22-1 Design of UAV Systems Integrated performance modelc 2002 LM Corporation Expectations - You will understand how to apply parametric relationships to

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Design of UAV Systems

Integrated performance modelc 2002 LM Corporation

Expectations - You will understand how to apply parametric relationships to an overall air vehicle

Lesson objective - to integrate the individual air vehicle parametric relationships into an overall ….

Integrated performance modeland show how it can be applied to estimates of ...

• Breguet range and endurance

and typical design problems

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During pre-concept design, the objective is not to come up with the best design but to identify the base range of design options (including their size, weight, cost, risk and schedule) and to define achievable requirements- If we do not employ a reasonably accurate and consistent

comparison process, invalid selections could be made on the basis of inconsistent, hasty or immature assessments

During conceptual design, the objective is similar but focuses on finding the best design and design features- This also requires use of an accurate and consistent

comparison process to ensure viable design decisionsWe now have the data and component methodologies to do integrated pre-concept and conceptual design comparisons for subsonic IC, TBProp and TBFan UAVs- Including simple geometry models that integrate key

configuration design features with the performance models to ensure physical constraints are not violated

Design objectives

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Concept exploration

Exploring a design space is an important part of the concept evaluation process- Typically, a wide range of concepts are drawn, analyzed

and compared and a few are selected for more study- Configuration designers sit in the middle of this

process and are skilled at laying out configuration study matrices to explore the most important design and operational issues

- They typically use specialized design and analysis tools that minimize the hand labor required to explore these options

- Students, however, seldom have such tools and configuration exploration becomes a time consuming process of draw-analyze-discover problems-redraw-reanalyze and very few options are really explored

We can bypass this laborious process by applying spreadsheet analysis tools to concept exploration

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Sizing vs. exploration

The term “sizing” is typically used to describe the process one goes through to calculate air vehicle size to meet design requirements- This is basically what an integrated spreadsheet

model will do- We input requirements and it outputs size and

weight- But because it rigorously maintains proper

geometric relationships as it adjusts to changing requirements, it can also be used to explore concepts- Most sizing programs don’t do this, they are

based on geometry inputs not internal geometry models

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Comparison approach

• Configuration comparisons need to be based on accurate and consistent data- We have data of both types

- Our spreadsheet models are rigorously consistent but of unknown accuracy

- Our parametric data is accurate by definition but of questionable consistency

- A combination of the two will give us what we need to compare the candidate configurations

• We will start by calculating performance against a consistent set of requirements- Range, takeoff distance, rate of climb at altitude, etc.

• Then we compare our results to parametrics - There will always be differences or issues to resolve

• Finally we will go back to our models to assess the impact of the differences or issues on the concepts

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The design process starts with definition of a “starting baseline” that meets mission requirements- The baseline objective is to define a solution that works

so that it can serve as a basis for comparison- Typically, starting baselines are conventional in

configuration and employ state-of-the-art technology- Later we can study more advanced approaches and compare the relative benefits and risks

The TBProp starting baseline selected in Lesson 15 generally meets these requirements- There is little about it that shouldn’t work

- The speed, required endurance and technology content are well within TBProp state-of-the-art

- One exception is the speed required at or below 10Kft- This could be a challenge for long endurance aircraft

Baseline definition

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Modeling approach

The configuration is described in both absolute and relative terms- Payload weight, volume, number of engines and fuselage

diameter are described in absolute terms (forebody, aftbody and length are relative to diameter)

- Aero and propulsion parameters (Cfe, e, Fsp0, f/a, etc.) are defined as absolute values

- Everything else (wing, tail area, engine size, nacelle size, etc.) is defined relative to a design, weight or performance parameter (AR, W0/Sref, BHp0/W0, Sht/Sref, BHp0/Weng, Waf/Sref, UWW, etc.)

Missions are described in absolute terms (taxi and takeoff times, operating radius, landing reserves, etc.)- One is the “design” mission, the other a “fall out”

- The UAV is sized or “scaled” (sc) to the design mission- The other mission is flown with a “fixed” (fx) air vehicle

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Performance calculations

• Performance calculations are based on a generic, unrefueled loiter mission which assumes….- Equal cruise out plus climb/cruise back distances,

constant cruise speeds (Vcr) and loiter altitudes (Hlo) assumed ; no outbound cruise input as 0 (minimum cruise back is set equal to climb dist.)

