Document created: 04 May 2004
Air University Review, July-August 1971

Aircraft Wake Turbulence

An Interesting Phenomenon Turned Killer

Andrew S. Carten, Jr..

In September 1970 the Office of the Air Force Deputy Inspector General for Inspection and Safety circulated the following message:

THE INVESTIGATION OF A RECENT T-38 FATAL ACCIDENT INDICATES THAT EXTREME TURBULENCE FROM TRAILING VORTICES OF A LARGE AIRCRAFT, 747 OR 707 TYPE, MAY HAVE BEEN A MAJOR FACTOR IN THE ACCIDENT. THE T-38 WAS MAKING A LANDING APPROACH AND APPARENTLY FLEW INTO THE TRAILING VORTICES OF A LARGE AIRCRAFT THAT HAD JUST COMPLETED A LOW APPROACH ON A 30° BISECTING RUNWAY. ALTHOUGH THE ACCIDENT INVESTIGATION IS STILL IN PROGRESS, IT IS SUSPECTED THAT THE T-38 PILOT LOST CONTROL OF HIS AIRCRAFT DUE TO WAKE TURBULENCE.

That message called attention to a safety problem that is becoming a crisis of serious proportions—the increasing incompatibility of the various types of aircraft constituting the traffic mix at busy airfields and in flight over heavily traveled air routes. The crisis results from the excessive wake turbulence generated by the new large swept-wing aircraft. In a way, it is a rerun of the clear air turbulence (CAT) crisis of the early 1960s except that aviation is in a better position to cope with the newer problem than it was when the CAT crisis developed.

The Air Force Office of Scientific Research and the Boeing Company conducted a symposium on aircraft wake turbulence during the summer 1970 in Seattle, which attracted many aircraft designers, aeronautical researchers, pilots, meteorologists, and operations personnel as well as representatives of various regulatory agencies. Several puzzling aspects of the wake turbulence picture were brought into focus at the symposium, and areas in need of investigation were identified. This article will stress some of the key points made there and will draw on other sources to explain the genesis of the problem, its operational impact, and possible alleviation techniques.1

Both as phenomena and as technical and operational challenges, clear air turbulence and aircraft wake turbulence exhibit striking similarities. In the belief that the wake turbulence problem will gain perspective if discussed in the light of CAT knowledge, I will review briefly the CAT situation and other related references.

the CAT crisis

In the initial stages of commercial jet transport operations in the United States (1959-1961), aviation had to face up to a serious problem for which it was not adequately prepared—the presence of random patches of very turbulent air at cruise altitudes, 25,000-40,000 feet. Since these patches are usually found in the absence of clouds, they provide no advance warning to pilots. This type of turbulence is called clear air turbulence to distinguish it from the well-known turbulence associated with convective clouds, which is generally avoidable through use of airborne radar. Several spectacular CAT-related incidents, involving both military and civilian aircraft, occurred during the early sixties and caused numerous personnel injuries and structural damage. A fatal B-52 accident during this period was blamed on CAT.

One probable reason for aviation’s unpreparedness was that earlier warnings from mountain wave and jet stream research programs had not reached enough people or were not properly heeded. Those warnings, incidentally, had been supplemented during the fifties by Strategic Air Command reports of clear air turbulence encountered by high-flying B-36, B-47, and B-52 aircraft. Another reason was the magnitude of the problem. The conversion to jets was proceeding rapidly while scientists and operations personnel struggled to obtain a working knowledge of CAT and CAT countermeasures.

Today, the causes of CAT are generally established, and reasonably effective methods have been developed for identifying areas in which CAT is likely to be present. In addition, pilots now employ special turbulence penetration techniques to reduce the hazard and avoid earlier problems caused by overreaction on encountering turbulence. Although the number of severe encounters is decreasing, CAT still deserves great respect, as the passengers on a PanAm 747 caught in turbulence attributed to CAT over Nantucket on 4 November 1970 will agree.2 Moreover, the routine circumnavigation of suspected CAT areas imposes a large economic and time penalty on civilian and military operations. Consequently research by the Air Force and other federal agencies continues, much of it at Air Force Cambridge Research Laboratories (AFCRL), under the purview of the Interagency Clear Air Turbulence Steering Group. An effective airborne CAT early-warning system is one of the major goals.

