Introduction
The purpose of this document is to provide a basic procedure to create model aircraft wing ribs. The
procedure is derived from methods that have been implemented in previous semesters of the airplane
section of AERO 402. Many of the methods were chosen to simplify the construction of physical wing
models built from these ribs. One should always design the preliminary layout of the airframe and know
the components that will pass through the ribs before beginning the process of designing wing ribs. The
components used in this example are: 1/8x1/4 inch spar caps, 1/16 inch thick shear webs, 1/16 inch
thick leading edge and trailing edge sheeting, a 1/8 inch diameter leading edge dowel rod, and a
trapezoidal trailing edge wedge with 1/16 inch and 1/8 inch bases and height of 1/2 inch.
It is assumed that the reader either has little knowledge of SolidWorks or may have forgotten since
having taken ENGR 111 and ENGR 112. Some of this document will therefore seem tedious to more
experienced readers.
Creating the SolidWorks Model
The wing planform and airfoil sections must have already been decided. This example will assume an
unswept wing with a constant 10-inch chord and NACA 0012-34 airfoil sections. Coordinates to plot the
airfoil can be generated using published equations such as in Theory of Wing Sections by Abbot and Von
Doenhoff. Coordinates for many airfoils along with their plotted shape can also be found at:
http://www.ae.illinois.edu/m-selig/ads/coord_database.html
This example will use a coordinate .dat file from the database in the above link. Import the data into
Excel using “Get External Data From Text”.
When importing the coordinates, ensure that
spaces are chosen as a delimiter. Once the
coordinates are imported, note that they are
for an airfoil with unit chord. Multiply all
coordinates by the chord. To use the
coordinates in SolidWorks, a .txt file will be
needed with X,Y, and Z data. For this example,
enter 0 in the Z-column for each X-Y pair, copy
and paste the three columns into Notepad,
and save as .txt format.
In SolidWorks, create a new Part and go to Tools->
Options->Document Properties->Units. Make sure the
inch-pound-second unit system is selected and select
ok. Go to Insert->Curve->Curve Through XYZ Points as
shown in the picture.
On the Curve File window click Browse. Use the pull-down arrow to select .txt file type and select the
airfoil coordinate text file.
The imported coordinates will be
shown in the table and a yellow
outline preview of theairfoil will be
shown in the part display area. Click
OK.
To use the curve for modeling, first create a Sketch on the Front Plane by
clicking Insert->Sketch and selecting the Front Plane in the window.
Alternatively right-click on the Front Plane in the “FeatureManager
design tree” (on the left of the screen) and left-click on Sketch.
Now select the curve and use Convert
Entities on the Sketch toolbar or use
Tools->Sketch Tools->Convert
Entities.
The airfoil curve can now be modified using the various Sketch
Entities and Sketch Tools. Zoom in on the trailing edge to see
that it does not quite connect. Select Line on the toolbar and
connect the two points at the trailing edge. (The coordinates
that were imported have a gap of 0.024 inches at the trailing
edge. For this example, the gap is trivial and need only be
closed. For other cases, including certain aerodynamic
analyses, more accurate airfoil coordinates should be used.)
Exit the Sketch. Create a Solid Body from the Sketch of
the airfoil using Extruded Boss/Base on the Features
toolbar or by going to Insert->Boss/Base->Extrude. Use
Blind End Condition to Extrude the Sketch by 1/16 inch.
Right click on the face of the rib and insert a Sketch
on the face. This Sketch will be used to define how
the spars intersect the ribs.
Select Line and check the “For
Construction” box in the Insert Line
window. Draw two vertical construction
lines in the Sketch. (If the lines do not
automatically snap to vertical, select them
and select Vertical in the Line Properties)
Use the Smart Dimension Tool to place the
lines at 2.5 inches and 7 inches from the
origin. These will be the locations of the
front and rear spars.
Create an I-beam cross section for the Front Spar to pass through the
rib. Draw the rough outline using Corner Rectangles as shown. (The
top and bottom rectangles will allow spar caps to pass through the ribs
while the middle rectangle will allow the shear web to pass through.)
Draw a pair of horizontal Construction Lines between the spar
placement line and the sides of the rectangle for each of the three
rectangles. Select each pair and add an Equal Relation between the two
lines to center each about the spar placement line.
Trim the lines (with the “Trim to closest” option) that are
shared edges between the shear web rectangle and the spar
cap rectangles.
For each of the spar cap rectangles, select the
edge of the rib and the point of intersection
for the left construction line and the left side
of the rectangle. Add a Coincident Relation.
Dimension the shear web thickness and the height and width of the spar
caps. The model created from these ribs used 1/8x1/4 inch cross-section
spar caps with 1/16 inch thick shear webs. (Note that the spar cap height will
be defined from the bottom of the spar cap rectangle to the construction
line. Thus when the spar cap is set in this cut, its cross-section will be entirely
enclosed by the perimeter of the airfoil shape. The rectangle is made to
extend beyond the construction lines to remove the small sliver of material
above the spar cap. This also prevents an error from creating “Zero
Thickness Geometry”)
Create a C-beam cross section for the rear spar in a similar manner. The
Collinear Relation may be necessary to align the shear web relative to
the spar caps.
Exit the Sketch and perform an
Extruded Cut with the Through
All end condition.
Choose to keep all bodies. The single rib is now separated
into three segments. Right click on the airfoil curve and
select Hide.
