Flight Energy Management Training

13 Pages·2017·0.53 MB·English

Available online at http://docs.lib.purdue.edu/jate
Journal of Aviation Technology and Engineering 3:1 (2013) 24–36
Flight Energy Management Training: Promoting Safety and Efficiency
Juan R. Merkt
DavisAviationCenter,JacksonvilleUniversity
Abstract
Poor aircraft energy management can lead to unsafe andinefficient operations. Despite their impact on safety and economy, energy
management skills are not adequately taught or evaluated in civilian pilot training. This paper 1) addresses the need for better energy
management training, 2) provides a conceptual and pedagogical framework for later curriculum development, and 3) suggests key
attributes of an effective training program. To make the case, the study uses energy management to link safety and efficiency. It then
synthesizesenergyprinciplesacrossdisciplinesandillustrateshowsuchprinciples,oncesimplified,becomepowerfulinstructionaltools.
Finally,itsuggeststhatanintegrated,energy-centered,top-downtrainingapproachwillleadtoabettermentalmodelofhowtheairplane
works and,in doing so,toenhanced energymanagement anddecision-making skills for safe andefficient operations.
Keywords: aircraft performance, controlled flight into terrain, energy efficiency management, energy balance, energy flow, energy gradient, energy
height,energymaneuverability,energysafetymanagement,energystate,flightcontrol,flightenergymanagement,kineticenergy,lossofcontrolinflight,
lawofenergyconservation,potentialenergy,specificexcesspower,runwayexcursion,totalenergycontrolsystem
Introduction
Aviation isa high-risk industry tasked withtransporting people and goods safely and efficiently. Unsafe and inefficient
operations pose a threatto public safety and confidence, and can cause significant financial losses tothe aviation industry
(BTRE,2006).Theincidenceofin-flightenergycrisesleadingtoaircraftaccidents(Airbus,2005;Boeing,2012;Jacobson,
2010) as well as the impact of wasted energy on increased operating costs (Airbus, 2004; Berglund, 2008; Boeing, 2004)
suggest a need for better energy management training for pilots.
AbouttheAuthor
JuanR.MerktisdirectoroftheDavisAviationCenteratJacksonvilleUniversity.Hehasbeeninvolvedinprogramsthateducateandtrainaspiring
airlinepilotssince1994.Hisresearchinterestsinvolveaircraftandhumanperformancelimitationsinextremeflightenvironments.Hiscredentialsinclude
aPh.D.degreefromHarvardUniversityandFAACommercialPilotandGoldSealFlightInstructorcertificates.MerktiscurrentlyservingontheBoardof
TrusteesoftheAviationAccreditationBoardInternational(AABI)andischairofitscriteriaandinternationalcommittees.HehasservedasanAABI
representativeintheNextGenerationofAviationProfessionals(NGAP)TaskForcecreatedbyICAOtoaddresstheworldwideshortageofskilledaviation
profe[e-mail].
http://dx.doi.org/10.7771/2159-6670.1072
J.R.Merkt / JournalofAviationTechnologyandEngineering 25
Energy is a fundamental, universal, and pervasive aircraft performance. This section examines more closely
concept.Viewinganairplaneinmotionasanenergysystem the usefulness of an energy-centered approach in linking
can enhance a pilot’s understanding of how the airplane safety (flight control) and efficiency (aircraft performance).
‘‘works’’ andhowtomanageits energyinflight.Airplanes From an energy management view, safety and efficiency
convertenergyastheymovethroughtheair,takingthermal can be regarded as ‘‘two sides of the same coin.’’
energyproducedfromburningfuelandtransformingitinto
the essentials of flight: altitude (potential energy) and Energy Safety Management
airspeed (kinetic energy). In the process of creating the
magicofflight,someoftheenergygainedfromburningfuel The safety side of energy management is primarily
is lost to the environment, generating a continuous flow of concernedwithflightcontrol.Onecandefineenergysafety
energy into and out of the airplane. Awareness of the management as the process of monitoring and controlling
aircraft’senergystateandflow,aswellasunderstandingthe theairplane’sflightpathandairspeedusingproceduresand
role of the flight controls for manually or automatically techniques designed to mitigate hazards caused by unsafe
managing its energy, can enhance safety and efficiency. or degrading energy states.
Once flying, an airplane will inexorably continue to Improper flight energy management can be deadly. A
transform and exchange various forms of energy until the significant number of fatal aircraft accidents have been
very end of the flight, sometimes with disastrous con- associated with poor management of vertical flight path
sequences. It is up to the pilot to ensure that the airplane’s (potentialenergy)and/orairspeed(kineticenergy).Resulting
energyismanagedsafelyandefficientlyfromstarttoend. fromtheinabilityofthepilot(orautopilot)tosafelymanage
Fighter pilots are well trained in energy management the airplane’s energy state or restore it following an upset,
skills—their lives depend on the energy tactics needed to lossofcontrolinflight(LOC-I)hasbecomethenumberone
outmaneuver and outperform enemy aircraft during air cause of fatal accidents in commercial (Boeing, 2012;
combat(Shaw,1985).Amongcivilians,gliderandaerobatic Jacobson,2010)andgeneralaviation(EAA,2011).Arecent
pilotsalsounderstandanduseenergymanagementskillsto example of loss of control accidents is the crash of Colgan
maneuver safely and efficiently (FAA, 2003; Szurovy & AirBombardierQ400inBuffalo,NewYork,whichresulted
Goulian, 1994). Unfortunately, the same cannot be said of inwidespreadmediaattentionandsweepingnewlegislation
most pilots being trained through traditional civilian for pilot qualifications (Jacobson, 2010). The next two
programsthatdonotappeartoadequatelyteachorevaluate leading causes of fatal accidents, controlled flight into
energy management skills.1 Such deficiency should be a terrain (CFIT), where an airworthy airplane is unintention-
cause for concern. The aviation industry is bracing for an ally flown into the ground (Boeing, 2012), and runway
unprecedented surge of pilot hiring worldwide (Boeing, excursions during landing, where an airplane undershoots,
2011) at a time when most of the pilot supply comes, not overruns,orcontactstherunwayabnormally(Boeing,2012),
from the military as in the past, but from civilian training can also be at least partially attributed to poor energy
programs(Duggar,Smith,&Harrison,2009). awarenessormanagement(Airbus,2005).Althoughvarious
This study examines the rationale and framework for contributing factors have been linked to each of the three
flight energy management training and provides general leadingcausesoffatalaccidents(Airbus,2005;Clark,2005;
guidelines for the design of such training programs. First, Cox, 2010;Jacobson, 2010),improperenergymanagement
it uses energy management to link flight safety and appearstobeasignificantcommondenominator.
