Experimental Propeller Performance Analysis of Distributed, Single and Isolated configurations

Distributed propulsion (DP) configurations are a promising concept for future aircraft systems. The main objective of the presented experiment is to investigate aerodynamic interactions of such configurations in detail and compare DP results to a single propeller configuration. The experimental setup at the Propulsion Test Facility, TU Braunschweig, features three co-rotating propellers. These are not attached directly to the wing, but are mounted on a separate carrier. This decoupling allows the forces and moments acting on wing and propeller to be considered separately. Additionally, different relative propeller positions are set up easily. In order to eliminate side wall effects, only the centre propeller and the centre wing element are subject of investigation for the distributed configuration. The periodically repeating outboard propellers reduce the wind tunnel interference while providing a true DP setup for the instrumented centre. Additionally to the DP setup, tests for conventional propeller wing configurations (only centre propeller installed) as well as isolated propeller and clean wing tests were performed. Thus, the DP effects on wing and propeller and the effect of the downstream wing on the propeller are clearly identified. The comparison between single propeller and distributed propulsion configurations shows that with distributed propulsion the drag increase is reduced from 326% to 216% compared to a clean wing. This effect is intensified by a greater thrust level and higher angles of attack. In order to identify the distributed propulsion effects, the forces acting on the propeller as well as the resulting efficiency of the propeller are compared between the distributed and single propeller configuration for two different relative propeller positions. The efficiency of the centre propeller is increased due to the outer propellers by approx. 2% to 3%.


Introduction
The results presented stem from the experimental setup of the Clean Sky 2 project CICLOP (Characterization of the Interaction between wing and Closely Operating Propeller).The distributed propulsion (DP) model from figure 1 was installed in the 2.4 m × 2.4 m closed test section of the Propulsion test facility, TU Braunschweig.Consisting of a wing and propeller diameters of either D = 0.6 m or D = 0.4 m, the experiment focuses on the centre wing segment and the centre propeller.The wing is a two-element airfoil with a slotted fowler flap.Based on a NACA 63(2)415 profile with modifications on the pressure side to house the slotted flap, the nominal chord length for the retracted flap is c = 0.8 m, resulting in operating conditions between Re = [2.1 ... 2.9] * 10 6 in the atmospheric wind tunnel.Both elements are instrumented with an individual load cell.The wing is equipped with 256 static and 10 transient pressure taps to study the propeller slipstream on the wing.The smaller D = 0.4 m propellers also have a smaller drive train and thus nacelle.The hub-to-tip-ratio is equal to the larger propellers.The outer wing elements and propellers serve as boundary conditions to allow a quasi-periodic setup.Due to this setup, the influence of the wind tunnel conditions on the centre wing and propeller are limited.The three propellers are not mounted directly to the wing but are positioned on an external carrier.This decoupling of propeller and wing allows not only the tip clearance but also the relative propeller height and distance to the wing to be easily changed.In addition, this design allows the forces on the wing and propeller to be measured and analysed fully separated.Both the propeller carrier and the wing are mounted on side plates.These can be continuously rotated along a spanwise axis to change the global angle of attack of the propeller and wing.Another advantage of the structure is the modularity of the components.Throughout the project, various propeller sets differing in design or diameter were investigated.All blades can be pitched individually.The flap angle can be adjusted without steps.In this study it is at take-off configuration with δ = 20°.Two different operating points were chosen.One at an inflow velocity of V 1 = 40 m/s and the other at V 2 = 55 m/s.In order to keep the blade tip speed constant at M a tip = 0.58, the rotational speed varies minimally from n 1 = 6160 RP M to n 2 = 6050 RP M .This leads to two tested advance ratios: J 1 = 0.65 and J 2 = 0.91.The aim of the setup is to generate experimental data showing the interaction of propeller and wing in distributed propulsion configurations.

Experimental Setup
This study is intended to take advantage of the presented flexible design of the assembly and focuses not only on the results of distributed propulsion configurations but also on the comparison of modified configurations.In addition to the main setup for distributed propulsion shown in figure 2 (a).It is possible to disassemble the outer two propellers from the DP configuration.The result is a single propeller (SP) configuration (b).The comparison between these two setups shows the direct influence of the outer two propellers on the centre wing segment and the centre propeller and thus the influence of distributed propulsion.For reference purposes,  the wing without propeller and support structure (c) and the isolated propeller without wing (d) were measured, whereas the isolated propeller results are not part of this study [1].Unless otherwise specified, the propeller analysed is the large propeller with D = 0.6 m and is placed in the reference position.Here, the propeller rotation axis is at the height of the leading edge of the wing z P /c = 0.The propeller plane is at a fixed distance from the leading edge with x P /c = −0.469.The gap between the propeller discs is also constant at y gap /c = 0.125.The entire layout of the setup was outlined by Oldeweme [2].

