The Influence of Propeller Position and Size on the Aerodynamics of Blown Wings

The drive to decarbonise the aviation sector is pushing aircraft designers towards more-electric propulsion architectures. One of the key benefits of electric propulsion is that multiple motors can be positioned around the airframe, without loss of motor efficiency. By placing many motors along the wing leading edge, each powering a propeller, increased lift can be achieved by blowing high velocity jets over the wing. This allows the wing to be reduced in size, decreasing its cruise drag, whist still maintaining the same lift during take-off and landing. It is shown that the use of small diameter jets allows high velocity jets to be used for a fixed landing thrust, increasing the lift coefficient. It is also shown that the wing must be fully immersed in the jet to avoid premature stall. This is only possible by placing the jet below the wing centreline, due to the wings’ up-wash. However, the use of small propellers placed below the wing centre-line requires many large nacelles, with a large associated drag. When the effect of nacelle installation drag is included, the optimum blown wing geometry to minimise drag is shown to be when the horizontal offset of the propeller is 25% wing chord in front of the wing leading edge, with no vertical offset of the propeller from the wing centre-line and with a jet diameter of 64% wing chord. This optimum is true for the wing and aircraft operating conditions studied in this thesis. This geometry achieves a 30% reduction in wing profile drag at cruise. By comparing 2D CFD modelling methods to 3D CFD and experimental data, it is shown that the lift coefficient of a blown wing is primarily driven by two things: the jet thrust, and jet position relative to the wing. 2D modelling of the blown wing is therefore able to capture the lift and drag without the need to use more complex models. 2D models can also be modified to account for the effect of propeller spacing. It is found that the propeller spacing can be increased, with a minimal reduction in the blown lift, provided that the total thrust of the propellers remains constant. Increasing the propeller spacing reduces the number and hence drag of the nacelles. The mechanism of blown wing stall when a flap is deployed is identified. It is shown that the flap stalls due to boundary layer diffusion resulting from a combination of the propeller jet and flap incidence. Moving the propeller down vertically increases the maximum lift coefficient, provided the jet does not pass below the wing, as the wing remains fully immersed in the jet up to higher flap angles. Increasing the propeller spacing leads to boundary layer separation at lower flap angles, however the growth of the separation with flap deployment is slowed when compared to closely spaced propellers, as less of the flap is affected by the propeller jet; as a result, spacing the propellers out also increases the maximum lift coefficient. 2D modelling of the blown wing lift coefficient captures the trends in maximum lift coefficient with different vertical offsets, but is unable to capture the increased un-stalled flap angle range afforded by increasing the propeller spacing. A power balance model for the aircraft shows that a blown wing can achieve a 12.2% reduction in power consumption in cruise if the leading edge propellers are stowed away after take-off. The optimum propeller position to achieve this is an axial offset of 25% chord upstream of the wing, a vertical offset of 5% chord below the wing chord-line and with a propeller diameter of 64% chord. If the leading edge propellers are not stowed away, and instead are also used in cruise, a 10.5% reduction in power consumption can be achieved. For this case, the propellers are positioned in the same horizontal location, but with no vertical offset from the wing chord-line and a larger diameter of 90% chord. Spacing propellers out offers a further 0.5% reduction in power consumption at cruise.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Hawkswell, George

Advisors dc:contributor.advisor

• Miller, Robert
• Pullan, Graham

• Blown Wing
• Distributed Electric Propulsion
• Electric Aircraft Propulsion

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/04a1f4d4-719c-4379-9381-0693c0a981b5/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.113020

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/375289