System Performance of Wing and Propellers in a Periodic Distributed Propulsion Experiment

The design task for distributed propulsion (DP) aircraft is more complex than conventional twin-engine designs due to the pronounced propeller wing interaction. DP concepts rely on a beneficial and robust interaction of propulsion and lifting surface. Additionally, a good DP design is optimised as a system such that each element is not optimised by itself (i.e. ηprop and CL/CD), but with consideration of the close coupled interaction. The evaluation of such an interaction driven setup is scope of this work. Thrust and torque of a periodic co-rotating DP wing are measured simultaneously with airfoil coefficients. Thereby the influence of propeller on the wing and vice versa are identified. Two different sets of propeller geometries with a diameter of D = 0.6 m are studied. One propeller set is designed for minimum induced propeller loss (MIL). The second propeller set is designed to have a constant induced axial velocity over the radius (CIV). We shall compare how the different strategies perform in the DP system. The two element wing has a span of B = 2.4 m and a reference chord of c = 0.8 m, operating at Re = 2.1 × 106. For this study, the propellers are pitched to meet a constant cT, J and Matip . The results focus on the system performance for the combined setup in take-off configuration. While the isolated propeller efficiency benefits from the integration in front of the wing by > Δηprop = 12%, the system efficiency suffers from increased drag on the trailing wing that is roughly tripled over the clean wing. Depending on the propeller position relative to the wing, interaction losses can be minimised so that a system efficiency gain over the isolated wing and propeller of > Δηsys = 4% is achieved.


Introduction
The complex interaction of propeller and wing in a DP configuration makes the exploitation of possible integration benefits hard to put into effect.This however is necessary for electrically powered regional propeller aircraft that integrate many propulsors with relatively low power density along the airframe.In the scope of the CleanSky2 project CICLOP, the interaction of propeller and wing was characterised in an extensive wind tunnel campaign in the Propulsor Test Facility Braunschweig.A large set of variations, including propeller position, thrust level, flap angle and more was carried out.In the presented work we focus on the system efficiency defined by Firnhaber [1] to evaluate the performance of a DP-Wing.This parameter serves as a global figure of merit for a DP wing section with periodic boundary conditions.It's goal is to characterise the efficiency of a combined DP wing with physical relevant relations, but without need for a mission definition on aircraft level.We compare the isolated efficiency of wing and propeller to the DP setup in order to quantify the integration effects precisely.Furthermore, we compare a propeller with a constant induced velocity (CIV), designed to work in DP, with a more classical minimum induced loss (MIL) propeller.Both propeller sets were designed using blade element momentum theory (BEMT) according to Pagano [2] and are described in detail by Oldeweme et al. [3].

Experimental Setup
The wind tunnel model is comprised of a wing of c = 0.8 m chord with a fowler flap based on a NACA 63(2)415 airfoil.In front of the wing, but mounted on a separate carrier, are three propellers of D = 0.6 m diameter.The two outboard propulsors serve as periodic boundary conditions for the central DP wing section.This centre wing section is instrumented with an internal force balance as well as stationary and instationary pressure taps.The centre propeller is equipped with a force balance measuring shaft torque and thrust.By introducing periodic DP extensions to the instrumented central section we minimise wind tunnel interference and generate clean periodic boundary conditions.The external mounting of the three propellers allows to change the relative position of propeller and wing.It allows the isolated analysis of wing and propeller.Figure 1 and 3 feature the experimental setup.
To compare the two propeller designs we keep the tip mach number M a tip = 0.58, advance ratio J = 0.65 and thrust level c T ≈ 0.125 equal between measurements.For each measurement we perform a alpha sweep from α = [−5...12] at U ∞ = 40 m/s.The dataset comprising the CICLOP results is available upon request in an online database.A detailed description of the instrumentation and model design is published by Oldeweme et al. [4].In the results presented we only consider the global parameters of this DP wing section such as lift, drag, thrust and torque.Detailed propeller wing interactions of this setup are published by Lindner et al. [5].

Methodology
Main objective of this paper is to provide a measure of the system performance of a DP wing introduced by Firnhaber [1].This is necessary in order to take the step from individual efficiency values for propeller and wing towards a combined evaluation of a periodic DP wing section.We arrive at a system efficiency η sys that combines wing and propeller efficiency in a physically relevant way.This is derived by introducing a horizontal force coefficient of the propeller thrust T normalised with wing area A and stagnation pressure q: Substituting this into the definition of the classic propulsive efficiency consisting of a power balance of T •U over P shaf t and adding the wings drag to the generated thrust C F,net = C D +C F,x , we arrive at: Which can be reduced into: This parameter therefore represents the power balance of the propeller but takes the drag caused by the slipstream into account.It omits the lift generated entirely, so the parameter η sys should be evaluated at the operating point that matches the lift requirement of the 2.5D wing section.Further comments on the applicability of this value and its extension are published in [6].

