NASA Technical Memorandum 4800

Supersonic Flying Qualities

Experience Using

the SR-71

Timothy H. Cox

Dryden Flight Research Center

Edwards, California

Dante Jackson

Analytical Services and Material, Inc.

Edwards, California

National Aeronautics and

Space Administration

Office of Management

Scientific and Technical

Information Program

1997

SUPERSONIC FLYING QUALITIES EXPERIENCE USING THE SR-71

Timothy H. Cox

NASA Dryden Flight Research Center

Edwards, California

Dante Jackson

AS&M

Edwards, California

Abstract CH

g

Approximately 25 years ago NASA Dryden Hight

Research Center, Edwards, California, initiated the IVSI

evaluation of supersonic handling qualities issues using KEAS

the XB-70 and the YF-12. Comparison of pilot

comments and ratings with some of the classical Let

handling qualities criteria for transport aircraft provided NASA

information on the usefulness of these criteria and insight

into supersonic flying qualities issues. A second research

s

study has recently been completed which again

addressed supersonic flying qualities issues through SAS

evaluations of the SR-71 in flight at Mach 3. Additional 1To2

insight into supersonic flying qualities issues was

obtained through pilot ratings and comments. A

These ratings were compared with existing military 0

specifications and proposed criteria for the High Speed

Civil Transport. This paper investigates the disparity 0c

between pilot comments and the Neal/Smith criteria xp

through a modification of the technique using vertical

speed at the pilot station. The paper specifically t_

addresses the pilot ability to control flightpath and pitch t_2tots0.

attitude in supersonic flight and pilot displays typical of

supersonic maneuvering.

Nomenclature

O')bwgain

fd'JbWphas¢

ADI attitude director indicator

bw bandwidth O)bw.t

CAP control anticipation parameter ¢Obw0

c.g. center of gravity tOsp

(0180o

Aerospace Engineer.

' Aerospace Engineer.

Copyright © 1997 by the American Institute of Aeronautics and

Astronautics, Inc, No copyright is asserted in the United States under

Title 17, U.S. Code. The U.S. Government has a royalty-free license to

exercise all rights under the copyright claimed herein for Governmental

purposes. All other rights are reserved by the copyright owner.

Good

Cooper-Harper rating

acceleration due to gravity

instantaneous vertical speed indicator

knots equivalent airspeed

lift curve slope of the aircraft, ibf

National Aeronautics and Space

Administration

LaPlace operator

stability augmentation system

high-frequency pitch attitude zero, rad/sec

the change in a parameter

pitch attitude, deg

pitch attitude command, deg

bandwidth time delay parameter, sec

phase angle, deg

phase angle at twice the phase crossover

frequency, rad/sec

gain-limited pitch attitude bandwidth

parameter, rad/sec

phase-limited pitch attitude bandwidth

parameter, rad/sec

flightpath bandwidth parameter, rad/sec

pitch attitude bandwidth parameter, rad/sec

short period frequency, rad/sec

phase crossover frequency, rad/sec

Introduction

handling qualities are essential for aircraft

performance and can be predicted during the design

process by analytic means. Handling qualities criteria

used to evaluate aircraft designs are defined by empirical

1

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data obtained from previous aircraft. A problem exists,

however, when an aircraft flies in a new flight regime.

Airplanes cruising at speeds greater than Mach 2 or

beyond are examples of this problem.

Flying qualities criteria do not address some of the

unique characteristics of high-speed flight because they

are based primarily on subsonic data. For example, many

of the current criteria assume that good flightpath

response follows from good pitch attitude response;

however for the high-speed case, this assumption may

not be valid. As Mach number increases, the lift curve

slope of an aircraft (Lct) decreases proportionally,

thereby increasing the lag between flightpath and pitch

attitude response. If this lag characteristic is too large,

the pilot's ability to control flightpath will be impaired.

Another unique characteristic of high-speed flight is

the decrease in pitch attitude change to achieve a rate of

climb. This characteristic, due primarily to the increased

velocity, implies that as the speed increases the pilot

must maintain precise control of pitch attitude to

establish the desired altitude response. Unless accurate

pitch attitude or rate-of-climb information is fed back to

the pilot, this characteristic could potentially cause

the pilot to overcontrol the aircraft. This increased

sensitivity to pitch attitude control in high-speed flight is

not as prevalent in subsonic flight. Nevertheless, flying

qualities criteria of pitch attitude are based on subsonic

data.

Approximately 25 years ago NASA Dryden Hight

Research Center researchers gained insight into these

issues by applying MIL-STD-8785B criteria I to YF-12

and XB-70 data. The longitudinal tasks for the YF-12

and XB-70 transport class aircraft, in up and away flight,

included precise flightpath tracking without gross

maneuvering, which was considered category C flight.

