Airfoil Roughness and Drag at Low Reynolds Number

ISEF Category: Engineering Technology: Statics and Dynamics

Subcategory: Aerospace and Aeronautical Engineering  ·  Difficulty: Advanced  ·  Setup: School Lab  ·  Time: Full Year

The Hook

A tiny bug splat can hurt a wing more than you think. On small airfoils, a little surface roughness can change lift, drag, and stall in a big way. That makes this project a real airplane problem, not just a model plane trick. You get to measure the effect instead of guessing.

What Is It?

Airfoils make lift because air moves differently over the top and bottom surfaces. When the surface stays smooth, the flow can behave in a predictable way. When dirt, dust, or insect remains stick to the wing, the flow can separate earlier, which means the air stops following the surface cleanly.

Think of it like riding a bike on a smooth road versus a road covered in gravel. The bike still moves, but it takes more effort, and control gets worse. Your roughness tiles act like controlled gravel spots. By changing their size, spacing, or placement, you can see how much performance drops on a small wing at low Reynolds number, which is a fancy way to say the airflow scale matches small aircraft, drones, and gliders.

Why This Is a Good Topic

This is a strong science fair topic because you can change one roughness variable at a time and measure a real aerodynamic response. It connects to drones, small aircraft, bird-like wings, and any design that flies at low speed. You can learn fluid mechanics, measurement design, calibration, and model validation without needing a university wind tunnel.

Research Questions

• How does roughness tile height affect the lift-to-drag ratio of a small airfoil at low Reynolds number?
• What is the effect of roughness tile placement near the leading edge versus mid-chord on stall behavior?
• Does roughness spacing change the drag penalty more than roughness height for the same total covered area?
• To what extent does surface contamination on one side of the wing change the measured lift asymmetry?
• Which airfoil shape loses the least lift when identical roughness patches are added?
• How does measured drag from the load-cell sting compare with XFOIL predictions for the same roughness condition?

Basic Materials

• Foam board or foam wing blanks for airfoil models.
• Access to a 3D printer for making roughness tiles.
• Digital calipers for measuring tile dimensions.
• Box fan or ducted fan tunnel setup.
• Homemade sting mount or rigid support arm.
• Load cell with amplifier or force sensor suitable for small aerodynamic forces.
• Microcontroller or data logger for force readings.
• Ruler or tape measure for positioning the wing consistently.
• Hot glue, double-sided tape, or thin adhesive for mounting tiles.
• Digital kitchen scale for checking model mass.
• Smartphone camera for documenting setup and surface placement.

Advanced Materials

• Laser-cut or CNC-machined airfoil models for tighter shape control.
• Multi-axis load cell or 6-axis force sensor for separate lift and drag measurement.
• Smoke wire, tufts, or flow visualization materials for observing separation.
• Pitot tube and differential pressure sensor for airspeed verification.
• Surface roughness gauge or optical profiler for checking tile geometry.
• High-resolution 3D printer for repeatable roughness features.
• Temperature and pressure sensors for air density correction.
• Mounting jig with angle-of-attack adjustment.
• Surface scan data from a flatbed scanner or photogrammetry setup.
• XFOIL or similar aerodynamic analysis tools for model comparison.

Software & Tools

• XFOIL: Predicts airfoil lift, drag, and stall trends so you can compare your measured data to a baseline model.
• Python: Organizes force data, computes averages, and plots lift-to-drag trends.
• ImageJ: Measures tile size, placement, and surface coverage from photos.
• Logger Pro: Records sensor output and helps you inspect force signals during testing.
• Excel: Tracks trials, calculates summary statistics, and builds graphs for your report.

Experiment Steps

1. Define the airfoil shape, Reynolds number range, and roughness feature you will test first.
2. Plan a baseline wing that stays identical across trials except for the roughness pattern.
3. Design a measurement setup that can separate force changes from fan noise, alignment error, and mount vibration.
4. Build a calibration plan that turns sensor output into force values you can compare across conditions.
5. Choose a data analysis method that converts your readings into lift, drag, and lift-to-drag ratio.
6. Set up an XFOIL comparison plan so you can test where the model matches, and where it fails.

Common Pitfalls

• Mounting the wing at slightly different angles between trials, which changes lift more than the roughness does.
• Letting the box fan create uneven airflow across the test section, which makes force readings drift from one side of the wing to the other.
• Using roughness tiles that are not identical in height or shape, which turns one variable into several.
• Measuring force without subtracting the empty mount baseline, which hides the small aerodynamic signal you want.
• Comparing XFOIL output directly to raw sensor values, which mixes model predictions with uncorrected experimental noise.

What Makes This Competitive

A stronger project will do more than show that roughness hurts performance. You can compare multiple roughness geometries, test whether placement matters more than height, and correct your data for baseline drag and tunnel variation. You can also check how well XFOIL matches real measurements across several Reynolds numbers, then explain where the model breaks down. That kind of careful analysis turns a simple wing test into a real engineering study.

Project Variations

• Test insect-like roughness only near the leading edge and compare it with uniform dust-like coverage across the chord.
• Compare two or three airfoil shapes, such as a cambered wing, a flat plate, and a symmetric section, under the same roughness pattern.
• Add flow visualization with tufts or smoke to see whether roughness changes where separation starts before you measure the force data.

Learn More

• NASA Glenn Research Center Aerodynamics pages: Search NASA for introductory airfoil and lift resources, plus small-Reynolds-number explanations.
• MIT OpenCourseWare Fluid Mechanics: Find free lecture notes and problem sets on boundary layers, lift, drag, and flow separation.
• XFOIL documentation and user notes: Search for the original XFOIL manual and example cases to understand airfoil prediction inputs and outputs.
• NACA airfoil database: Search NASA archives for classic airfoil coordinates and airfoil performance reports.
• PubMed: Search for review articles on insect contamination, surface roughness, and aerodynamic performance in small aircraft or blades.
• NOAA National Weather Service aviation resources: Use background material on air density, wind, and atmospheric conditions that affect air testing.