|
Background
As aircraft configurations
continued to advance into the 1950s, designers
began to recognize the potential advantages of
“T-tail” configurations, wherein the
horizontal tail was moved to the top of the vertical
tail for enhanced efficiency and reduced weight.
By arranging the vertical and horizontal tails
in a T, designers could locate the horizontal tail
in a relatively benign flow field. In this location,
the flow downwash from the wing would not reduce
the stabilizing effects of the horizontal tail
at the relatively low angles of attack associated
with cruise conditions. This advantage was also
obtained during the landing approach, when the
downwash effects became even stronger because of
the deflected wing trailing-edge flaps. In addition
to placing the horizontal tail in a region of less
downwash, the T-tail position provides more tail
length (with a swept vertical tail); thereby less
tail area is used for the required tail contribution
to stability. Also, the endplating effect on the
vertical tail makes the vertical tail more effective
and permits a reduction in vertical fin size. As
a result of the increased efficiency of the horizontal
and vertical tails, the surface areas could be
reduced relative to a tail size for the conventional
low-tail configuration and thereby result in a
significant weight savings.
Wake flow patterns before
and after entry into deep-stall
condition. Note impingement of low-energy stalled
wake
on horizontal tail in poststall, deep-stall situation.
A British aircraft BAC 111,
similar to this, experienced a fatal deep-stall
accident.
In the early 1960s, four new
T-tail jet transports (the de Havilland Trident,
the Boeing 727, the BAC 111, and the McDonnell
Douglas DC-9) emerged within the highly competitive
transport market. These aircraft shared many common
configuration features in addition to the T-tail
arrangement: aft-fuselage-mounted engines and all-moving
(variable-incidence) horizontal tails, for instance.
In Britain, the de Havilland Aircraft Company,
Ltd. (who later merged with Hawker Siddeley Aviation,
Ltd.), developed the Trident transport, which first
flew on January 9, 1962. In the United States,
Boeing brought forth its new model, the 727, which
first flew on February 9, 1963. The BAC 111 first
flew later that year on August 20, 1963. The Douglas
DC-9 provided additional competition with a first
flight that occurred a few years later, in February
1965.
The BAC 111 suffered a major
setback during early flight testing of the prototype
on October 22, 1963; the aircraft was destroyed
and crew members lost their lives in a fatal accident
during tests to evaluate the stall characteristics
of the configuration. The accident investigation
board conducted an exhaustive investigation of
the aerodynamic, stability, and control characteristics
of the BAC 111 and concluded that the cause of
the accident was an unrecoverable deep-stall phenomenon,
which was precipitated by the geometric and aerodynamic
characteristics of the configuration. In particular,
wind-tunnel tests indicated that at high angles
of attack near and above those associated with
wing stall, the low-energy wakes of the stalled
wing and fuselage-mounted engine nacelles impinged
on the horizontal tail and significantly reduced
its stabilizing effect. Also, the low-energy wakes
severely reduced the effectiveness of the horizontal
tail as a longitudinal control. These characteristics
manifest themselves as an insidious poststall condition
in which the angle of attack of the aircraft would
increase to very large values (in excess of 40∞)
in response to the loss of stability, and the pilot
would be unable to recover from the condition because
of the loss of horizontal tail control effectiveness.
During this entire sequence, the attitude of the
aircraft fuselage would vary over a relatively
small angular range, and the uncontrollable aircraft
would descend steeply in an almost horizontal,
wings-level attitude with a high rate of descent
to an unsurvivable crash.
The BAC 111 design was subsequently
modified to incorporate a stick pusher, modified
leading-edge camber, and powered elevator controls
to prevent the aircraft from entering high-angle-of-attack
conditions. As news of the BAC 111 accident and
its causal factor spread throughout the technical
community, other design teams raced to examine
the characteristics of their own T-tail aircraft.
For example, Douglas immediately returned to the
wind tunnel and conducted additional testing for
the DC-9, which had not entered flight testing.
Douglas had followed conventional wisdom regarding
preliminary assessments of stall characteristics
of conventional configurations during the DC-9
development program. The transport industry approach
to wind-tunnel testing did not include high-angle-of-attack
testing much beyond stall, and few studies of very-high-angle-of-attack
characteristics had been conducted. However, when
Douglas engineers examined the newly acquired high-angle-of-attack
data, they found that the baseline DC-9 configuration
would exhibit locked-in deep-stall characteristics
similar to those exhibited by the baseline BAC
111. Following in-depth analysis of the aerodynamic
data, Douglas designed an under-wing leading-edge
fence (which they named a “vortillon”
which is short for vortex generating pylon) that
provided additional flow energy at the tail for
nose-down recovery at and slightly above the stall
angle of attack. In addition to analyzing wind-tunnel
data, Douglas also conducted some rudimentary piloted
flight simulations before deciding on final modifications
for the DC-9. The final modifications developed
to prevent the DC-9 from entering a dangerous deep
stall included the vortillons (which assisted in
immediate poststall recovery, but had little effect
at the deep-stall condition), an increase in the
span of the original horizontal tail, a stick shaker,
visual and aural stall warnings, and a standby
power system that provided full nose-down elevator
capability for deep-stall recovery. (The original
aerodynamic tab system was not capable of providing
sufficient elevator angle at very high angles of
attack.) These modifications, which were incorporated
prior to the first flight of the DC-9 on February
25, 1965, proved effective in preventing deep stall
for the DC-9 throughout its service life.
