Lesson objective to show how to

1
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
2
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
3
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
4
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
5
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
6
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
7
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
8
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
9
Review - fuselage geometry
24-9
10
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
11
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
12
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
13
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
14
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
15
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
16
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
17
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
18
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
19
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
20
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
21
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
22
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
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
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
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
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
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
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
29
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
30
Spreadsheet demonstration
Notional values
24-30
31
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
32
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
33
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
34
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
35
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
36
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
37
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
38
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
39
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
40
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
41
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
42
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
43
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
44
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
45
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
46
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
47
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
48
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
49
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
50
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
51
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
52
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
53
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
54
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
55
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
56
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
57
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
58
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
59
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
60
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
61
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
62
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
63
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
64
Intermission
24-61