Design of UAV Systems Objectives
Lesson objective - to discuss
including …
• Rationale
• Applications
• Limits
Expectations - You will understand when and how to use parametric relationships c 2002 LM Corporation Air vehicle parametrics 15-1

Design of UAV Systems Many types of parametrics
Range/endurance related parametrics
• Speed (V)
• Lift-to-drag ratio (L/D)
• Specific fuel consumption (SFC)
• Fuel fraction (FF)
• Range factor (RF)
• Specific range (SR)
• Example problem
Propulsion related parametrics
• Internal combustion
• Turboprop
• Turbojet & turbofan
• Afterburners
Weight related parametrics
• Fuel
• Payload
• Structure
• Systems
This lesson
Covered under propulsion
Covered under weights c 2002 LM Corporation Air vehicle parametrics 15-2

Design of UAV Systems Why parametrics?
During pre-concept design we need reasonable, not design specific, solutions *
• Good enough to support technology readiness, cost, risk and schedule estimates
• Parametrics enable Pre-Concept Design studies that don’t require the user to specify a design
During conceptual design we need to systematically explore a wide range of potential concepts
• Parametric design methods allow even small teams to evaluate and compare (quantitatively) a wide range of concepts and technologies
During both phases, speed and accuracy are critical
• Parametric design methods can significantly reduce
“design” and analysis time and produce credible results
* Customers should avoid the temptation of specifying the design solution, they almost always get what you ask for and it may not be the best available c 2002 LM Corporation Air vehicle parametrics 15-3

Design of UAV Systems Definitions
From Webster’s New Collegiate Dictionary
• Parameter – any set of physical properties whose value determine the characteristics or behavior of something
Our definition
• Design parametric – fundamental design parameter whose value determines the design or performance characteristics of a design
- Usually (but not always) a multi-variable relationship
e.g., wing loading (W0/Sref), Swet/Sref, etc.
• Parametric design – Parametric based design approach to define, size, estimate performance and do trade offs on classes of conceptual air vehicles
- Different from the traditional approach c 2002 LM Corporation Air vehicle parametrics 15-4

Design of UAV Systems
How is it different?
• Traditional conceptual design starts with a sketch
- See RayAD Chapter 3, Sizing from a Conceptual Sketch
• The sketch or drawing is analyzed
- Using a variety of techniques
Aerodynamics from geometry and parametrics
Weight “fraction” parametrics from historical data
Propulsion from parametrics or “cycle decks”
Performance from fuel or weight fractions and Breguet range and/or endurance equations
• The concept is “sized” to meet mission requirements
- Based on results of the first analysis
• A scaled drawing is made and analysis inputs generated
• Higher fidelity analyses is performed
Based on actual configuration areas and features
• Performance is calculated and compared to mission requirements and/or team expectations
• The process is repeated until expectations are met c 2002 LM Corporation Air vehicle parametrics 15-5

Design of UAV Systems
What’s the problem?
The process is time and data intensive
- Teams plan to evaluate a wide range of concepts but often never get beyond the first concept or sketch
Particularly for student design teams
The first concept gets most of the attention
- Lots of effort expended to make it meet expectations
Teams start to fall in love with the concept
- Alternatives get little attention
“Qualitative” comparisons eliminate the competition
Errors and disconnects start to surface and/or requirements problems emerge
- Teams scramble to recover, fixing errors gets priority
Trade studies to improve performance are defined but seldom completed
- Too much work, not enough time
Everybody hopes the reviewers won’t see the flaws
- And wish they had more time c 2002 LM Corporation Air vehicle parametrics 15-6

Design of UAV Systems
The alternative?
Use simple analytical geometry models instead of concept drawings to generate data for aero, weight and propulsion analysis and mission performance
- Physically capture important design variables but minimize the time and effort required to assess them
Use full mission integrated spreadsheet analysis to evaluate performance
- Size for the actual mission, reduce dependence on configuration insensitive rule-of-thumb estimates
Empty weight fraction, fuel for climb, etc.
Quickly and systematically evaluate a range of concepts
- Select preferred concepts and technologies based on data
Draw and analyze the preferred concept
- Confirm vs. discover how it really performs c 2002 LM Corporation Air vehicle parametrics 15-7

