A broad research program to identify maneuvering requirements for advanced fighters and the corresponding design criteria to aid in making critical design tradeoffs is being conducted under the NASA High-Angle-of-Attack Technology Program (HATP). As part of this activity, NASA and the U.S. Navy are conducting cooperative research to develop high-angle-of-attack control margin requirements. This paper will summarize the status of this program. Following some background information, the simulation study conducted to develop a set of preliminary guidelines for nose-down pitch control is reviewed, and the results of some very limited flight tests are described

Foster, John V.

Ogburn, Marilyn E.

English

NASA Technical Reports Server

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OU'ILINE
A broad research program to identify maneuvering requirements for
advanced fighters and the corresponding design criteria to aid in
making critical design tradeoffs is being conducted under the NASA
High-Angle-of-Attack Technology Program (HATP). As part of this
activity, NASA and the U.S. Navy are conducting cooperative research
to develop high-angle-of-attack control margin requirements. This
paper will summarize the status of this program. Following some
background information, the simulation study conducted to develop a
set of preliminary guidelines for nose-down pitch control is reviewed
and the results of some very limited flight tests are described.
295
296
CONTROL EFFECTIVENESS REQUIREMENTS FOR
MULTI-AXIS RELAXED STABILITY COMBAT AIRCRAFI"
A well-defined set of criteria exists and has been used for the design
of combat aircraft for performance considerations. These criteria
address the use of concepts such as relaxed stability in pitch and yaw
and reduced control surface size to enhance performance
characteristics. However, detailed, validated design criteria to define
minimum maneuvering requirements, particularly for flight at low
speeds and high angles of attack, do not currently exist. These
criteria are needed to aid in making the critical design tradeoffs
between performance and maneuvering requirements, which often
result in conflicting design characteristics. The use of advanced
propulsive and aerodynamic control effectors will aid in achieving
the control moments to meet enhanced maneuvering requirements;
however, these control concepts cannot be utilized most effectively
for making design tradeoffs until design criteria for maneuvering are
determined.
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KEY DESIGN ISSUE
The first element in the development of control margin requirements
is nose-down pitch control at high angles of attack. The key design
issue is the level of nose-down pitching moment required for tactical
maneuvering and for safety of flight. Guidelines are needed to help
the designer determine that, for his particular airplane, the Cm
characteristics illustrated by curve 1 are unacceptable whereas the
curve 2 characteristics are desirable.
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SCOPE OF ACTIVITY
A cooperative program was formed by NASA and the Navy to
address this design issue and is often referred to by the acronym
HANG (high-angle-of-attack nose-down guidelines). This chart shows
the overall outline of the activity. The first step involved analysis
and a simulation study which were used to develop a set of
preliminary guidelines. Flight testing for validation of these
guidelines has been initiated. The final output of this work will be a
set of flight-validated design criteria and specifications for flight test
demonstration.
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APPROACH TO DEVELOPMENT OF PRELIMINARY GUIDELINES
The set of preliminary guidelines was developed in two steps. The
first involved analysis of information from previous simulation and
flight tests of relaxed static stability aircraft to correlate aircraft
response in recoveries to available nose-down aerodynamic pitching
moment. The next step comprised a more detailed systematic study
on the Langley Differential Maneuvering Simulator.
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PRELIMINARY GUIDELINE BASED ON EXISTING AIRCRAFT
A key parameter for the definition of required nose-down pitching
moment is the value of Cm at the minimum point (Cm*), as illustrated
in the figure. Determination of the smallest value of Cm* that is
acceptable from a pitch recovery point of view is important in
making critical configuration design tradeoffs. Because Cm can be
related to a first-order definition of pitch acceleration by using the
aircraft inertia and geometry information, these characteristics were
examined for some existing aircraft and correlated with the known
high-(_ nose-down capabilities of these aircraft. The results were
used to generate the plot shown which defines a preliminary
guideline for Cm* based only on the airplane mass and geometry
characteristics. For a given configuration, the designer needs only to
calculate the value of (Iy/S_) and use the chart to determine the
minimum levels of Cm* required for "satisfactory" and
"unsatisfactory" nose-down control characteristics. Although this
result provides a useful, easy-to-apply guideline, particularly during
very preliminary design studies, it was felt that a more
comprehensive criterion that applies to more than just one point on
the Cm curve is needed. As a result, a systematic, parametric study
using piloted simulation was conducted as the first step in
developing such a design guide.
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DIFFERENTIAL MANEUVERING SIMULATOR
The Langley Differential Maneuvering Simulator (DMS) was used for
the simulation study. As illustrated in the drawing, the DMS is a
domed facility with extensive state-of-the-art features that make it a
very effective tool for air combat research and high-angle-of-attack
flight dynamics studies. These features include a visual scene
produced by a computer-generated imaging system, programmable
displays and force-feel systems, and artificial "g" cues. A key aspect
of the subject simulation study was the systematic variation of the
key parameters which define the nose-down pitching moment
capability. The parametric variations were anchored to an existing
comprehensive F-18 math model, and a specific evaluation
methodology was developed to determine the relative merits of the
pitch response as the parametric variations were made.
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PRELIMINARY DESIGN GUIDELINE
The results of the simulation evaluation showed clearly that the
