1Undergraduate Student, AIAA Student Member

Vehicle Sketch Pad – User Manual

Aaron Botwick1

Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville,

VA 22903

Vehicle Sketch Pad is an open source NASA program used for conceptual design of aircraft. The

software was built to produce CAD models of an aircraft with aircraft-specific geometric parameters

as the driving inputs of the design. The user is able to start with discrete aircraft components such as a

fuselage or a wing, then use parameters such as aspect ratio, taper ratio, chord and sweep to modify

the part. The use of such parameters allows for much faster production of an aircraft conceptual

design. Upon completion of the design, the program is able to compute values for things such as the

center of gravity (CG) and total wetted area of the aircraft. The program is also able to produce

geometric meshes of the design for export into other programs. In order to use the program, the user

should have a good knowledge of aircraft terminology.

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I. Uses

Vehicle Sketch Pad was at first used as a tool for producing conceptual design for each member

or each team to present as a base concept for the aircraft. Upon narrowing down the options, it was

used for preliminary sizing as well as an important visual aid for discussion and decision-making for the

aircraft. Decisions regarding the configuration of the aircraft require a tool such as VSP in order to

determine the geometry necessary to accommodate each crucial feature of the aircraft. Changes to the

aircraft were made in VSP, and then parameters such as CG and wetted area were calculated in order to

understand the system-wide consequences associated with such changes.

II. Getting started with a design

Left clicking and dragging allows the user to manipulate the view angle of the aircraft. Right

clicking and dragging allows the user to translate the aircraft within the view screen. Holding down the

center scroll button on the mouse and dragging left or right will adjust the view by zooming in or out.

Pressing the letter ‘C’ on the keyboard at any point will center the view. The following commands are

used to revert to principle views of the aircraft:

F1 - Side View

F2 - Top View

F3 - Front View

F4 - Isometric View

In order to start a design, open the geometry directory by selecting ‘Geom’ from the VSP main

menu and then selecting ‘Modify’. Next to the button that reads “add”, a component such as FUSE or

MS WING will be selected in order to add the component. Pressing the “add” button creates the

component. The menu contains components labeled POD, FUSE, HAVOC, EXT STORE, MS WING, BLANK,

DUCT, PROP, ENGINE, HWB, FUSE2, and CABIN LAYOUT. For purposes of preliminary conceptual design,

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only MS WING (multi-section wing), FUSE2 should be used. ENGINE and PROP can be used to

demonstrate a rough idea of the external configuration of the propulsion system.

a. FUSE2

FUSE2 is the best option for creating the fuselage of an aircraft. It allows for detailed shaping of

the piece. Select FUSE2 from the Add menu on the geometry directory, and press add. The following

base geometry will appear.

Figure 1: The base geometry created by adding a FUSE2 component

Figure 2: The four tabs used to modify a FUSE2 component

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XForm is used for the location and orientation of the piece. Shape is used to adjust the length of

the aircraft. The tangent options will be discussed after XSec, as they are used for fine tuning the shape

of the fuselage.

XSec is used to modify the location and shape of the cross sections. XSec is the likely to the be

the most heavily used tab in shaping the fuselage. Under XSec, the type of cross sectional shape can be

selected under the ‘type’ option. Examples are points, ellipses and circles. Note that the fuselage must

end in a point at each end in order to produce geometric meshes of the aircraft. The option also exists to

freely modify the cross sectional shape by selecting ‘edit’ under the type menu, and to import a shape

by selecting ‘file’. The sizes of the cross sectional shapes can be edited once the type of shape is

selected. Cross sections can be added and deleted as well as copied and pasted. The location of a specfic

cross-sectional shape can be controlled by selected Loc under the XSec tab. The offset in the Z and Y

directions can be adjusted by selecting Y off or Z off. Below is an example of a shape obtaining by

making modifications under the XSec tab.