Design of UAV Systems Example mission
18
19
End cruise at W = W0 - (1-Klr)*WF
17
Start cruise at
W = W0 - Kttoc*WF
4 5 6
16 15
2
1 0
3
Border -
Penetrate/Loiter
14
12 13
7 8 9
10 11
Border -
Loiter/Penetrate
Border -
Standoff
Kttoc = (taxi-takeoff-climb fuel)/Wf
Klr = landing fuel reserves/Wf
Wf = fuel weight
Notation
0 Engine start
1 Start taxi
2 Start takeoff
3 Initial climb
4 Initial cruise
5 Start pre-strike refuel
6 End pre-strike refuel
Start cruise
7 Start loiter
8 End loiter, start cruise
9 Start ingress
10 Combat
11 Weapon release
12 Turn
13 Start egress
14 End egress, start cruise
15 Start post-strike refuel
16 End post-strike refuel
17 End cruise
18 Start hold
19 End hold
Terminology
Standoff -
Distance from loiter or combat to border (+/-)
Standback -
Distance from refuel to border
Ingress -
To target at penetration speed
Egress -
From target at penetration speed
Range (Rge) = 2*Radius(R) c 2002 LM Corporation Air vehicle parametrics 15-8

Design of UAV Systems The UAV challenge
• Parametric design requires historical data for use in preliminary sizing & analysis and reality checks
- There is a limited amount of good data available on
UAVs (from public release sources)
- A lot of the stuff is marketing hype and useless for design
• We will use available UAV data and fill in the gaps with manned aircraft data
- Example problems will be structured to show you how to do it c 2002 LM Corporation Air vehicle parametrics 15-9

Design of UAV Systems Next subject
Range/endurance related parametrics
• Speed (V)
• Lift-to-drag ratio (L/D)
• Specific fuel consumption (SFC)
• Fuel fraction (FF)
• Range factor (RF)
• Specific range (SR)
• Example problem
Propulsion related parametrics
• Internal combustion
• Turboprop
• Turbojet & turbofan
• Afterburners
Weight related parametrics
• Fuel
• Payload
• Structure
• Systems
This lesson
Covered under propulsion
Covered under weights c 2002 LM Corporation Air vehicle parametrics 15-10

Design of UAV Systems Range related parametrics
• Based on factors from the Breguet range equation
- For jet aircraft (See RayAD 3.5)
R = [Vcr

(L/Dcr)/TSFCcr]

Ln(Wi/Wj) where
R = Cruise range
(15.1)
Vcr = Cruise speed
L/Dcr = Cruise lift-to-drag ratio (LoDcr)
TSFCcr = Cruise thrust specific fuel consumption and
Vcr

L/Dcr/TSFCcr = RF (range factor) = W

SR
SR = Specific Range = V/Fuel flow)
- For propeller aircraft (more about this later) where
RF(nm) = 325.6
 p

(L/Dcr)/SFCcr
 p = propeller efficiency

0.8 (for constant speed prop)
(15.1a) c 2002 LM Corporation Air vehicle parametrics 15-11

Design of UAV Systems Endurance related parametrics
• Based on Breguet endurance equation factors
- For jet aircraft (RayAD 3.7)
E = [(L/Dlo)/TSFClo]

Ln(Wi/Wj) where
E = Endurance (hrs)
(15.2)
L/Dlo = Loiter lift-to-drag ratio (LoDcr)
TSFClo = Loiter thrust specific fuel consumption and
(L/Dlo)/TSFClo = EF (endurance factor) = Wbar/WdotF
Wbar = Average loiter weight (lbm)
WdotF = Fuel flow (lbm/hr)
- For propeller aircraft (more about this later also)
E = EF

Ln(Wi/Wj) (15.2a) where
EF = 325.6
 p

[(L/Dlo)/(Vlo

SFClo]
Vlo = Loiter speed;
 p = propeller efficiency c 2002 LM Corporation Air vehicle parametrics 15-12

Design of UAV Systems Weight fraction version
• The Breguet equation is often expressed in the form of weight fractions (more in Lesson 18) where:
Empty weight fraction (EWF)

Empty weight/Gross wt.
Fuel fraction (FF)
Payload fraction (PF)

Fuel weight/Gross wt.

Payload wt./Gross wt.
Misc weight fraction (MiscF)

Misc. weight/Gross wt.
• Where by defintion
Gross weight (W0)

Empty weight (We) + Fuel Weight
(Wf) + Payload weight (Wpay) + Other weight (Wmisc)
• Dividing through by W0 and solving,
FF = 1 - EWF - PF - MiscF
• Maximum range and endurance occur when
(15.3) or
(Wi/Wj)max = (1 - Kttoc*FF)/(1-(1- Klr)*FF) (15.4)
Rmax = RF
 ln[(Wi/Wj)max] (15.5)
Emax = [(L/Dlo)/TSFClo]
 ln[(Wi/Wj)max] (15.6) c 2002 LM Corporation Air vehicle parametrics 15-13

