Aerodynamic Forces on a Wing in a Subsonic Wind Tunnel AerodynamicForces Lab -1 Learning Objectives 1. Familiarization with aerodynamic forces 2. Introduction to airfoil/wing basics 3. Use and operation of wind tunnels and standard instrumentation: sting balances, Pitot-static probes, pressure transducers 4. Analysis of aerodynamic force/moment data e.g., lift and drag curves, drag polars 5. Uncertainty analysis AerodynamicForces Lab -2 1
Lab Info Location Montgomery Knight 104 low speed wind tunnel When next week and the week after there will also be a 2 nd lecture next week Safety do not cross safety barrier near wind tunnel entrance if wind tunnel main power switch is on only enter and exit wind tunnel using stairs make sure all loose items (including people) are removed from wind tunnel before starting the fan handle the sting balance rod with GREAT CARE and make sure not to bend or twist it AerodynamicForces Lab -3 Aerodynamic Forces In general, aerodynamic bodies experience 3 forces and 3 moments (about some defined coordinate system) forces: lift, drag, side moments: pitch, roll, yaw AerodynamicForces Lab -4 pictures from ODU and NASA Aerodynamics 2
Wings and Airfoils Wing is major source of lift for most (fixed-wing) aircraft related to helicopter rotors, propellers, turbomachinery, Cross-sectional shape of a wing is known as airfoil rectangular wing has constant (unchanging) crosssectional shape Airfoil Nomenclature Wing Leading Edge Mean Camber Line Thickness Trailing Edge AerodynamicForces Lab -5 Airfoil v Angle of Attack (or Pitch) c Aerodynamics Chord Line Aerodynamic Forces on an Airfoil Dominant components for a wing/airfoil flying directly into the wind are Lift lift drag pitch Yaw v Pitch Roll Drag c/4 c Pitching moment usually Chord defined about aerodynamic center Line (AC), where moment independent of pitch for thin airfoils, AC ~ quarter-chord location AerodynamicForces Lab -6 Aerodynamics 3
AerodynamicForces Lab -7 Aerodynamic Coefficients Forces and moments are normalized by wing/airfoil geometry and dynamic pressure of the oncoming flow LiftForce lift coefficient C L qs DragForce drag coefficient C D q S moment coefficient dynamic pressure C M q PitchingMoment q Sc p o stagnation (total) pressure p 1 2 2 v static pressure Aerodynamics Bernoulli s equation valid for low speed (incompressible) flows Aerodynamic Performance From thin airfoil, predict that for small to moderate angles of attack Stall Point lift increases linearly C L with angle of attack For higher angles lift reaches peak = stall Drag Polar for required lift (e.g., weight for constant altitude) gives needed thrust ( = drag for constant velocity) AerodynamicForces Lab -8 C L C D 4
Sting Balance We will use a 6-component sting balance (Aerolabs) to make our measurements sting balance also know as internal balance (resides in wind tunnel) Pitch and yaw moments from two parallel strain gauges located about electrical center AerodynamicForces Lab -9 Measurements AerodynamicForces Lab -10 Data Datalogger Output What data will you get from the balance? a file with a time stamp for each reading from the balance followed by Normal1 Normal2 Side1 ` AF Raw forces Side2 Normal N1 N 2 Side S1 S2 Axial Pitch N1 N 2L 2 Roll(Mom.) Yaw S1 S2L 2 L=2.545 Normal Side Reduced forces SF Pitch(Mom.) AF Yaw(Mom.) Pitch and yaw angle data incorrect NF Measurements 5
Model Positioning System and Model Sting balance is connected to Model Positioning System (MPS) that can change pitch and yaw angles Rectangular wing connected to sting balance by aerodynamic support structure Flow Direction adjustable flap sting balance Bottom view of wing AerodynamicForces Lab -11 Measurements Pitot Static Probe Will use a Pitot-static probe to measure wind tunnel dynamic pressure p o p q p o p 1 2 2 v Stagnation and static ports connected to two ends of differential pressure transducer Baratron (capacitance-based) v C 1 F p AerodynamicForces Lab -12 Typical Capacitance-Type Press Transducer Measurements 6
GT Low Speed Wind Tunnel Open return type tunnel (vs. closed) Blown fan at entrance Rectangular test section (42 40) Fan Section Settling Section 132 Test Section 42 242 Top view of tunnel AerodynamicForces Lab -13 Wind Tunnel Experiment Perform measurements at 20 angles of attack (AOA) called pitch sweeps because MPS will dwell at each pitch angle for awhile and then move to next angle Record 50 force/moment measurements per AOA using computer #1 with Aerolab software Simultaneously measuring wind tunnel dynamic pressure (q ) using computer #2 with Labview software mean and rms of >1000 samples per AOA With multiple measurements can estimate random uncertainties AerodynamicForces Lab -14 7
Experiment Measure with and without wing attached support structure has its own aerodynamic loads Measure without wind on tare system (non-aero forces: weights) Measure with wind on for 2 flap angles zero (symmetric airfoil) non-zero (cambered) 2 wind speeds high speed (33-36 mph) low speed (21-24 mph) AerodynamicForces Lab -15 Lab Info Location Montgomery Knight 104 low speed wind tunnel When next week and the week after there will also be a 2 nd lecture next week Safety do not cross safety barrier near wind tunnel entrance if wind tunnel main power switch is on only enter and exit wind tunnel using stairs make sure all loose items (including people) are removed from wind tunnel before starting the fan handle the sting balance rod with GREAT CARE and make sure not to bend or twist it AerodynamicForces Lab -16 8