The researchers showed for both the YF-122,3'4 and the

XB-702,5 positive correlation between category C

criterion on the control anticipation parameter (CAP)

and pilot comments and ratings. The researchers also

indicated the requirements for short period damping may

be relaxed although these results were not considered

conclusive. 4.5 Researchers also gained insight into

unique issues of supersonic cruise flight, such as the

usefulness of an inertial vertical speed display 2.3 and

the problems associated with unstable long period

dynamics. 2,6

Recently a second study was completed which extends

the research of supersonic flying qualities issues

conducted by the XB-70 and YF-12 programs through

evaluations of the SR-71 aircraft in flight at Mach 3. Data

includes pilot comments and ratings of the SR-71 flown

with a well-defined vertical altitude plane change

maneuver. Data from this study were used to assess the

applicability of handling qualities criteria, especially

those used in the design of the High Speed Civil

Transport program. 7

A comparison of the pilot's ability to directly control

flightpath and pitch attitude in supersonic f/ight and an

evaluation of the vertical speed display are specifically

addressed in this report. The Neal/Smith criteria, 8

bandwidth criteria on pitch attitude, 8 and bandwidth

criteria on flightpath 9 are used to evaluate pilot control

of flightpath and pitch attitude. Throughout the analysis,

results of these criteria are compared with the piloted

evaluations. In all cases, criteria for category C flight

were assumed to be applicable to the high-speed tasks.

Aircraft Description

The SR-71 aircraft (fig. 1) is a twin engine, delta-wing

airplane designed to cruise at a speed of Mach 3.2 and to

altitudes above 80,000 ft. The SR-71 aircraft is powered

by two Pratt & Whitney (West Palm Beach, Florida) J-58

afterburner engines with axisymmetric, variablegeometry, mixed compression inlets. Centerbody spikes

and bypass doors located on the forward part of the

nacelle are automatically modulated to control the

oblique and normal shock positioning that is associated

with flying at high supersonic speeds. Data gathered in

this report occurred with doors and inlets in this

automatic configuration.

The majority of the cockpit contains conventional

instrumentation. Some of the main cockpit instruments

used during this evaluation include a pressure-driven

instantaneous vertical speed indicator (IVSI) and a triple

display indicator that shows altitude, equivalent

airspeed, and Mach number in a digital format. The IVSI

is a circular gauge with a needle indicating vertical

speed to a resolution of I00 ft/min. The resolutions of

the triple display indicator parameters are 50 ft, 1 kn,

and Mach 0.01, respectively. Because a lag in the

response of the IVSI exists at high altitude, a horizontal

needle displaying inertial vertical speed located on the

attitude director indicator (ADI) provides a reference for

climb and descent rates. This gives SR-71 pilots a more

precise and reliable vertical speed indicator than the

IVSI.

Wing trailing-edge elevons are used symmetrically as

elevators and differentially as ailerons to provide

longitudinal and lateral control, while twin all-movable

vertical tails supply directional control. The pilot

2

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Figure 1. SR-71 aircraft.

EC94 42883-04

controls consist of a conventional stick for pitch and roll

inputs and rudder pedals for yaw inputs. The SR-71

aircraft has a conventional response with angle-of-attack

and normal acceleration changes commanded by the

pitch stick. The controls are irreversible and fully

powered by two independent 3000 lb/in 2 hydraulic

systems that operate actuating cylinders at each control

surface.

The SR-71 flight control system provides a stability

augmentation system (SAS) to increase damping about

all three axes. This is accomplished with conventional

feedback of roll rate, pitch rate, and yaw rate. In addition,

lateral acceleration is used in the yaw axis to reduce the

severity of engine unstarts.

An autopilot is available to reduce the workload

involved in flying the SR-71 aircraft. The autopilot

includes attitude hold (in pitch, roll, or both), Mach

number hold, and knots equivalent airspeed (KEAS)

hold. Normal aircraft maneuvering is executed by pitch

and roll attitude inputs through thumbwheels, while

acceleration and deceleration to and from Mach 3 are

performed with KEAS hold engaged on the autopilot.

The autopilot mode is used routinely in the climb,

cruise, and descent portions of the flights. However for

the handling qualities evaluations, all autopilot modes

were disengaged, and maneuvers were performed

manually with the stick.

Maneuver Description and Pilot Evaluations

Three maneuvers were flown at Mach 3 to evaluate the

handling qualities characteristics of the SR-71 aircraft: a

steady level turn, an ascending turn, and a vertical plane

altitude change. These maneuvers were considered to be

3

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typicalof maneuverstobe flown by a large supersonic

transport ah'craft. Reference 7 describes each maneuver

and documents pilot comments and ratings. For the

purpose of this paper, only the description and evaluation

of the vertical plane altitude change is considered.

