The price of aircraft fuel has continuously increased in recent years. Nowadays, escalating fuel cost represents a big challenge to keeping commercial aviation competitive with other transportation alternatives. The aviation community therefore has been looking for ways to reduce aircraft fuel consumption.
Center of gravity optimization is one possible way to reduce fuel consumption of long range aircraft. Generally speaking, the center of gravity of an aircraft is that point from which the aircraft could be suspended (e.g., as from a cable) while remaining level. Positioning the aircraft's Center of gravity (CG) slightly aft can save fuel by reducing drag. When the aircraft center of gravity is in the aft position, the lift of the tail is less negative than a forward center of gravity due to the smaller moment arm between the lift generated by the wing and the weight. Consequently less angle of attack is necessary to create the lift necessary to offset the weight plus the less negative lift generated by the tail.
Optimum center of gravity in terms of fuel saving is obtained when the aircraft center of gravity is maintained exactly at the aft limit of the aircraft's certified center of gravity versus weight envelope (“CG Envelope”). Due to system limitations, the center of gravity generally is maintained in a region defined as a control band (i.e., a small region located nearest the aft limit of the CG envelope). The control band width is derived from the center of gravity control system characteristics.
Aircraft center of gravity is affected by a number of factors including for example location of passengers and cargo. In addition, in certain types of aircraft, fuel distribution can affect center of gravity. Some long range aircraft have auxiliary fuel tanks to increase the fuel available required to complete long range missions. The main function of the auxiliary tanks in such aircraft is to increase the fuel quantity available. However, the distribution of fuel within the different fuel tanks of a multi-tank aircraft can affect the aircraft's center of gravity and can be used to change the aircraft center of gravity position. For example, the amount of fuel in the inboard and outboard wing tanks of a sweptwing airplane affects both lateral and longitudinal balance of the aircraft. Specifically, the aircraft is tail-heavy when more fuel is within the outboard wing tanks, and nose-heavy when there is more fuel in the inboard wing tanks. For this reason, fuel-use scheduling in swept-wing aircraft operation is said to be critical. See e.g. Aircraft Weight and Balance Handbook, FAA-H-8083-1A (2007).
It is possible to schedule consumption of fuel between the main tanks and the auxiliary tanks in order to maintain proper center of gravity. During flight, the aircraft consumes fuel continuously which in turn changes the aircraft's center of gravity. The way which the center of gravity displaces as fuel is used from fuel tanks is a particular characteristic of each aircraft design.
It is also known to transfer fuel between tanks. For example, the Concorde supersonic passenger jet used a fuel management system to transfer fuel between the various tanks during flight. Such a system was used because of an aerodynamic effect that occurs around the speed of sound. At these speeds, the point where the lift acts on a wing tends to move around by a large amount. As an aircraft transitions from subsonic to supersonic speeds, the point where the lift is generated by the wing acts tends to move further back. Aerodynamicists refer to this behavior as a shift in the center of pressure, and it is caused by the creation of shock waves on the surface of the wing. The changing center of pressure has a tremendous impact on the stability and controllability of a plane. The Concorde's engineers chose to adjust the weight distribution of the plane to balance out the changes in aerodynamic lift. Their solution was to transfer fuel between different tanks to move the plane's center of gravity aft or forward. The tanks used in this process were known as “trim tanks” since their purpose was to keep Concorde in a trim condition during different phases of flight to maintain stability.
The fuel transfers on Concorde are carried out by the flight engineer from his fuel control panel. On Concorde this is one of the most important and time consuming jobs for the engineer. The panel allows the engineer to set up the transfers to be carried out automatically and stop when the relevant quantities of fuel have been moved to the correct tanks. On the other hand, the advent of computer technology, reliable software, and a desire by airlines to cut costs by reducing flight deck crew, has generally eliminated the requirement for flight engineers on modern airliners. Pilots and copilots due to the intense workload do not have the time and may not even have the expertise to finely adjust aircraft center of gravity during flight through intermittent, manually-actuated fuel transference. Nevertheless, intermittent transfer of fuel between tanks is used nowadays in some narrow body aircraft. The intermittent form is based on transference through fuel packages. The intermittent transference requires a thick control band due to booster pumps and valves that are able to transfer fuel only through a constant flow.
It would be desirable to equip an aircraft with an automatic system to detect minor changes in the aircraft center of gravity and control the continual fuel transference between tanks to maximize the time during which the center of gravity is maintained in the aft position.
It would also be desirable to provide continuous or continual transference of fuel using an integration to estimate or predict the aircraft center of gravity. Through the dynamic continuous or continual transference of fuel between tanks on board the aircraft, the aircraft center of gravity can be maintained in the optimal position during flight with minimum error.
An exemplary illustrative non-limiting system to control the fuel transference between tanks includes means to determine the fuel quantity, a method to predict the aircraft mass distribution after or during the aircraft loading; components such as plumbing, booster pumps and valves; and a controlling unit responsible to manage the distribution. The pumps may provide continuously-variable flow rate control so that the pumps remain on while providing a controllable flow rate from zero to a predetermined maximum.
Continuous transference makes use of a more sophisticated control system to monitor fuel usage and dynamically transfers fuel between tanks based on prediction or estimation. Continuous transference of fuel can be provided through a variable flow that is managed by a highly integrated system that includes electronic operated pumps or valves, a Center of Gravity Estimation Box, and several sensors to estimate the mass distribution in the aircraft. The continuous form can operate in a reduced control band due to the capacity to transfer fuel continually exactly in the quantity necessary to maintain the center of gravity close to the envelope aft limit while taking fuel burn rate into account.
Continuous transference provides certain technical advantages when compared to the intermittent transference such as reduced component cycling (which can provide extended component life) and an optimized after center of gravity due to the reduced band control thickness.
In one exemplary illustrative non-limiting implementation, the control system transfers fuel from the main tanks to the auxiliary tanks at the beginning of the flight to maintain optimum center of gravity during flight. In advance of landing, the control system transfers sufficient fuel from the auxiliary tanks back to the main tanks (or ceases to transfer additional fuel from the main tanks when the amount of fuel the main tanks contain reaches a predetermined lower limit) to comply with flight regulations (e.g., requiring sufficient fuel in the main tanks during landing to allow alternate landing procedures).
Additional non-limiting exemplary features and advantages include:                An Active Center of Gravity Control System with continuous or continual fuel transference logic        Center of Gravity Estimation Box (CGEB) that receives and processes signals from sensors and inputs        A CGEB that converts information in terms of center of gravity to fuel flow required to be transferred between fuel tanks        A fuel transfer system, comprised of pumps and/or valves, able to modulate the fuel flow according to the output of the CGEB to take the aircraft CG to the optimum position and maintain it there        A plurality of fuel tanks disposed in different zones of the aircraft with communicating fuel lines able to transfer fuel between the tanks according to the CGEB.        