Measurements of Boeing 747, Lockheed C5A and Other Aircraft Vortex Wake Characteristics by Tower Fly-by Technique
Flight tests have been conducted by the Federal Aviation Administration (FAA) at both the Environmental Science Services Administration/Atomic Energy Commission (ESSA/AEC) facility, Idaho Falls, Idaho, and the National Aviation Facilities Experimental Center (NAFEC), Atlantic City, New Jersey, during the period 18’February 1970 through 3 August 1970 to gather quantitative data on aircraft vortex wake characteristics using the tower fly-by technique. Aircraft tested included the Boeing 747, 707–300 and 727–100, the Douglas DC-8-63F, DC-8-33 and DC-9-10, the Lockheed C5A, the Convair 880, and the Learjet 24. A 200-foot and a 100-foot tower was used at the ESSA and NAFEC test sites, respectively. Vortex flow visualization for vortex characteristics and movement was provided by colored smoke grenades mounted on the towers and by injecting CORVUS-type smoke oil into the outboard jet engine exhausts of certain aircraft with wing mounted engines.
Vortex flow velocities were obtained using hot-film/hot-wire sensors.
Measured tangential velocities were approximately double those velocities predicted by certain theory (which assumes an elliptical lift distribution) .
Distinct vortex characteristics peculiar to certain model aircraft and configurations per aircraft model were noted: (1) T - tail aircraft with engines mounted on the fuselage, i.e., the B-727 and DC-9 were observed to produce much higher tangential velocities, on the order of 175-200 feet/second, than aircraft with engines mounted on the wing, e.g., the B-707 and B-747 which were on the order of 130 feet/second; (2) For “clean” configuration and small flap deflections, the vortex systems of all the aircraft were observed to be of a tubular form, relatively small in diameter, very clearly structured and very persistent. For greater flap deflections, vortices of tubular form were much less evident. With the exception of the B-727 and DC-9, when maximum flap deflections were employed this characteristic was not observed, the vortex flow field appearing much larger in diameter, i.e., field of influence; (3) From both recorded data and visual flow observations of the tubular-type vortex system when highest vortex tangential velocities were recorded, the core diameters as outlined by the tower smoke appeared to be small: from approximately 5–6 feet for the larger B-747 and C5A aircraft to about 1–3 feet for the CV-880, B-727, and DC-9 type aircraft.
Particular attention was given vortex axial flow phenomena by observing and photographically recording entrainment of the colored smoke. When tubular-type vortex systems passed the tower, smoke was clearly entrained and spontaneously moved within the vortex system along its axis in one or both directions, i.e., up and/or down flight path. A vortex has a low-pressure area within its cylindrical wall analogous to a tornado or waterspout. Once the wall is penetrated, as by the instrumented tower upon vortex passage, there is an immediate injection of relatively higher pressure ambient air which carries the colored smoke in one or both axial directions in the attempted pressure equalization process.
It is possible that the tower technique may cause premature vortex instability onset when the vortex tube passes through the tower with subsequent valid data acquisition on that particular vortex made questionable.
This is an interim report on the data processed and analyzed to date.
KeywordsTangential Velocity Flight Test Vortex System Ground Effect Federal Aviation Administration
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