There have been many cases in which the crew of a multi-engine airplane had to use engine thrust for emergency flight control. Such a procedure is very difficult, because the propulsive control forces are small, the engine response is slow, and airplane dynamics such as the phugoid and dutch roll are difficult to damp with thrust. In general, thrust increases are used to climb, thrust decreases to descend, and differential thrust is used to turn. Average speed is not significantly affected by changes in throttle setting. Pitch control is achieved because of pitching moments due to speed changes, from thrust offset, and from the vertical component of thrust. Roll control is achieved by using differential thrust to develop yaw, which, through the normal dihedral effect, causes a roll. Control power in pitch and roll tends to increase as speed decreases. Although speed is not controlled by the throttles, configuration changes are often available (lowering gear, flaps, moving center-of-gravity) to change the speed. The airplane basic stability is also a significant factor. Fuel slosh and gyroscopic moments are small influences on throttles-only control. The background and principles of throttles-only flight control are described

Burcham, Frank W., Jr.

English

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"Background and Principles of Throttles-Only Flight Control"
Table of Contents
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CONTENTS:
• Abstract (p. 1)
• Background &Introduction (p. 2)
• Roll Principles (p. 3)
• Pitch Principles (p. 4)
• Phugoid (p. 5)
• Manual Phugoid Damping Technique (p. 6)
• Speed Control (p. 7)
• Speed Effects on Propulsive Control Power (p. 8)
• Stability and References (p. 9)
Author: Frank W. Burcham Jr.
Affiliation: NASA Do,den Flight Research Center
Phone: 805-258-3126
Fax: 805-258-3744
Address: M. S. D2033, P. O. Box 273, Edwards, CA 93523-0273
159
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e-mail: Bill_Burcham@QMGATE.DFRF.NASA.GOV
160
Background and Principles of Throttles-Only Flight Control
Frank W. Burcham
NASA Dryden Flight Research Center
Edwards, CA
Abstract
There have been many cases in which the crew of a multi-engine
airplane had to use engine thrust for emergency flight control. Such
a procedure is very difficult, because the propulsive control forces
are small, the engine response is slow, and airplane dynamics such
as the phugoid and dutch roll are difficult to damp with thrust. In
general, thrust increases are used to climb, thrust decreases to
decend, and differential thrust is used to turn. Average speed is not
significantly affected by changes in throttle setting. Pitch control is
achieved because of pitching moments due to speed changes, from
thrust offset, and from the vertical component of thrust. Roll control
is achieved by using differential thrust to develop yaw, which,
through the normal dihedral effect, causes a roll. Control power in
pitch and roll tends to increase as speed decreases. Although speed
is not controlled by the throttles, configuration changes are often
available (lowering gear, flaps, moving center-of-gravity) to change
the speed. The airplane basic stability is also a significant factor.
Fuel slosh and gyroscopic moments are small influences on throttles-
only control. The background and principles of Throttles-Only
flight control are described in this paper.
161
Background and Introduction
The crew of a multi-engine aircraft with a major flight control system
failure may use throttle manipulation for emergency flight path control.
Differential throttle control generates yaw, which through dihedral effect, results
in roll. Collective throttle inputs may be used to control pitch. Crews of DC-10, B-
747, L-1011, and C-5A aircraft have used throttles for emergency flight control ref
1.
To investigate the use of engine thrust for emergency flight control, the
National Aeronautics and Space Administration's Dryden Flight Research Center
(NASA Dryden) at Edwards, California, has been conducting a study including
flight, ground simulator, and analytical studies. One objective is to determine the
degree of control power available with engine thrust for various classes of
airplanes. This objective has shown a surprising amount of control capability for
most multi-engine airplanes, ref 2.
A second objective was to provide awareness of throttles-only control
capability and suggested manual throttles-only control techniques for pilots.
Dryden conducted simulation and flight studies of several airplanes, including
the B-720, Lear 24, F-15, B-727, C-402, and B-747, refs 2&3. A third objective was
to investigate possible augmented control modes that could be developed for
future airplanes. An augmented control system that uses pilot flight path inputs
and airplane sensor feedback parameters to provide appropriate throttle
commands for emergency landings was developed. This augmented system was
evaluated on a B-720 transport airplane simulation, ref 4, and a simulation of a
conceptual megatransport, ref 5.
Recently, simulation studies and flight tests have been conducted to inves-
tigate the details of throttles-only control for the F-15 airplane, and to investigate
the performance of a PCA (Propulsion Controlled Aircraft) augmented system.
The PCA system was installed on the NASA F-15 research airplane. The
objectives of the flight program were to demonstrate and evaluate PCA
performance in up-and-away and landing approach flight, over the speed range
from 150 to 190 knots at altitudes below 10,000 ft. There was also an option, if
PCA performance was adequate, to attempt PCA landings.
The F-15 has since completed a 36 flight series of tests, including actual landings
using PCA control. Recoveries from upset conditions including 90 deg bank at a
20 deg dive have also been flown. Altitudes to 38,000 ft and speeds up to 320
knots were flown. Six guest pilots have flown the PCA system.
