https://people.bath.ac.uk/jldb20/pubs/Thomas.etal2014Advancesinair.pdf

Advances in air to air refuelling

Wind tunnel data from a tanker wing and receiver aircraft model at varying vertical separation have been compared with theoretical results. In the aerodynamic model the tanker wing is represented by a horseshoe vortex while the aerodynamic loads on the receiver are determined by the vortex lattice method and lifting-line theory, although an approximate method is used to determine the side force on the fin. In the longitudinal case data were obtained for low, mid and high tailplane positions and, with the exception of the pitching moment results, fairly good agreement is obtained between theory and experiment. The relatively small differences are due mainly to the wind tunnel boundary interference effect which could not be quantified for the pitching moment measurements. The lateral aerodynamic interference was determined by banking the tanker wing and displacing it sideways and by yawing the receiver model. Fairly good agreement is obtained between the theory and experiment for the most significant terms which are the rolling moments due to bank and sideways displacement. The effect of the sidewash due to the tanker wake on the receiver in yaw is found to be relatively insignificant. Over the range of bank, yaw and sideways displacements tested the results are almost linear.

The aerodynamic interference between tanker and receiver aircraft during air-to-air refuelling

Application of a wake roll-up method coupled with the vortex lattice method and approximate expressions for the receiver fuselage effect have been used to determine the induced loads on a Hercules receiver aircraft behind a KC10 tanker. The induced loads depend strongly on the vertical position of the receiver wing and fin relative to the tanker wing wake. In the case of steady sideslip there is a large decrease in the directional stability of the receiver as quantified by the gradient of the rudder angle versus sideslip. This is due mainly to the combined effects of the yawing moments due to bank, yaw and side displacements. Minimum directional stability corresponds to the tip of the receiver fin intersecting the tanker wing wake. The associated aileron angle is two to three times the value in free air in agreement with flight test data. Solution of the linearized equations of motion gives three lateral characteristic oscillations for the air-to-air refuelling case. These include the usual Dutch roll osc...

Lateral and Directional Stability and Control in Air-to-Air Refuelling:

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

Mechanisms contributing to the “bow wave” effect of a receiver on a tanker are investigated, and a modeling method for dynamic simulation is developed Vortex lattice and Cart3D codes are used to calculate the aerodynamic forces and moments for a C-141B behind a KC-135 The vortex lattice code significantly underpredicts the bow wave effect, whereas the Cart3D code shows good agreement with available flight data The modeling method uses stream functions for inviscid flow around solid bodies The flowfield induced by the presence of aircraft bodies is superimposed on the flowfield generated by horseshoe vortices The resultant flowfield is approximated by the effective uniform translational and rotational velocity components, which are used in the buildup equations for the aerodynamic force and moment coefficients of both tanker and receiver aircraft This method applied to a flying wing behind a KC135 shows good agreement with Cart3D results It is concluded that the major contribution to the “bow wave” 

Bow Wave Effect in Aerial Refueling: Computational Analysis and Modeling

: This report investigated the feasibility of using an analytical approach and the vortex lattice method (VLM) to evaluate aerodynamic interference effects present during aerial refueling. While KC-10 tanker and a B-52 receiver were studied, the method applies to any tanker receiver combination. Major assumptions include: linear potential flow; tanker receiver are represented by wing planforms; fuselage effects are small; and the rolling- up process of the tanker's wing tip vortices are not considered. The analytical approach uses a lifting line followed by a semi-infinite vortex sheet to represent the tanker. Three linear lift distributions for the tanker's wing were used and equations were derived for the induced downwash at any point in space. Points on the receiver's wing were selected to indicate the tanker's effect on the receiver's flowfield. Results of the analytical equations when compared with analytical predictions were found to overpredict the induced downwash by 25-35%. VLM was used to study the effects of: rectangular vs swept wings on induced downwash; tanker tailplane on the receiver; change of angle of attack of one aircraft due to the presence of the other; and the presence of the tanker on th receiver's pitching moment. VLM results varied from the Douglas prediction for induced downwash by only 5% and were even more accurate at lower tanker angles of attack.

E. H. Hoganson

Papers

A Study of the Aerodynamic Interference Effects during Aerial Refueling