With the battle against CAT only partially won, it is ironic that the era of the second-generation jet transports—the so-called jumbo jets, 747, C-5, etc.has been marred from the start by the presence of a second turbulence-oriented hazard, aircraft wake turbulence.

CAT versus aircraft wake turbulence

Both CAT and aircraft wake turbulence can cause loss of control of encountering aircraft. Both frequently occur in clear air, without warning.

Many reported brief CAT incidents may actually have been penetrations of wake turbulence, and airborne instrumentation concepts for detecting CAT remotely are also being applied to wake turbulence detection.

Major dissimilarities also exist between the two phenomena. CAT occurs naturally, and usually in preferred locations (i.e., over mountainous country and near the jet stream), and at preferred heights (i.e., near the tropopause and in regions of high-altitude wind shear). Aircraft wake turbulence is a man-made atmospheric disturbance found behind all aircraft in flight, wherever they may be. Severe CAT can toss very large aircraft around quite handily (witness the 747 incident over Nantucket) while wake turbulence in its most severe form is a threat principally to medium-sized (DC-9, 737) and small aircraft. This does not mean that larger aircraft arc unaffected by wake turbulence. The threat is simply less obvious.

A typical aircraft wake occupies a much smaller volume of the sky than that occupied by a typical “patch” of CAT, which can extend for hundreds of miles and be 2000 to 3000 feet thick. The wake also tends to dissipate more rapidly than the CAT. However, more extensive and more persistent volumes of wake turbulence may be expected under busy air lanes. Also, wake turbulence can hover over runways, where aircraft are most vulnerable to unexpected loss of altitude or control. The combination of poor visibility and a wake encounter during landing operations is especially dangerous.

 the trailing vortex

Aircraft wake turbulence is not only a mark of the jumbo jet era, it is to some degree a product of it since the most dangerous atmospheric disturbances of this type are those associated with the trailing vortex wakes of the jumbo jets.* When multiple, regularly spaced vortices emanate from a source, the rings combine to form a spiral or helix (e.g., curling cigarette smoke).3 This is essentially what happens at or near the wingtips of an aircraft in flight. Aircraft vortex wakes are, in effect, two parallel, rapidly rotating, spiral “tubes” of air, up to 35 feet in diameter, trailing downstream. (Figure 1) Although less severe than natural tornadoes, they exhibit a tornado-like drop of pressure in their cores. They are called “streamwise,” “trailing,” or “wingtip” vortices or, more simply, aircraft wake turbulence.  

Figure 1. Hazard potentials of trailing vortices (source: NASA)

Figure 1. Hazard potentials of trailing vortices (source: NASA)

At cruise altitudes, they extend 10 to 40 miles behind the generating aircraft. They are frequently visible against a blue sky when, under proper conditions of air saturation, the familiar condensation trails produced by engine exhaust persist and are entrained in the vortex tubesanother example of flow visualization. (The reader is invited to verify this phenomenon by skywatching.)

Are aircraft wakes really turbulent?

One might question the use of the word “turbulence” when discussing vortex wakes. By definition, turbulence is an irregular velocity fluctuation imposed on the mean airflow. It is made up of extremely variable eddies of a wide range of sizes and directions (e.g., the wide and violent fluctuations of surface winds observable on a gusty day). CAT is believed to have this random distribution of eddy sizes and directions despite its origin in organized natural wave and vortex motions of the atmosphere.

Aircraft vortex wakes, on the other hand, have a highly organized circular flow, a relatively stable geometry, and tangential velocities which remain constant for long periods of time. They are most dangerous during their organized stage. When they degenerate into random turbulence, they lack the power to cause serious upsets to following aircraft. The use of the word “turbulence” is justifiable, nonetheless, in that an aircraft crossing the vortices experiences buffeting characteristic of turbulence encounters.

early  investigations of trailing vortices

Aircraft vortex wakes were known long before the advent of jet aircraft and the discovery of CAT. Early (1900-1930) aeronautical experimenters (Lanchester, Prandtl, Von Kármán) postulated or demonstrated the presence of wingtip vortices—findings that have been updated in subsequent studies. As aerodynamic shapes have become more sophisticated, vortices have been used not only to explain the dissipation of lift and drag energy but also as actual sources of lift and control. In the T-tail aircraft, vortex management prevents deep stalls. The highly swept wing of the supersonic transport (SST) relies on vortex-generated lift for safe handling qualities at low speed.4

