At each wingtip of an aircraft, a strong vortex is generated during flight as the flow from the lower wing side is turning around the wingtip to follow the lower pressure at the upper wing surface. The energy of the vortex is linear proportional to the lift generated by the wing and is related to the wing induced drag. For more than 50 years the aircraft manufacturers are trying to reduce the tip vortex by defining specific wing tip devices; some devices are a “prolongation“ of the wing, but changed in a vertical or 45° declined planform relative to the wing planform. Others are like a specifically designed end plate (Airbus A320!). Despite these tip devices there is still a very strong tip vortex, which is generated at each wing tip. The question remains, if there are means and possibilities to recover energy from the forming wing tip vortex and use or store this energy on board?
During the years 1985 and 1990 Airbus has shown and presented an idea of a Wing Tip Turbine (WTT) [see FIG. 1].
FIG. 1: Research project Wing Tip Turbine from Airbus
This WTT concept contained a propeller fixed on an axle behind the wing tip. Thus, the forming vortex energy could be transfered to an electric generator integrated in the wing tip.
The WTT had several options:
- Generate electrical power during Takeoff and Landing in such a quantity that the Ram Air Turbine (RAT) could be replaced.
- During cruise flight the WTT was expected to either turn and generate power or be fixed in a specific position to act like a wing tip fence and improve cruise drag.
The tests have shown that for both flight phases a positive effect could be shown, but the additional masses and complexity was negatively seen and the business case and an industrial solution was not recommended.
Despite the decision from Airbus around 20 years ago, the situation today with much higher fuel prices is suggesting that a new trial with also some broader geometrical investigation should be started again and may come up with a positive overall benefit.
Benefits to the Air Transport system
The target for the vortex energy recovery should be mainly focused on cruise flight and the final solution should demonstrate an overall positive energy recovery. This means the shaft energy produced by the wing tip device (propeller, impeller, ..) should be optimized for the total cruise phase including climb and descend. This energy produced could either be used on board and the classical electrical energy system could be reduced accordingly or the energy could be stored.
The energy recovery has to clearly dominate the additional mass, cost complexity of the new wing tip device system. In addition, there will be most probably a reduction in vortex strength behind the aircraft which will help the air transport system by reducing the separation distances between aircraft in cruise and approach conditions (however, investigating this aspect is not the focus of the proposed study).
The target is a 3% overall drag reduction, respectively energy generation. This would be a sufficiently large additional energy on board, which also would mean that the engine offtakes have to be adjusted accordingly and the overall engine efficiency will be improved. This aspect is of major importance for novel more/ all-electric aircraft system architectures. But different benefit strategies will have to be developed in parallel to the geometrical positioning of the energy recovery device (ERD) to optimise the overall benefit.
Likelihood of Public acceptance
The public acceptance of a WTT system should be no problem. The aircraft wing tip is modified and different to current shapes. It is more a marketing exercise to show that aircraft with this sort of devices is of high tech standard and is recovering some energy or leading to the consumption of less fuel.
This idea is not new (see the WTT exercise from Airbus!). However the focus is different. The Energy Recovery Device (ERD) has to have a good performance in cruise. In cruise, due to fuel burn, the aircraft mass and therefore also the wing tip vortex will change and a good compromise for the whole cruise phase has to be achieved.
Credibility of the physics
The physics of the wing tip flow are fairly well known. There is a strong wing tip vortex which is forming and generated at the tip. The credibility of the physics is not the issue.
The main problem is to define a technical concept which:
- Defines a good position behind the wing
- Not too far behind, to keep torsion loads on the wing box reasonable
- Not too big for airport handling and for cruise tip speed (noise)
- ERD Turning device either propeller (speed?), impeller, or ??
- Retractable for ground handling ??
Criteria for successful Incubation
All the issues can not be solved in the incubation phase. Basic aerodynamic studies are required prior to more detailed evaluations to understand the vortex generation at different airspeeds. A reasonable good position for the ERD behind the wing has to be identified and a first estimate about the amount of energy recovery should be obtained. But criterion for a successful incubation is an estimate of energy recovery, a design carpet of geometrical ERD position, size and type of ERD and the related benefit (metric still to be defined).
Timescale for incubation
A 2 year incubation project seems sufficient to start with, although aerodynamic studies might require more time if comprehensive wind tunnel testing is included.
Budget required for incubation
A 1,5 M€ budget should be sufficient. There is a need for a wind tunnel test. A wing model should be available, but the rotating devices (at least 2 different) have to be developed, manufactured and tested. Also a specific sensor system is needed to relate wing lift (vortex strength) and energy recovery. In the wind tunnel test the wing and ERD device should be separated but both have to be well instrumented.
Partnering needs for incubation
A research center with wind tunnel facilities, but also a partner (University) with overall aircraft design capabilities to check the design constraints are required. Also an electrical engineering specialist to give advice for the possible generator requirements and geometrical aspects of the generator sizing (3 partners in total).
Related and supporting Capabilities
A mid term plan should be setup to define the different steps, necessary fo the longer technology development process. But the incubation phase should concentrate on the aerodynamic flow field and the best way for energy recovery (a geometry screening!).
Scalability of the idea
For smaller propeller driven aircraft, the concept may be easier as propellers can be used as tip devices. For the faster aircraft (Ma=0.78 and higher) there will be a problem to use propellers. Impellers could be a solution.
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