conceptual design of a ducted fan‐based vertical takeoff and landing tactical unmanned aerial vehicle...

7/30/2019 ConceptualDesignofaDuctedFanBasedVerticalTakeoffandLandingTacticalUnmannedAerialVehicle

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ConceptualDesignofaDuctedFanBasedVerticalTakeoff

andLandingTacticalUnmannedAerialVehicle

Jonathan

D.

KeithEngineerandManagingMember

EmpiricalSystemsAerospace,LLC

PismoBeach,CA93448

[email protected]

Ryan

S.

WoodChiefExecutiveOfficer

FrontlineAerospace,Inc.

Broomfield,CO80020

[email protected]

Abstract

VSTAR(VerticalTakeoffandLandingSwiftTacticalAerialResource) isatacticalunmannedaerialvehicle(UAV)

withauniqueintegrationofhistoricalaerospacedesignconcepts. Usingasingleductedfanforverticaltakeoffand

landing operations, VSTAR offers payload flexibility throughout its range of missions. This range of flexibility

comesthroughtheplacementofcargoatthevehiclescenterofgravity,whichisinlinewithductedfanusedfor

verticaltakeoff

and

landing

(VTOL)

operations.

In

addition

to

the

unique

placement

of

the

payload,

the

use

of

two

turbineengines,placedforbalanceandfunctionalintegration,allowsVSTARtooperatemoreefficientlyduring

conventional forward flight. This approach increases range and endurance capabilities while providing greater

reliability and safety for the mission. The configuration of the wing provides distinct advantages from both a

structuralaswellasaerodynamicperspective. Theseaerodynamicbenefitsallowformissionadaptabilitythrough

the use of wingtip extensions and inflight wing planform modifications. Finally, VSTAR incorporates the

MicroFire engine recuperator to provide heightened fuel efficiencies for its turboshaft engines. Through the

integrationofthesedesignconcepts,theVSTARarchitectureallowsforhighflexibilityandexcellentcompetition

amongthecurrenthighdrag,rotorbasedunmanneddesigns.

Nomenclature

CL

WingLift

Coefficient

C Cost($)

E Emptyweight(lb)

F Fuelweight(lb)

N Noise(db)

FM Figureofmerit

O Observability

P Payloadweight(lb)

R Range(n.mi.)

RES Reservefuelweight(lb)

TOGW Takeoffgrossweight(lb)

TRAP Trappedfuelweight(lb)

V Velocity(kts.)

PresentedattheAHSInternationalSpecialists'Meeting

on Unmanned Rotorcraft, Scottsdale,Arizona, January

2022,2008.2008by J.D.KeithandR.S.Wood.

PublishedbytheAHSInternationalwithpermission.

Introduction

Vertical

takeoff

and

landing

vehicles

have

garnered

interest from thepubliceversince the firsthelicopter

tookflight. Byofferingtheabilitytotakeoffand land

inremoteareasandonvariousterrains,VTOLvehicles

arecontinuallydesigned to leverage these capabilities

while maintaining the speed and efficiencies of

conventionalaircraft. However,despitetheworkthat

has been done on VTOL aircraft over the years, the

VTOL UAV sector remains rather young; few

operational offerings exist and of those, the majority

arelowdiskloading,rotorbaseddesigns.

Over the years, aircraft designers have taken a

myriadof

approaches

in

order

to

achieve

the

forward

flight efficiencies experienced by conventional tube

andwing aircraft. Low disk loading, rotorbased

designsoftenhave themostdifficult timeovercoming

thishurdle,astherotorisntusedduringforwardflight

yetbecomesdifficulttohidewhennotinuse. Design

strategiesforslowingtherotorhelpreducetheoverall

drag but in the end will be at a considerable

disadvantagewhencomparedtoafixedwingdesign.


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Inthefigurebelow(Figure1),theFaireyRotodyneis

shown to illustrate a historical example of an aircraft

designers attempt to merge VTOL capabilities with

forward flight efficiencies using a conventional wing

andaseparateforwardpropulsionsystem

Figure1TheFaireyRotodyne2

Throughout the design of the VSTAR, these

challengeswere

considered

and

addressed

in

aunique

manner, providing a system that incorporates VTOL

capabilitieswithefficientforwardflightoperations.

