weight estimation using cad in the preliminary rotorcraft...

33rd European Rotorcraft Forum

Kazan, Russia, September 11-13th, 2007

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Weight Estimation Using CAD In The Preliminary Rotorcraft Design

M. Emre Gündüz1, Adeel Khalid

2, Daniel P. Schrage

3

1Graduate Research Assistant, Daniel Guggenheim School of Aerospace Engineering,

Georgia Institute of Technology , Atlanta, GA. 30332, USA

Email: [email protected]

2Systems Engineer, Avidyne Corporation

55 Old Bedford Road, Lincoln, MA. 01742, USA


3Professor, Daniel Guggenheim School of Aerospace Engineering, Georgia Institute of

Technology , Atlanta, GA. 30332, USA


Key words: Concept, rotor, design, weight, computer, CAD

Abstract: Weight estimation of aircraft, including rotorcraft has always been a

critical part of design process. Since most aircraft are designed based on a baseline

similar to the concept at hand, current methods utilized for weight estimation rely on

either extrapolation using statistics and historical data, or analysis software based on

performance requirements. These approaches provide the designer with a rough

approximation at the concept generation stage, but they may not always be adequate

in the subsequent stages of design. They usually supply a total weight for each

subsystem of the aircraft, such as transmission group, rotor group, etc. They do not

assign a weight for each part in a subsystem assembly.

A method to be applied using Computer Aided Design (CAD) software during vehicle

engineering analysis for calculating weights of each component as well as the entire

aircraft is proposed. It helps acquiring detailed weight assessment earlier in the design

stage, and in turn, brings development costs down. This method depends on CAD

drawings of all components of the aircraft. The CAD program is linked with design

and analysis software. After any design change, the total weight can be recalculated

automatically, enabling detailed weight information in every design iteration. A

comprehensive weight optimization involving component size and shape

modifications can also be performed.

CAD software that permits assembly generation can also be utilized to obtain exact

location of center of gravity of the aircraft, for every conceivable passenger or

payload distribution during the mission.

For proof of concept, a Schweizer TH 330 helicopter and several other helicopter

rotors are modeled and calculated blade weights are compared with weights estimated

using Prouty’s equations [1]. It was found that even crude model of a helicopter rotor

with correct dimensions and airfoil shape may result in a better approximation to the

actual weight of the blades.



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1. INTRODUCTION

Estimating the helicopter weight has always been a challenge for weight engineers in

the conceptual and preliminary design stages. Several techniques have been suggested

in the past including the group weight estimation. Usually historical data from the

helicopters of same or similar class is used to approximate weights of individual

components. For example, the weights of avionics group can be obtained in the initial

design stage using rough estimates of avionics based on historical data. Prouty et al

[1] suggest lumping the weights of helicopter components into groups and using

historical data to formulate approximate empirical models. These techniques yield

results that are inaccurate and therefore do not provide satisfactory component

weights for the weights engineers. Lack of correct or accurate weight information

leads to incorrect estimation of cost and that results in several expensive design

iterations. One of the objectives of concurrent engineering is to bring detailed design

information early in the design stage. As shown in Figure 1 [3], more design

information early in the design provides more design freedom and reduces committed

cost. Weight engineers need the detailed design information as early as possible so

that other disciplines that are dependent on the weights group can perform accurate

analyses.

Figure 1. Design freedom, Knowledge and cost relationship [3]

Use of vehicle engineering early in the design stage is suggested in this paper for

accurate weight estimation. Detailed Computer Aided Design (CAD) models can be

linked with design and analysis software. Modern CAD models also help in

determining the weights. By linking the design software with CAD packages, detailed

and accurate weight information is brought to the design database, which is accessible

by other disciplinary experts. This technique helps the designers to estimate helicopter

weights in the preliminary design stage with a high degree of accuracy using the

actual CAD drawings of the new helicopter under consideration.



