Document created: 04 May 2004
Air University Review, July-August 1971
Andrew S. Carten, Jr..
September 1970 the Office of the Air Force Deputy Inspector General for Inspection and Safety
circulated the following message:
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.
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
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
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.
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.
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.
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
CAT and aircraft wake turbulence can cause loss of control of encountering
aircraft. Both frequently occur in clear air, without warning.
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.
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.
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.
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)
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 tubes—another example of flow
visualization. (The reader is invited to verify this phenomenon by skywatching.)
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.
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
nonetheless, in that an aircraft crossing the vortices experiences buffeting
characteristic of turbulence encounters.
investigations of trailing vortices
vortex wakes were known long before the advent of jet aircraft and the discovery
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
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.
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.
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.
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.
wake turbulence as a product of
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
aircraft vortex generation involves aerodynamic theories not generally familiar,
a brief discussion may be of interest.
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.
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.
magnitude of the circulation depends on a number of variables. It increases or
decreases depending on which of the variables is
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.
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
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 ground—conditions 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
importance of attitude and of aspect ratio
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.
vortex sheet and
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.
photograph taken during an early AFCRL cloud-seeding operation graphically
illustrates the trailing vortex formation with cloud tops serving as the flow
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.)
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
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.
look at the crisis
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.
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
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.
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.
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
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.
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
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
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).
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.
evidence of filament vortices was one of the highlights of the Seattle
symposium. It helped to explain flight tests reported by the FAA,
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
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)
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.
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
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.
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
*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.
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.
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.
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
B. Quinn, “Cigarettes, Vortices, and V/Stol,” Air Force Research Review,
M. Sforza, “Aircraft Vortices: Benign or Baleful?” Space/Aeronautics, April
1970, pp. 41 ff.
V. Plank, “Fog Modification by Use of Helicopters,” AFCRL Report 70-0593, 23
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.
D. N. Jones, “Introduction to Jet-Engine Exhaust and Trailing Vortex Wakes,
“Technical Report 226, Air Weather Service, April 1970.
W. A. McGowan, “Avoidance of Aircraft Trailing Vortex Hazards,” NASA Report,
C. Crow and J. H. Olsen, “The
Duration of Trailing Vortices: 747 and 707,” Boeing Company Flight
Sciences Laboratory Technical Communication 008, October 1969.
B. MacCready, Jr., “An Assessment of Dominant Mechanisms in Vortex-Wake
Decay,” Aircraft Wake Turbulence Symposium, Seattle, Washington,
1-3 September 1970.
W. McCormick, J. L. Tangler, and H. E. Sherrieb, “Structure of Trailing
Vortices,” Journal of Aircraft, Vol. 5, No. 3 (May-June
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.
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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