efficiency. It then applies a multidisciplinary approach to Severalindustry/government-ledinitiativeshaveaddressed
develop the conceptual and pedagogical framework for the above leading causes of fatal accidents. Such initiatives
future training programs. Finally, it suggests key attributes include the upset recovery training aid (URTA), aimed at
of an effective energy management training program. decreasing loss-of-control-in-flight accidents (FAA, 2008);
the terrain alert warning system (TAWS), developed for
Energy Management as an Integrating Approach reducing controlled-flight-into-terrain accidents (Cox, 2010);
andtheapproachandlandingaccidentreduction(ALAR)tool
Merkt(1992)suggestedusingtheairplane’spowercurve kit,focusedondecreasingrunwayexcursionaccidents(Clark,
as a tool to demonstrate principles of flight control and 2005). Although TAWS has been successful in reducing
CFIT accidents (Jacobson, 2010), CFIT is still the second
highest cause of jet fatalities worldwide (Boeing, 2012).
1Energymanagementcompetenciesforpilottrainingarenotaddressedin
FAA airman certification and training regulations (FAA, 2011a), FAA NeithertheURTAnorALARinitiativesappeartohavebeen
airman practical test standards (FAA, 2011b), or AABI aviation effectiveindecreasingaccidentscausedbyloss-of-control-in-
accreditation criteria (AABI,2012a). Inaddition,‘‘energymanagement’’ flight(Jacobson,2010) and runwayexcursions(Cox,2010),
doesnotappearasatopicinaviationcoursetitlesordescriptionsinflight
respectively.
education curricula as listed in the 2011-12 catalogs from the 27
The above initiatives constitute disparate remedial
universities and colleges that have AABI-accredited flight education
programs(AABI,2012b). measures and may not sufficiently address the root causes
26 J.R.Merkt / JournalofAviationTechnologyandEngineering
ofenergycrisesinflight.Basicenergysafetytrainingcould stallspeed,whichcouldresultinlossofcontrol.Aproperly
goalong wayinhelpingpilotsacquiretheknowledgeand trainedpilot,caughtbetweentwounsafesituations(running
skills needed to operate safely. Such basic training could outoffuel versusstalling), maydecidetoslow down toan
incorporate common mitigation strategies designed to intermediate speed that would still prolong time aloft and
prevent and detect unsafe energy states, and serve as the provideahighermarginofsafety.
foundation for advanced drills, including emergency Notethatinaddressingtheintegrationofenergysafetyand
recovery techniques such as URTA (Jacobson, 2010). efficiency, emphasis is on altitude and airspeed control and
related performance for flight in the ‘‘up-and-down’’ vertical
Energy Efficiency Management plane, since most of the airplane’s time and energy are spent
flyingstraight-and-level,climbing,anddescending.However,
The efficiency side of energy management is primarily someoftheairplane’senergyisalsospentmaneuveringinthe
concerned with aircraft performance. One can define horizontal plane. Maneuvering along a curved flight path
energyefficiencymanagementastheprocessofmonitoring requires coordination of multiple flight controls (not just
and controlling the airplane’s performance using proce- throttleandelevator)torotatetheairplanearounditsthreeaxes
dures and techniques aimed at reducing the amount of fuel andchangeitsattitude.Obviously,maneuveringcontrolerrors
consumed per unit distance or time. can impact flight safety. For example, overbanking or
Energy efficiency is often associated with range or excessive yawing during a turn could lead to an initial loss
endurance performance in cruise flight (Hurt, 1965). of control in flight. On the other hand, maneuvering
However, energy efficiency also applies to other phases performance can affect energy efficiency. Thus, excessive
of flight and even ground operations. Thus, energy- maneuvering can drain energy because of the increased load
centered training may be ideal to develop energy manage- factor and resultant higher induced drag during turns (Hurt,
mentskillsforefficientoperations(Merkt,1992).Although 1965). In addition, as the airplane rotates around its axes,
this view of aircraft performance emphasizes fuel-optimi- stabilityalsobecomescriticalasitcanaffectbothenergysafety
zationstrategies, training should also addressother aspects andefficiency.Forinstance,movingthecenterofgravityaftto
of performance and assess trade-offs. Fuel efficiency is improvefueleconomyreduceslongitudinalstability,whichin
important, but there are times when other factors (e.g., turn makes controlling the airplane on the vertical plane
safety, time-related performance, ATC restrictions) may inherently less safe (Barnard & Philpott, 1989). Thus, an
take precedence over energy economy. integrativeenergyapproachseemsidealtoaddressinteractions
Fuel consumption is the second highest cost to airline among various aspects of control, stability, and performance
operators (Stolzer, 2002; Vause, 2004), and proper energy andexaminetheirrelativeimpactonsafetyandefficiency.
management techniques can save fuel and decrease cost of Having established that an energy view is useful in
flight operations (Airbus, 2004; Berglund, 2008; Boeing, linkingflightsafetyandefficiency,thenextsectionfocuses
2004). Although fuel economy depends on many factors, on general energy principles that can be applied to flight
someofwhicharebeyondaflightcrew’scontrol(e.g.,pre- controlandaircraftperformance.Asitwillbecomeevident,
flightactionsbygroundpersonnelandin-flightrestrictions the energy connection between control and performance is
by air traffic control), in-flight procedures and piloting well rooted in the science of motion.