In general, the effect of the vortex on the generating aircraft seems to have been stressed rather than the hazards to other aircraft. Investigators from NASA (Kraft, McGowan, Wetmore) and from the Royal Aircraft Establishment (Squire, Dee, Fose, Kerr), among others, published explicit, experimentally supported warnings on vortex hazards in the 1955-1964 time period; but these warnings, like the early CAT warnings, appear to have been largely unheeded.

the mounting evidence

Inevitably, vortex wakes were recognized as more than just interesting and sometimes useful phenomena, particularly after reports of upsets to light planes operating near the larger first-generation jets (e.g., Boeing 707, B-52). A multiple-fatality takeoff accident to a commuter aircraft at New York’s Kennedy Airport in 1969 is typical. The loss of control was blamed on the residual wake of a 707. The National Transportation Safety Board has records of at least 98 wake-caused accidents in the past five years, with numerous fatalities.

action at last

There is a strong correlation between the weight of an aircraft and the strength of its vortices. With growing concern over hazardous wakes from the 300,000-pound first-generation jet transports, aviation was finally forced—in 1969—to examine the implications of wakes from the 700,000-pound C-5and 747 aircraft.

Preliminary calculations showed the 747 and C-5, as well as the heavier versions of the earlier DC-8 and 707 (and their Russian and British equivalents), to have powerful wakes indeed. Early in 1970 the Federal Aviation Administration  (FAA) and the Air Force imposed rigid separation distances (10 miles, later reduced to 5 for FAA-controlled operations) between aircraft weighing more than 300,000 pounds and following lighter aircraft, distances which many felt to be too conservative. (Previous separation distance had been 3 miles.) By reducing the number of aircraft movements per hour, this regulation has added to airfield congestion. With more jumbo jets entering service, theorists and experimenters have been working overtime to establish the true characteristics of the wakes of these planes, with emphasis on measured vortex diameters, tangential velocities, decay times, and horizontal and vertical transport. To appreciate their findings, one must delve a little into existing wake turbulence theory.

aircraft wake turbulence as  a product of lift

Many people think of aircraft wakes in terms of “prop wash” or jet engine exhaust. The inflight turbulence associated with the power plants is actually short-lived. The principal cause of the trailing vortices which constitute the wakes is the lift generated by wing surfaces. As proof of this, wake turbulence is present behind unpowered aircraft and even birds in flight. Geese are believed to adjust their positions in formation flight instinctively to achieve maximum lift from the vortex wakes generated by their mates, the energy saved permitting longer-range flight. (Presumably, the birds take turns in the lead position.) Helicopter pilots in Southeast Asia (SEA) refueling operations are reported to be using the wakes of tanker aircraft to decrease rotor workloads during hookup, an obvious range-extending maneuver. (Helicopters themselves generate powerful [and dangerous] vortex wakes which have been useful in fog-clearing operations in SEA, a technique tested at the Army’s Cold Region Research and Engineering Laboratory and developed at AFCRL.)5

vortex theory

As aircraft vortex generation involves aerodynamic theories not generally familiar, a brief discussion may be of interest.

The key word in this discussion is “circulation,” the concentric airflow about a body, such as that around a cylinder. Our smoke ring is an example of this circular air motion; a spinning golf ball has circulation. When airflow past a body is added to the circular airflow about the body, lift at right angles to the airflow past the body is the result. In the case of the golf ball, it could mean a long drive, a hook, or a slice, depending upon the direction of the propulsive force.

Aircraft wings have circulation about them. When the flow past the wing (produced by the forward motion of the aircraft) is added to the circulation about the wing, lift is produced. As the lift increases, there must be a corresponding increase in circulation. This has an important bearing on the strength of the spinning vortex tubes trailing from the wingtips. Their circulations are essentially the circulations “spilled” from the wing, and their tangential velocities (i.e., peripheral speeds) are directly proportional to those circulations. In other words, the larger its circulation, the greater the force a trailing vortex can impose on an encountering object, such as the wing of a following aircraft. If the ability of the following aircraft to overcome the suddenly imposed load is exceeded, loss of control (often fatal at low altitudes) will occur.

The magnitude of the circulation depends on a number of variables. It increases or decreases depending on which of the variables is changed or held constant. Thus, in discussing vortex strength, one must know how heavy the aircraft is, at what altitude and speed it is flying, and what its wingspan is. (In this paragraph and the next, let us assume a constant wingspan.) Since lift can be equated with aircraft weight, increased weight requires increased circulation. One can expect very strong updrafts, downdrafts, and lateral wind components, therefore, in the wake of a heavy aircraft.

