Loading...

PPT – Lesson objective to show how to PowerPoint presentation | free to download - id: b3286-ZDA1N

The Adobe Flash plugin is needed to view this content

Objectives

- Lesson objective - to show how to
- Put it all together
- With a focus on
- The air vehicle

Expectations - You will better understand how to

approach air vehicle design

24-1

? 2002 LM Corporation

How do we start - review

- 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 starting baseline
- Size and weight range and endurance
- Analyze the other approaches
- Compare results and select preferred baseline
- Define preferred overall system
- Reasonable balance of cost, risk and

effectiveness - Document results

Today

24-2

What kind of air vehicle - review

- Operates from 3000 ft paved runway (defined

reqmnt) - Loiters over an area of interest (defined

reqmnt) - At h 10-17Kft, 158nm-255 nm from base (derived)
- Baseline loiter time 12 hrs, do trade study on

6 and12 hr (system engineer, team decision) - Fly circular pattern, 2 minute turns (derived)
- Maximum coverage area 200nm x 200 nm (defined)
- WAS for 10 sqm moving targets in 2 minutes

(defined) - Dashes 141 nm to target in 30 min. (derived

reqmnt) - Once per hour (follow-up customer response)
- Based on WAS sensor or other information
- Images targets from 10 Kft (derived reqmnt)
- Operates in all weather
- - 60 good weather, 30 bad but flyable, 10

terrible weather (unflyable) - This conflicts with our 100 availability

assumption

24-3

Our first decision- review

- It is a very important one
- What is the best propulsion cycle for the

mission? - Internal combustion (IC), turboprop (TBProp) and

turbo fan (TBFan) engines can all meet baseline

speed (280 kt) and altitude (10-17Kft)

requirements - We bring our team together for the decision
- Speed and altitude is at the upper end of IC

capability, reliability required will be a

challenge for an IC engine - TBProp is good cycle for low-medium altitude

operations - TBFan is best at altitudes gt 36 Kft but has best

reliability - We select the TBProp as our starting baseline and

agree to evaluate a TBFan as the primary

alternative - IC alternative decision will be based on size

required - Conventional wing-body-tail configuration(s)

selected - Evaluate innovative concepts during conceptual

design - We document our decisions as derived

requirements

24-4

Next decision- review

- How many engines?
- Generally determined by available engine size
- The smallest number of engines will always 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
- We design for balanced field length (BFL) 3000

ft - Approximate BFL 1500 ft ground roll to lift off

speed, 1500 ft to stop if engine fails at liftoff - Later we will calculate performance over the

entire mission and ensure that all requirements

can be met - This is what we will do today

24-5

Review reqmnt disconnect

- Initial system assessment assumed 100 air

vehicle availability, weather now 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 needs to provide 89

area coverage (range increase to 102km) to

achieve overall 80 (threshold) target coverage - We decided to leave the baseline alone and finish

the first design cycle before making the change? - During any design cycle, there will always be

design and requirement disconnects - If we change baseline every time we find a

disconnect, we would never complete even one

analysis cycle - Orderly changes occur at the end of an analysis

cycle

24-6

Review - fuselage considerations

- Our methodology sizes the fuselage as a

cylindrical center section with elliptical fore

and aft bodies - The fuselage is defined in absolute and relative

terms - Fuselage equivalent diameter (Df-eq) is absolute

but is iterated to assure volume required

available - Relative variables are length to equivalent

diameter ratio (Lf/Df-eq), and forebody and

aftbody length ratios - At a maximum speed of 282kts, a relatively low

fineness ratio (Lf/Df-eq) can be used with

minimum drag impact - We select a nominal value of 7.0 (cigar shape)

to minimize wetted area (a weight and drag

driver) - If we assume the fuselage forebody length

1Df-eq and the aftbody 2Df-eq, center section

length (Lc) ratio will be 4/7 or Lc/Lf 0.571

24-7

Review - fuselage volume

- To get started we put payload in the fuselage

center section, close to the vehicle center of

gravity - It accommodates a payload weight of 720 lbm and

a volume of 26.55 cuft (density 27.1 pcf) - It also carries some fuel (amount TBD, density

50 pcf at packing factor PF 0.8 or installed

density 40 pcf) - And it carries airframe structure and some

systems (landing gear, etc., nominal installed

density 25 pcf) - We assume other systems are in the fore

aftbodies - We assume center section volume (Vc) is allocated

entirely to payload at a packing factor (PF)

0.7 - Therefore, Vc required 26.55/0.7 37.9 cuft
- Later the spreadsheet will size for actual volume

required

25 pcf is a reasonable estimate for installed

electrical, mechanical systems including

avionics, landing gear and engines

24-8

Review - fuselage geometry

24-9

Review - engine installation

- Simple engine installations are always best

unless there are over-riding considerations - Such as high speed, stealth, thrust vectoring,

etc. - Otherwise, complexity reduces overall performance
- Nacelle geometry is driven by engine installation
- TBProp nacelles should be low drag, minimum

length - Our methodology models nacelles like

mini-fuselages - Cylindrical center section, elliptical fore and

aft bodies - Nacelle type is defined by an input wetted area

fraction (vs. a typical podded nacelle) - 1.0 typical podded commercial jet transport

nacelle - 0.5 nacelle attached to fuselage (e.g. Global

Hawk) - 0.0 engine buried in the fuselage (e.g.

