19-1 Design of UAV Systems Parametric weightsc 2002 LM Corporation Lesson objective - to discuss Parametric weight methods ….the minimum level of fidelity

19-1

Design of UAV Systems

Parametric weightsc 2002 LM Corporation

Lesson objective - to discuss

Parametric weight methods

….the minimum level of fidelity required to predict air vehicle weights for pre-concept and conceptual design assessments of subsonic UAVs

Expectations - You will understand how to apply the basics and to avoid unnecessary detail

19-2



Importance

These are the fundamental weight relationships needed to define an air vehicle for a conceptual UAV system

19-3



Editorial comment

The first part of this lesson will be a fairly conventional, albeit UAV slanted, discussion of weight fractions (empty weight, fuel and payload)

• Assumed weight fractions are traditionally used as a starting point for air vehicle sizing

• The advantage is simplicity, the weaknesses is high sensitivity and the inability to capture important configuration or technology features

Therefore, assuming weight fractions is not my favorite way of sizing air vehicles

- At the end of the lesson we will discuss another method that is almost as simple and does a better job of capturing design and technology features

Nonetheless, it is important that you understand the weight fraction method and how it is applied

19-4



Discussion subjects

Parametric weights• Weight categories• Weight fractions

• Empty weight• Fuel• Payload• Miscellaneous• Performance

• Bottoms-up weights• Geometry based weights• Conceptual design weights

19-5



Weight definitions - review*

* For additional information see RayAD 3.2-3.5 & 6.2 and RosAD.1 2.0-2.4

Weights are typically defined in categories such as

W0 = We + Wpay + Wf + Wcr + Wmisc (19.1)

W0 = Gross weight ≈ Takeoff weight We = Empty weightWpay = Payload weightWf = Fuel weightWcr = Crew weight (for UAV = 0)Wmisc = Other weights (trapped fuel, oil, pylons,

special mission, equipment, etc.)

where

19-6



Empty weight

Empty weight is also defined in categories such as:We = Waf + Wlg + Weng + Wfe + Wos (19.2)

Waf = Airframe (structure) weightWlg = Landing gear weightWeng = Propulsion system weightWfe = Fixed equipment weight (avionics, etc)Wos = Other systems

These categories are useful for concept design

• Their weights are typically driven by different design issues. For example:- Airframe weight often scales with wetted area- Landing gear weight scales with takeoff weight- Fixed equipment weight is constant, etc.

Later we will use equations 19.1 and 19.2 to do what we will call a “bottoms-up” weight estimate

whereLater we will combine both of these into one We category –systems+avionics or Wspa

19-7



Weight fractions - review

Another commonly used form of weight parametric.

From Equation 19.1We/W0 + Wpay/W0 + Wf/W0 + Wmisc/W0 = 1 (19.3)

where by definition

We/W0 = Empty weight fraction (EWF)Wpay/W0 = Payload fraction (PF)Wf/W0 = Fuel Fraction (FF)Wmisc/W0 = Misc. weight fraction (MWF)

There is a similar form of Equation 19.2

EWF = Waf/W0 + Wlg/W0 + Weng/W0 + Wspa/W0 + Wfe/W0 (19.4)

RosAD.5 Appendix A tabulates these weight fractions for a wide range of manned aircraft

19-8



Weight fractions - review

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

0.40

0.50

0.60

0.70

0 100000 200000

Nominal Gross Weight

No

min

al E

WF

SE Prop

ME Prop

Biz Jet

Reg TBP

Jet Transp

Mil Trainer

Fighters

Mil. PBC

FW UAV

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

0.00

0.10

0.20

0.30

0.40

0.50

0 100000 200000

Nominal Gross Weight

No

min

al F

F

SE Prop

ME Prop

Biz Jet

Reg TBP

Jet Transp

Mil Trainer

Fighters

Mil. PBC

FW UAV

Typical value

• Empty weight fraction and fuel fraction are key design parametrics

- They vary widely with design mission and vehicle class- There are physical constraints on what they can be

• Range and/or endurance, speed, maneuver, payload and technology level are primary drivers.