Component Efficiency
In a simple aircraft design strategy, the efficiency of wing and propeller are often evaluated on their own.Integration effects are thereby omitted.We therefore state the propulsive efficiency of the spanwise periodic propellers as well as the glide ratio of the isolated wing.By then comparing these isolated measurements to the true DP setup of wing and propellers, we quantify the integration effect.The two propeller sets of equal diameter but different design strategies, i.e.MIL and CIV, are compared.3 b)).The pitch angle is chosen such that the thrust output of the two propellers is similar as measured by the thrust coefficient c t .By comparing the MIL propeller (left) to the CIV propeller (right), the expectation that a MIL propeller is better performing for a given thrust is confirmed.This satisfies the design goal of the minimum induced loss (MIL) propeller.Especially at high advance ratios, closer to the individual efficiency peak for each pitch angle θ, the MIL propeller outperforms the CIV design by ∆3 % at the maximum efficiency for the pitch angle of θ = 16.5°(17.25)°.

Propeller Efficiency
By adding the wing and periodic propellers to the wind tunnel setup, the propeller operating point is shifted.Oldeweme [3] develops the mechanism of how the wings flow field affects the propeller for this setup.Summarized briefly, the propeller is placed in the region of decelerated flow in front and below of the wing.Furthermore, with increasing angle of attack the normal velocity component through the disc is reduced.Both effects lead to lower effective advance ratio in which the propeller operates.The thrust increase by this mechanism is seen in the scatter plot for the pitch angle of θ = 16.5°(17.25)°.Here, the arrow points in the direction of increasing angle of attack for otherwise fixed operating conditions.Because the wings circulation grows with angle of attack, both effects of the thrust increase are intensified.Thrust output linearly increases with the wings circulation.Even more pronounced than the thrust increase is the increase of propulsive efficiency (marked with * ) due to the wing integration: The increase in efficiency does not correspond to the isolated propeller performance map.Considering that we observe a thrust increase due to lower effective advance ratio, we would expect based on Figure 2 that the propulsive efficiency reduces according to the lower advance ratio.This is however is not measured.Applying first principles of propulsive efficiency, the propulsion efficiency gain results from placing the propeller in a locally lower flow velocity.Due to the wings circulation the required power at the same thrust output is reduced.After wing integration we therefore do not operate in the domain of the isolated propeller map but on a new propeller map where efficiency's are elevated.Besides the thrust and efficiency increase that is shared by both propeller designs, we observe that the CIV propellers efficiency benefits more from the wing integration than the MIL propeller at equal thrust.The maximum overall efficiency is ∆η prop = 4.5% greater for the CIV over the MIL propeller.The efficiency trend shown as a scatter cloud with arrows in Figure 2 is presented in Figure 4 over α.The grey error band indicates the 90 % confidence interval as calculated from repetition measurements plotted as black squares.Three observations are notable: • After wing installation, the propeller efficiency is increased.The CIV propeller benefits more strongly from the wings interaction with a minimum gain of η prop = 5% • The MIL propeller has performance advantages only in the isolated propeller case.
• The wings beneficial influence reaches a maximum at around α = 7°for the CIV propeller.
Beyond this angle of attack efficiency decreases again.
We conclude from this observation, that isolated propeller maps do not represent the operating conditions that occur when the propeller is placed in the wings flow field.Both thrust output and efficiency are increased in the DP setup over the isolated propeller.In this DP setup a propeller design that is more tolerant to the flow field of the wing is therefore superior to a purely efficiency-driven propeller design and must be taken into account for such high lift configurations.Further investigations reveal that this is due to wing-to-propeller effects and not due to propeller-to-propeller interaction [3].