techniques can have the largest impact on fuel savings
(Airbus, 2004; Boeing, 2004). Boeing estimates that a 1 Conceptual Framework for Energy
percent reduction in fuel used per flight can result in fuel Management Training
savings of up to 135,000 gallons per airplane per year
(Boeing, 2004). At current jet fuel prices, that’s up to $0.5 Barring a few related findings,2 a literature review
million in savings per airplane per year. revealed a lack of energy management textbooks, studies,
Basic energy efficiency training could go a long way in orcurriculadesignedfortrainingcivilianpilots.Traditional
assisting pilots develop energy conservation skills to save textbooks on flight theory for pilots (e.g., Barnard &
fuel and reduce operating costs. Obviously, flight crews
mustnotcompromisesafetyforefficiency.Abenefitfroman 2Inhisclassicbook,StickandRudder,Langewiesche(1944)emphasizes
integrated approach, incorporating both energy control and theroleoftheelevatorforexchangingaltitudeandairspeedashedescribes
the ‘‘zoom’’ technique and the ‘‘law of the roller coaster.’’ In a short
performance,isthatitmayenhancetheabilitytoassessthe
training guide for pilots, Airbus (2005) outlines energy management
balancebetween safetyandefficiency,helping pilotsmake
principlestocontrolflightpathandairspeedduringtheapproachphase.In
critical decisions. As an example, consider an unsafe his online book, See How It Flies, Denker (2008) explains energy
situation where fuel is running low. The pilot could slow conversions and the effect of the flight controls on energy. Amelink,
downtomaximumendurancespeed,atthe‘‘bottom’’ofthe Mulder, van Paassen, and Flach (2005) apply total energy control
principles to describe how pilots can control aircraft speed and altitude.
power curve, in order to minimize fuel consumption and
Finally, Merkt(1990; 1991;1992)exploresthe instructional merits of a
maximizetimealoft.However,slowingdownformaximum
‘‘top-down,’’energy-centeredapproachfortrainingpilots.Thetop-down
endurance would bring the airplane dangerously close to approachisdiscussedlaterinthispaper.
J.R.Merkt / JournalofAviationTechnologyandEngineering 27
Philpott, 1989; Dole, 1989; Hubin, 1992; Hurt, 1965) Equation 1 defines how much of the airplane’s total
neglect or insufficiently address energy management con- mechanical energy is stored as potential energy (altitude)
cepts. Fortunately, flight energy concepts rely on funda- and kinetic energy (airspeed) at any given time.
mental physics. In addition, energy management models Turning to the forces of motion, Newton’s second law
have been applied successfully to address a wide range of (SF 5 ma) provides a simplified longitudinal point-mass
problemsdealingwithself-poweredmotion,fromdesigning equation depicting the summation of the forces acting on
saferautomatic flightcontrolsystemsfor passengeraircraft the airplane along its flight path (Anderson, 2007):
(Lambregts,1983),andhelpingfighterpilotswinaircombat
T{D{Wsinc~ma ð2Þ
battles (Shaw, 1985), to elucidating how animals move
around efficiently (Pennycuick, 2003). Physics principles Where:
and energy models from other disciplines provide the
T 5 thrust
conceptualframeworkfortheproposedtrainingprogram.
D 5 drag
W 5 aircraft’s weight
Physics: Laws of Energy Conservation and Motion
c 5 angle of flight path with the horizontal
m 5 aircraft’s mass
Energy management concepts are founded on basic
a 5 aircraft’s acceleration
principles of physics, namely the law of energy conserva-
tion, which defines the energy state of the airplane at any Equation2 assumesthatthrust(T)actsalmostnearlyinthe
given time (Goodman & Warner, 2001), as well as directionoftheflightpath.Formostsituations,exceptforflight
Newton’s laws of motion that govern the airplane’s at very high angle of attack, this is a reasonable assumption
translational (and rotational) movement through the air (Anderson,2007;vonMises,1945).NotethatEquation2can
(Anderson, 2007). Energy and motion equations, such as be applied to climbing, level, or descending flight simply by
those described below, provide the foundation for an making flight path angle c . 0, 5 0, or , 0, respectively.
energy-centered study of aircraft control and performance. Likewise,theequationcanbeappliedtoaccelerating,constant
Appliedtoflight,thelawofenergyconservationdefines speed,ordeceleratingflightbymakingaccelerationa.0,50,
the total mechanical energy of the airplane—its energy or,0,respectively.Certainly,whenc50anda50,thenT
state—as the sum of its gravitational potential energy 2 D 5 0, which represents the familiar steady-state equation
derived from height above the ground plus its kinetic forlevel,unacceleratedflight—wheretheairplaneismaintain-
energyacquiredfromspeedthroughtheair(Amelinketal., ingaconstantaltitudeandairspeed.
2005; Goodman & Warner, 2001; Rutowski, 1954): Reorganizing the terms in the above equation:
E~E zE ~mghz1= mV2 ð1Þ T{D~Wsinczma ð3Þ
pot kin 2
Where: Equation 3 is a simple but significant rearrangement of
thetermsinEquation2,sinceitallowsustocombineflight
E 5 aircraft’s total mechanical energy
path angle (c) and acceleration (a) as a function of the
E 5 gravitational potential energy (mgh)
pot difference between thrust and drag (T 2 D). The
E 5 kinetic energy (K mV2)
kin significance is that flight path angle (c) is associated with
m 5 aircraft’s mass
altitude changes, just as acceleration (a) is associated with
g 5 gravitational acceleration
airspeed changes. But there is more to these energy and
h 5 aircraft’s altitude above a ground reference
motion equations than meets the eye.