Design of UAV Systems Typical weight fractions
Typical value
Caution
- Within any vehicle class, weight fractions can vary widely
- Nonetheless, most initial concept design procedures start with an assumed empty weight fraction (EWF)
- This can cause problems, as we will see later
- Later we will introduce an alternative approach c 2002 LM Corporation Air vehicle parametrics 15-14

Design of UAV Systems Database variation examples
SE-Prop ME-Prop Biz Jet Reg. TurboJet Transp.Mil. Trainers Fighters Mil PBC FW UAV
GW
Min
Mean
Max
EWF
Min
Mean
Max
FF
Min
Mean
Max
1817
2750
11574
0.437
0.595
0.791
0.059
0.125
0.283
2183
6625
10325
0.555
0.623
0.689
0.115
0.178
0.298
4550
20000
68200
0.479
0.548
0.622
0.291
0.359
0.417
5732
14550
57250
0.517
0.577
0.662
0.149
0.230
0.334
44000
220000
775000
0.428
0.536
0.610
0.199
0.326
0.536
2238
4188
11100
0.461
0.697
0.789
0.101
0.213
0.381
5291
29975
91500
0.381
0.472
0.639
0.101
0.212
0.387
50706
158730
769000
0.327
0.491
0.732
0.208
0.405
0.674
4
1245
25600
0.359
0.605
0.870
0.111
0.266
0.566
c 2002 LM Corporation Air vehicle parametrics 15-15

Design of UAV Systems Next
Range/endurance related parametrics
• Speed (V)
• Lift-to-drag ratio (L/D)
• Specific fuel consumption (SFC)
• Fuel fraction (FF)
• Range factor (RF)
• Specific range (SR)
• Example problem
Propulsion related parametrics
• Internal combustion
• Turboprop
• Turbojet & turbofan
• Afterburners
Weight related parametrics
• Fuel
• Payload
• Structure
• Systems
Covered under propulsion
Covered under weights c 2002 LM Corporation Air vehicle parametrics 15-16

Design of UAV Systems Cruise speed ranges
Typically determined by propulsion system type
Internal combustion(IC)* …..…….
Turboprop (TBP)* ………………..
High BPR turbofan (TBF)………..
Low BPR TBF without AB………..
Low BPR TBF with AB…………...
Turbojet (TBJ) without AB………..
Turbojet with after burner (AB)…..
Turbo Ramjet (TRJ)………………
Ramjet (RJ)………………………..
Scramjet (SRJ)…………………….
Rocket………………………………
50 - 300 Kts
200 - 350 Kts
350 - 500 Kts
350Kts - M1.5
400Kts - M2.5
350Kts - M1.0
400Kts - M3.0+
M2.0 - M3.5+
M2.0 - M5.0+
M5.0 - M12+
M1.0 - M25+
* Typical operating regime - higher speeds have been demonstrated c 2002 LM Corporation Air vehicle parametrics 15-17

Design of UAV Systems Variation with altitude
70000
60000
50000
40000
30000
20000
10000
0
Typical economic cruise/loiter speeds
Fighter/Attack
Bomber/transp.
Civil - TBProp
Civil ICProp
Global Hawk
Predator
100 200 300 400
Cruise Speed (Kts)
500 600 c 2002 LM Corporation Air vehicle parametrics 15-18

Design of UAV Systems Typical L/D ranges
Condensed from RosAD.1,Table 2.2*
Single & twin engine -
Cruise prop …………...
STOL ……………………………………
5 - 7
Business jets …………………………..
Regional turboprop…………………….
11 - 13
Jet transports…………………………..
8 - 10
4 - 7
4 - 6
Average 8.9
Loiter
10 - 12
8 - 10
12 - 14
14 - 16
14 - 18
10 - 14
6 - 9
7 - 9
11.4
(+25%)
Global Hawk** …………………………. 33 - 34
* Also see RayAD Fig 3.6 ** Flight International, UAVs, page 28,5 /1/01 c 2002 LM Corporation Air vehicle parametrics 15-19

Design of UAV Systems Typical SFC ranges*
Determined by propulsion and fuel
IC** ………………………………….
TBP*** ………………………………
High BPR TBF …………………….
Low BPR TBF (without AB)……….
Low BPR TBF (with AB)…………..
TJ (without AB)…………………….
TJ (with AB)………………………...
TRJ…………………………………..
M4 RJ (Hydrogen/Hydrocarbon)….
M8 SRJ (Hydrogen/Hydrocarbon)..
Rocket (Hydrogen/Hydrocarbon)…
Cruise
0.4
0.5
0.5
0.8
2+
0.8
2+
2+
0.9/2
1.5/3.5
8/10
Loiter
0.5
0.6
0.4
0.7
-
0.7
-
-
-
-
-
* Data from Roskam, Raymer and others
** IC SFC = Fuel Flow/HP; Turbine SFC = Fuel Flow/Thrust
*** Turboprops use both forms - SFC(hp) and TSFC (Lbf) c 2002 LM Corporation Air vehicle parametrics 15-20

