NASA Technical Memorandum 104308

A Flying Qualities Study of

Longitudinal Long-Term

Dynamics of Hypersonic Planes

Timothy H. Cox, G. Sachs, A. Knoll, and R. Stich

April 1995

National Aeronautics and

Space Administration

https://ntrs.nasa.gov/search.jsp?R=19990110676 2018-05-11T19:24:27+00:00Z
NASATechnicalMemorandum104308

A Flying Qualities Study of

Longitudinal Long-Term

Dynamics of Hypersonic Planes

Timothy H. Cox

NASA Dryden Flight Research Center

Edwards, California

G. Sachs, A. Knoll and R. Stich

Technical University of Munich

Munich, Germany

1995

National Aeronautics and

Space Administration

Dryden Flight Research Center

Edwards, California 93523-0273
A FLYING QUALITIES STUDY OF LONGITUDINAL LONG-TERM

DYNAMICS OF HYPERSONIC PLANES

Timothy H. Cox*

NASA Dryden Flight Research Center

EO. Box 273

Edwards, California 93523-0273

G. Sachs,** A. Knoll, + and R. Stich +t

Technische Universitat Miinchen

Lehrstuhl f. Flugmechanik

Arcisstrasse 21

80290 Miinchen

ABSTRACT

The NASA Dryden Flight Research Center and the

Technical University of Munich axe cooperating in a

research program to assess the impact of unstable longterm dynamics on the flying qualities of planes in hypersonic flight. These flying qualities issues are being investigated with a dedicated flight simulator for hypersonic

vehicles located at NASA Dryden. Several NASA

research pilots have flown the simulator through welldefined steady-level turns with varying phugoid and height

mode instabilities. The data collected include pilot ratings

and comments, performance measurements, and pilot

workload measurements. The results presented in this

paper include design guidelines for height and phugoid

mode instabilities, an evaluation of the tapping method

used to measure pilot workload, a discussion of techniques

developed by the pilots to control large instabilities, and a

discussion of how flying qualities of unstable long-term

dynamics influence control power design requirements.

INTRODUCTION

Much of the work in recent years focused on proposals

and evaluations of hypersonic vehicle concepts) Many of

these concepts required development of technology in

fields such as structures and propulsion, and researchers

have appropriately focused in these areas. However,

deficiencies also exist in the flying qualities criteria for

*Aerospace Engineer.

**Director, Institute of Flight Mechanics and Flight Control. Associate

Fellow ALAA.

+Research Assistant.

+tResearch Assistant.

Copyright © 1995 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

fights under the copyright claimed herein for Governmental purposes. All

other fights are reserved by the copyright owner.

hypersonic vehicles. One example where the criteria are

deficient is in the impact of long-term dynamics on hypersonic flying qualities. 2 The long-term dynamics consist of

the phugoid and height modes of motion.

The current military standard 3 does not adequately

address the phugoid and height mode flying qualities

requirements at hypersonic speeds. The phugoid flying

qualities criteria are based on data from the subsonic

regime. However, substantial differences exist between

phugoid dynamics in subsonic and hypersonic flight. From

the constant energy equation, a phugoid mode excited at

high speeds produces greater altitude excursions for a

given velocity excursion than a phugoid mode excited at

low speeds. To make matters worse, tight control of altitude becomes more important in hypersonic flight than in

subsonic flight because of engine performance sensitivity

to dynamic pressure. Thus, the current criteria and data

collected on the phugoid mode may not be applicable for

the supersonic and hypersonic speed regimes.

The height mode is a mode of motion that becomes

more significant in hypersonic (and supersonic) flight than

in subsonic flight. The height mode, not addressed by the

military standard, is typically a first-order response resulting from thrust variation with density gradient or speed. A

more detailed description of the phugoid and height mode

characteristics has previously been given. 4

Berry addressed the issue of long-term dynamics 5 by

examining the impact of the propulsion system on the

phugoid and height mode dynamics of a hypersonic lifting

vehicle developed by Etkin. 6 Berry discovered that airbreathing propulsion systems can produce flight regions of

unstable height and phugoid modes. Berry thought that

safe pilot control of these instabilities would have strong

implications on redundancy management and backup systems required for the flight control system of hypersonic

vehicles. The flying qualities impact of these unstable

modes was, however, beyond the scope of his work.
Theneedforthisdataled theNASADrydenFlight

Research Center and the Technical University of Munich

into a cooperative research program to assess the flying

qualities impact of unstable long-term dynamics on hypersonic planes. The primary objective of the research is to

develop and validate flying qualities criteria for these

dynamic characteristics of hypersonic vehicles. These

issues are investigated with a dedicated flight simulator

for hypersonic vehicles and an SR-71 aircraft located at

NASA Dryden. Previous results from this cooperative

research have been documented. 7-1° The results presented

in this paper are as follows:

• the design guidelines for height and phugoid mode

instabilities

• the piloting techniques developed to control high

degree of instabilities

• the evaluation of the tapping method used to

measure pilot workload

• the influence of flying qualities of unstable longterm dynamics on control power design

requirements

NOMENCLATURE

A state matrix

B control matrix

BTL

C

CD v

Cm r

Cm8 e

GHAME

H

basic tapping level

output matrix

change in the nondimensional drag coefficient with respect to speed changes

change in the nondimensional moment coefficient with respect to flightpath change

change in the nondimensional moment coefficient with respect to elevator change

Generic Hypersonic Aerodynamic Model

Example

altitude, ft

J

KEAS

K 6

LTL

N

a given run at each instability level

equivalent airspeed, knots

control system gain, Cmr/Crnse

loaded tapping level

total number of runs flown at each instability

NASP

PML

P

p'

i'ms

Prms

q

r

Fills

S

u

V

X

x

Y

Ot

"y

A

_a

_ap

8

¢.

