1. Reduced structural and payload weight
Some of the ideas dealt with in other sections may have relevance here: the use of ground power augmentation (section 2.2) may serve to reduce engine thrust or the use of detachable or ground located undercarriages (section 4.1.3). However, the most effective approach to a better relationship between power, weight and payload seems to rest with new concepts for the aircraft design. The concept that appears to receive most effort to bring it forward is the Blended Wing Body [BWB]. The savings of drag achieved by its tail-less nature are significant and the L/D function in designs being studied is about 15% higher than current design conventions.
Reducing the structural weight of a given concept will bring about benefits dependent upon its operational use. Broadly based figures suggest that the percentage taken off the weight of the aircraft, engines and systems will produce a percentage saving in fuel burn per tonne-kilometre of 1 – 1.5 times greater.
It may be possible to reduce the carried weight of the cabin crew by installing server systems that dispense food, drink and incidental items to passengers in flight. Concepts of zero-baggage or self loading baggage might also serve to reduce the baggage weight carried. It is not known at present how significant the net savings would be.
2. Reduced fuel weight
On long range sectors especially, the carriage of the fuel carried by the aircraft itself implies a considerable cost in fuel used. A number of ways have been suggested to optimise the balance between fuel carried over long stages and the additional fuel used for landing more often. In the UK “Greener by Design” paper5 this optimisation is dealt with more extensively and the conclusion drawn that for many aircraft an optimum stage length is about 4000 Km. This would allow savings on long haul flights of 10-20%.
Already a commonplace for military aircraft the technology is well developed. The benefits would arise from reducing the fuel carried for the latter parts of a journey – this fuel could be loaded much nearer to the destination and allow smaller aircraft with smaller fuel loads to operate. Such a technology could be extended to commercial operations and this has been studied in some depth by Dr Raj Nangia. His preliminary studies indicated significant savings of fuel burn by this method but more study is required to identify practically achievable net benefits when the expenditure of fuel by the tanker fleet are taken into account6. Flight re-fuelling of airliners might become routine, saving some of the fuel used to lift massive fuel loads from the ground and carry it halfway around the world.
Every schoolboy is familiar with pictures of military aircraft flying in tight formation advancing toward the enemy. Every naturalist is familiar with the sight of skeins of ducks flying into the sunset in their typical “V” formation.
The idea put forward is to transport both of these ideas into the air transport system leading to significant benefits in fuel consumption, fuel carriage and in better ATC.
The obstacles of tight formation flying are to avoid on a regular, reliable and secure basis any adverse effects from the weight dependent vortices from the wings of the lead aircraft. Vortices are a function of aircraft weight and commercial airliners are substantially heavier than all other aircraft that have attempted close formation flying.
Conceptually the benefits are very considerable. If the reduced drag that ducks create by wing warping to reduce their individual effort in flying long distances can be harnessed then long range travel could be substantially more economical. Research already conducted shows that cruise fuel savings could be 15-40%.Wings flying within about 0.8m of each other can experience 60% drag reduction7. Practically achievable fuel savings (and fuel carriage) in the order of 10% have been projected. Groups of aircraft (such as on the busy intercontinental routes) could be treated as a single entity by ATC as they sometimes are now. In the further future groups of similarly routed aircraft could form “fixed” formations in which the aircraft became a single flying system under a single control only dissolving as aircraft wished to drop off nearer to destination. The technical resolution of the problem of reliable and secure close flight would be very relevant to other concepts (such as the Cruiser/Feeder above).
3. Reducing systems weight
The section (2.2) dealing with the use of ground power augmentation will be relevant to any attempts to reduce aircraft systems weight. Additionally ideas were put forward that would also bear upon this aim. The benefits of achieving some reduction would naturally play through to overall fuel burn, to economy and to reduced climate impact.
The concept of launched aircraft with some form of airport retained take-off undercarriage was discussed. Were such an idea to be feasible it would bring substantial benefits by eliminating one of the heaviest systems on the aircraft. On initial considerations it is relatively easy to imagine how the take off might be accomplished with the undercarriage being left behind. Landing would be much more demanding. Firstly the loads upon landing are much heavier so the prospect of any lighter weight “landing only” undercarriage is immediately to be rejected.
Landing at normal speeds would have catastrophic consequences. The only idea presently presented that seems to hold out any promise at all of bringing this to fruition is landing within a landing tube (see section 3.2.2) where the “touch-down” speed is effectively zero or extremely low.
Parafoils are used to deliver military supplies. The advantage over a traditional parachute is that these parafoils are steerable. The current loads that can be used with these systems are relatively small.
If the technology could be developed into a system where aircraft could make parafoil assisted landings, noise produced by aeroplanes during decent and landings could be substantially reduced. One could imagine an airport where aircraft are launched by MAGLEV systems whilst runways are only used to recover aircraft using parafoil assisted landings. As the final landing speeds would be extremely low, a simple skid undercarriage would be needed saving at least 5% of the total aircraft weight. Landing strips could be much shorter than the current runways.
The parafoil technology would need to be further developed. First one needs to consider the additional weight that would be carried. Second, the parafoil glider should be able to operate in cross wind conditions. Third, the accuracy of the system should be extremely high with an accuracy of about one meter.
Another issue that needs further research would be the mechanism to open and retrieve the parafoil. This should be done totally automatically. Steering the parafoil should be automatic as well with the possibility of manual override.
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