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Once a link has been established between the CAD software and the design package,

updated weight information can be obtained as the design changes. Component

weights, and subsequently overall empty and gross weights do not have to be

recalculated every time the design changes. This approach guarantees automated

weight estimation. Additionally, once the weight calculation is automated, it can also

be optimized. This approach enables the designer to obtain detailed component

weights as opposed to group weights. The level of detail and precision of weights

depends on the level of accuracy and detail of the actual CAD model and component

material properties. This approach is evolutionary in nature. If the existing helicopter

weights information already exists, then this approach can be used to calculate and

update the weight information for a new design where all the changes in the design

are captured. This approach is also visual in nature. It is compatible with weight

reports used in industry and military, such as MIL-STD-1374. Using the CAD model,

the designer can also calculate the center of gravity and moments.

2. METHODOLOGY

Helicopter design consists of several disciplines that interact with each other. In an

optimization problem, design information flows between disciplines at every system

iteration as shown in Figure 2 [2]. One of the key disciplines in the preliminary

design process is the weight engineering. Traditionally, weights are estimated in the

initial design stage both by extensive experience and by good judgment about existing

and future engineering trends. Multiple linear regressions can be used to derive

equations for each aircraft component from weights data of previous aircraft. This

determines sensitivity with respect to every parameter that logically affects the weight

of the component [1]. The resulting equations are continually modified as more

modern helicopters are added to the database and as detail design of specific

components is accomplished. Prouty [1] lists a set of regression equations for

preliminary design weight estimates based on work done by Shinn et al [5, 6]. These

equations are used to determine the initial system weight estimates of fuselage,

landing gear, nacelle, engine installation, propulsion subsystems, fuel systems, drive

systems, cockpit control, instruments, hydraulics, electrical, avionics, furnishings and

equipments, air conditioning and anti icing, and manufacturing variations. This type

of analysis can be used for prediction of almost all the group weights that comprise

the helicopter empty weight, and is particularly applicable for the structural groups.

The weight prediction can be refined as the design progresses and applicable values

for increasing numbers of design parameters are determined. However in today’s

design environment where the design is iterative in nature and design variables

change from one iteration to the next, it can be very time consuming and tedious to

use the regression equations. Additionally the group weights obtained using

regression equations are estimates that may have significant error. These estimates are

often not suitable for new designs.

The new approach proposed in this paper is to calculate weights by using a vehicle

engineering or CAD package. CAD packages have improved significantly in the past

decade. In this particular study, CATIA by Dassault Sytemes is used to calculate

component weights. Detailed parameterized component CAD drawings are developed

using CATIA V5R16. These drawings are then linked with the Phoenix Integration

design software called ModelCenter. This link allows the designer to dynamically

change the part designs parametrically from ModelCenter by changing the variables



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as they get updated from one system level iteration to another. The updated empty

weight information is then used by other disciplines, which are dependent on weights

discipline for their calculations. This approach automates the weight estimation

process while providing an accurate weight estimate.

System

Optimization

Stability and

Control

Weight

Iteration

Aerodynamics

Figure 2. Interdisciplinary dependency and design iteration [2]

A wrapper is developed that helps establish a link between ModelCenter and CATIA.

Important design variables are specified in ModelCenter. By changing these variables,

the corresponding dimensions are updated in CATIA. This process is indicated in

Figure 3. The wrapper facilitates this process. Corresponding material properties are

specified in CATIA. These material properties are based on the best available

information at the early design stage. In this research, historical data is used for the

specification of material densities. In majority of new designs, the material details

may not be available at the early design stage or the designer may decide to keep the

material as a variable so the material information can be updated or new materials can

be added as the design progresses. The design variables may change significantly

from one iteration to another or over several iterations in a design process. For

example during the helicopter sizing process, the designer may decide to start with a

small rotor and then as the design matures, the rotor size may increase, as is the case

in most designs. This change in the rotor dimensions is reflected in the CAD

drawings. As the CATIA drawings get updated, the volumes and weights are

calculated automatically in CATIA and the updated information is sent back to

ModelCenter as shown in Figure 3. The component weights are then integrated in

ModelCenter to find group weights. Group weights are added to find the vehicle

empty weight. This updated weights information is then sent to various other

disciplines or system level optimizer in an optimization problem, as indicated in

Figure 2. The Component Weights (CW) are added in ModelCenter to get Group

Weights (GW). For example rotor blades, flexures and hub weights are added to get

the rotor group weight. Similarly group weights are added to get the vehicle Empty

Weight (EW). The addition process in ModelCenter is shown in Figure 4. This entire process of sending new variable information from ModelCenter to CATIA, update of

CAD drawings, new component weight calculations and addition of component and

group weights to get the vehicle empty weight is automated, which enables system



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iterations. With this methodology, the weights discipline’s works load is significantly

reduced.