19-9



EWF variation

Within a given aircraft class, EWF will also vary - widely

Single Engine - Propeller

0.400

0.500

0.600

0.700

0.800

0 2000 4000 6000 8000 10000 12000

GTOW (lbs)

Minimum = .437Average = .59Median = .59Maximum = .791

Data From Roskam - Table 2.4

Data source - Roskam, (RosAD.1)

19-10



Fuel Fraction variation


0.000

0.100

0.200

0.300

0.400

0.500

0 2000 4000 6000 8000 10000 12000

GTOW (lbs)



• Ditto for fuel fraction (FF). Design is about choices. Fuel and EW fractions reflect these choices

Data source - RosAD.1, Table 2.4

19-11



UAV weight fractions

Data sources - Janes UAVs, Shepard UAVs, AUVSI

• Current UAVs are designed primarily for endurance. Empty weight and fuel fractions correlate accordingly

UAV Empty Weight Fractions

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0 8 16 24 32 40 48

Max Endurance (hrs)

Piston

Jet

Piston

Jet

UAV Fuel Fractions

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0 8 16 24 32 40 48

Max Endurance (hrs)

Piston

Turboprop

Jet

Jet

Piston

Global Hawk

Global Hawk

19-12



Payload fraction

• Payload fraction (PF) is another fundamental design driver

- Most aircraft designs are driven primarily by payload requirements

• Payload definitions- Internal/external stores and removable mission

equipment are considered payload- For manned aircraft, passengers are defined as

payload, crew members and their equipment are not

- In order to correlate manned and unmanned aircraft our payload fraction will include crew weight, crew equipment and payload in a single equivalent “ payload” parametric

19-13



PF comparisons

UAVs

0.00

0.10

0.20

0.30

0 10000 20000 30000Gross weight (Lbs)

Piston

Jet

Jet Transports

0.00

0.10

0.20

0.30

50000 250000 450000 650000

GTOW (lbs)


Multi Engine - Propeller

0.05

0.15

0.25

2000 4000 6000 8000 10000 12000

GTOW (lbs)



0.10

0.20

0.30

0.40

0 2000 4000 6000 8000 10000 12000

GTOW (lbs)


19-14



Miscellaneous weight fraction

Miscellaneous weights can be initially estimated as a gross weight fraction- A typical value would be 1%

- A small number but one we should not ignore- It is better to guess at a number than to leave it out

- We might forget to put it back in!….or as a percentage of useful load

- Useful load is defined as gross weight minus empty weight- A typical value would be 2%,

19-15



Typical application

How do the example TBProp and TBFan UAV empty weights compare to manned aircraft?- Based on Predator B and C, we assumed empty weight

fractions of 0.44 and 0.39 - The assumptions do not fit manned aircraft data

Are Predator B/C designed that much differently from their manned TBProp and TBFan counterparts?

Business Jets

0.400

0.500

0.600

0.700

10000 15000 20000 25000 30000

GTOW (lbs)

Data From Roskam - Table 2.7Regional Turboprops

0.400

0.500

0.600

0.700

5000 20000 35000 50000

GTOW (lbs)


19-16



Application cont’d

Regional Turboprops

0.1

0.2

0.3

0.4

0.5

0.6

0.7

5000 20000 35000 50000

GTOW (lbs)



Business Jets

0.1

0.2

0.3

0.4

0.5

0.6

0.7

10000 15000 20000 25000 30000

GTOW (lbs)



The calculated TBProp and TBFan fuel fractions (FFs) were 0.175 and 0.354. - Both fit manned aircraft parametric data- What happened?

- By definition, FF is not affected by assumed EWF

19-17



One more application

How do TBProp and TBFan UAV “payload” fractions PFs (0.375 and 0.25) compare to manned aircraft?- They don’t

- Payload fraction is a design choice but…. - They also don’t fit UAV payload fraction parametrics

either (chart 19-13)- Another indication of questionable sizing results

Business Jets

0

0.05

0.1

0.15

10000 15000 20000 25000 30000

GTOW (lbs)


Regional Turboprops

0.05

0.1

0.15

0.2

0.25

0.3

5000 15000 25000 35000 45000

GTOW (lbs)


