15-1 Design of UAV Systems Air vehicle parametricsc 2002 LM Corporation Lesson objective - to discuss Air vehicle parametrics including … Rationale Applications

15-1

Design of UAV Systems

Air vehicle parametricsc 2002 LM Corporation

Lesson objective - to discuss

Air vehicle parametricsincluding …

• Rationale

• Applications

• Limits

Objectives

Expectations - You will understand when and how to use parametric relationships

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

15-2



Many types of parametrics

Covered under propulsion

This lesson

Covered under weights

15-3



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 designDuring 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

15-4



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

15-5



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

15-6



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 competitionErrors 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

15-7



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 themUse 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

15-8



Example mission

Kttoc = (taxi-takeoff-climb fuel)/Wf Klr = landing fuel reserves/WfWf = fuel weight

Start cruise at W = W0 - Kttoc*WF

0

2 3

4 5 6 7 810 11

12 13151617

18

19

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)

Terminology

1

14

9

0 Engine start1 Start taxi2 Start takeoff3 Initial climb4 Initial cruise5 Start pre-strike refuel6 End pre-strike refuel

Start cruise 7 Start loiter8 End loiter, start cruise9 Start ingress

10 Combat11 Weapon release 12 Turn13 Start egress14 End egress, start cruise15 Start post-strike refuel16 End post-strike refuel17 End cruise18 Start hold19 End hold

Notation

End cruise at W = W0 - (1-Klr)*WF

Border -Penetrate/Loiter

Border - Loiter/Penetrate

Border - Standoff

15-9



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

15-10






Next subject



This lesson

15-11



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) (15.1)

R = Cruise range Vcr = Cruise speedL/Dcr = Cruise lift-to-drag ratio (LoDcr)TSFCcr = Cruise thrust specific fuel consumption

VcrL/Dcr/TSFCcr = RF (range factor) = WSRSR = Specific Range = V/Fuel flow)

- For propeller aircraft (more about this later)

RF(nm) = 325.6p(L/Dcr)/SFCcr (15.1a)

p = propeller efficiency 0.8 (for constant speed prop)

where

where

and

15-12



Endurance related parametrics

• Based on Breguet endurance equation factors- For jet aircraft (RayAD 3.7)

E = [(L/Dlo)/TSFClo]Ln(Wi/Wj) (15.2)

E = Endurance (hrs)L/Dlo = Loiter lift-to-drag ratio (LoDcr)TSFClo = Loiter thrust specific fuel consumption

(L/Dlo)/TSFClo = EF (endurance factor) = Wbar/WdotFWbar = Average loiter weight (lbm)WdotF = Fuel flow (lbm/hr)

- For propeller aircraft (more about this later also)

E = EFLn(Wi/Wj) (15.2a)

EF = 325.6p [(L/Dlo)/(VloSFClo]Vlo = Loiter speed; p = propeller efficiency

where

where

and

15-13



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) Fuel weight/Gross wt.

Payload fraction (PF) 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

(15.3)• Maximum range and endurance occur when

(Wi/Wj)max = (1 - Kttoc*FF)/(1-(1- Klr)*FF) (15.4)Rmax = RFln[(Wi/Wj)max]

(15.5)Emax = [(L/Dlo)/TSFClo]ln[(Wi/Wj)max]

(15.6)

or

15-14



Typical weight fractions

- 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

Typical Empty Weight Fractions(weight data from Roskam, Janes UAVs)

0.40

0.50

0.60

0.70

0 100000 200000

Nominal Gross Weight

No

min

al E

WF

SE Prop

ME Prop

Biz Jet

Reg TBP

Jet Transp

Mil Trainer

Fighters

Mil. PBC

FW UAV

Typical Fuel Fractions(weight data from Roskam, Janes UAVs)

0.00

0.10

0.20

0.30

0.40

0.50

0 100000 200000

Nominal Gross Weight

No

min

al F

F

SE Prop

ME Prop

Biz Jet

Reg TBP

Jet Transp

Mil Trainer

Fighters

Mil. PBC

FW UAV

Caution

Typical value

15-15



SE-Prop ME-Prop Biz JetReg. TurboJet Transp. Mil. TrainersFighters Mil PBC FW UAV

GWMin 1817 2183 4550 5732 44000 2238 5291 50706 4

Mean 2750 6625 20000 14550 220000 4188 29975 158730 1245Max 11574 10325 68200 57250 775000 11100 91500 769000 25600

EWFMin 0.437 0.555 0.479 0.517 0.428 0.461 0.381 0.327 0.359

Mean 0.595 0.623 0.548 0.577 0.536 0.697 0.472 0.491 0.605Max 0.791 0.689 0.622 0.662 0.610 0.789 0.639 0.732 0.870

FFMin 0.059 0.115 0.291 0.149 0.199 0.101 0.101 0.208 0.111

Mean 0.125 0.178 0.359 0.230 0.326 0.213 0.212 0.405 0.266Max 0.283 0.298 0.417 0.334 0.536 0.381 0.387 0.674 0.566