DarkStar) - We assume a single, attached, aft mounted engine
- L/Dnac 4 k1 0.2 k2 0.4 Dnac/Deng

1.25 nacelle Swet fraction 0.5

Change from lesson 20

24-10

Wing considerations (expanded)

- Our design methodology sizes the wing separate

from the fuselage - We have 4 primary decisions to make size

(planform area or Sref), shape (Aspect ratio or

AR and taper ratio or ?), sweep (?) and thickness

ratio (t/c) - Planform area will be determined by wing loading

(W0/Sref), a primary design variable - A reasonable value for a turboprop is ? 30-60 psf

(PredatorB RayAD Table 5.5) - We pick a value of 30 and later will refine the

estimate to ensure takeoff/cruise/loiter

requirements are met - AR is a primary wing design variable determined

by speed, maneuverability and lift-to-drag (L/D

or LoD) ratio - High AR generally means high LoD (gt20), low

maneuverability (a few gs) and low speed (lt350

kts) - For long endurance we select a starting value of

20

24-11

Wing - continued

- Taper ratio (?) is a secondary wing design

variable that drives wing drag due to lift

achieved vs. a theoretical minimum (see RayAD

Fig. 4-23) - A nominal value is 0.5 selected and needs no

further pre-concept design trade - Wing sweep is driven by speed, at a maximum speed

of 282 kts we have no need for wing sweep - Wing t/c has a major impact on wing weight, the

higher the t/c, the lighter the wing weight - High t/c increases drag but trades favorably

against wing weight at low speed - At 282 kts we select a nominal maximum value (t/c

0.13), it needs no further pre-concept design

trades - Of the wing design variables selected, only

W0/Sref and AR need to be traded for our speed

range

24-12

Review - wing volume

- Another concept design wing consideration is

volume available for fuel - Wing fuel volume is defined in terms of percent

wing chord and span available for tankage - Typically wing tanks start at the wing root or

fuselage attachment and can extend to or near the

wing tip - For our UAV application we assume the wing tank

starts at 10 span and extends to 90 span (?1

0.1, ?2 0.9). We estimate tank chord at 50

wing chord (Kc 0.5) and fuel packing factor at

0.8 - These initial estimates are not upper limit

values - The tanks could extend from fuselage centerline

to wing tip if required (?1 0, ?2 1) but it

is unlikely that tank chord will exceed the

assumed 50 - Fuel density again is estimated at 50 pcf at PF

0.8

Another change

24-13

Tail considerations

- During pre-concept design, our primary concern is

tail type and size - We use parametric (historical) data to estimate

both horizontal and vertical tail size required - For V-tails we size using projected areas
- During conceptual design we will resize to ensure

adequate stability and control and handling

qualities - Our geometry model defines horizontal tail area

(Sht) and vertical tail area (Svt) as fractions

of Sref or - Sht Kht?Sref and Svt Kvt?Sref
- Where for an average air vehicle
- Kht .25 and Kvt .15
- Average V-tail area would be 0.39Sref
- Our UAV will use an average V-tail area fraction

Another change

24-14

Review - aerodynamic model

- Our aerodynamic model estimates lift and drag

from geometry and input values of equivalent skin

friction coefficient (Cfe) and Oswald wing

efficiency (e) - We will assume a state-of-the art Cfe value of

0.0035 to reflect our assumption of good surface

smoothness (See RayAD Table 12.3) - Wing efficiency (e) is estimated at a value of

0.8 using parametric data for an unswept wing at

AR 20 - The model uses these inputs to calculate minimum

and induced drag coefficients (Cd0 and Cdi) - Lift coefficients are calculated from weight (W),

Sref and flight dynamic pressure (q) where - Cl W/(q?Sref)
- Loiter and climb q are assumed to be at max L/D

24-15

Review - weight model

- Bottoms-up weight estimates are based on a

combination of methods - Airframe weight estimates use input unit weights

and calculated wetted or planform areas - Propulsion weight is based on T0/Weng or

Bhp0/Weng - Landing gear weight (Wlg) is based on an input

gross weight (W0) fraction where Wlg Kwlg?W0 - Other system weights (Wsys) use another input

weight fraction where Wsys Ksys?W0 - We will use nominal values from RayAD Table 15.2

adjusted for a typical turboprop UAV where - Wing unit weight (Uww) 3.25 psf
- Tail unit weight (Utw) 2.6 psf
- Fuselage/nacelle unit weight (Ufpnw) 1.8 psf
- Klg 0.05 and Ksys (or all-else empty ) 0.12
- We also include an empty weight margin (5)

Another change

24-16

Review - volume model

- Volume requirements are calculated while

iterating bottoms-up weight and geometry - Fuel, payload, system and landing gear weights

are used to estimate fuselage and pod (if any)

volume required - Fuel volume fuel weight/( fuel density?PF )
- Payload volume 26.55 cuft (chart 11-61)
- Landing gear volume gear weight/25 pcf
- Other systems volume other systems weight/25

pcf - Volume available is calculated by the geometry

model using input estimates of useable volume per

component - Nominal value 0.7 for fuselage and pods (if

any) - Nominal value for nacelles is a configuration

variable - In our baseline, we assume the nacelle is

unavailable for anything except the engine, inlet

and nozzle - Df-eq is adjusted to equate volume available and

volume required plus 30 margin (or PF 0.7/1.3

0.54)