PreliminaryDesign

In order to determine the initial sizing of the V

STAR vehicle, calculating a preliminary takeoff gross

weight and empty weight were required. As is often

done in the first stages of a new aircraft design,

historical aircraft were used to generate the initial

weighttrend,

utilizing

aircraft

that

perform

amission

in

amannersimilartotheaircraftbeingdesigned. Forthe

VSTARs initialsizing,aircraftbeyondhistoricalVTOL

UAVs were required in order to provide enough data

points to generate a sufficient trend, thus Analytic

Services (ANSER) V/STOL wheel was used to help

definethehistoricalweighttrend.

Figure2ANSERsV/STOLWheel1

Using select vehicles from the ANSER wheel, a

weighttrendwasgeneratedandatrendlinewasfitto

the data (Figure 3), creating the weight equation

(Equation 1) that defines the empty weight to the

takeoffgrossweight for the typeof aircraftchosen in

the

sample.

While

this

grouping

of

aircraft

not

onlyincludes contemporary VTOL UAVs such as the Bell

Eagle Eye, italso incorporatesaircraftdesigned in themidtwentieth century that utilize a wide variety of

VTOLapproaches.

0.4988.

(1)

Figure3VTOLWeightTrendforWeightSizing

OfnoteinFigure3isthetrendgeneratedbyaircraft

using different propulsion approaches. As mentioned

earlier, both VTOL UAVs and manned VTOL aircraft

were used in the study; in addition, both propeller

based aircraft along with jetbased aircraft were

included in the study. Despite this seemingly large

difference between propulsion options, the weight

trend displays minimal error in relation to the

generatedtrendline.

TheprimarydesigncriterionfortheVSTARwasto

filltheneedforanautonomousresupplyaircraft. Such

a mission requires dense payloads to be delivered to

troops

in

remote

locations,

often

very

quickly,

while

maintainingassmallanaudibletrace(noisesignature)

as possible. These design factors drove VSTARs

sizing in various ways, the first of which was through

theselectionofpayloadweights.

Utilizing the above equation along with the initial

missionprofile,aircraftassumptions(Table1),andthe

weight fraction method3, the VSTARs initial takeoff

gross weight and empty weight were estimated to be

100

1,000

10,000

100,000

1,000 10,000 100,000

AircraftEmptyWe

ightlbs.)

Aircraft Takeoff Gross Weight (lbs.)


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2,360 lb and 1600 lb respectively (Table 2). As seen

representedbythe largerdatamarkerabove inFigure

3, the initial takeoff gross weight estimated by the

trendlineandweightfractionmethodfallstowardsthe

bottom of the data set but remains inline with the

datainthegraph.

Table1InitialMissionProfileandAssumptions

MissionRequirements

Item Value Units

Payload 400 lb

Range 400 nmi

CruiseSpeed 300 kts

CruiseAltitude 15,000 ft

MissionAssumptions

LoiterSpeed

180

kts

MaxCL 1.6

WingLoading 85 lb/ft2

No.ofEngines 2

CruiseThrottle 85%

CruiseTSFC 0.55 lb/lbhr

CruiseCL 0.30

StallSpeed 125 kts

Table2InitialWeightEstimationsforVSTAR

Category Weight(lb)

WTOGW 2,360

WRES 130

WTRAP 26

WFUEL 602

WEMPTY 1,600

WPAYLOAD 398

WE/WTOGW 61.6%

WF/WTOGW 23.2%

WP/WTOGW 15.3%

AsnotedinTable1above,apayloadweightof400lb

was selected in order to size the vehicle. While the

takeoffgrossweightof2,360 lbmayappeartoyielda

smallvehicletocarry400lb,themainelementsofthe

resupply mission are to deliver dense payloads, e.g.

water, food, batteries, and ammunition. Recognizing

the nature of the payloads around which the vehicle

wasdesigned leadtothefirstmajordesign innovation

ontheVSTAR.