Two pilots flew the maneuver a total of five times. Pilot

ratings and comments evaluating each maneuver using

predefined adequate and desired performance margins

were collected immediately after performing each task

and during postflight briefings.

Maneuver Description

The vertical plane altitude change at constant KEAS

entailed a wing level pullup to capture a 2000-ft altitude

increment at a climb rate of 1000 ft/min. Once the target

altitude was established, it was to be held for an

additional 10 see. Constant airspeed was to be

maintained throughout the maneuver. The pilots

evaluated this maneuver with two variations of feedback

displays: one using the IVSI and the other using the

inertial vertical speed on the ADI.

Adequate margins for the maneuver were +300 ft

deviation from target altitude and + 10 KEAS deviation

from target airspeed. Desired margins for this maneuver

were +100 ft deviation from target altitude and

+5 KEAS deviation from target airspeed.

Summary of Pilot Evaluations

Pilots' comments using the IVSI and the inertially

derived vertical speed are summarized below for the

vertical plane altitude change. Figure 2 presents the

Cooper-Harper (CH) ratings for pilots A and B.

IVSI

Flying the maneuver with the IVSI as a vertical speed

indicator made establishing and maintaining the desired

rate of climb very difficult. This is caused by sluggish

initial response and an excessive delay between the stick

input and a reaction in the IVSI gauge. The altitude

change was able to be performed but not without high

concentration and some loss of performance. The ability

of the pilot to hold airspeed was decreased because of the

excursions in rates of climb. These problems warranted

ratings of CH = 5-7 (levels 2-3).

Inertially Derived Vertical Speed

Flying the maneuver using the inertial derived vertical

speed for feedback made achieving desired performance

relatively easy. The desired rate of climb was easy to

Cooper41arper

10

9 --

8

7

6

5

4

3

2

1

Pilot

[] A

_ e

Level 3

[]

<_] Level 2

I

IVSI

<I]

Level 1

I I

Inerthllly derived

vertical speed

970372

Figure 2. Pilot ratings for the vertical plane altitude

change maneuver.

establish. As long as enough range of throttle motion in

afterburner was available, the airspeed was easily

maintained as well. The only difficulty arose when

searching to establish rate of climb and when leading the

aircraft as it approached the target altitude. This minor

compensation required in establishing flightpath added

to the basic concentration necessary to fly this airplane

and warranted CH = 3 (level 1).

Pilot Display of Vertical Speed

This section presents insight on the pilot display of

venical speed necessary for typical maneuvering in highspeed flight. As was mentioned earlier, the vertical plane

altitude change maneuver was flown using two types of

vertical speed indicators as feedback to the pilot: a

pressure-driven one (IVSI) and an inertially derived one.

Figure 3 compares the altitude time histories of two

2000

A altitude

from initial 1000

condition,

ft

IVSl

500 f_min 7 _ Inertial

/

,

50 10O 150 20O

Time,llec

970373

Figure 3. Comparison of the vertical plane altitude

change maneuver using the IVSI and an inertiaily

derived vertical speed indicator.

4

American Institute of Aeronautics and Astronautics
vertical plane altitude change maneuvers. One maneuver

was flown using the IVSI and the other was flown with

the inertially derived vertical speed. This figure plots the

increment in altitude from the initial altitude for each of

the maneuvers to provide a common reference point for

comparison.

It is clear from observing the maneuver flown with the

IVSI that the pilot was unable to establish the correct rate

of climb (1000 ft/min). Instead, the pilot overshot rate of

climb by a factor of 3 and then overcompensated to

500 ft/min. By reducing the rate of climb to 500 ft/min,

the pilot was able to hit the target altitude within the

desired performance of 100 ft. The pilot comment on this

maneuver was "rate of climb [is] all over the place.

Could not keep rate of climb anywhere near where I

wanted it." The corresponding pilot rating was CH = 5.

The difficulty in performing the maneuver derived from

the lags in the pressure-driven IVSI. The static pressure

decreases which are associated with high altitude make

changes in altitude difficult to measure. Thus, the pilot

tends to overdrive rate of climb. Figure 4 shows the

relative amounts of lag contributed by the IVSI and the

flightpath response lag, l/T02, as a function of

altitude. 2At an altitude of 70,000 ft, nearly 12 sec of lag

is present, one-half of which is caused by the IVSI. One

pilot concluded that the "vertical velocity indication has

a very long lag, and the pilot cannot use it as a feedback

variable."

In contrast, the ability of the pilot to establish the

1000 ft/min rate of climb was greatly improved when the

maneuver was flown with the inertially derived vertical

speed (fig. 3). Although the pilot misjudged the target

altitude by 500 ft, he was able to establish the desired

altitude precisely. Pilot comments indicated that this

maneuver was "to some extent a nonevent. You establish

the 1000 ft/min and then sort of nod off. You establish a

new trim and then be alert enough to see when you want

to start to level off. I held 75,500 right on at the level off.

