Sabre Dance (aerodynamics)
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aerodynamics Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...
, pitch-up is an uncommanded nose-upwards rotation of an aircraft. It is an undesirable characteristic that has been observed mostly in experimental swept-wing aircraft at high subsonic Mach numbers or high angle of attack.


History

Pitch-up problems were first noticed on high-speed test aircraft with swept wings. It was a common problem on the
Douglas Skyrocket The Douglas D-558-2 Skyrocket (or D-558-II) is a rocket and jet-powered research supersonic aircraft built by the Douglas Aircraft Company for the United States Navy. On 20 November 1953, shortly before the (17 December) 50th anniversary of p ...
, which was used extensively to test the problem. Before the pitch-up phenomenon was well understood, it plagued all early swept-wing aircraft. In the
F-100 Super Sabre The North American F-100 Super Sabre is an American supersonic jet fighter aircraft that served with the United States Air Force (USAF) from 1954 to 1971 and with the Air National Guard (ANG) until 1979. The first of the Century Series of ...
it even got its own name, the Sabre dance. In aircraft with high-mounted tailplanes, like the
F-101 Voodoo The McDonnell F-101 Voodoo is a supersonic jet fighter which served the United States Air Force (USAF) and the Royal Canadian Air Force (RCAF). Initially designed by McDonnell Aircraft Corporation as a long-range bomber escort (known as a ''p ...
, recovery was especially difficult because the tailplane was placed directly in the wing wake during the pitch-up, causing
deep stall In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', p. 486. Aviation Supplies & Academics, 1997. This occurs when t ...
(although the T-tail was meant to prevent pitch-up from starting in the first place). Deployment of the braking parachute and a considerable height above the ground were essential for a chance at recovery.


