FEA Study of Ribs/Spar

Task Description

Replicate https://semarakilmu.com.my/journals/index.php/CFD_Letters/article/view/113/62 to determine the internal stresses on the ribs and determine if design changes need to be made. Main area of concern will be the square cutout for the spar but it will also be interesting to see how the stress is around the elliptical holes.

Constraints

Relevant Contacts

Assignees

Task Progression/Updates

Author: @Sohee Yoon Date: 2024/09/15

Ansys Analysis

Setup for the simulation:

• Before downloading the Solidworks Assembly as a STL file, I had to remove the airfoil skin to better replicate the study

• Materials had to be assigned to the individual components; ABS for LE and TE covers, Aluminium for the wingspar, and Balsa wood for the ribs based off of these properties American Balsa Wood

• The research study runs a Static Structural Analysis

• There is a mesh error which seems to be caused by the TE covers, hence for this FEA study it is removed

• The forces acting on each rib was calculated based on how the study calculated their load

  • Assume 10 kg drone: 10 kg x 9.81 m/s = 98.1 N (over two wings) → 98.1 N / 2 = 49.05 N (over one wing) → 49.05 N / 8 (ribs) = 6.131 N per rib

• The fixed point and node points are similar to the study. Two types of simulations were run:

  1. Force on the node points

  2. Force on the bottom face of the ribs

Open

(a) Full Wing View

Open

(a) Zoomed-in First Rib

Open

(a) Zoomed-in Behind the First Rib

Ansys Study Conclusion:

• (b) could not be simulated because of errors related to the spar ~ will look into that in the future

• Unlike the research study, there wasn’t a lot of stress near the end of the spar (the part attached to the drone)

• There were no stresses along the other cut outs except for the spar cut out

• The main areas of stress lie along the corners of the spar cut out and the LE spacer cut out

Solidworks Analysis

Setup for the simulation:

• Similar to the Ansys setup, however, the two types of simulations that were run:

  1. Force on the top face of the ribs

  2. Force on the bottom face of the ribs

• The wing spar had to be fixed in place to prevent any bending or movement in the simulation

• Note: the TE covers are present in this simulation

Open

(a) Full Wing View

Open

(a) Zoomed-in First Rib

Open

(a) Zoomed-in Behind the First Rib

Open

(b) Full Wing View

Open

(b) Zoomed-in First Rib

Open

(b) Zoomed-in Behind the First Rib

Solidworks Study Conclusion:

• Surprisingly the Ansys and Solidworks studies visually showed different results but this may be caused by the material properties, the fixed points, and location of forces.

• The Solidworks study resembled the original research article the closest as the other cut outs had stresses along the edges.

• There is barely any stress visible near the LE and TE cut outs/spacers

• (a) and (b) deemed similar results, slight difference in the amount of stress at the top/bottom but almost identical

• Both the Solidworks and Ansys studies display high stress concentrations near the corners of the cut outs, especially for the wing spar cut out

Final Conclusion from this study:

• Because Balsa wood doesn’t have a defined yield point (yield strength), the modulus of elasticity is used to determine the point of fracture, 3.0 GPa.

• Based on the Ansys results, there is concern near the corners of the spar cut out as the colours indicate a greater stress value than the modulus of elasticity, but the Solidworks results point in the opposite direction where no point goes over the limit.

  • Future steps → fillet the corners of the spar cut out and run the simulation on Ansys to see any changes

  • Possibly run the Solidworks simulation more closely or similar to Ansys to have obtain a better comparison. Ex: Fix the point instead of the whole spar, try applying force on node points instead of the surface

Author: @Nathaniel Li Date: Sep 16, 2024

Sims Feedback

• This looks great, exactly what we were looking for!

• Seems like our predictions were correct regarding the corners of the spars in both SW and Ansys sims → main focus for us will be to decrease this

• Continue with Ansys sims and attempt SW sim if time permits

Next Steps

1. Changes to ribs:

   1. Fillets on the spar corners → Adding a fillet in the ribs is ideal for force distribution but we also need to think about how the spar would fit into the new shape/lock in place

   2. An idea I had would be to make a 3d print that fits onto the spar and then slides into the rib? not sure if this makes sense but in my head it seems like the spar needs to a rectangular cutout to slot into for good fit

2. Future sims:

   1. Run the sim again with the same parameters to see changes

   2. Simulate maximum AOA/lift scenario?

      1. This is to see what the limits of our design would be

      2. Will need to determine the lift force and angle it correctly so more work

Author: @Nathaniel Li Date: Oct 1, 2024

New Sim Conditions

• I will try to run the sim (in ANSYS) with the updated rib with the following new conditions found

• @Sohee Yoonto try in SW

• Ran a some calculations using fixed wing calculator to determine lift forces instead of following the study:

  • Hope this gets more realistic/related numbers for our use

  • Takeoff Lift Force 159.8447 N

    Cruising Lift Force 173.5784 N

    Max Lift Force 200.3639 N

  • Takeoff lift < cruising lift because of velocities chosen

  • Max lift force is assuming max CL at cruise speed

    • ie. cruising and attempting max climb rate

• For a “worst” case scenario, will use max lift force of 200 N

• The forces acting on each rib was calculated:

  • 200 N (over two wings) → 200 N / 2 = 100 N (over one wing) → 100 N / 8 (ribs) = 12.5 N per rib

Author: @Sohee Yoon Date: Oct 5, 2024

Solidworks Analysis #2 with New Sim Conditions:

Before the simulation:

• Previously, when I ran the simulation I did not set the von Mises stress scale but this time I set the Max to be the yield strength of balsa wood provided by Solidworks material property (2e+7 Pa). Hence, anything red means that the balsa wood will fracture.

  • I verified the value was accurate since wood doesn’t really have a defined yield strength and this source (American Balsa Wood) states that the Modulus of Rupture is 2.16e+7 Pa (which is quite close to the value Solidworks provided)

• The setup of the analysis is basically the same as before, note that the shape of the ribs have been adjusted slightly.

Three Situations were simulated based off of Nathaniel’s calculations above:

1. Takeoff Lift Force: 159.8447 N → 79.92235 N (Total over one wing)

2. Cruising Lift Force: 173.5784 N → 86.7892 N (Total over one wing)

3. Max Lift Force: 200.3639 N → 100 N (Total over one wing)

4. Miscellaneous Tests: 200 N and 300 N

* All situations have the force placed at the Top (a) or Bottom (b) face of the ribs

Conclusion from this study:

• Takeoff, Cruising, and Max lift forces pass the simulation as none of them resulted in red spots!

  • Why is this different from the last time we simulated on Solidworks? This is because I manually set the max value on the von Mises stress scale to relate to the Balsa wood property. Before, it was automatically set by Solidworks to around ~5.5e+5 (less than 2e+7), therefore it was inconclusive whether the red parts meant the ribs would fracture.

• When the force is placed on the bottom, it results in slightly more stress than when placing the force on the top of the ribs (this was concluded by comparing the Max stress points). But because of how small the difference is we can assume they’re (force on top and bottom) identical.

• Since the Max lift force passed the simulation, I tried running a few tests with greater forces (200 N and 300 N) to see where the greatest amount of stress appear

  • Based on the simulation, it looked like the middle cutout experienced the most amount of stress, specifically near its corners. This is unlike our previous simulation, where there was a dense amount of stress on the spar cutouts.

    • Possible reason: The bottom half of the new rib shape is closer to the chord line, hence there isn’t a lot of support/material on the bottom which might be causing more stress towards the middle of the cutouts

    • Possible reason: The way the LE and TE supports are assembled might be supporting the front and back of the ribs more than before