For a given aircraft weight, circulation will vary with changes in air density and in aircraft velocities. The low, slow “dirty configuration” (i.e., flaps down) period of flight is usually considered the time of maximum vortex production. At cruise altitudes and high cruise speeds, the increase in aircraft velocity (reduced circulation) more than cancels out the effect of reduced air density (increased circulation). There are reduced cruise speed conditions, however, when the vortices at altitude can be as severe as those near the groundconditions common to military missions (e.g., refueling and formation flights) where maximum cruise efficiency is not realizable. (The high-velocity filament described later is a further complication.)

the importance of attitude and of aspect ratio

Aircraft turning motions and changes in attitude (e.g., increased climb rate) will also cause variations in vortex strength (discussed further under vortex alleviation methods). In addition, circulation will increase if the wingspan is decreased. This means that heavy aircraft of low aspect ratio, i.e., with very short wingspans such as the highly swept SST, will have very large circulations and high vortex tangential velocities. With high aspect ratio (long span) wings, added lift is possible without increased circulation.

the vortex sheet and roll-up

Aerodynamicists have developed a vortex model that features a thin vortex “sheet” flowing from the trailing edges of the wing. The sheet undergoes a rolling-up process at some distance behind the wing. (Figure 2) The roll-up is aided by the pressure differential between the bottom and upper surfaces of the wing, which imparts a characteristic rotational movement to the airflow at each wingtip (clockwise at the left wingtip, viewed from the rear of the aircraft, and counterclockwise at the right wingtip). The resultant mutual interaction between the two rolled-up vortices causes them to sink below the aircraft—a very important consideration in wake turbulence avoidance and dissipation determinations.

Figure 2. Vortex sheet.

Figure 2. Vortex sheet.

A photograph taken during an early AFCRL cloud-seeding operation graphically illustrates the trailing vortex formation with cloud tops serving as the flow visualization agent.

Photography can show engine exhaust smoke being caught up in the vortex wrap-up process of a landing C-5. Here the exhaust serves not only as a flow visualization agent but also as a vortex locator. There is evidence that the exhaust of T-tail aircraft with aft-mounted engines (unlike the C-5’s wing-mounted engines) is not caught up in the vortices, in which event the smoke would not help to locate the vortices. (The 747’s exhaust is “clean” and offers no clue at all.)

the vortex radius

In 1950 Spreiter and Sacks of Ames Aeronautical Laboratory of the National Aeronautics and Space Administration (NASA) helped to bridge the gap between the pioneering work of the Prandtl school in Germany (based for the most part on straight, high aspect ratio wings) and the need for data on the new swept, low aspect ratio wings. They showed that wing geometry greatly influences vortex roll-up time.6 With delta wings, for example, the roll-up is extremely rapid (within a chord length or less of the trailing edge and well within the tail region). Using certain energy cutoff assumptions, they also postulated that the core radius of each roiled-up vortex (i.e., the distance from the center of the core to the region of maximum tangential velocity) is approximately one-tenth the distance between the two vortices. This relationship allows us to estimate how completely a specific following aircraft will be enveloped by a particular trailing vortex. The Spreiter and Sacks theories were used extensively in vortex strength calculations released in the past year or two.

Typical is the accompanying Air Weather Service table.7 It shows that an aircraft with a 32-foot wingspan would be completely captured by the vortex of a 747 if it were to fly into the vortex endwise. It would experience induced roll rates far beyond its capacity to control them. Larger aircraft would be affected in proportion to their size. (In Boeing tests, a 737 was rolled 30° in a hair-raising encounter with a 747’s wake 100 feet above the ground.) Transverse crossings of the 747 wake by other aircraft would subject them to severe updrafts and downdrafts in rapid succession, with possible structural damage. (Figure 1) Flight between the vortices would place the following aircraft in the downwash region and seriously degrade its rate-of-climb performance. The table shows that wakes from other large aircraft are dangerous too, depending on the size and performance of the encountering aircraft.

another look at the crisis

At this point the reader may ask: “Why the crisis, if trailing vortices have been known almost since the beginning of flight and if established theories explain their relationship to other flight parameters?” The crisis derives from the unavoidable commingling of aircraft of all sizes in the same airspace. To date, segregation by size at airfields and in the airlanes has been very difficult to implement, and conditions have become increasingly intolerable for smaller aircraft. The first sweptwing jet transports and bombers accelerated the trend; the jumbo jets have sharply increased the peril.