Design of UAV Systems Real cruise SFCs
From previous chart
IC** ………...
TBP*** ……..
Cruise
0.4?
Loiter
0.5
HBPR TBF ..
LBPR TBF…
0.5
0.5?
0.8?
0.6?
0.4?
0.7?
Notation
0 = Sea level static cr = Typical cruise altitude & speed c 2002 LM Corporation Air vehicle parametrics 15-21

Design of UAV Systems Range factor (RF)
For typical fighters (@ subsonic cruise*) we will derive range factors for UAVs during the course
F-100 4920NM
F-101 4530NM
F-102 5390NM
F-104 4500NM
F-105 5200NM
F-106 5400NM
F-111 6450NM
F3D
F3H
F4D
F-4
3750NM
4480NM
3820NM
4200NM
F-86 4870NM
F-89 3970NM
* From RAND N-2283/2-AF, Dec 1987, approved for public release
From 15.1 and 15.1a, Range factor (RF)
For jets (nm)

KTAS*(L/Dcr)/TSFCcr
For prop (nm)

325.6*
 p*(L/Dcr)/SFCcr c 2002 LM Corporation Air vehicle parametrics 15-22

Design of UAV Systems Specific range (SR)
• A simple performance parametric used in many flight manuals
R = SR*

W (15.7) where and
SR = V/Wfdot (NM/Lbm-fuel)
Wfdot = Fuel flow (lbm/hr)
- Typically used for optimum (constant Mach) cruise above 36 Kft
- Or high-q dash performance c 2002 LM Corporation Air vehicle parametrics 15-23

Design of UAV Systems
Range related parametrics
• Speed (V)
• Lift-to-drag ratio (L/D)
• Specific fuel consumption (SFC)
• Fuel fraction (FF)
• Range factor (RF)
• Specific range (SR)
• Example problem
Propulsion related parametrics
• Internal combustion
• Turboprop
• Turbojet & turbofan
• Afterburners
Weight related parametrics
• Fuel
• Payload
• Structure
• Systems c 2002 LM Corporation Air vehicle parametrics
Next
Covered under propulsion
Covered under weights
15-24

Design of UAV Systems Example problem - review
• Five medium UAVs, four provide wide area search
(two are comm. relay), fifth does positive target ID
• WAS range required (95km) not a challenge
• No need to switch roles, simplifies ConOps
• No need for frequent climbs and descents
• Base communications and relay distances reasonable
• 158nm & 212 nm
• Reasonable dash speed (282kts)
• WAS and ID operating altitude
27.4 Kft differences reasonable
• But………….
• What kinds of air vehicles?
• What propulsion?
• How big will they be?
• How will they perform?
• What will they cost?
158 nm
100 nm
27.4 Kft
212 nm
10 Kft 27.4 Kft
200 nm x 200 nm c 2002 LM Corporation Air vehicle parametrics 15-25

Design of UAV Systems Positive ID - review
• We have a threshold requirement for positive (visual image) target identification (ID) 80% of the time
• To design our baseline for the threshold requirement
• We have to be able to operate at or below 10 Kft for 30% of the target identifications
• 50% of the time we can stay at altitude and 20% of the time we won’t see a target (unless we image at <= 5 Kft)
• This places 10Kft efficient cruise, loiter and climb and descent rate requirements on the air vehicle
Atmospheric conditions (customer defined)
Cloud ceiling/visibility
Clear day, unrestricted
10Kft ceiling, 10 nm
5Kft ceiling, 5 nm
1Kft ceiling, 1nm
Percent occurrence
50%
30%
15%
05% c 2002 LM Corporation Air vehicle parametrics 15-26

Design of UAV Systems Derived requirements - review
Derived requirements (from our assumptions or studies)
• System element
• Maintain continuous WAS/GMTI coverage at all times
• One target recognition assignment at a time
• Assume uniform area distribution of targets
• Communications LOS range to airborne relay = 158 nm
• LOS range from relay to surveillance UAV = 212 nm
• Air vehicle element
• Day/night/all weather operations, 100% availability
• Takeoff and land from 3000 ft paved runway
• Cruise/loiter altitudes = 10 – 27.4Kft
• Loiter location = 158 nm (min) – 255 nm (max)
• Loiter pattern – 2 minute turn
• Dash performance =141 nm @ 282 kts @