_)ep

8_

8

rp

8r

0

2

National Aerospace Plane

perceptual motor load

roll rate, deg/sec

rms variation of the parameter altitude or

KEAS for a given run

rms variation of the parameter altitude or

KEAS across the total number of runs at a

given instability

pitch rate, deg/sec

yaw rate, deg/sec

root mean square

Laplace operator

control input vector

true airspeed, ft/sec

element in matrix A modified for height

mode variation

state vector

change of x with respect to time

output vector

angle of attack, deg

sideslip angle, deg

flightpath angle, deg

change in a parameter

aileron position, rad

lateral stick deflection, in.

elevator position, rad

longitudinal stick deflection, in.

rudder position, rad

rudder pedal position, in.

throttle position, percent of full throw

dutch roll damping

pitch attitude, deg

bank angle, deg
headingangle,deg

Oar dutchrollfrequency, rad/sec

SIMULATORDESCRIPTION

The space shuttle and the National Aerospace Plane

(NASP) programs tl successfully applied the fixed-base

flight simulator used in this research in previous hypersonic flight research. For this study, the hypersonic vehicle

aerodynamics and propulsion system were linearized

models based on the Generic Hypersonic Aerodynamic

Model Example (GHAME) at Mach 10 and an altitude of

110,000 ft. The GHAME model represents a generic,

unclassified hypersonic vehicle incorporating six degrees

of freedom; oblate earth equations of motion; and a turbojet, ramjet, and scramjet propulsion system combination.

A simple, conventional control system (pitch rate feedback) designed for the linearized model provided Level 1

longitudinal short-term and lateral-directional flying qualifies at the flight condition (fig. 1). 3 The pilot-vehicle

Control

anticipation

parameter,

g-1 sec-2

10.00

1.00

0.10 =

m

m

_N \N\\\\\\\\\\___N ,. \\\

Level 1 _,

Category C

_ Baseline

_ Mach 10 data_

Level 1

Category B

0.01 I I I I I IIII I

0.10 1.00

Short period damping

I I I II III

10.00

95O006

(a) Longitudinal short-period flying qualities design

2,5 m

Dutch roll

frequency,

rad/sec

2.0

1.5

1.0

.5

0

©

\

Baseline Mach 10 data

,N.N.N.\\\\_,\\\\\\N.\\\\N.N.\_\\\ \\_\\\\

Level 1 - Category C flight phase

,.\\\\\\\\\\N\\\\\\\\\\\\\\\\\\\\\\\

Level 1 - Category B flight phase

I I I I I

.1 .2 .3 .4 .5

Dutch roll damping

95OO07

(b) Lateral--directional dutch roll flying qualities design.

Figure 1. Baseline Mach 10 flying qualities designs.
.5-

_dr°)dr,

rad/sec

.4

.3

.2

.1

7 °

Baseline Mach 10 data

-- Level 1 - Category C

flight phase

\\ _.\_.NNNNN\_N\N'_\NN_,NN\ N

\\\N\\\N\N\\\NNN\N\ \ \\\\

Level 1 - Category B

flight phase

I I

0 .5 1.0 1.5

Roll mode time constant, sec

95OOO8

(c) Lateral--directional roll mode and dutch roll mode flying qualities design.

Figure 1. Concluded.

interface was a conventional center stick and rudder pedal.

The appendix details the control system and linear equations of motion for the flight condition used in this study.

The cockpit instrumentation was based on a configuration used for the space shuttle. A central control panel and

a simulated head-up display (fig. 2) provide the pilot with

cues adequate to perform hypersonic maneuvering.

Included in the head-up display were vertical and longitudinal accelerations, which proved useful for flightpath and

speed control during space shuttle and NASP research. A

fixed scale and arrow with rolling digits displayed vertical

speed while a moving tape and arrow with rolling digits

inside indicated altitude. A moving scale and fixed arrow

displayed the heading with a resolution of 0.5 °. Because

of experience gained from initial simulator runs, it was

decided to display an inertial vertical speed indicator in

the form of a diamond on the pitch ladder.

TEST PROCEDURE

Pilots flew a well-defined steady-level turn to investigate the flying qualities implications and characteristics of

various levels-of-height and phugoid instabilities. The

definition of the maneuver was to perform a 30 ° bank turn

and capture a 12 ° heading change. This maneuver had to

be performed while immediately eliminating a vertical

speed initial condition and maintaining constant airspeed

and altitude.

Adequate performance was defined as capturing the

final heading within + 1°, maintaining speed within

± 10 knots of the initial speed, and maintaining altitude

within ± 600 ft of the initial altitude.

Desired performance was defined as capturing the final

heading within ± 0.5 °, maintaining speed within ± 5 knots

of the initial speed, and maintaining altitude within + 300

ft of the initial altitude.

These performance criteria were chosen arbitrarily as

reasonable but tight constraints and are shown in Table 1.

The turning portion of the maneuver lasted approximately

2 min and termination of the maneuver occurred 1 min

after capturing the final heading. Initial conditions were

wings level at Macb 10 and an altitude of 110,000 ft. A

vertical speed initial condition of 20 ft/sec was introduced

to increase pilot workload.

Table 1. Adequate and desired performances.

Steady Level Turn

Controlled Adequate Desired

parameters performance performance

Target heading ± 1o +0.5 °

Target altitude ±600 fl ±300 ft

Trim speed (KEAS) +10 kn _+_5kn

The pilots involved in the test program were experienced in high-speed flying. Their experience includes a

wide spectrum of supersonic and hypersonic vehicles,

including the SR-71, YF-12, and X-15.

4
Figure2.Simulatedhead-updisplaywithmodificationsforhypersonicflight.

Thepilotsevaluatedphugoidmodeinstabilitieswith

time-to-double-amplitudes downto 1.7secandheight

modeinstabilitiesdownto 3.5sec,whichwereempiricallydeterminedlimitsof controllabilityforthisstudy.

Similarlevelsof instabilitywerepreviouslystudied12

usingalargesupersonictransport modelwithanaperiodic

phugoidtime-to-double-amplitude of 4.6 sec in the

approachandlandingcondition.Aftereachrun,thepilot

gaveCooper-Harper ratings13(fig. 3), comments, and

estimationsofworkload.