Final change

24-17

Review - propulsion model

- Our propulsion model is a simplified cycle deck

used to represent both turboprops (TBP) and

turbofans (TBF) - Engines are sized at sea level static conditions

(h0, V0) based on input values of thrust or

power to gross weight required (T0/W0 or Bhp0/W0) - The models predict performance at other values of

altitude and speed by assuming that power or

thrust vary primarily with airflow (WdotA) - Differences between TBFs and TBPs are determined

by input values of bypass ratio (BPR), fan

specific thrust (T0-fan/W0dotA-fan) and a

reference speed (V0) - Our UAV studies will use the TBP and TBF values

in Lesson 18, chart 18.33

24-18

Review - air vehicle performance

- Air vehicle performance is estimated using

calculated values of gross weight (W0), empty

weight (We or EW) and fuel weight (Wf) - The mission is calculated forward and backward
- Forward calculations use simplified performance

models to estimate fuel required for engine

start-taxi-takeoff, climb and cruise out to

initial loiter location - Another calculation works backward from empty

weight and calculates fuel required for landing

reserves and loiter, cruise back, dash from

target, combat over the target (including payload

drop) and dash to target - The sum of the two subtracted from the starting

fuel weight is the amount of fuel available for

loiter - A Breguet endurance calculation using the pre and

post loiter weights then predicts operational

endurance

24-19

Review - mission description

- We will define our mission to meet maximum

distance requirements for each of the two mission

types - WAS cruise out 255nm at 27.4Kft
- Baseline operational endurance is 12 hr, with

trade study options for 6 hr and 24 hr endurance - Positive ID mission cruise out 200 nm _at_ TBD Kft
- We will size for 12 hrs over the surveillance

area, including loiter and ingress/egress - The positive ID mission requires a
- 282 kt dash (out and back)
- Based on requirement for
- 1 target ID per hour
- 3000ft balanced field length
- takeoff and landing requirements
- are assumed
- - Clto 1.49, Bhp0/W0 0.092

24-20

WAS mission definition

See Lesson 21 performance

WAS MISSION Engine start taxi time 30

min Start taxi thrust level 10 Takeoff (max

thrust time) 1 min Climb cruise out distance

255nm Cruise altitude 27.4Kft Cruise speed

TBD Ingress/egress altitude n/a Ingress/egress

speed n/a Ingress/egress dist. 0 Cruise back

distance 255 nm Landing loiter time 1

hr Landing fuel reserves 5

24-21

ID mission definition

POSITIVE ID MISSION Engine start taxi time

30 min Start taxi thrust level 10 Takeoff

(max thrust time) 1 min Climb cruise out

distance 200nm Cruise altitude ?10Kft Cruise

speed TBD Ingress/egress altitude

10Kft Ingress/egress speed 282

kts Ingress/egress dist. N?282 nm where N

number of searches Cruise back distance 200

nm Landing loiter time 1 hr Landing fuel

reserves 5

24-23

Review - spreadsheet model

- Configurations are defined in absolute and

relative terms - Payload weight, volume and number of engines are

described in absolute terms (forebody, aftbody

and length are relative to diameter) - Fuselage diameter can be input as an absolute

value or as a variable to meet volume

requirements - - Aero and propulsion parameters (Cfe, e, Fsp0,

f/a, etc.) are defined as absolute values - - Everything else (wing, tails area, engines,

nacelles,etc.) is defined in relative terms (AR,

W0/Sref, BHp0/W0, Sht/Sref, BHp0/Weng, Waf/Sref,

UWW, etc.) - Missions are described in absolute terms
- Takeoff times, operating radius, speed, altitude,

etc - Most variables are input via worksheet Overall,

some are input via worksheet Mperf - - Mperf inputs are used to converge the overall

solution

24-24

Overall worksheet inputs

- 48 Nacelle k1 0.2
- 49 Nacelle k2 0.4
- 50 Nacelle w/h 1.0
- 51 Nacelle Swet fract. 0.5
- Nac. Non-prop PF 0.0
- 54 Number of pods 0
- 55 Pod offset/(b/2) n/a
- 56 Pod D-eq/Df-eq n/a
- 57 Pod L/D-eq n/a
- 58 Pod k1 n/a
- 59 Pod k2 n/a
- Pod w/h n/a
- Pod PF 0.0
- 65 Taper ratio 0.5
- 66 Thickness ratio 0.13
- 67 Tank chord ratio 0.5
- 68 Tank span ratio 1 0.1
- 69 Tank span ratio 2 0.9
- 72 Horiz tail area 0.39

- Row Description Value
- 08 Volume margin 1.3
- 09 Headwind (kt) 0
- Climb V/Vstall 1.25
- 11 Loiter V/Vstall 1.1
- 13 Idle time (min) 30
- 14 Idle power () 10
- 15 Takeoff time (min) 1
- 16 Takeoff param 220
- 17 Takeoff CL 1.5
- 18 Takeoff altitude 0
- 31 Landing loiter (min) 60
- 32 Landing reserve .05
- 34 of fuselages 1
- 35 Fuse. offset/(b/2) 0
- 36 Df (starting value) 2.29
- 37 Lf/Df-equiv 7
- 38 Fuselage k1 .143
- 39 Fuselage k2 .286