This is easy to do." The pilot rating for this maneuver

was much improved also, CH = 3 (level 1). The

conclusion is that to accurately control flightpath the

pilot must have an inertially derived vertical speed

indication.

Pilot Control of Pitch Attitude and Flightpath

This section presents insight into whether it is more

appropriate for the pilot to control pitch attitude or

flightpath for maneuvering typical of high-speed flight.

Pilots typically control flightpath by commanding a pitch

attitude change and then waiting for the desired

flightpath to develop. This technique applies to normal,

Tlmelag,

8ec

16

14

12

10

8

6

4

2

0

50,000 55,000 60,000 65,000 70,000 75,000 80,000

Altitude, ft 97o374

Figure 4. Relative time lag comparison of flightpath response and altimeter. 2

Altimeter

lag

FIIghtpath

response

lag

5

American Institute of Aeronautics and Astronautics
subsonic flight when the pilot uses pitch attitude as the

feedback parameter either through visual cues obtained

by looking out the canopy window or through the ADI.

For conventional aircraft in subsonic flight, this

technique is generally successful. On the other hand,

how does this apply to high-speed flight, where the

increase in lag of the flightpath response may make this

technique unusable? In such a case, direct pilot control of

flightpath may be more appropriate. To directly control

flightpath, the pilot must use either flightpath or vertical

speed as the feedback parameter.

Several analysis techniques are used to evaluate the

pilots' ability to control pitch attitude and flightpath:

pitch bandwidth analysis, pitch bandwidth as a function

of flightpath bandwidth analysis, frequency response

analysis, and Neal/Smith analysis. To evaluate the

analysis techniques, the results from criteria are

compared with the ratings and comments from the

maneuvers using the inertially derived vertical speed

indication.

Reproduced YF-12 data were used to supplement the

SR-71 handling qualities data for some of the analysis.

Enough information existed in the reference 3 report to

extract the test conditions of the YF-12 data. Because of

the similarity of the YF-12 and SR-71 aircraft in the

longitudinal axis, a flight-validated SR-71 linear

simulation was used to model the YF- 12 data.

P.ilr,h.l_algi_i_a

To analyze pitch attitude control, the pitch bandwidth

criterion as defined in MIL-STD-17978 was evaluated

with SR-71 and reproduced YF- 12 data. The gain-limited

bandwidth, defined as the frequency at the magnitude

which is 6 dB above the magnitude at the phase

crossover frequency, and phase limited bandwidth,

defined as the frequency where a 45 ° phase margin

exists, were calculated from a pitch attitude from stick

deflection frequency response. The lesser of the two

frequencies was considered the bandwidth frequency.

This criterion places limits on the bandwidth frequency

as a function of the time delay, which is estimated from

the phase at twice the phase crossover frequency:

(1)

The calculation of the bandwidth frequency for SR-71

and YF-12 data with the SAS turned on proved

straightforward. However, the YF-12 data included four

test points where the SAS was turned off. A typical

example of the pitch attitude bandwidth calculation for

low damped, SAS turned off, YF-12 data is shown in

figure 5. The calculation of the phase bandwidth value,

the frequency where the phase is -135 °, is

straightforward. However, the calculation of the gain

20--

0

Magnitude, - 20 --

dB

_40

-60

(Llbw,gain = 1.8-_

(_ow,gain = 0.8-_

_- (ObW, gain = 0.19

I I I I II II

-lOO

- 20o

0 h

0 = - 135 f

=14 I

(_bw, phase I" -_ I IIII t

.5 .6.7.8.9 14) 2

Frequency,rad/aec

6 dB

I I I I llll

_- Ol 80°

_ = - 180

I I I I I L l llll

-300.1 .2 .3 .4 3 4 5 6 7 8 910

970375

Figure 5. Example pitch attitude bandwidth calculation typical of a low damped YF- 12 test point.

6

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bandwidth value is less obvious. Applying the definition

of the gain-limited bandwidth to the data in figure 5

results in three possible gain bandwidth values: 1.8, 0.8,

and 0.19 rad/sec. Note that a slightly increased phase

crossover frequency, (o180o, would prevent this

phenomenon from occurring. If the value of 1.8 rad/sec

were considered the gain bandwidth value, then the

phase bandwidth value would be compared to the

criterion since it is less than 1.8 rad/sec. However, if

either the 0.8 or 0.19 rad/sec values are the appropriate

gain bandwidth value, then these values would be

compared to the criterion because they are less than the

phase bandwidth value.

