Actuators/sensor failures and structural/airframe failures are two main sources for aircraft damages. A high degree of redundancy is able to solve actuator/sensor failures easily without any technique challenge to flight control system design, so this kind of failures are not the objective the reconfigurable flight control systems are going to deal with. Obviously, most of civil aviation disasters are resulted from structural/airframe failures, such as United Airlines Flight UA232, EI AI Flight 1862 of the Israeli Airline, American Airlines Flight 587 and so on. Therefore, structural/airframe failures are the author’s objective.

After extensive background research I have finally concluded on the subject of my research. This will be “Drag reduction with the use of plasma actuators". So, what does this mean?

Drag, as a force acting on an airplane is caused by several phenomena and it is classified, according to them, into various types. These are:

• Profile drag which is caused by the diversion of the flow around the aircraft shape
• Viscous drag which is caused by the friction between the air and the airplane skin
• Wave drag which is caused by shock waves that are formed in parts of the plane that the flow is accelerated beyond the speed of sound.
• Induced drag which is formed by the vortices that are formed at the wingtips

For the category of aircraft CleanEra is focused on, the main contribution in total drag is due to viscous and induced drag. Out of the two the most promising for sizeable improvement is viscous drag. One of the most promising techniques and one that has been researched for years is laminar to turbulent flow transition delay. This technique is based on the fact that laminar flow viscous drag is one order of magnitude smaller than turbulent flow. If transition to turbulent flow is delayed for a considerable percentage of the chord length of the wing, then the total drag is reduced dramatically.

My research proposal is based on the technique of transition delay for drag reduction. Previous research has dealt with various ways of achieving this but very few actual solutions and applications have resulted. This is due to the complexity of the proposed systems, their limited maintainability and their energy consumption. One new technology that is beginning to find its way into aerodynamics and flow control is the use of plasma actuators.

Plasma actuators operation is quite straightforward. By applying a large voltage between two electrodes lying in a spanwise position on the wing and separated by a dielectric barrier, the air in between them is ionized. Due to the alternating voltage, ions from the ionized air move from one electrode to the other and by doing so they also collide with neutral air particles. Through the collisions, momentum is transferred to the free air and this is perceived in a macroscopic scale as a, tangential to the wing surface, body force acting on the flow. This can be used in a variety of ways for flow control.

The way to implement this technique for transition delay is to operate the actuators in pulsed mode with a duty cycle between 20% and 70 %. The resulting pulses can be used for direct wave cancellation of the instabilities that lead to transition. This is very energy efficient as it consumes only 10 W/m length of actuator. The challenge with this technique is to create a sensing and control system that can detect and cancel the microscopic flow instabilities as well as to look into the mechanisms of transition as little is known about these.

I appreciate any coments or questions you have regarding these thoughts.

An aircraft is a three-dimensional solid body moving through air and as such it is subjected to a variety of external forces. These can be summarized as gravitational forces, inertial forces and aerodynamic forces. Aerodynamic forces can be further categorized into lift and drag, the former providing the aircraft the ability to defy gravity and fly, while the later resisting to the forward movement of the aircraft is the 100 year-old nemesis of aeronautical engineering.

With rising fuel prices as well as the ever growing demand for cleaner, more environmentally friendly transport, drag reduction on aircraft is emerging as one of the most promising technologies to that end. Through drag reduction techniques, aircraft fuel efficiency as well as aircraft-related pollution can be considerably reduced.

My work in the CleanEra team is to investigate and develop drag reduction technologies that combine two main characteristics:

·        providing considerable reductions on aircraft drag thus resulting in fuel economy and cleaner operation

·        being technologically and economically feasible for adoption on new commercial aircraft concepts targeted for production in the next few decades

There is a very broad field of techniques for aircraft drag reduction under various levels of know-how and feasibility. These techniques address different forms of drag such as viscous drag, wave drag, drag-due-to-lift and pressure drag. The majority of these are based on flow control by means of suction/blowing, vortex generating, active shape morphing etc.

The basis of the work conducted for the project will be to find a way to combine and effectively use some of these technologies and/or to develop some new ones, in order to produce a drag-reducing system that will be both efficient and feasible for the revolutionary green aircraft

Airplanes must have the following general flight characteristics:

• The airplane must have sufficient control power to maintain steady state, straight line flight throughout the design flight envelop.
• It must be possible for the airplane to be safely maneuvered from one steady state flight condition to another.
• Cockpit control forces should be within acceptable upper and lower limits under all excepted conditions throughout the design flight envelop.

Characteristics mentioned above are a subset of the so-called ‘flying or handling qualities’ of an airplane. Requirements for good flying or handling qualities of civil airplanes are qualitatively and quantitatively specified in JAR-VLA that The CleanEra Team investigated three weeks ago. One of all the objectives on which we are able to and intend to do some further research is the landing gear system.

