Formation flight is currently being investigated as a means to reduce drag and improve fuel efficiency in commercial aviation. In an attempt to approximate passenger impact in formation flight, a simple aerodynamic model for two aircraft in formation was developed. The wing trailing vortices were assumed to shift in an ideal fashion within atmospheric turbulence resulting in aerodynamic disturbance loads acting on the trailing aircraft. As the sensitivity of the human body to vibrations is frequency dependent, a spectral representation of atmospheric turbulence was incorporated. Monte Carlo simulations were done for various levels of turbulence intensity. The predicted acceleration responses of the trailing aircraft were used to determine the passenger comfort levels by applying the criteria of standard 2631-1 from the International Organization for Standardization and comparing with comfort levels experienced in an aircraft flying in isolation in the same turbulent atmosphere. A significant increase in d...

Tentative Study of Passenger Comfort During Formation Flight Within Atmospheric Turbulence

The interaction of meandering tip vortices shed from a leading wing with a downstream wing was investigated experimentally in a water tunnel using flow visualization, particle image velocimetry measurements, and volumetric velocity measurements. Counter-rotating upstream vortices may exhibit sudden variations of the vortex core location when the wing-tip separation is within approximately twice the vortex core radius. This is caused by the formation of vortex dipoles near the wing tip. In contrast, co-rotating upstream vortices do not exhibit such sensitivity. Large spanwise displacement of the trajectory due to the image vortex is possible when the incident vortex is further inboard. For both co-rotating and counter-rotating vortices, as long as there is no direct impingement upon the wing, there is a little change in the structure of the time-averaged vortex past the wing, even though the tip vortex shed from the downstream wing may be substantially weakened or strengthened. In the absence of the downstream wing, as well as for weak interactions, the most energetic unsteady modes represent the first helical mode |m| = 1, which is estimated from the three-dimensional Proper Orthogonal Decomposition modes and has a very large wavelength, on the order of 102 times the vortex core radius, λ/a = O(102). Instantaneous vorticity measurements as well as flow visualization suggest the existence of a smaller wavelength, λ/a = 5–6, which is not among the most energetic modes. These two-orders of magnitude different wavelengths are in agreement with the previous measurements of tip vortices and also exhibit qualitative agreement with the transient energy growth analysis. The very long wavelength mode in the upstream vortex may persist during the interaction, and reveal coupling with the trailing vortex as well as increased meandering.

/pdf/experiments-on-tip-vortices-interacting-with-downstream-1jm4t0i2hy.pdf

Experiments on tip vortices interacting with downstream wings

The influence of wake from a leading aircraft, Boeing 757, on a following aircraft, Cessna 750 Citation X, is investigated using the unsteady vortex lattice method. The effect of the wakes from both wings and each other is considered and the new position of the wakes from both aircraft calculated. As the midspan of the Cessna wing approaches the Boeing 757 trailing vortex core, the roll moment increases to its maximum value. Strong changes appear in aerodynamic loads of the Cessna as it moves near the Boeing 757’s wake. Rapid change occurs as the Cessna moves in transverse or vertical directions near the Boeing’s wake. The unsteady aerodynamic loads on wings moving along different paths are calculated. The Boeing 757’s wake strongly affects the aerodynamic loads and the wake of the Cessna. Also included in the present results are numerical simulations of wakes and velocity vectors in the Trefftz plane detailing the influence of the wake from the leading aircraft, the Boeing 757.

https://arc.aiaa.org/doi/pdf/10.2514/1.C10416E

Aerodynamic Interference of a Large and a Small Aircraft.

Birds fly in formation to benefit from the upwash wake vortices created by their peer leader. Similar strategies can be employed on aircraft for fuel saving or for range extension. However, further understanding of wake vortices in the near field and mid field is needed for close formation flight (< 10 wingspans). This paper introduces a new method for wake vortex detection using small UAV formation. A NonLinear Dynamic Inversion (NLDI) based formation control law is adapted to the WVU Phastball UAV platform. The designed controller showed good tracking performance during close formation flight (5-10 wingspans) with typical errors of less than 3 meters during straight legs. Then, multiple sensor measurements from leader-follower UAV formation flights were used for the wake vortex detection, including angle of attack, angle of sideslip, accelerometers, and gyros. The collected flight data showed the feasibility of using UAV formation for the wake encounter detection.

Wake Vortex Detection with UAV Close Formation Flight

The aim of this research is to develop realistic models of aer odynamic cross-coupling effects that can be incorporated in real-time or near real-time si ulations of Unmanned Aerial Vehicles (UAVs) in close formation flight. These woul d permit the assessment of the risks and issues associated with wake vortex evolution a nd encounter and the analysis of their consequences on the design of automatic contro l systems and the development of safe and reliable operating procedures. A number of w ake vortex modelling techniques that can be used in formation flight simulations a re reviewed. A novel Wake Vortex Model (WVM) is developed, implemented, verified, val id ted and successfully integrated within a Matlab /Simulink simulation environment. The code, named ELL because it is based on Weissinger’s extended lifting line th eory, meets the following requirements: (i) it is generic and can easily be adapted to a cc modate any wing planform and air vehicle configuration; (ii) it is computational ly rapid enough to be used in real-time or near real-time simulations; (iii) and it is su fficiently representative to support studies of aerodynamic interaction between multiple a ir vehicles during formation reconfiguration and air-to-air refuelling simulations. Si mulink test scenarios of two Aerosonde UAVs are developed to test and validate the use of E LL within simulation models, and the simulation environment is interfaced with v isualisation tools in order to facilitate the evaluation of multiple air vehicle dynami c interaction.

Wake Vortex Modelling and Simulation for Air Vehicles in Close Formation Flight

The aerodynamic forces and moments on the Hercules receiver aircraft, due to its horizontal and vertical position and bank, yaw, and pitch attitudes in the wake of the KC10 tanker aircraft, are assessed relative to the receiver's aerodynamic characteristics in free air. Large changes in lift, drag, and pitching moment are predicted near the tanker wake centerline. As the receiver is displaced sideways towards the tanker wingtip vortices it experiences large side force and yawing moment and particularly high rolling moment. The most significant term due to the receiver attitude is the rolling moment due to bank. b C»t CL C. ^'V. cm '" H ,

M. Jouma'a

Papers

Trailing Vortex Effects on Large Receiver Aircraft