Database variation examples




15-16



Next



15-17



Cruise speed ranges

* Typical operating regime - higher speeds have been demonstrated

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 Kts200 - 350 Kts350 - 500 Kts350Kts - M1.5400Kts - M2.5350Kts - M1.0400Kts - M3.0+M2.0 - M3.5+M2.0 - M5.0+M5.0 - M12+M1.0 - M25+

15-18



Variation with altitude

Typical economic cruise/loiter speeds

10000

20000

30000

40000

50000

60000

70000

0 100 200 300 400 500 600

Cruise Speed (Kts)

Cru

ise

alt

itu

de

(ft

)

Fighter/AttackBomber/transp.Civil - TBPropCivil ICPropGlobal HawkPredator

15-19



Typical L/D ranges

* Also see RayAD Fig 3.6 ** Flight International, UAVs, page 28,5 /1/01

Condensed from RosAD.1,Table 2.2*

Single & twin engine - prop …………...STOL ……………………………………Business jets …………………………..Regional turboprop…………………….Jet transports…………………………..Military trainers………………………...Fighters…………………………………Supersonic cruise……………………..Average

Cruise8 - 105 - 710 - 1211 - 1313 - 158 - 104 - 74 - 6

8.9

Loiter10 - 128 - 1012 - 1414 - 1614 - 1810 - 146 - 97 - 911.4

Global Hawk** …………………………. 33 - 34

(+25%)

15-20



Typical SFC ranges*

Determined by propulsion and fuelIC** ………………………………….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)…

Cruise0.40.50.50.82+0.82+2+

0.9/21.5/3.5

8/10

Loiter0.50.60.40.7- 0.7-----

** IC SFC = Fuel Flow/HP; Turbine SFC = Fuel Flow/Thrust*** Turboprops use both forms - SFC(hp) and TSFC (Lbf)

* Data from Roskam, Raymer and others

SFC variation - TBF

0.2

0.4

0.6

0.8

1

0.2 0.4 0.6 0.8 1TSFC0 (lmb/hr lbf)

15-21



Real cruise SFCs

SFC Variation - TBP

0.2

0.4

0.6

0.8

0.2 0.4 0.6 0.8SFC0

SFC Variation - Piston

0.2

0.4

0.6

0.8

0.2 0.4 0.6 0.8SFC0 (lbm/hr Hp)

Cruise0.4?0.50.5?0.8?

IC** ………...TBP*** ……..HBPR TBF ..LBPR TBF…

Loiter0.50.6?0.4?0.7?

From previous chart

Notation 0 = Sea level static cr = Typical cruise altitude & speed

15-22



Range factor (RF)

* From RAND N-2283/2-AF, Dec 1987, approved for public release

For typical fighters (@ subsonic cruise*) - we will derive range factors for UAVs during the course

F-100 4920NMF-101 4530NMF-102 5390NMF-104 4500NMF-105 5200NMF-106 5400NMF-111 6450NM

F3D 3750NMF3H 4480NMF4D 3820NM F-4 4200NMF-86 4870NMF-89 3970NM

From 15.1 and 15.1a, Range factor (RF) For jets (nm) KTAS*(L/Dcr)/TSFCcrFor prop (nm) 325.6*p*(L/Dcr)/SFCcr

15-23



Specific range (SR)

• A simple performance parametric used in many flight manuals

R = SR*W (15.7)

where SR = V/Wfdot (NM/Lbm-fuel)

and Wfdot = Fuel flow (lbm/hr)

- Typically used for optimum (constant Mach) cruise above 36 Kft

- Or high-q dash performance

15-24



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



Next



15-25



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 differences reasonable• But………….

• What kinds of air vehicles?• What propulsion?• How big will they be?• How will they perform?• What will they cost?

100 nm

200 nm x 200 nm

158 nm

27.4 Kft

10 Kft27.4 Kft 27.4 Kft

212 nm

15-26



• 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

Positive ID - review

Cloud ceiling/visibility Clear day, unrestricted 10Kft ceiling, 10 nm 5Kft ceiling, 5 nm 1Kft ceiling, 1nm

Percent occurrence 50%30%15%05%

Atmospheric conditions (customer defined)

15-27



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

Derived requirements - review

15-28



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

15-29



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?

15-30



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

15-31



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

15-32



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

15-33



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

15-34



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-to-weight 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

15-35



• 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 (out and back), L/D will be lower and SFC will be higher• Both will have the same take off and landing requirements• Answer….

Next decision

100 nm

158 nm

200 nm x 200 nm

255 nm

141 nm simpler to design for WAS and calculate fallout ID mission performance

15-36



Mission notation

0 Engine start1 Start taxi2 Start takeoff3 Initial climb4 Initial cruise5 Start pre-strike refuel6 End pre-strike refuel

Start cruise 7 Start loiter8 End loiter, start cruise9 Start ingress

10 Combat11 Weapon release 12 Turn13 Start egress14 End egress, start cruise15 Start post-strike refuel16 End post-strike refuel17 End cruise18 Start hold19 End hold

Notation

Border - Standoff

Border - Loiter/Penetrate

Border -Penetrate/Loiter

0

2 3

4 5 6 7 810 11

12 13151617

18

19

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)

