Nazarbayev University Repository

The results of the first experiment indicate that propeller placement plays a crucial role in determining the vertical force generated by a model aircraft, especially at different throttle settings. To isolate the impact of the propellers on the wing, the thrust generated by the propellers was measured and accounted for later in the analysis. This approach allowed the evaluation of the air coupling effect of the propellers on the wing.

First, placing propellers too close to the wing can result in a degradation of overall performance.

Introduction

• Background
• Literature Review
• Aerodynamic Effects: Stall
• Aerodynamic Effects: BLI
• Propeller/Wing Interaction
• DEP: Numerical Studies
• DEP: Experimental studies
• Summary of the Literature & Research Gaps
• Research Aim and Objectives

The propulsion system can be placed along the wing span to re-accelerate the flow. The flow field above the wing is influenced by the flow field of the propellers when the propulsion is placed upstream of the wing. The longitudinal position of the propeller relative to the wing has minimal effect on the lift distribution of the wing.

One of those gaps is the influence of the horizontal distance between the trailing edge of the wing and the propeller on aerodynamics. Assess the influence of the distance between trailing edge and propulsion elements on the net forces acting on the wing. Assess the indirect influence of the propellers on the lift and drag forces to better understand how these are affected by the flow acceleration around the wing.

Figure 1.1 STOL aircraft examples [11, 12]

Experimental Setup

• Introduction
• Design description
• Baseline wing
• Propeller characteristics
• Brushless Motor
• Wind Tunnel
• DEP Configuration
• Measurement process description
• Angle of Attack
• Freestream Velocity & Pressure
• Lift and Drag
• Propeller RPM
• Experiment procedure
• Experiment #1: Effect on Net Forces
• Experiment #1: Effect on Lift & Drag
• Experiment #2: Steady Level Flight

The powered propellers are attached to the wing via mounts and a 5 mm rod supported by the side and center wing struts. A pitostatic tube is fixed to the front rear cross member or upstream of the test model in the working part of the wind tunnel. The study will focus on four different configurations that differ in the placement of the propellers relative to the trailing edge of the wing.

The propellers will be driven by an electric motor and the throttle on the propellers will be kept at 20% and 40% throughout the experiment. The experiment will involve measuring the net horizontal and vertical forces acting on the wing model for each of the four configurations. The standard deviation is a measure of the amount of variation or spread in the data.

Presumably, this arrangement would increase lift by accelerating the flow on the upper surface of the wing. In the second part of the first experiment, the aim is to distinguish between the lift generated by the propellers and the actual lift produced by the wing. To isolate the direct contribution of the propeller, it is essential to measure its thrust independently of the wing.

The goal is to adjust both the propellers' throttle and the freestream speed to achieve zero net horizontal and vertical force on the wing (Figure 2.14). The throttle of the propellers is controlled using a motor controller, and the free flow speed is controlled using the wind tunnel fan. The wing angle of attack is set to an initial value and the propeller throttle is set to a low value.

The throttle on the propellers is gradually increased while the net horizontal and vertical force on the wing is monitored using force sensors.

Figure 2.1 Baseline Wing Table 2.1 Wing dimension

Results & Discussion

Experiment 1: Propeller Position Effect on Net Forces

• Net Vertical Force
• Net Horizontal Force

At 20% throttle, the second best configuration was with the propellers positioned 0.71C vertically and 0.11C horizontally from the trailing edge of the wing. The proximity of the propellers to the wing could disrupt airflow over the wing and result in reduced lift. This effect may be particularly significant for the first and second configurations where the propellers were mounted close to the wing.

The airflow from the wing could have interrupted the flow around the propellers, leading to turbulent and non-uniform airflow, which could reduce the efficiency and performance of the propellers. Therefore, placing propellers above the wing can increase the lift generated by the wing, which can improve the aircraft's performance. Therefore, propeller performance can be affected by the interaction between the flow from the wing and the flow generated by the propellers, which can vary depending on the location and orientation of the propellers relative to the wing.

The second configuration, with the propellers located 0.22C vertically and 0.33C horizontally from the trailing edge, resulted in the lowest net horizontal force at both throttle settings. This configuration appears to strike a good balance between the propellers' horizontal and vertical distances from the trailing edge, minimizing their exposure to the low-pressure area above the wing. The third configuration, with the propellers located 0.71C vertically and 0.33C horizontally from the trailing edge, appears to have been affected by the flow from over the wing as the propellers were completely over the trailing edge.

The first configuration, with propellers positioned 0.22C vertically and 0.11C horizontally from the trailing edge, also had high horizontal thrust. This may be due to the positioning of the propellers close to the wing, which is likely to degrade their performance due to the low pressure region created by the wing.

