A clean era: reality or wishful thinking

Developing technologies for the aircraft of the future

 

The world has changed from the early days of aviation and will continue to change in the future. It is impossible to predict the future of aviation without looking at the environment the aircraft has to operate in. Due to changes in the world the goals of aircraft design are continuously changing. It’s still impossible to predict what the future will bring, but it is clear that three challenges will dominate the near future of aviation. The first challenge will be the continuous growth of China and India and the need for more fuel to sustain this growth. This will result in a scarcity of energy. The second will be the environment, becoming more and more of a concern and requiring action. The third challenge aviation will have to to face is the changing world economy, due to the financial crisis, making it harder to acquire funds and to invest in new technologies.

by: Ir. M.J.T. Schroijen, PhD Aerospace Engineering student, team member of CleanEra

In this changing world the CleanEra project is trying to look into technologies which might aid or enable the existence of aviation in the future. CleanEra is an acronym for Cost-effective Low Emissions And Noise Effective Revolutionary Aircraft. The project started in May 2007 and has grown to eight PhD students, aided by several students under guidance of a project leader from industry and backed-up by the knowledge available at the faculty of Aerospace Engineering. Not only Delft University of Technology is working on this project, also several partners from industry are involved: KLM, Stork, NIVR and the European SADE project.

The mission statement born from the previously explained change in environment is: 'To develop new technologies for (a) revolutionary conceptual aircraft design(s) optimized for environment and passenger friendliness and investigate the feasibility of these technologies and their integration.'

Translated into the four goals of CleanEra, the future aircraft should produce:

1. No emission footprint (e.g. carbon neutral)
2. Minimum noise footprint
3. Increased quality and safety
4. Easy maintenance

It is a huge challenge to reach the goals described in the mission statement. Many projects have focused or are focusing on one of these goals. To give just a random selection of these projects:

1.  The Silent Aircraft Initiative which had the goal of reducing the noise produced by aircraft.
2. The Cryoplane project which had the goal investigating the potential of liquid hydrogen in aviation.
3. The CleanSky project is developing technologies which are readily usable by industry complying with the Vision2020 goals.

So how does CleanEra differ from these projects? CleanEra is improving performance of the complete aircraft life cycle using a new approach. This approach is quite revolutionary because CleanEra is looking at improvements in multiple areas simultaneously; this increases the risk of not achieving the predicted performance. In industry only one technology improvement is investigated to reduce this risk, this approach however also limits the achievable improvements. CleanEra is looking into multiple potential technologies to force a step change in the performance of the aircraft. A structured approach is therefore necessary to be able to contain and identify possible risks. Being aware of the implications of a technology in areas it is not intended for. Before evaluating the risks of not achieving the predicted performance, one has to be aware of what this performance should be. Understanding the way an aircraft is used during its future life is therefore essential. The fact that the aircraft has an influence on many people and companies (airlines, manufacturers, passengers, airports etc.) results in a complex system of needs and expectations. A correct interpretation of, and a good balance between these can already be quite a challenge. Being aware of what we want to achieve, we can look into ways of achieving that. Already mentioned in that respect are technologies but CleanEra is also looking into novel configurations. Examples are the blended wing body and joined wing. CleanEra is investigating which of these configurations has the most potential of achieving the desired goals.

In conclusion the multi-technology approach we are using involves a lot of risk, making a good understanding and tracking of these risks necessary. Before predicting the performance of a technology we have to define how this performance is defined by looking at the needs of the various people and companies using the aircraft. All technologies are integrated and affect the design; therefore the implementation is taken into account when determining the effect of a technology.

Technologies
With any new design one tries to maximize the benefits (the needs) while reducing the drawbacks (the costs). The objective is identifying those technologies which improve performance while reducing the drawbacks or even turn these drawbacks into improvements. Looking at the four goals of the CleanEra project and looking for technologies and methodologies to get improvements in the areas identified as being important in the future, several promising technologies emerged.

Eliminating emission footprint

The first goal is an area in which much improvement has been made in the past. This focus results from the direct relation between carbon dioxide emissions and fuel consumption (and therefore direct operating costs). The performance of an aircraft design with respect to fuel consumption is called fuel efficiency. This parameter relates how efficiently fuel is converted into thrust or power (thermodynamic and propulsive efficiency), how efficiently this thrust is used to transport a certain weight through the air (aerodynamic efficiency) and how effective the payload is carried by the structure of the aircraft (structural efficiency). From the aircraft design perspective we are therefore investigating technologies to improve these three efficiencies to reduce the amount of fuel needed for a given mission. It is readily clear that increasing one of the efficiencies at the cost of decreasing another can deteriorate the overall efficiency of the aircraft. We therefore have to look into the effects of a technology on all three aspects.