82 Model Bhp0 default 83 Eng Fsp 90 84 Fan (prop)

Fsp 5 85 Ref speed (kts) 50 86 Bypass

ratio 133 87 Prop efficiency 0.8 88 Fuel/air

ratio default 89 Engine L/D-eq 2.5 92 Starting

W0 default 93 Engine Hp0/Weng 2.25 94 Eng. inst.

wt. factor 1.3 95 Land gear fraction .05 96 System

wt.fract. 0.12 97 Fusenac unit

wt. 1.8 98 Wing unit wt. 3.25 99 Horiz tail unit

wt. 2.6 100 Vert. Tail unit wt. 2.6 101 Empty wt.

margin .05 102 Misc. wt. Fraction .02 104 Fuel

density 50 105 Fuel PF 0.8 106 Engine rho

(unstl) 22 107 LG rho (instal) 25 108 System

rho (instl)25 109 Payload rho (instl) 27.12

24-25

Mperf worksheet inputs - WAS

- Row Description Value
- h4 (kft) 27.4
- h7-cruise (kft) 27.4
- h7-loiter (kft) 27.4
- h8-loiter (kft) 27.4
- h9-10,13-14 (kft) 27.4
- h11-12 (kft) 27.4
- h14 (kft) 27.4
- h17 (kft) 27.4
- V-cruise 180
- V-ingress ( egress) 282
- Op dist (nm) 255
- Ingress/egress (nm) 0
- Combat (min) 0
- Max climb M 0.48
- T factor (cruiseclmb) 1
- T factor (op loiter) 1
- T factor (ingress/combat) 1
- SFC factor (cruiseclmb) 1

Design mission definition

- Row Description Value
- 52 Df-equiv 0 to iterate
- 2.29 fixed Df
- 56 W0/Sref 30
- 57 Fuel fraction TBD
- 58 Additional fuel 0
- 61 Bhp0/W0 TBD
- 64 Payload retained (lbm) 720
- 65 Payload dropped (lbm) 0
- Aspect ratio 20
- Wing efficiency (e) 0.8

24-26

Mperf worksheet inputs - ID

- Row Description Value
- h4 (kft) 10
- h7-cruise (kft) 10
- h7-loiter (kft) 10
- h8-loiter (kft) 10
- h9-10,13-14 (kft) 10
- h11-12 (kft) 10
- h14 (kft) 10
- h17 (kft) 10
- V-cruise 180
- V-ingress ( egress) 282
- Op dist (nm) 200
- Ingress dist.(nm) 141
- Combat (min) 0

Secondary mission definition

- Row Description Va
- 58 Additional fuel 0.0
- 64 Payload retained (lbm) 720
- 65 Payload dropped (lbm) 0
- Aspect ratio 20
- Wing efficiency (e) 0.8
- Lamda 0.5

24-27

Initial sizing

- The spreadsheet iterates the air vehicle to meet

input weight, geometry,volume and propulsion

requirements - Bottoms-up weights must be iterated by definition
- Geometry is adjusted with each weight iteration

to maintain proper fuselage-wing-tail

relationships - Engine and nacelle size is adjusted as required
- Waf/Sref and volume required/available are the

variables used to converge weight and geometry

during iteration - Waf/Sref is used as an input to the weight model

and an output from the geometry model - Fuselage diameter is adjusted to meet volume

required - When the values converge, mission model

performance estimates will be valid, even though - - Mission range may be short (or long)
- - Climb rate may be inadequate (even negative)
- Cl may be too high (exceeding stall margins)

24-28

Speed and performance margins

- Civil/military certification requirements and

good operating practice specify that certain

speed and performance be mainatined. Typical

values - Takeoff (V/Vstall ? 1.1)
- Climb (V/Vstall ? 1.20)
- Cruise (V/Vstall ? not defined)
- Landing approach (V/Vstall ? 1.2-1.3)
- Service ceiling 100 fpm
- UAVs have not yet established criteria but safety

and good practice will dictate something similar - One difference will be operational loiter speed

margin, to get high LoD we need to operate at

V/Stall ? 1.1 - For design project purposes, we will apply the

above margins except we require enough thrust

margin for 300 fpm (Ps 5 fps)

24-28a

Performance convergence

- Worksheet Mperf accepts new inputs to improve or

adjust performance - Fuel fraction (FF) is adjusted to meet range

and/or endurance requirements - Bhp0/W0 or T0/W0 is adjusted to meet takeoff or

rate of climb requirements or achieve consistency

(see below) - W0/Sref is adjusted to improve LoD or takeoff

distance - AR and wing efficiency (e) can also be traded to

improve overall performance - The values are adjusted by hand until a

satisfactory solution is achieved - This includes ensuring adequate (and consistent,

if configurations are being compared) margins

such as residual ROC, T-D and stall margin - Bhp0/W0 or T0/W0 is further iterated to achieve

the desired level of consistency

24-29

Spreadsheet demonstration

Notional values

24-30

Spreadsheet results

- Engine size mismatch for WAS and ID mission
- Negative Ps at 10 Kft, 282 Kt - requires Bhp0/W0

increase to 0.10 - Cruise speeds near LoDmax yielded best

performance - 161 kts for WAS _at_ 17 Kft, 144 kts for ID at 10Kft
- Positive ID was the driving mission
- Baseline 12 hour operational endurance WAS air

vehicle sized to W0 3304 lbm, EW 1912 lbm - For 12 IDs in 12 hrs, W0 16534 lbm, EW