The papers to follow present the principles of throttles-only flight control, flight
tests of manual and augmented propulsion-only flight control for the F-15, the
PCA design, development, and implementation, test techniques, and results, and
pilot comments.
In this paper, the principles of throttles-only flight control are presented. These
principles are rather simple but are not well-understood becuase the effects are so
much smaller than normal flight control forces that they are often ignored.
162
Bank Angle Control
As shown below, bank angle is controlled by using differential
thrust, which generates sideslip. The sideslip, through the dihedral
effect present on the F-15 and most airplanes, results in roll rate. Roll
rate is controlled to establish a bank angle which results in a turn and
change in aircraft heading.
Full differential thrust for the F-15 yields a roll rate of about 15
deg/sec at a speed of 170 kts. Because bank angle is controlled by
sideslip with throttles-only flight control, the turns are typically not
properly coordinated.
Throttles-OnlyRollControl
1=-15, 170 knots
NASA
93-30
"°EThrottle 60Anglo,deg 40
20
Left
Right
-2.0
Sideslip,
dog -1.0
0
--12 xl0 3
--0 Net thrust
per engine,
--4 Ib
--0
I I
16-
12-
8-
4-
0
=
Iv
0 1 2 3 4 5
Time, so(:
163
Principles of throttles-only control - Pitch axis
For pitch control due to throttle changes; several effects occur, as shown below.
1. Flight path angle change due to speed stability. Most stable airplanes, including the F-15
exhibit positive speed stability. Over a short period of time (appro× 15 see), a thrust increase
will cause a speed increase, which will cause a lift increase. With the lift being greater than the
weight, the airplane will climb, which causes a pitch rate increase. (If allowed to continue for a
longer period of time, this effect will be oscillatory, see "phugoid" on the next page. 1he degree
of change to the flight path angle is proportional to the difference between the initial trim
airspeed and the current airspeed, hence, the flight path angle tends to increase as speed
increases.
2. Pitching moment due to thrust line offset. If the engine thrust line does not pass through the
center of gravity (CG), there will be a pitching moment introduced by thrust change. For many
transport aircraft, the thrust line is below the CG, and increasing thrust results in a desirable
nose-up pitching moment, the magnitude being a linear function of the thrust change. This is
the desirable geometry for throttles-only control, because a thrust change immediately starts the
nose in the same direction as will be needed for the long term flight path angle change. The
effect is more a function of change in thrust than in change in speed, and occurs near the time of
the thrust increase. For the 1::-15, the thrust line passes within plus or minus an inch of the
vertical CG, depending on fuel quantity, and this effect is small.
3. Flightpath angle change due to the vertical component of thrust. If the thrust line is inclined
to the flight path, as is commonly the case, an increase in thrust will increase the vertical
component of thrust, which will cause a direct increase in vertical velocity, ie, rate of climb, and
a resulting increase in flightpath angle. For a given aircraft configuration, this effect will
increase as angle of attack,(a) increases (ie, as speed decreases)
For the F-15, the combined effects of the engine thrust is to produce a nose up pitching
response that peaks at approximately 2 deg/sec for a throttle step from power for level flight
(PLF) to intermediate power on both engines.
Pitch Axis Effects due to Thrust Increase
Thrust line slightly below the CG
Change In
Flight path=,
angle "_
due to:
Change
in Thrust
Change In
Airspeed
f Speed
stablllty
Thrust
Offset
Vertical
component
of thrust
Overall
Thrust IncreaseJ
ii ....... :- I
...... ust
-" "°°°°°...Phugoid
[
0 2 4 6 8 10 12
Tlme, sec
164
Phugoid
The phugoid is the longitudinal long period oscillation of an airplane. It is a
motion in which kinetic and potential energy (speed and altitude) are traded.
The phugoid oscillation is excited by a pitch, or velocity change, and will have a
period of approximately a minute, and may or may not damp naturally. An
example of an F-15 phugoid with the gear down and flaps up is shown below.
The oscillation was excited by a step increase in thrust, which results in an oscilla-
tory climb with very light damping. Although a very low amplitude phugoid is
usually considered to be a constant angle of attack maneuver, if the amplitude is
not small, there can be significant angle of attack variations resulting from pitch
rate damping, as shown in this example. Properly sized and timed throttle inputs
can be used to rapidly damp unwanted phugoid oscillations; these techniques are
discussed in ref 2 and 3, and shown on the next page
6
4
Flightpath, 2
deg
0
-2
180
Airspeed,
kts 170
Angle of
attack,
deg
160
9.0
8.5
8.0
0.5
Pitch
rate, 0
deg/sec -0.5
-1.0
Power 7 , , ,
0 25 50 75 100
lever
angle,
deg
Time, sec
165
Manual Phugoid Damping Technique
The phugoid, a pitch oscillation in speed and altitude, may be damped
with a properly timed and sized throttle input
Suggested technique for damping a phugoid oscillation
1. Determine trim speed from the average of VMAX and VMIN (set bug)
2. Observe trim power setting (EPR or %N1 or %N2) (set bug)
3. Just prior to reaching the top of a cycle, sharply increase power setting
(to get speed back to the trim speed as the flight path is approximately level)
4. As soon as the speed increases to the trim speed, rapidly reduce power
setting to trim
The phugoid oscillation should now be much smaller
Airspeed I VMAX
I
i I
"ate°'F
climb & __
flight pathE Jangle _ : II
I I
I I
Altitude I i
Power
setting
Add a lot of _1 I
thrust just before I_
reaching the top I
of a cycle J i
It
'_ Note trim
power setting
Time
Reduce thru_'_m
_> as soon as trim
speed is reached
166
Speed control
Once the flight control surfaces of an airplane are locked at a given
position, the trim airspeed of most airplanes is only slightly affected by
engine thrust. Retrimming to a different speed may be achieved by other
techniques, such as variable stabilizer control, center-of-gravity (CG) control,
lowering of flaps, landing gear, and reducing weight. In general, the speed
will need to be reduced to an acceptable landing speed; this implies
developing nose-up pitching moments. Methods for doing this include
moving the CG aft, lowering of flaps, extending the landing gear, or
burning off or dumping fuel. Shown below are some of these effects for the
F-15.