This phenomenon results because of the large, shelflike characteristic created by the significant difference

between 1To2 and the short period frequency as well

as the low short period damping. The low short period

damping characteristic produces the three possible gain

bandwidth values. Standard procedure for the

application of this criterion would be to choose the lesser

value, 0.19 rad/sec, as the gain bandwidth. The rationale

is that if the pilot tries to close the loop at the higher gain

bandwidth values a tendency to oscillate will occur

because of the lightly damped peak. The large, shelf-like

characteristic produces a wide separation between the

three gain bandwidth values. The existence of a large

shelf is generally an indication of poor handling

qualities, because the gain margin is very sensitive to

slight changes in phase. 8The results of the analysis using

the lowest gain bandwidth (represented by 0.19 rad/sec,

fig. 5) and the phase bandwidth are compared in the

following discussion.

Figure 6 shows the pitch bandwidth results. The pilot

ratings for each point are plotted next to the point. The

analysis indicates that good pilot control of pitch attitude

is possible with the SAS turned on. The level 1 pilot

ratings of these two points correlate with the analysis.

When the SAS is turned off and the damping decreases,

pitch bandwidth decreases. In addition, pilot control of

pitch attitude deteriorates. The amount of deterioration

in the predicted pilot control of pitch attitude depends on

whether the gain or phase-limited bandwidths are used in

the analysis. Use of the gain-limited bandwidths

drastically reduces the pitch bandwidth to around 0.25,

near the level 3 border. The pilot ratings, which are

mainly level 2, appear to correlate better with the phaselimited bandwidth values. Although the reason behind

this phenomenon is unknown, it may be that ignoring the

two lower frequency gain-limited bandwidths is

appropriate for this type of transfer function.

Although the pitch bandwidth analysis is supported

with pilot evaluations and indicates that good pitch

attitude control is possible, it may be a misleading result.

"Cp,

aa¢

.35

.3O

.25

.2O

.15

.10

.O5

0 SAS off - phase bandwidth

0 SAS off - gain bandwidth

X SAS on

Level 3

__ 3.0 3.8 _ Level 1

I I I I I

.5 1.0 1.5 2.0 2.5 3.0

CObwo,rad/sec

970376

Figure 6. MIL-STD-1797 pitch bandwidth criteria for category C flight.

7

American Institute of Aeronautics and Astronautics
Because the vertical speed indicator is the primary

feedback display, the pilot ratings and comments appear

driven more by the flightpath response than the pitch

attitude response. Thus, it is uncertain whether the

bandwidth analysis, which assumes pilot feedback of

pitch attitude, accurately reflects the pilot evaluations.

The level 2 pilot evaluations of the SAS turned off data

could reflect degraded flightpath response and not

degraded pitch attitude response. In this case, the pilot

may not even care about pitch attitude response, and the

correlation of the pilot ratings with the phase-limited

pitch attitude bandwidth may be coincidental.

The pitch attitude bandwidth analysis for the SAS

turned on data in figure 6 may also be misleading

because of the existence of the large shelf in the pitch

attitude frequency response. Reference 8 documents the

potential of a frequency response which has a large shelf

and a gain-margin-limited bandwidth frequency to

provide bandwidth frequency estimates that give

optimistic predictions of handling qualities. Although

the SAS turned on bandwidth frequency estimates are

phase margin limited, the gain-margin-limited

frequencies are very close to the phase-margin-limited

frequency, and both fall right on the edge of the shelf.

Small changes in gain could provide for large reductions

in phase margin, providing misleading predictions of

pitch attitude control. Thus, although good pitch attitude

bandwidth exists, it may not necessarily relate to good

flying qualities if the pilot were to control pitch attitude.

Pitch Bandwidth as a Function of Flightpath Bandwidth

&0a!xm

Because pilot comments and ratings appear linked to

flightpath response, applying an additional criterion on

flightpath bandwidth may be insightful. A criterion on

flightpath bandwidth as a function of pitch attitude

bandwidth was evaluated against SR-71 and reproduced

YF-12 data (fig. 7). Phase bandwidth values of pitch

bandwidth are used for the SAS turned off YF-12 data.

The SAS turned on data falls well within the level 1

region for both flightpath and pitch attitude bandwidth.

This analysis indicates that good pilot control of pitch

attitude and flightpath control is possible. However, the

addition of the SAS turned off YF-12 data reduces the

pitch attitude bandwidth and increases the flightpath

bandwidth enough to be near the border between levels 2

and 3. Because pilot ratings appear more dependent on

flightpath than on pitch attitude, the increase in flightpath

bandwidth could be a factor which degrades the SAS

turned off evaluations although the pilot ratings are

better than what would be predicted by this criterion.

Still, it is unclear whether the degradation results from

the increase in flightpath bandwidth or the decrease in

pitch attitude bandwidth.

1.4

1.2

1.0

.8

.4

.2

o s,so,

30 xs,so°

4.3 Z/ .).4 3.0x

jj ....

J \\

Level 2

Level 3

I I I I i I

.5 1.0 1.5 2.0 2.5 3.0

O)bw O, rad/sec 970377

Figure 7. Criteria on pitch bandwidth vs. flightpath bandwidth.

