This thesis describes an Engineering Doctorate project in Distributed Propulsion carried out from 2004 to 2007 at Cranfield University. Distributed propulsion is a propulsion system arrangement that consists in spreading the engine thrust along the aircraft span. This can be accomplished by distributing a series of driven fans or the engines themselves. The aim of this project is to determine the feasibility of distributed propulsion for civil aviation in the medium term (with small gas turbines) and long term (with driven fans) from a technical and economic perspective. The effect of distributed propulsion was assessed by creating a long-range subsonic airliner baseline with conventional technology for the small gas turbines study, and an equivalent blended wing body baseline for the driven fans study. Different distributed propulsion effects were modelled and integrated together to produce optimised baselines with different technological parameters. The feasibility of small gas turbine distributed propulsion was found to be limited by the excessive fuel consumption associated with small gas turbines. Although advanced heat exchanger technology could improve their performance, the resulting cost advantage might not be large enough to justify the development costs. The feasibility of distributed driven fans depends on the availability of superconductive elements, as electrical power transmission seems to be the only promising transmission method in the long run. If superconductive elements are applied, distributed driven fans could afford fuel burn reductions of more than 50% relative to current technology. As both distributed propulsion concepts rely on small propulsive units, their enabling technologies coincide with those required to develop future unmanned aerial vehicles. UAVs therefore represent the most appropriate technological avenue to develop technologies with the potential to become distributed propulsion enablers. Future work should therefore concentrate on improving engine performance and cost for unmanned aerial vehicles.

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Eckert and Weise first measured time-averaged energy separation behind a circular cylinder in 1943- Although a significant amount of work has been carried out on the subject since Eckert and Weise's discovery no time-resolved measurements have been made of the phenomenon at high subsonic Mach numbers. The aim of this project was to make these measurements, along with those of surface pressure. Energy separation and base pressure are investigated at high subsonic Mach numbers, behind a circular cylinder in crossflow, for the first time.;The measurement of energy separation has involved developing a novel operating procedure for a high frequency response thin film total temperature probe, allowing it to be heated in stagnant conditions while keeping the wind tunnel running, hi addition the analysis of the results has involved the development of a fully automated phase lock averaging routine.;The principal original contribution of this work is to demonstrate clearly that unsteady energy separation occurs as a result of vortex shedding at high subsonic Mach numbers. The time-resolved measurements show how the areas of increased and decreased total temperature and total pressure are related. The results also give, a good qualitative description of the shape of the vortex street, showing the presence of interconnecting ribs and the areas of maximum entropy increase and thus drag creation.;The surface pressure results have permitted the study of how Strouhal number, drag, base drag and vortex shedding mechanisms change with Mach number. Of particular note they show that the vortex mechanism present in the permanent shock wave regime does not replace that present in the intermediate shock wave regime but rather develops from it.

Mainly owing to the dwindling fossil oil resources and the environmental concerns of discharging greenhouse gases into the atmosphere, it is essential to find an alternative to kerosene for civil aviation. The overall objective of this thesis is to evaluate the potential of reducing the environmental impact of civil subsonic aviation by using hydrogen fuel. Mainly due to the complex interactions among a number of different fields affected by the introduction of hydrogen in aviation and due to the nature of the research question, the chosen scientific approach for this thesis is to carry out a broad study covering a number of selected fields. Engine and combustion chamber design are studied in detail, along with the cruise altitude for minimum environmental impact. Airport implications, and available and envisioned methods for hydrogen production, are discussed. From a technical point of view, it seems to be feasible to use hydrogen for aero gas turbines. In terms of pollutant emissions, hydrogen use offers the possibility of a significantly reduced number of emission species, resulting in only H₂0 and small quantities of NOₓ emissions. For minimum environmental impact, the results suggest that cryoplanes should cruise at an altitude of about 2-3 km below where conventional aircraft cruise today. If the priority is to lower the mission fuel consumption, the results indicate that an engine employing increased combustor outlet temperature, overall pressure ratio and by-pass ratio, seems to be the most attractive choice. The mission NOₓ emissions, on the other hand, seem to be reduced by using engines with a weak core and lowered by-pass ratio. Ignoring the cost implications, from an airport infrastructure point of view, it seems feasible to change to hydrogen use. With respect to the availability of energy, it would be reasonable to change from kerosene to liquid hydrogen as fuel for all civil aviation refuelling in Sweden.

Damage to the lipskin of an engine nacelle can have a significant impact on engine performance and safety, especially in combination with other sources of inlet flow distortion. While the degradation of performance in aircraft propulsion systems as a result of inlet distortion has been well-studied, the impact of lipskln damage upon inlet distortion levels is not well understood. Current practice can result in the grounding of aircraft in order to perform repairs if the damage exceeds the specified tolerance in size and the position on the lip, with these tolerances varying from aircraft to aircraft. If these tolerances are overly conservative, aircraft operating times may be adversely affected. The effect of lipskin damage upon the performance of the inlet of the CF34-3A engine has been investigated using the commercial Computational Fluid Dynamics Package ANSYS CFX in a nacelle model which included the fan. The fan was included in order to capture the interaction between the blades and the upstream distortion. The fan was found to have a stabilising effect upon the flow upstream of the fan, reducing the inlet distortion due to a redistribution of the flow. Other sources of inlet distortion have also been studied, with the effects of high angles of attack and crosswinds having been modelled in isolation, and combined with cases of lipskin damage. A typical take-off configuration was primarily used to assess the effect of the damage due to this being the most critical segment of the flight envelope. The resulting distortion levels have been compared to the engine manufacturers limits in order to assess the severity of the damage.

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Civil aviation, providing transport to connect people, cultures and economies, is situated at the heart of globalisation. Since its earliest days, it has grown along with every other part of the industrialised society and experienced growth rates exceeding that of global GDP. Projections suggest that future air traffic emissions will play an increasingly important role in the contribution to global warming, which is regarded to be a serious threat to earth’s socio-ecological systems. Air traffic contributes to the overall anthropogenic radiative forcing, a metric denoting perturbations in the earth’s radiation budget, by the emission of greenhouse gases and aerosols, and also by the generation of high ice clouds, commonly known as contrails. Recent studies suggest that the radiative forcing resulting from contrails is potentially higher than that of all other air-traffic pollutants combined. In light of this, contrail avoidance is attracting increasing interest from the aeronautical community. An important contribution to the understanding of the problem in a wider context is made in this thesis, alongside proposals for short, mid and long term strategies for contrail avoidance. These are in particular the optimisation of the aircraft for contrail avoidance, the application of remotely induced heat to suppress contrail formation, and a novel engine concept that exhibits the potential for a reduction of all emissions simultaneously. Aircraft optimisation deals with the adaptation of existing technology for more environmentally compatible air transport, whereas the latter two approaches are breakthrough technologies of a more disruptive character covered by several patents resulting from this research. Short and mid term strategies are accompanied by an increase in carbon dioxide emissions. A study examining the long-term impact of aviation carbon dioxide emissions relative to that of contrails suggests that in order to achieve more sustainable air transport, the avoidance of contrails is inevitable. However, as the short-term impact of contrails is less severe, postponing contrail avoidance until the associated increase in carbon dioxide emissions is less significant could be a better way to deal with the problem.