10 Kft
• Payload weight and volume = 720 lbm @ 26.55 cuft
• Payload power required = 4700 W c 2002 LM Corporation Air vehicle parametrics 15-27

Design of UAV Systems How do we start ?
• Analyze the problem
- What does the air vehicle have to do?
- Is any information missing?
• Look at some potential solutions
- What are the overall design drivers?
Payload weight and volume
Range and endurance
Speed and propulsion type
• Pick a starting baseline
• Analyze it
- Size/weight; range/endurance; cost and support
• Define and analyze the other approaches
- Compare results and select preferred baseline
• Define/trade preferred overall system
- Reasonable balance of cost, risk and effectiveness
• Document results c 2002 LM Corporation Air vehicle parametrics 15-28

Design of UAV Systems What kind of air vehicle?
• One that operates from a 3000 ft paved runway
• One that provides WAS over an area of interest
- At h = 27.4 Kft, 158nm - 255 nm from base,
- Fly circular pattern, 2 minute turns
- Maximum coverage area = 50nm x 50 nm each
• One that can ID targets at 141 nm in 30 minutes
- Based on analysis of WAS sensor information
- Based on other information
• One that can image targets from 10 Kft
• Once per hour (at maximum fly out distance)
• But how long must it loiter?
- 6 hours, 12 hours, 24 hours or even longer?
• …and what is the definition of “all weather”?
- Typhoons included?
c 2002 LM Corporation Air vehicle parametrics 15-29

Design of UAV Systems Getting answers
• Confer with team and/or ask the Systems Engineer
- And insist on definitive (quantifiable) answers
• Some typical responses –
- Loiter time:
What the team wants to say “interesting question, what are the trades?”
What the team needs to say “lets baseline a 12 hour loiter and do a trade study on the effects of from 6 to 24 hours?”
All weather definition : “Statistics indicate terrible
(unflyable) weather 10% of the time
Note: this conflicts with our 100% availability requirement c 2002 LM Corporation Air vehicle parametrics 15-30

Design of UAV Systems Other resources
• Lessons to follow – the basic understanding, analysis methods, models and parametric data for preliminary sizing and estimating overall mission performance
- Lesson 16: (Standard atmosphere) : Simple models that describe atmospheric properties as functions of altitude and speed
- Lesson 17: (Aerodynamics) : first-order aerodynamic prediction methods that capture key configuration features
- Lesson 18: (Parametric propulsion) : simplified engine models applicable across the performance envelope
- Review 19: (Parametric weights) simplified weight models that capture key configuration features
- Lesson 20 : (Parametric geometry) : simplified geometry models required to generate aerodynamic and weight inputs
- Lesson 21: (Flight mechanics) simplified physics based relationships used to predict flight performance by mission segment
- Lesson 22: (Integrated performance) : Spreadsheet models to perform initial sizing and calculate overall mission performance
• Plus parametric data from real air vehicles needed to test and validate simplified model predictions c 2002 LM Corporation Air vehicle parametrics 15-31

Design of UAV Systems Our first decision
It is a very important one
- What is the best propulsion cycle for the mission?
Internal combustion (ICprop), turboprop (TBProp) and turbo fan (TBFan) engines can all meet the baseline speed (280 kt) and altitude (10-27Kft) requirements
We bring our team together for the decision
- Speed is at the upper end of IC capability and high availability required will be a real challenge for IC engines
- TBProp is a good cycle for low-medium altitude operations
- TBFan is best at altitudes > 30 Kft and has best reliability
We select a … TBProp for our starting baseline and agree to evaluate a TBFan as the primary alternative
- IC alternative decision will be based on size required
We start with conventional wing-body-tail configurations
- We can evaluate more innovative concepts during conceptual design c 2002 LM Corporation Air vehicle parametrics 15-32

Design of UAV Systems Conflicting requirements
• System analysis thus far assumed 100% air vehicle availability and now weather limits availability to 90%
- This will affect SAR sizing (primarily)
We assumed SAR operation 100% of the time, therefore, the SAR only needed 80% area coverage
At 90% availability, the SAR would need to provide
89% area coverage (range increase to 102km) to achieve overall 80% (threshold) target coverage
• What should the we do, leave the baseline alone or resize the payload and start over?
Answer – leave it alone!
- During any design cycle, there will always be design and requirement disconnects
- If we change baselines every time we find a disconnect, we would never complete even one analysis cycle
• Orderly changes occur at the end of an analysis cycle c 2002 LM Corporation Air vehicle parametrics 15-33

