F-16 Modular Common Inlet Duct Flight Test Support
As
the F-16 propulsion systems have evolved, the requirement to increase
the inlet throat and capture area for increased engine mass flow led
to the development of the Modular Common Inlet Duct (MCID) for the Block
30D aircraft. Initial flight testing with this inlet configuration revealed
deep stall behavior that was more oscillatory and more difficult to
recover from. Bihrle Applied Research had collected a substantial set
of static and dynamic wind tunnel test data for General Dynamics and
the US Air Force, with evidence of degradation in the dynamic characteristics
for this configuration. Following a flight test deep stall encounter
with a center line tank configuration that required over 30 pitch rock
cycles before recovery, Bihrle was tasked to review the flight results
and support configuration modifications to alleviate the deep stall
conditions.
During the review of the data, the MCID configuration effects were
added to the Bihrle in-house F-16 engineering simulation, and our engineers
were able to replicate the extended recovery deep stall observed in
flight. Review of the flight data revealed a number of issues that delayed
recovery. The oscillatory nature of the deep stall with this configuration
aggravated the ability of the pilot to synchronize his pitch rocking
inputs, as the roll oscillation phasing gave the illusion of a nose
down pitch break. Since the yaw rate limiting function of the flight
control system at high angles of attack had priority over the longitudinal
control, the yaw on the airplane during this oscillatory condition was
severely limiting the available nose down control. Following these insights,
the yaw rate limiting component of the flight control system was modified
in the engineering simulation to respond less immediately to the yaw
rate buildup, thus enabling greater pitch control. With this modification
in place, previously difficult recoveries were now immediate. This change
to the yaw rate limiting circuit was recommended to the Air Force, approved,
and successfully flight tested. The success of this modification on
the deep stall recoveries was sufficient to lead to the retrofitting
of all F-16 configurations.
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