pilots were evaluating the short-term pitch response. A simple
pushover maneuver starting from l g stabilized trim at high angles of
attack to recover to low angles of attack was used as the primary
evaluation maneuver. It was determined that two primary figures of
merit were needed because the pilots said that they judged two
response characteristics during the recovery: (1) the pitch
acceleration almost immediately following the forward stick input
and (2) the pitch rate buildup over the initial part of the recovery.
The two figures of merit that best correlated the pilots' ratings and
comments with the airplane response were the maximum pitch
acceleration achieved within one second of the initiation of forward
stick movement and the pitch rate at two seconds into the recovery.
The pilot ratings covered a range of values that indicated good to
poor response. Confidence levels are shown about the mean value
for several pilot ratings. The pitch acceleration results generally
agree well with the preliminary guideline discussed earlier that was
based on previous flight and simulation experience and with the
results from other related work. The current results have also been
checked using piloted simulations of several relaxed static stability
configurations besides the F-18, and full-scale flight validation has
been initiated.
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FLIGHT TESTS
The objectives of the flight tests are to validate and refine the test
methodology used in the development of the guidelines and the
numerical guideline values. A two-phase flight program is being
conducted. Phase I, a very limited study to validate the test
methodology using a Navy F-18 and the NASA HARV, has been
completed. The second phase, which is in progress, is a much more
detailed study to validate the guideline values, using the HARV with
thrust vectoring controls to provide a wide range of nose-down pitch
response.
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NATCFLIGHTTESTRF_ULTS
Flight tests of a Navy F-18 were conducted at the Patuxent River
Naval Air Test Center. Two Navy pilots performed and rated 32
pushover maneuvers. The center of gravity was varied using a fuel-
transfer e.g. control system. The flight program was very successful
in achieving its primary objective which was the verification of the
fundamental simulation methodology and validation of the key
figures of merit (_land q). As a very limited, preliminary check of
the guidelines developed in the simulation study, the results from
flight for the maneuvers with the most linear pitch acceleration
response (similar to the simulation responses) were compared with
the simulation data shown earlier. The plot shows fairly good
correlation for the danax in 1 second metric. A comparison of the
flight results with the preliminary guideline values for the initial
pitch acceleration response shows good agreement between flight
and simulation.
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NATC FLIGHT TEST RESULTS
Similarly, the correlation for the pitch rate in 2 seconds metric was
also fairly good. Again, it should be noted that this flight data base is
very small so that no conclusions can be drawn based on this data
alone. Definitive refinement and validation of the guidelines will be
accomplished in the second phase of the flight test program which is
currently in progress.
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SUMMARY OF FLIGHT TEST RESULTS TO DATE
The limited amount of flight testing that was completed in the first
phase of these tests satisfied the primary objective which was to
validate the simulation study methodology by confirming the
suitability of the pushover as the primary evaluation maneuver and
the utility of the rating scale. These preliminary flight tests also
provided much information that has been very valuable in the
preparation for further tests. The resuRs verified that the pilots'
opinions of the recoveries are based on the short-term pitch
response, and good agreement for the numerical response values was
obtained between the individual pilots' simulation and flight results
for maneuvers in which the character of the response was
comparable.
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TOTAL Cm REQUIREMENT
A method for designing the Cm versus angle of attack curve was
developed using the preliminary guideline values. The overall
design process is accomplished in four steps, the first three of which
require separate calculations of the Cm required at each angle of
attack. The first step, which satisfies the requirements for the
pushover maneuver, is described in a previous publication on this
work. The second and third steps involve the calculation of Cm
values based on the nose-down pitch control power needed to oppose
the inertia coupling generated during commanded roll maneuvers
and uneommanded roll/yaw motions. The final step is to select the
largest value of Cm computed for any of the first three steps at each
angle of attack.
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SUMMARY AND PLANS
In summary, the research activity described in this paper has
resulted in the development of preliminary nose-down pitch control
guidelines. The appropriate study methodologies were developed
and validated by limited flight test programs and extensive
simulation results have been obtained from which the guidelines
were derived. There is still much work to be done to reach the final
goal of a complete set of fully-validated design criteria and
specifications for demonstrating in flight that the criteria have been
met. The analysis of existing data will continue, but the main
activity will be to complete experiments in progress using the HARV
with thrust vectoring, primarily to validate the numerical guidelines.
Using the HARV with thrust vectoring controls enables the evaluation
of a wide range of nose-down response. The capability to vary the
level of pitch vectoring that is obtained for forward stick inputs
means that the character and the level of the pitch response during
recoveries from high angles of attack can be specified. A systematic
parametric variation of nose-down response is being made and a
large number and variety of maneuvers including the pushover
maneuver are being performed, rated, and analyzed. In addition, a
complementary effort is underway to develop similar design
guidelines for the roll/yaw axes.
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Development of high-angle-of-attack nose-down pitch control margin design guidelines for combat aircraft

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