7996lbm - Also required increased diameter fuselage (to 4.5

ft) to accommodate additional fuel required - Changing wing loading (W0/Sref) yielded little

benefit - Higher loiter and cruise speeds offset smaller

wing - Lower increased wing size offset smaller engine
- Changing aspect ratio (AR) was of little benefit
- Increased AR (25) yielded small weight improvement

Earlier example problem

24-31

WAS concept

W0 3080 lbm EW 1744 lbm AR 20 Sref

77sqft Swet 381 sqft Payload 707 lbm Fuel

603 lbm Power 373 Bhp TBProp Max endurance

15.3 hrs Max speed 350 kts

Note not to scale

39.2

Earlier example problem

This air vehicle can stay on station at 17Kft for

12 hours at an operating radius of 255 nm

2.82

19.7

24-32

ID concept

W0 16534 lbm EW 7996 lbm AR 20 Sref 413

sqft Swet 366 sqft Payload 707 lbm Fuel

7660 lbm Power 2000 Bhp TBProp Max endurance

57.3 hrs Max speed 350 kts

Note not to scale

Earlier example problem

This air vehicle can perform 12 IDs in 12 hours

at 10Kft at an operating radius of 200 nm

24-33

Parametric comparisons

- During every step of the PCD process, we always

test our performance estimates vs. data on known

aircraft - This is essential to ensure our results make

sense - Critical comparisons for our concept are defined

by Breguet range and endurance equation variables - LoD, SFC and weights (airframe, propulsion and

EW) - LoD comparison
- We would compare our estimates to RayAD Fig 3.5

but our vehicle is beyond Raymers parametric

range - For AR20, Sref 77, Swet 381 and Sref 413,

Swet 1614 our wetted ARs (A/Swet/Sref)

4.0 and 5.1 vs. Raymers maximum value of 2.4 - But from the trend of the data our assessed

values of LoDmax 27-30 look slightly optimistic - We can also compare to Global Hawk with a

reported LoDmax of 33-34 at an estimated wetted

AR ? 7

24-34

LoD comparison

- This data shows that our model LoDmax estimate

may be optimistic by about 5 - We will put a 10 multiplier on our Cdmin

estimate

- to correct for it (Cell B25 1.1)
- Why do you suppose we corrected a 5 high LoD

estimate by increasing minimum drag by 10? - Could we have done it another way?

Model estimate

Corrected value

Manned aircraft data source LM Aero data

handbook

24-35

SFC comparisons

Data source Roskam AP

Note turboprop SFC is defined in terms of

horsepower. Our turboprop model converts

horsepower to thrust and uses TSFC for

performance calculations

24-36

Weight comparisons

This data shows that our calculated weights

are low compared to regional turboprops but high

compared to U-2 and Global Hawk - But the

results are close enough for now Another weight

related issue is operating a high AR wing at 280

kts at low altitude (flutter and gust potential)

24-37

Final comparison

GA Altair (Predator B variant) W0 7000 lbm EW

? Sref 315 sqft AR 23.5 Payload 750

lbm Fuel 3000 lbm Power 700Hp

TPE-331-10T Endurance 32 hrs Max speed 210

kts

With these inputs our concept would have a 49

hour endurance at 50 Kft but require a 45

airframe weight reduction

24-38

Overall conclusions

- Data comparison shows that our model estimates

are reasonable, although some are probably

optimistic - We have already decided to put a factor on our

drag estimates to reduce LoDmax to the data trend

line - We will also should put a 10 multiplier on

ingress-egress SFC to put it in the middle of the

parametric range - But we will have to wait for conceptual design to

see if our weights are optimistic or pessimistic - Some people, however, will put on additional

margins to ensure early estimates will be

achievable - Typically 5-10 on SFC and drag and 10-20 on

weight - Putting additional margins on our estimates,

however, should not be necessary since our

parametric data already shows they should be

generally achievable - Adding more margin would be overly conservative

and negate otherwise valid design solutions

24-39

Adjusted baseline

Note not to scale

W0 3178 lbm EW 1792 lbm AR 20 Sref 79

sqft Swet 391 sqft Payload 707 lbm Fuel 651

lbm Power 384 Bhp TBProp Max endurance 15.3

hrs Max speed 350 kts

Earlier example problem

This air vehicle has 10 drag and 280 Kt SFC

multipliers and can stay on station for 12 hours

at 17Kft or perform 2.8 ID missions at 10Kft in

2.8 hours

24-40

Balancing mission requirements

- Since size requirements for vehicles to do the

WAS and ID missions are so different, we will do

a study to determine which size vehicle can do

both missions at the lowest cost using the

following approach - Size WAS concepts for 6, 12, 24 and 48 hours of

loiter - ID mission performance will be a fallout
- We will then calculate the number of aircraft

required for 24/7 surveillance for 30 days for

both missions - We will do simple weight based cost estimates
- Air frame and systems less installed propulsion