V 5 aircraft’s velocity What may not be apparent by looking at the above
In Equation 1, using energy as the common ‘‘currency’’ equationsisthefactthatthelawofenergyconservationand
tomeasurehowhighandfastanairplane isflying, thelaw Newton’s second law of motion are intimately linked
ofenergyconservationallowsustocombinetheairplane’s (Goodman&Warner,2001).Thiscloserelationisthecrux
altitude (h) and airspeed (V) and examine their relative of the energy view of flight control and performance.
contribution to its total energy.3 To put it differently, Accordingtothelawofenergyconservation,energycanbe
converted from one form to another, but it can never be
created or destroyed. Thus, the conservation law allows an
3Inthispaper,thetermsaltitudeandheight(h)areusedinterchangeablyto
airplane to exchange stored kinetic and gravitational
refertothe verticaldistance(e.g.,feet)abovea referenceon theearth’s
surface. Usually, altitude is measured from sea level, while height is potential energy while its total mechanical energy remains
measuredfromgroundlevel.Thetermsairspeedandvelocity(V)arealso constant. During such exchange, one form of energy (e.g.,
usedinterchangeably.Notethatairspeedisascalarquantityrepresenting potential energy) increases at the expense of another (e.g.,
themagnitudeoftheairplane’sspeedthroughtheair(e.g.,knots),while
kinetic energy). Simply put, airspeed can be traded for
velocity is a vector quantity denoting both magnitude and direction.
altitude and vice versa. The conservation law also dictates
Energy (or work), on the other hand, is a scalar quantity that can be
measuredinunitssuchasfoot-poundsorjoules. that energy can be added to or removed from the energy
28 J.R.Merkt / JournalofAviationTechnologyandEngineering
stored in an open system (i.e., the flying airplane). Of mechanical energy increases, decreases, or remains the
course, the energy gained is not created—it is transferred same. Any change in total mechanical energy is, in turn,
in from another system acting as the energy ‘‘source.’’ distributedoverpotential(altitude)and/orkinetic(airspeed)
Likewise,theenergylostisnotdestroyed—itistransferred energy.Forexample,ifT2Disgreaterthanzero,thereis
out to another system acting as the energy ‘‘sink.’’ Once a positive net energy flow into the airplane, allowing it to
airborne, the airplane gains energy through engine thrust climband/oracceleratetoanew(higher)energystatelevel.
(T), the propulsive force generated from burning fuel, and In sum, the left-hand side of the balance equation controls
loses energy through drag (D), the retarding force that thenettransferofenergy intooroutoftheairplanebythe
releasesheatintothesurroundingair(Amelinketal.,2005; actionoftwoforces(TandD)—henceregulatingtherateof
Rutowski,1954).Consequently,energyflowscontinuously change of total mechanical energy; while the right-hand
intoandoutoftheflyingairplane.Moreimportantly,there side controls the distribution of the net transferred energy
is a fundamental relationship between the energy flowing between altitude (E˙ ) and airspeed (E˙ ).6 Thus, in
pot kin
through the airplane and changes in its energy state. examining basic principles of flight energy management,
Since energy cannot be created or destroyed, an energy Newton’s second law of motion (which dictates the rate of
balance must always exist between 1) the net flow of change of the airplane’s total mechanical energy) is as
energy through a system (i.e., energy gain minus loss) and
importantasthelawofenergyconservation(whichgoverns
2) the resultant change in total energy within the system
the airplane’s ability to transfer, distribute, store, and
(Cengel, 2007). The airplane’s energy balance, central to
exchange energy).
both flight control and aircraft performance, can be
As described in the next three sections, the relation
expressed as follows4:
between energy and motion laws has been exploited
E {E ~DE ð4Þ differently by two groups of engineers and a group of
T D
biologists in their energy approach for studying motion.
Where: Using the same set of energy and motion equations, one
group of engineers has focused on developing principles
E 5 energy gained through thrust
T
for energy-based flight control (e.g., Lambregts, 1983),
E 5 energy lost through drag
D
while asecond grouphasconcentratedonfindingrulesfor
DE 5 change in total mechanical energy
energy-based aircraft performance (e.g., Rutowski, 1954).
Theenergybalanceequationcanalsobewrittenonaper
Biologists,ontheotherhand,havetakenanalogousenergy-
unit time basis, converting it into a time rate equation,
force relations one step further in order to examine the
where the dot over the letter symbolizes rate5:
energy-based integration of motion control and perfor-
E_ {E_ ~E_ ð5Þ mance(e.g.,Dickinsonetal.,2000).Inaddition,theability
T D
of some animals to travel ultra-long distances without
Expressing the difference E˙ 2 E˙ as a net total energy ‘‘refueling’’ has prompted biologists to develop energy
T D
flow E˙ , and the rate of change of the airplane’s total models addressing not only the efficiency of extracting
T-D
mechanical energy E˙ as the sum of the rates of change of mechanical energy from fuel but also that of converting
potentialenergy(E˙ )andkineticenergy(E˙ ),theenergy mechanical energy into motion (e.g., Pennycuick, 2003).
pot kin
balance equation takes on a more useful form:
Engineering: Total Energy Control System
E_T{D~E_potzE_kin ð6Þ
An important model for aircraft energy control comes
As implied by Equation 6, the net total energy flow
(E˙ )—a direct function of the difference between thrust from NASA-funded research conducted by Tony
T-D
and drag (T 2 D)—determines whether the aircraft’s total Lambregts while working at the Boeing Company
(Lambregts, 1983). Applying energy concepts to design
safer and more efficient automatic flight control systems,
4Rutowski(1954)championedtheenergybalanceapproachforanalyzing
aircraft performance. As explained in the next section, the same energy Lambregts developed the Total Energy Control System
methodcanbeusedforsolvingflightcontrolproblems.NotethatEquation (TECS), a multi-input, multi-output (MIMO) automated
4representsanapproximationoftheenergybalancerelationshipbecauseit system that integrates the autopilot (which operates the
neglectstoaccountforthechangeintotalmechanicalenergycausedbythe
elevator) and the autothrottle for decoupled vertical flight
changeinaircraftweightasfuelisgraduallyburnedinflight(Rutowski,
path (altitude) and airspeed control. In MIMO automatic
1954).Theeffectofweightchangemaybenegligiblewhenapplyingthe
energyapproachtosolveshort-termcontrolproblems(e.g.,Amelinketal., control systems, flight path and airspeed error signals are
2005),butitiscriticaltosolvelong-termperformanceproblemssuchas cross-fed to both autopilot and autothrottle. In contrast, in
thoseinvolvingclimbandrangecalculations(e.g.,Pennycuick,2003).For
conventional single-input, single-output (SISO) systems,
acompleteanalysisoftheenergybalanceequationasitappliestoaircraft
performance,seeRutowski(1954).