Approximate Parameters and Wake Values for Selected Aircraft

To appreciate the aircraft wake problem, one has only to compare it with the corresponding problem in boating, which also has greatly increased in volume and variety of traffic. Small, unpowered or lightly powered watercraft must frequently share the same river, lake, or harbor with high-powered cruisers and outboards. With boats, safe coexistence can be assured through simple speed control. With aircraft, the problem is more difficult, on several counts. Not only are aircraft wakes usually invisible; their effects can be so catastrophic that little margin for error is possible. Reducing a boat’s speed automatically minimizes its wake generation, since the momentum imparted to the water thrust behind by the boat is proportional to the square of the boat’s speed. With aircraft, the opposite is true. Circulation and wake turbulence increase when aircraft velocity is decreased; the hazard to other aircraft landing or taking off is actually greatest when it should be least. The flight-no-flight transition zone is too sensitive to be subjected to such large unexpected upsetting forces. This is especially true of STOL and VTOL aircraft where forward velocity is at a minimum and critical dependence is placed on high lift devices.

the needs identified

To those responsible for the safe coexistence of various sizes of aircraft in the terminal area, the minute-to-minute location of destructive vortices is of vital concern. Unless they are positive that vortices from a previous large aircraft movement have drifted out of the path of a following lighter aircraft, they must impose large time and distance separations. An alternate possibility is to have lighter aircraft take off “short,” so that their flight path is consistently above the trajectory of the sinking vortices from a preceding heavy aircraft, or to land “long,” which achieves the same result.8 This method does not cover all situations, however.

With respect to terminal area operations, a primary goal is to develop methods of predicting vortex presence, movement, and decay under various traffic and environmental conditions. A longer-range goal is to develop instrumentation that will actually detect and track the vortices. A third goal is the alleviation of the problem by vortex destruction or dissipation techniques.

The problem is somewhat easier at jet cruise altitudes. Most planes that fly at these levels are not endangered by wake encounters, although all are affected to some degree. Small, short-span fighters and executive jet transports pose a special problem. They are upset easily by a strong wake, as was graphically demonstrated when a small NASA transport was probing the wake of a C-5 and rolled through 180° against the pilot’s input. (Complete capture is relatively infrequent in such tests, as the wake tends to cast aside the would-be penetrator.)

At cruise altitudes, small jet aircraft should stay above the long-lived vortices of larger ones. In the event of an unexpected encounter, altitude is a saving factor and should allow recovery. Unfortunately, this was not the case in the 1966 B-70 accident at Edwards AFB in which a closely following chase aircraft was suspected to have been rolled over by the B-70 vortex wake, and both aircraft were destroyed.

vortex decay

When aircraft weight is transferred from wheels to wings during takeoff, lift is generated, and the concomitant circulation initiates trailing vortex formation, a process that ceases only on landing. What happens to the vortices is of concern now.

Cruise altitude vortices usually level off at about 1000 feet below the altitude of the aircraft as their density comes into equilibrium with that of the surrounding air. Decay processes then take over. Two commonly mentioned decay mechanisms are the mixing action of eddy viscosity and the interaction of the vortices with each other. The eddy viscosity mechanism is relatively slow and is marked by many unknowns, particularly the value of the eddy viscosity coefficient. The interaction mechanism is based on instability modes in the vortices, which propagate sinusoidal waves through the wake, bringing the two vortices in contact with each other periodically and causing them to break into vortex rings. (This occurrence can be seen in the sky when condensation trails are present.) Crow of Boeing believes that these rings quickly disintegrate into chaotic and harmless turbulence.9 Others see the rings as still quite dangerous. Crow also suggests that the vortices of the 747 (about 1.8 times as powerful as those of the 707) should break up faster than 707 vortices. He reasons that the instability propagates much more rapidly in vortices of higher intensity.

Neither the triggering mechanism nor the ideal instability wavelength for vortex breakup is known. A wavelength of 5 to 10 wingspans has been suggested. MacCready has proposed mechanical excitation of the proper wavelength.10 Atmospheric turbulence and stability also play an important but not completely understood part in the decay of the vortex wakes.