Wing Efficiency
Having established the isolated propeller performance and the point in the propeller map we operate in the DP setup, we now evaluate the aerodynamic performance of the isolated wing in the same fashion.Here, the lift over drag ratio is chosen as the relevant parameter.Depending on the optimisation objective L/D is the relevant parameter of the aircraft in the climb phase.
In Figure 5 we plot this ratio as it is measured by the internal balance of the periodic wing section.The clean wing, that is without any propellers or support structure serves as the comparative baseline (ref. .In this range, the wake of the nacelle directly intersects the wing.Outside this α range the nacelle wake is deflected above or below the wings surface, where its effect on the wing is slightly less pronounced.The nacelle and strut integration increases the drag of the periodic wing section, leading to the observed reduction of L/D.At high α > 10 the clean wing efficiency drops sharply because of a sudden stall, while the wing with nacelles features a more moderate drop in lift beyond α max .The general behaviour of this airfoil at high lift is elaborated by Lindner [5].The point of separation induced lift breakdown on the airfoil corresponds loosely to the drop of efficiency at high α observed in Figure 4.
Integrating the propellers further reduces the aerodynamic efficiency by > 20 counts.Note that L/D is plotted in Figure 5 on a logarithmic scale for clarity.The propellers are operating at equal thrust in the conditions from Figure 2. The lifting component of the propeller as well as the thrust is omitted from the wing forces in Figure 5.The separate force measurement of propeller and wing makes this individual force balancing possible.
The CIV propeller has a small benefit over the MIL propeller regarding L/D of the wing.This is likely because the propeller slipstream is more homogeneous and therefore has less spanwise pressure gradients generating longitudinal vortices.The measured drag difference between MIL and CIV in this DP setup is relevant with respect to the 90% confidence interval taken from four repetition measurements in the CIV setup.The reduction of L/D originates from the drag increase after propeller installation and not lift reduction.The wings lift is augmented due to the increased stagnation pressure and is equal for equal thrust of both propellers [5].Later in the evaluation of the system efficiency we will derive that this drag difference of the wing due to propeller design is the driving factor for differences in system efficiency of the DP setup.
We conclude that the propeller integration reduces the wings efficiency as measured with L/D, while roughly half of the drag increase is due to nacelles and support structure and the other half due to the propeller slipstream.The isolated evaluation of propeller and wing can quantify the integration effects for both components.This is necessary to understand the underlying effects.For the periodic DP wing section both propeller and wing are relevant.Their combined influence on the relevant system efficiency is evaluated in the following chapter.Only at system level a sound answer regarding the overall efficiency of a DP wing can be given.

DP-System Efficiency
As derived in Chapter 3 we use a power ratio of the net thrust output and the shaft power as figure of merit.This power ratio remains after reformulating the thrust coefficient with freestream velocity and wing area.We again compare the system efficiency for the CIV and MIL propeller at equal thrust.In Figure 6 this system efficiency is plotted over angle of attack of the DP setup.The two propeller designs (CIV and MIL) are compared.Efficiency steadily decreases with increasing angle of attack.This decrease is driven by the drag increase at higher α, as we show later.Figure 6 also features the virtual system efficiency when propeller and wing are combined posteriori from isolated wing and propeller measurements.The spread between this virtual DP dataset ("Comb.")and the true DP wing quantifies the efficiency drop due to integration effects.Increased drag in the wing (both friction in the wake of the propeller and spanwise lift variations causing longitudinal vortices) cause the lower system efficiency of the DP setup.We identify the wing drag as the governing influence of the system efficiency by breaking down the individual parameters:   Variation of horizontal force components thrust and drag.
To understand the origin of the decreasing system efficiency we separate the variable components from Equation 3 into separate curves.Figure 7 shows the relative change of the net thrust T − D as well as the required shaft power P for the propeller at constant RPM and pitch angle.Both net thrust T − D and shaft power P have linear influence in Equation 3. Freestream velocity U remains unchanged through the experiment.Due to the linear influence we therefore plot the relative change of the values referenced to the arbitrary operation point at α = 0 in Figure 7.Although the power required increases over α, it is clear that the drop in system efficiency observed in Figure 6 is governed by reduction of net thrust.

Comparison of CIV and MIL propeller reveals what we already observed on component level:
The CIV propeller benefits in two ways.Firstly, the propulsive efficiency is increased in all operation points except zero angle of attack (ref.Figure 4).Secondly, the drag on the airfoil is lower for the CIV propeller, leading to greater excess thrust T − D over most of the polar.
To further analyse the reduction of net thrust, we split this horizontal force into its components, drag coefficient C D and thrust coefficient C F,x .We plot this in Figure 8. Note, that the sign of the thrust is flipped for clarity, the propeller is generating a positive thrust.Both drag and thrust are increasing.Thrust increases over α due to increasing inflow angle into the propeller disc reducing the normal component through the disc, effectively reducing the advance ratio and therefore yielding higher thrust (ref.to Section 4.1).We observe this thrust increase to be true up to α = 7°, above which the thrust output reduces (MIL) or stagnates (CIV).On the other hand, the wings drag increases more rapidly with increasing angle of attack in the familiar fashion for a high lift two element wing.The drag outgrows the increases in thrust over angle of attack.
We can therefore conclude: On system level, the wing has the greater influence over to the propeller regarding overall efficiency.While the shaft power required increases over the operation range of a wing, the wings drag increase and therefore net thrust output of the DP wing decreases by a greater factor.The net thrust reduction is more intense by an order of magnitude as seen by the steeper gradient in Figure 7.For the optimisation task of a DP system this means that drag decrease on the wing due to adapted wake profiles in the propeller slipstream can be made at the expense of some propulsive efficiency.The propeller position relative to the wing at x p /c = −0.47,z p /c = 0.0 presented so far is not selected to achieve a maximum system performance.Lindner et.al [6] show that the system performance is sensitive to the placement of the CIV propeller.Selecting the best performing propeller position, that sits closer and below the wing at x p /c = −0.19,z p /c = −0.13, the system performance even exceeds the combined-from-isolated test case as shown in Figure 9.The arrows show the change due to more beneficial interaction of propeller and wing at otherwise equal settings.If placed well, a DP setup yields higher system efficiency over isolated wing and isolated propeller, but only for a limited operation range α.It should be noted that this observation holds true for conventional two-engine designs as well, but is emphasised for DP configurations with propellers along the full wing span.