Figure4 showstheworkloadscaledevelopedforthis

studytoprovideinsightonpilotworkloadindependent of

performance. Forthisworkloadscale,thepilotestimates

thecomplexityof thesidetaskspossiblewhileperforming

themaneuver. If theairplaneis controllable, thepilot

decidesif simplesidetasks(forexample, radiocommunications)orcomplexsidetasks(for example,flyingin a

complexair trafficcontrolenvironment)arepossible.

Afterthesedistinctions, thepilotmustestimatethelevelof

compensation requiredtoperformthemaneuver, resulting

inaratingfrom1to 13.

A timehistoryrecordedfromeachtestrunprovided

informationonpilotperformance. Pilotperformancewas

measuredthroughout theentiremaneuverastherootmean-square(ms)variationfromtheinitialconditionof

thekeyparametersaltitudeandairspeed.Pilotperformancewasmeasuredfor onlythosetestrunswherethe

pilotmaintainedcontrol.Thefollowingequationdescribes

thesummaryofpilotperformanceforallthepilotsateach

instability:
Deflclencles

warrant

improvement

Deflclencles

require

Improvement

y

Aircraft Demands on the pilot in selected

characteristics task or required operation

Excellent - Highly Pilot compensation not • factor for

desirable desired performance

Good - Negligible Pilot compensation not a factor for

deficiencies desired performance

Fair - Some mildly Minimal pilot compensation

unpleasant required for desired performance deficiencies

Minor but annoying Desired performance requires

deficiencies moderate pilot compensation

Moderately objection- Adequate performance requires

able deficiencies considerable pilot compensation

objectionable Adequate performance requires

but tolerable extensive pilot compensation

deficiencies

Adequate performance not obtainable

with maximum tolerable pilot compensation; controllability not In question

f Major deficiencies

Major deficiencies

Improvement mandatory

Pilot

rating

Considerable pilot compensation r_

is required for control

Intense pilot compensation Is required

to retain control

Control will be lost during some

Major deficiencies porUon of required operation

95O009

Figure 3. Cooper-Harper rating scale. 13
I Yes

Are complex side

tasks possible?

Yes

Are simple side

tasks possible?

Yes

Is It controllable?

I Complex side tasks (e.g.

I flying in • complex ATC

_-! environment without

I sny support) and

I flying requires:

Compensation not noticeable

Minimal but noticeable compensation

Small but more notlceeble compensatlon

Moderate compensstlon

Consldereble compensatlon

Moderate compensatlon

Consldersble compensatlon

Immense compensstlon

Simple side tasks (e.g.

radio communications)

and flying requires:

No more side tasks

possible (bsckseater

required)

I [ Moderate compensation

I onsiderable compenssUon

I mmense compensation

Not tolerable compensation

Alrcraft runs out of control

X

X

X

[9]

I o]

131]

95001O

Figure 4. Workload rating scale.

EN

Prms = _J= 1N

where

P = altitude or airspeed

Prms = the rms variation of the parameter about its

initial condition for each run

N = the total number of runs flown at each

instability level

Prms = the rms variation about the initial condition

across all runs at each instability

The Prms parameter provides a summary of the performance achieved across all the runs at a given instability.

The phugoid and height mode instabilities were implemented through unstable feedback in the control system

and modification of the aerodynamic models. Phugoid

mode instabilities were implemented by positive feedback

of flightpath angle (fig. 5). Height mode instabilities were

implemented by modifying the drag coefficient due to

speed, C D (fig. 6). This implementation allowed phugoid

and heightVmode instabilities to be set independently from

each other. The eigenvectors of the phugoid and height

modes implemented with these methods were verified to

have accurate characteristics. All phugoid instabilities

evaluated were in the aperiodic region; however, the

baseline phugoid case was periodic and neutrally stable.

The baseline height mode case was stable with a

time-to-half-amplitude of 77 sec. Tables 2 and 3 show the

phugoid and height mode instabilities evaluated.

7
Table 2. Phugoid mode instabilities evaluated.

Root location, Time to double

K_i rad/sec amplitude, sec

0 0

4.2 0.05 13.6

10 0.16 4.4

20 0.30 2.3

25 0.36 1.9

30 0.42 1.7

Table 3. Height mode instabilities evaluated.

Root location, Time to double

CDv rad/sec amplitude, sec

0.00001 -0.01

-0.00003 0.02 40.8

-0.00006 0.04 18.2

-0.00008 0.05 13.6

-0.00018 0.12 5.9

-0.00024 0.16 4.4

-0.00030 0.20 3.5

.04

Imaginary .03

exls,

rad/sec .02

.01

0

AaeP + Aircraft I A7

dynamlca Cm I ]

• KS= "Y

- Cm

__hugoid mode

/- Height mode \

I , I,

.01 .02 .03 .04 .05

Reel axis, red/sec

950011

Figure 5. Aperiodic phugoid mode instability characteristics.

.05 100:1

scale

Imaginary .03

axis,

rad/sec

.02 --

.01 --

X

-.01

.0380Xp'_

.0381 __

.0382_

Phugold

S Height mode

I

0 .01 .02 .03 .04 .05

Real axis, rad/sec

950012

Figure 6. Aperiodic height mode instability root contours.
Toincreasepilotworkload,thetestrunsincludedatmosphericdensityperturbationsforthepilotto fly through.

Thedisturbancesoccurredwithin1to20secofeachother

intheshapeof acosinewavelasting20sec.Theamplitude

ofthedisturbance, whosesignwasrandomlymodelled, was

2.5percentof thestandarddensityatanaltitudeof 110,000

ft andconsistent withatmosphericvariationsobservedfrom

YF-12flights.14

Duringsomeofthetestruns,asecondarysidetaskforthe

pilotwasinstitutedtomeasurepilotworkload. Themethod,

calledtapping,requiredthepilottotapabuttonlocatedon

thestickasrhythmicallyaspossible.Thetheorystatesthat

asthepilotworkloadincreases, theirregularityof thetappingincreases. Acomparisonbetweentheregularityofthe

tappingforagivenrun(loadedtappinglevel,LTL)andthe

regularityofthetappingduringstraightandlevelflightwith

nodisturbances (basictappinglevel,BTL)yieldsameasure

ofthepilotworkloadorperceptual motorload(PML).The

PMLiscalculatedasfollows:

PML= (LTL-BTL)/BTL

Informationonthetappingmethodasit wasappliedto

thisresearchhaspreviouslybeengiven,l0 General information on the tapping theory has also been reported

previously. 15

RESULTS AND DISCUSSION

The following section discusses how phugoid and height

mode instabilities affect Cooper-Harper ratings, compares

the effects of workload and pilot performance on CooperHarper ratings, and analyzes pilot technique in controlling

large instabilities. The effect of control power on

Cooper-Harper ratings is examined, and the tapping

method is evaluated.