Description

Wings generate pressure distributions on their upper and lower surfaces which produce a single force acting at a point known as the " center of pressure", or CoP, which is normally located between ⅓ and ½ of the way back from the
leading edge The leading edge of an airfoil surface such as a wing is its foremost edge and is therefore the part which first meets the oncoming air.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 305. Aviation Supplies & Academics, ...
. This upward and rearward leaning force is replaced by an equivalent pair of forces called lift and drag. The longitudinal position at which these forces act and the magnitude of the forces change with
angle of attack In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference line on a body (often the chord line of an airfoil) and the vector representing the relative motion between the body and the fluid through which it is m ...
. In addition a varying pitching moment exists for any force location other than the CoP. These changes lead to a requirement to
trim Trim or TRIM may refer to: Cutting * Cutting or trimming small pieces off something to remove them ** Book trimming, a stage of the publishing process ** Pruning, trimming as a form of pruning often used on trees Decoration * Trim (sewing), or ...
aircraft as they change their speed or power settings.Ion Paraschivoiu
"Subsonic Aerodynamics"
Presses inter Polytechnique, 2003, §1.9
Another major consideration for aircraft design is a vector addition of all of the weight terms of the parts of the aircraft, including the wing. This too can be reduced to a single weight term acting at some point along the longitudinal axis of the aircraft, the "
center of gravity In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weight function, weighted relative position (vector), position of the distributed mass sums to zero. Thi ...
", or CoG. If the wing is positioned so its CoP lies near CoG for the aircraft, in level flight the wing will lift the aircraft straight up. This reduces any net forces pitching the aircraft up or down, but for a number of reasons the two points are normally slightly separated and a small amount of force from the
flight control surfaces Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude. Development of an effective set of flight control surfaces was a critical advance in the development of aircraft. Ea ...
is used to balance this out. The same basic layout is desirable for an aircraft with a swept wing as well. On a conventional rectangular wing, the CoP meets the aircraft at the point on the chord running directly out from the root. While the same analysis will reveal a center of pressure point for a swept wing, its location may be considerably behind the leading edge ''measured at the root of the wing''. For highly swept planforms, the CoP may lie behind the
trailing edge The trailing edge of an aerodynamic surface such as a wing is its rear edge, where the airflow separated by the leading edge meets.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 521. Aviation Supplies & Academics, 1997. ...
of the wing root, requiring the wing to meet the aircraft at a seemingly far-forward location. In this case of a swept wing, changes to the CoP with angle of attack may be magnified.Malcolm Abzug & Eugene Larrabee, "Airplane Stability and Control", Cambridge University Press, 2005, p. 177 The introduction of swept wings took place during a move to more highly tapered designs as well. Although it had long been known that an elliptical planform is "perfect" from an
induced drag In aerodynamics, lift-induced drag, induced drag, vortex drag, or sometimes drag due to lift, is an aerodynamic drag force that occurs whenever a moving object redirects the airflow coming at it. This drag force occurs in airplanes due to wings or ...
standpoint, it was also noticed that a linear taper of the wing had much the same effect, while being lighter. Research during the war led to widespread use of taper, especially in the post-war era. However, it had been noticed early on that such designs had unfavourable stall characteristics; as the tips were more highly loaded in high angles of attack, they operated closer to their stall point. Although this effect was unfavourable in a conventional straight wing aircraft, on a swept-wing design it had unexpected and dangerous results. When the tips stall on a swept wing, the center of pressure, the average lift point for the wing as a whole, moves forward. This is because the section still generating considerable lift is further forward. This causes further nose-up force, increasing the angle of attack and causing more of the tip area to stall. This may lead to a chain reaction that causes violent nose-up pitching of the aircraft. This effect first noticed in the
Douglas D-558-2 Skyrocket The Douglas D-558-2 Skyrocket (or D-558-II) is a rocket and jet-powered research supersonic aircraft built by the Douglas Aircraft Company for the United States Navy. On 20 November 1953, shortly before the (17 December) 50th anniversary of p ...
in August 1949, when a 0.6 G turn suddenly increased out of control to 6 G. This was not entirely surprising; the effect had been seen earlier in
wind tunnel Wind tunnels are large tubes with air blowing through them which are used to replicate the interaction between air and an object flying through the air or moving along the ground. Researchers use wind tunnels to learn more about how an aircraft ...
simulations. These effects can be seen at any speed; in the Skyrocket they occurred primarily in the transonic (the Weil-Gray criteria) but with more highly swept and tapered planforms, like on the
North American F-100 Super Sabre The North American F-100 Super Sabre is an American supersonic jet fighter aircraft that served with the United States Air Force (USAF) from 1954 to 1971 and with the Air National Guard (ANG) until 1979. The first of the Century Series of ...
, the effect was common at low speeds as well (the Furlong-McHugh boundary), when the aircraft flew at higher angles of attack in order to maintain lift at low speeds.Kenneth Spreemann
"Design Guide For Pitch-Up Evaluation And Investigation At High Subsonic Speeds Of Possible Limitations Due To Wing-Aspect-Ratio Variations"
NASA ™ X-26, 1959, p. 5
In addition, swept wings tend to generate span wise flow of the
boundary layer In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces a no-slip boundary condi ...
, causing some of the airflow to move "sideways" along the wing. This occurs all along the wing, but as one moves towards the tip the sideways flow increases, as it includes both the contribution of the wing at that point, as well as span wise flow from points closer to the root. This effect takes time to build up, at higher speeds the span wise flow tends to be blown off the back of the wing before it has time to become serious. At lower speeds, however, this can lead to a considerable buildup of the
boundary layer In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface formed by the fluid flowing along the surface. The fluid's interaction with the wall induces a no-slip boundary condi ...
at the wing tip, adding to the problems noted above. Finally, while not directly related to the effects above, it was common during the early jet age to use
T-tail A T-tail is an empennage configuration in which the tailplane is mounted to the top of the fin. The arrangement looks like the capital letter T, hence the name. The T-tail differs from the standard configuration in which the tailplane is ...
designs in order to keep the aerodynamic surfaces well clear of the jet engine area. In this case it is possible for a pitch-up event to cause the turbulent air behind the wing to flow across the horizontal stabilizer, making it difficult or impossible to apply nose-down pressure to counteract the pitch-up. Aircraft with low-mounted tail surfaces did not suffer from this effect, and in fact improved their control authority as the wing's wake cleared the controls surfaces, flowing above it. This was not always enough to correct for the problem, however; the F-86 continued to suffer from pitch-up in spite of increasing nose-down pressure from the tail surfaces.