Design of UAV Systems Next decision
• How many engines?
• Generally determined by available engine size
• The smallest number of engines will generally be the lightest and lowest drag
• How big will they be?
• Engine size is determined by thrust or horsepower-toweight required to meet performance requirements
• One sizing consideration is takeoff; others are speed, acceleration and maneuver
• Initially we size for takeoff (see RayAD, page 99)
• We assume a 3000 ft takeoff balanced field length
• Balanced field length means the air vehicle can accelerate to takeoff speed, have an engine failure and brake to a safe stop within the specified length
• We assume half the distance to reach takeoff speed
• Later we will calculate performance over the entire mission and ensure that all requirements can be met c 2002 LM Corporation Air vehicle parametrics 15-34

Design of UAV Systems c 2002 LM Corporation Air vehicle parametrics
Next decision
• Which mission do we size for?
1. WAS with maximum cruise out = 255nm at 27.4Kft
• Baseline operational endurance is 12 hr, with trade study options for 6 hr and 24 hr endurance
2. ID mission with cruise out = 200 nm @ TBD Kft
• Maximum ID distance constant at 141 nm, 282 kts
• WAS missions are performed at best loiter speed (max
L/D) and SFC
• ID missions are at max. speed
200 nm x 200 nm
(out and back), L/D will be lower
100 nm and SFC will be higher
• Both will have the same take off and landing requirements
• Answer…. simpler to design for
WAS and calculate fallout ID mission performance
158 nm
255 nm
141 nm
15-35

Design of UAV Systems Mission notation
17 16 15 14
12 13
4 5 6 7 8 9
10 11
18
19
2
1 0
3
Border -
Penetrate/Loiter
Border -
Loiter/Penetrate
Border -
Standoff
Notation
0 Engine start
1 Start taxi
2 Start takeoff
3 Initial climb
4 Initial cruise
5 Start pre-strike refuel
6 End pre-strike refuel
Start cruise
7 Start loiter
8 End loiter, start cruise
9 Start ingress
10 Combat
11 Weapon release
12 Turn
13 Start egress
14 End egress, start cruise
15 Start post-strike refuel
16 End post-strike refuel
17 End cruise
18 Start hold
19 End hold
Terminology
Standoff -
Distance from loiter or combat to border (+/-)
Standback -
Distance from refuel to border
Ingress -
To target at penetration speed
Egress -
From target at penetration speed
Range (Rge) = 2*Radius(R) c 2002 LM Corporation Air vehicle parametrics 15-36

Design of UAV Systems Defined & derived requirements
Defined
Remain airborne 24/7

90% of the time
Derived – payload, distances and altitudes
Payload : Wpay = 720 lbm
Cruise/loiter altidude: Hcr = Hlo = 27.4 Kft
Operating radius: D3-4+ D4-7 = D17-14 = 255 nm
Ingress/Egress: D8-14 = 0
Assumptions – typical values (design independent)
Landing fuel reserves; Klr = 5%; MiscF = 1%
Propeller efficiency:
 p = 80%
First cut estimates – refine later (design dependent)
Taxi/takeoff/climb fuel: Kttoc =10%
Average rate of climb: ROCavg = 1500 fpm
Average climb speed: Vcl = 0.8 Vcr (more about this later)
Parametric estimates – Next chart (design dependent)
Unknowns – Gross weight (W0); Fuel fraction (FF) c 2002 LM Corporation Air vehicle parametrics 15-37

Design of UAV Systems Parametric estimates
Chart 15-14 shows nominal empty weight fractions for manned TBProps (EWF = 0.58) and UAVs (EWF = 0.6)
- Predator B/Altair shows EWF = 0.44; probably more representative of our concept
Chart 15-18 shows typical economic cruise/loiter speeds at 27Kft to be in range of 180-300 kts
- We select lower value (180 kts) to maximize performance ( for both cruise and loiter )
Chart 15-19 shows typical TBProp cruise/loiter LoDs
- Regional TBProp: LoDcr = 11-13, LoDlo = 14-16
- Global Hawk LoDlo much higher (33-34) @ AR = 25
- We will select intermediate values @ 23 and 25
Charts 15-20 (table) and 15-21 (plot) TBProp cruise &
loiter SFCs conflict (not unusual for parametric data)
- The plot is from our engine database (real TBProps) so we use it and estimate SFCcr = SFClo = 0.4
c 2002 LM Corporation Air vehicle parametrics 15-38