200/lbm EW-Weng for ICProp (Lesson 8-45), 400

for TBProp, 800/lb for TBFan - Payload 5000 per pound
- Engine 150/lbm for ICProp, 700/lbm for

TBProp, 1000/lb for TBFan - Finally we will do a simple cost effectives

comparison to select our preferred size concept

24-41

WAS sortie rate elements

- In order to estimate number of aircraft required

we have to perform a preliminary sortie rate

analysis - See Lesson 7 (Sortie Rate) chart 10,
- We will use the maintenance and planning times in

chart SRR-10 as representative values - The nominal mission ground times required are
- Maintenance and flight preparations 180 minutes
- Preflight checks - 6 minutes
- Post landing checks and taxi - 25 minutes
- The remaining elements of the sortie are
- Engine start-taxi-takeoff - 31 minutes
- Time to climb to 17 Kft 6.7 minutes
- Outbound and return cruise time - 184 minutes
- WAS - 6, 12, 24 or 48 hours
- Landing loiter 60 minutes
- Land 3 minutes

- Use RAND data and adjust for UCAV vs. UAV
- Include maintenance time f(flight hrs)

24-42

Sortie rate elements

UCAV unique

Include in flight time

Therefore SR(UAV) ? 24hours/1.68?FT

4.9 SR(UCAV) ? 24hours/1.68?FT 5.9

http//www.rand.org/publications/MR/MR1028/

24-42a

ID sortie rate elements

- The ID mission sortie is identical to the WAS

mission except for the flight times where the

spreadsheet values are - Time to climb to 10 Kft 3.5 minutes
- Outbound and return cruise time - 163 minutes
- IDs 1 hour each

Earlier example problem

Still valid

Still valid

24-43

WAS coverage requirements

- Time required to fly a WAS sortie are
- 6 hr loiter - 496 min. 6 hrs 14.26 hrs
- 12 hr loiter - 20.26 hrs
- 24 hr loiter - 32.26 hrs
- 48 hr loiter - 56.26 hrs
- The number of missions an air vehicle can fly in

30 days vs. the number required, therefore, are - 6 hr loiter - able to fly 50.5 missions vs. 480

required - 12 hr loiter - can fly 35.5 missions vs. 240

required - 24 hr loiter - can fly 22.3 missions vs. 120

required - 48 hr loiter - can fly 12.8 missions vs. 60

required - The number of flight vehicles required,

therefore, are - 6 hr loiter - 480/50.5 9.5 ? 10
- 12 hr loiter - 240/35.5 6.8 ? 7
- 24 hr loiter - 120/22.3 5.4 ? 6
- 48 hr loiter - 60/12.8 4.7 ? 5

Earlier example problem

24-44

WAS air vehicles required

- The total number of air vehicles required are

greater than the number required to meet flight

requirements - We assume one air vehicle is always on standby in

case one of the flight vehicles has a problem - And we assume all vehicles vehicle under go

maintenance at rate of 3.4hrs 0.68Flight Time - The total number of air vehicles required for

continuous WAS mission coverage, therefore, are - 6 hr loiter - 10 1 11
- 12 hr loiter - 7 1 8
- 24 hr loiter - 6 1 7
- 48 hr loiter - 5 1 6

Earlier example problem

24-45

WAS air vehicle cost

- At a nominal air vehicle cost of 400 per pound

of empty weight and a nominal payload cost of

5000 per pound, we can calculate WAS costs as

follows - 6 hr loiter - 12 air vehicles 6.6M, Payloads

38.9M - Total cost 45.4M
- 12 hr loiter - 9 air vehicles 5.7M, Payloads

28.3M - Total cost 34.0M
- 24 hr loiter - 8 air vehicles 7.3M, Payloads

24.7M - Total cost 32.1M
- 48 hr loiter - 7 air vehicles 16.9M, Payloads

21.2M - Total cost 38.1M

- Earlier example problem
- You should include engines as separate cost

element

24-46

ID mission requirements

- Assuming one target identification per hour, the

times required to fly an ID sortie are - 1 ID 471.5 minutes 1 hrs 8.86 hrs
- 2 IDs - 9.86 hrs
- 4 IDs - 11.86 hrs and 8 IDs - 15.86 hrs
- The number of missions an air vehicle can fly in

30 days vs. the number of IDs required are - 1 ID able to fly 81.3 missions vs. 720 required

- 2 IDs - can fly 73.0 missions vs. 360 required
- 4 IDs - can fly 60.7 missions vs. 180 required
- 8 IDs - can fly 45.4 missions vs. 90 required
- The number of flight vehicles required,

therefore, are - 1 ID - 720/81.3 8.85 ? 9
- 2 IDs - 360/73.0 4.93 ? 5
- 4 IDs - 180/60.7 2.96 ? 3
- 8 IDs - 90/45.4 1.98 ? 2