Mitigation

As the primary causes of the pitch-up problem are due to spanwise flow and more loading at the tips, measures to address these issues can eliminate the problem. In early designs these were typically "add-ons" to an otherwise conventional wing planform, but in modern designs this is part of the overall wing design and normally controlled via the existing
high-lift device In aircraft design and aerospace engineering, a high-lift device is a component or mechanism on an aircraft's wing that increases the amount of lift (force), lift produced by the wing. The device may be a fixed component, or a movable mechanism w ...
s. The first known attempt to address these problems took place on the platform where they were first noticed, the Douglas Skyrocket. This took the form of a series of
vortex generator A vortex generator (VG) is an aerodynamic device, consisting of a small vane usually attached to a lifting surface (or airfoil, such as an aircraft wing) or a rotor blade of a wind turbine.
s added to the outboard portions of the wing, breaking up the boundary layer. However, this was found to have almost no effect in practice. Nevertheless, a similar solution was attempted on the
Boeing B-47 Stratojet The Boeing B-47 Stratojet (Boeing company designation Model 450) is a retired American long-range, six-engined, turbojet-powered strategic bomber designed to fly at high subsonic speed and at high altitude to avoid enemy interceptor aircraft ...
where it proved considerably more effective. This may have been helped by the presence of the
podded engine A podded engine is a jet engine that has been built up and integrated in its nacelle. This may be done in a podding facility as part of an aircraft assembly process. The nacelle contains the engine, engine mounts and parts which are required to r ...
s, whose vertical mountings acted as barriers to span wise flow. More common solutions to the problem of spanwise flow is the use of a
wing fence Wing fences, also known as boundary layer fences and potential fences are fixed aerodynamic devices attached to aircraft wings. Often seen on swept-wing aircraft, wing fences are flat plates fixed to the upper surfaces parallel to the wing ch ...
or the related dogtooth notch on the leading edge of the wing. This disrupts the flow and re-directs it rearward, while also causing the buildup of stagnant air inboard to lower the stall point. This does have an effect on overall airflow on the wing, and is generally not used where the sweep is mild. To address the problems with spanwise loading, a wider variety of techniques have been used, including dedicated
slats Super Low Altitude Test Satellite (SLATS) or Tsubame was a JAXA satellite intended to demonstrate operations in very low Earth orbit (VLEO, below 200 km), using ion engines to counteract aerodynamic drag from the Earth's atmosphere which i ...
or flaps, the use of washout or automated control of the
aileron An aileron (French for "little wing" or "fin") is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll (or movement around ...
s. An unusual solution tried on the
XF-91 Thunderceptor The Republic XF-91 Thunderceptor (originally designated XP-91) is a mixed-propulsion prototype interceptor aircraft, developed by Republic Aviation. The aircraft would use a jet engine for most flight, and a cluster of four small rocket engines ...
prototype fighter was to give the wingtips a wider chord than the wing roots. The idea was to increase wingtip efficiency and cause the wing roots to stall first.
Angle of attack In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference line on a body (often the chord line of an airfoil) and the vector representing the relative motion between the body and the fluid through which it is m ...
sensors on the aircraft can also detect when the angle of attack approaches the attitude known to result in pitch-up and activate devices like the
stick shaker A stick shaker is a mechanical device designed to rapidly and noisily vibrate the control yoke (the "stick") of an aircraft, warning the flight crew that an imminent aerodynamic stall has been detected. It is typically present on the majority of ...
to warn the pilot, and the
stick pusher A stick pusher is a device installed in some fixed-wing aircraft to prevent the aircraft from entering an aerodynamic stall. Some large fixed-wing aircraft display poor post-stall handling characteristics or are vulnerable to deep stall. To preven ...
which overpowers the pilot and forces the nose of the aircraft down to a safer angle of attack. Twist or washout built into the wingtips can also alleviate pitch-up. In effect, the angle of attack at the wingtip becomes smaller than elsewhere on the wing, meaning that the inboard portions of the wing will stall first. A commonly used solution to pitch-up in modern combat aircraft is to use a control-canard. Another modern solution to pitch-up is the use of slats. When slats are extended they increase wing camber and increase maximum
lift coefficient In fluid dynamics, the lift coefficient () is a dimensionless quantity that relates the lift generated by a lifting body to the fluid density around the body, the fluid velocity and an associated reference area. A lifting body is a foil or a com ...
. Pitch-up is also possible in aircraft with forward-swept wings as used on the
Grumman X-29 The Grumman X-29 was an American experimental aircraft that tested a forward-swept wing, canard control surfaces, and other novel aircraft technologies. The X-29 was developed by Grumman, and the two built were flown by NASA and the United St ...
. With forward-swept wings the span wise flow is inboard, causing the wing root to stall before the wingtip. Although at first glance it would appear that this would cause pitch-''down'' problems, the extreme rear mounting of the wing means that when the root stalls the lift moves forward, towards the tips.