- Equal length ingress/egress penetration distances, Vpen and Hpen assumed; no ingress/egress input as 0

- Combat assumed to follow ingress, combat defined by minutes at max thrust; no combat input as 0

- Payload drop follows combat; no payload drop = 0• Fuel remaining after cruise, ingress/egress, combat,

landing loiter defines operational loiter time available• The model also calculates max range and endurance

- Calculated at initial cruise/pre-landing loiter weights- Assumes mid-mission payload drop

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Mission definition

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

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Spreadsheet approach

Three (3) spreadsheets (IC, TBP and TBF) are available, each of which has similar modules that calculate data needed to assess performance including:- A bottoms-up weight model (Worksheet name = Wt-iter)- A geometry model that adjusts its configuration for each

weight iteration (Name = Geom-iter)- It also calculates volume and Swet based aero & weights

- 2 atmosphere models (Names = Atmos-sc and Atmos-fx)

- Each calculates atmospheric properties for one mission - 2 propulsion models (Names = IC/TBP/TBF-sc and -fx)

- Each calculates engine performance for one mission- A mission performance module (Name = Mperf)

There is also an input worksheet for most (but not all) independent variables- Most variables are input via worksheet Overall, but some

are input via worksheet Mperf- Mperf variables are used to converge the overall solution

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Model versions

Previous versions of the spreadsheet model held fuselage size constant

- Everything else (wing, tails, engine size,etc) varied as the configuration adjusted to individual input values

Airframe weights were calculated two ways- The bottoms up weight worksheet calculated overall

weights using an input value of Waf/Sref- The geometry model calculated another value of

Waf/Sref from wetted areas and input unit weights- Input and output values of Waf/Sref had to be hand

iterated to convergence (easy but time consuming)- Then volume required vs. available was hand interated

A new version of the spreadsheet internally iterates weight, geometry and volume but it is developmental

- You will need to check the numbers carefully- The description that follows is for the latest version

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Starting solution

A first pass calculation is made to get started- Input values of FF, T0/W0 or Bhp0/W0, etc and input

(default) gross weight (W0) and fuselage diameter (Df) are used by the bottoms-up weight worksheet (Wt-sc) to make one weight iteration- Outputs include W0, Wf, Weng, Wlg and Wsys - Sref is calculated using an input value of W0/Sref- Component weights and input payload, fuel and

subsystem densities are used to calculate fuselage volume required

- The outputs are passed to the geometry worksheet (Geom-sc) where wetted areas and fuselage diameter are calculated and then passed back to Wt-sc

- This process continues until weight and volume converge- The converged wetted areas are then passed to the

mission performance spreadsheet (Mperf) and used to generate aerodynamic coefficients

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Starting solution – cont’d

Converged weights are also used by Mperf to calculate an engine scale factor (ESF) or thrust multiplier - The propulsion worksheets generate thrust and fuel flow

for a fixed size engine - ESF is defined as Shp0 or T0 for the scaled vehicle ÷ the

default engine model (Shp0scale or T0scale) orESF = ((Shp0/W0)*W0/Neng)/Shp0scale

- We scale the propulsion module Shp or Ta values by multiplying them by ESF. No adjustment is needed for SFC or TSFC since our proplusion model is not size dependent

Worksheet Mperf then uses all of the inputs to do a complete mission analysis by segment from engine start through landing - Results are shown in column B of worksheet Mperf- And they may be totally inadequate!

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Performance convergence

Inadequate performance shows up in many forms, e.g.- Mission range may be short- Climb rate may be inadequate (even negative)- Cl may be too high (exceeding stall margins)

Worksheet Mperf requires that you to make new inputs to correct these deficiencies- For example, increase fuel fraction (FF) to increase

range, increase T0/W0 to improve climb rate, change aspect ratio to reduce drag (and increase weight), increase speed/reduce altitude to reduce cruise Cl

- These values are adjusted by hand until a satisfactory solution is achieved

When convergence is achieved, it is important to ensure there are adequate (and consistent) 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

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Secondary mission

Once the model has converged to an acceptable level of performance for the design mission….