Phugoid and Height Instability Impact on CooperHarper Ratings

Figure 7 shows the Cooper-Harper ratings as a function

of phugoid real-root locations without use of the tapping

method. Five data points, generated by three pilots, were

averaged at each instability. Eighty-three percent of the

ratings fell within a + 1 pilot-rating band, illustrating the

consistency of the results. The ratings stay relatively constant at low phugoid instabilities until a root location of

0.15 rad/sec, where the ratings degrade sharply. Level 2/3

and Level 3/Uncontrollable borders occur at unstable root

locations of 0.23 and 0.44 rad/sec respectively, or 3 and

1.4 sec time-to-double-amplitude respectively.

Figure 8 shows the nontapping Cooper-Harper ratings

as a function of height mode instability. Seven data

points, generated by four pilots, were averaged at each

instability. Seventy-three percent of the ratings fell within

a + 1 pilot-rating band, illustrating the consistency of the

results. Similar to the phugoid data (fig. 7), relatively

constant ratings occur for the low instabilities, degrading

sharply as instability increases above a root location of

approximately 0.015 rad/sec. This result indicates a

"cliff-like" characteristic in the height mode data. Comparing the slopes of the height mode and phugoid mode

Cooper-Harper data (figs. 7 and 8) indicates that the

"cliff'" characteristic exists in the phugoid data to a lesser

degree. Level 2/3 and Level 3AJncontrollable borders

occur at unstable root locations of approximately 0.045

and 0.14 rad/sec respectively, corresponding to 15 and 5

sec time-to-double-amplitude.

Cooper-Harper

rating

10 F-.,,,,ot..,,og///

9 _-- -- Average Cooper- J / I /

l Harper rating / / /

81"-83percentofthe / /" /'1

1 data fall within / _ ,,/ '

7_-_+lpilotrating/ / /

t . . ,/ __/,_/ 0.44 -_

56rT- : /_]1 '/_--Slope 18] 0._23 23 = I

4 1 I I I I I I )

0 .1 .2 .3 .4 .5

Phugoid root location, rad/sec

950013

Figure 7. Average Cooper-Harper rating as a function of phugoid instability (nontapping data).

9
Cooper-Harper

rating

I- I

6 _,.__/_ : 73 percent °f the

5 _" f I data fall within _+1

pilot rating k3

2

-.01

m +_1 pilot rating

0.045 m Average Cooper-

'- Harper rating

I ] I I

.04 .09 .14 .19 .24

Height mode root location, rad/sec _so01,

Figure 8. Average Cooper-Harper rating as a function of height mode instability (nontapping data).

Assuming Level 2 ratings are acceptable for backup

control modes of hypersonic vehicles, these data indicate a

time-to-double-amplitude of 15 sec for the height mode

and 3 sec for the phugoid mode are acceptable limits.

However, the apparent "cliff-like" tendencies in the ratings may warrant extra margin in design. Comparison of

the phugoid and height mode data suggests the pilot is

more sensitive to the height mode instability than the

phugoid mode instability, as the height mode borders

occur at larger time-to-double-amplitudes. This result

might be expected because the height mode produces a

divergence in energy as well as flightpath, thus requiring

both throttle and pitch control to correct a deviation. The

phugoid mode, on the other hand, is primarily a constant

energy mode and deviation can be corrected with the pitch

controller alone.

Comparison of Workload and Performance Effects on

Cooper-Harper Ratings

In the previous section, all of the evaluations included

both controllable and uncontrollable cases. In this section,

the comparison of performance measures and corresponding pilot ratings includes only the controllable cases

because the performance was calculated only for those

cases.

The trends in the Cooper-Harper rating data and the

workload rating data for the controllable phugoid instabilities without tapping (fig. 9) are similar. Both CooperHarper and workload ratings start out relatively fiat and

then degrade after a root location of 0.16 rad/sec.

Comparison of Cooper-Harper and workload ratings for

the height mode (fig. 10) shows similar results. The

reduced Cooper-Harper rating of 7 at the height root location of 0.15 rad/sec was the only controllable run of the 7

test runs and therefore does not represent a meaningful statistical average. These results suggest that pilot workload

was the primary consideration driving the Cooper-Harper

rating. However, a question about the influence of pilot

performance on Cooper-Harper ratings remains.

Figure 11 shows the pilot's performance, displayed as a

percentage of desired performance, as a function of

phugoid instability. The pilot performance on speed stays

relatively flat until a root location of 0.3 rad/sec and then

degrades, similar to the Cooper-Harper and workload ratings. However, the degradation in speed performance

becomes no worse than approximately desired speed at the

most unstable phugoid tested. The altitude performance

data show some degradation as the instability increases but

remains relatively flat and well within desired performance. Figure 9 shows even at the highest root locations

of approximately 0.4 rad/sec, where control is marginal

(Cooper-Harper ratings of 8-9), the performance data (fig.

11) remain approximately less than or equal to desired performance. Therefore, despite some degradation in performance in speed, the pilots achieved levels of performance

within the desired range even as the ratings deteriorated to

the point of uncontrollability.

Figure 12 shows the pilot's performance, displayed as a

percentage of desired performance, as a function of

controllable height instability. Similar to the phugoid case,

the pilots maintained desired performance on speed and

altitude despite the degradation of Cooper-Harper ratings

to 9 (fig. 10).

10
Ratings

13--

12- --e--

11 --

10-

9-

8 r,- "e

7 -

6-

4 I

0 .1

Cooper-Harper ratlngs

Workload ratlngs

/O

/

/

I I I I

.2 .3 .4 .5

Phugold root, rad/sec

950015

Figure 9. Average Cooper-Harper and workload ratings of the controllable cases

as a function of phugoid instability (nontapping data).