Figure 3. Weight iteration between CAD (CATIA) and Design (ModelCenter) software

Figure 4. Component, Group and Empty weight calculation in ModelCenter

3. IMPLEMENTATION

The above mentioned methodology is implemented by means of modeling an entire

helicopter in CATIA, and comparing its weight calculations with the actual values.

Schweizer TH330 is modeled parametrically, as shown in Figure 5, and calculated rotor group weights are compared to provide rotor group weight of the rotorcraft by

the manufacturer.

Parametric design is crucial in this method because of the necessity to modify the various design

parameters quickly when switching between various concepts. For instance, with the CATIA

and ModelCenter models developed in this study, it is possible to modify the blade length, chord

lengths at the root and tip of the blade, and number of blades easily within ModelCenter without

Empty Wt.

Users

ModelCenter • Change design

variables

CATIA • Update drawings

• Calculate comp. Weights

New Design

Component Weights

CW – Component Weights

GW – Group Weights

EW – Empty Weight



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making any changes in CATIA. In addition it is relatively easy to switch between different airfoil

cross-sections, such as NACA0012, NACA23012, or SC1095 as long as the airfoil shape is

available in a table format of (x, y) coordinates in terms of chord length (c), as shown in Figure 6. The table values are fed into CATIA to obtain the exact blade cross-sections of unit chord length.

It is then possible to scale the cross-section to the desired chord length at the tip or at the root.

The number of blades can be changed from ModelCenter and the CATIA drawings get updated

as shown in Figure 7. The wrapper file integrated in ModelCenter and the linkages between design variables

and actual physical dimensions in CATIA are shown in Figure 8. The variables that are passed to CATIA are the same parameters that change and

update drawings and are summarized in

Figure 9.

Figure 5. Isometric and parametric views of Schweizer TH 330 modeled in CATIA

X/c Y/c

1 0.00126

0.992704 0.002274

0.979641 0.004079

0.964244 0.006169

0.947231 0.008434

0.929323 0.010765

0.910956 0.013101

0.892372 0.01542

0.873723 0.0177

0.855041 0.019931

0.836311 0.022119

0.817558 0.024266

0.798819 0.026366

0.780088 0.028414

0.761336 0.030413

0.74256 0.03237

0.72378 0.034284

� �



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Figure 6. Airfoil geometry modeled in CATIA

Figure 7. Main rotor group designed in CATIA with inputs from ModelCenter

The model also allows changes in hinge offset. The possibility of making such

changes easily on the rotor enables the authors to obtain a rough model of most

conventional single-main-rotor helicopter rotors. For simplicity, potentially low-

weight components such as control linkages are removed from the design to keep it

more general, and the hinge-offset section is modeled as a single beam. Any forward

or backward sweep of the blade, or any built-in twist angles throughout the blade span

are also ignored, since they do not significantly affect weights.



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Figure 8. CATIA wrapper shown in ModelCenter design environment

Since there is very little knowledge about the details of the design in the conceptual

stage, it is acceptable to make use of historical weight data to make a relatively

reliable weight estimate. The main purpose of this research is to introduce high

fidelity tools early in the design process. It is, however, highly possible to have little

or no information about the design necessary to use those tools. For example, weight

of a blade depends highly on its inner structure and materials. Exact weights of the

blade can be calculated using CATIA if structural information and material

distributions within the blade are well defined. However it is very unlikely to have

knowledge of this level of detail in the beginning of design. Therefore as a first

approximation, blade, hinge offset and rotor hub in this research are modeled as

separate solid components, composed of one single material each. The density of each

material is predefined by the user, based on weights of other similar helicopter rotors

published by their manufacturers, i.e., historical data. A method for finding the

densities is explained below. Since the volume of any rotor blade can be matched

using CATIA, given necessary dimensions, the only data required is the density of the

material of the blade, in order to calculate the blade weight. Volumes are calculated

by CATIA, and weights are obtained from either published information or historical

data.

