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Boeing T-45 Goshawk Inverted Spin/Spiral Mode and Hinge Moment Testing
The
Boeing (formerly McDonnell Douglas) T-45 Goshawk design was based on
the British Aerospace Hawk land-based aircraft with design modifications
that made the aircraft more suitable for carrier-based operations.
Bihrle's involvement in the T-45 began with an investigation of the aircraft's basic high angle-of-attack and spin characteristics. Flight-test experience with the British Hawk aircraft had indicated that the aircraft was very reluctant to spin and that attempts to intentionally spin
the aircraft usually resulted in a spiral with rapidly increasing airspeed. Bihrle's results indicated that the T-45 would exhibit similar characteristics. Flight test of the actual T-45 aircraft subsequently verified these predictions as well as an oscillatory-divergent nature that if
allowed to build would couple into a negative angle of attack post-stall gyration.
Flight
tests of the T-45 exhibited some unusual
characteristics in the inverted angle-of-attack region. These included
an inverted spiral mode where the pilot had difficulty controlling the
rudder. Also, unlike the upright modes, the inverted modes occurred
at large angles of sideslip. The inability of the T-45 pilot to deflect
the rudder as needed complicated the technique required to recover from
the inverted spin/spiral modes. The pilot was unable to deflect the
rudder against the spin as desired even while developing an average
400-lb load on the rudder pedal.
Bihrle, at the Navy's request, conducted an investigation on understanding the spin/spiral behavior and the implications of alternate control sequencing on spin entry and recovery as well as the feasibility of implementing a powered rudder. Wind tunnel tests were conducted to collect
the aerodynamic data necessary to extend the simulation database for an accurate representation of the aircraft's inverted characteristics, including the spin/spiral modes at large sideslip angles. Bihrle's simulation and spin mode analysis indicated that the application of full rudder
against this spin would lead to quick recovery. As a powered rudder actuator was considered to alleviate the controllability problem, McDonnell Douglas required hinge moment data for the rudder during conditions representative of the spiral mode. Bihrle conducted additional wind tunnel
tests to measure rudder hinge moments at inverted angles of attack for selected rudder deflections over a range of sideslip angles and rotation rates. A unique two- component hinge moment strain gauge balance was created for this effort. This hinge moment balance was mounted completely
within the model's vertical fin, avoiding airflow disturbance. The design also allowed the rudder to be set at any deflection angle. Rudder hinge moments were also obtained by integrating the vertical tail surface pressure measurements made by pressure tapping the vertical
tail including the rudder, which corroborated the hinge moment balance results.
The
rudder hinge moment data from these tests were used to improve the T-45
simulation, which previously contained minimal hinge moment modeling
at inverted angles of attack and sideslip, and was therefore unable
to properly model this rudder condition. In addition, the rudder hinge
moments acquired with a deflectable internal strain gauge balance and
from the integrated pressure data correlated well with flight data.
Based on these results, recommendations were made to provide a consistent
recovery technique for all upright and inverted T-45 spin/spiral modes
as well as the validity of a powered rudder for the aircraft.
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