Ratings

13-

Cooper-Harper ratings

12 -- - -e-- - Workload ratlnga

/D.

%

/

11 -- ,' _'

/// %%

10 - s J" b

//

'F r" /%

j \

sl " I I I

-.01 .04 .09 .14

Height mode root location, rad/aec

.19

95OO16

Figure 10. Average Cooper-Harper and workload ratings

of the controllable cases as a function of height instability (nontapping data).

11
Desired

performance

achieved,

percent

200 -

150 -

100

50

Altitude

-.-e- - Speed

-- ,0

e"

/

/

I

/

"_ .4)- ..

I I I I I

.1 .2 .3 .4 .5

Phugoid root, rad/sec

950017

Figure 11. Percentage of desired performance achieved by the pilot for nontapping, phugoid mode data.

Desired

performance

achieved,

percent

100 --

Altitude

--.4-- Speed

80--

60

40

20

S

s

s

s

s

s %

-./ ,_ / I \

,,A f" "m

o I I I I

-.01 .04 .09 .14 .19

Height root, racl/sec

950018

Figure 12. Percentage of desired performance achieved by the pilot for nontapping height mode data.

The obvious conclusion is that pilot workload and not

pilot performance is the major influence on CooperHarper rating. This result implies that a pilot flying aircraft

with these unstable modes can achieve an acceptable level

of performance, but the workload required to achieve this

level of performance increases dramatically with instability. In measuring the flying qualities impact of these

modes, a measure of workload is necessary.

Figures 11 and 12 show that the speed performance was

more critical than the altitude performance. With phugoid

roots greater than 0.3 rad/sec, the speed performance

degraded, whereas the altitude performance remained flat.

Also, pilots achieved a lower percentage of altitude

deviation than speed deviation across all instabilities. This

lower percentage of altitude deviation may be influenced

by the fact that equivalent airspeed is more difficult

to control because it is a function of Mach and altitude.

The choice of desired airspeed performance of _+_5knots

equivalent airspeed (18 lbs/ft 2 dynamic pressure) may also

be an influence. Tight performance on speed was chosen

because of the sensitivity of the propulsion systems to

12
dynamicpressureandbecausetightcontrolof dynamic

pressureiscriticaltothesuccessofasingle-stage-to-orbit

vehiclemission.

Analysis of Pilot Technique for Large Instabilities

In general, the data presented show that pilots can successfully control aircraft models with quite large phugoid

and height mode instabilities. For example, phugoid instabilities with a time-to-double-amplitude of 1.7 sec were at

times controllable, although the workload was very high.

Inherent to the design of air-breathing hypersonic vehicles

is the shaping of the aft fuselage to act as a nozzle. This

design creates significant coupling between the vertical

and longitudinal axes when the thrust is modulated.

Figure 13 shows this principle with the linearized version

of the GHAME at Mach 10. A positive amplitude throttle

step produces a significant increase in vertical and longitudinal speed. This coupling characteristic is potentially

quite useful to the pilot in controlling large instabilities.

The following evaluation of the data determines whether

or not the pilot took advantage of this coupling

characteristic.

Figure 14 shows the time history of a run split into three

regions, A, B, and C, to show how one pilot controlled

a phugoid instability with a time-to-double-amplitude of

1.7 sec. At the beginning of region A, the pilot observed

the negative trend on vertical speed and reacted with full

positive aft stick deflection, thereby reducing the slope of

the vertical speed but not negating it. At this point, the

pilot would have lost control of the vehicle unless

Throttle

position,

percent

60

50

40

570

I I

Equivalent

airspeed,

kn

560

550

20

I I I I

Vertical

speed,

Wsec

10 -

I I

Pitch

acceleration,

deg/sec 2

-.05 0 2.5

I I I

5.0 7.5 10.0

Time, sec

95OO19

Figure 13. Throttle step showing vertical speed and longitudinal speed coupling.

13
Throttle

position,

percent

Longltudinal

stick

postlon,

in.

Equivalent

alrspeed,

kn

Vertical

speed,

fVsec

100

5O

-5

-- A

Region

B _ C

I

I

I !

I !

I I

I __ t I

I I

I . I

I I

I I

I I

f I r

f tl I

580 [-- _ I i

L

f I !

I I t

570 [_

55o I I

5O

- 50

130

-- I J y

I | /e"_ I

I

I !

,\ /_ I

I '1 II

140 150 160

Time, eec

Figure 14. Pilot technique controlling phugoid mode instability.

I

170

950020

something else was done to arrest the vertical speed.

Region B shows the pilot's reaction. The pilot increased

the throttle sharply at the beginning of region B, thereby

creating enough positive vertical speed to negate the

downward trend. Once the pilot observed the sign reversal

of the vertical speed, the pilot sharply decreased the throttle.

At the end of region B, the pilot was confident control

was retained and thus reduced the stick deflection off its

limit. However, the vertical speed kept climbing positively

and went unnoticed until approximately 160 sec, which is

the beginning of region C. At this point, the pilot reversed

the above procedure by reducing the stick deflection.

Again, it was not enough to arrest the trend of the vertical

speed, so the pilot applied a negative, sharp throttle increment. Unfortunately, the throttle input was too late and the

pilot lost control.

Nevertheless, Figure 14 shows the pilot used

sharp throttle inputs to take advantage of the coupling

between longitudinal and vertical axes to control a

vehicle otherwise uncontrollable. This technique with

the throttle, a "bang-bang" technique, increased controllability and minimized airspeed deviations. Note that

the engine model for this simulator experiment was a

simple, linear model with no time delay. A more

realistic engine modeled with filters or time delays

would reduce the effectiveness of this "bang-bang"

technique.

14
Effect of Control Power on Flying Qualities

Because the maximum lift coefficient was not exceeded,

the inability to control vertical speed with full aft stick

deflection (fig. 14) implies the pilot ran out of control

power. However, the pilot supplemented the control power

with the coupling of thrust between the vertical and longitudinal axes. The success of this technique implies the

pilot was not limited by the capacity to handle the large

instabilities, but rather by the available control power. If

this implication is true, increasing the control power

would allow the pilot to fly aircraft models with even

higher instabilities.