Earlier example problem

24-47

Equivalent WAS coverage

- WAS sortie equivalent IDs are
- 6 hr loiter 1.5 IDs
- 12 hr loiter 2.8 IDs
- 24 hr loiter 5.1 IDs
- 48 hr loiter 9.5 IDs
- The number of ID missions a WAS air vehicle can

fly in 30 days vs. the number required,

therefore, are - 1.5 IDs - can fly 76.8 missions vs. 478.9

required - 2.8 IDs - can fly 67.7 missions vs. 261 required
- 5.1 IDs - can fly 55.4 missions vs. 141.2

required - 9.5 IDs - can fly 41.4 missions vs. 75.9 required

- Total number of ID vehicles required, therefore,

are - 6 hr loiter or 1.5 IDs - 478.9/76.8 6.2 ? 8
- 12 hr loiter or 2.8 IDs - 261/67.7 3.9 ? 5
- 24 hr loiter or 5.1 IDs - 141.2/55.4 2.5 ? 4

- 48 hr loiter or 9.5 IDs 75.9/41.4 1.8 ? 3

If WAS and ID vehicles are identical, a 2nd

back up is not required

Earlier example problem

24-48

ID air vehicle cost

- At a nominal air vehicle cost of 400 per pound

of empty weight and a nominal payload cost of

5000 per pound, ID costs are - 1.5 IDs 8 air vehicles 4.2M, Payloads

24.7M - Total cost 29.0M
- 2.8 IDs - 5 air vehicles 2.9M, Payloads

14.1M - Total cost 17.0M
- 5.1 IDs 4 air vehicles 3.1M, Payloads

10.6M - Total cost 13.7M
- 9.5 IDs 3 air vehicles 5.6M, Payloads

7.1M - Total cost 12.7M

Earlier example problem

24-49

Total cost

- The most cost effective single vehicle solution

for both missions is an 18 hour WAS vehicle that

can also perform 4 IDS - Therefore ID air vehicles launch once every 4

hours while WAS air vehicles launch once every 18

hours for an average of 7.3 missions per day

24-50

Resulting configuration

W0 3911 lbm EW 2153 lbm AR 20 Sref 98

sqft Swet 464 sqft Payload 707 lbm Fuel

1016 lbm Power 473 Bhp TBProp Max endurance

21.4 hrs Max speed 350 kts

Note not to scale

Earlier example problem

This air vehicle can stay on station for 18 hours

at 17Kft or perform 4 ID missions at 10Kft in 4

hours

24-51

What it really looks like

W0 3911 lbm EW 2153 lbm AR 20 Sref 98

sqft Swet 464 sqft Payload 707 lbm Fuel

1016 lbm Power 473 Bhp TBProp Max endurance

21.4 hrs Max speed 350 kts

Earlier example problem

Looks like a ½ scale TBProp Global Hawk

This air vehicle can stay on station for 18 hours

at 17Kft or perform 4 ID missions at 10Kft in 4

hours

Approximately to scale

24-52

TBProp status

- We have completed our first pre-concept design

cycle - We have explored the basic concept and found that

one 4000 lbm class vehicle can meet both WAS and

ID mission requirements at minimum cost - The vehicle size is reasonable and the internal

volume available should accommodate the required

payloads, propulsion, systems and fuel - We have shown that the required weight,

aerodynamic and propulsion performance levels are

consistent with the state-of the art and should

be achievable - However, we have not completed pre-concept design
- We still have a requirement problem resulting

from the assumption of 100 availability vs. 90

flyable days - We also need to conduct goal vs. threshold and

and explore alternative TBProp architectures

(Charts 8-59/63) - And we need to evaluate alternate propulsion

concepts

24-52

Alternative propulsion concepts

- One of our early decisions was to compare TBFan

and IC engine concepts against our TBProp

baseline - But only if an IC engine of appropriate size is

available - However, the minimum size TBProp required to

perform the ID mission is 420 Hp - This minimum power required exceeds the size of

the largest available IC engine - Therefore, we can drop IC the engine from our

study on the basis of size incompatibility - TBFan concept evaluation will be straight-forward

with few decisions required - At the relatively low speeds and altitudes

associated with our mission, there is only one

viable option - A fuel efficient high bypass ratio (BPR) engine
- We select a nominal BPR5 as being representative

of high efficiency engines of this type

24-53

TBF alternative

- We develop a spreadsheet model nearly identical

to a TBProp, the major differences being engine

definition - From PCD Review Part 1.5, PRR-14, nominal T0/Weng

5.5 installed thrust loss ? 10 (for good

installation) - From PRR-26 TBFan parametric data we select a fan

specific thrust value of 25 sec for BPR 5 - From PRR-22 we select the remaining model inputs
- Geometrically, the only difference will be the

nacelle - TBFan nacelles are modeled as open-ended

cylinders where by definition k1n k2n 0 - We assume nominal values of Lnac/Dnac 4 and

Dnac/Deng 1.25 - Takeoff performance will also be different, a

3000 ft balanced field length for a jet (ground

roll of 1500 ft) requires a thrust based takeoff

parameter of 100

24-4

Overall TBFan inputs

- Col Description Value
- 03 R-start (nm) default
- 17 Headwind (kt) 0
- 18 Clmax 1.2
- 19 V/Vstall 1.25
- 25 LoD start default
- 26 SFC start default
- 27 EWF start default
- 28 Kttoc start default
- 31 Idle time (min) 30
- 32 Idle power () 10
- 33 Takeoff time (min) 1
- 34 Takeoff param 100
- Takeoff CL 1.5
- Takeoff altitude 0
- Landing loiter (min) 60
- Landing reserve .05
- of fuselages 1
- Fuse. offset/(b/2) 0