5Energy(orwork)perunittimeispower,whichcanbemeasuredinunits 6SeeFigure1inthenextsectionforavisualrepresentationoftheenergy
suchashorsepowerorwatts. balanceequation.
J.R.Merkt / JournalofAviationTechnologyandEngineering 29
flightpathandairspeederrorsareindependentlycontrolled really energy controls, where the throttle acts as the total
by the autopilot and autothrottle, respectively (Lambregts, energycontrollerandtheelevatorastheenergydistribution
1983). The problem with SISO strategies is that, without controller (Lambregts, 1983; Lambregts et al., 2008).
coordinated use of the controls, the change in one variable Areservoiranalogy(Figure 1),adaptedfromAmelinket
(e.g., altitude) is always coupled with an unwanted change al. (2005), illustrates the energy-based role of the throttle
in the other variable (e.g., airspeed). In addition, under andtheelevatorforaltitudeandairspeedcontrol.Asshown
certain conditions, the inherent limitations of a typical inthediagram,theairplanegainsenergythroughthrust(T)
SISOautomaticcontrolsystemcanleadtounsafesituations andlosesenergy throughdrag(D)atratesindicatedbyE˙
T
suchaslossofcontrolinflight(Lambregts,Rademaker,& and E˙ , respectively. The net total energy flow E˙ ,
D T-D
Theunissen, 2008). resulting from the difference between thrust and drag,
Significantly, the TECS/MIMO control system was determines whether the airplane’s total energy (E)—the
designed from the ground up following energy manage- sum of the energy contained in the altitude and airspeed
ment principles (Lambregts, 1983). The airplane has two ‘‘reservoirs’’—increases, decreases, or remains constant.
primarydevicestocontrolaltitudeandairspeed:thethrottle Thethrottleregulatesthenettotalenergyflow(E˙ )while
T-D
and the elevator. The question is: which device controls the elevator controls the distribution of this energy flow
altitude and which one controls airspeed? Given that between altitude and airspeed. In other words, the throttle
altitudeandairspeedareintrinsicallyrelatedtotheaircraft’s and elevator control the airplane’s energy balance
total energy, Lambregts reasoned that the problem of (Equation 6)—with the throttle controlling the left-hand
controlling these variables is basically one of managing ‘‘energy transfer’’ side (E˙ ) and the elevator controlling
T-D
energy. From an energy management view, he recognized the right-hand ‘‘energy distribution’’ side (E˙ + E˙ ) of
pot kin
thatthethrottle,whichcanrapidlyincreaseordecreasethe theequation.Thus,whenthethrottleincreasesthrustabove
amount of thrust (energy gain), should be used to regulate drag (T 2 D . 0), there is a net positive energy flow
therateofchangeoftheaircraft’stotalmechanicalenergy. raising the airplane’s total energy, and when the throttle
Eventhoughthelatterisafunctionofboththrustanddrag reduces thrust below drag (T 2 D , 0), there is a net
(Equation 5), the amount of drag (energy loss) mainly negative energy flow decreasing the total energy. This
varies due to long-term changes in airspeed or limited positive or negative energy flow is distributed by the
deploymentofhighlift/dragdevicesthatcanonlyincrease elevatorintooroutofthealtitudeandairspeedreservoirsat
drag (Amelink et al., 2005; Lambregts, 1983). Therefore, ratesindicatedbyE˙ andE˙ ,respectively.Finally,when
pot kin
most changes in total energy—demanded by new or thethrottleadjuststhrustequaltodrag(T2D50),thenet
corrective maneuvers—are initiated by changing thrust, energyflowiszero,buttheenergystoredinthealtitudeand
not drag. In the long-term (i.e., once the targeted steady airspeed reservoirs can be exchanged using the elevator
flight condition is achieved), engine thrust can be re- while total energy, intheshort-term, remains constant. For
trimmed to compensate for changes in drag and satisfy the a more extensive analysis of the reservoir analogy and its
new total energy demand (Lambregts, 1983). Next, limitations see Amelink et al. (2005).
Lambregts noted that the elevator, which can be used to Fromasafetyperspective,itisnoteworthythateveryin-
trade the aircraft’s potential energy for kinetic energy and flight energy crisis starts as a deviation from the desired
vice versa, effectively controls the distribution of energy verticalflightpathand/orairspeed.Akeyadvantageofthe
between altitude and airspeed. Thus, notwithstanding energy approach to flight control is that it provides an
the debate about ‘‘single-input, single-output’’ strategies effective strategy for correcting such deviations. Since the
(Table 1), neither the throttle nor the elevator controls aircraft’s total mechanical energy is distributed over
altitude and airspeed independently. Rather, to control potential andkineticenergy, onecan distinguishtwotypes
altitude and speed effectively, both devices need to of energy state deviations: 1) total energy errors and 2)
be coordinated following energy management rules. energydistributionerrors(Amelinketal.,2005).Inthefirst
According to TECS principles then, the flight controls are case,theairplanehastoolittleortoomuchtotalenergy,but
Table 1
Strategiesusedtocontrolairspeedandaltitude.
Strategy Throttlecontrols: Elevatorcontrols:
SISO1 Airspeed Altitude
SISO2 Altitude Airspeed
TECS/MIMO Totalenergyrate(E˙) Distributionofenergybetweenaltitude(E˙ )andairspeed(E˙ )
pot kin
Note:In‘‘single-input,single-output’’(SISO)controlstrategies,wherethecontrolsarenotcoordinated,adesiredchangeinonevariable(e.g.,airspeed)is
alwayscoupledwithachangeintheothervariable(e.g.,altitude)resultinginunwantedandinefficientenergydeviations.Incontrast,intheenergy-based
‘‘multi-input,multi-output’’(TECS/MIMO)controltechnique,thethrottleandelevatorarecoordinatedtodecouplealtitudeandairspeed.Forathorough
analysisofthesecontrolstrategiesseeAmelinketal.(2005)andLambregtsetal.(2008).