The densities to be entered into CATIA model are obtained by first calculating

‘weight/volume’ information of several helicopters. These approximated densities for

each helicopter are then inserted into the equation below:

100/5/1

ctipapp VVds ⋅⋅= (1)



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where dapp is approximated density, Vtip is the tip speed of blades, and Vc is cruise

speed of the rotorcraft. Vtip and Vc are given in design specifications, and dapp is

calculated as

designCADfromvolumeestimated

componenttheofweightd app = (2)

This expression of s combines the most important design parameters in blade design,

implicitly including rotor radius and chord length within estimated density. The

necessary values for each rotorcraft are given in Table 1.

s value for each helicopter model is plotted against the years in which the particular

models were first released for sale or operation, as shown in Figure 10. This plot is used for forming a regression equation to find the density for a helicopter component

based on its first production year, to compensate for new technologies. Technological

advances may not be identical for every component in a rotorcraft; therefore this

method may be applied for each component separately in order to find different

density estimations for each part.

One may think that usage of historical data causes the weight estimation results to be

same as the case where no high fidelity tools are utilized, thus those tools are futile. In

the traditional approach of weight calculation, when the approximate size of the rotor

is being estimated, usually a predefined weight equation based solely on statistical

analysis of historical data is utilized. These equations still require specific data from

the rotorcraft under development, but this data is not used for calculating an entity

directly related to the helicopter; it is only needed for substituting the values in a

regression equation. The proposed method is a step towards minimizing dependency

on historical data in weight calculations. Although historical data is still employed, it

is not the major source of information any more. It is only needed for a crude

approximation to the overall density of the component under consideration. The

design data of the newly-designed rotor is used for calculating component volumes of

the same helicopter, rather than being used in a regression equation. Previous rotor

data is a starting point for helicopter sizes in the same ballpark.

Table 1. Helicopters considered in analysis

Make and Model Year

Root chord (ft.)

Rotor Radius (ft)

No. of blades

Hinge Offset ratio

tip speed (fps)

cruise speed (fps)

Aerospatiale/ Eurocopter AS 350B 1974 0.984 17.53 3 0.038 698.8 210.9762

Eurocopter BO 105LS 1981 0.895 16.14 4 0.14 716.7 221.1031

MBB/ Kawasaki BK 117 1979 1.049 18.04 4 0.12 723.6 226.1665

McDonnell Douglas MD 500E 1982 0.56 13.2 5 0.14 680.5 226.1665

Schweizer/ Hughes 300C 1988 0.563 13.42 3 0.014 661.3 135.024

Agusta A109 1971 1.1 18.04 4 0.027 725.5 239.669

Robinson R22 1975 0.6 12.58 2 (0.62)* 699.1 163.7176

Sikorsky UH-60A 1974 1.73 26.83 4 0.047 725 268.3618



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Sikorsky CH-53E 1974 2.44 39.5 7 0.063 732 253.3537

Sikorsky S-76A 1977 1.29 22 4 0.038 675 244.7324

Bell JetRanger 206 1977 1.083 16.665 2 (0.098) 670.2 194.0981

* For rotorcraft with teetering rotors, the term “hinge offset ratio” represents the ratio of blade structure at the root enabling connections and controls to the blade structure with airfoil shape. Values for such rotorcraft are presented in parentheses.

A unique blade weight is estimated using CATIA model for the new design, and the

estimate is automatically updated for minute changes in blade geometry. As the

design progresses, interior structure of the blade and other components will be formed

eventually, thus crude density approximations will in turn be eliminated from the

design without affecting CAD models. Using the same models provide a smooth

transition between design, analysis and manufacturing stages of a product.

4. RESULTS & DISCUSSIONS

Effect of production year is incorporated by fitting a linear curve for the points in

Figure 10. The equation for that curve is used for finding approximate values of s.

