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diStributed Open-rotor AiRcraft

Final Report Summary - SOAR (diStributed Open-rotor AiRcraft)

Executive Summary:
The open-fan wing aircraft concept offers a unique class of aircraft, powered by a crossflow fan that exhibits unconventional aerodynamic characteristics neither like those of a fixed-wing aircraft nor helicopter. Among its advantages are very good stability, reduced fuel-consumption and ultra short take-off and landing capabilities (USTOL).
The cross-flow fan is mounted horizontally atop of a supporting wing structure; inside the fan a trapped vortex develops which, in addition to forward thrust, creates a major part of lift for the aircraft.

The German Aerospace Centre (DLR) is coordinator of the SOAR project, and analysed potential markets and missions for return on investment, new aircraft purchase volume, and a willingness to pay a premium for low speed aircraft. After the identification of competitor aircraft for the chosen markets, a conceptual aircraft design study was performed to determine the basic performance and cost factors for the open-fan wing technology integration.
FanWing Ltd (FANWING), providing crucial patented aspects of crucial parts of their FanWing propulsion design, optimised the technology further in SOAR to find the optimal blade profile, angle and rotor cavity position, and evaluate the open-fan wing's aerodynamic performance to derive scaling laws for this type of aircraft.
The Von-Karman Institute (VKI) simulated the open-fan wing design with Navier-Stokes computational fluid dynamics (CFD) methods, designed and built a customised balance to measure forces on the wind tunnel model, and together with the project partners performed several weeks of wind tunnel tests to validate findings of the CFD computations.
The Saarland University (USAAR) developed and implemented a electric fan drive system involving a brushless synchronous torque motor and various sensors. They also created a planetary gear mechanism to enable synchronous adjustment of the blade pitch angle between test runs without time-consuming manual adjustment.
During the collaborative wind tunnel tests at VKI, three sets of up to 16 carbon-fibre fan blades were tested at different pitch angles within the rotor, at different angles of attack for the entire wing model, and at different rotational as well as wind tunnel air speeds. After the first testing campaign, the best airfoil candidate was selected and further derived candidates were tested in consecutive runs. Complementing the rotor tests, the wing shape was systematically modified and tested though shorter and longer tail lengths and the addition of a specialised flap for thrust direction.
The most promising markets are the high-volume agricultural application and the high-premium fire-fighting areas. In the agricultural market, a significant cost savings was observed in owner-operator model with farmers flying aircraft themselves with high utilization and storing them in the vicinity of the fields. For the fire-fighting business case, the high premium that operators pay for aircraft that are able to scoop water provides ample margin for the FanWing to compete on price for the same payload while also allowing the operator to fly at a lower speed for more precise retardant application. Other potential uses are short range cargo and civilian air tourism.

Project Context and Objectives:
During the first project months of the project, the open-fan wing model design was completed. Matching the wing, the rotor blades and their mould were designed and built.
The wing section of the model - including the cross-flow fan - was built by FanWing Ltd (FANWING) and by subcontracting relevant parts of machining work and production of the carbon fibre-reinforced plastics fan blades and shaft.

Saarland University (USAAR) specified the fan motor which was subsequently built by a subcontractor according to USAAR's design criteria. The engine is built as a brushless synchronous torque motor capable of delivering 45 Nm of torque at 1250 rpm.
After integration of the motor, the actuated wing model was completed. USAAR also implemented an open-fan wing model with the X-Plane flight simulator software to provide a handy educative and experimental platform for people interested in learning to fly this type of aircraft.

In preparation of the wind tunnel tests and serving as a validation reference, the Von-Karman Institute (VKI) performed unsteady Navier-Stokes computational fluid dynamics (CFD) studies for the open-fan wing technology.

Because of the unconventional nature of the open-fan wing design regarding production of lift and thrust, VKI designed and built a customised external balance for the measurement of forces of and on the wing model, taking special care to observe safety factors of the expected forces to avoid damage of the wind tunnel.

