first the software is introduced. Next analytical methods are
revised with sufficient equations taken from flight mechanics. In the equations some
assumptions are needed different than in the flight mechanics, because of making part of the
calculations with aerodynamic software. After that requirements to finish the project are
stated.
1. Introduction to the software
AVL - Vortice lettice method software for aerodynamic and aircraft dynamic
stability analysis, originally written by Mr. Mark Drela and Mr. Harold
Youngren, is available free on GPL license on web page:
http://web.mit.edu/drela/Public/web/avl/.
Although, the program has text user interface and simple graphics, it
demonstrated it’s usefulness and is still improved and updated. Model
sailplanes designed with use of AVL can be seen on:
http://www.charlesriverrc.org/articles.htm.
AVL first steps:
- Binary version of the program appropriate for the system they use, example:
avl327.zip (name of the downloaded file may change with the versions). After
unpacking the program is ready to use without any installation.
- Text documentation for AVL avl_doc.txt, which contains detailed description of AVL
functionality.
- Short tutorials session1.txt and session2.txt. It is strongly recommended to go through
this tutorial to get familiar with the software.
- Files with run cases runs/ directory, which contain example geometry of planes, mass
distributions, flight conditions (run files) and other.
Jacek Mieloszyk, Agnieszka Kwiek – Materials for project for flight dynamics 1

The Faculty of Power and Aeronautical Engineering of Warsaw University of Technology – Aircraft Design Department
Fig. 1 Example of computations in AVL
PANUKL - Panel method software for aerodynamic analysis developed under supervision
of dr Grabowski at Aircraft Design Department on The Faculty of Power and
Aeronautical Engineering. The software is available free. It has user-friendly
GUI for grid preparation, analysis and post processing. The software,
examples and manual is available at:
http://www.meil.pw.edu.pl/add/ADD/Teaching/Software/PANUKL
PANUKL first steps:
- Download one of the versions on Windows or Linux and install on your system.
Opening it for the first time you will be asked to create work dirs for PANUKL. It’s an
important step, because if the work dirs won’t be created program may not be able to
make aerodynamic analysis.
- Manual for PANUKL may be opened from help menu. Remember to set English
language.
- Open and compute examples attached to PANUKL.
- In case of computations failure logs can be found in “C:\Users\ComputerUser\.panukl”
(Windows), “home\.panukl” (Linux).
Jacek Mieloszyk, Agnieszka Kwiek – Materials for project for flight dynamics 2

The Faculty of Power and Aeronautical Engineering of Warsaw University of Technology – Aircraft Design Department
Fig. 2 Example of computations in PANUKL
XFOIL - 2D panel method for viscous aerodynamic analyze of airfoils. The software
includes boundary layer analysis and computes all components of airfoil
drag. Examples and manual is available at:
http://web.mit.edu/drela/Public/web/xfoil/
XFOIL first steps:
- Binary version of the program appropriate for the system they use, example:
xfoil6.96.zip (name of the downloaded file may change with the versions). After
unpacking the program is ready to use without any installation.
- Text documentation for XFOIL xfoil_doc.txt
- Short tutorial sessions.txt . It is strongly recommended to go through this tutorial to
get familiar with the software.
Jacek Mieloszyk, Agnieszka Kwiek – Materials for project for flight dynamics 3

The Faculty of Power and Aeronautical Engineering of Warsaw University of Technology – Aircraft Design Department
Tips & Trics:
Fig. 3 Example of computations in XFOIL
- Reading manuals is unavoidable, but it is possible to use the software efficiently after
one day of learning.
- If you start AVL or XFOIL from script or shell you will be able to read why it crashed
even if it crashed.
- Read what is displayed on screen, it might tell you about your mistakes.
- Type “?” to get list of available commands in AVL and XFOIL.
- Modifying example files is butter than building your own from the beginning. This
let’s you avoid mistakes in the text files.
- Make small changes in the configuration files and control many times if you are still
able to load them. Make backup of files from time to time.
- Pay attention to units. It is advised to use SI units: meters and kilograms.
Jacek Mieloszyk, Agnieszka Kwiek – Materials for project for flight dynamics 4