Estimated densities are then found by solving the following modified equation of s

100/5/1

ctipest VVds ⋅⋅= (3)

to find dest. These estimated densities are then multiplied by CATIA blade volumes to

obtain estimated weights. Results of these calculations are given in Table 2. These weights may be compared to Poruty’s blade weights in terms of closeness to the

published blade weight of the rotorcraft. For example, published total blade weight of

Sikorsky CH-53E is 2120 lbs. Prouty’s equations yield 2264 lbs, and estimation using

the method described in this paper gives 2189 lbs. Although the CAD model crude

and generalized for multiple rotorcraft, it still manages to approximate the actual data

better than a statistics-based method.

This approach, when implemented on new designs or design modifications, can

greatly reduce the design time, cost and effort of the weight engineers and help the

system designers for overall design optimization.

In this research, ModelCenter is used to demonstrate the integration of different

software including CAD, Spreadsheets and Design software. Information flow

between these software is facilitated. It is shown that by linking the design software

with CAD packages, detailed and accurate weight information can be brought back at

the initial design stage. As the design matures and more information becomes

available, the weights information is update automatically. This information is also

passed to other disciplines dependent on weights. The entire process is automated,

thus significantly reducing the weight engineers’ work. For future work, more

parameterized components of helicopter, developed in the CAD package, can be

linked directly with ModelCenter. This will ensure that as the user changes the design

parameters or as the design information gets updated from other disciplines involved

in the design cycle, precise updated weights information is calculated automatically.



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Figure 9. Updated CATIA Parameters

s vs. year

y = -1.8057x + 3691.1

0

20

40

60

80

100

120

140

160

1970 1975 1980 1985 1990

Year

s

Figure 10. Plot of release year versus s values



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Table 2. CATIA results and estimated densities

Make and Model

CATIA blade volume (ft

3)

Weight / Volume

( appd )

Estimated density

( estd )

Estimated Blade Weights (lbs)

Prouty's Blade Weights (lbs)

Aerospatiale/ Eurocopter AS 350B 4.028 43.700 61.10669 243.2404 176.0252457

Eurocopter BO 105LS 3.678 71.778 26.206512 97.49366 176.9745477

MBB/ Kawasaki BK 117 5.769 41.787 25.39844 155.8142 241.0732115

McDonnell Douglas MD 500E 1.162 52.323 33.79633 45.32606 95.32845078

Schweizer/ Hughes 300C 1.039 66.021 214.0503 220.2993 68.59666497

Agusta A109 9.722 26.043 32.57084 348.3768 253.1979527

Robinson R22 0.805 66.287 157.819 160.6157 53.36181113

Sikorsky UH-60A 20.176 33.050 17.38828 333.7174 666.825784

Sikorsky CH-53E 101.666 20.852 20.2695 2189.057 2264.261225

Sikorsky S-76A 9.85 37.176 36.8463 248.1934 366.1893129

Bell JetRanger 206 2.571 74.120 70.1363 209.0921 134.9539122

5. REFERENCES

[1] Prouty, W. Raymond, “Helicopter Performance, Stability, and Control”,

Krieger Publishing Company, Malabar, Florida, 1995

[2] Khalid S. Adeel, “Development and Implementation of Rotorcraft Preliminary

Design Methodology using Multidisciplinary Design Optimization”, Ph.D.

Dissertation, Georgia Institute of Technology, December 2006

[3] Mavris, D.N., DeLaurentis, D.A., Bandte, O., Hale, M.A., A Stochastic

Approach to Multi-disciplinary Aircraft Analysis and Design, AIAA 36th

Aerospace Sciences Meeting and Exhibit, January 12-15, 1998

[4] http://www.flug-revue.rotor.com/Frtypen/FRSch330.htm

[5] Shinn, “Impact of Emerging Technology on the weight of Future Aircraft,”

AHS 40th forum, 1984

[6] Schwartzberg, Smith, Means, Law, & Chappell, “Single Rotor Helicopter

Design and Performance Estimation Programs,” USAAMRDL, SR 10, 77-1,

1977

[7] http://www.pagendarm.de/trapp/programming/java/profiles/NACA4.html

[8] http://www.ae.uiuc.edu/m-selig/ads/coord_database.html

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