Figure 15 shows the impact of modifying the control

power on one pilot's Cooper-Harper ratings of phugoid

instabilities. By varying the control surface deflection limits, three different control power variations are compared:

the baseline control power, 0.5 of the baseline control

power, and 1.5 of the baseline control power. The pilot

controlled much higher levels of phugoid instability with

the increased control power, as evidenced by the increase

in the Level 2/3 border from root locations of 0.23 to

0.32 rad/sec and Level 3/Uncontrollable border from root

locations of 0.44 to 0.60 rad/sec.

These data indicate that the long-term modes are flyable,

even for large instabilities, as long as enough control

power is available. As the available control power lessens,

the bandwidth required increases and the workload

necessary to maintain acceptable or controllable

performance increases, resulting in an increase in

Cooper-Harper and workload ratings.

Figure 16 shows this relationship by plotting the vertical

speed response of two phugoid instabilities to a step input

Cooper-Harper

rating

10--

9

7 --I

6

fl

0

I

I

I

I il) Ii

.1 .2 .3

/

II I, I

.4 .5

Phugoid root location, rad/sec

.6 .7

95OO21

-e- Baseline

-i- 0.5 baseline control power

•..4)- 1.5 baseline control power

Figure 15. Effects of available control power on Cooper-Harper pilot ratings of phugoid mode instabilities.

Vertical

speed,

Wsec

100

5O

0

Time to double

amplitude = 1.7

I I

-//////////////////////////////////////_._I I

Control power limit _...._. _ J, -- " - ,'--_..__ Tlme to double

I I I I _ - I I ) I I, I amplitude= 2.3

1 2 3 4 5 6 7 8 9 10 sec

Time, sec

95OO22

Figure 16. Effects of control power on pilot workload requirements.

15
ofstickdeflection and the vertical speed control power limit,

which is the point where full reversed stick deflection just

arrests the vertical speed. Decreasing the time-to-doubleamplitude reduces the reaction time necessary for the pilot to

maintain controllability, thereby increasing the concentration and effort required of the pilot. Decreasing the control

power available, which is equivalent to lowering the "control power limit" boundary (fig. 16), has the same effect.

Therefore, the relationship between the unstable phugoid

mode and the control power available impacts the flying

qualities of a hypersonic vehicle and may influence the control power requirements of the design.

Not only could this hypothesis explain why the CooperHarper ratings were primarily a function of the workload

rating, as discussed earlier; it also could explain the "clifflike" ratings. For low levels of instabilities, the bandwidth

required is low enough not to influence the pilot ratings. As

the instability increases, or as the control power decreases,

the bandwidth required begins to approach the maximum

available from the pilot-vehicle system, resulting in deteriorated ratings.

Tapping Method Evaluation

As stated earlier, the main objective of the tapping task

was to measure the degree of PML, or workload, on the pilot

as the instabilities increased. Since tapping was designed as

a secondary task, no degradation in pilot ratings should

occur in the main task because of tapping. Figures 17 and 18

Cooper-Harper

rating

10-

8 - /°1 /

/',e

r - ,"

5 _-- -4"-- NO Tapping

4 I I I I --I-T;pping I

0 .1 .2 .3 .4 .5

Phugold root locatlon, rad/aec 95OO23

Figure 17. Average Cooper-Harper ratings of tapping and nontapping data as a function of phugoid instability

Cooper-Harper

rating

10 in

,L ".-2"

I /iS/

5 1

4 -

3 -

2

-.01

--e-- No Tapping

-- •-- Tapping

I I I I

.04 .09 .14 .19

Height root location, rad/sec _4

Figure 18. Average Cooper-Harper ratings of tapping and nontapping data as a function of height mode instability.

16
comparetappingandnontappingCooper-Harper ratingsas

afunctionof thephugoidandheightmodeinstabilitylevelsrespectively. Exceptfortheratingat arealphugoid

rootof approximately0.3rad/sec(fig. 17),theratings

showlittle differencebetweentappingandnontapping

data.Fromthisobservation, it canbeconcludedthesecondarytaskdidnotinterferewiththemaintask.

Butthequestiononhowwellthetappingestimatedpilot

workloadremains.Figure19showstheaverageof the

pilot's estimationof workload with tapping for

the phugoid and height mode instabilities. As the phugoid

instability increases, the workload stays relatively

constant at first and then degrades sharply at a root location of approximately 0.3 rad/sec. The workload

for increasing height mode instabilities exhibits a similar

trend, degrading at a root location of 0.024 rad/sec.

These observations match intuition as well, as one

would expect workload to increase with increasing

instability.

13

7

0

12

11

Average

workload 10

rating

9

m

I I I I I

.1 .2 .3 .4 .5

Phugold root location, rad/sec

95O025

(a) Phugoid mode data.

13

12

11

Average

workload 10

ratlng

9

8

7 I I I I

-.01 .04 .09 .14 .19

Height root locatlon, rad/sec

95O026

(b) Height mode data.

Figure 19. Average workload scale ratings as a function of height and phugoid instabilities (tapping data).

17
Figure 20 shows all PML data as a function of phugoid

and height mode instabilities and overplots the average

PML at each instability. The phugoid data (fig. 20(a))

shows an increase in PML after the root location of 0.3

rad/sec, which was the point where the average pilot

workload rating began degrading sharply. Nevertheless,

the scatter in the PML data, which is as large as the

previously noted increase in PML, obscures any trends

exhibited by averaged PML data and prevents the

conclusion that any significant correlation exists between

increasing instability and PML.

In addition, no correlation between PML and increasing

instability is observed in the height mode data (fig. 20(b))

because the large degree of scatter in PML obscures any

trends exhibited by the averaged PML data. Although

consistent trends between PML and instability exist in

some individual series of runs, these trends exhibit both

PML

Average PML

I I I I

.2 .3 .4 .5

Phugoid root, rad/sec

950027

(a) Phugoid mode data.