30 J.R.Merkt / JournalofAviationTechnologyandEngineering
Military Science: Energy Maneuverability Theory
Military researchers have long been interested in
predicting optimal trajectories of fighter aircraft during air
combat (Cliff, 1998; Kelley, Cliff, & Weston, 1986;
Rutowski, 1954; Wendl, Grose, Porter, & Pruitt, 1974).
Air combat models rely partly on energy maneuverability
(EM) theory,animportant conceptlinkingtheenergy state
of an airplane withits potential performance (Kelley etal.,
1986; Shaw, 1985). Fighter pilots use energy tactics to
outmaneuver and outperform their opponent during air
combat (Kelley et al., 1986; Shaw, 1985).
One of the core concepts in EM theory is energy height
or specific energy (E), which represents the idealized
s
maximum altitude that the airplane would reach from its
currentaltitudeifitconvertedallitsexistingkineticenergy
into potential energy (Cliff, 1998; Shaw, 1985). E is
s
measuredinunitsofheight(e.g.,feet)anditcorrespondsto
thetotalmechanicalenergy oftheairplane—thesum ofits
potential and kinetic energies—divided by its weight:
Figure 1. The reservoir analogy (adapted from Amelink et al., 2005)
depicts the role of the throttle and elevator in controlling the aircraft’s E=W~(cid:2)mghz1= mV2(cid:3)=W
2
energy balance (see Equation 6). The throttle controls the ‘‘valve’’
regulatingthenettotalenergyflowwhiletheelevatorcontrolsthe‘‘valve’’ Noting that W 5 mg, one can simplify the above
regulating the distribution of energy between altitude and airspeed. The
equation to obtain:
solid arrows represent energy flows, not relative direction of forces or
airflows. Note that thrust (E˙T) and drag (E˙D) energy flows are E ~hzV2=2g ð7Þ
unidirectional. Once energy from fuel is converted to thrust, it cannot s
flowback intothe fuel tanks.Likewise, energy lostthroughdragto the
environmentisunrecoverable.Incontrast,thenettotalenergyflowE˙ Plottedonanaltitude-velocitygraph,Eslinesofconstant
T-D
(resulting from the difference between thrust and drag) as well as the energy height describe the energy state of the airplane for
altitude (E˙ ) and airspeed (E˙ ) energy flows can switch directions as any given combination of altitude and airspeed (Figure 2).
pot kin
energymovesintooroutthereservoirs.Finally,notethatstoredenergy Thus, each E line on the graph represents an idealized
s
can also be exchanged between reservoirs without being lost to
instantaneous ‘‘zoom’’ along which kinetic (airspeed) and
theenvironment.
potential(altitude)energymaybeexchangedbackandforth
whilemaintainingaconstantenergystate(Anderson,2007;
itsrelativedistributionoverpotentialandkineticenergymay
becorrect.Intotalenergyerrors,altitudeandspeeddeviations
tend not to be inversely correlated, and thus both variables
usually deviate in the same direction (e.g., low-and-slow or
high-and-fast). In the second type of error, the aircraft may
havetheproperamountoftotalenergybutitsdistributionover
potentialandkineticenergyisincorrect.Inenergydistribution
errors, altitude and speed deviations tend to be inversely
correlatedandthusbothvariablestypicallydeviateinopposite
directions(e.g.,high-and-sloworlow-and-fast).Accordingto
TECSprinciplesthen,totalenergyerrorsshouldbecorrected
by increasing or decreasing energy using the throttle, while
energydistributionerrorsshouldbecorrectedbyexchanging
energy between altitude and speed using the elevator
(Amelink et al., 2005). When both total energy and its
distribution are off, the pilot (or automatic flight control
system) may need to mix the use of the flight controls
accordingly to bring the airplaneback into its properenergy
state. In addition, once short-term control is achieved and Figure 2. Altitude–velocity diagram, adapted from Anderson (2007),
showinglinesofconstantspecificenergy(E)orequalenergyheight.In
energy deviations are corrected, the airplane will need to be s
this graph, (pressure) altitude is plotted in thousands of feet and (true)
trimmedforlong-termcontroltomaintainthedesiredvertical
airspeedinknots.Forsupersonicaircraft,itiscommontoplotairspeedin
flightpathandairspeed(Amelinketal.,2005). Machnumberinsteadofknots.
J.R.Merkt / JournalofAviationTechnologyandEngineering 31
Cliff,1998;Shaw,1985).Inotherwords,byestablishingthe The link between specific excess power (P) and energy
s
aircraft’s current E or energy height, the pilot can quickly height (E) can be made explicit by applying the energy
s s
determinetheabsolutemaximumaltitudeorairspeeditcould balance principle described earlier—where the difference
attainthroughenergyexchangealone(Rutowski,1954). betweenenergygainandlossmustequalthechangeintotal
To grasp the significance of energy height, consider the energy of a system (Cengel, 2007). However, this time,
three identical airplanes (i.e., of equal weight) depicted in energy rates are expressed on a per unit weight basis.
Figure 2.AirplaneAiscruisingatanaltitudeof8,000 feet Specific excess power (P) represents the airplane’s rate of
s
at approximately 200 knots while airplane B is cruising at energy gain minus energy loss per unit weight, while the
an altitude of 3,000 feet at about 400 knots. Although rate of change of energy height (E˙ ) represents the rate of
s
these two airplanes have a different combination of change of the airplane’s total mechanical energy per unit
potential and kinetic energies, their total energy is the weight. Thus, according to the energy balance principle,
same and both are capable of zooming to a maximum specific excess power must equal the rate of change of
altitude of 10,000 feet by trading speed for altitude. In energy height (Rutowski, 1954):
other words, their energy height is 10,000 feet. Now,
P ~E_ ð9Þ
examine airplane C cruising at 11,000 feet at roughly s s
450 knots.Thisairplanehasahigherenergystatethanthat SinceP 5(T2D)V/W(Equation8)andE˙ 5˙h+V˙V/g
s s
of either airplane A or B (Es 5 10,000 feet) since the (derived from Equation 7), Equation 9 can be re-stated as
former can zoom to a maximum altitude of 20,000 feet follows:
withoutaddingpower.TheonlywayforairplanesAandB
toreachtheenergyheightofairplaneC(E 520,000 feet) ðT{DÞV=W~h_zV_V=g ð10Þ
s
is for the formertoincreasetheir totalenergy. In short, an
Equation10simplysaysthatspecific[positive,negative,
airplane can move back and forth along a constant energy
or zero] excess power P—a function of T 2 D—can be
line (Figure 2) by exchanging energy (i.e., redistributing s
distributed between altitude (˙h) and airspeed (V˙V/g)
potential and kinetic energies while maintaining total
changes. The close parallel between the energy balance
energy constant). In contrast, an airplane cannot move
equation and Newton’s force equation described earlier
across energy lines by simply exchanging energy—in
(Equation 3) is no coincidence. The former can easily be
ordertogofromoneenergyheighttoanother,theairplane
derived from the latter (Anderson, 2007; Rutowski, 1954),
mustincreaseordecreaseitstotalenergy.Andthisbrings
pointingtotheintimaterelationbetweenthelawsofenergy
us to the next important concept in EM theory.