The Faculty of Power and Aeronautical Engineering of Warsaw University of Technology – Aircraft Design Department
2. Flight mechanics methods revision
Estimation of friction drag
Total aircraft drag force consists of four components: pressure drag, friction drag,
induce drag and wave drag. This project assumes that a Student estimates characteristic which
include drag coefficient of whole airplane. Some components of drag have to be computed,
other have to be estimated analytically.
Both 3D programs are capable of computing only inviscid flow, therefore friction drag
of an airplane has to be estimated analytically from http://meil.pw.edu.pl/zm/ZM/ml_3l_en
with some modifications of the way of using equations. Part of the equations estimating the
pressure drag has to be omitted because this part is computed numerically. Moreover in the
analytical equations minimum drag of the wings includes friction drag and pressure drag.
Only friction drag should be added, because pressure drag is calculated by AVL or PANUKL.
To obtain the friction drag XFOIL can be used. The procedures in more detail are described
below.
Fuselage and nacelles
Pressure drag of fuselage is calculated numerically and only friction drag has to be
estimated from analytical formula (1). Value ηf is called slenderness coefficient or aspect ratio
coefficient of a fuselage. It is used to estimate pressure drag of the fuselage dependent on the
shape of the fuselage. Since geometry is defined in software this part of drag is already
calculated numerically and value of ηf has to be 1. Value ηMa is correction of fuselage drag due
to air compressibility effect. If airplane flies slow effect of air compressibility don’t have to
be considered. Otherwise it should be calculated numerically by setting appropriately solver.
In the equation (1) ηMa has to be always 1.
Wing and horizontal stabilizer
S
C C
Df
friction
wet
f Ma
(1)
S f
In case of wing and horizontal stabilizer drag is described by equation (2). Part of drag
dependent on lift coefficient is well calculated numerically. Only part of the minimum drag
and drag of gaps in the wing has to be included analytically. Minimum drag still contains
components of friction and pressure drag.
Vertical Stabilizer
C
C
DH DH
min
C
Dgap
2
LH
C
It is assumed that vertical stabilizer adds minimum drag, while plane isn’t turning and
doesn’t produce induced drag (3). Minimum drag again contains components of friction and
pressure drag.
DV
C DV CDgap
C
eH
min (3)
Jacek Mieloszyk, Agnieszka Kwiek – Materials for project for flight dynamics 5
(2)

The Faculty of Power and Aeronautical Engineering of Warsaw University of Technology – Aircraft Design Department
Other Parts – Parasite Drag
Small parts drag, that aren’t modeled in the airplane computational grid, should be
estimated according to flight mechanics guide.
Estimation of friction drag
For minimum drag of wings (4) only friction drag is needed because pressure drag is
computed numerically and here should be equal 0. Assuming 2D flow on the wing the drag
coefficient computed from XFOIL will be approximately equal to 3D wing. It is possible to
obtain only friction drag after XFOIL analysis.
Airplane drag
C D CDfriction
CDpressure
min (4)
Drag coefficient of a whole airplane is (5). Computations should end for maximum lift
coefficient achieved by the airfoil used in the airplane. Corrections for the maximum lift
coefficient can be made from flight mechanics.
S S S
C
S S S
f
V
H
C Dfriction wing CDfuselage
CDV
CDH
CDparasite
Dairplane CD
3D _ analysis _
(5)
3. Flaps analysis
The last part of the Project is the estimation of the lift coefficient due to flaps or other
super lift devices. The minimal airspeed in landing configuration should be checked and
compared with agreed earlier assumptions or Airworthiness regulations (eg. JAR 23.49 – Stall
speed). This part can be computed analytically or numerically by appropriate geometry
change due to flap deflection.
4. Project requirements
Download one of the 3D programs (AVL or PANUKL). Basing on user guide and
available examples prepare numerical model of your airplane. Next step is calculation of
aerodynamic characteristic (lift coefficient, drag coefficient and pitching moment coefficient
as a function of angle of attack) using previously selected program. Next, use XFOIL to
calculation wing friction drag. For rest components of the aircraft make analytical calculations
of friction drag. Finally calculate complete airplane’s drag using equation (6).
The result of this part of the project should be basic aerodynamic characteristics of the
whole aircraft: CL(Alfa) CD(Alfa) Cm(Alfa), polar drag CD(CL) Fig. 4. Results of
computation have to be presented in numerical and graphical form as well including example
of pressure distribution on the aircraft. All computations and assumptions must be
documented clearly. The report should be presented numerical and analytical part of
calculations.
Jacek Mieloszyk, Agnieszka Kwiek – Materials for project for flight dynamics 6

The Faculty of Power and Aeronautical Engineering of Warsaw University of Technology – Aircraft Design Department
Fig. 4 Aerodynamic characteristics
Jacek Mieloszyk, Agnieszka Kwiek – Materials for project for flight dynamics 7