19-18



Wrap up - weight fractions

Within any vehicle class, weight fractions can vary widely- Yet most conceptual sizing procedures start with assumed empty weight, fuel or payload weight fractions - Often the result is a significant difference between

initial size estimates and subsequent ones based on higher fidelity methods

- Lots of effort is spent analyzing the wrong size concept- Therefore, we will use another sizing approach

Nonetheless, weight fraction are still useful for parametric comparison- We can use them to test the validity of our calculated weight estimates

- If they don’t fit within the data range, we need to make sure we understand why

19-19



One last subject

Performance weight fractions

• Raymer and Roskam also use gross weight fractions to make preliminary fuel consumption estimates for some mission segments

- Examples from RayAD Table 3.2

Warmup and takeoff = 0.97Climb = 0.985Landing = 0.995

• Notional values are really not required

- Physically relevant mission segment calculations can replace notional values with little additional work

• We will will address this further in lesson 21

19-20



Next subject


• Empty weight• Fuel• Payload• Miscellaneous• Performance


19-21



Bottoms-up weights

A bottoms-up estimate is a process for estimating weights in categories, each of which is influenced by similar design drivers as discussed earlier, e.g. - Payload weights are defined by mission requirements- Fuel fraction is determined by mission requirements and aero-propulsion performance

- Airframe weight is influenced by wing-body-tail Swet, etc.- Landing gear is driven by maximum vehicle weight (W0)- Engine weight is driven required air vehicle thrust-to-weight (TO/W0), etc.

Our initial bottoms-up UAV estimate categories will be defined by combining equations 19.1-19.4 or W0 = [Wpay+Wfe]+[(Waf/Sref)Sref]+ [FF+(Wlg/W0) +(Weng/T0)(T0/W0)+Wos/W0]W0 +[Wmisc/(W0-We)](W0-We) (19.5)

19-22



Bottoms-up weight inputs

A variety of sources will provide parametric data for bottoms-up weight estimates - Payload weight and fuel fraction will be input as variables- Airframe weight (initially) will be estimated from parametric data - We will use an airframe weight parametric (Waf/Sref)

- A similar parameter (We/Sref) will also be used for parametric empty weight comparisons

- Later we will use airframe unit weights (e.g. RayAD Table 15.2) and geometry to refine the estimates

- RayAD Table 15.2 weight fractions are used for landing gear and systems plus avionics (aka, “all else empty”)

- Lesson 18 propulsion parametrics will provide engine thrust-to-weight or power-to-weight inputs

- A nominal 2% useful load will be used to account for miscellaneous weights

19-23



UAV weights

• Little detailed UAV weight data is available - We will assume that selected and/or adjusted manned

aircraft weight data can be used until more UAV data becomes available

• Parametric comparisons indicate that manned and unmanned aircraft weights are comparable (exc. GH)

Empty Weight Comparisons (data from Roskam and J anes)

0

10

20

30

40

50

60

70

0 25 50 75 100 125 150

GTOW/Sref (psf)

EW/S

ref (

psf)

Biz J etSE Piston PropME Piston PropReg TurboJ et TransJ et fightersMil TrainProp UAVsJ et UAVs

Empty Weight Comparisons (data from Roskam and Janes)

0

10

20

30

40

50

0 25 50 75

GTOW/Sref (psf)

EW

/Sre

f (p

sf)

Biz JetSE Piston PropME Piston PropReg TurboJet TransJet fightersMil TrainProp UAVsJet UAVs

Expanded scale

Global Hawk

TR-1Global Hawk

19-24



UAV weights – cont’d

There are reasons why manned aircraft weight data should be applicable- Landing gear weight will

be 3-6% W0 whether manned or unmanned

- Engine T0/W0 and Bhp0/W0 will be no different for UAVs

- Most system weights should scale with empty or gross weight whether manned or not

- Payload avionics, however, will be UAV unique

And for now we will assume that airframe weights are comparable and correlate like EW/Sref

Airframe Weight Comparisons - (data from Roskam and Janes)

0

5

10

15

20

25

0 25 50 75

GTOW/Sref (psf)

