Upset Dynamics and Deep Stall of the NASA Generic T-Tail Transport Aircraft

Abstract

Loss of control in flight is the largest contributor to commercial aviation
accidents. Especially T-tailed aircraft are well known for their often unforgiving
stall characteristics, yet their behaviour under high angles of attack is still not
fully understood. The Generic T-Tail Transport (GTT) is a geometrical model of a
typical T-tailed, regional jet transport aircraft with rear-mounted engines. It was
defined by NASA to conduct research of flight dynamics under upset conditions. The
research presented in this thesis is based on the GTT model, which has been provided
to the University of Bristol by the NASA Langley Research Center.
A 3.7% scale model of the GTT aircraft was designed and used in experimental
investigations. One wind tunnel and two free-flight models, equipped with actuated
control surfaces, on-board instrumentation, and custom flight computers, were
manufactured in-house. Advanced composite construction techniques were applied to
fulfil the dynamic scaling requirements under the expected high structural loads.
A wide-envelope aerodynamic model was created from wind tunnel force and
moment measurements and compared against NASA data. In addition to the original
GTT geometry, the impact of high-lift devices, landing gear, and engine nacelle size
was also assessed. Tuft patterns were captured to document flow separation on the
wings, stabilisers, and fuselage. Further understanding of the aircraft’s
unconstrained stall behaviour was gained from free-flight testing. Control excitations
within the linear flight envelope and several stall tests were conducted to identify
analytical models.
During an impending stall, the GTT exhibits an unstable pitch-up tendency and a
kinematically coupled wing rock. Both phenomena are primarily driven by tip stalling
of the main wing. At high angle of attack, the main wing produces nonlinear
downwash characteristics and wing wake impingement at the horizontal. This causes
a reduction of control authority and can lead to unrecoverable deep stall conditions.
Once in a deep stall, the aircraft’s trajectory follows a steep spiralling descent with a
very limited amount of lateral controllability. Generally accepted stall recovery
procedures are still suitable under these conditions. It is recommended to use the
results from this work to improve flight training simulation models for increased pilot
awareness of reduced stability characteristics and unintuitive instrumentation cues
under upset conditions.


Date of Award	17 Jun 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Mark H Lowenberg (Supervisor) & Simon A Neild (Supervisor)