Figure 3: An example of a FUSE2 piece after editing the shapes and Z off values of the cross sections

After the general shape of the fuselage is obtained, the details of the curvature can be adjusted

by using the tangent options under the Shape tab. Adjusting the top tangent of a section will change the

manner in which the top line is fitted across that section and the sections in front of and behind it. This

translates to the shape of the “bulges” that occur before and after each cross section. Adjusting top and

bottom tangents will change the shape as viewed from the side, while right and left tangents will change

the shape as viewed from above. Note that symmetry can be toggled for Top/Bottom and Left/Right

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tangents by pressing the buttons labeled ‘Top/Bot Sym’ or ‘Left/Right Sym’. Below is an example of a

shape that can be obtained from the previous shape shown in figure 3 can by further modifying the XSec

tab and modifying the tangent options under the Shape tab.

Figure 4 : A more developed fuselage shape created with a FUSE2 component

b. MS WING

This geometry is used to create components such as the wings, vertical tails, rudders and canards of

an aircraft. Adding MS WING result in the following base geometry, automatically named “Ms_Wing_0”

in the geometry directory:

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Figure 5: Base geometry created by adding MS WING (top view)

An MS WING component can be edited with direct aircraft design parameters such as aspect

ratio, planform area, and sweep. Selecting Ms_Wing_0 in the geometry directory will automatically

bring up the geometry modification menu for the MS WING. Adjusting some values, particularly under

the ‘Plan’ and ‘Sect’ tabs, may result in undesired changes to the shape of the wing. This is because

some other parameters are locked in place as design drivers, and must be edited separately in order to

achieve the desired shape. For example, adjusting the span of the wing will change the entire shape of

the wing because the sweep is not being adjusted. Back and forth modification is usually necessary to

achieve the desired wing planform.

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Figure 6: Geometry modification options for an MS WING component

The first tab ‘Gen’ shows options for modifying the name and color of the component, as well as

the option to attach it to another piece. Take note that the material option only affects the component

when it is the shade option is selected for the piece under the geometry directory.

The second tab ‘XForm’ contains location, rotation and symmetry options for the piece. Note

that the bar for the parameters is dragged left or right in order to adjust the values, and more

importantly that discrete values can be obtained by typing them directly into the value box. This is true

for all geometric inputs in VSP.

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The third tab ‘Plan’ contains options for the overall planform dimensions of the wing. The aspect

ratio of the wing is shown in grey under the Span, Proj Span, Chord and Area options.

The fourth tab ‘Sect’ contains options for modifying each individual section of the wing. This tab

is the likely to be used the most in the design of the wing. Under the ‘Driver’ tab, different options are

provided such as Span-TC-RC. This option provides the user with input controlling the span, tip chord

and root chord in order to manually shape that section of the wing. Wing sections can be added, deleted

as well as copied and pasted. Multiple sections are needed to create different taper ratios and sweeps at

different locations of the wing - many aircraft wings have these characteristics. Take note that the MS

WING base geometry contains two sections, numbered 0 and 1.

The fifth tab ‘Dihed’ contains options for the dihedral angle for each section joint of the wing.

The sixth and final tab ‘Foil’ contains options for modifying the airfoil for each wing section, or importing

a custom airfoil.

i. MS WING as a Wing

The base geometry of the wing is strange and almost always requires drastic changes in order to

get to a reasonable starting place for an aircraft’s wings. Adjusting the basic parameters of the base

geometry under the Plan tab usually results in unexpected changes that make it difficult for a beginner

user to achieve a reasonable starting wing planform for a design. The easiest way to produce a more

standard wing is to navigate to the Sect tab, select section 1, adjust the sweep to 15 degrees, adjust the

span to 15, and adjust the root chord (RC) to 6. Then select section 0, adjust the span to 20, sweep to 15

degrees, then adjust the root chord to 11. After that, navigate to the Dihed tab, and adjust Dihed 2 to 0

degrees. The result is the following wing planform, which is must easier to edit with general inputs such

as span and chord under the Plan tab.