Conclusion
By comparison of two propeller designs at otherwise equal conditions installed in a DP configuration of a high lifting wing, we derive that the choice of propeller design is relevant regarding overall efficiency.This is most evident for conditions outside the design space of the propeller.We quantify both the individual behaviour of propeller and wing as well as their combined efficiency in a DP setup.On system level, the wing drag governs the total efficiency of the DP wing section.This is shown by balancing the individual terms of the system efficiency.Because the choice of propeller design affects the drag caused on the wing, the influence of the propeller on the DP system manifests indirectly rather than directly due to it's propulsive efficiency.This makes a DP optimisation task complex.In a optimised wing and propeller setup, some propulsive efficiency has to be given up to allow for low drag on the trailing wing in the slipstream.With the figure of merit introduced here, this balance can be evaluated.The propeller designed with constant induced velocity wake profile (CIV) increases the system efficiency by a maximum of ∆η sys = 8.25% over a MIL design at equal thrust.This is caused for the most part by drag reduction on the airfoil in the wake of the slipstream.Comparing the superimposed measurements of isolated wing and isolated propeller sets, no significant efficiency difference is measured.The benefit of different propeller designs is therefore interacion driven.
Placing the propeller position downwards and close to the leading edge greatly improves the system efficiency over the position where the propeller axis coincides with the wing chord.Gains of +36% η sys with values above the isolated wing and propeller are measured, featuring an overall positive effect of distributed propulsion.

Figure 2 .
Figure 2. Isolated propeller performance map for MIL (left) and CIV (right) prop.and DP operating conditions along arrows.

Figure 2
Figure 2 features the performance map of the two propeller designs over advance ratio.The performance maps are obtained by placing a single propulsion unit in the test section (without neighbouring propellers and wing, ref.Figure3 b)).The pitch angle is chosen such that the thrust output of the two propellers is similar as measured by the thrust coefficient c t .By comparing the MIL propeller (left) to the CIV propeller (right), the expectation that a MIL propeller is better performing for a given thrust is confirmed.This satisfies the design goal of the minimum induced loss (MIL) propeller.Especially at high advance ratios, closer to the individual efficiency peak for each pitch angle θ, the MIL propeller outperforms the CIV design by ∆3 % at the maximum efficiency for the pitch angle of θ = 16.5°(17.25)°.By adding the wing and periodic propellers to the wind tunnel setup, the propeller operating point is shifted.Oldeweme[3] develops the mechanism of how the wings flow field affects the propeller for this setup.Summarized briefly, the propeller is placed in the region of decelerated

Figure
Figure 2 features the performance map of the two propeller designs over advance ratio.The performance maps are obtained by placing a single propulsion unit in the test section (without neighbouring propellers and wing, ref.Figure3 b)).The pitch angle is chosen such that the thrust output of the two propellers is similar as measured by the thrust coefficient c t .By comparing the MIL propeller (left) to the CIV propeller (right), the expectation that a MIL propeller is better performing for a given thrust is confirmed.This satisfies the design goal of the minimum induced loss (MIL) propeller.Especially at high advance ratios, closer to the individual efficiency peak for each pitch angle θ, the MIL propeller outperforms the CIV design by ∆3 % at the maximum efficiency for the pitch angle of θ = 16.5°(17.25)°.By adding the wing and periodic propellers to the wind tunnel setup, the propeller operating point is shifted.Oldeweme[3] develops the mechanism of how the wings flow field affects the propeller for this setup.Summarized briefly, the propeller is placed in the region of decelerated

Figure 3 .
Figure 3. Sketch of the wind tunnel configurations investigated.

Figure 4 .
Figure 4. Propeller efficiency in isolated and DP setup.

Figure 5 .
Figure 5. Wing efficiency for isolated wing, with nacelles and in DP Setup.

Figure 3 a
)). Adding the propeller support structure and nacelles, L/D is reduced by more than 20 counts over the full measured α range.The nacelle wake shows as a distinctive dip in the L/D between α = [−4... − 2]

Figure 6 .
Figure 6.System efficiency as measured in DP setup and superimposed from isolated cases.

Figure 7 .
Figure 7. Variation of components in system efficiency formula.
Figure 8.Variation of horizontal force components thrust and drag.

Figure 9 .
Figure 9. System efficiency of improved interaction due to prop.placement.