2

PML 1

B

• • $

Average PML

-1 I I I I

-.01 .04 .09 .14 .19

Height root, rad/eec

950028

(b) Height mode data.

Figure 20. Perceptual motor load as a function of instability for height and phugoid mode data.

18
increasing and decreasing PML as a function of increasing

instability. The lack of correlation between phugoid and

height mode instabilities and PML is inconsistent with the

pilot's estimation of workload (fig. 19). The pilot's estimation shows a strong correlation between workload and

increasing instability.

In some series of runs, PML increased as the number

of the runs evaluated in the series increased. After the

pilots evaluated a few runs in a series, they occasionally

complained that their fingers became tired because of the

tapping. This effect could increase the irregularity of their

tapping and produce increasing PML with the number of

the runs evaluated in the series. This factor may indicate

pilot fatigue is sometimes a more significant factor in the

tapping workload measurement than the increasing instability. However, data from previous research 15 show no

indication of degradation caused by fatigue in test runs of

comparable length in time. No conclusive correlations

were observed when the data were evaluated for individual pilots.

For this experiment, the tapping method was inconsistent in measuring pilot workload. No reason for the inconsistency has been isolated to date because the various

effects could not be separated out with certainty.

SUMMARY

The NASA Dryden Flight Research Center and the

Technical University of Munich are cooperating to

research the flying qualities impact of unstable long-term

dynamics on planes in hypersonic flight. These issues

were investigated with a dedicated flight simulator for

hypersonic vehicles located at NASA Dryden. The

results presented in this paper are as follows:

. Level 2 handling qualities could be maintained to a

time-to-double-amplitude of 15 sec for the height

mode and 3 sec for the phugoid mode. Level 2 is appropriate for the design of a manual backup control

mode; however, the trends in the ratings indicated

"cliff-like" tendencies that may warrant extra margin

in design. The borders of controllability were a timeto-double-amplitude of 5 sec for the height mode and

1.4 sec for the phugoid mode. A technique developed

by the pilots to take advantage of the coupling of thrust

between the vertical and longitudinal axes allowed

these large instabilities to be flyable.

. The pilots were able to maintain a steady level turn at

or within desired performance margins even at the

highest levels of instabilities, but at a high cost of

workload, as reflected by the increased Cooper-Harper

ratings.

. The flying qualities impact from the long-term dynamics influence control power design requirements. The

influence is shown by the deterioration of

the Cooper-Harper ratings that accompanies decreasing available control power. As instability increases,

the bandwidth required approaches the bandwidth the

pilot is capable of producing with the aircraft. At this

point, workload and Cooper-Harper ratings deteriorated. Decreasing control power has a similar effect.

4. A method for measuring pilot workload, the tapping

method, did not work as shown by the inconsistent results for these experiments.

REFERENCES

IL6tzerich, K., "Comparative Assessment of Future

Aerospace Planes," AIAA-93-5014, Nov. 1993.

2Berry, Donald T., National Aero-Space Plane Flying

Qualities Requirements, NASP TM- 1084, 1989.

3U.S. Department of Defense, "Flying Qualities of Piloted Vehicles," MIL-STD-1797, Mar. 1987.

4Sachs, Gottfried, "Flying Qualifies Problems of Aerospace Craft," AIAA-90-2804, Aug. 1990.

5Berry, Donald T., "Longitudinal Long-Period Dynamics

of Aerospace Craft," AIAA-88-4358, Aug. 1988.

6Etkin, Bernard, "Longitudinal Dynamics of a Lifting

Vehicle in Orbital Flight," Journal of the Aerospace Sciences, Vol. 28, 1961, pp. 779-788, 832.

7Sachs, G., Knoll, A., Stich, R., and Cox, T.,

"Flugeigenschaftsuntersuchungen mit dem HyperschallFlugsimulator der NASA," DGLR Jahrbuch 1993 I,

Sep. 1993, s. 77-83.

8Sachs, G., Knoll, A., Stich, R., and Cox, T., "Simulations- und Flugversuche fiber Flugeigenschaften von Hyperschall-Flugzeugen," Zeitschrift fiir Flugwissen-schaften

und Weltraumforschung, Band 19, Heft 1, Feb. 1995.

9Sachs, G., Knoll, A., Stich, R., and Cox, T., "Hypersonic

Simulator Experiments for Long-Term Dynamics Flying

Qualities," AIAA-93-5088, Nov. 1993.

l°Sachs, G., Knoll, A., Stich, R., and Cox, T., "Simulator

and Flight Tests on Aerospace Plane Long-Period Control

and Flying Qualities," A/AA-94-3508, Aug. 1994.

llBerry, Donald T., National Aerospace Plane Flying

Qualities Task Definition Study, NASP TM-100452, 1988.

19
12Grantham, WilliamD.,Nguyen,LuatT.,Neubauer,

M.J.,Jr.,andSmith,PaulM.,"SimulatorStudyoftheLowSpeedHandlingQualitiesof aSupersonicCruiseArrowWingTransport ConfigurationDuringApproachandLanding," Proceedings of the SCAR Conference, NASA

CP-001, Nov. 1976.

13Cooper, George E., and Harper, Robert P., Jr., The Use

of Pilot Rating in the Evaluation of Aircraft Handling

Qualities, NASA TN-D-5153, 1969.

14Berry, Donald T., and Gilyard, Glenn B., "Airframe/Propulsion System Interactions--An Important Factor in Supersonic Aircraft Flight Control," AIAA-73-831,

Aug. 1973.

15Michon, J. A., "Tapping Regularity as a Measure

of Perceptual Motor Load," Ergonomics, Vol. 9, No. 5,

1966, pp. 401--412.

20
Appendix

LINEAR EQUATIONS AT MACH 10 AND AN

ALTITUDE OF 110,000 FEET

This appendix describes the linear equations of motion

that were used in the flying qualities experiment. Basic

aircraft equations and control system block diagrams are

presented for both longitudinal and lateral--directional

axes.

