How to Set Up and Control Aerofoil Shapes in a Wind Tunnel

An aerofoil has key anatomical features: leading edge, trailing edge, camber, and chord line. The chord line runs from the furthest point of the leading edge to the aft point of the trailing edge, and is essential because angle of attack is measured between relative airflow and the chord line. Angle of attack is the fundamental factor determining whether an aircraft flies or stalls. Regardless of aircraft type, it will have an aerofoil, chord line, and relative airflow, making angle of attack universally important. The critical angle of attack is where airflow can no longer cling to the wing surface, causing a stall. Understanding these concepts is essential for all pilots.

In basic aerodynamics, the chord line connects the leading edge and trailing edge of an airfoil, the mean camber line is equidistant from both upper and lower surfaces, camber is the distance between the mean camber line and chord line, and the angle of attack is the angle between the chord line and the relative wind direction.

Airfoil profiles are defined by several key parameters. The forward-most edge is called the leading edge, while the trailing edge is at the back. A straight line connecting these points forms the chord line. The angle between the chord line and the flow direction is the angle of attack. The mean camber line is drawn midway between the upper and lower surfaces. Camber describes how curved an airfoil is, with positive camber indicating upward curvature, negative camber indicating downward curvature, and zero camber for symmetrical airfoils. Both camber and angle of attack significantly influence lift generation.

The chord line connects the leading and trailing edges of an airfoil. The camber line is the curve equidistant from the upper and lower surfaces. The angle of attack is the angle between the chord line and relative airflow. Positive camber (curved upper surface) generates lift at zero angle of attack. Negative camber (curved lower surface) is rarely used. Symmetric airfoils have identical upper and lower surfaces.

A wind tunnel consists of key components: inlet lip, settling chamber, convergent, propeller group, and test section. The settling chamber maintains constant velocity, while the convergent accelerates flow. Two fundamental equations govern tunnel analysis: the continuity equation (area × velocity = constant) and Bernoulli's equation (total pressure constant along streamlines). In an open test section, static pressure equals atmospheric pressure. The honeycomb structure aligns velocity vectors with the tunnel axis. Wire meshes straighten flow and produce pressure losses proportional to velocity squared. The convergent must avoid abrupt changes to prevent flow separation. The test section has maximum velocity and minimum static pressure. Blockage limits are 10% for open and 5% for closed test sections. Wind tunnels are classified by: (1) Impulsion vs. Aspiration (propeller upstream or downstream), (2) Open vs. Closed test section, (3) Circuit open vs. circuit closed. The boundary layer is the region where viscous effects are significant, requiring a fluid flow and solid surface. The no-slip condition applies: velocity is zero at the wall. As distance from the wall increases, velocity increases until reaching free stream velocity. The boundary layer thickness (δ) is defined where velocity reaches 99% of free stream velocity. At the leading edge, thickness is zero. Initially, flow is laminar (ordered). At a critical Reynolds number, transition to turbulent flow occurs. Turbulent boundary layers grow faster and offer more friction resistance.

Boundary layer wind tunnels simulate atmospheric boundary layer conditions for structural testing. The facility uses eight axial fans to drive airflow through honeycomb straighteners, optional Irwin spires for large-scale mixing, and roughness elements in the development section to grow the boundary layer. Standard tests include flow characterization (velocity probes), rigid pressure-tapped models, rigid models with high-frequency force balances measuring reaction forces, aeroelastic models accounting for structural flexibility, and cyberphysical models where characteristics change in real-time. Limitations include Reynolds number mismatch, inability to simulate large-scale turbulence effectively, usable profiles only up to ~25% of gradient height, and challenges with stationary/non-neutral profiles and higher-order flow characteristics.

This segment introduces the wind tunnel practical exercise. The instructor explains the main components: (1) Diffuser (grupo propulsor) at the bottom providing airflow; (2) Test section (cámara de ensayo) for experiments; (3) Settling chamber (cámara de tranquilización) ensuring uniform flow; (4) Convergent accelerating airflow; (5) Diffuser decelerating airflow after the test section. The instructor describes operating conditions: subsonic, continuous operation, open circuit with aspiration. The settling chamber has the largest diameter (lowest velocity), while the test section has the smallest diameter (highest velocity). The airflow direction is from left to right, and the test section is closed with an aluminum wall.

