⚡ electrical engineering

Experimental Investigation of the Wake Flow of a Propeller-Wing Configuration in the Laminar Wind Tunnel

This study experimentally investigates the mutual aerodynamic interference between a laminar wing and three distinct propeller geometries in a wind tunnel, revealing that spanwise variations in wing loading induce lateral shearing in the propeller slipstream and asymmetric thrust distribution, effects that diminish as the advance ratio increases.

Original authors: Iftekher Ahamed, Niko Konrad, Ulrich Deck, Thorsten Lutz

Published 2026-07-06

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CC BY 4.0

Original authors: Iftekher Ahamed, Niko Konrad, Ulrich Deck, Thorsten Lutz

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a busy highway where a powerful truck (the propeller) is driving right next to a long, sleek bridge (the wing). Usually, we think of the truck's wind blowing straight back and the bridge just sitting there. But in this study, researchers wanted to see what happens when these two actually interact. They built a special, very smooth wind tunnel to watch this "traffic jam" of air in slow motion.

Here is what they found, explained simply:

1. The "Squashed" Wind Tunnel

The researchers used a high-tech probe (like a tiny, super-sensitive weather station on a stick) to measure the air behind the truck and under the bridge. They found that the wind coming off the truck doesn't stay in a perfect circle. Instead, the bridge pushes and pulls on it, squashing it sideways.

The Analogy: Imagine blowing a perfect ring of smoke through a straw. Now, imagine someone holding a flat board right next to the smoke ring. The board doesn't just block the smoke; it stretches the ring out, making it oval-shaped and twisting it. That's what the wing does to the propeller's wind.

2. The "Uneven" Push

The study showed that the wind doesn't push evenly on all sides. Because the bridge creates its own downward wind (downwash), it messes with the truck's engine intake.

The Result: One side of the propeller's wind gets a harder push than the other. It's like if you were trying to blow up a balloon, but someone was blowing air against one side of it. The balloon would get lopsided. The researchers found this caused an imbalance in pressure, making the wind swirl differently on the "up" side versus the "down" side of the propeller.

3. The "Splitting" Vortex

The most dramatic finding involves the "tip vortices." These are the swirling tornadoes of air that spin off the very tips of the propeller blades.

The Analogy: Think of a spinning top that leaves a trail of smoke. When that smoke trail hits the bridge, it doesn't just bounce off. The bridge acts like a knife, slicing that single smoke trail into two smaller trails. One part goes over the top of the bridge, and one goes under. They travel separately for a while before eventually trying to merge back together.
The Twist: As these split trails move, they make the bridge's own wake (the air behind the bridge) wiggle and curve back and forth, like a snake moving in rhythm with the spinning propeller.

4. Speed Matters

The researchers tested this at different speeds.

Fast Forward: When the propeller spins very fast relative to the forward speed (high "advance ratio"), the wind is so strong that the bridge has a harder time messing with it. The wind stays more circular.
Slow Motion: When the propeller spins slower relative to the forward speed, the bridge has a much bigger effect. The wind gets squashed and twisted much more dramatically.

5. The "Turbulence" Hotspots

Finally, they looked at how "rough" or chaotic the air was.

The Finding: The air is calmest in the middle of the streams and most chaotic where the truck's wind and the bridge's wind crash into each other. This is where the air gets the most "shaken up," creating high turbulence. Interestingly, the very center of those spinning tornadoes (the vortex cores) was the most turbulent part of all.

The Bottom Line

This paper is a detailed look at how a propeller and a wing "fight" over the air they share. They found that the wing distorts the propeller's wind, splits its swirling tornadoes, and creates an uneven push. Understanding this "dance" is crucial for engineers who want to design planes that are more efficient and quieter, especially for new designs where engines are placed right next to wings.

What they didn't do: The paper focuses strictly on measuring and describing these wind patterns in a wind tunnel. It does not test these findings on real airplanes in the sky, nor does it propose specific new engine designs yet; it simply provides the "map" of how the air behaves so engineers can use that map later.

Technical Summary: Experimental Investigation of the Wake Flow of a Propeller-Wing Configuration in the Laminar Wind Tunnel

Problem Statement and Context
The increasing global demand for commercial air traffic necessitates advanced aerodynamic solutions to reduce emissions while sustaining growth. Within the Transregio Collaborative Research Center Synergies of Highly Integrated Transport Aircraft (SynTrac), distributed propulsion is investigated as a means to improve overall efficiency through aerodynamic integration. While previous research has established the importance of wake-boundary layer interactions and the spatial structure of propeller wakes, there remains a need for detailed, two-dimensional analysis of the mutual interference between a propeller and a laminar wing. Specifically, the unsteady nature of these interactions, including the deformation of the propeller slipstream and the splitting of tip vortices, requires further experimental characterization to understand loss mechanisms and secondary flow phenomena.