InitialConfigurationandVehicleLayout

Realizingthat

VTOL

aircraft

are

very

sensitive

to

the

aircraftscenterofgravity(c.g.),VSTARwasdesigned

around the payload bay, providing flexibility in the

weightsofpayloads thatVSTARcancarry. Inorder

toaccomplishthisdesignfeature,VSTARisdesigned

with a counterrotating ducted fan used for vertical

takeoff and landing. This central fan, featuring an

emptyhubinthecenter,allowspayloadtobeplacedat

the center of the fan and thus minimizes the concern

surrounding payload weight limits. Figure 4 below

shows the payload integration at the center of the

vehiclesliftfan.

Figure4VSTARPayloadIntegration

Byplacingthepayloadatthecenterofthevehicles

vertical lift vector, the importance placed on the size

and the weight of the payload are minimized, leaving

only the engines, fuel, permanent systems, and the

vehiclesstructuretobebalancedaroundthecenterof

lift. In addition to the flexibility in shapeand weight,

the bottomloading aspect and semispherical field of

viewofthepayload integrationallows foramyriadof

payloadtypes

to

be

integrated

into

V

STAR,

ranging

from internal resupply payloads, for which it was

designed, to external surveillance and/or

communicationequipmentandevenweaponry.

Asseeninthefigureabove,thecentralfanwasalso

designedwithaseriesofcontrolvanesonboththetop

and bottom of the aircraft. After full transition to

forward flight has occurred, the vanes of the vehicle

are designed to close, providing lower drag for the

LiftFan

(w/Control

Vanes)


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vehicle than an open duct. In addition, the vehicles

bottom vanes aid the vehicle in stability and control

during hover operations by constantly attenuating

themselvesandcontrollingtheductsexitflow.

Thepresenceofaduct in themiddleof theaircraft

presentedanearlydesignchallengewithrespecttothe

integrationof

the

aircrafts

wing.

In

order

to

properly

position a conventional wings lifting point with the

aircrafts c.g., the wings torque box would be in the

middleoftheduct. Intheearlystagesofthevehicles

layout,thiswasruledout,asthestructurecouldcause

disruption intheductflowandpotentiallyplacestress

on the duct that would adversely affect its geometric

efficiencies.

In order to circumvent these problems, VSTAR is

designed with a diamond box wing (Figure 5), a

modification of ajoinedwing approach first proposed

byJulianWolkovich4. Whileallowingthewingstorque

boxestobeplacedforeandaftofthecenterduct,the

diamond

box

wing

approach

provides

two

additional

benefits: exact c.g. placement and exhaust suction

minimization.

Figure5VSTARsDiamondBoxWing

Inthepreliminarydesignphase,theabilitytochange

the fuselage station of the wing joints, essentially

changingthesweeponboththe frontandrearwings,

allowsforthewingsystemscenterof lifttobeplaced

where desired. Additionally, the lack of a horizontal

surfacedirectlynexttotheexhaustofthecentralduct

helps reduce the downward suction that often occurs

withVTOLvehiclesandthelowpressureregioncreated

bytheexitingflow.

For forward propulsion,VSTAR isdesigned witha

rearmountedductedfan. Inordertopowerboththe

rearmounted ducted fan and the central lift fan, V

STAR incorporates two RollsRoyce Model 250

turboshaftengines. Figure6showstheenginelinkage

system while Figure 7 depicts the engine placement

insideofVSTARsfuselage.

Figure6VSTAREngineLinkageSystem

Figure7VSTAREngineIntegration

Inordertopowerthecentral liftfan,bothenginesare

coupledtogetheratthecentraltransmissionhub,using

gears to reduce thespeed and combine the powerof

bothengines. Thisapproach,showninFigure6,utilizes

belt drives and linkages in order to allow the rear

engine to power the rearmounted ducted fan. With

thetwoenginesusedtopowerthevehicle fortakeoff

and landing and only one engine used for forward

propulsion,V

STAR

incorporates

aredundant

engine

system whereby either engine can power the rear

mountedductedfanorprovideincreasedpowertothe

rearfanfora"dashspeed"exceeding400knots. Due

toVSTARsdesignforoperatinginremote,frontline

areas,engineredundancyallowsthevehicletosustain

damage while continuing its conventional flight

operations, even providing reduced descent rates

throughthepartialpoweringofthecentralliftfan.

