Boeing Mid-Wing Aircraft
A Boeing patent application filed in 2009 and published in June 2012 detailed a double-deck airplane that would move the wings from their traditional spot at the bottom of the fuselage barrel to the middle. Airbus' double-decker A380 and Boeing's partially double-decker 747 have their wings at the bottom of the fuselage. This design was also disclosed in application US20130099053 A1 of April 25, 2013. It comes with the usual caution that Boeing and other technology heavy companies patent lots of concepts that stand little chance of becoming reality, and in this case the application had not been translated into an actual patent as of mid-2014.
The design would accommodate the larger fans of efficient super-high bypass ratio engines, such as geared turbofan or open-rotor engines, Boeing said in the patent application. The mid-wing position eliminates the need to raise the fuselage higher off the ground and minimizes the impact on the capacity and layout of the airplane. The wing would pass through the cabin's lower level, as shown in the images above, with passages between the fore and aft sections and spaces that could be used for such facilities as galleys, rest areas, lavatories, a lounge, a play area and storage.
Conventional airliner and cargo aircraft are configured as high or low wing aircraft. The wings are positioned above or below the passenger or cargo compartment within the fuselage. The wings are attached to the fuselage through a wing structural box. The fuselage is attached at the top or bottom to the wing structural box depending on whether the aircraft is configured as a high-wing or low-wing aircraft. This wing structural box is typically very heavy since it needs to be substantial enough to bear a large portion of the wing loads and support the fuselage.
Mid-wing aircraft have the wings positioned at the sides of the fuselage at a position between the top and bottom of the fuselage. The wing structural box for mid-wing aircraft passes through the middle of the fuselage where a cargo or passenger compartment would be located in a cargo aircraft. There are performance advantages to a mid-wing configuration; however, because of the structural box passing through the fuselage and because the performance advantages are typically less of a consideration with cargo and passenger aircraft, mid-wing aircraft configurations are conventionally reserved for high-performance aircraft such as fighter aircraft and aerobatic aircraft. Moreover, the wings on these mid-wing aircraft are typically very thin for performance reasons, with the engines mounted to or within the fuselage. Because of the loads experienced by the thin wings, the spars include spar caps or chords that are structurally substantial, increasing the weight of the wing assemblies as compared to thicker wing spars. Moreover, the loads carried by thin wings also typically require substantially thicker aircraft skin than the skin used on thicker wings.
Advanced designs for high-capacity commercial and military airplanes require operating efficiency combined with reduced emissions and low noise. In order to meet these requirements, super-high bypass ratio jet engines, such as geared turbo fan or open-rotor jet engines may be used. These engines typically employ larger-diameter engine fans, rotors and/or nacelles which, because of their size, may place design constraints on other components of the airplane.
For example, larger-diameter engines mounted beneath the primary lifting wing on the airplane may require excessive inboard wing shear and associated large weight penalties on a low-wing airplane configuration, or alternatively may require that the wing be positioned at a higher level on the fuselage in order to provide sufficient ground clearance beneath the engines. This higher placement of the wing on the fuselage may in turn place constraints on the configuration of payload-carrying decks within the fuselage.
Traditional commercial aircraft have been designed around a simple circular tube to carry passengers and cargo and a wing. The traditional airplane configuration places all passengers on one deck and cargo on a lower deck. To configure an airplane in the traditional approach, a passenger count and seat width dimension is decided and then the airplane is wrapped around this seat arrangement. The close wrapping of cargo and passengers at the same time is not a traditional approach.
When designing a new airplane, many other factors need to be considered. The world airplane market is becoming increasingly sensitive to fossil fuel burn, which can be measured by the airplane specific fuel consumption and emissions. A direct correlation to fuel burn can be drawn to the airplane wetted area: the smaller the wetted area, the lower the drag on the airplane. Aircraft noise is also becoming more of an issue, especially during airport operations and during approach and departure. Furthermore, the commercial aviation industry has traditionally surveyed North America and the European markets, where the air transportation infrastructure can be crowded, and one of the parameters to gauge an aircraft concept is the footprint size of the airplane.
A smaller footprint size for a given passenger capacity is desirable. In addition, as labor becomes more expensive worldwide and airlines rely on revenue cargo operation for profits, containerized cargo assists airlines not only turn the airplane quicker between flights, but also helps airlines lower labor costs and potential personnel injury issues. Faster airplane turn times have a great value to an airline. The ability to utilize dual boarding and departure operations can help decrease airplane turn times. Most airplanes do not allow for dual deck utilization for passengers and containerized cargo.
The escalating cost of aviation jet fuel, enactment of and anticipated growth in carbon-related taxation regulations has created an enthusiastic industry-wide resurgence into Prop-fan or Open-Fan technology, as demand for travel continues to climb with sustained pressure to minimize fare increases. Concurrently; due to increased travel the legal noise limits imposed by the United States Federal Aviation Administration (FAA) and the International (ICAO) agencies for engine and aircraft certification have become more stringent.
In many countries, local aviation authorities have imposed a combination of fees, curfews, and quotas aimed to offset the growth of noise exposure and costs associated with abatement, including sound-proofing homes. For enforcement, numerous airports have installed microphones in noise sensitive communities that currently force operators to sacrifice payload and or range to avoid violation of these local noise policies. Furthermore, it is anticipated that local air-quality or other carbon related environmental costs could be imposed.
Additionally, evolving demand for expanded point-to-point service beyond traditional narrow-body markets and operational flexibility drives designers to further emphasize fuel efficiency relative to other tradeoffs. Improved fuel consumption may be obtained by reducing cruise speed however this may increase flight time and be undesirable to passengers, the net fuel benefit may be fairly small, challenges with air traffic integration may be created, the number of revenue flights in a given day might be reduced, and this approach may actually result in an increase of other airline operating costs. In order to achieve the cruise speeds of today's jet powered aircraft (.about.Mach 0.8) counter rotation open fan (CROF) systems are needed, since single rotation/stage turboprops are practically limited to cruise speeds of roughly Mach 0.7 due to insufficient specific thrust.
Counter Rotating Open Fans have complex noise sources that single rotation turboprops do not have; specifically propeller wake interaction and tip vortex interaction noise. Both of these noise sources may result in external environmental noise which affects airport communities, cabin noise which affects passenger comfort, and airplane structural sonic fatigue. A complex trade between these noise sources and net propulsive efficiency exists for various design approaches. Wake interaction noise, which is undesirable, and propulsive efficiency, which is desired, tend to decrease with greater spacing between the fans. However, vortex interaction noise, which is undesirable, may actually increase with spacing depending on free stream Mach number, local flow effects, angle of attack and downstream propeller row diameter due to the stream tube contraction after the first row of blades.
Vortex interaction avoidance is typically of highest priority to designers, however the only means in present designs to accomplish this is to "crop" or reduce the diameter of the aft or down-stream rotor. This however may carry a performance penalty as the aerodynamic efficiency may be compromised in the same fashion as a fixed wing due to loss of span/aspect ratio. A key challenge for the designer is that vortex interaction is affected by several factors.
The strength of the vortex is influenced mostly by blade tip loading and the path of the tip vortex is greatly affected by free-stream momentum and angle of attack. With lower cruise speeds the tip vortex collapses toward the root of the aft/downstream rotor interacting with it and causing vortex interaction noise. Because of this designers of CROF engines in the prior art typically have chosen an aggressive degree of spacing and cropping (10% or greater) so that vortex interaction is avoided under limiting operational conditions such as, for example, the highest thrust rating and climb trajectory with maximum vortex plume contraction. This may result in the airplane reduced performance for all operational conditions.
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