Design of UAV Systems Solution approach
Calculate cruise ranges and range factors (15.1a)
Time to climb = 27.4 Kft/1500 fpm = 0.30 hr
Climb speed

0.8

180 = 144 kts
Climb distance = 43.8 nm
R4-7 = 255 - 43.8 = 211.2 nm
R14-17 = 255 nm
RFcr = 325.6
 p

(L/Dcr)/SFCcr = 14977.6 nm
Calculate outbound initial/final cruise weight ratios (15.1)
R4-7 = 14977.6 nm

Ln(W4/W7) or….
W4 = 1.014

W7
Calculate inbound initial/final cruise weight ratios (15.1)
R14-17 = 14977.6

Ln(W14/W17) or… W14 = 1.017

W17
Calculate initial/final loiter weight ratios (15.2 and 2a)
EFlo = 325.6
 p

[(L/Dlo)/(Vlo

SFClo] = 90.4 hr
E7-8 = 12hrs = EFlo

Ln(W7/W8) or… W7 = 1.142

W14
- Note: W8

W14 (no ingress/egress) c 2002 LM Corporation Air vehicle parametrics 15-39

Design of UAV Systems Size estimate
By definition the initial and final cruise weights (W4 and
W17) are given by (see chart 15.8)
W4 = W0

[1-FF

Kttoc] where Kttoc = 0.1
W17 = W0

[1-FF(1-Klr)] where Ktlr = 0.05
Therefore:
W4 = W0

[1- 0.1

FF] = 1.014

W7 = 1.014

1.142

W14
= 1.014

1.142

1.017

W17
= 1.014

1.142

1.017

W0

[1 - 0.95

FF] or
FF = 0.175
Then from 15.3
FF = 1 -EWF -MiscF - PF = 1 -0.44 -0.01 -720lbm/W0 or …..W0 = 1918 Lbm
And maximum range and endurance (from Eq 15.5-6) are
Rmax = 2453 nm and Emax = 14.8 hrs c 2002 LM Corporation Air vehicle parametrics 15-40

Design of UAV Systems Parametric comparison
• Whenever we calculate a performance parameter or size a vehicle, we should always ask ourselves if the calculation makes sense
- In this case, the sizing results should make sense since we used parametric data from similar aircraft as inputs
- Nonetheless, we should still make a reality check using our UAV data spreadsheet ASE261.UAV data.xls
Which shows that we have a problem
UAV Fuel Fractions
• Compared to other TBProp
UAVs, our calculated FF is
0.80
0.70
0.60
0.50
Piston
Turboprop
Jet
Jet
Piston low for Emax = 14.8 hrs
- Other TBProp UAVs
0.40
0.30
0.20
require higher FFs for this level of performance
0.10
0 8 16 24 32 40 48
- The data shows our inputs must be optimistic
Max Endurance (hrs) c 2002 LM Corporation Air vehicle parametrics 15-41

Design of UAV Systems Issue resolution
• There are many possible explanations for why our estimated fuel fraction is low
- LoDs and SFcs were estimated, not calculated
- Ditto for empty weight fractions, speeds, etc.
• What should we do?
- Press on with a higher value of fuel fraction?
- Stop and try to resolve the issues
- Proceed with the knowledge that our performance estimates are optimistic
• We can press on and sort it out later
- Our spread sheet design and analysis methods are designed to handle uncertainties and disconnects
- Corrections can be made with a few input changes or multipliers on performance parameters
• However, if we were using traditional design methods, we would need to resolve the issue or risk a major down stream redesign or disconnect c 2002 LM Corporation Air vehicle parametrics 15-42

Design of UAV Systems TBFan alternative
Defined Same assumptions as TBProp
Remain airborne 24/7

90% of the time
Derived – payload, distances and altitudes
Payload : Wpay = 720 lbm
Cruise/loiter altidude: Hcr = Hlo = 27.4 Kft
Operating radius: D3-4+ D4-7 = D17-14 = 255 nm
Ingress/Egress: D8-14 = 0
Assumptions – typical values (design independent)
Landing fuel reserves; Klr = 5%; MiscF = 1%
Propeller efficiency:
 p = 80%
First cut estimates – refine later (design dependent)
Taxi/takeoff/climb fuel: Kttoc =10%
Average rate of climb: ROCavg = 1500 fpm
Average climb speed: Vcl = 0.8 Vcr (more about this later)
Parametric estimates – Next chart (design dependent)
Unknowns – Gross weight (W0); Fuel fraction (FF) c 2002 LM Corporation Air vehicle parametrics 15-43