MissionAdaptivePlanform

Due to the uniqueness of the diamond box wing

arrangement, VSTARs configuration can be

manipulatedtoincreasetheoverallwingplanformarea

without affecting the overall stability and balance of

the aircraft. Various design approaches that leverage

this feature were studied early on in VSTARs

configuration and include both removable wingtip

Engines

Engines


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extensions (increase the area about the aircrafts

centeroflift)aswellasinflightdeployableextensions.

With the ability to add bolton extensions onto the

wingtipsofdiamondboxwing(Figure8),VSTARhas

the potential to increase the aircrafts overall liftto

dragratio,thereby increasing itsrangeandendurance

duringoperations.

Figure8

Wingtip

Extensions

Attached

to

VSTAR

Because the extensions center of lift are placed at

thesamefuselagestationasthemainwingscenterof

lift,theoveralleffectontheaircraftsstaticstability is

minimal, allowing any number of extension

configurations, whose size is dictated only by the

structuralcapabilities of the diamondbox wing, to be

placedontotheaircraft.

Beyond a bolton extension, the features of the

diamond box wing allow for inflight modifications to

the

wing,

similar

to

the

principles

achieved

from

state

oftheart wing morphing designs. By integrating

pivoting extensions into the top and bottom of the

wings endplates (Figure 9), VSTAR is designed with

the capability to increase its overall planform area

duringflight.

Figure9VSTARWingtipwithMAP

This design approach, termed Mission Adaptive

Planform (MAP),couldallowVSTAR flight segments

of heightened endurance times (extension deployed),

followed by a high cruise speed segment (extension

retracted). This integration of an adapting planform

canprovidea flightvehiclewithastrategicadvantage

againstotherdesigns.

InadditiontoallowingthewingplanformareaofV

STARto

be

scaled,

it

is

perceived

that

the

coupling

of

the diamond box wing and aircraft c.g. location allow

for the vehicle as a whole to be scaled up and down

withminimaleffectsonthenumberofdesigniterations

required. Figure 10 depicts the 2D configuration

scalabilitythattheVSTARcouldundergo,depending

onmissionrequirements.

Figure10FlexibilityandScalabilitywith

VSTAR

This above graphic demonstrates the flexibility that

theVSTARplatformprovidedduringtheconceptual

design

phase

of

the

vehicle.

Designing

to

a

specific

missionwasachieved,but furtherexploration intothe

synergistic design features within the VSTAR

configurationprovidethesecapabilities.

MicroFireRecuperatorIntegration

The ideaofgasturbinerecuperators isnotnewand

has been a viable fuel saving concept since the

inventionofthegasturbine;thetechnicaldifficultyisin

thespecificengineintegration,takingcaretokeepthe

overall

system

weight

low

so

as

not

to

negate

the

fuel

savingsachieved. Figure11outlinesthebasicpremise

behind the recuperator as is currently integrated into

thetwoModel250turboshaftenginesontheVSTAR.


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Figure11TheWorkingsofaRecuperatorAs noted in the figure above, a recuperator is a

special purpose counterflow heat exchanger used to

recoverwasteheatfromexhaustgases. Inmanytypes

ofprocesses,combustionisusedtogenerateheat,but

therecuperatorservestoreclaimthisheat inorderto

recycleit.

In

agas

turbine

engine,

the

incoming

air

is

compressed,mixedwithfuel,andthenburnedtodrive

aturbine. Therecuperatortransferssomeofthewaste

heat intheexhausttothecoolercompressedair,thus

preheating it before entering the fuel burner stage.

Since the gases have been preheated, less fuel is

needed to heat the gases to the turbine inlet

temperature. Byrecoveringsomeoftheenergyusually

lost as waste heat, the recuperator can make a gas

turbinesignificantlymorefuelefficient.

Historically, inaerospaceandaircraftapplicationsof

Carnot cycle recuperators, the fundamental technical

challenges

preventing

the

recuperators

implementation have been weight, volume and

sufficient performance to create a strong value

proposition for potential users. Frontline Aerospaces

patentpending microchannel heat exchanger

technology creates an opportunity forVSTAR using

the Model 250 engine. The key performance

specificationsand features forVSTARsMicroFire

include:

ExhaustDropinRetrofitontheModel250 MassFlowRate3.5lb/s SystemWeight


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configurators to determine what effects the

recuperatorhadontheVSTAR sperformance.