The leading edge rib segment must be cut to fit a
1/8 inch diameter leading edge dowel rod and
1/16 inch thick sheeting. On the leading edge rib
segment, insert a sketch on the face shown.
Select the leading edge and use the Offset Entities
Sketch Tool as shown. Use the reverse option and
use 1/16 inch for the offset.
Create a horizontal construction line through the
origin. Use the Mirror Entities Sketch Tool to
mirror the offset curve about the horizontal
construction line.
Draw a circle with 1/8 inch diameter. Make the center of the
circle coincident to the horizontal construction line. Select the
circle and the offset curve to add a Tangent Relation as shown.
Draw a horizontal line, tangent to the circle, from the top of
the circle to the top offset curve. Mirror this tangent line
about the construction line or simply draw one at the bottom.
Select the bottom offset curve, right click on the
existing Symmetric Relation, and delete the relation.
(Otherwise there would be undesired results when
the curves and circle are trimmed.) Starting toward
the leading edge trim the portions of the curves and
the circle that lie between the horizontal lines. The
sketch should be left looking line the sample image.
Draw lines to create a closed section as shown in
the image. Exit the Sketch and perform an
Extruded Cut Through All.
The trailing edge segment of the rib must be cut to fit a
trailing edge wedge and 1/16 inch sheeting. The trailing
edge wedge used for the physical models has a trapezoidal
cross-section with bases of 1/16 inch and 1/8 inch and a
height of 1/2 inch. Insert a sketch on the trailing edge rib
segment. For reference, sketch the cross section of the
trailing edge wedge using construction lines for all but the
1/8 inch base. Make the endpoints of the 1/16 inch base
of the trapezoid coincident with the edges of the rib.
Just as with the leading edge rib
segment, use Offset Entities to
create curves offset by 1/16 inch
from the rib edges.
Connect the endpoints of the
1/8 inch base to the offset
curves with horizontal lines.
Starting at the trailing edge
trim segments of the curves
until the sketch looks as
shown.
Optionally, a small spike can be added as shown.
The models that were built in this example used
1/16 inch birch plywood for ribs while the trailing
edge wedge was balsa. The spike held the balsa in
place while glues were applied. The spike must be
small since it will create a stress concentration in
the wedge and can cause cracks in the balsa
wedge.
As with the leading edge segment, draw
lines to create a closed section and use
and Extruded Cut Through All.
Unlike the leading and trailing edge segments, the central rib segment will use stringers rather than
sheeting. The stringers used have 1/8 x 1/8 inch cross section. Since the stringers function to support
the skin and to maintain the aerodynamic shape of the wing, the placement of the stringer notches will
be driven largely by the curvature. Stringers should be placed closer in areas of greater curvature and
can be placed further apart where the curvature is less. A stringer should always be located at any local
extremum. For simplicity this example will use four stringers on each of the upper and lower surface.
Insert a sketch on the central rib segment.
Place a vertical construction line with
endpoints coincident on the top and
bottom edges of the rib. Select the top
edge and the construction line and add a
Perpendicular relation. Repeat with the
bottom edge. For a symmetrical airfoil,
this is the location of maximum thickness
and there exists a local extremum on each
of the upper and lower surfaces.
Draw a rectangle for a stringer notch and use
horizontal construction lines to center it about
the vertical construction lines. As with the spar
cap cuts made earlier, set the endpoint of one of
the horizontal constructions lines to be
coincident with the rib edge and use this line to
define the stringer height.
To place the remaining
stringer notches, first draw
horizontal constructions lines
as shown. There should be
two equal lines between the
front spar cut and the vertical
construction line and three
between the vertical line and
the rear spar cut. The lines should all be collinear. The most forward and the most aft lines should
intersect the midpoints of the forward and aft rib edges. (Use a Midpoint Relation between the edge
and the endpoint that lies on that edge.)
Use construction lines and Midpoint Relations to center each stringer notch with
the intersection points of the horizontal construction lines. Draw and dimension
the edges for the 1/8x1/8 inch cross section the edges of the square are not
aligned with horizontal and vertical axes.
The top edge of each stringer should be made to lie as
nearly along the airfoil curve as possible. To accomplish
this first add a Coincident relation between either of the
top corners of the stringer notch and the curved rib edge.
Then add a Perpendicular relation between either the left
or the right edge of the stringer notch and the curved rib
edge. As with the notches for spar caps, extend the sketch
above the defined cross-section to remove any small
material slivers.
Place each of the top surface stringer
notches using the above procedure.
Mirror the notches about the one of
the horizontal construction lines.
Exit the Sketch and perform and
Extruded Cut Through All.
The rib design is now functional in the rib will hold the structural components in their proper locations
relative to the spars and carry the loads from the various components to the spars. There is still
however a substantial amount of material (and therefore weight) that is unnecessary to carrying the
loads to the spars.
Determining how much material to remove and from where can be a very in depth process. For this
example, lightening holes will be placed somewhat arbitrarily. In general, the primary considerations
when adding the lightening holes are to avoid creating large stress concentrations and to avoid
amplifying the existing stress concentrations. The lightening holes shown are therefore somewhat
conservative.
Use an Extruded Cut Through All on
each rib segment to create the
lightening holes. Use Ellipse and
Circle Sketch Entities to avoid
creating new large stress
concentrations and use
construction lines to space them
away from corners and edges
where stress concentrations exist.
For the purpose of this example, the wing rib model is now complete. The rib design can now be
prepared to be cut for a physical wing model.