Greener by design – the technology challenge

Greener by design – the technology challenge

Keywords: Aircraft engineering, Environmental issues,Design

A group including participants from every part of the aerospace industry, the Royal Aeronautical Society, environmentalists and government departments was formed in 2000 and became known as the Greener by Design Steering Group. Various sections were established; Operations, Technology, and Market-Based Options, of which Technology is significantly larger than the others and is the subject of this article.

Key questions are asked in the study on technology, which has been restricted essentially to subsonic jet transport aircraft. Its theme is the potential to mitigate the environmental impact of the continuing growth in air travel by the introduction of new technology and new design aspects. The environmental issues it addresses are noise, local air quality and climate change, the last named being taken as the most important in the long term. It has been addressed in the report by the creation of a set of 13 self-consistent aircraft design concepts,of differing levels of technology, which are compared in terms both of fuel burn and of radiative forcing of climate. The time horizon for the study is 2050. The main findings are outlined on each of the three issues.

Noise

Current noise regulation is contained in Chapter 3 of Annex 16 in ICAO, which specifies maximum noise levels at each of three defined measuring points: to the sideline of the runway at take-off; under the flight path at take-off; and under the flight path on final approach. At present some 90 per cent of the total civil fleet, passenger and freight, meet the Chapter 3 requirements. It is noteworthy that more than 20 per cent of the fleet, including all A340s and Boeing 777s, could meet a noise target 14dB below the Chapter 3 limits. Note that the rating is the sum of noise measurement at three points, so that, for example, a reduction of 14dB might be the sum of reductions of, say, 5db at the sideline and take-off flyover points and 4dB under the landing approach. In January 2001 CAEP/5 agreed on a total increase in stringency of 10dB, with a minimum of 2dB at each measuring point, to come into effect on new types certificated from 1 January 2006.

Since the beginning of the jet age, aircraft noise has been reduced by some 20dB. This is partly the result of continuing research to reduce engine noise and partly the result of continuing reductions in engine specific thrust in order to improve fuel economy and operating costs. The variation of some key engine parameters with specific parameters is shown in the illustration. The specific thrusts of today's large engines are around the economic optimum. Further reduction in specific thrust in order to reduce noise would result in increased aircraft weight, drag, hence fuel burn and operating costs (see Figure 1).

Figure 1 Variation of fuel burn, noise and weight with specific thrust

Reduction of specific thrust is only one route to further noise reduction,however, as aircraft noise sources are many and varied, and there are active research programmes in Europe and the USA with ambitious noise reduction goals. For example, NASA claims in the Advanced Subsonic Transport Noise Reduction programe, to be able to demonstrate in 2001 a reduction of 10dB relative to the noise standards of 1992, with further goals of 10dB within ten years and 20dB within 25 years. Within Europe, research is directed towards a similar goal of 10dB reduction within the next decade. Although the noise exposure of communities around airports has been reducing for some time due to the progressive retirement from the fleet of older and noisier aircraft, this benefit will end within the next five years or so. It is hoped that research programmes will help compensate for this otherwise partial loss of control of noise reduction. In the longer term, although NASA has declared a further goal of a 20dB reduction below current levels in 25 years time, this is not likely to be achievable with the classic swept wing aircraft with podded underwing engines. Some radically different configuration in the future such as the blended wing-body with engines mounted above the wing-body, combined with noise-reducing operating procedures, could make the 20dB reduction goal a realistic goal.

Local air quality

Improvements in aero-engine combustor performance over the past 30 years have considerably reduced the contribution of aircraft operations to air pollution in the vicinity of airports, although it remains a serious environmental issue at busy locations. The regulations for aircraft seek to limit the emission of a range of pollutants, the main ones of which, from modern engines, are the oxides of nitrogen, NO and NO₂, collectively termed NO_x. These result from the combination of the nitrogen and oxygen of the air under the conditions of very high pressure and temperature prevailing in the combustor.