During three following test campaigns, the open-fan wing model and systems were tested thoroughly in the wind tunnel, measuring forces and capturing high-speed video for Particle Tracing Velocimetry (PTV). The PTV flow patterns proved extremely useful in the study of the trapped vortex.
In the first phase, the model, drive system and wind tunnel balance were installed and the data acquisition system installed and tested. The second phase concentrated on rotor optimisation by testing a focused set of key performance points of angle of attack (AoA), fan rotational speed (RPM), and wind speeds representing take-off, loiter, cruise, dash and auto-rotation landing. Six sets of fan blade shapes were tested with five different blade angle settings each. The best combination candidate was then selected and the optimisation of the fixed wing geometry proceeded, in steps modifying lower shroud, entry and exit heights. Also setups of with less than all 16 blades were tested and did perform not siginificantly worse.
In the third phase the fixed wing and rotor optimisation was continued at the key performance points with regards to modifications including the rotor cavity and, leading edge position and other modifications. This completed the wind-tunnel design optimisation in the wind tunnel.

The full data on wing-flow diagrams and airspeed was gathered and first CFD conclusions drawn. These activities resulted in an evaluation report.

The consortium decided to invest into a supplemental test phase within a reduced test environment, to further investigate the scaling laws for the open-fan wing, especially the existence and integrity of the vortex inside the cross-flow fan.

In parallel to the wind tunnel tests, Deutsches Zentrum für Luft- und Raumfahrt (DLR) analysed potential markets and missions for an open-fan wing configuration. These missions and markets determined the vehicles' conceptual design in comparison to selected competitors’ aircraft. An economic analysis was performed for these designs.
The dissemination activities included the creation of a public website, production of a project video, holding of a workshop and presentation at the Paris Airshow, and collection of a listing of other dissemination activities including all scientific publications and press releases.

Project Results:

The FanWing was shown to have scenarios with lower operating costs than its competitors in both the 500kg-2500kg payload class and the 10000kg payload class. In the smaller payload class in the agricultural operation market, the open-fan wing’s low takeoff distance enables it to be used in owner-pilot model while its competitor aircraft cannot be used in this way. However, the FanWing will only have a cost advantage in this scenario if the owner needs to use the aircraft as often or more often than a business operating chartered agricultural flights. In the 10000kg payload class, the FanWing can potentially disrupt the market premiums currently paid by the market by employing scooping capability of the competitor aircraft. The FanWing can further enhance its economic competitiveness by improving its cruising speed or, by further lowering its takeoff distance so that the FanWing can be based at more mission sites by owner pilots or be towed there by a service.
In a generic market, the Fan Wing needs one of two things to be economically successful: the market must pay a premium for low-speed maneuvering missions and/or the low takeoff distance must enable an owner-pilot model in the same market with the owners having a high utilization rate. Alternative fuels or energy sources would not enhance the open-fan wing’s competitiveness because the operating costs are highly dominated by aircraft ownership costs, not fuel costs. This is due to the low utilization rates for the selected target markets which is in contrast to an airline model with high utilization rates. The FanWing can reduce its CO2 relative to a baseline aircraft only when it enables and is used in an owner-pilot model due to longer mission times with similar fuel burn rates. There is also evidence from the bench test that the fuel burn and/or energy required for a FanWing mission may be improved due to presence of the vortex.

