AE 3903/4903Airfoil Design

Lecture 1

OVERVIEW

• Introductory Remarks

• Your first analysis tool – Panel Method

Introductory Remarks

• Airfoil Design is an “open-ended” process.– There may not be a unique shape meeting the design

specs.– Each one of you may come up with different shapes

for the same design specifications.– In some cases, no airfoil shape may exist that will

meet the design specifications. You may need to settle for the closest or “optimum” answer, rather than the “exact” answer.

– Airfoil design is often an iterative process, where small but steady improvements are made to the airfoil shape until the design specifications are closely met.

Example: Design an airfoil that has the best L/D ratio

at a specified lift coefficient.

Cl

Cl

CdTarget lift

Two airfoils may have the same minimum “Lift to Drag ratio” at theSame target lift coefficient.

In the above example, both airfoils meet the design specifications (example lowest L/D at a specified Lift)

Their off-design performance may differ significantly, however.

Target lift

Airfoil 1

Airfoil 2

Airfoil 2

Airfoil 1

Example 2Sometimes there may be no airfoil shape

that will meet the design specifications

x/c

Cp

Designer specifies a desiredPressure distribution

A fish tail shape may occur

An open-ended trailing edgeMay occur.

If this occurs, the design specifications may need to be modified.For example, the designer may need to change the target pressure

Distribution.

Design Specifications

• Design specifications (or design criteria) that the airfoil should meet may vary from one application to another. Here are some examples.– Target Cl given, lowest L/D is required.– Target pressure distribution, known to have good

boundary layer characteristics, is given. This requires expertise on the part of the designer.

– Higher lift stall angle is required or specified.– Higher moment stall angle is required or specified.– Trailing edge (or leading edge) separation is to be

eliminated.– Laminar flow is required.

Single Point vs. Multi-Point Design

• In some design problems, it is enough to meet a single operation point. This is called a single point design.– For example, design an airfoil that has the lowest L/D

at Cl=0.5, at a cruise Mach number of 0.75.• In other cases, the design must meet

satisfactory performance at two or more conditions. This is called a multi-point design.– Example, satisfactory subsonic cruise performance

(lowest L/D at M=0.78 at Cl=0.5), and acceptable supersonic dash performance ( L/D ~ 4 at M=2.0)

Steps to a Successful Design

• Acquire analysis tools – potential flow analysis of flow over an airfoil, boundary layer analysis

• Validate the tools before you use them.• Specify design criteria.• Start with a known airfoil shape, as close to the

target as you can.• Use an iterative process to gradually improve

the starting geometry/shape.• Use the new design in a 3-D analysis (or tunnel

test) to make sure that the new airfoil does indeed perform better.

For this course, choose from one of the following

• Laminar airfoil design• General aviation airfoil Design• Supercritical airfoil Design• Wind Turbine Airfoil Design• Tools needed: panel method or CFD method

(supplied), laminar and turbulent boundary layer analyses (supplied), transition prediction method (supplied), design method (needed)

• Verify your design in a 3-D lifting line or CFD method (supplied)

Assignment IDue on September 10, 2002

• Write a 5 page report summarizing– Design problem you have decided to study (one of the four

choices)– Why did you choose this problem– Literature survey of previous work related to your specific design– What are some of the design criteria that have been used in past– The specific design point you would like to work on

• You may change your mind later. If you decide to work on a different problem, just let me know. However, the sooner you settle on the design problem, the sooner you will get it done.

• All work must be done independently. It is Ok to exchange your sources of information.

Some Sources

• NASA Technical Report Server http://techreports.larc.nasa.gov/cgi-bin/NTRS/

• Papers by Eppler (laminar airfoil, general aviation airfoil)

• Papers, books by: (a) Tranen, (b) Bauer-Garabedian-Korn on supercritical airfoil

• Papers and website by Prof. Selig at U. Illinois• National Renewable Energy labs web site:

http://www.nrel.gov/wind• I will complement this with papers by my

students, colleagues, and me.

First Set of Tools

• Panel method

• Laminar Boundary layer Analysis

• Turbulent Boundary layer Analysis

• Transition prediction methods

• We will start on panel method today. The source code and sample input are available at our AE 2020 web site: http://www.ae.gatech.edu/~lsankar/AE2020