Trim speed is affected by changes in weight. As weight is reduced (such as
by burning fuel), (assuming that the CG remains constant) the lift remains
constant, so the airplane tends to climb. To maintain level flight, the throttle
setting must be reduced to reduce speed until lift and weight are again in
balance. For the F-15, flying at low speed and approaximately level flight,
this effect reduces trim speed by approximately I knot every I to 2 minutes.
Other effective ways of slowing the F-15 include moving the air inlets to
the full-up emergency position, and lowering the flaps. Landing gear
extension on the F-15 has essentially no effect on trim speed. Center of
gravity control using fuel transfer was studied for the F-15 and was feasible,
but was not implemented due to funding constraints.
2604 F-15, mid weight, CG 1 Weight
2401-\ I- F-15, Flaps down Light
k\
Heavy
Trim 220 !" _ o
Airspeed, I" \ Stabilizer
kts 200 I- _ Angle,
I" ,X deg "
140 ['_ -31 I I i I I I I
Inlets Lower 120 160 200 240
"Emergency" flaps Trim airspeed, kts
I67
Speed effects on propulsive control power
The propulsive forces (differential thrust for yaw for lateral control and
collective thrust for flightpath control) tend to be relatively independent
of speed, whereas the aerodynamic restoring forces that resist the
propulsive forces are proportional to the dynamic pressure, which is a
function of speed squared. This relationship results in the propulsive
control power being approximately inversely proportional to the speed.
The figure below shows these effects for the F-15. The maximum roll
rate for a full differential thrust step varies from 8 deg/sec at 300 kts to 18
deg/sec at 150 kts. The maximum pitch rate, occurring approximately 8
sec after the throttles were stepped from power for level flight (PLF) to
intermediate, varies from 0 at 300 kts to 2 deg/sec at 170 kts.
Maximum
roll rate,
deg/sec
20
Note: Maximum roll rate occurs
about 2.5 sec after thrust change
Maximum
pitch rate,
deg/sec
10
A
0 -,
140 160 180 200 220 240 260 280 300
Airspeed, kts,
2.5
2.0 Note: maximum pitch rate occurs
| _ about 10 to 12 sec after thrust increase
|
1.0
140 160 180 200 220 240 260 280 300
Airspeed, kts,
168
Airplane Stability
The flight controls-failed stability of an airplane is an important
consideration for throttles-only control. Large transport airplanes
typically have good basic static stability. Yaw dampers may be used for
increasing the dutch roll mode stability, but good pitch, roll, and yaw
static stability is usually built in. This stability remains if the flight
control system should be lost. For fighter airplanes, the airframe may
have lower levels of static stability, with adequate stability being
achieved with mechanical and/or electronic stability augmentation.
Thus in the case of flight control system failure in a fighter, the basic
short period stability may be considerably reduced, and the control
requirements for a PCA system will be more difficult. (The previous
comments do not apply to the long-period phugoid stability which will
likely be a problem for both fighter and transport aircraft)
References for PCA Background and Principles
1. F.W. Burcham Jr., Trindel Maine, C. Gordon Fullerton, and Ed Wells:
Preliminary Flight Results of a Fly-By-Throttle Emergency Flight Control
System on an F-15 Airplane, NASA TM 4503, 1993
2. Burcham, Frank W. Jr., and Fullerton, C Gordon: Controlling Crippled
Aircraft - With Throttles, NASA TM-104238, Sept 1991.
3. Dornheim, Michael A.: "Research Pilot Devises Rules of Thumb for
Engine-Only Control of Disabled Aircraft", Aviation Week and Space
Technology, June 24, 1991, pp 43.
4. Gilyard, Glenn B., Joseph L. Conley, Jeanette L. Le, and Frank W.
Burcham, Jr., "A Simulation Evaluation of a Four-Engine Jet Transport
Using Engine Thrust Modulation for Flightpath Control, AIAA-91-2223,
June 1991.
5. Gerren, Donna: Design, Analysis and Control of a Large Transport
Aircraft Utilizing Engine Thrust as a Backup System for the Primary Flight
Controls. NASA CR-192938, Mar 1993
169


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