conservation and motion (Goodman & Warner, 2001).
The rate of change of an aircraft’s total energy—
Moreover, the same set of force and energy equations
allowing it to go from one energy height to another—is
equally apply to flight control and aircraft performance.
definedbythespecificexcesspowerorP,whichisexcess
s Historically, the development of energy-based perfor-
power divided by the airplane’s weight (Anderson, 2007;
mance models (e.g., energy maneuverability) has preceded
Cliff,1998;Shaw1985).P describestheaircraft’snettotal
s that of energy-based control models (e.g., TECS). The
energy flow per unit weight as a function of the difference
earliestmodelofenergymaneuverabilitytracesbacktothe
between thrust (T) and drag (D):
early 1940s when Fritz Kaiser, a German flight test
P ~ðT{DÞV=W ð8Þ engineer, developed the concept of energy height (which
s
he called ‘‘Gesamtho¨he,’’ meaning resulting height) work-
Thus,apositivePsrepresentstheabilityofanairplaneto ing on the Messerschmitt 262—the first jet aircraft to
increase its energy stateinorder toclimb (without slowing become operational toward the end of World War II
down) or accelerate (without losing altitude) to a higher (Merritt, Cliff, & Kelley, 1985). Kaiser was mainly
energy height (E). Note that being a specific energy rate, interested in determining energy climb performance of
s
P has the units of length/time (e.g., feet/min). The P theMe262(Merrittetal.,1985).Rutowski,workingatthe
s s
capabilities of fighter aircraft are displayed on the same Douglas Aircraft Company, expanded and applied EM
altitude-velocity graphs that are used to depict specific theory to include both climb and range performance
energy or energy height (E) lines. By plotting a series of (Rutwoski, 1954). More recently, Schneider (2010) elabo-
s
excess power (P) contours (derived from power-required/ rated EM theory incorporating the realm of negative P
s s
power-availablecurvesatdifferentaltitudes)onanaltitude- (‘‘negative excess power’’) to analyze the safety and
velocity diagram, the pilot is able to determine the most efficiency of high-speed, steep angle dives. However, it
optimal path to go from one energy level to another. In was USAF Col. John Boyd who coined the term ‘‘energy
other words, this energy approach provides the optimal maneuverability’’ in the 1960s (Coram, 2004). Boyd
solution to get from any initial altitude and airspeed to advanced the theory to new levels by comparing how the
another final altitude and airspeed within the airplane’s P capabilitiesofdifferentfighteraircraftvarywithaltitude,
s
performance envelope (Rutowski, 1954). For illustrations airspeed, and other factors such as G-loading, and
of P contours, see Schneider (2010). determining how an energy advantage could be attained
s
32 J.R.Merkt / JournalofAviationTechnologyandEngineering
in air combat by maneuvering and rapidly changing from Next, Pennycuick drew a simple connection between
one energy state to another (Coram, 2004; Schneider, energyheightanddistanceflown,allowinghimtolinktwo
2010). distinct and fundamental processes that make up energy
efficiency in flight: 1) converting chemical (fuel) energy
Biology: Structure-Function Integration and into mechanical energy and 2) converting mechanical
Energy Efficiency energy into motion. The first process, defining how
efficiently mechanical energy is generated for flight, is
Biologistshavehadalong-standinginterestinstudying therealmofengineperformance;whilethesecondprocess,
the integration of structure and function in living defining how efficiently mechanical energy is spent in
organisms (Russell, 1916). Structure-function relations flight, is dictated by airframe aerodynamic performance
have been extremely valuable to physiologists seeking to (Anderson, 2007; Pennycuick, 2003).
unravel the underlying principles of animal locomotion To establishtheconnectionbetweenheightanddistance
(Dickinson et al., 2000), where structure is determined (and later between engine and aerodynamic performance),
by the anatomical features that control movement Pennycuick used gliding flight as a starting analogy. As a
(Alexander, 2003; Dickinson et al., 2000) and function glider descends, the distance flown (Y) is proportional to
is measured through the physiological performance of the starting height (h):
running, swimming, or flying animals (Schmidt-Nielsen,
Y~hN ð11Þ
1972). One of the main themes that have emerged from
these studies is that motion control and performance are Equation 11 simply states that the horizontal distance
tightly integrated through common mechanisms govern- covered (Y) is equal to the product of the starting height
ing the use of locomotive forces and energy transfer, (h)—representingtheinitialstoreofpotentialenergy;times
exchange, and storage (Dickinson et al., 2000). As the glide ratio (N)—a dimensionless number defining the
suggested in this paper, this view is akin to focusing on gradient along which the bird or glider descends. The
the energy-based integration of flight control and aircraft gradientN,whichconvertsheightintodistance, isequalto
performance, encapsulated in the airplane’s energy the lift:drag ratio L/D—itself a function of airspeed. As a
balance equation. result,aglidercoversthefarthestdistance(Y)fromagiven
Sincetheenergynotusedforlocomotionisavailablefor starting height (h) flying at one particular speed—the
other important biological activities (e.g., growth and ‘‘best-glide’’ speed—where drag is minimized and L/D is
reproduction),itfollowsthatanimalsshouldbeeconomical maximized (Pennycuick, 2003). At any other faster or
intheuseofenergyformovingaround(Alexander,2003). slower speed, the angle of descent is steeper shortening
Examples of energy-efficient strategies in flight include the distance covered as the glider descends. Thus, the
foraging behavior of hummingbirds (Chai, Chang, & performance number N or L/D is at the heart of
Dudley, 1998) and long-distance migration of geese aerodynamic performance (Anderson, 2007; Pennycuick,
(Cutts & Speakman, 1994). Given the significance of 2003).
