Students from the Faculty of Aerospace Engineering, Delft, Netherlands have been actively competing in prestigious International design competitions that are organised by the American Institute of Aeronautics and Astronautics (AIAA). This year, two teams, one undergraduate and the other graduate, both achieved second places in the competitions.
Dutch Designs can be typically characterized as minimalist, experimental and innovative. Not only do these traits apply to chairs and buildings designed by famous Dutch engineers, they also extend to aircraft designs created by students of the Aerospace faculty.
Every year AIAA organizes aircraft design competitions in three different categories: undergraduate, undergraduate individual, and graduate. Students are asked to submit a proposal for a new aircraft design under a strict set of requirements that can vary widely between the categories. In 2014/2015, two teams competed, one in the undergraduate category and one in the graduate category. The undergraduate team was tasked to design a military transport aircraft, while the graduate team had to come up with a new air-taxi concept for large urban areas.
The US armed forces is focusing on modernizing their current fleet by implementing new technologies to extend the lifetime and to advance the performance of their aircraft. Even though this transformation has resulted in considerable improvements, it is expected that significant advancement will be achieved by introducing a new generation of aircraft. Therefore, AIAA issued a request for proposal (RFP) for a next-generation strategic-airlift military transport aircraft with an expected entry into service (EIS) in 2030. The aircraft to be designed is aimed to replace the military’s largest air transport aircraft, the C-5 Galaxy. The RFP asked for an aircraft that could carry a greater payload, both in terms of weight and volume, and at the same time have twice the range of the Galaxy. As part of the 2014 fall Design Synthesis Exercise, a group of nine aerospace students (Figure 1) responded to this RFP with a new design proposal.
For the first design stage, the team split up into three smaller groups, all tasked to bring forward their most promising concept. The first group investigated a canard blended wing body that has a pressurized central fuselage section and two unpressurized cargo bays embedded in the wing. The second proposal was an integrated wing body with a modular cargo compartment that can be exchanged for different missions. The third concept, which the team regarded to be the most suitable, was an ultra-wide body aircraft, relying on the conventional tube and wing configuration.
The ultra-wide body aircraft, mostly attributed to its shape, was conveniently named the WHALE (Wide Hull Airlift for Long Endurance), designed with a V-tail, truss braced wings and a lifting fuselage nose. The key design goals of this concept are decreasing the turn-around time, allowing more versatility in cargo transport and an increase in the aerodynamic efficiency.
The WHALE features a tapered fuselage, much like a glider, with a maximum width of almost twelve meters in the forward section and a total length of over seventy meters (see Figure 2). The dimensions were mainly driven by the necessity to carry 44 pallets. Opening up the forward cargo door would give access to four pallets simultaneously, effectively reducing the loading and unloading time. The fuselage was shaped such that the landing gear could be retracted into the fuselage, requiring no need for drag-intensive landing-gear pods on either side. Additionally, the use of struts to support the wings allowed for a slender wing design with a large span (eighty meters) without a high penalty in structural weight. The most crucial aspect to the success of the WHALE is its propulsion system. In the context of an EIS in 2030, a new engine technology was selected: the Counter-Rotating PropFan (CRPF) engine. This new technology promises large reductions in fuel consumption and is currently being developed. The CRPF system has however not yet been scaled up to provide a similar thrust output as the most powerful high-bypass-ratio turbofan engines. Even though this resulted in a total of six engines needed to propel the aircraft, the reduction in fuel consumption outweighed the added weight and drag. The WHALE is capable of transporting any required piece of military hardware over trans-Atlantic distances without refueling whilst having an additional 72 seats available to carry personnel without compromising the cargo volume. Its versatility and efficiency expands the capabilities of the strategic airlift at a lower cost than the C-5 Galaxy.
Considering the increasing amount of traffic congestions in large metropolitan areas, AIAA issued an RFP for designing an air taxi system based on an innovative vehicle with VSTOL (Vertical and Short Take-Off and Landing) capabilities, allowing commuters to get in and out of a city at a faster pace and a moderate price, with low environmental impact. The required vehicle had to operate on both runways and helipads, carry a maximum of nine passengers, be part of a 30-year service starting in 2020 (EIS) and break-even in the shortest time possible. Finally, some form of electrical propulsion had to be part of the design. The graduate team (see Figure 3) was tasked to bring design and engineering skills together with entrepreneurial and business skills for their final proposal.
The strategy the team adopted featured the creation of small departments, each one dealing with a different discipline, such as: performance, flight dynamics, structures, powertrain, noise, and network & operations. The first stage of the project included the proposal of the network and the evaluation of different designs proposed by three groups. The first explored the field of tilt-rotors, presenting a fixed-wing aircraft with three rotatable and electrically powered fans. The second proposed the adoption of a forward-swept tilting wing vehicle, carrying an electrically powered propeller on each semi-wing. The third proposal, which was eventually deemed by the team to be the most suitable for the project, was a Gyrodyne (Figure 5), featuring a helicopter-like main rotor, actively propelled during vertical takeoff and landing, and a fixed wing. During cruise and horizontal takeoff and landing, the main rotor would be disconnected from the powertrain and put in autorotation, carrying a variable percentage of the total required lift. Two wing-mounted propellers would provide the required thrust during cruise, and provide counter-torque in hovering.
The powertrain design was perhaps the most challenging task. After having concluded that a fully electrical power plant was not viable with today’s technology, the team opted for a hybrid system with a constant-power gas turbine. A dual acting electric generator/motor would direct the energy towards the main-rotor or thrusters depending on the flight leg while batteries absorbed or fed fluctuations. The redundancy of the powertrain design, and the capability of autorotation to land in a few meters, even in case of a total power loss, represents an unmatched safety enhancement.
Given the unconventional configuration, a hybrid design procedure was necessary, resorting to numerous references from aircraft literature including sources on helicopter design procedures and autogyro design. All relevant disciplines were integrated in an automated design tool built in the Matlab environment.
As for the network and business plan, San Francisco Bay was deemed to be the best market for introducing this aircraft, where a fleet of 16 vehicles cruising at 160kts would serve a total of 10 operating sites covering an interwoven network of short-range routes. With 750 daily flights, the system would reduce traveling time by up to 75% and costs by up to 58% when compared to normal taxi transportation.
Tradition in the making
It is not the first time that students from TU Delft have won prizes in the AIAA design competitions. This is the fourth consecutive year that TU Delft has been ranked in one or more categories. In 2015/2016 two teams will be competing as well. The question of their victory remains to be seen. At least they have good predecessors to aspire to.
Dr. Sander Hartjes and Dr. Daan Pool for coaching the undergraduate team. In addition, Dr. Marilena Pavel is acknowledged for providing pivotal guidance for the Gyrodyne design.