 

 

^{Seams resulting from the presence of high lift devices have                Seperation of functions, gray the pressure vessel and
a significant effect on the performance of the aircraft                          blue the aerodynamic shell}

Propulsive efficiency
Increasing propulsive efficiency is a challenging task. Most efficiency gains in the past have already been achieved by an increase in engine performance; e.g. the increase in by-pass ratio, the higher pressure ratios in engines combined with higher turbine entry temperatures. CleanEra is looking at the overall engine design to see if there are other possibilities to improve the engine performance, for instance by intermediate cooling or gearing of the fans. NOx production for instance is proportional to the maximum temperature in the combustion chamber. Reducing NOx with current technology means reducing turbine entry temperatures which increases fuel consumption and therefore CO2 emissions. A new technology called flameless combustion might be able to decrease the maximum temperature in the combustion chamber by spreading the amount of fuel over a larger area, thus reducing the maximum temperature while still obtaining the same turbine entry temperature. On top of that the effect of alternative fuels is being taken into account.

Aerodynamic efficiency
To obtain a higher aerodynamic efficiency CleanEra is looking at several options: CleanEra uses advanced computational fluid dynamics models coupled with optimizers to analyze and optimize the overall shape of the aircraft to obtain the maximum possible lift over drag ratio.

Technologies for increasing the lift over drag ratio are focused on laminar flow in cruise flight. Reducing drag can be done actively by using plasma actuators to control the boundary layer and keeping it from transitioning to turbulent. The actuators apply a body force onto the boundary layer damping disturbances using a limited amount of power. Passively it can be done by increasing the smoothness of the surface. The most important disturbance factors on the wing of current aircraft are the seams due to the presence of high lift and control surfaces. Making them seamless could therefore reduce the drag in cruise.

                                                                           ^{Alternative interiour concept for a blended wing body to increase
                                                                                                     passenger comfort during travel.}

Structural efficiency
The third efficiency gain is the reduction of structural weight, by decoupling of the functions of the structure. The aerodynamic structure which has to keep its shape, no longer has to carry the pressure loads of the cabin. These pressure loads are transferred by a second structure which is allowed to deform, therefore requiring less material. This might be particularly interesting for non cylindrical cabin shapes as encountered in Blended Wing Bodies (BWB). Besides technologies we are also investigating alternative procedures to reduce the structural weight. One of these procedures is an external launch system; think of for example a cart accelerated by a Maglev track omitting the weight of the landing gear.

Minimizing noise
The second goal focuses on noise reduction. Noise is becoming more and more an issue at airports. Even though aircraft have become quieter over time, the increase in air traffic and more articulate people living near airports increase the noise burden. The noise produced by aircraft is called Equivalent Perceived Noise Level (EPNL) and is determined at three flight phases, take-off, flyover and approach. A better understanding of the sources of noise in these phases as well as the propagation of the noise in air allows noise to be included in the early stages of aircraft design. Optimal placement of engines and noise shielding are promising.

A second option investigated is related to the operation of the aircraft. Reducing throttle in the approach might reduce the EPNL as well as the fuel consumption of aircraft. These so called continuous descent approaches are more complicated to fly. A technology which might aid in this respect is adaptive control, a control system which adapts itself to the state of the aircraft. This technology has the added benefit of increased safety, as the control system can adapt itself in case of failure of a control system which increases the chances of survival of the aircraft.

Improving the passenger experience
The third goal is related to the passenger experience. Both safety and comfort can be increased by the previously mentioned adaptive control system. Increasing passenger comfort is also related to cabin design; CleanEra is looking into the seating opportunities provided by alternative configurations e.g. BWB. This is done in close cooperation with KLM.

Improving maintenance
The fourth goal relates to the durability of the aircraft. Coatings provide a protective layer on the aircraft against UV light and moisture. Reducing the number of repairs reduces the amount of material needed and therefore waste.

Future
As previously mentioned: the impact of a technology on the overall performance of the aircraft is still unknown, not to mention the impact of the technologies in other areas. To make things more concrete, reducing drag with a new technology, adding a lot of weight or requiring a lot of power, could turn out to be deteriorating instead of improving the overall performance of the aircraft. We are therefore investigatin g the effects of these new technologies not only in their intended area but also the effects on the overall performance of the aircraft, which is a complex task.

Whether the aircraft of the future will truly be a clean aircraft and if we are able to enter a truly clean era is still to be determined, but the progress shown looks promising.

^{Plasma actuators keep the boundary lyer from transforming 
from laminar to turbulent.}

For more information: www.cleanera.tudelft.nl