In the engines of today's long-range aircraft, the air enters the combustor at a pressure typically 35 to 40 times the pressure in the engine inlet and a temperature more than three times the inlet temperature. It passes through a peak temperature in the flame and is cooled by dilution, leaving the combustor at the turbine inlet temperature, which may be as high as 1,750°K, and undergoing a rapid drop in temperature and pressure through the turbine. It is the high temperatures and pressures in the combustor which promote chemical reaction between the nitrogen and oxygen of the air to form NO_x.

The modern combustor is a highy evolved compromise which achieves virtually 100 per cent combustion of the fuel, good uniformity of inlet temperature into the turbine, low overall pressure drop, stable operations over a wide range of fuel/air ratios and air flow velocities, high-altitude re-light capability,durability (typically 5,000 cycles to first repair), and low NO_xemissions.

The evolution of the modern combustor can be considered as the characteristics of four successive generations:

• 1.

  Generation 1: the can-annular type (1950) and (1a) the improved can-annular type (1990 +).

• 2.

  Generation 2: the large annular type.

• 3.

  Generation 3: the compact annular (1980).

• 4.

  Generation 4: the staged low emission combustor.

Each generation marks a substantial step forward in combustor design, with generation 3 currently established as the dominant type. Even so, within that generation there is a spread in NO_x emissions of ±20 per cent about the mean at higher pressure ratios. The designs lying close to the lower limit of the generator 3 range are typical of the most recent combustors and those on major engines about to reach production.

Generation 4 is the first result of research to develop combustors giving NO_xemissions significantly lower than the generation 3 designs. One such "staged"configuration is the dual annular combustor (DAC) and is the only type currently in service, but it is effective in reducing NO_x only on smaller engines. The concept of the two separate combustion zones is that the pilot stage should provide good operational performance at low power while the main stage gives low NO_x at full power. So far, this technology has shown average NO_x emissions approximately 12 per cent below the best of the generation 3 designs at engine pressure ratios in the range 25 to 30. At high pressure ratios (above 35), however, demonstrated NO_x levels from generation 4 combustors have been in the same range as the best of the generation 3 designs.

The generation 4 combustors require more complex control systems and are likely to increase weight, maintenance costs and possible fuel burn. It is feasible to retrofit them to existing engines but this would require replacement of many other parts of the engine core at an estimated price of approximately one third the price of a new engine. It is therefore thought likely that generation 4 systems will be considered only for new production engines.

For the future, still more radical combustor designs are likely to be explored in both Europe and the USA. One declared target of the NASA UEET Program is a reduction of 70 per cent in NO_x emissions relative to the ICAO 1996 (CAEP/2) standard. The initial target pressure ratio at which these reductions are aimed is 55:1.

More advanced combustor schemes are aimed at 2nd generation SST (see Figure 2), although there are difficulties in applying such concepts in the high pressure, high temperature environment of a modern high bypass ratio engine. Some of the features are incorporated, however, in the EC EEFAE programme. The ANTLE programme is part of the same European effort, led by Rolls-Royce.

The conclusion is that NO_x reduction is of great importance both for the airport environment and for global warming. Research is needed to develop practicable combustors with reduced NO_x emissions, backed by basic experimental and theoretical research into nitrogen-oxygen reaction kinetics on the basis of which realistic future goals for NO_xemissions might be set. Research is also needed to establish more accurately the contribution of NO_x emissions at altitude to radiative forcing and the dependence of this effect on flight altitude.

Climate change

As stated, it is considered that the impact on climate change is considered potentially to be the environmental issue which poses the most serious long-term threat to the future growth of air travel. The objective is to identify the means of reducing the contribution of air travel to climate change. An indication of this is the radiative forcing (RF) which is defined as the global,annual average radiative imbalance to the atmosphere-land-ocean system caused by anthropogenic perturbations. To a first approximation, the current contribution of aviation to RF is proportional to its current rate of creation of the shorter lived contributors which, in turn, is related to its annual rate of fuel burn.