Methodology
The study was conducted in the Laminar Wind Tunnel (LWT) at the Institute of Aerodynamics and Gas Dynamics (IAG), University of Stuttgart. The experimental setup consisted of a laminar wing (span $b=2.3$ m, chord $c=0.35$ m, designed for $Re = 1.25 \cdot 10^6$ and $c_l = 0.73$ ) mounted horizontally. A single, two-bladed, fixed-pitch propeller ( $R=0.15$ m) was positioned upstream of the wing on a pylon in a tower configuration, avoiding a nacelle-pylon interface to isolate interaction effects.

Three distinct propeller geometries were tested at advance ratios ( $J$ ) ranging from 1.07 to 2.14. A Vectoflow FRAP-Pro five-hole probe was utilized to traverse a predefined measurement plane downstream of the wing. The probe operated at a sampling rate of 100 kHz, with data subsequently downsampled to 10 kHz via block averaging to reduce computational load while preserving flow physics.

Data processing involved two primary averaging techniques:

Temporal Averaging: Measurement points were mapped to an equidistant grid ( $250 \times 50$ ) and arithmetically averaged to characterize the mean flow field.
Phase Averaging: A rotary encoder provided the propeller azimuth angle ( $\Psi$ ), allowing measurement points to be subdivided into phase bins ( $\Delta\Phi = 6^\circ$ ). This resolved unsteady effects and instantaneous blade positions.

Flow quantities, including pressure coefficient ( $c_p$ ), tangential velocity ( $V_t$ ), and turbulence intensity ( $T_u$ ), were derived from calibrated pressure measurements.

Key Results
The experimental data revealed several critical aspects of the propeller-wing interaction:

Lateral Shear and Wake Deformation: The propeller slipstream undergoes lateral shearing due to spanwise variations in wing loading. This deformation is most pronounced at higher advance ratios ( $J \approx 2.11$ ), where the wake deviates from an idealized circular shape. As the advance ratio decreases ( $J \approx 1.07$ ), the lateral shear becomes less pronounced, and the wake retains a more circular profile.
Asymmetric Thrust and Pressure Distribution: The presence of the wing modifies the propeller inflow, leading to an asymmetric thrust distribution. The descending blade side ( $y/R < 0$ ) exhibits locally increased pressure coefficients, while the ascending side shows different characteristics. This results in a horizontal pressure imbalance and a shift in the operating point.
Tip Vortex Splitting and Evolution: Phase-averaged analysis confirms that blade tip vortices interact with the wing by splitting into two sub-vortices upon reaching the leading edge. These sub-vortices convect independently along the suction and pressure sides of the wing. At the trailing edge, they partially re-merge, though lateral shear persists. The splitting process induces spanwise flow along the wing surface.
Tangential Velocity and Downwash Interaction: The wing-induced downwash (approximately 1.5 m/s) interacts with the propeller swirl. On the ascending blade side, the downwash counterbalances the tangential velocity, reducing local swirl. Conversely, the descending side exhibits higher swirl. This superposition creates an asymmetric swirl distribution within the slipstream.
Turbulence Intensity: Turbulence levels are highest in the cores of the tip vortices ( $T_u \approx 7-8\%$ ) and in regions where the propeller and wing wakes overlap. The interaction zones show increased turbulence compared to isolated configurations, particularly near separation zones and pressure gradients.

Significance and Claims
The authors assert that this study provides a fundamental understanding of the wake shape and its evolution in a propeller-wing configuration. By resolving the two-dimensional wake field both spatially and temporally, the research demonstrates that:

Mutual interference induces significant horizontal asymmetries in the flow, altering local total pressure and effective angles of attack.
The wing substantially impacts the shape of the turbulent propeller wake, with deformation diminishing at lower advance ratios.
Phase-averaged measurements are essential for identifying the splitting and independent convection of tip vortices, a phenomenon obscured in quasi-steady evaluations.

The paper concludes that these findings offer initial insights into loss mechanisms and secondary flow phenomena. The authors state that a deeper comprehension of these aerodynamic phenomena will enable the derivation of aircraft-specific design parameters. Future work is proposed to validate and improve numerical simulations and to investigate complementary parameters, such as varying the number of propellers along the wingspan or the vertical position of the propeller, to exploit synergistic effects more effectively.