Additionally, the initial estimates of the aircrafts

overall lift and drag found in Digital DATCOM were

supplementedbyalternateDATCOMmodels, including

bothwingtipextensionandMAPmodelstodetermine

thepreliminary

effects

of

such

additional

aerodynamic

surfaces. These configurations were distilled into

classic drag polars which were incorporated into the

missionperformancecodeforfuelburnestimates.

Inordertocomparethevariousdesignmodifications

suchasthewingtipextensionsandtheMAP,payload

range diagrams were generated for each of the

configurations and are shown below in Figures 13

through 15 and include the baseline configuration,

MAP configuration, and endurance configuration (ten

foot wing extensions) respectively. Each of the

diagramsbelow contains performancenumbersbased

on VSTAR with the recuperator and StandardAeros

Model250

engine

modifications.

Figure13PayloadRangeDiagram(Baseline)

Figure14PayloadRangeDiagram(MAP)

Figure15PayloadRangeDiagram(WingExtension)

As is evident by Figure 15, the extensions, coupled

with the MicroFire recuperator and StandardAeros

engine upgrades gives VSTAR a substantial increase

in

range

and

endurance.

By

adding

extensions

to

increase the efficiency, the new wing configuration

complementsthe recuperator toprovideanew realm

offlightcapabilitiesandmissions.

MilitaryMissionsComparison

Although VSTAR was designed originally for a

logistics resupply mission and the payload, range

performancemetricswereoptimizedforthatpurpose.

It is worth considerable discussion around other key

military

missions

and

how

other

competitive

tactical

UAVsperform.

Thecoremilitarymissionsofinterestforthisclassof

UAVare:

Logisticsresupplyofdeployedtroops

Endurancefocusedonlongflightendurance

andpersistentIntelligence,Surveillance,

TargetAcquisition,andReconnaissance

(ISTAR)applications

UCAVUnmannedCombatAerialVehicle

HunterKillermostlyanISRendurance

missionwithweapons

ClandestineResupplyquiet,

fast,

behind

enemylinesresupply

CasualtyEvacuationevacuation ofthe

woundedfromthebattlefield

Eachofthesemissionshavedifferentaircraftdesign

requirements and ideal performance metrics. At the

preliminarydesignphase it ishard tojuggle, letalone

0

100

200

300

400

500

600

0 1,000 2,000 3,000 4,000 5,000

Payload

(pounds)

Range(nauticalmiles)

Baseline

FlightSpeed:~185kts.

0

100

200

300

400

500

600

0 1,000 2,000 3,000 4,000 5,000

Payload

(pounds)


MAP

FlightSpeed:(~300kts.)

0

5

10

15

20

25

30

0

100

200

300

400

500

600

0 1,000 2,000 3,000 4,000 5,000

Endu

rance

(hours)

Pa

yload

(pounds)


UltraEndurance

FlightSpeed:~153 kts.


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knowallthekeyfunctionalrequirementsandtechnical

specifications.Nevertheless,mostaircraftdesignteams

will agree that speed, range, payload, noise, IR

signature, radar signature, cost, life cycle cost,

survivability and durability, all weather capability and

landing zone flexibility areallvalid metrics tooutrank

andcompare

to

different

UAV

designs.

Determining

the

perfect (just and fair) analytical metric to compare

competingUAVdesignsisthefinalgoal.

Figure 16 shows theperfect metric which requiresfour key elements: 1) a simple and transparent

calculation(intheaircraftdesigncaseanequationthat

blends the key performance metrics i.e. speed,

payload, cost), 2) a common interpretation of the

metrics(designersallknowwhatspeed,range,payload

are and can agree), 3) credible independent data

publishedbytheDODviaUASroadmaps(forexample),

and4)the militarymissionthatwillclearlybenefitor

losefromanincreaseordecreaseintheperfectmetricscore.

Figure16RequirementsforaPerfectMetric

The question then turns to finding the reasonable

(perfect metric) equations with which to judge eachmission. Table 3 shows the equations derived by

reasonable consensus amongst VSTARs aircraft

design and advisory team. With over one hundred

years of experience designing aircraft, this team

includes the following individuals: Darold Cummings

(Boeing Technical Fellow, ran Boeing PhantomWorks

exploratory concepts for five years), Rick Foch (Naval

ResearchLabs,

over

40

UAV

designs

to

his

credit),

Dave

Hall(formerLockheedandNASAAmesaircraftdesigner

and lead configurator), and Empirical Systems

Aerospace (conceptual aircraft design firm, detailed

designs and tools completed for Boeing and NASA).