Design of UAV Systems TBFan alternative
Chart 15-14 shows nominal empty weight fractions for manned TBFans (EWF = 0.55) and UAVs (EWF = 0.6)
- Predator C shows EWF = 0.39; probably more representative of our concept
Chart 15-18 shows jet aircraft economic cruise/loiter speeds at 27Kft to be in range of 250-525 kts
- We select a lower value (300 kts) for both cruise and loiter (but not the lowest since RF

Vcr)
Chart 15-19 shows typical TBFan cruise/loiter LoDs
- BizJet TBFan: LoDcr = 10-12, LoDlo = 12-14
- Global Hawk LoDlo much higher (33-34) @ AR = 25
- We will select intermediate values @ 22.5 and 23.5
Charts 15-20 (table) and 15-21 (plot) TBFan cruise &
loiter SFCs conflict (not unusual for parametric data)
- The plot is from our engine database (real TBFans) so we use it and estimate TSFCcr = TSFClo = 0.65
c 2002 LM Corporation Air vehicle parametrics 15-44

Design of UAV Systems
TBFan cont’d
Calculate cruise ranges and range factors (15.1a)
Time to climb = 27.4 Kft/1500 fpm = 0.30 hr
Climb speed

0.8

300 = 240 kts
Climb distance = 72 nm
R4-7 = 255 - 72 = 183 nm
R14-17 = 255 nm
RFcr = Vcr

(L/Dcr)/TSFCcr = 10385 nm
Calculate outbound initial/final cruise weight ratios (15.1)
R4-7 = 10385 nm

Ln(W4/W7) or….
W4 = 1.018

W7
Calculate inbound initial/final cruise weight ratios (15.1)
R14-17 = 10385

Ln(W14/W17) or… W14 = 1.025

W17
Calculate initial/final loiter weight ratios (15.2 and 2a)
EFlo = (L/Dlo)/SFClo = 36.2 hr
E7-8 = 12hrs = EFlo

Ln(W7/W8) or… W7 = 1.394

W14
- Note: W8

W14 (no ingress/egress) c 2002 LM Corporation Air vehicle parametrics 15-45

Design of UAV Systems
TBFan cont’d
By definition the initial and final cruise weights (W4 and
W17) are given by (see chart 15.8)
W4 = W0

[1-FF

Kttoc] where Kttoc = 0.1
W17 = W0

[1-FF(1-Klr)] where Ktlr = 0.05
Therefore:
W4 = W0

[1- 0.1

FF] = 1.021

W7 = 1.018

1.394 W14
= 1.018

1.394

1.025

W17
= 1.018

1.394

1.025

W0

[1 - 0.95

FF]
FF = 0.354
Then from 15.3
FF = 1 -EWF -MiscF - PF = 1 -0.39 -0.01 -720lbm/W0 or …..W0 = 2914 Lbm
And maximum range and endurance (from Eq 15.5-6) are
Rmax = 3885 nm and Emax = 13.5 hrs c 2002 LM Corporation Air vehicle parametrics 15-46

Design of UAV Systems Expectations
• You should understand
(1) How to analyze requirements to meet mission altitude, speed, operating distance and loiter time requirements
- What is defined
- What to assume
- What to estimate and later refine
- What to solve for
(2) How to calculate fuel fraction and gross weight
- To meet operating distance and loiter time requirements
(3) How to use parametric data
- To assess/select inputs
- To check results c 2002 LM Corporation Air vehicle parametrics 15-47

Design of UAV Systems Second half approach
40 individual design projects

10 team projects
- Every team member responsible for team product
Team grade shared (1/3)
- Everybody responsible for one element (1/3 credit)
System engineering or system, payload or support elements
- Every team member responsible for at least one configuration concept (1/3 credit)
Each team will carry multiple configuration concepts through logical configuration evaluation/comparison
- Configuration down selects must be based on quantitative vs. qualitative assessment
Top 10 first half (1H Top 10) projects will be revealed
- Teams should form around each of the 10 projects
Team leads (System Engineers) = 1H Top 10 c 2002 LM Corporation Air vehicle parametrics 15-48

Design of UAV Systems Homework
1. Establish your project design teams. List names (team grade)
2. Document your project plan (team grade)
Schedule your project activities
Who is responsible for what element/task
3. Select 4 air vehicle concepts (one per team member) to be evaluated during the 2 nd half of the semester (team grade)
4. Size your configuration concept (individual grades)
Calculate FF and W0
Compare your calculations to parametric data and assess the results c 2002 LM Corporation Air vehicle parametrics 15-49

Design of UAV Systems Intermission c 2002 LM Corporation Air vehicle parametrics 15-50