Features of the flow around the model of a small-sized aircraft of a classical layout with a distributed electric propulsion

. The article presents the results of experimental studies on the influence of the work of a distributed electric propulsion on the flow around the straight wing of a small-sized aircraft. The flow structure was studied at natural (flight) Reynolds numbers, taking into account the angle of attack. The results of the experiments are visualization patterns obtained using the soot-oil coating method.


Introduction
As is known, the wing is an important structural element of any aircraft.It creates lift, allows maneuvering, provides lateral stability and controllability of the aircraft, the flight performance of the aircraft directly depends on its shape and size.
During the operation of the wing, various adverse phenomena can occur, including separation and global stall.These phenomena are undesirable and require elimination, since they lead to inevitable energy losses and adversely affect the aerodynamic quality of the aircraft.A stall is accompanied by a sharp drop in lift, an increase in resistance to movement, and an increase in vibrations, which in turn leads to a deterioration in the stability and controllability of the aircraft in extreme conditions.In this regard, for many years, one of the important tasks of aerodynamics has been the problem of improving the flow around the wing.The improvement of the flow is understood as an increase in the laminar flow zone and the elimination of separated flows at high angles of attack.
When solving the identified problem, it is necessary to take into account aerodynamic interference, since in most cases it leads to an increase in the total drag and to a decrease in the lift force of the aircraft.Based on these considerations, research aimed at studying the influence of various elements of the aircraft, in particular the distributed electric propulsion, on the structure of the wing flow, is a very actual task.
Currently, the concept of the distributed electric propulsion is associated with the future of the aircraft industry.Numerous studies show that this technology will reduce the noise level, fuel consumption, increase the stability and controllability of aircraft, and will also allow creating more efficient aerodynamic schemes of aircraft [1][2][3].One of the features of aircraft with the distributed electric propulsion is the close integration of the distributed electric propulsion with the wing surface.
The purpose of this research is to experimentally study the influence of the work of the distributed electric propulsion on the vortex structure of the flow on a straight wing.The work is a continuation of a series of studies devoted to the study of separated flows and the possibilities of controlling the flow around various layouts of models of small-sized aircraft [4][5].The studies were carried out at natural Reynolds numbers, which are typical for small-sized aircraft.The results are flow patterns obtained by soot-oil coating method.

Research methodology
To achieve this goal, a basic model of a small-sized straight-wing aircraft was designed (Fig. 1).The model has the following geometric dimensions: wingspan -0.75 m, chord -0.15 m, fuselage length -0.7 m.The fuselage consisted of the lower and upper parts, which were fastened with bolts.This engineering solution made it easy to dismantle the fuselage during the experiments.Behind the trailing edge of the wing was a distributed power plant, including 6 engines (impellers).The engines were installed so that the axis of rotation of the propeller was at the level of the trailing edge.Figure 2 shows the geometric dimensions of the propeller.Using a hot-wire anemometer, the speed was measured E3S Web of Conferences 459, 03005 (2023) https://doi.org/10.1051/e3sconf/202345903005XXXIX Siberian Thermophysical Seminar in the wake of the engine (Fig. 3).The maximum value was 50 m/s at 32800 rpm.The studies were carried out in the T-324 subsonic low-turbulence wind tunnel of the Khristianovich Institute of Theoretical and Applied Mechanics, Siberian Branch of the Russian Academy of Sciences in Novosibirsk.The T-324 wind tunnel is a closed-type installation and has a closed test section with dimensions of 1 × 1 m 2 and a length of 4 m.This wind tunnel is characterized by a low degree of free flow turbulence in the test section (less than 0.04%), therefore it is mainly used for experiments aimed at studying the process of laminar-turbulent transition.The experiments were carried out at angles of attack α=0⁰, α=5⁰ and α=10⁰ and at a slip angle β=0⁰.The oncoming flow velocity was 25 m/s, and the Reynolds number along the chord was Re=2.5×10 5 .The velocity control was carried out with a Pitot-Prandtl tube connected to an alcohol pressure gauge.Three flow modes were considered: − model without distributed electric propulsion; − model with blocked engines (imitation of malfunction); − model with engines operating at maximum rotational speed.The main method used to visualize the flow was the method of "soot-oil" coatings.Typically, when using this method, the surface of the model is covered with a mixture of various oils with pigments that give the color of the mixture (most often transformer oil and carbon black).In this case, a mixture of titanium dioxide powder and kerosene was used.The principle of the method is as follows.First, the model is installed in the test section of the wind tunnel, then the upper surface of the wing is covered with a mixture and the wind tunnel is launched.When flowing around, particles of titanium dioxide powder thicken and settle, forming white and black stripes in accordance with the limiting streamlines.After complete evaporation of kerosene, the resulting flow pattern is recorded using a camera.Thus, the method makes it possible to obtain qualitative information about the flow structure on the wing surface.
Another visualization method was the method of silk threads, which was used to determine the critical angle of attack or flow instability.Silk threads, about 1.5 cm long, were glued to the wing at one end.During the experiments, silk threads were turned in the direction of the flow on the surface of the model, which made it possible to conclude that there was a stalled or attached flow around.

