Aircraft Lateral Trim Using Internal Fuel Transfer & Differential Thrust In Formation Flight

Abstract

This paper investigates alternative moment generation mechanisms for aircraft as fuelsaving techniques in formation flight. Aircraft flight generates wake vortices that induce a nonuniform wind distribution in the wake of the aircraft. A follower aircraft, flying in the wake of the leader aircraft, can experience induced wind components and gradients with various magnitudes and directions, depending on the location within the induced nonuniform wind field. It has been demonstrated that there is a "sweet spot" within the wake of the leader aircraft where the follower aircraft experiences upwash, which leads to drag reduction. When an aircraft flying in formation flight is at the "sweet spot", the aircraft experiences lower drag but at the same time experiences induced aerodynamic moments. The deflections of control effectors required for trimming the aircraft under the effect of these moments induce additional drag, which reduce the benefits of fuel efficiency. This report investigates two different mechanisms of moment generation for alleviating the drag induced by the deflection of the control effectors: (1) internal fuel transfer among fuel tanks and (2) differential thrust. It was seen that an elimination of the control effector deflections led to a reduction in the thrust required and thus savings in fuel. Internal fuel transfer, in the configuration studied, was seen to reduce the aileron deflection of the follower by generating a rolling moment, and differential thrusting reduced the rudder deflection by the generation of a yawing moment. Furthermore, the combination of the two techniques provided the highest fuel savings of 11 percent by reducing the need for the aileron and rudder deflections. The results of this analysis will allow for further studies ultimately leading to the development of a fuel-saving method of flight with various applications.

Application of Sweet Spot Determination to a Conventional Pair of Aircraft

“…When the EQ-II is commanded to fly in a steady-state cruise condition at the sweet spot and is trimmed, the required thrust level is seen to be 3% higher than that if the EQ-II were to fly solo (baseline case). 6,7 This is in contradiction with the common expectation that the required thrust be reduced by the induced upwash effect of the trailing vortices of the lead aircraft. However, a closer look at the control surface deflections clearly explains the observation.…”

“…Also, it was impossible to significantly affect the elevator deflections using internal fuel transfer as the configuration of the EQ-II fuel tanks can provide only a lateral CG shift of the EQ-II by fuel transfer. 7 These results form the motivation for the work presented in this report. The formation flight study of the KC-135R -EQ-II pair showed that the unconventional control surfaces on the EQ-II were too costly for formation flight.…”

“…7 One novel idea that was investigated to reduce the induced drag from the control surface deflections was to utilize alternate trimming methods such as lateral internal fuel transfer among the fuel tanks to generate a rolling moment and reduce the need for the aileron. 7 Another was to employ differential thrusting to generate a yawing moment and reduce/eliminate the need for rudder deflection. 7 A combination of these two methods of drag alleviation provided a decrease in thrust from the baseline of more than 11%.…”

“…7 Another was to employ differential thrusting to generate a yawing moment and reduce/eliminate the need for rudder deflection. 7 A combination of these two methods of drag alleviation provided a decrease in thrust from the baseline of more than 11%. This is smaller than the expected 13% because there was not enough fuel transfer capability in the EQ-II configuration studied to completely eliminate the aileron deflection.…”

“…5 To this effect, the authors of this work have done studies on formation flight on a pair of aircraft, a KC-135R tanker aircraft as the leader and an Equivalent Model II (EQ-II) aircraft as the follower. 6,7 The EQ-II aircraft was used to represent the performance and maneuvering capabilities of a subsonic cruise, long range, tailless aircraft. 8 The trailing EQ-II has unconventional independent control surfaces for 3-Dimensional control.…”