Aerodynamic Forces on a Wing and Airfoil Lab Manual, Exercises of Aerospace Engineering

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Jeff Jagoda

Copied liberally from A. Steinberg

Lab Manual: https://gtae.gitbook.io/ae2610/wing

Aerodynamic Forces on a

Wing and Airfoil

Familiarization with aerodynamic forces in a wind tunnel

• Introduction to airfoil and wing basics

Use and operation of:

The Low Turbulence Wind Tunnel

• Pitot Probes

Load Cells

• Pressure Transducers
• Analysis of load cell force/moment data to obtain aerodynamic loads

e.g., lift and drag curves, drag polars

Understand different types of uncertainty in data and learn the

basic steps in analyzing/reporting them

Laboratory Objectives

• Lift and drag analysis is crucial to almost every

aerospace structure and more!

• In AE: Wings, Helicopter Blades, Wind Turbines,

Compressor Blades, etc.

Other: Cars, Buildings, Bicycles, etc.

• Wind tunnel testing to acquire lift, drag, and

moment data is one of the most useful and

reliable ways to test aerodynamic bodies

• Validation of theoretical and computational solutions

As close to real-life scenario as possible before test

flights

• Often uses scale models

Flow visualization

• Many different scenarios can be tested!

Different airspeeds

• Different levels of turbulence

Crosswind performance

Experimental Motivation

• Aerodynamic bodies experience 3 forces and 3

moments:

• Forces
  • Lift balanced byGravity
  • Drag balance by Thrust
  • Side

• Moments

   Pitch 

  • Roll
  • Yaw
• Typically dependent on speed, density, wing area,

body geometry

Theory: Aerodynamic Forces

SIDE

• Dominant components for a wing/airfoil flying directly into the wind are

   Lift, drag, pitch 

  • Lift is normal to flow
  • Drag is parll fo flow
  • At chords/

Where Forces act = aerodynamic center

Theory: Forces on an Airfoil

• Pitching moment usually defined about aerodynamic center

(AC), where moment independent of pitch

for thin airfoils, AC ~ quarter-chord location

• Forces and moments are normalized by wing/airfoil geometry and dynamic pressure of the

oncoming flow

• Lift coefficient:
• Drag coefficient:
• Moment coefficient:
• Dynamic pressure:

L

LiftForce

C

q S



D

DragForce

C

q S



M

PitchingMoment

C

q Sc



2

0

q p p  v

  

Bernoulli’s equation

Valid for low speed

(incompressible) flow Stagnation

(total) pressure

Static pressure

Planform area, cord*span

q=dynamic pressure= (rU

2

/2)=((air

density*airspeed)^2/2)

Theory: Aerodynamic Coefficients

• Drag Polar

A correlation between the lift and drag coefficients for

a given wing/airfoil geometry

• For required lift (e.g., weight for constant altitude),

gives needed thrust (=drag for constant velocity)

• Stall - Loss of lift after increasing AoA too much! - Peak of Lift curve

The AoA at which this occurs is called the stall angle

Theory: Aerodynamic Performance

• Low speed, subsonic, open return wind tunnel

The air “circulates” from outlet to inlet but is not routed directly back to the inlet

(recirculating wind tunnel)

Low speed allows us to use Bernoulli’s equation

• Upstream blowdown – fan at inlet pushes air downstream to the test section
• Roughly square test section (42” x 42”)
• Max speed ~16 m/s or ~35mph

Equipment: Low Turbulence Wind Tunnel

• A look at the fan - Collection of bent flat plates

Run by a motor connected to a VFD

• VFD: Variable Frequency Drive

A device that changes the RPM of the motor

running the fan based on controller input

(electrical impulses)

Equipment: LTWT Fan and Motor

• ATI Gamma 6-component load cell to make our

measurements

• Array of strain gauges bonded to a machined

block of steel

• Steel block is designed such that strain gauges can

be placed in locations to resolve forces/moments

• Strain gauge voltages used with a calibration

matrix to obtain forces / moments

Equipment: The Load Cell

• Load cell is connected to a stepper motor that varies angle-of-

attack

• Stepper Motor Basics:

  • A microcontroller controls a stepper motor driver

    We use an ESP 32—a fancier Arduino Nano 

Stepper motor driver sends impulse electrical current to a

brushless DC motor

• Impulses based on microcontroller inputs

Each pulse of current turns the shaft through one precise, fixed

increment of movement

• Multiple pulses needed to turn the shaft multiple increments

Open loop feedback control

• Stepper motors count steps—the count gets refreshed each time the motor is

power cycled

• No true “zero” position!
• Think of ways to zero the angle of attack!

VERY precise movement!

• Wing model connects directly to load cell

Equipment: Model Positioning System

• Finite rectangular wing with NACA 0012

airfoil geometry

Symmetric wing

• 27” span and 12” chord

Removable endplate to test the article as

both a wing and airfoil

• Vertically mounted at the ¼ chord location

Removes moments created by wing mass

Easier to resolve pitching moment

Equipment: Test Article

• Stagnation and static ports connected to two ends

of differential pressure transducer

• Baratron is a capacitance-based transducer

Diaphram displaces based on differential pressure

Displacement interpreted as a change in capacitance in

circuit

• Output as a DC voltage change

Voltage change proportional to differential pressure

experienced

Equipment: Baratron Pressure Transducer

1

C F p

   

• Labview program talks to the stepper motor, baratron, and load cell - Controls: - Angle of attack via stepper motor
• What data will you get from the VI?

A file containing the raw data as follows (1000 samples/sec):

• Angle-of-attack (deg)
• Fx (axial force, N)
• Fy (normal force, N)
• Tz (pitching moment, Nm)
• Baratron voltage (V)

You are required to:

Transform load cell forces into lift/drag

Transform Baratron voltage into dynamic pressure and then wind speed

Perform Uncertainty Analysis

Data Acquisition: Labview VI