Vortices generated below about 1500 feet usually sink to just above the ground, 50 to 100 feet. Their speed of descent varies with the type of aircraft and with the local atmospheric conditions. A figure of 450 feet per minute is typical of large sweptwing jet transports. As the vortices approach the ground, they spread apart and move sideways, at right angles to the runway, at the same 450 feet per minute, roughly 5 miles per hour. An opposing surface wind of 5 miles per hour will block horizontal movement of one vortex while doubling the movement of the other, transporting it perhaps to a runway intersection or to a parallel runway. (Figures 3 and 4) The time to disappearance of the vortices depends on the degree of low-level turbulence present, on mutual interaction, and on a number of other factors. No-wind days usually make for the most dangerous conditions, since wake-destroying turbulence is at a minimum and the calm instills a sense of complacency.

Figure 3 (left) illustrates vortex movement near the gorund with no wind, and Figure 4 (right) shows vortex movement near the ground with a crosswind (after W. A. McGowan).

Figure 3 (left) illustrates vortex movement near the gorund with no wind, and Figure 4 (right) shows vortex movement near the ground with a crosswind (after W. A. McGowan).

vortex detection, high-intensity cores

NASA has been particularly active in investigating instrumentation for detecting the presence and movement of vortices near runways. This work involves the use of a CO2 Doppler laser velocity-measuring system adapted from a CAT detection system. A high-intensity, small-radius vortex jet or filament (accompanying photos), which is noted in NASA field experiments with the CO2 laser and a collocated smoke generator, suggests a more complicated vortex picture than covered by the Spreiter and Sacks theories. These filaments (also noted by the FAA and the Australian Department of Civil Aviation) were generated by low-flying test aircraft in the “clean” (i.e., wheels up, flaps up) configuration. (In actual operations, this configuration is partially achieved in a “go-around” situation when a landing is aborted.) The filament seems common, even at high flap settings, to T-tail aircraft with aft-mounted engines (DC-9, 727), the clean wings apparently permitting tighter vortex wrap-ups. The FAA has recorded on film both the persistence of the filament and its propensity towards sudden destruction. It can be seen to whip around in the sky and then suddenly burst when it touches the smoke tower. It frequently displays a very strong axial component, probably the result of smoke being entrained in the core’s intense vacuum. Acoustic verification of the filaments has also been reported by FAA personnel, who talked of “walking” the filament across the test zone by listening for the disturbed air sound.

This evidence of filament vortices was one of the highlights of the Seattle symposium. It helped to explain flight tests reported by the FAA, by NRC (Canada), NASA, and AFFDL, in which tangential velocities higher than predicted by theory were encountered. The evidence also made it clear that much remains to be learned about the trailing vortex, especially in connection with modern aircraft. The Spreiter and Sacks theory, which places the highest tangential velocities at a core radius equal to one-tenth the vortex span, apparently correlates best with wakes generated by aircraft in the “dirty” configuration, such as at landing. Where the filament is present, tangential velocities at least twice as high as predicted have been encountered, and the vortex filament radius is only about one-third the expected value for the core radius. There is evidence also that even when the filament is present there is an induced vortex (external to the filament) whose radius and tangential velocities are in accord with the older theory. Experiments by McCormick, et al.,11 who use a different energy cutoff value than Spreiter and Sacks, correlate better with the filament phenomenon.

vortex alleviation

Several vortex “alleviation” (breakup) mechanisms or techniques were discussed at Seattle. One suggestion was a porpoising of the aircraft during landing and taking off. Changes in angle of attack would presumably alter circulation, introducing periodic instabilities in the vortices. Passenger discomfort ruled out this idea. Another suggestion, which would use somewhat the same technique, involved an adaptation of the Load Alleviation and Mode Stabilization (LAMS) system, in which automatically induced symmetrical oscillatory movements of the aircraft wing control surfaces would introduce perturbations in the vortices without disturbing the passengers. (The usefulness of LAMS in alleviating structural loading and possible upset tendencies in vortex encounters was also discussed, although the application seems practical only for large aircraft.) A third proposal featured vortex alleviators on the wings of the generating aircraft—essentially flat plates lifted into the airstream. NASA-Ames is currently working on this technique, but results to date have been inconclusive. A suggested “brute force” elimination of vortices near a runway via a gigantic suction system was felt to be impractical.