LONGITUDINAL EQUATIONS

Figure A-1 shows the longitudinal block diagram. Pilot

inputs into the control system are represented as longitudinal stick deflection, 8 , and throttle position, 8r. The ep

linear aircraft equations of motion are represented by the

state-space equation, which is defined as follows:

A_

B

--0.06 1

-4.0 -0.11

-34.5 0.13x10 -3

-0.10xl05 0.70x10 -2

0 1

--0.75x I0--3 -0.19x10--5

-1.34 0

0.21x10 -4 1

0 0

0 0

-0.53xi0 -6 0.12x10 "6 0.25x10-_

0.12x10 -7 -0.24x10 --8 0.17x10-1

X 0.28x10 -5 -26.42 |

0.61x10 -5 0.12x10 -5 0.10x 105J

0.48x10 --7 -0.23x10 -10 0.17x10-_ 1

X _

q

V

H

0

U _

5£

8r

C___

57.3 0 0 0 0

0 57.3 0 0 0

0 0 1 0 0

0 0 0 1 0

0 0 0 0 57.3

-57.3 0 0 0 57.3

8ep

_ i= Ax + Bu___

y=Cx

950029

Figure A-1. Longitudinal block diagram.

21
Phugoidinstabilitieswereintroducedby varyingthe

controlsystemgain,K_i, with the values listed in table 2

in order to obtain their corresponding instabilities. The

rest of the control system gains remained constant. Height

mode instabilities were introduced by varying the value of

CDv (table 3). Varying CDv had the effect of varying only

the element X in matrix A; all the other elements in both

the A and B matrices were constant. Table A-I shows the

KS., CDv, and X values for both phugoid and height mode

variations.

LATERAL-DIRECTIONAL EQUATIONS

Figure A-2 shows the lateral-directional block diagram.

Pilot inputs are described by 8a and 8r for the lateral

• , P .

stick deflectton and rudder ped_lPmputs respecuvely. The

linear aircraft equations of motion are represented by the

state-space equation. The lateral--directional control system gains and state-space matrices remained constant

throughout both phugoid and height mode evaluations.

The state-space matrices are as follows:

Table A-I. Constants required for implementing height

and phugoid mode variations.

K8 CDv X

Phugoid mode variations

0

4.2

10

20

25

30

0 --0.22x10 -2

Height mode variations 0

0.00001 -0.89xi0 -2

-0.00003 O.18xlO -1

-0.00006 0.38x10-1

--0.00008 0.51xlO -1

--0.00018 0.12

--0.00024 0.16

--0.00030 0.2

--0.79x10 -1 0.19xl0--1

-0.43x10 -3 --0.62x 10--2

0.143 -0.99

1 0.14

0 1

0.96 0.18x10 --5 -0.42x10-7

3.76 0.70x10 -5 -0.16x 10--6

-0.17x10 -1 0.31x10-2 0.30x 10-8

-0.48x10 -3 -0.86x 10--5 0.61 xlO--4

--0.69x 10-4 -0.53x10-3 0.86x10-5

B_

5.1 1.2

0.20x10 -1 --0.75x 10-t

0.91x10 --4 0.86x10-3

0 0

0 0

P

r

u_..

_a

8

I"

C_

57.3 0 0

0 57.3 0

0 0 57.3

0 0 0

0 0 0

0

0

0

57.3

0

0

0

0

0

57.3

22
_i=Ax+Bu

y=Cx

Figure A-2. Lateral--directional block diagram.

23
REPORT DOCUMENTATION PAGE FormApproved

OMB No. 0704-0188

: Pu bhc r®PO rl.qlQbu fd_r._otlhls coltectlonofin forrn at ionisestim albeit o average 1 houtperresponse, inc[udingthetime|orrevl'ewinglnstr uctions,searchingexistingdata sources.

gathenn g a ndmainta_ning th a data needed, and completing and rev_ewmgthe collection of in lormation. Send comments re<Jarciing this burden estimate or any other aspect of this

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

April 1995 Technical Memorandum

4. TITLE AND SUBTITLE S. FUNDING NUMBERS

A Flying Qualities Study of Longitudinal Long-Term Dynamics of

Hypersonic Planes

6. AUTHOR(S)

T. Cox, G. Sachs, A. Knoll, and R. Stich

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

NASA Dryden Flight Research Center Technische Universit_it Miinchen

P.O. Box 273 Lehrstuhl f. Flugmechanik

Edwards, California 93523-0273 Arcisstrasse 21

80290 Mtinchen

9.SPONSORING/MONOTORING AGENCYNAME(S)ANDADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

Technical University of Munich

Munich, Germany

WU 466-70-64

8. PERFORMING ORGANIZATION

REPORT NUMBER

H-2034

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM- 104308

11.SUPPLEMENTARY NOTES

Prepared for the American Institute of Aeronautics and Astronautics Sixth Intemational Aerospace

Planes and Hypersonics Technologies Conference in Chattanooga, Tennessee, April 3-7, 1995.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified--Unlimited

Subject Category 05

13. AB_T_tACT (Maximum 200 worda)

12b. DISTRIBUTION CODE

The NASA Dryden Flight Research Center and the Technical University of Munich are cooperating in a research program to assess the impact of unstable long-term dynamics on the flying qualities of planes in hypersonic flight. These

flying qualities issues are being investigated with a dedicated flight simulator for hypersonic vehicles located at NASA

Dryden. Several NASA research pilots have flown the simulator through well-defined steady-level turns with varying

phugoid and height mode instabilities. The data collected include pilot ratings and comments, performance measurements, and pilot workload measurements. The results presented in this paper include design guidelines for height and

phugoid mode instabilities, an evaluation of the tapping method used to measure pilot workload, a discussion of

techniques developed by the pilots to control large instabilities, and a discussion of how flying qualities of unstable

long-term dynamics influence control power design requirements.

14.SUBJECTIP..HMS

Hypersonics, Flying Qualities, Phugoid Mode, Height Mode, Long-Term Dynamics,

Pilot Workload, Workload Estimation, Tapping Method

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION

OF REPORT OF THIS PAGE OF ABSTRACT

Unclassified Unclassified Unclassified

i

NSN 7540-01-280-5500 Available from the NASA Center for AeroSpace Information, 800 Elkridge Landing Road,

Linthicum Heights, MD 21090; (301)621-0390

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25

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