New aircraft technologies
Many of the ideas collected in this survey concern the design of the aircraft, or have implications for its design.
Rather than present them as a simple list of disparate ideas they have been collected together under eight headings.
These range from new ideas for flight mechanics (under Global Flight Concepts) to the induction of sleep to give passengers a sensation-less flight (under Passenger Experience). Several of the concepts examined in other principal sections have implications for the design of the aircraft – whether in the Cruiser/Feeder concept or the Airport of the Future. In general where individual subjects have been treated at length in other sections they are not repeated here.
We have become accustomed to the "standard" airliner of the early 21st Century. It has a familiar form and most of them have the family characteristics of large twin engines, a cylindrical fuselage, a lower freight bay and upper passenger compartment, swept back wings and a tricycle undercarriage. Some argue that this form is the conclusion of evolution and that it simple demonstrates the limiting form of the idea. Others take the view that any form is only the product of the circumstances that produced it and if these change the evolutionary form will change and can be changed. The ideas presented here follow this path.
Prompted by the pressures for environmental sensitivity some ideas focus on ways to make dramatic, or at least important, savings in the amount of fuel used by the world’s airliners. Previously dismissed contributions to economy of fossil fuel lie behind the thinking of several concepts.
1. The glider-like airliner
Gliders have very high aspect ratio wings. These low drag wings allow them to sustain altitude in the lightest of upward thermals (about 1 fpm) and thereby to carry out long distance flight on no fuel at all. Their glide ratio is extremely shallow – in the order of 1 in 55 compared with a typical airliner of 1 in 15 (B747). Powered gliders are somewhere between a conventional a/c and a glider. Their small engines can be used to gain or to sustain altitude and the consumption of fuel is still only small.
The concept is for airliners with some of the characteristics of a powered glider. It would have high aspect ratio wings and be fitted with substantial engines for climb out but much less powerful than those in current use. Its cruising speed would inevitably be much lower – perhaps in the M0.4 range.Take-off speeds would be lower and runway lengths much reduced. The normal mode of operation would be using the engines but cruising
power demands would be very much lower. A glide ratio of perhaps 1 in 27 would produce a hybrid having many of the advantages of the glider whilst retaining most of the flexibility of the modern airliner.
The benefits are mainly in the consumption of fossil fuel. A 200 seat airliner on a 1000 nm leg will use something like 10 tonnes of fuel. A glider-like a/c of the same capacity would use perhaps an eighth or a quarter of this amount.
The lower speed of the aircraft brings some disadvantages: the aircraft is less able to earn revenue and it offers slower and thereby less attractive journey times. Against this it will be a cheaper product to make and for medium legs the extra journey time will not be hugely significant (the change from say M0.83 to M0.4 would add about 30% to total door-to-door journey times on a 1000 nm leg).
2. Wing in ground effect craft
These WIGE craft have been produced for many years – notably by the former Soviet Union. They give the promise of considerable savings in fuel through operating near to the surface (within about half a span) and gaining from the ground effect of the airflow over the craft.
Large and small craft have been shown to be feasible. A body of design knowledge exists for their design and construction. In the era when most WIGE were subject to experimentation within the FSU the considerations of fuel saving were of a different kind to today. It may now be the time to re-examine such WIGE craft and to adapt them for commercial operation.
The potential benefits lay in their fuel efficiency. For a 200-seat craft over a 1000nm leg the fuel saved might amount to 50-60% or 5 or 6 tonnes.
The disadvantages of these craft are that speeds are generally in the range 150-250 knots and the routes for their operation must be suitable for very low flying craft – i.e. either sea-lanes or barren land. Technically the craft work satisfactorily when properly designed although a natural disposition to pitch instability needs to be carefully considered. (WIGE are also covered in section 3.2.3.)
3. High speed blimps
The technology of lighter-than-air craft has progressed substantially since the heyday of the great German Airships of the 1920’s. At the most obvious level it is no longer necessary to use hydrogen as the lift gas with its attendant dangers. In recent times, several companies have come up with concepts for airships for special purposes including heavy lift operations. The idea put forward here is for an examination of a high-speed airship which might overcome the disadvantages of the relatively slow speed (around 100 knots) of conventional craft.
The essence of a pure airship is that the lift and thrust components of flight are provided for quite separately. Lift is from a large gas enclosure and this lift is almost independent of speed. Thrust is provided by a number of engines which do not provide any component of lift.With modern designs these conventional approaches have been questioned. The very modern Zeppelin NT has rotating engines that can contribute to lift as well as forward speed. Nevertheless the speed is 125 Km/hr in level flight. The Ohio Airships Dynalifter“ is a hybrid style of craft with wings that contribute to lift and control as speed is achieved. A substantial fraction of the weight is lifted by the wings and this is claimed to be a benefit in landing since the craft will sit securely on the landing ground once at rest. Speeds of 100-200 knots are forecast for this type of craft. The Cargolifter heavy lift airship was one of the concepts designed by the company carrying up to 160 tonnes at speeds up to 90 km/hr.