A wind tunnel is a machine that blows air on an object and accurately measures the effects. Design depends on the test object: airplanes need large tunnels with high speeds, while smaller objects like golf balls can use smaller tunnels. The test section must balance size - small enough for high speeds but large enough to avoid restricting airflow. The contraction section (4:1 ratio) accelerates air to 130 mph, while the diffuser recovers energy efficiently. Fans are placed at the rear to ensure smooth, uniform airflow rather than turbulent air from the front. The honeycomb section straightens air as it turns corners, functioning like an aerodynamic lens. Every air particle is a vector requiring correct angle and speed. Reynolds number determines when airflow qualities become consistent - for full-sized cars, speeds above 100 mph produce consistent air flow qualities. Wind tunnel design uses spreadsheets (Excel) as calculation tools, generating 3D point clouds that define the tunnel shape. Wind tunnels are bolted to the earth and don't need high-tech materials like carbon fiber or titanium. Construction uses concrete floors, steel I-beams, and wooden spars - similar to house building. Aerodynamics expertise comes from combining formal education with hands-on experience. Wind tunnels are designed for specific applications: the GM tunnel was for production cars, the NRC tunnel was all-purpose, neither was designed for race cars. Race car wind tunnels have unique requirements like very close ground clearance and dependence on airflow under the car. Boundary layer control keeps air moving along the floor because air naturally wants to slow down and stall. The system uses an air-to-air intercooler to keep injected air within 2° of ambient temperature. The HVAC system has 250 tons of cooling capacity. Wind tunnels generate significant heat because all electrical energy eventually turns into heat. Cars enter through a large door in the diffuser side. Curved walls reduce blockage effects by matching natural airflow around a car on a racetrack - air expands around the car and then collapses around it, creating a teardrop shape. Slotted walls serve as backup if the curved wall design doesn't work. The curved wall design was validated empirically by covering slots with aluminum covers. The tunnel uses 22 fans (18 in a rectangle plus 4 on top), each about 100 horsepower, selected for quiet operation near residential areas. Test cars are bolted to a balance in the basement, not supported by tires. The six-axis balance measures three forces (drag, side force, lift) and three moments (roll, yaw, pitch). Rolling roads move the floor at the same speed as airflow to simulate vehicle motion. Boundary layer is dead air along a stationary surface that drags down and stalls, creating a ramp of dead air that gets thicker downstream. Wind tunnels use suction (removing dead air) and blowing (energizing the boundary layer) to maintain thin boundary layers. Combining suction and blowing adds energy without changing mass flow, which is more efficient than moving floors. The entire floor system provides the correct boundary layer (zero thickness) regardless of how low the car sits. The front layer is a massive vacuum cleaner sucking away dead air. Multiple suction slots (1-5) incrementally remove air to keep the boundary layer thin. Forward-facing suction slots remove air while rearward-facing jets replace it. Tangential jets blow air out of very thin openings, speeding up air along the floor. The Kanda effect enables air to turn 90 degrees without raising surfaces above the flush floor. Tire rollers with electric motors spin at airflow speed, and turntables yaw the car about the front axle center line to simulate cornering conditions.

Manometers are pressure measuring instruments that use liquid columns to measure pressure differences; a piezometer measures pressure at a single point, a U-tube manometer measures pressure difference between two points, and a differential manometer measures pressure difference between two points in a system.

Pressure measurement employs multiple technologies: manometers use liquid columns (mercury preferred for accuracy), barometers measure atmospheric pressure with one sealed end, and pressure transducers convert pressure to electrical signals for rapid, precise measurements. Wind tunnel balances measure aerodynamic forces with three main types: three-component balances measure forces along three axes, six-component balances measure at six discrete points (A-F) for comprehensive force distribution analysis, and external balances measure forces outside the primary test section. These integrated measurement systems enable comprehensive aerodynamic characterization in wind tunnel experiments.

This video demonstrates how to calibrate and operate a lift and drag force balance system in a wind tunnel for aerodynamic measurements. The process involves setting up the force balance by mounting it upside down on a calibration fixture, connecting it to a digital readout, and applying known weights to calibrate the voltage output for both lift (up to 3.5 kg) and drag (up to 2.2 kg). After calibration, airfoils are mounted on the stinger using bolts and set screws, aligned to specific angles of attack, and tested in the wind tunnel. The voltage readings from the force balance are then converted to actual force values using the calibration curves established during the initial setup.

This section covers instruments for measuring mass and force. Mechanical balances compare mass by balancing objects with standard weights using a horizontal beam with knife edges. Types include mechanical balance and platform balance for heavy items. Electronic balances measure mass digitally without standard weights, displaying readings and calculating prices. Force meters (Newton meters) measure force directly in Newtons using a spring mechanism. The section explains working principles, reading methods, and differences between instruments.

When an airplane flies steadily at constant velocity, forces are in equilibrium: drag equals thrust, and lift equals total weight. This allows determination of aircraft weight capacity and thrust requirements. To measure airfoil efficiency, small copper tubes are built into the model and connected to manometers. The manometer principle involves liquid in a reservoir that remains at constant level when pressure is constant. When pressure increases, liquid is forced into the reservoir, lowering the field level; when pressure decreases, liquid rises above the reservoir level. Each manometer can be connected to different points on the airfoil to record pressures at various locations.