Experimental results
Below are photographs of the obtained flow patterns.For a better understanding of the observed flow, a graphic diagram is attached under each photo.The flow is directed from top to bottom.At α = 0⁰, on the model without the distributed electric propulsion, local separation zones formed in front of the elevons (Fig. 4).In the front part of the wing, the flow is laminar, it flows from the leading edge to the separation zone, near this area a laminar-turbulent transition occurs and then the turbulent flow attaches the wing surface.The presence of blocked engines behind the trailing edge of the wing affected the location of the separation bubbles (Fig. 5).The bubbles shifted towards the leading edge, while the laminar flow zone decreased.In addition, separation zones formed in front of the engines due to locking.Turning on the engines was accompanied by a shift of the local separation zones back towards the trailing edge, and the separation areas in front of the engines decreased (Fig. 6).
Figures 4 and 5 show titanium dioxide particles about 500 µm in size as dots.Particles got on the surface of the wing when applying the mixture.During the experiment, they played the role of roughness, behind each of them a turbulent trace appeared, which broke the separation zone into parts.A similar effect can be achieved by the point installation of protrusions on the leading edge of the wing.
Figure 7 shows that an increase in the angle of attack to 5⁰ on the model without engines was accompanied by a displacement of the bubbles to the front of the wing.In the modes with the engines blocked and on, the bubbles were located on the leading edge parallel to it (Fig. 8 and 9).The difference between these modes is that two separations occurred on the wing in the flow mode with the engines blocked.First, a separation of the laminar boundary layer occurred on the leading edge, the flow changed from laminar to turbulent motion and reattached to the wing surface.Then, in the rear part of the wing, the turbulent boundary layer detached without its subsequent reattachment.The operation of the engines contributed to the elimination of separation of the turbulent boundary layer.At α = 10⁰ the flow structure on the wing has completely changed.When there was no distributed electric propulsion behind the trailing edge of the wing, a large-scale vortex appeared on each console (Fig. 10).The installation of engines contributed to the occurrence of global stall (Fig. 11).On the entire surface of the wing, the flow became reverse, a stagnant zone appeared on the leading edge in front of the stall zone, and vortices formed at the side edges of the wing.Turning on the engines did not lead to an improvement in the flow around the model (Fig. 12).

Conclusions
Within the framework of this work, a set of experimental studies of the influence of the work of a distributed electric propulsion on the flow around a model of a small-sized aircraft with a straight wing was carried out.The results of the research are flow patterns obtained by the "soot-oil" visualization method.The main results of the work are as follows: • The restructuring of the flow structure is shown depending on the angles of attack on a straight wing with and without a distributed installation; • It was found that the presence of a distributed electric propulsion at subcritical angles of attack leads to displacement of the separation bubbles towards the leading edge, as well as to a decrease in the length of the laminar flow; • It has been demonstrated that at subcritical angles of attack on a wing with blocked engines, two types of separation can occur simultaneously: separation of a laminar boundary layer and separation of a turbulent boundary layer without its subsequent reattachment.
Running engines at maximum rotational speed helped to eliminate the separation of the turbulent boundary layer.• At the critical angle of attack, the presence of the distributed electric propulsion behind the trailing edge of the straight wing resulted in a global stall.Turning on the engines did not lead to an improvement in the flow around the model.

Fig. 1 .
Fig. 1.Draft model of a small-sized aircraft with a straight wing.

Fig. 3 .
Fig. 3. Profiles of the average velocity in the trace of a running engine at maximum rotational speed.

Fig. 9 .
Fig. 9.The flow on the surface of the wing with running engines at α = 5⁰.

Fig. 10 .
Fig. 10.The flow on the surface of the wing without engines at α = 10⁰.

Fig. 11 .
Fig. 11.The flow on the surface of the wing with blocked engines at α = 10⁰.

Fig. 12 .
Fig. 12.The flow on the surface of the wing with running engines at α = 10⁰.