Figure 2 Staged combustors

Fuel burn per passenger or kilogram of freight per km can characterise the contribution of an individual aircraft to the total RF relative to the benefit it produces. A key performance parameter is the calorific value of the fuel x overall efficiency of the engine x L/D (lift/drag). Airframe weight as a fraction of all-up weight is the other key factor influencing fuel efficiency. The overall propulsion efficiency is frequently written as the product of the thermal efficiency of the gas turbine x the propulsive efficiency of the jet. The L/D ratio is a measure of aerodynamic efficiency and because of various factors, the value is always slightly below its maximum possible, which represents one of the fundamental constraints imposed by nature on the aircraft designer.

An assessment of the influence of design range on fuel economy has been made which shows the envelope of payload fuel efficiency (PFE) plotted against range R for an aircraft designed for that particular range. The diagram also shows the variation of PFE for maximum ranges of 15,000km, 7,500km and 5,000km, each with a full payload. The results are broadly consistent with an example of the Boeing 747-400 operated by Japan Air Lines on various ranges. The poorer fuel efficiency of the long range aircraft is explained simply. A substantial fraction of the fuel used over the first third of its journey is to carry the fuel for the last two thrirds of the journey, and also to carry the structure needed to carry the extra fuel load. This suggests that to carry a given payload over 15,000km in a single leg rather than three requires an aircraft which has a 40 per cent greater empty weight and which burns 40 percent more fuel in completing the mission. Further, the long range aircraft has a maximum take-off weight almost 80 per cent higher than its medium range counterpart. The implications of this are considered later in the report.

The effect of aircraft emissions on climate change varies with flight altitude primarily through the varying effects of shorter-lived secondary products of the emissions. The effects also vary strongly with latitude, climate and season and understanding of them is incomplete. Atmospheric research is needed to obtain a more complete understanding of the relationship between flight altitude and the effect on climate change of all components of engine exhaust emissions.

The dominant configuration for medium and long range civil aircraft has low,swept wings and is powered by turbofan engines mounted in pods beneath the wings, the A380 being the most recent example. Some of the forecasts for future fuel efficiency made in other areas are not considered realistic. The likely evolution of the dominant design over the next 50 years is considered.

There are three ways in which engine development can contribute to reducing fuel burn: improving thermal efficiency, including the introduction of more complex thermodynamic cycles; impoving propulsive efficiency; and reducing the weight of the engine and its related systems. Studies of engine thermal efficiency indicate a clear conflict between the goals of achieving improved fuel economy and costs on the one hand, and reducing greenhouse effect on the other (see Figure 3). It is evident that combustion research, not only to reduce NO_x but to improve basic understanding of nitrogen-oxygen physical chemistry at high temperatures and pressures, is needed for its relevance both to global warming and to pollution around airports. In the longer term global warming may become the the more important.

Figure 3 Variation of SPC with overall pressure ratio and turbine entry temperature

Alternative engine cycles is one way of increasing thermal efficiency. One technology, the inter-cooled recuperative (ICR) cycle, has many practical obstacles to its introduction, not least the achievement of efficient, low pressure loss heat exchange at an acceptable weight penalty. This may exist but has yet to be demonstrated, although research continues.

When considering propulsive (Froude) efficiency, indications are that todays high bypass ratio engines are at an economic optimum for current technology. It appears that the net effect of any further, significant reduction in specific thrust would be to reduce fan noise but increase operating costs, aircraft weight and fuel burn.

The ducted fan is one possibility for further increase in propulsive efficiency. A small increase in component efficiencies; the reduction of nacelle drag by laminar flow control; and the reduction of fan cowl weight by wider use of lightweight structutal materials, could collectively lead to a significant reduction in specific thrust, gain in propulsive efficiency and reduction in noise. As far as engine weight is concerned, the primary contributors are:advances in the aerodynamic design of turbomachinery; and advances in materials and manufacturing processes. An example of the latter is the potential progression from a conventional compressor disc plus blades to a bladed disk(blisk) and on to an integrally bladed ring (bling) made from a titanium matrix with silicon carbide fibres. The concept of a more electric engine together with the more electric aircraft has also been advocated as a means of reducing maintenance costs, improving reliability and also reducing weight.

