TITLE AND SUBTITLE A Comparison of Airborne Wake Vortex Detection Measurements With Values Predicted From Potential Theory

An analysis of ight measurements made near a wake vortex was conducted to explore the feasibility of providing a pilot with useful wake-avoidance information. The measurements were made with relatively low-cost ow and motion sensors on a light airplane ying near the wake vortex of a turboprop airplane weighing approximately 90 000 lb. Algorithms were developed which removed the response of the airplane to control inputs from the total airplane response and produced parameters which were due solely to the ow eld of the vortex. These parameters were compared with values predicted by potential theory. The results indicated that the presence of the vortex could be detected by a combination of parameters derived from the simple sensors. However, the location and strength of the vortex cannot be determined without additional and more accurate sensors. Introduction The limited rate at which airplanes can land at an airport is a serious problem facing the commercial airplane industry, particularly in Instrument Meteorological Conditions (IMC). One factor which limits the landing rate is the Instrument Flight Rules (IFR) longitudinal spacing required between trailing airplanes in the landing pattern. These longitudinal spacing requirements were imposed to reduce the operational hazard which might be associated with the vortices trailing from the wingtips. One proposed solution for this problem is to provide the pilot with warning and avoidance information should the airplane approach a dangerous vortex. A theoretical analysis indicated that, using conventional airborne sensors, a warning could be generated at a distance which would provide the pilot with ample time to avoid the vortex (ref. 1). Angle of attack and angle of sideslip sensors mounted on the detecting airplane were thought to be su cient by themselves for measuring the vertical and horizontal components of the vortex velocity, while airplane rolling sensors could be used to measure the lateral gradient of the vertical component of the vortex velocity. These velocity components and gradients could then be used in closed-form equations based on a dipole approximation of the wake vortex to determine the location and strength of the wake vortex. Depending on the accuracy of the sensors, it was predicted that the wake vortex of a large airplane could be detected, located, and quanti ed at a distance of almost 500 ft. The National Aeronautics and Space Administration (NASA) and the Federal Aviation Administration (FAA) recently undertook a ight test to investigate these concepts. The preliminary analyses of the ight data indicated that, under certain circumstances, the vortex eld could be detected, although at smaller distances than had been anticipated (refs. 2 and 3). These preliminary analyses were based on post ight inspections of time histories of the ight data and did not attempt to locate or determine the strength of the wake vortices by using only the sensors on the detecting airplane. This paper presents a more detailed analysis of the same ight data. The primary purposes of the present analysis were (1) to develop candidate vortex ow parameters and algorithms and (2) to determine if the location and strength of the vortex could be determined theoretically as well as experimentally with a combination of vortex ow parameters. Measurements on the detecting airplane of the ow angles, velocity, angular rates, attitude, and control position were used in the algorithms. These algorithms produced the following vortex ow parameters: di erential angle of attack, di erential angle of sideslip, angle of attack, vertical velocity, and rolling rate. A potential theory model with two vortices rotating in opposite directions was used to calculate theoretical values of the vortex ow parameters for comparison with the measured vortex parameters. In addition, the potential model was used to determine the theoretical combination of parameters required to ascertain the location and strength of the wake vortex as well as to develop an estimate of the distance at which a warning of a vortex presence could be generated.