Table 3.1 Net Vertical & Net horizontal forces at α = 10°

Experiment 1: Effect on Lift & Drag

This may be because the propellers are located closer to the wing in this configuration, which allowed the wing to prevent flow separation at higher angles of attack. The propellers helped activate the boundary layer and keep the airflow attached to the wing, resulting in increased lift. At smaller angles of attack, the third and fourth configurations outperformed the other two, most likely because the propellers were positioned vertically above the wing surface, resulting in flow acceleration (Figure 3.8).

This configuration helped increase the lift coefficient by activating the boundary layer of air around the wing. After an angle of attack of 10°, flow separation occurred and the positioning of the propellers did not allow the airflow to remain attached to the wing. As a result, the lift coefficient decreased and the performance of these configurations became worse compared to the other two configurations.

Figure 3.6 Lift Coefficient in each Position

Experiment 2: Steady Level Flight

For example, at an AOA of 2 degrees, the required throttle setting is 42.5%, while at an AOA of 20 degrees, the required throttle setting is only 33%. This trend suggests that wing-generated lift increases as AOA increases, allowing the aircraft to maintain level flight with less propeller power. For example, at an AOA of 2 degrees, the required freestream speed is 15.1 m/s, while at an AOA of 20 degrees, the required freestream speed is only 9.27 m/s.

As the AOA increases, the shape of the wing generates more lift, reducing the free stream velocity required to maintain stable level flight. At lower AOAs, increasing the throttle setting generates more lift with relatively little increase in drag, allowing the aircraft to maintain stable level flight with less power. At higher AOAs, increasing the throttle setting generates more drag with relatively little increase in lift, decreasing the overall efficiency of the system.

This trend suggests that the wing reaches a limit to the amount of lift it can generate as AOA increases, causing the required freestream speed to level off. This limit can be due to factors such as stall, where the airflow over the wing becomes turbulent and lift is reduced, or separation, where the airflow over the wing separates from the surface and lift is lost. These trends can help explain the behavior of the wing with two pusher propellers under different operating conditions and can inform the design and optimization of similar aircraft.

At this angle, a throttle level of 26.5% and a freestream speed of 11.42 m/s was sufficient to maintain wing height. To optimize the performance of the wing with two pusher propellers, the specific performance metric of interest must be considered.

Figure 3.11 Throttle vs AOA

Conclusion & Future Work

Conclusion

After 10° AOA, the freestream velocity began to decrease, indicating a reduction in lift generation by the wing and compensated by the propulsive lift generated by the propellers. The report's findings can be used to optimize propeller placement for maximum performance in the design and development of aircraft with similar configurations, and to understand the trade-off between energy consumption and lift generation in stable flight.

Limitations

Future Work

Ultimately, the results of this study contribute to a better understanding of the complex interaction between blades and propellers and may help improve the performance and efficiency of aircraft in the future. Gohardani, "A Synergistic Look at the Prospects of Distributed Propulsion Technology and the Electric Aircraft Concept for Future Unmanned Aerial Vehicles and Commercial/Military Aviation," Progress in Aerospace Sciences, vol. Meck, "Sustainable Aircraft Design - A Review of Optimization Methods for Electric Propulsion with Derived Optimal Number propulsion engines,” Progress in Aerospace Sciences , vol.

Teperin, "Parametric Studies on Airfoil-Boundary Layer Ingestion Propulsion System," SAE International Journal of Aerospace, vol. ZUMWALT, "Effects of wingtip-mounted propellers on wing lift and induced drag.," Journal of Aircraft, vol. Deng, "Validation of an actuator disk model for numerical simulation of propeller," Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering, vol.

Xu, “Effects of Distributed Propellers Slipstream on the Aerodynamic Properties of Wing,” Lecture Notes in Electrical Engineering, pp. Varela, “Computational Study of the Propeller Position Effects in Wing-Mounted, Distributed Electric Propulsion with Boundary Layer Ingestion in a 25 kg Remotely Piloted Aircraft,” Drones, vol. Wang, “Effects of Distributed Propulsion Critical Variables on the Aerodynamic and Propulsion Performance of Small UAV,” Lecture Notes in Electrical Engineering, pp.

Khang, “An Investigation on the effects of Distributed Propulsion System on Aerodynamic Performance of a Fixed Wing”, Proceedings of the URECA@NTU, 2021. Wu, “The Impact of Distributed Propulsion on the Aerodynamic Characteristics of a Blended-Wing-Body Aircraft ”, Lucht- en ruimtevaart, vol.