Potential Impact:
The development of an open-fan wing aircraft could disrupt the analysed markets of crop-dusting in the payload range of 500kg – 2500kg and fire-fighting in the payload class of 10.000kg for certain missions and business cases.
In the 500kg-2500kg payload class scenario, the selected target market is agricultural application. The baseline competitor aircraft for the agricultural application market is the Air Tractor 502 which is used for crop dusting. The Air Tractor 502 is one of the most popular models for crop dusting currently in production. A total of 7800 aircraft, a large volume, have been built for this purpose. Air Tractor prices were not published so its acquisition price was also assessed using the Raymer-Dapca IV evaluation method but, with no change factors applied using the Markish model.
The direct operating results for the 500kg-2500kg payload class can be seen below in the figure. In the business operator role, the FanWing exhibits both higher costs in all three categories. The higher ownership costs result from higher empty weight and power requirements. The higher fuel and pilot costs are the result of a five-hour mission time vs. a three-hour mission time for the competitor baseline aircraft with roughly the same fuel burn per hour and a lower cruise speed for the FanWing. The baseline competitor aircraft also is designed for a high speed (30% faster than the open-fan wing’s maximum speed) chemical application segment, further lowering its mission time.
In the owner pilot operating model, the fuel costs are lower due to the shorter mission distance, however the ownership costs are higher due to having fewer flights per month over which to amortize the costs. The pilot costs are zero in this case because the owner doesn't hire a pilot and flies the FanWing on his or her own.
Lastly, a scenario in which the FanWing achieves a lower cost in than the baseline aircraft is when the owner pilot needs to fly the same number of times per month (20) as a business operator. The baseline aircraft could not operate in the same way from the owner’s field because its takeoff field length is prohibitively long, more than 3 times longer than that of the FanWing.
In the 10.000kg payload class scenario, the selected target market is firefighting. The baseline competitor aircraft for the for the firefighting market is the Bombardier CL-415 which has the unique ability to scoop more water and mix it with more fire retardant for additional application after it has dispensed its initial payload . This unique ability gives the CL-415 the ability to be as productive as an aircraft with three times the payload but with lower takeoff weight, fuel burn and acquisition costs. As a result the CL-415 commands a 100% premium (two times the acquisition price) over aircraft with same power-plant and takeoff weight that are designed for other applications. A summary of the 10.000kg payload class is shown in the figure below.
The firefighting mission of 5 mission segments: the departure to the fire (30 nautical miles), 3 segments of a low altitude application and round trip flight to refill the water supply (12 nautical miles), and return to the field (30 nautical miles). The total mission distance is 96 nautical miles or 180 kilometers. The mission time for the FanWing is 1.65 hours compared to 0.78 for the baseline CL 415. The smaller difference in mission time is due in part to the fact that the firefighting baseline competitor is designed for a slow application speed. Unlike the agricultural baseline competitor (AT502) which has a high application speed.
A Publically (government) owned firefighting aircraft operation was considered the only viable operating model for this mission because the application frequency and thus application revenue are highly uncertain. Also in most societies, firefighting is considered a public service while farmland is privately owned and maintained. The government operator was assumed to operate an average of 10 flights per month for either training or live fire application.
The direct operating results for the 10.000kg payload class can be seen below in Figure 4. In the government operator role, the FanWing exhibits significantly overall operating. The lower ownership costs result from the absence of the 100% price premium that is currently enjoyed by the Bombardier CL-415. The CL-415 does not have any scooping competitors in its payload classes or in smaller or larger payload classes. The acquisition costs dominate the operating costs because of low (10 flights per month) utilization rate. In comparison, an airline might operate an aircraft 100 flights per month. The higher fuel and pilot costs are again the result of a longer mission time. It would not be possible to reliably lower these costs for the FanWing as was possible in the agricultural mission because the location of the fires will be unknown and will vary season to season. Even with higher pilot and fuel costs, the FanWing with scooping capability enjoys a significant operating cost advantage over the CL-415.
Due to the complex aerodynamics of the open-fan wing design, prediction of further economic and social benefits is difficult. However new markets can open and develop when the technology is developed to its full potential.
The open-fan wing design was widely promoted during the project by a number of press releases, open-house days, articles in scientific and popular press, a workshop with aviation specialists, and the production of a documentary-style video. Cf. to deliverable D4.1 D4.2 D4.3 and/or next sections for more information.

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