Terminology

1

14

9

15-37



Defined & derived requirements

Defined Remain airborne 24/7 90% of the time

Derived – payload, distances and altitudesPayload : Wpay = 720 lbmCruise/loiter altidude: Hcr = Hlo = 27.4 KftOperating radius: D3-4+ D4-7 = D17-14 = 255 nmIngress/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 fpmAverage climb speed: Vcl = 0.8 Vcr (more about this later)

Parametric estimates – Next chart (design dependent)Unknowns – Gross weight (W0); Fuel fraction (FF)

15-38



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

15-39



Solution approach

Calculate cruise ranges and range factors (15.1a)Time to climb = 27.4 Kft/1500 fpm = 0.30 hrClimb speed 0.8180 = 144 ktsClimb distance = 43.8 nmR4-7 = 255 - 43.8 = 211.2 nmR14-17 = 255 nmRFcr = 325.6p(L/Dcr)/SFCcr = 14977.6 nm

Calculate outbound initial/final cruise weight ratios (15.1) R4-7 = 14977.6 nmLn(W4/W7) or…. W4 = 1.014W7

Calculate inbound initial/final cruise weight ratios (15.1) R14-17 = 14977.6Ln(W14/W17) or…W14 = 1.017 W17

Calculate initial/final loiter weight ratios (15.2 and 2a)EFlo = 325.6p[(L/Dlo)/(VloSFClo] = 90.4 hrE7-8 = 12hrs = EFloLn(W7/W8) or…W7 = 1.142 W14

- Note: W8 W14 (no ingress/egress)

15-40



Size estimate

By definition the initial and final cruise weights (W4 and W17) are given by (see chart 15.8)

W4 = W0[1-FFKttoc] where Kttoc = 0.1W17 = W0[1-FF(1-Klr)] where Ktlr = 0.05

Therefore:W4 = W0[1- 0.1FF] = 1.014W7 = 1.0141.142 W14 = 1.0141.1421.017 W17 = 1.0141.1421.017W0[1 - 0.95FF]

FF = 0.175Then from 15.3

FF = 1 -EWF -MiscF - PF = 1 -0.44 -0.01 -720lbm/W0or…..W0 = 1918 Lbm

And maximum range and endurance (from Eq 15.5-6) areRmax = 2453 nm and Emax = 14.8 hrs

or

15-41



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

• Compared to other TBProp UAVs, our calculated FF is low for Emax = 14.8 hrs- Other TBProp UAVs require higher FFs for this level of performance

- The data shows our inputs must be optimistic

UAV Fuel Fractions

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0 8 16 24 32 40 48

Max Endurance (hrs)

Fu

el F

ract

ion

(F

F)

Piston TurbopropJetJetPiston

• 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

15-42



Issue resolution

• 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

15-43

TBFan alternativeDesign of UAV Systems


Defined Remain airborne 24/7 90% of the time

Derived – payload, distances and altitudesPayload : Wpay = 720 lbmCruise/loiter altidude: Hcr = Hlo = 27.4 KftOperating radius: D3-4+ D4-7 = D17-14 = 255 nmIngress/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 fpmAverage climb speed: Vcl = 0.8 Vcr (more about this later)

Parametric estimates – Next chart (design dependent)Unknowns – Gross weight (W0); Fuel fraction (FF)

Same assumptions as TBProp

15-44

TBFan alternativeDesign of UAV Systems


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

15-45

TBFan cont’dDesign of UAV Systems


Calculate cruise ranges and range factors (15.1a)Time to climb = 27.4 Kft/1500 fpm = 0.30 hrClimb speed 0.8300 = 240 ktsClimb distance = 72 nmR4-7 = 255 - 72 = 183 nmR14-17 = 255 nmRFcr = Vcr(L/Dcr)/TSFCcr = 10385 nm

Calculate outbound initial/final cruise weight ratios (15.1) R4-7 = 10385 nmLn(W4/W7) or…. W4 = 1.018W7

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 hrE7-8 = 12hrs = EFloLn(W7/W8) or…W7 = 1.394W14

- Note: W8 W14 (no ingress/egress)

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TBFan cont’d

By definition the initial and final cruise weights (W4 and W17) are given by (see chart 15.8)

W4 = W0[1-FFKttoc] where Kttoc = 0.1W17 = W0[1-FF(1-Klr)] where Ktlr = 0.05

Therefore:W4 = W0[1- 0.1FF] = 1.021W7 = 1.0181.394 W14 = 1.0181.3941.025W17 = 1.0181.3941.025W0[1 - 0.95FF]

FF = 0.354Then from 15.3

FF = 1 -EWF -MiscF - PF = 1 -0.39 -0.01 -720lbm/W0or…..W0 = 2914 Lbm

And maximum range and endurance (from Eq 15.5-6) areRmax = 3885 nm and Emax = 13.5 hrs

• 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

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Expectations

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

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Second half approach

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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 2nd 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

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IntermissionDesign of UAV Systems


Documents

15-1 Design of UAV Systems Air vehicle parametricsc 2002 LM Corporation Lesson objective - to discuss Air vehicle parametrics including … Rationale Applications