For general aerodynamic improvements, aircraft with turbulent boundary layers(virtually all of todays jet transport aircraft) could benefit from variable camber wings; riblets; active stability; and passive and active shock control on the upper surface of wings at transonic conditions. Aerodynamic integration of the engine and pylon with the wing is also thought to be worthy of refinement. Natural and active laminar flow control is a promising area of investigation; of the latter, hybrid laminar flow control (HLFC) employs a combination of suitable aerofoil shaping and boundary layer suction.

From the various considerations mentioned, up to 2050 it is forecast (which excludes hybrid laminar flow control and the ICR engine, as well as IDF propulsion), technological advances in three areas: in aerodynamics a reduction of 10 per cent in drag at zero lift relative to 2001 design standards; in propulsion, overall efficiency is assumed to increase by 8 per cent, achieved in a large part by increase in overall pressure ratio and turbine inlet temperature, but including improvements in combustor technology; and the use of lighter airframe materials enabling operating empty weights to be reduced by 15 per cent.

Other possibilities include two aerodynamically efficient alternatives to the dominant configuration. One is the blended wing-body with turbulent boundary layers; and a large BWB or delta wing with full laminar flow control, similar to that proposed by Handley Page. Alternative fuels are also a possibility, the only one likely to be seriously considered being hydrogen.

Conclusions

In the neigbourhood of airports, we can expect the environmental impact of aircraft noise to continue to reduce over at least the next decade and the increase in emissions to be appreciably less than the increase in traffic.

Current research should lead to further reductions in noise and emissions from future production aircraft but the rate of alleviation of community impact will decline as the last of the older aircraft are withdrawn from service.

The contribution of air travel to climate change has been taken in this study as the most serious of the environmental effects. The potential to reduce it by the adoption of specific technological advances has been investigated by the creation of a self-consistent set of long and medium range aircraft configurations. The conclusions are as follows.

For lone range aircraft three areas were identified. Where range is concerned, the payload fuel efficiency of the current and projected families of large wide-bodied aircraft, typically designed to operate over ranges from 13,000km to 16,000km, is substantially inferior to that of an aircraft of the same technology standard designed to carry the same payload over a range of 5,000km or less. A full system study of the feasibility of undertaking long distance travel in stages not exceeding 7,500km is recommended.

Dealing with engine overall pressure ratio (OPR), current evidence shows that NO_x emissions increase as engine pressure is increased. This trend appears to be the case for engines with combustion chambers of current design. It is possible that more advanced combustion systems will lead to higher optimum pressures ratios but this remains to be determined.

Concerning cruise altitude, the modelling of the variation of greenhouse effect with cruise altitude adopted in this report suggests that the effect might be substantially reduced by crusing at lower altitudes. Contrail formation, however, is not taken account of, and a much fuller understanding of the atmospheric effects of aircraft emissions is needed, paticularly of the variation of these effects with flight altitude, latitude, climate and season.

Improvements to the fuel burn of the dominant configuration have been estimated at 30 per cent to 35 per cent. Other technology could be introduced but would require some incentive. Hybrid laminar flow control (HLFC) offers reductions in fuel burn and geenhouse effect for swept wing aircraft. Turbofan engines with substantially reduced NO_x emission is another possibility. The blended wing body offers greater aerodynamic efficiency and beyond this the large, wholly laminar flying wing. The contra-rotating ducted fan offers further reductions in fuel burn, but there are probems to be solved. In the longer term, hydrogen is the only alternative fuel to become available. Overall, a range of research and technology programmes are required to be addressed, with many specifically related to climate change.