The landing gear system, a significant contribution to the total weight of the airplane and noise emission during take-off and landing, is a conventional aircraft tricycle configuration consisting of a nose landing gear and a left and right main landing gear. Using of new materials and configurations might be able to make the size smaller and the weight lighter. Furthermore, complete elimination combined with unconventional take-off and landing techniques might be able to make the airplane cleaner, safer and more comfortable. Here are several examples which can lead to that direction:

Conventional Vs Unconventional Design

My earlier task with CleanEra project was to evaluate the existing and future aircraft configurations.

One of the main debates among the team-members is should CleanEra retain the present conventional aircraft configuration or should we venture into the unconventional configurations in order to meet the requirements  set in “European Unions Vision 2020” or even beyond such as

• Increase in quality and safety

• Reduction of the environmental impact

• 50% decrease of CO2 and 80% NOx emissions

• 50% decrease of perceived noise

• Pioneering the Air Transport of the Future

• Easy maintenance

Conventional configurations are also progressing well in achieving the above requirements with the introduction of Airbus A320 and now with the birth of B787-Dreamliner. Should we change since we are bored with the same design for the last 60 years starting with the introduction of B-47 in 1947 ? Or should we  changed to the unconventional configurations such as Blended Wing Body (BW, Flying wing, Box/Tandem Wing, or Multi Hull/Fuselage etc since scientifically, theoretically and experimentally one of these configurations may able to achieve a faster and better result in meeting the above requirements?

Would the public/passengers able to accept the idea of flying in a flying saucer shape aircraft or Blended Wing Body aircraft for example, no matter how safe and reliable the unconventional configuration aircraft wil be?

I really would like to hear the public/readers general opinions, views, fears or anticipations in flying in unconventional configurations if there is such aircraft introduced in the near future. Thank you.

When speaking about 'green aircraft' one may immediately think about emissions and noise production. The major contributor to noise generation and emissions is still the engine of the airplane. One solution in order to reduce the engine noise and emissions, and also to satisfy the 'Vision 2020' requirements, would be further optimizing the combustion cycle and design of the advanced turbofans that are used today. But how far can they still be improved and could there exist an other propulsion system that would be even more ultra-green?

I believe that there exists an other propulsion system that is more eco-friendly en that not may be excluded in the future: counterrotating propellers. Although they have a fuel reduction of 30% (which also means less emissions) there are still some large technical obstacles that have to be overcome. One of the major problems regarding propellers are the reliability and the noise generation (external but also in the cabin itself). My PhD will deal with the topic of noise optimization of these counterrotating propellers (CPR). Can the CRP design still be improved in order to reduce the noise generation? How can the noise sources of the CRP be modelled accurately? Does there exist a possibility to shield the CRP? Is a pull or push CRP preferable?

Like I already mentioned before, CRP have a fuel reduction of 30% but they have some other important advantages, such as an efficiency of nearly 90%, a shorter runway length is needed, no asymmetrical torque effects and they can fly as fast as turbofans. Although these advantages seem very attractive, there are also some disadvantages/problems that have to be solved: reduction of the cabin noise (and external noise) to improve the passengers comfort, the anti-icing problem during flight, improvement of the pitch adjustment system. Also the image that CRP have at the moment has to change because a lot of people think they are based on old technologies and that they are less safe as turbofans.

How the aerospace industry will look like in 20 years is still an open question. One thing is sure... the air transport will increase dramatically over the next 20 years and noise and  emissions will be key issues in the further development of the industry. In order to share the visions of different parties on the future aerospace industry regarding noise the National Institute of Aerospace (NIA) and NASA organized the workshop 'Revolutionary Aircraft for Quiet Communities', which was held in Hampton (virginia). I went to this workshop and presented there the objectives and goals of CleanEra. The workshop was in the first place very interesting because a lot of interesting people (representing companies and organisations like NASA, Boeing, Lockheed Martin, Cambridge University, European Commision, Dassault,...) gave their opinion of how they see the implementation of  the noise reduction factors in current and revolutionary designs. A lot of information and knowledge was shared and a clear view was given of the current problems concerning noise reduction. 

I can go on about this workshop for hours so if you still have questions or you want to know more please contact me personally. Of course I'm also very curious about what experts think about future airplane with CRP and how to solve the current noise problem of CRP...

The Clean-Era project consists of a team that develops new technologies to make the future aviation more environmentally & consumer friendlly.

My research is about non-cylindrical pressure fuselages that will contribute to greener aircrafts for the future. To lower the emisions there are 4 aspects that determine the efficiency of the aircraft:

1. Lower the weight of the aircraft

2. Improve the Lift over Drag (L/D) ratio

3. More efficient engines

4. Use a more environmentally friendly fuel

Non-cylindrical pressure fuselages have impact on the first two aspects.

Future aviation will probably consist of flying wings for large aircrafts and lifting fuselages for medium sized aircrafts. Changing the shape of the cylindrical fuselage will cause weight penalties but the challenge is to change it in such a way that the better L/D ratio exceeds the increment in weight.  

 Some concepts of future fuselages are variants of the multibubble-fuselage and the multidome-fuselage.

Francois Geuskens