Biz JetSE Piston PropME Piston PropReg TurboJet TransJet fightersMil Train

TR-1

Note - Roskam definition includes landing gear in airframe weights, we do not

19-25



Typical application

Example TBProp UAV bottoms-up weight estimate

- For W0/Sref = 30, we calculated Bhp0/W0 = 0.092 to meet our 1500 ft ground roll requirement (chart 18-22)

- From our Breguet range analysis (chart 15-40) we estimated W0 = 1918 lbm and can calculate Sref = 1918/30 = 63.9 sqft

- From chart 19-24 we can estimate Waf/Sref = 9 psf and calculate Waf = 575.4 lbm

- From Shp0/W0 we know BHp0 = 176.5

- Chart 18-13 shows that a TBP of this size produces about 2.25 Shp/lb so that Weng (uninstalled) = 78.4 lbm

- Using RayAD Table 15.2 installation factor (Kinst) = 1.3 we calculate installed engine weight = 101.9 lbm each

19-26



Application cont’d

TBP weight calculation (lbm)Waf 575.4 Wpay 720Weng (instl) 101.9 WF 375.7Wlg 95.9 Wmisc 18.3 Wspa 230.2 W0 2117.4We 1003.4

Note that W0 differs from our initial value (1918 lbm)

- From RayAD Table 15.2 we assume Wlg/W0 = 0.05 and calculate Wlg = 95.9 lbm

- We use the RayAD Table 15.2 Wspa or “all else empty” factor of 12% to estimate system and avionics weights- In doing this we are assuming that the additional systems and avionics needed for manned aircraft are offset by the systems and avionics unique to a UAV (may not be valid)

- We assume Wmisc = 2% of useful load- Payload weight is given & FF = 0.175

19-27



Weight iteration

Converged TBP weights (lbm)Waf 739 Wpay 720Weng (instl) 131 WF 431Wlg 123 Wmisc 23Wspa 296 W0 2463We 1288 EWF = 0.52

• One characteristic of a bottoms-up weight estimate is a requirement to iterate the solution to convergence- We do this by brute force using spreadsheet analysis methods and after a number of iterations (17) the following bottoms-up weight estimate results- Copy bottoms up weight equations “n” times, update W0 each iteration (See ASE261.BUWeights.xls)

19-28



Next subject


• Empty weight• Fuel• Payload• Miscellaneous• Mission segment

• Bottoms-up weights• Geometry related weights• Conceptual design weights

19-29



Geometry related weights

RayAD Table 15.2 lists airframe component unit weights (weight per unit area) for three vehicle types- Unit weight factors can be used to do an airframe

component weight build-up when areas are known:- Fuselage (Wfuse) = SwetFus*Uwf (19.6)- Wing weight (Wwing) = SrefExp*Uww (19.7)- Horizontal tail (Wht) = Sht*Uwht (19.8)- Vertical tail (Wht) = Svt*Uwvt (19.9)

- Uwf = Fuselage weight /Swet-fus- Uww = Wing weight / SrefExp - SrefExp = Exposed wing area - Uwht = Tail weight /Horizontal tail area (Sht)- Uwvt = Tail weight /Vertical tail area (Svt)

- Where for simplicity we assume fuselage weights include engine nacelles or - Uwfpn = Fuselage+nacelle/Swetfpn = Uwfpn

where

19-30



Airframe weight

Therefore, by definition, airframe weight is given byWaf = Wfuse + Wwing + Wht + Wvt =

Uwfpn*Swetfpn + Uww*Sref*(Srefexp/Sref) + Uwht*Kht*Sref + Uwvt*Kvt*Sref

Waf/Sref = Uww*(Srefexp/Sref) + Kht*Uwht

+ Kvt*Uwvt + Uwfpn*Swetfpn/Sref (19.10)Combining equations 19.3, 19.4 and 19.10:W0/Sref = Waf/Sref /((1 - FF - PF)/(1 - Kwmisc) - Kwprop

- Kwlg - Kwspa) (19.11)

Kwmisc = Misc. wt /(useful load W0 - We)Kwprop = Installed propulsion weight fraction

