Evaluating and augmenting fuel-saving benefits obtained in aircraft formation flight

Abstract

When an aircraft flies, it generates wake vortices, which induce a non-uniform wind distribution in its wake. A trail aircraft, placed in the wake of a lead's aircraft vortices experiences this non-uniform wind distribution with varying directions and magnitudes, depending on the location within the wake. It has been demonstrated that there is a "sweet spot" within the wake of a leader in which a trail can experience upwash which leads to reduced drag. Through this mechanism, aircraft can save significant amounts of fuel by flying at the sweet spot of the lead aircraft's wake. This dissertation provides two metrics of obtaining the sweet spot and evaluating the benefits to the trail aircraft at the sweet spot: a static and a dynamic study. The static study, similar to wind tunnel tests in which aircraft are statically placed in formation without trimming, investigates the induced aerodynamic forces and moments on the trail aircraft as it varies its position within the wake of the lead aircraft, and assigns the relative location of maximum lift-to-drag ratio as the static sweet spot. The dynamic study, similar to flight tests which account for trim, analyzes the control surface deflection and thrusts for the trail aircraft as it varies its position within the lead's wake and assigns the location of minimum thrust as the dynamic sweet spot. The static and dynamic analyses are applied to aircraft formations with different relative sizes and varying configurations of trail aircraft such as a flying-wing and a conventional aircraft. Results indicate that sweet spot locations and associated bene ts are dependent on the weight of the leader and the relative sizes of the aircraft pair in the formation. This dissertation then augments the fuel-savings by investigating alternate lateral trimming methods to reduce the need for drag-inducing control effector deflections required at the static sweet spot. Internal fuel transfer and differential thrusting are employed to increase the thrust saved at the static sweet spot, making it comparable to the dynamic sweet spot. Formation simulations of extended durations are also studied to understand the impact of significant weight variations in the leader on the formation benefits. In longduration flights, the lead and trail aircraft weights decrease due to fuel burn. Since the upwash generated by the leader decreases with lift and weight as fuel is burned, the magnitudes of the non-uniform wind induced and thus bene ts for the trail decrease. This decrease is investigated for a 6.5-hour formation simulation. Although there is a reduction in the benefits with time, the overall benefits of flying in formation is significant enough to motivate formations of such long durations. Finally, the feasibility of formation flying is also considered from a perspective of comfort levels for passengers or aircrew in the trail aircraft. Using international standards for likely reactions of a person subjected to discomforts characterized by vibrations, it is shown that there is no additional detrimental degradation of comfort levels for a person onboard a trail aircraft in formation as compared to a solo flight.