Similarities between the current aircraft wake turbulence crisis and the earlier clear air turbulence have been pointed out. It has also been shown that the problem of aircraft wake turbulence has been growing over the years as a result of the tremendous quantitative increases in vortex strengths that have come with low aspect ratio heavy aircraft. The need to operate various sizes and kinds of aircraft at the same airfield at the same time is shown to be the crux of the problem.

Flight tests have demonstrated that vortex cores and tangential velocities are frequently different from those predicted from theory. This is especially true of aircraft in the “clean” configuration and of T-tail aircraft with aft-mounted engines. The need for additional knowledge and the search for methods of assuring peaceful coexistence were summed up at the 1970 Symposium on Aircraft Wake Turbulence.

Conservative separations, in time and in distance, between the large aircraft and the aircraft small enough to be affected by wake turbulence will be in order for some time. The peculiar requirements of military operations, which tend to maximize vortex production at times, will dictate against use of the same separation criteria for civilian and military flights. The use of smoke trails to mark the presence of wake vortices has some merit for daytime use but probably would not be allowed in today’s world of antipollution pressures. A vortex-detection system, capable of identifying minute-to-minute conditions in the vicinity of an airfield, will be developed and should become standard equipment at major airfields within the next decade. Vortex alleviation methods may eventually bring about quick dissipation of wakes, but this cannot be predicted with certainty.

Air Force Cambridge Research Laboratories

*A vortex is a parcel of air in circular motion. A common example is the cigarette smoke ring; the smoke enables one to see the air in motion. It also typifies the kind of flow visualization on which research in fluid dynamic depends heavily.

Notes

1. The proceedings of the 1970 Aircraft Wake Turbulence Symposium are being published and will be available later this year. I must defer to that publication for a proper listing of papers given and their authors as well as for a more definitive treatment of the subject matter. In this article, it is impossible to give proper credit to individuals.

Another symposium on turbulence and CAT was held in Washington on 22-24 March 1971, with over a thousand people in attendance. Its findings did not differ significantly from those of the earlier one in Seattle.

2. References to the 707, 727, 737, and 747 are to Boeing commercial transports; DC-8 and DC-9 references are in McDonnell-Douglas commercial transports.  Air Force equivalents, C-135 and C-9, may be substituted for 707 and DC-9 references.

3. B. Quinn, “Cigarettes, Vortices, and V/Stol,”  Air Force Research Review,  July-August 1970.

4. P. M. Sforza, “Aircraft Vortices: Benign or Baleful?” Space/Aeronautics, April 1970, pp. 41 ff.

5. V. Plank, “Fog Modification by Use of Helicopters,” AFCRL Report 70-0593, 23 October 1971.

6. J. R. Spreiter and A. H. Sacks, “The Rolling Up of the Trailing Vortex Sheet and Its Effect on the Downwash Behind Wings,” Journal of the Aeronautical Sciences, January 1951, pp. 21-32.

7. D. N. Jones, “Introduction to Jet-Engine Exhaust and Trailing Vortex Wakes, “Technical Report 226, Air Weather Service, April 1970.

8. W. A. McGowan, “Avoidance of Aircraft Trailing Vortex Hazards,” NASA Report, October 1970.

9. S. C. Crow and J. H. Olsen, “The Duration of Trailing Vortices: 747 and 707,” Boeing Company Flight Sciences Laboratory Technical Communication 008, October 1969.

10. P. B. MacCready, Jr., “An Assessment of Dominant Mechanisms in Vortex-Wake Decay,” Aircraft Wake Turbulence Symposium, Seattle, Washington, 1-3 September 1970.

11. B. W. McCormick, J. L. Tangler, and H. E. Sherrieb, “Structure of Trailing Vortices,” Journal of Aircraft, Vol. 5, No. 3 (May-June 1968), pp. 260-67.


Contributor

Andrew S. Carten, Jr. (M.S., Tufts University) is Chief, Equipment Engineering and Evaluation Branch, Aerospace Instrumentation Laboratory, Air Force Cambridge Research Laboratories. During World War II he served as a staff weather officer, Eighth Air Force. He joined the Air Force Cambridge Research Center in 1954 as Chief, Design Engineering Branch, Atmospheric Devices Laboratory, and in 1960 assumed his present position. He is author of numerous published articles and papers presented at meteorological conferences.

Disclaimer

The conclusions and opinions expressed in this document are those of the author cultivated in the freedom of expression, academic environment of Air University. They do not reflect the official position of the U.S. Government, Department of Defense, the United States Air Force or the Air University.


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