- Col Description Value
- Dn-eq/Dengine 1.25
- Nacelle k1 0
- Nacelle k2 0
- Nacelle Swet fract. 0.5
- Nacelle w/h 1.0
- Number of pods 0
- Pod offset/(b/2) n/a
- Pod D-eq/Df-eq n/a
- Pod L/D-eq n/a
- Pod k1 n/a
- Pod k2 n/a
- Pod w/h n/a
- Taper ratio 0.3
- Thickness ratio 0.13
- Tank chord ratio 0.5
- Tank span ratio 1 0.1
- Tank span ratio 2 0.9
- Tank pack factor 0.8

Changes from TBProp shown in red

- Col Description Value
- of engines 1
- Model Bhp0 default
- Eng Fsp 90
- Fan Fsp 25
- Ref speed (kts) 100
- Bypass ratio 5
- Installed T0 0.9
- Fuel/air ratio default
- Engine L/D-eq n/a
- Engine density n/a
- Starting W0 default
- Engine T0/Weng 5.5
- Eng. inst. wt. factor 1.3
- Land gear fraction .05
- System wt.fraction 0.1
- Fusenac unit wt. 3
- Wing unit wt. 5
- Horiz tail unit wt. 3

Earlier Spreadsheet

24-55

Mperf TBFan inputs

Changes from TBProp shown in red

- Col Description Value
- h4 (kft) 17
- h7-cruise (kft) 17
- h7-loiter (kft) 17
- h8-loiter (kft) 17
- h9-10,13-14 (kft) 10
- h11-12 (kft) 10
- h14 (kft) 17
- h17 (kft) 17
- Vcruise 200
- V-ingress/egress 280
- WAS op dist (nm) 255
- ID op dist (nm) 200
- WAS dash (nm) 0
- ID dash (nm) 141
- Combat (min) 0
- Max climb M 0.48
- T factor (cruise) 1
- T factor (loiter) 1

- Col Description Value
- Airframe weight factor 1
- Fusenac Swet factor 1
- Df-equiv 3.04
- Waf/Sref (input) TBD
- W0/Sref 40
- Fuel fraction TBD
- Payload retained (lbm) 707
- Payload dropped (lbm) 0
- T0/W0 TBD
- Aspect ratio 20

Earlier spreadsheet

24-56

TBFan WAS concept

W0 4865 lbm EW 2454 lbm AR 20 Sref 122

sqft Swet 517 sqft Payload 707 lbm Fuel

1656 lbm Engine 1299 Lbf TBFan Max endurance

15.4 hrs Max speed 280 kts

Note not to scale

Earlier example problem

- This air vehicle can stay on station at 17Kft for

12 hours at an operating radius of 255 nm - It is 41 heavier than a TBProp with the same

performance

24-57

TBFan ID concept

W0 5660 lbm EW 2761 lbm AR 20 Sref 141

sqft Swet 573 sqft Payload 707 lbm Fuel

2133 lbm Engine 1511 Lbf TBFan Max endurance

18.8 hrs Max speed 280 kts

Note not to scale

Earlier example problem

- This air vehicle can perform one ID at 10Kft at

an operating radius of 200 nm - It is 39 heavier than a TBProp with the same

performance

24-58

TBFan conclusions

- The TBFan alternative is bigger and about 40

heavier than the TBProp baseline for both design

missions - The relatively low-speeds and altitudes required

really are optimum for TBProp operations - TBFan cycles are better suited for higher speeds

and altitudes - We can now confidently drop the TBFan concept

from further consideration - And document the results of our alternative

concept study as rationale for our future

exclusive focus on TBProp engines - We will also document the rationale for selecting

an 18 hour WAS capability for our preferred

baseline to meet both WAS and ID mission

requirements

24-59

TBProp continued

- Even though we have concluded that the TBProp is

the best overall solution to meet mission

requirements, we still need to address some

unresolved issues - The impact of 10 of the weather being unflyable

vs. our assumption of a 100 flight rate vs. the

threshold requirement for 80 target coverage - The cost effectiveness of designing for threshold

vs. goal performance - The effectiveness of alternative
- See Lesson 3, charts 13-15
- The support concept required
- Overall system life cycle cost

24-60

Homework

- Using spreadsheet TBProp.AE261Example.xls and

total mission procurement cost as the figure of

merit, for the TBProp example, do the following

trades (one trade for each team member,

individual grades) - Aspect ratios (AR) of 10-20-25-30 at W0/Sref 30
- W0/Sref of 15-30-45-60 at AR 10
- Aspect ratios (AR) of 10-20-25-30 at W0/Sref 60
- W0/Sref of 15-30-45-60 at AR 30
- 2. Select best combination of W0/Sref and AR and

use TBProp.AE261Example.xls to trade 12-24-48 hr

WAS loiter times (team grade). Select the best

loiter time and explain why it turned out that

way - 3. Use TBProp.AE261Example.xls to determine best

WAS and ID cruise speeds. Explain why (team

grade) - 4. Discuss ABET issues 5 and 6 and document

your conclusions (one paragraph each team

grade)

24-61

Intermission

24-61