Results
In the following, the influence of distributed propulsion on the wing and propeller will be obtained.First, the dependence of the thrust level on the lift and drag of the wing is investigated.
Based on this, the differences to the single propeller configuration are shown and sensitivities identified.Finally, the effects on the propeller are determined for two different propeller positions.For the results presented here the a constant induced velocity (CIV) propeller was used.Outside of the scope of this paper are other blade geometries (minimum induced loss, MIL) at the same diameter and thrust output [1,3].

Distributed Propulsion
Figure 3 shows the lift and drag of the wing over angle of attack α.On the one hand, the pitch angle is increased at constant rotational speed, so that the thrust level changes.On the other hand, measurements were made at the two different advance ratios, resulting in three pitch angles for each advance ratio.Both lift and drag are clearly dependent on the thrust level.However, it is important to note that the largest pitch angle does not provide the highest lift and drag.This is due to the fact that the thrust generated is also dependent on the inflow velocity and thus on the advance ratio.Therefore, the largest pitch angle of J 1 provides the largest lift and drag.Both are directly dependent on the thrust level, but this in turn is strongly correlated with the advance ratio.
To further investigate this thrust-lift dependency, figure 4 shows the lift gain of the distributed propulsion configuration (a) in comparison to the clean wing (c).
On the left, the lift gain is shown in relation to the standard thrust coefficient for two angle of attacks α = −1°and α = 7°.The lift gain of J 1 is higher for both angles, as already observed in figure 3. Also a linear dependence of advance ratio and lift gain may be assumed based on figure 4 (left).However, a relationship between the two operating points cannot be determined.The lift gain caused by increasing advance ratio from J 1 to J 2 is different for both angles of attack.In order to find a robust trust-lift relation, De Rosa [4] has shown numerically and Lindner [5] experimentally, that a thrust coefficient related to the inflow velocity (CTR) is better suited for a comparison of two different advance ratios.Therefore the lift gain over ( is shown on the right of figure 4. Here, the thrust-lift-dependency becomes obvious for four angles of attack.The regression between the points shows the non-linear relationship, especially at low angles of attack.It is important to note that in this setup, the axis of rotation of the propeller is at the level of the leading edge of the wing.At high angles of attack, the propeller slipstream therefore impacts less the wing and the interactions decrease.The slipstream interaction for varying propeller position of this experiment is published by Lindner [5].

Single Propeller Comparison
Next, a comparison of the distributed propulsion configuration from figure 2 (a) with the single propeller configuration (b) will be made to identify the DP-effect on wing and propeller.
Figure 5 shows lift and drag of both configurations in comparison to the clean wing configuration (c).The propeller for the DP: Ref. Position and single propeller setup is in the reference  position.In this orientation, the lift is higher for both propeller configurations at small angles of attack, but maximum lift C L,max is almost the same for all three configurations.A maximum lift gain is only achieved by moving the propeller below the leading edge, shown by the "Low & Close" position (z P /c = −0.2,x P /c = −0.19).A detailed propeller position analysis and the resulting impact on the wing was done by Lindner [5].On the right it shows that the clean wing configuration has the lowest drag due to the lack of a slipstream.The SP configuration generates at maximum 326% more, while the DP configuration (ref.position) generates 216% more drag on the wing.It is also obvious that the higher the angle of attack and thus the lift, the greater the deviation between the two configurations.In conclusion, this sub-optimal propeller placement drastically increases the drag of the trailing wing, while the lift augmentation is very limited.
Based on this, figure 6 shows lift gain and drag increase over thrust level using CT R for two angles of attack.The same sensitivities can be seen in the SP configuration as with distributed propulsion systems.The lift gain is again quadratically dependent on the CT R and thus on the thrust.As already indicated in figure 5, at α = 3°a slight offset between the lift gain of DP and SP can be seen.As the angle of attack increases, i.e. at α = 7°the lift gain is the same for both configurations.Focusing on the drag increase, it can be seen that with a small angle of attack of α = 3°and a small thrust level CT R = 0.15, both configurations generate the same drag.
If the thrust level is increased, the drag increase for the DP setup is relatively linear, while for a single propeller the increase is non-linear and thus higher.The larger the thrust level, the distributed propulsion becomes more advantageous compared to a single propeller configuration in terms of drag savings.This means, that the increased stagnation pressure and friction in the fast slipstream is not the only source of drag due to propeller installation, because the thrust outputs of DP and SP are kept equal.
If the angle of attack is now increased to α = 7°as shown to the right of figure 5, two aspects become clear: Firstly, the difference between the two configurations is already present at low thrust levels.The advantage of distributed propulsion thus increases in general at higher angles EASN-2023 Journal of Physics: Conference Series 2716 (2024) 012004 of attack.And secondly, the difference between the drag of the two configurations intensifies with increasing thrust levels.Thus, the advantages become clear especially at high angles of attack and high levels of thrust.This is especially important as the setup presented in this study is a high-lift configuration with a two element airfoil.