8

American Institute of Aeronautics and Astronautics
Closer scrutiny of the flightpath bandwidth data

shows some problems, however. This criterion sets

requirements on the amount of lag between pitch attitude

and flightpath response. The flightpath bandwidth values

for the SAS turned off data in figure 7 are above the

level 1 region. This implies that the flightpath response

lags the pitch attitude, not by too much as is expected,

but by too little. This result is counter intuitive because

physically the lag between pitch attitude and flightpath

increases with Mach number. If flightpath bandwidth

were the cause of the degradation in pilot ratings

between the SAS turned on and turned off data, the SAS

turned off data would be expected to be below the level

1 flightpath bandwidth limit (at 0.6), not above. Thus,

pilot evaluations are inconclusive in supporting these

criteria.

One possible explanation for this data is that the upper

limit on flightpath bandwidth was set based on the need

to separate the pitch attitude and flightpath responses for

precision approach and landing. In this flight phase, both

pitch attitude and flightpath response need to be

controlled by the pilot. For the data in figure 7, the pilot

ratings appear to be based only on flightpath control. The

separation requirement between flightpath and pitch

attitude response may not be as important for these

conditions. The pilot simply may not care as much about

the smaller pitch attitude response in cruise flight as

compared to landing.

Frequency Response Analysis

Additional insight on the issue of whether it is more

appropriate to control flightpath or pitch attitude is

provided through figure 8. This figure presents a

comparison of the magnitude of the pitch attitude and

flightpath from stick defection frequency responses with

the SAS turned on. These frequency responses were

generated with an SR-71 linear model that was validated

with flight data. A large shelf exists in the pitch attitude

frequency response which is normally associated with

bad pitch attitude control. The large shelf exists because

of the significant separation in 1/Toz (which causes the

lag between flightpath and pitch attitude) and short

period frequency (tOsp). However, the large shelf is

actually the cause of good flightpath control. For the

frequency range where the large shelf exists, the

flightpath frequency response is characteristic of an

integrator, or l/s. The 1/s characteristic is typical of

aircraft with good flying qualities. Thus, pilot control of

flightpath should be better than pitch attitude control.

Other types of control systems could remove the shelf -

improving pitch attitude control, such as pitch rate

command systems. However, such systems would cause

flightpath control to deteriorate.

Neal/Smith Analysis

The Neal/Smith criterion typically involves closing the

loop around a pitch-attitude-to-stick-deflection transfer

function and a lead-in-lag compensator. This procedure

is modified by the addition of pure time delay, to meet

specific closed-loop characteristics. The characteristics

of the closed-loop frequency response are defined as

-90 ° of phase at the bandwidth frequency and no less

than -3 dB of droop (fig. 9). The bandwidth frequency

represents the piloting task which is being conducted and

is generally chosen based on fight phase. Criteria are

established based on the lead required of the

compensator to meet the characteristics and the

maximum amplitude, or resonant peak, of the frequency

response of the closed-loop system; for example, the

compensator and airplane.

50

Magnitude, 0

dB

m I i

...... O) i

i I

p

Flightpath t l l llllll i i i ii lll Lil it

.02 .04 .06 .08.10 .2 .4 .6 .8 1.0 2 4 6 8 10

Frequency, racVsec 970378

Figure 8. Comparison of pitch attitude and flightpath from stick position frequency response.

9

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IO_cl,

dB

deg

10

0

-3

-10

0

-9O

- 180

F

I

I

Frequency,

rad/sec

Bandwidth

frequency

970379

Figure 9. Illustration of application of Neal/Smith criteria.

To examine the issue of whether pilot control of pitch

attitude or flightpath is more appropriate, the Neal/Smith

criteria were applied to SR-71 flight data at Mach 3.0

using frequency responses of pitch attitude from stick

deflection and vertical speed from stick deflection. The

vertical speed data were obtained by integrating the

normal acceleration at the pilot station. (The inertially

derived vertical speed was calculated by the navigation

system, which is located just aft of the cockpit.) The data

presented here use 0.3 sec as the time delay of the

compensator. Three bandwidths ranging from 1.5 to

2.5 rad/sec were analyzed with the pitch attitude

frequency response to evaluate the results for increases

in the demands of the task. Bandwidth from 1.0 to

2.0 were analyzed for the vertical speed frequency

response.

Figure 10 presents the results comparing the two

frequency responses. For the compensator using pitch

attitude, significant amounts of pilot lag are required,

driving the flying qualities into the level 2 region. Level 1

ratings from bandwidths of 1.0 to 1.5 result with the

compensator using vertical speed, which was consistent

with pilot comments. In addition, the slope of

degradation (Aresonant peak/Apilot lead) as bandwidth

increases using vertical speed is much less than when

using pitch attitude. This indicates that the flying

qualities are less sensitive to increasing demands of the

maneuver when using vertical speed instead of pitch

attitude. Therefore, the Neal/Smith analysis supports

using direct pilot control of flightpath as opposed to

direct control of pitch attitude.