Sabre dance

When a swept wing starts to stall, the outermost portions tend to stall first. Since these portions are behind the center of pressure, the overall lift force moves forward, pitching the nose of the aircraft upwards. This leads to a higher
angle of attack In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference line on a body (often the chord line of an airfoil) and the vector representing the relative motion between the body and the fluid through which it is m ...
and causes more of the wing to stall, which exacerbates the problem. The pilot often loses control, with fatal results at low altitude because there was insufficient time for the pilot to regain control or eject before hitting the ground. A large number of aircraft were lost to this phenomenon during landing, which left aircraft tumbling onto the runway, often in flames. One of the most notorious incidents was the loss of F-100C-20-NA Super Sabre 54-1907 and its pilot during an attempted emergency landing at
Edwards AFB Edwards Air Force Base (AFB) is a United States Air Force installation in California. Most of the base sits in Kern County, but its eastern end is in San Bernardino County and a southern arm is in Los Angeles County. The hub of the base is Ed ...
,
California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...
on January 10, 1956. By chance, this particular incident was recorded in detail on
16 mm film 16 mm film is a historically popular and economical gauge of film. 16 mm refers to the width of the film (about inch); other common film gauges include 8 and 35 mm. It is generally used for non-theatrical (e.g., industrial, educ ...
by cameras set up to cover an unrelated test. The pilot fought desperately to regain control due to faulty landing technique, finally rolling and yawing to the right before striking the ground with the fuselage turned approximately 90 degrees to the line of flight. Anderson, 1993 states the F-100 was noticeably underpowered for its day and had very pronounced "backside" tendencies if airspeed was allowed to decay too much. The brand new F-100C was flown by Lt. Barty R. Brooks, a native of Martha, Oklahoma and a
Texas A&M Texas A&M University (Texas A&M, A&M, or TAMU) is a public, land-grant, research university in College Station, Texas. It was founded in 1876 and became the flagship institution of the Texas A&M University System in 1948. As of late 2021, T ...
graduate, of the 1708th Ferrying Wing, Detachment 12,
Kelly AFB Kelly Field (formerly Kelly Air Force Base) is a Joint-Use facility located in San Antonio, Texas. It was originally named after George E. M. Kelly, the first member of the U.S. military killed in the crash of an airplane he was piloting. In ...
,
Texas Texas (, ; Spanish language, Spanish: ''Texas'', ''Tejas'') is a state in the South Central United States, South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2 ...
. The aircraft was one of three being delivered from North American's
Palmdale Palmdale is a city in northern Los Angeles County in the U.S. state of California. The city lies in the Antelope Valley region of Southern California. The San Gabriel Mountains separate Palmdale from the Los Angeles Basin to the south. On Aug ...
plant to
George AFB George Air Force Base was a United States Air Force base located within the city limits, 8 miles northwest, of central Victorville, California, about 75 miles northeast of Los Angeles, California. Established by the United States Army Air Co ...
, California, but the nose gear pivot pin worked loose, allowing the wheel to swivel at random, so he diverted to Edwards, which had a longer runway.Deadly Sabre Dance
/ref> On approach, at a high angle of attack, the fighter exceeded its flight envelope, and, too far into stall condition, lost directional control with fatal results. These scenes were inserted in the movie '' The Hunters'', starring
Robert Mitchum Robert Charles Durman Mitchum (August 6, 1917 – July 1, 1997) was an American actor. He rose to prominence with an Academy Award nomination for the Best Supporting Actor for ''The Story of G.I. Joe'' (1945), followed by his starring in ...
and
Robert Wagner Robert John Wagner Jr. (born February 10, 1930) is an American actor of stage, screen, and television. He is known for starring in the television shows '' It Takes a Thief'' (1968–1970), ''Switch'' (1975–1978), and ''Hart to Hart'' (1979– ...
, in the movie ''
X-15 The North American X-15 is a hypersonic rocket-powered aircraft. It was operated by the United States Air Force and the National Aeronautics and Space Administration as part of the X-plane series of experimental aircraft. The X-15 set spee ...
'' with actor
Charles Bronson Charles Bronson (born Charles Dennis Buchinsky; November 3, 1921 – August 30, 2003) was an American actor. Known for his "granite features and brawny physique," he gained international fame for his starring roles in action, Western, and war ...
playing the pilot, and in the made for TV film ''Red Flag: The Ultimate Game'', although in ''The Hunters'' and in ''Red Flag: The Ultimate Game'', the aircraft supposedly represented were respectively an F-86 and an F-5E. The incident was also commemorated in the fighter pilot song "Give Me Operations" (set to the tune of the California Gold Rush song "What Was Your Name in the States?"): ::"Don't give me a One-Double-Oh ::To fight against friendly or foe ::That old Sabre Dance ::Made me crap in my pants ::Don't give me a One-Double-Oh."''The Unhymnal - Unofficial songbook of the Clemson University bands'', edit circa 1967, Clemson University, Clemson,
South Carolina )''Animis opibusque parati'' ( for, , Latin, Prepared in mind and resources, links=no) , anthem = " Carolina";" South Carolina On My Mind" , Former = Province of South Carolina , seat = Columbia , LargestCity = Charleston , LargestMetro = ...
.


See also

*
Mach tuck Mach tuck is an aerodynamic effect whereby the nose of an aircraft tends to pitch downward as the airflow around the wing reaches supersonic speeds. This diving tendency is also known as tuck under. The aircraft will first experience this effect ...
*
Wing twist Wing twist is an aerodynamic feature added to aircraft wings to adjust lift distribution along the wing. Often, the purpose of lift redistribution is to ensure that the wing tip is the last part of the wing surface to stall, for example when exec ...


References and Notes


Bibliography

*{{cite web, author=Loftin, LK, Jr., title=Quest for performance: The evolution of modern aircraft. NASA SP-468, url=http://www.hq.nasa.gov/pao/History/SP-468/cover.htm, accessdate=2006-04-22
Deadly Sabre Dance
Retrieved 2011-11-24 Aerodynamics Aviation risks