Cargolifter suffered insolvency in 2002 and the present fate of the project is uncertain.
It is certain that new technologies make the construction and use of airships more practical.With hybrid technologies, some of the handicaps of the format may be overcome. Looking for very high speeds will continue to be a compromise between lift, size and construction since an envelope of minimum cross-section would be too long to be practical. Similarly, an envelope of compact length would represent a considerable cross sectional drag.
4. Flying boats
The days of the great flying boats seem to be well past. The unexplored potential of the Spruce Goose, the might of the PanAm Yankee Clippers, the sturdy service of PBY’s and Sunderlands all seem to be from the history books.
However, as we look at the new challenges of the future there are reasons to think that the second age of the flying boat may be coming about.We hear, for example, about congestion at hub airports, and we know the opposition that is raised to any airport extension.We know also that these reactions are set to get worse and not better.
New technologies could be applied to new designs of flying boat and might include better resistance to corrosion, more controlled approach and landing, and more convenient entry and exit arrangements when compared to their forebears. Very large aircraft could be considered given the space and water surface for landing.
5. Flying lower and slower
A high cruising speed has always been a design parameter in airliner design. High speeds mean high productivity and thus low operating cost.Turbofan engines are optimised to operate at high altitude and aircraft are optimised to fly at speeds up to Mach 0.89 for large and about Mach 0.80 for regional aircraft.
Initial studies in the past have indicated that a regional jet seating 100 people and powered by a turbofan engine, flying at Mach 0.77 at 37.000 ft, would need 82.5 minutes to fly a 400nm mission. If the aircraft was powered by a counter-rotating prop-fan engine flying at Mach 0.72 at FL 31.000 ft, the time required would be 83.4 minutes. This represents an increase in time over a 400nm stretch of 1%.Yet fuel consumption would be 35% lower and NOX emissions are estimated to be 50% lower. The effect of flying at lower speeds on long haul flights would of course be more substantial. A trade off between flying slower versus the longer flying time and the consequences for total fuel consumption need to be calculated.
Flying at high altitude creates contrails. It is believed that contrail formations make a significant contribution to global warming. The formation of contrails is depending on the ambient atmospheric conditions. Contrails start to appear when the outside temperature is less than -40 degree Celsius.To avoid contrails, aircraft would need to fly substantially lower especially during wintertime. The temperature conditions can vary significantly between regions and may vary on a daily basis. Consequently, some have suggested adapting preferred flight altitudes in flight plans on a real time basis to minimise contrail formation (note: flying above 40.000 ft also reduces contrails as the humidity level at or above that altitude is low. But flying at such altitudes may have other negative effects due to other emissions).
The current generation of aircraft is not very well suited to fly at lower altitudes like 20.000 ft however. Flying lower would result in longer journey times and hence more fuel burn. On a 6000nm trip this could result for some particular aircraft in a flight time of one to three hours more and up to 30% to 60% more fuel-burn and hence CO2 emissions depending on the flight altitude. Such severe consequences are difficult to accept with current aircraft.
Different fuels will not mitigate the contrail problem, except in the case nuclear energy could be used to power aeroplanes. The alternative would be to design aircraft and engines that would be optimised to fly at lower altitudes.
Trade-offs are needed to demonstrate the optimum, related to the environmental impact of flying high and fast versus low and slow. These studies need to take a full systems view accounting for total carbon emissions, NOx as well as the investment return on the aircraft.
Flying at lower speed or altitude is against the long-established trend in air transport as it results in longer journey times, loss of efficiency and the need to procure more aircraft. On long haul flights, it may even make stopovers necessary again unless mid air refuelling is introduced. But, if the protection of our environment calls for this concept, we should seriously consider changing technological developments in that direction.
6. UAV’s and autonomous flight operations
During the recent years UAV’s (uninhabited aerial vehicles) have become common goods in the military domain. UAV’s in the military are used for dull, dangerous and dumb missions. Two types of UAV vehicles exist: UAV’s controlled by an operator on the ground and UAV’s that fly autonomous missions.
Pilot-less aeroplanes are attractive to civil aviation as well. The cost of the crew is a substantial part of the Direct Operating Cost [DOC] of airliners and the replacement of the crew by automated systems seems to be attractive. However there are serious safety concerns. Even today aircraft can auto-land and fly using the autopilot. However, technology is not fool proof and human intervention is needed from time to time to reset the systems.
One could imagine a future in which planes would be flown in a totally automatic mode. Advanced self-separation and automated station keeping, auto-takeoff and auto-land will be feasible. One could think of a safety pilot who would monitor the onboard systems as an interim phase before accepting fully pilot-less aeroplanes.
Manual override capability would be available to the ground-based operator.