Pilot evaluations of the SR-71 aircraft compare

favorably with the Neal/Smith analysis using vertical

speed in figure 10. The pilot ratings and comments of the

SR-71 aircraft noted that leading the aircraft was

required to acquire the target altitude during the vertical

plane altitude change. One pilot commented that "a great

deal of lead is required [to acquire the target altitude] in

terms of time." However, the lead is not significant

enough to reduce the ratings from level 1 to level 2 for

the inenially derived vertical speed data (fig. 2). The

Neal/Smith analysis using the vertical speed at the pilot

station predicts level 1 flying qualities up to 1.5 rad/sec

(category C bandwidth requirement) while requiring

pilot lead. Direct pilot control of vertical speed could

result in large, objectionable pitch rate overshoots.

Figure 11 presents a step input into a validated batch

simulation at Mach 3 to illustrate the pitch rate overshoot

that occurs when a small vertical speed increment is

made. No pilot comments indicating an abrupt initial

response were noted. Thus, pilot ratings and comments

appear to be based on the vertical speed response, while

the pitch attitude response goes unnoticed. Although the

10

American Institute of Aeronautics and Astronautics
Resonant

peek,

dB

14

12

10

8

6

4/ -

X 1.5

2--

0

--2 --

-4

m 4 to 2.5

/

:/

X 2.0

_\\\

I J

- 20

0 Vertical speed

Level 3 X Pitch attitude

Category C bandwidth

frequency _

o ---_4.s

1.0 Level 1

I I_1 I

0 20 40 60 80

Pilot load, deg

970380

Figure 10. Neal/Smith results using pitch attitude and vertical speed from stick position frequency responses as a

function of bandwidth frequency.

Pitch rate,

deg/lmc

.5

--.5

I I

Vertical speed, 20

Wee c 10

0 1 2 3 4 5 6

Time, eec

1 I I I

7 8 9 10

970381

Figure 11. Comparison of pitch rate and vertical speed responses at Mach 3.

11

American Institute of Aeronautics and Astronautics
16

14!

Resonant

pes_ 6

dB

4

12

10--

8--

2--

0

-2 m

--4'

-40

\\\\\\\\\"

-20

2.0 = bw X

Level 2

2 0 bw

1,0 = bw _,,

\\

Level I

1 I I

D 20 4D 60 80

Pilot lead, (leg

970382

0 Vertical speed at pilot

station

X Vertical speed at e.g.

Figure 12. Neal/Smith analysis of the vertical speed at the center of gravity and the vertical speed at the pilot station

to stick deflection frequency response.

pitch rate overshoots are relatively large, the absolute addressed

magnitude of the pitch attitude changes are small at high reported:

speeds, so the overshoot characteristics are not a factor in

pilot opinion. 1.

The NeaYSmith analysis can also be used to illustrate

the affect of the location of the inertial vertical speed

measurement. Figure 12 shows the results for the inertial

system in its present location near the cockpit and at the

center of gravity. The cockpit location data provide lead

due to the pitch acceleration, thereby reducing the

amount of lead required by the pilot. The cockpit

location data also reduce the sensitivity of the response

for increased demands of the task.

Conclusions

Pilot comments and ratings of the SR-71 aircraft flown

at Mach 3 were collected using a vertical plane altitude

change and compared to established handling qualities

criteria. The objective of this study was to extend some

of the supersonic flying qualities research conducted

25 years ago with the XB-70 and YF-12 programs using

these new data. Analysis of pilot displays of vertical

speed critical to performing the maneuvers and

comparisons of the pilots' ability to directly control pitch

attitude and flightpath were among the issues specifically

2.

in this study. The following results are

The pilots' ability to perform the maneuvers was

strongly influenced by the information displayed to

the pilot. Feedback of vertical speed based on an

inertial reference frame was critical to the pilots'

ability to perform accurate flightpath control. Pilot

ratings and comments of maneuvers flown with a

pressure-driven vertical speed indicator showed

that performance was significantly degraded.

Analysis indicates that direct pilot control of

flightpath for a conventional airplane in supersonic

flight has significant advantages over pilot control

of pitch attitude. A modification of the Neal/Smith

analysis using the vertical speed at the pilot station

as feedback for pilot control produced level 1

results for a bandwidth up to 1.5 rad/sec. Pilot

ratings and comments are consistent with the

level 1 Neal/Smith analysis using vertical speed at

the pilot station. In comparison, the analysis using

pitch attitude feedback to the pilot produced level 2

results for the same bandwidths. In addition, the

large shelf in the pitch attitude from stick deflection

frequency response, typically associated with poor

flying qualities, supported the Neal/Smith analysis

using pitch attitude frequency response.