Hawker Siddeley Trident
jet transport.
Even after the BAC 111 experience
and the international concern and research activities
it stimulated, the deep-stall phenomenon continued
to cause problems in civil aviation. For example,
deep stall was found to be responsible for crashes
experienced with the Hawker Siddeley Trident transport.
On June 3, 1966, one of the first production Trident
aircraft crashed during its first flight as a result
of entering a deep-stall condition, with all four
crew members killed. The aircraft was carrying
out the first of a series of production test flights
to qualify for a Series Certificate of Airworthiness.
After completing a large part of the required tests,
the stall tests were begun. Three approaches to
stall were made to check the stall warning and
stall recovery systems. The fourth stall test was
made at an altitude of 11,600 ft in the landing
configuration and with the stall warning and recovery
systems inoperative. The Trident entered a deep
stall with the nose going up to a 30∞–40∞
attitude. The aircraft turned to the left, the
right wing dropped, and the plane went into a flat
spin to the right. The investigation board concluded:
“During a stalling test, decisive recovery
action was delayed too long to prevent the aircraft
from entering a superstall (deep stall) from which
recovery was not possible.” Later, on June
18, 1972, another Trident entered a deep stall
immediately after takeoff from Heathrow airport
in severe weather and crashed, with a loss of 118
lives.
Langley Research and Development
Activities
Immediately following the
BAC 111 crash, Langley responded by mapping out
a research program directed at understanding the
physical phenomena and factors responsible for
the deep-stall problem. Langley’s Edward
C. Polhamus visited England and was briefed on
the BAC crash and analysis data. Upon his return,
the staff of the Langley 7- by 10-Foot High-Speed
Tunnel formulated and conducted a generic research
program with systematic variations of T-tail configurations
to provide industry with appropriate design guidelines,
analysis procedures, and wind-tunnel test techniques
to avoid the potential for unacceptable deep-stall
behavior for T-tail aircraft. Under the leadership
of Robert T. Taylor and Edward J. Ray, a parametric
wind-tunnel study was immediately begun with generic
configurations that included a range of some of
the more critical configuration features, such
as the relative locations of the engine nacelles
and the horizontal tail. The data resulting from
the studies served not only to provide insight
into the causes and cures of the problem, but the
method of approach and analysis techniques provided
designers of future aircraft with a general understanding
and sensitivity to the deep-stall problem.
The wind-tunnel testing at
Langley was augmented by extensive piloted simulator
studies at the Langley and the Ames Research Centers
to establish potential pilot recovery procedures,
recognition cues of the deep stall, and assessments
of various modifications designed to limit angle-of-attack
excursions during stall testing. At Langley, the
simulator and analytical studies were led by Martin
T. Moul, Lindsey J. Lina, and Raymond C. Montgomery.
With a representative DC-9 cockpit, the researchers
were able to define levels of satisfactory recovery
and an index for aerodynamic control design, as
well as give recommendations for deep-stall recovery
criteria.
Applications
The Langley researchers dedicated
themselves to provide timely, valuable dissemination
of the results to all sectors of the civil aircraft
industry. In addition to active interchanges with
interested industry representatives, the results
of the studies were highlighted at several national
symposia in 1965. With a rapid growth in the popularity
of the aft-mounted engines and T-tail arrangement
incorporated by both large jet transports as well
as business jets, the data and procedures generated
in the Langley program have had a significant impact
on industry’s awareness of the potential
danger, and industry has adopted a general approach
to design and testing for stall and poststall characteristics.
Armed with this information, industry has been
able to design aerodynamic and flight control concepts
to prevent the occurrence of deep stalls. Because
this pioneering work was conducted with representative
generic configurations, it provided a fundamental
explanation of the phenomenon and general sensitivity
and approach to avoid this catastrophic problem.
Collectively, the results of this research provided
an intrinsic tool for easy understanding and direct
application throughout industry.
Cessna Citation, a current
T-tail aircraft, designed to avoid potential for
deep stall.
|