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Figure 7: The edited MS WING - more useful as a base geometry for standard aircraft configurations

ii. MS WING as a tail

Shown below is the combination of the example FUSE2 shape from the previous section

combined with the MS WING main wing, tail and vertical tail surfaces. Copying and pasting the above

MS WING will make a good start for a tail component. Make sure to copy the MS WING piece, and then

select aircraft before pasting the piece. If the piece is pasted while the original MS WING is selected, it

will be parented to the original MS WING piece. This is unnecessary and bad for the organization of the

geometry directory. After copying and pasting the piece, nothing will seem to happen because they are

at the same location, so first adjust the X location of the new piece to a value close to the length of the

fuselage of the aircraft in order to see the piece. Then adjust the scale factor of the piece to about 0.5.

Adjust the X and Z location, span, chord and sweep of the MS WING tail piece as desired.

ii. MS WING as a vertical tail/rudder

Copying and pasting the above MS Wing (under the geometry directory) will serve as a good

start for a vertical tail component. Again, make sure to copy the MS WING piece, and then select aircraft

before pasting the piece. Also remember that after copying and pasting the piece, nothing will seem to

happen because they are at the same location, so first adjust the X location of the new piece to a similar

location to the tail piece in order to see it. Then select NONE under the symmetry option on the XForm

tab and adjust X rotation to 90 degrees. After that, deleting section 1 and adjusting the span and chord

to 10 and 4, respectively, the piece becomes a good base geometry for a vertical tail. Below is an

example of a rough aircraft configuration that can be obtained by combining a FUSE2 fuselage piece

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with MS WING wing, tail and vertical tail components. Engines and/or props can easily be added to such

a design if desired.

Figure 8: Example aircraft created with MS WING and FUSE2 components

The aircraft can be rendered to appear as a solid object by highlighting each component in the geometry

directory and selecting ‘shade’.

Figure 9: The same aircraft with components shaded

More complex aircraft can be created by using MS WING and FUSE 2 components. Some examples are

shown below.

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Figures 10 and 11 : examples of aircraft configurations created with VSP

III. Calculations, Meshes and Exports with VSP

Once an aircraft designed has been produced, VSP can calculate values for center of gravity (CG)

and wetted area. Meshes can also be created for geometric imports for programs such as SolidWorks.

The calculations are based solely on geometry, so they should be taken as rough estimates rather than

accurate values.

a. Examples of Useful Calculations

i. Center of Gravity

Center of gravity is a very important parameter to keep in mind when producing the initial

concept of an aircraft. It is one of the strongest driving factors in the overall configuration of the aircraft.

VSP is able to produce a calculation of center of gravity based on the geometry of the design. Keep in

mind the actual internal components and relative weights of different parts of the aircraft are not

included in the calculation, making it a very rough estimate. To carry out this calculation, select ‘Mass

Prop’ from the VSP main menu, turn on the Draw CG option and press compute. Note that producing

this mesh or any other mesh automatically visually hides the rest of the aircraft, which can be turned

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back on by selecting them on the geometry directory and selecting ‘Show’ as opposed to ‘NoShow’.

Figure 12 : The visual and numerical results of running a CG calculation

ii. Wetted Area

Wetted area refers to the total amount of surface area exposed to air on the external surface of

an aircraft. It is an important factor in reducing drag. To run this calculation, select ‘Geom’ from the

main menu, and then select ‘CompGeom (Union)’. The Comp Geom window contains values for the total

wetted area as well as a breakdown of each component. Note that for components such as wings, there

are two items listed. This is because the program counts the left and right side of each component

separately before adding everything together. Below are the results from a Comp Geom calculation.

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Figure 13: The visual and numerical results of running a wetted area calculation

b. Exports

Geometries can be exported from VSP in order to provide geometric imports for other programs.

This can be very useful for programs such as SolidWorks. In order to export the file, simply select export

from the File tab on the main menu and select the type of export desired. Note that meshes created for

wetted area and center of gravity calculations can be exported to a specific location by selecting the

destination for the output in each of the calculation menus.

IV. Additional Information

For additional information, examples and help with Vehicle Sketch Pad, refer to

www.openvsp.org. The user manual can be found at www.openvsp.org/ files/VSP_Manual.pdf.