Predicting High-Lift Aerodynamics of the Dornier 228 Using Fidelity CFD

Dornier 228 wind tunnel model installed in the RUAG LWTE (Pal et al., 2024)

High-lift configurations are where aircraft aerodynamics become complicated. When flaps are deflected, flow is bent aggressively, pressure gradients intensify, and separation begins to lurk around every corner. When using CFD to predict aircraft performance, the real debate usually begins with the choice of turbulence model.

The paper “Computational and Experimental Investigation of Dornier 228 Aerodynamic Performance Prediction in High-Lift Configuration” directly addresses this challenge by comparing in-tunnel and free-air CFD simulations, performed using Cadence Fidelity CFD, with wind-tunnel experiments carried out in the RUAG Large Subsonic Wind Tunnel in Emmen (LWTE). The study evaluates the accuracy of predicted lift, drag, and flow behavior under complex high-lift conditions, with the primary objective of assessing the reliability of CFD in one of the most nonlinear and sensitive regimes of aircraft aerodynamics.

Why are High-Lift Configurations Difficult to Simulate?

High-lift configurations operate at low speeds and high angles of attack, especially during takeoff and landing. To remain airborne, aircraft rely on flaps and modified control surfaces that increase camber and effective wing area. While this improves lift, it also creates strong pressure gradients, thick boundary layers, curved streamlines, and complex wake structures. From a simulation perspective, these flows are demanding because they depend heavily on turbulence modeling, near-wall resolution, and separation prediction. Small numerical changes can cause large shifts in predicted stall angle or drag.

These challenges become especially evident when applied to a real aircraft configuration, where geometric complexity and coupled flow effects must be captured reliably, making the Dornier 228 an ideal test case for assessing the practical capabilities of CFD in high-lift analysis.

The Dornier 228 Test Case

The Dornier 228 is known for its excellent short takeoff and landing performance, achieved through an efficient high-lift system. In the studied configuration, the aircraft uses deflected flaps and modified ailerons that significantly alter the flow field around the wing. The purpose of the study was to test whether CFD could reproduce the aerodynamic behavior observed in experiments, particularly for lift, drag, pitching moment, and stall characteristics. Answering this required both carefully executed wind tunnel experiments and high-quality numerical simulations.

Wind Tunnel Testing

Wind tunnel tests were carried out in the RUAG Large Wind Tunnel in Emmen, Switzerland, using a scaled model of the Dornier 228 in high lift configuration. Measurements of lift, drag, and pitching moment were taken over a wide range of angles of attack, including conditions close to stall. Corrections were applied to ensure the results represented free-air behavior as closely as possible, and an uncertainty analysis was performed to assess the reliability of the data, especially near stall, where unsteadiness and nonlinear effects increase.

Large Wind Tunnel Emmen layout (Pal et al., 2024)

CFD Simulation

On the computational side, CFD simulations of the full aircraft geometry in the high-lift configuration were performed using Fidelity CFD. Both tunnel and free-air simulations were carried out. This allowed us to distinguish between true aerodynamic effects and artifacts caused by tunnel confinement. The mesh was refined in critical regions, including the flaps, trailing edges, and near-wall areas, to capture boundary-layer development and separation.

Aircraft Surface Mesh using Fidelity Hexpress (Pal et al., 2024)

Two turbulence models were used to understand their impact on prediction accuracy. The simulations solved the Reynolds Averaged Navier Stokes equations using the Fidelity Flow Solver.

Modeling both tunnel and free-air conditions proved important. By including the tunnel walls in the CFD simulation, the simulations could reproduce experimental conditions more realistically and help identify which differences were due to wall effects rather than weaknesses in turbulence modeling. This approach improved correlation between CFD and corrected experimental data and strengthened confidence in the validation process.

Results and Comparison

CFD results generated using a best-practice design assessment grid with Fidelity Flow Solver show good agreement with experimental data. Across the linear angle-of-attack range (−2° to +11°), lift, drag, and pitching moment coefficients agree within 1–5%, with slope differences of approximately 6% (lift), 3% (drag), and 10% (pitching moment). At critical and post-stall conditions, discrepancies increase to around 9%, reflecting the higher sensitivity of separated flows.

Lift coefficient as a function of angle of attack for the corrected and uncorrected wind tunnel data, as well as free air and in-tunnel CFD results (Pal et al., 2024)

Drag coefficient as a function of angle of attack for the corrected and uncorrected wind tunnel data, as well as free air and in-tunnel CFD results (Pal et al., 2024)

Pitching moment coefficient as a function of angle of attack for the corrected and uncorrected wind tunnel data, as well as free air and in-tunnel CFD results (Pal et al., 2024)

Local flow features on the aileron also correlate well with experimental observations. Despite minor differences in the flap and aileron deflection angles, CFD captures the location and extent of the separation and recirculation regions observed in oil-flow visualization. Additional postprocessing confirms that key vortical structures and recirculation patterns are consistent between wind-tunnel and free-air simulations, indicating minimal influence from tunnel wall and support interference.

a) Experimental oil flow visualization on the aileron; and b) surface streamlines based on viscous stress vector for the solution obtained with the Fidelity Flow PBS solver (Pal et al., 2024)

Lessons Learned for High-Lift Scenarios: Predict Early, Validate Always

• High-lift performance is mission-critical: It directly influences takeoff and landing distances, stall margins, and operational flexibility – and drives key design decisions early in the design cycle.
• CFD is now design-relevant, not just exploratory: The accuracy of Fidelity CFD helped in reliably capturing key high-lift flow physics and support flap optimization and drag reduction, significantly reducing reliance on costly experimental iterations.
• Experiments remain indispensable: Wind-tunnel testing provides validation, physical grounding, and confidence in CFD predictions, especially for complex, nonlinear flow behavior.

The Dornier 228 study reinforces that the most effective high-lift design strategy combines CFD-driven insight with targeted experimental validation. When used together, simulation and testing transform complex high-lift flows from design risk into an optimizable engineering problem.

Reference:

Botond Iosif Pal, Artemii Sattarov, Luca Mangani, Olivier Thiry, Daniel Steiling, Mario Rüd, and Philippe Stephani. "Computational and Experimental Investigation of Dornier 228 Aerodynamic Performance Prediction in High-Lift Configuration," AIAA 2024-4432. AIAA AVIATION FORUM AND ASCEND 2024. July 2024.

For deeper insights into the study conducted by Cadence and RUAG on high-lift prediction of Dornier 228, read the AIAA Paper.