Below are the outranking comparison equations

(Equations2to6)foreachmission:

Table3MissionComparisonEquations

Mission Equation

Logistics

(2)

Endurance

(3)

Hunter/Killer

(4)

UnmannedCombat

AerialVehicle(UCAV)

(5)

CasualtyEvacuation/

Clandestine

Re

Supply

(6)

Some data are not known; thus the metrics of life

cyclecost, IR and radar observability were not

included. Secondary aspects were also omitted, such

as survivability and landing zone flexibility due to

insufficientandpotentially inconsistentdata.Thedata

for competing VTOL UAV aircraft were compiled

(Appendix A) and each equation was applied to the

availabledata.

UAVcostingcanbedifficult.Whatisintegratedintoa

productionperunitproductcost isacomplexblendof

volume,

development

costs,

payload

options,

and

maintenance contracts. In order to be fair but

potentiallynotthatveryaccurate (+/15%),regression

equationscreatedbyTechanomics, Inc,basedontheir

work for theUS Army, areused. Theyhave evaluated

dozens of UAV platforms, including all of the

documentsandcostbreakdowns,andcreatedcredible

metrics using TOGW, payload, endurance, production

year,prototypes,etc.ThecostdatausedinAppendixA

aregenerated fromthesepubliclyavailableequations.

In Figures 17 and 18, it can be seen that VSTAR

outperforms competitors by a sufficient margin that

slightamountsoferrorinthespeed,weight,endurance

time,

etc.

will

not

significantly

change

the

overall

outcomes. Figure 17 shows a normalized logistics

missioncomparisonofthecompetitorsandpresentsV

STARasthreetotentimesbetterthanthealternative

designs.


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Figure17CombatReSupplyLogisticsMission

When each of themissionsdiscussedarecompared

within a radar diagram, the results become even

moredramatic(Figure18and19).

Figure18ComparisonofVSTARwithIts

Competitors

Figure19Comparison ofVSTARCompetitorswith

EachOther

Theseresultsstemfromseveralkeydesignelements

oftheVSTARplatform. TheMicroFirerecuperator

improvesfuelconsumptionandrange,whilethewing

morphing planform with its folding down and bolton

wing tip extensions dramatically improve lift anddrag

ratios. Theducted fanVTOLdesignhas relatively low

noiseand

drag

compared

to

those

of

helicopters

while

the diamondbox and high aspect wings allow for

efficient high speed flight (nearly three times that of

helicopters).

Conclusion

Throughaconventionalconceptualdesignapproach,

a unique blend of historical and stateoftheconcepts

has produced the VSTAR platform. Through the

implementation of recuperator technology as well as

the possibility to increase the wings planform with

minimal design impacts, the VSTAR is poised as an

efficient,effective UAVwith potential that extends its

originallogisticsresupplydesignmission.

References

1Hirschberg, M. J., V/STOL: The First HalfCentury,

American Helicopter Society, URL:http://www.aiaa.org/tc/vstol/VSTOL.html [cited 22

December2008].

2Fairey

Rotodyne,

Wikipedia,

Retrieved

January

4,

2009,URL:http://en.wikipedia.org/wiki/Rotodyne

3Roskam,J.R.,AirplaneDesignPartI:PreliminarySizing

of Airplanes, Roskam Aviation and EngineeringCorporation,Ottawa,1985.

4Walkovich, J., Joined Wing Aircraft, US Patent

3942747,March9,1976.

5Advanced Engine Concept Assessment, US Army

ResearchandDevelopmentCommand,RDECOMTR04

D

35,

Contract

Number

DAAH10

03

C

0050.

6The USAF Stability and Control DATCOM, Volume I,

Users Manual, McDonnell Douglas AstronauticsCompany,St.Louis,1979.


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AppendixA

conceptual design of a ducted fan‐based vertical takeoff and landing tactical unmanned aerial vehicle...

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