The introduction of this system is related to the reliability of systems, to safety concerns and to security issues. At no time should terrorists be able to intercept the communication with the aircraft and take over their control. Highly secure data links would be needed to ensure these situations cannot occur.
The introduction of pilot-less aircraft in civil aviation could be feasible at first in all cargo UAV’s. There are substantial cargo movements in Europe. In the north/south direction, the cargo has to pass the Alps and the Pyrenees. In the east/west direction surface traffic has to cope with transport infrastructures in the East that are not yet of the same quality as in Western Europe. As rail infrastructure is limited and passenger trains have higher priority than cargo, the average speed of cargo rail is extremely low. Inland shipping is an option but is only slow to re-develop. Therefore, trucking has become the most favoured way to ship goods. However, European highways are already saturated and will be completely blocked in a few years. Unless the personal flying vehicle is introduced quickly, the most desirable option will be to ship goods by air. Aerial freighters could fly standard routes that could easily be handled by UAV’s. The idea of a UAC(argo)V is not new. Its development should take into account the certification related issues.
How to ensure that the vehicle will operate safely? How can we track the aircraft and who would be able to take over manual control. What will be the importance of the interference with other traffic? What about the liability issues?
One possible scenario is to fly these aeroplanes at night and to create special flight corridors for cargo aircraft. These could use direct routes and be monitored and controlled by a single authority. The UACV would fly standard patterns. There would be an automated station keeping and avoidance system installed with autonomous features to fly holding patterns in case of a disruption in traffic.
The next step in flight automation could be the autonomous small flying vehicle. The difference between the cargo liner and the personal vehicle would be that the cargo-liner would fly the same pattern every day whilst the personal aircraft would need to be very flexible.
If the technology proves to be safe and reliable even large passenger aircraft could become pilot-less. Here the key word will be safety perception rather than technology.
7. Vortex control
Vortices have been present behind aircraft since the beginning. As aviation developed the physics of vortex generation has also extended. The importance of vortex management lies in the central factor that determines their strength – the weight of the generating aircraft. So as we move to heavier and heavier aircraft the problem becomes more acute. The forces are considerable. Vortex circular wind speeds of up to 300 ft/sec are generated. The fundamental cause of vortices is well known. They are twin contra-rotating spirals of air that rotate in the underwing to over-wing direction at each wing tip.
Vortices are important to airport efficiency and safety. Where large heavy aircraft are using the airport, the avoidance of vortices can become a limiting factor to airport capacity. This occurs by the application of longer separation distances between heavy leading aircraft and lighter following aircraft. Where inadequate allowance is given to the possibility of vortices affecting following aircraft accidents can happen, with disastrous results given the low altitude of the event. Vortices are at their most serious in just the conditions that apply to landing -heavy, low, and slow.
Vortices also occur in flight but they sink and die away normally within the clearance distances that are usually sustained. They become more important when concepts such as formation flight and linked aircraft are applied. For these reasons, research to understand, to modify and ultimately to control vortex formation has a high priority on both sides of the Atlantic. The introduction of the A380 will raise new issues for resolution as it leads the field in weight. European research programmes like the AWIATOR programme in FP6 has vortex management at its centre.
In the longer term, the control of vortex formation and the ability to modify its behaviour might also bring new opportunities. Arranging the vortices to be favourable to formation aircraft would bring the concept of grouped flying very much into play. Flying onto large structures such as imagined under the Airborne Metro would need this phenomenon to be very well understood.
8. Invisible aircraft
Airports are centres of economic activities. As a consequence airport attract business activities, both directly related to the air station function and businesses that are depending on air freight and easy access to air transport. Consequently, many people want to live near to their working place and thus near to the airport. Cities and airports expand and because of the increasing air traffic, there are complaints from the people living near the airport about aircraft noise, pollution and smell. Experience has shown that when traffic increases even at constant noise levels due to improvements in aircraft technologies, the complaints about nuisance from aircraft noise tend to rise.Noise is therefore not an absolute issue but a question of perception.
One solution to alleviate the problem is to make aircraft invisible and to make them silent. In the military domain work is carried out to create visual stealth. The active camouflage technologies range from using light to illuminate the aircraft, amongst others by using fluorescent panels. Research is also focused on electro optical camouflage using electro-chromic polymer materials. Aircraft could be covered with a coating of LCD’s.
Photosensitive receptors scan the surrounding of the aeroplane and a picture is displayed on the LCD’s. This technology would make the aircraft virtually invisible as it blends with the surrounding.
Anti noise would be used to counter the noise of the aircraft. Already tests are performed to see if anti noise technology can be used to compensate for aircraft noise inside a house. The technology could be expanded to create anti noise in areas located near to the departure and arrival tracks at airports.
Aircraft themselves can be made silent by avoiding airframe noise produced by high lift devices , the undercarriage and aircraft cavities.
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