liker
NASA
Technical
Paper

National Aeronautics and
Space Administration
Ofce of Management
Scientic and Technical
Information Program

3624

1996

The X-31A Quasi-Tailless
Flight Test Results

John T. Bosworth and P. C. Stoliker

Dryden Flight Research Center
Edwards, California
A quasi-tailless flight investigation was launched using the X-31A enhanced fighter maneuverability
airplane. In-flight simulations were used to assess the effect of partial to total vertical tail removal. The rud-
der control surface was used to cancel the stabilizing effects of the vertical tail, and yaw thrust vector com-
mands were used to restabilize and control the airplane. The quasi-tailless mode was flown supersonically
with gentle maneuvering and subsonically in precision approaches and ground attack profiles. Pilot ratings
and a full set of flight test measurements were recorded. This report describes the results obtained and em-
phasizes the lessons learned from the X-31A flight test experiment. Sensor-related issues and their impor-
tance to a quasi-tailless simulation and to ultimately controlling a directionally unstable vehicle are
assessed. The X-31A quasi-tailless flight test experiment showed that tailless and reduced tail fighter air-
craft are definitely feasible. When the capability is designed into the airplane from the beginning, the ben-
efits have the potential to outweigh the added complexity required.
1

ABSTRACT

A quasi-tailless flight investigation was launched using the X-31A enhanced fighter maneuverability air-
plane. In-flight simulations were used to assess the effect of partial to total vertical tail removal. The rudder con-
trol surface was used to cancel the stabilizing effects of the vertical tail, and yaw thrust vector commands were
used to restabilize and control the airplane. The quasi-tailless mode was flown supersonically with gentle maneu-
vering and subsonically in precision approaches and ground attack profiles. Pilot ratings and a full set of flight
test measurements were recorded. This report describes the results obtained and emphasizes the lessons learned
from the X-31A flight test experiment. Sensor-related issues and their importance to a quasi-tailless simulation
and to ultimately controlling a directionally unstable vehicle are assessed. The X-31A quasi-tailless flight test ex-
periment showed that tailless and reduced tail fighter aircraft are definitely feasible. When the capability is de-
signed into the airplane from the beginning, the benefits have the potential to outweigh the added complexity
required.

INTRODUCTION

Early aircraft designers realized the necessity of using a vertical tail to provide directionally stable airplane
designs. This practice has carried through to modern commercial and military aircraft designs, the majority of
which include vertical tails and rudder control surfaces. For transport aircraft, the need to control the large yawing
moments created in an engine failure condition dictates tail size. With the advent of thrust vectoring and
full-authority flight control systems, significantly reducing or eliminating the vertical tail became a viable design
option.
Potential advantages of a reduced tail size include decreased drag, reduced weight, and reduced structural
complexity. For military applications, reduced radar cross-section is an additional advantage. These advantages
must be balanced against the disadvantages of the added weight, complexity, and reliability requirements of a
thrust vector system. If sufficient thrust vectoring system reliability cannot be achieved, emergency systems may
also be required. In addition, because the thrust vector control power is proportional to the engine power setting,
some flight conditions that normally require low-power settings, such as landing, may require larger drag devices,
such as speed brakes, to maintain necessary higher power settings.
The X-31A enhanced fighter maneuverability airplane provided a unique opportunity to demonstrate the abil-
ity of a thrust vector system to provide the stabilizing and maneuvering moments equal to that of a vertical tail
and rudder (refs. 1-5). A quasi-tailless investigation was launched which used destabilizing feedbacks to the rud-
der for in-flight simulation of partial to total vertical tail removal. The rudder control surface was used to cancel
the stabilizing effects of the vertical tail, and yaw thrust vector deflections were used to restabilize and direction-
ally control the aircraft.
This report summarizes the observations and lessons learned from the X-31A quasi-tailless flight experiment.
The quasi-tailless mode was flown at one supersonic flight condition with high power settings and in a large sub-
sonic flight envelope with reduced power settings. Handling qualities assessments were made using a precision
approach and landing task and a simulated ground attack task. Pilot ratings and a full set of flight test measure-
ments were recorded during the experiment. These measurements included pilot inputs, aircraft response, and
control surface activity. Comparisons between the simulation-predicted results and flight test results are present-
ed. Emphasis is placed on areas where differences were observed.
2