energy savings in animal locomotion, biologists have Byanalogy,inpoweredflight—wherefuelenergy,rather
sought general models for predicting and testing energy thanpotentialenergy,isusedtoovercomedrag—theinitial
efficiency. One such model was developed by C. J. store of fuel can be transformed into a fuel energy height
Pennycuick, a biologist at the University of Bristol, UK (hfuel),animaginaryheighttowhichagivenamountoffuel
(Pennycuick,2003).Thefactthatsomemigratingbirdscan energy could ‘‘lift’’ a bird (or airplane) if it wereconverted
fly astonishing airline-like distances of over 6,000 miles into work (Pennycuick, 2003). In other words, hfuel is the
non-stop withoutrefueling (Piersma &Gill, 1998) prompted maximum height that could theoretically be reached from
Pennycuick—himselfapilot—tofocusonrangeperformance. converting all the fuel onboard into potential energy. Fuel
The starting point in Pennycuick’s model is the concept energy height(hfuel) takes into accountseveral factors such
of energy height—described in the previous section— astherelativeamountoffuelatdeparture,theefficiencyby
representingthemaximumheightthatanairplane(orinthis which fuel energy is progressively converted into work
case, a bird) would reach from its present height by againstgravity,andthegradualweightlossasfuelisburned
(Pennycuick, 2003). Expressed in units of length, h is a
converting all of its stored kinetic energy into potential fuel
measure of engine performance in its simplest form—
energy. However, Pennycuick generalized the concept of
defininghowefficientlymechanicalenergyisgeneratedfor
energyheightandredefineditasavirtualheightthatabird
flight.Completingtheanalogy,range(R)inhorizontalflight
wouldreachifitconvertedanystoredenergy—suchasfuel
isproportionaltotheinitialfuelenergyheight(h ):
energy—into potential energy by doing work against fuel
gravity (Pennycuick, 2003). An immediate benefit of R~h N ð12Þ
fuel
Pennycuick’s approach is that stored fuel energy can be
treated the same way as stored potential or kinetic energy Asfuelisprogressivelyconsumed,thebird(orairplane)
when analyzing energy utilization in flight. comes ‘‘down’’ from its initial fuel energy height along a
J.R.Merkt / JournalofAviationTechnologyandEngineering 33
virtualenergygradientN(Pennycuick,2003).Asingliding height—and established their usefulness for exploring and
flight, N measures aerodynamic performance—governing defining the role of the flight controls and the aircraft’s
how efficiently mechanical energy is spent during flight. performance limits, the next section focuses on the merits
Forbirdsandpropelleraircraftinpoweredflight,Nisequal of a top-down energy approach as an instructional tool.
to the lift:drag ratio, exactly as it is in gliding flight
(Pennycuick, 2003); while for jet aircraft N is equal to the Pedagogical Basis for Energy Management Training
ratio of the square root of the coefficient of lift to the
coefficient of drag (Anderson, 2007). Maximum range, for The instructional value of an energy-centered approach
a given fuel energy height, is achieved at the airspeed for pilot training was examined some years ago (Merkt,
where L/D (or C 1/2/C in jets) is maximized. In sum, 1990; 1991; 1992). Figure 3 illustrates two opposite
L D
measuring range (R), a quintessential representation of approaches to learning principles of flight. The traditional
energy efficiency in long-distance flight, boils down to approach is comparable to that employed by early
calculating and multiplying two quantities (Equation 12): aeronautical engineers designing airplanes (e.g., Warner,
1) an engine performance number or fuel energy height 1936), while the alternate approach is analogous to that
(h )inunitsoflength(e.g.,feetormiles)—defininghow used by physiologists studying animal motion (e.g.,
fuel
effectively mechanical energy is extracted from fuel—and Schmidt-Nielsen, 1972).
2) a dimensionless aerodynamic performance number or
energy gradient (N)—dictating how efficiently mechanical Bottom-up ‘‘Designer’’ Approach
energy is converted into mileage.
Inhisuniqueapproachthen,Pennycuick(2003)tookthe Traditionally, engineers begin by examining the under-
energyheightconcept—formulatedyearsearliertoenhance lying causes (e.g., fluid properties), as opposed to top
aircombattactics for fighter jetpilots(Merritt etal.,1985; consequences (e.g., flight performance), of aerodynamic
Shaw, 1985)—and expanded it to solve the mystery of phenomenainordertounderstandprinciplestheycanapply
long-range migration in birds. Interestingly, Rutowski to design airplanes (Warner, 1936). For design engineers,
(1954) used yet a different energy approach, focusing on the ‘‘finished product’’ at the end of the process is not just
the airplane’s energy balance to arrive at an equivalent the physical airplane but its performance, best represented
rangeequation,againunderliningtheusefulnessofenergy- by power-curves or drag polars (Anderson, 2007; von
based methods for solving performance problems across Mises,1945).Inanutshell,designengineersuseabottom-
disciplines. up approach (Figure 3). This approach is most obvious in
Having examined energy management concepts—such traditional aerodynamics textbooks (e.g., von Mises, 1945)
as energy balance, specific excess power, and energy aswellasthosedealingwiththeoryofflightforpilots(e.g.,
Figure3.Twooppositeapproachestothestudyandpracticeofflight(adaptedfromMerkt,1990).Thedesigner(engineer)orientedapproachstartsby
studyingthe causes of aerodynamic phenomena (e.g.,fluid properties) and proceedsup to higher-level consequences (e.g.,aircraft performance). The
operator(pilot)orientedapproachstartswiththehighestconsequencesofaerodynamicprocessesandproceedsdownwardtounravelunderlyingprinciples.