- The solution is passed to the “fixed” model (Col C, Mperf) where mission performance is calculated with size and geometry fixed - The primary independent variables are fuel and

payload but you can modify some of the propulsion variables to support fixed airframe size engine trades

- But the primary purpose of the “fixed” model is to perform alternate mission performance assessments- Varying fuel, payload and mission parameters

- During conceptual design this feature can be used to run performance for a wide range of missions for the preferred design

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Spreadsheet example

Operational loiter (hrs) 6.39 6.78 Loiter equiv dist (LED) 1169 1080 Dist to clmb+cruise 200 200Maximum cruise range (nm) 1372 1472Climb+cruise w/HW 169 169Max cruise range w/HW 1180 1266Max Endurance 7.3 9.4

Df 9.00 9Waf/Sref - geom 17.31 17.30

Waf/Sref 17.24 17.30b 86.3 86.3Df/b 0.104 0.104W0 37230 37270W0/Sref 60.00 60.06FF 0.2030 0.203Wfuel (total) 7558 7558Wing fuel @ 50 ppcf 4541 4541Wpay 10000 10000Wpay (dropped) 0 10000WE 19672 19712Bhp0/W0 0.123 0.123TOP Bhp0/W0 req'd 0.123 0.123Bhp0 4579 4579

Scaled Fixed

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Parametric comparisons

We should always test spreadsheet results against parametric data to make sure they are reasonable, e.g. - Below are Waf/Sref examples for regional TBProp and

business jet models. The TBProp looks good but the Biz Jet is conservative - an airframe weight factor of 0.85 will correct it

Similar comparisons should be done for aerodynamic and propulsion estimates

Business Jets(airframe structure only - Roskam)

5

10

15

20

30 45 60 75 90

GTOW/Sref (psf)

Regional Turboprops(airframe structure only - Roskam)

5

10

15

20

25 35 45 55

GTOW/Sref (psf)

InitialFinal

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Wing design-There are two primary wing options to reduce size and

weight - Increase wing loading for reduced wing area, wing

weight and wetted area and higher Clcr- Increase aspect ratio (AR) for improved LoD

- Practical limits- Increasing W0/Sref or AR will improve cruise

performance until some constraint (e.g. takeoff distance or thrust available) changes the trend

- In reality there are other constraints (e.g. wing flutter) that will limit AR and wing loading (W0/Sref)

- To ensure that we don’t stumble into one of them, we should do our optimization within known limits for the configuration type. E.g. for BizJets, typically

W0/Sref ≤ 80 and AR ≤ 14

Other considerations

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• Aerodynamics

- Our model keeps fuselage size and payload fixed- When we optimize the overall vehicle by increasing wing

loading we change fundamental area ratios - If we increase the regional TBProp W0/Sref by 50%, Swet

will drop by 25%, Swet/Sref will increase by 40% and LoDmax will decrease by 18%*

- To maintain LoD as we reduce Sref, AR has to increase

- Another constraint is lift coefficient for LoDmax**. As we increase AR in order to maintain LoD, we quickly reach a point where the Cl required gets too close to Cl-stall (≈1.2)

* LoDmax ≈ 0.5*sqrt((*e*AR/Cfe)*(Swet/Sref)) ** Cl(LoDmax) = sqrt (*AR*e*Cfe*(Swet/Sref))

Typical stall speed margin > 1.25; HALE vehicle stall margin < 1.1

Considerations cont’d


5

10

15

20

25 35 45 55

GTOW/Sref (psf)

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Considerations cont’d

• Airframe

- When we try to reduce wing area while keeping fuselage size fixed, we also change fundamental area ratios

- Recall that wing weights per unit reference area (Sref) run about two times fuselage unit weights per unit Swet

- As we reduce wing size relative to the fuselage, therefore, overall airframe unit weight

increases. For example ….

- If we increase baseline W0/Sref by 50%, Waf/Sref should increase about 30%

- You can see this effect in the parametric weight data

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Technology considerations

• We can also use our spreadsheet methods to evaluate technology impacts, e.g. using composites to reduce airframe weight (which cost more) to see if we get payback in terms of reduced overall empty weight

- Example 25% airframe weight reduction at 35% cost increase (Project RAND, Advanced Airframe Structural Materials, R-4016-AF)


5

10

15

20

25 35 45 55

GTOW/Sref (psf)

- We can capture this effect by putting a 0.75 multiplier on airframe unit weight and a 1.35 multiplier on airframe cost per pound

• We can also see a small cross functional impact - Propulsion- Aero

Aluminum

Composites

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Expectations

You should now understand

• Integrated geometry and performance parametrics

• Where they come from

• How they are used

• The limits of their applicability

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Intermission

Documents

22-1 Design of UAV Systems Integrated performance modelc 2002 LM Corporation Expectations - You will understand how to apply parametric relationships to