NOMENCLATURE
Acronyms

AGL
above ground level
AOA
angle of attack
ARPA
Advanced Research Projects Agency
ATLAS
Adaptable Target Lighting Array System
CHR
CooperHarper rating
CIC
close in combat
DASA
Daimler-Benz Aerospace, Germany
EFM
enhanced fighter maneuverability
FMOD
Federal Ministry of Defense, Germany
FORTRAN
Formula translation
FQ
flying qualities
HARV
High Angle of Attack Research Vehicle
JAST
Joint Advanced Strike Technology
KIAS
knots indicated airspeed
NASA
National Aeronautics and Space Administration
PLA
power lever angle
QT
quasi-tailless
RI
Rockwell International, Downey, California
TC
test conductor
USAF
United States Air Force
USN
United States Navy

Symbols

state derivative matrix
control derivative matrix
rudder surface yaw control effectiveness derivative
thrust vector system yaw control effectiveness derivative
destab
destabilization

g


unit of acceleration, 32.174 ft/sec

2

K

destabilization feedback gain matrix

p

roll rate, deg/sec

r

yaw rate, deg/sec
u
control input vector
x
state vector
angle of sideslip, deg
differential flap position, deg
A
B
c
n R
c
n
TVV


df
3
rudder position, deg
thrust vector deflection, deg
angle of bank, deg

Subscripts and Superscripts
denotes time derivative
denotes perturbation quantities
1
matrix inverse or pseudoinverse

Sign Conventions

Angle of sideslip
Positive nose left
Differential flap
Positive right trailing-edge down (right-left)/2.0
Lateral acceleration
Positive out right wing
Lateral stick
Positive right roll
Roll rate
Positive right wing down
Rudder surface
Positive trailing-edge left
Yaw rate
Positive nose right
Yaw thrust vector command
Positive nose left

BACKGROUND

The X-31A program provided an opportunity to fly a quasi-tailless investigation. The availability of the aircraft
and program resources allowed for an effective demonstration with a minimum of overhead cost.

Program Description

The experimental X-31A airplane was designed for enhanced fighter maneuverability (EFM) especially in the
slow-speed flight environment (fig. 1). Two X-31A aircraft were built by Rockwell International (RI), Downey,
California, and Daimler-Benz Aerospace (DASA), Germany, using joint funding from the Advanced Research
Projects Agency (ARPA) and German Federal Ministry of Defense (FMOD). The initial program goals were rapid
demonstration of EFM technologies, investigation of EFM tactical exchange ratios, development of design require-
ments and a database for future fighter aircraft, and development and validation of low-cost prototype concepts.
Under the auspices of the International Test Organization, which is composed of representatives from ARPA,
FMOD, DASA, RI, United States Navy (USN), United States Air Force (USAF), and National Aeronautics and
Space Administration (NASA), poststall envelope expansion and close-in-combat (CIC) evaluations were per-
formed at the NASA Dryden Flight Research Center, Edwards, California. This flight test program successfully
demonstrated the ability of the airplane to stabilize and maneuver in a controlled fashion up to 70

°

angle of attack
(AOA) (refs. 3-5).
Upon completion of the initial program goals, an investigation was undertaken to demonstrate the ability of the
thrust vector system to replace some or all of the functions of a vertical tail. The Joint Advanced Strike Technology
(JAST) program funded a portion of the quasi-tailless flight test experiment. R TVV
4

Aircraft Description

The X-31A airplane is a single-seat fighter configuration with a takeoff gross weight of approximately
16,000 lb. A single GE-F404 engine (General Electric, Lynn, Massachusetts) with an uninstalled gross thrust of ap-
proximately 16,000 lb at sea level powers this airplane. The planform includes a delta wing and a relatively small
canard (fig. 2). The wing area, span, and mean aerodynamic chord are 226.3 ft

2

, 22.8 ft, and 12.4 ft, respectively.
The length is approximately 43.3 ft.
Figure 1. The X-31A in poststall flight.

Figure 2. The X-31A planview.
..
.
.
.
.
.
.
..
.
.
.
.
.
.
22.8 ft
(7.1 m)
43.3 ft
(13.2 m)
11.6 ft
(3.5 m)
14.6 ft
(4.5 m)
7.4 ft
(2.3 m)
Area, ft2
(
m2
)
Wing
Aspect ratio
226.3 (21.0)
2.3
3.2
1.2
23.6 (2.2)
37.6 (3.5)
Canard
Vertical
Surface dimensions
960226
Empty
Max
12,000 (5,450)
16,200 (