Propeller-Propeller Interaction
Finally, the influence of the outer two propellers on the centre propeller will be investigated.For this purpose, the results of the smaller propeller with a diameter of D = 0.4 m will be used, since this drive train is equipped with a 6K load cell and thus all forces can be compared.Due to the changed diameter, but same advance ratio J 1 , same inflow velocity V 1 and same blade tip speed M a tip , the rotational speed changes to n 1 = 9250 RP M .Figure 7 shows the forces and the propeller efficiency.The forces are now displayed on the body fixed coordinate system that rotates with angle of attack and split into their three vector components (axial, lateral, upwards).On the left side in the reference position (x P /c = −0.489,z P /c = 0) and on the right side in another position, where the propeller rotation axis is below the leading edge of the wing (z P /c = −0.2) and the propeller plane is closer to the wing (x P /c = −0.114).The gap between the propeller discs is y gap /c = 0.375 due to the smaller diameter.First, it is apparent that the off-axis forces (F lat and F up ) do not change for either configuration in both positions.are the forces that do not act in the thrust direction of the propeller.F lat acts in the lateral direction, i.e. in the direction of the span of the wing.This force is mainly caused by the horizontal inflow imbalance due to the wings pressure field.F up acts in 90°offset to it.This force is mainly caused by the lateral inflow imbalance due to the upwards/downwards rotating blade.The phenomenon measured here is described in detail by Oldeweme [1].Both forces are dependent on the angle of attack.The outer two propellers therefore have no influence on the forces of the centre propeller in these two directions.
However, a difference between the two configurations can be seen in the forcecomponent in thrust direction F ax and in the propeller efficiency η prop .In both propeller positions, the centre propeller of the DP configuration generates more thrust and has a higher efficiency.Previous experimental studies of this setup and also De Vries [6] have shown that moving the propellers spanwise, i.e. reducing the blade tip gap y gap to very small distances, has no effect on the forces and only a minimal impact on efficiency.Thus, although there is no direct propeller-to-propeller interaction, a second-order effect is detected.The outer two propellers change the pressure field of the wing through their slipstream in such a way that this in turn has a upstream feedback on the centre propeller.Only changing the tip spacing has very limited influence on the propeller force components as published by Oldeweme [1].This interaction has the result of increasing efficiency and is present for both investigated positions.

Conclusions
In this study, the capabilities of the experimental setup of the Clean Sky 2 project CICLOP were presented and examined.Besides the investigation of changing different parameters in a distributed propulsion configuration, this flexible setup also allows to analyse wing and propeller separately and to test single propeller configurations.
In distributed propulsion configurations, the influence of the thrust level of the propeller on the lift and drag of the wing is significant.For two different operating conditions, it was shown that the use of a thrust coefficient related to the inflow velocity makes a comparison of both advance ratios possible.Thus, a reasonable correlation and interpolation between thrust and lift gain could be shown.The influence of the outer two propellers in distributed propulsion setups on the wing and on the propeller was investigated using a single propeller configuration.The results show that in reference position the maximum lift remains unchanged, but drag reductions due to the outer propellers are large.These reductions become larger with increasing angle of attack and thrust levels.No direct propeller-propeller influence could be detected, since the off-axis forces remain the same and spanwise shifts do not cause significant changes.Nevertheless, the outer two propellers still have a positive influence on the centre propeller via the upstream effect of the wing, so that the efficiency increases by 2% to 3%, regardless of the propeller position studied.

Figure 1 :
Figure 1: The CICLOP distributed propulsion model installed in the Propulsion Test Facility (PTF), TU Braunschweig.
(a) Distributed Propulsion: Three propeller and wing.(b) Single propeller: Only centre propeller and wing.(c) Clean wing without support structure.(d) Isolated centre propeller without wing.

Figure 3 :
Figure 3: Thrust dependency for two advance rations on lift (left) and drag (right) of the wing.

Figure 4 :
Figure 4: Comparison of lift-thrust dependency with coefficient normalised with rotational speed (left) and freestream velocity (right).

Figure 6 :
Figure 6: Influence of global angle of attack α = 3°(left) and α = 7°(right) for distributed and single propeller configuration on thrust dependency of lift and drag.

Figure 7 :
Figure 7: Propeller forces and efficiency for the reference position (left) and for a low and close position (right).