12

AmericanInstitute of Aeronauticsand Astronautics
.

.

Application of criteria on pitch attitude and

flightpath bandwidth was inconclusive in the

evaluation of pitch attitude and flightpath control.

Although the pitch bandwidth criterion showed

good pitch attitude control was possible and

correlated with pilot ratings, this criterion may not

be sufficient to predict good supersonic flying

qualifies. Ratings were based on flightpath control

using vertical speed indications to the pii :_t.

Application of the flightpath bandwidth ctiterion

showed that good flightpath control using pitch

attitude was possible. However, the flightpath

bandwidth analysis was inconsistent with degraded

high-speed SAS turned off ratings and comments.

The SAS turned off flightpath bandwidth data fall

above the minimum level 1 value in a region where

flying qualities limits are defined by poor

consonance between pitch attitude and flightpath in

the approach and landing phase. This factor may

not be as influential for the high-speed evaluations

because the pilots used direct feedback of vertical

speed in cruise.

References

1 "Flying Qualities of Piloted Airplanes:' Military

Specifications MIL-STD-8785B, Aug. 1969.

2 Berry, Donald T., "Some Handling Qualities Problems

of Supersonic Cruise Aircraft" Proceedings from NASA

Aircraft Safety and Operating Problems Conference,

vol. 2, NASA SP-271, May 1971, pp. 25-38.

3 Berry, Donald T., "A Summary of YF-12 Handling

Qualities," YF-12 Experiments Symposium, NASA

CP-2054, Sept. 1978, pp. 31-57.

4 Berry, Donald T., Donald L. Mallick, and Glenn B.

Gilyard, "Handling Qualities Aspects of the NASA

YF-12 Experience," Proceedings of SCAR Conference,

NASA CP-001, Nov. 1976, pp. 193-213.

5 Berry, Donald T. and B. G. Powers, "Flying Qualities

of a Large, Supersonic Aircraft in Cruise and Landing

Approach," AIAA 70-566, May 1970.

6 Berry, Donald T. and G. Gilyard, "Airframe/Propulsion

System Interactions_An Important Factor in

Supersonic Aircraft Flight Control," AIAA 73-831, Aug.

1973.

7 Cox, Timothy H. and D. Jackson, Evaluation of High

Speed Civil Transport Handling Qualities Criteria with

Supersonic Flight Data, NASA TM-4791, Apr. 1997.

8 "Flying Qualities of Piloted Vehicles," Military

Specifications MIL-STD-1797, Mar. 1987.

9 Hoh, Roger H., "Recommendations for Approach

and Landing Flying Qualities," Systems Technology,

Inc., Hawthorne, California, Working Paper 2631-1,

Dec. 1988.

13

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REPORT DOCUMENTATION PAGE OMB FormApproved NO.0704-0188

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE

August 1997

4.TITLE AND SUBTITLE

Supersonic Flying Qualities Experience Using the SR-71

s. _(s)

Timothy H. Cox and Dante Jackson

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Dryden Flight Research Center

P.O. Box 273

Edwards, California 93523-0273

9.SPONSORING/MONITORINGAGENCYNAME(B)ANDADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

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11. SUPPLEMENTARY NOTES

Presented as AIAA 97-3654 at the AIAA Atmospheric Right Mechanics Conference, New Orleans, Louisiana, August 11-13, 1997.

Timothy H. Cox, Dryden Flight Research Center, Edwards, California; Dante Jackson, Analytical Services and Material, Inc., Edwards,

California.

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13. ABSTRACT (Maximum 200 words)

Approximately 25 years ago NASA Dryden Flight Research Center, Edwards, California, initiated the

evaluation of supersonic handling qualities issues using the XB-70 and the YF-12. Comparison of pilot

comments and ratings with some of the classical handling qualities criteria for transport aircraft provided

information on the usefulness of these criteria and insight into supersonic flying qualities issues. A second

research study has recently been completed which again addressed supersonic flying qualities issues through

evaluations of the SR-71 in flight at Mach 3. Additional insight into supersonic flying qualities issues was

obtained through pilot ratings and comments. These ratings were compared with existing military specifications

and proposed criteria for the High Speed Civil Transport. This paper investigates the disparity between pilot

comments and the Neal/Smith criteria through a modification of the technique using vertical speed at the pilot

station. The paper specifically addresses the pilot ability to control flightpath and pitch attitude in supersonic

flight and pilot displays typical of supersonic maneuvering.

14. SUBJECT TERMS

Bandwidth criteria, Flightpath bandwidth, Flying qualities, High Speed Civil

Transport, Neal/Smith criteria, SR-71 aircraft, Supersonic flying qualities.

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