= Kint*(T0/W0)/(Neng*T0/Weng)Kwpint = Propulsion installation weight factor (≈ 1.3)Kwlg = Wlg/W0 (≈ 0.3 - 0.6)Kwspa = (Wsystems+Wavionics)/W0 (≈ 0.10 - 0.17)

or

where

19-31



Application

We can use Eq 19.9 to check the Chart 19-24 airframe weight parametric using typical area ratios for the 3 aircraft types in RayAD Table 15.2

Fighters Bombers & transports General aviation

Uww (psf) 9.0 10.0 2.5Uwht (psf) 4.0 5.5 2.0Uwvt (psf) 5.3 5.5 2.0Uwfpn(psf) 4.8 5.0 1.4Overall* 19.6 35.2 6.9

Airframe Weight Comparisons - Manned Aircraft (data from Roskam)

0

10

20

30

40

0 50 100 150

GTOW/Sref (psf)

Biz JetSE Piston PropME Piston PropReg TurboJet TransJet fightersMil Train

• Comparison with Chart 19-23 shows good agreement for fighters at Swet/Sref = 4, bombers at Swet/Sref = 6.5 and general aviation aircraft at Swet/Sref = 4.5

• Raymer’s unit weights look good! * Landing gear included

19-32



Next subject


• Empty weight• Fuel• Payload• Miscellaneous• Mission segment


19-33



Conceptual design weights

Conceptual design weight estimates are typically based on statistical weight methods (see RayAD 15.3)- Component aircraft weights are compiled and

statistically analyzed - RayAD Equations 15.1-15.59 are examples available

from US government public release documents- Individual companies typically have their own

proprietary weight equations that reflect actual internal design and manufacturing capabilities

- For student design projects, Raymer’s equations are more than adequate

Even though our spreadsheet analysis methods are most applicable for pre-concept design studies, they can also be used for concept studies and tradesduring the early phases of conceptual design - However, statistical weight equations should be used to

generate multipliers to correct the pre-concept design weight estimates for the baseline design

19-34



Expectations

You should now understand

• Basic weights and weight parametrics

• Where they come from

• How they are used

• The limits of their applicability

19-35



Other expectations - ABET

• Each final report should contain a section (one paragraph or longer) addressing each of the following points. Note that not all of these issues may be relevant to your project, but you should think about them before concluding that they are irrelevant and justify your decision. These sections should be included in your index and should be mentioned in your executive summary.

1. Economic Issues – How will your project, if done, affect the economy of the US and perhaps the world. Does your project require resources that are difficult to obtain?

2. Environmental Issues – How will your project, if done, affect the environment of the earth? Discuss any positive and/or negative factors.

3. Sustainability Issues – Are there sustainability issues with your design. Is it meant to be one shot or the backbone for later work.

4. Manufacturability Issues – Will your spacecraft I facility be manufactured on earth, in space, on Mars, or where. Will it be assembled and then flown or flown in parts and assembled later.

19-36



Other expectations

• ABET (Continued)

5. Ethics Issues – Are there ethical issues associated with your project? If so, identify and discuss them.

6. Political Issues – Are there political issues associated with your project? If so, identify and discuss them.

7. Health and Safety Issues – Are there health and safety issues associated with your project? If so, identify and discuss them.

8. Social Issues – Are there social issues associated with your project? If so, identify and discuss them.

9. Global Impact – What is the global impact of your project? Discuss it.

19-37



Homework

1. Write a spreadsheet program to calculate bottoms up weights with W0/Sref, Wpay, FF, Kmisc, BHp0/W0, Bhp0/Weng, Kinst, Wlg/W0, Waf/Sref and Wspa/W0 as inputs - (team grade)

2. Run your spreadsheet for the example problems (charts 19-25/27) and compare results (team grade)- Identify any errors in my example problems

3. Use the team spreadsheets to calculate bottoms up weights for your proposed air vehicle (individual grade)

4. Compare your spreadsheet results to ASE261.BUWeights.xls and identify differences (individual grades)

5. Discuss ABET issues #1 and #2 and document your conclusions (one paragraph each– team grade)

2nd week

Documents

19-1 Design of UAV Systems Parametric weightsc 2002 LM Corporation Lesson objective - to discuss Parametric weight methods ….the minimum level of fidelity