I doubt there is anyone on this planet that would dare say they do not like Nerf Guns. It’s hard to explain why it’s so much fun to successfully hit another object with a projectile. It’s too bad that Nerf darts aren’t the most accurate. Well, that won’t be the case for long. Keep following along.
Here at GoEngineer, we take silly little questions and create big engineering challenges. Every. Single. Day. My story begins with this question:
So why are Nerf darts so inaccurate?
I choose to believe this is due to pressure irregularities caused by a blunt object moving space with no spin. Time to prove me wrong. The first thing that comes to mind is modifying a Nerf gun to give a spin to the dart but this would take some serious planning and eventually become a real engineering job so I chose to modify the dart instead.
Will changing the shape of the head to have vanes and transfer some linear motion to rotational motion give me more accuracy? I may be losing some forward velocity but I can make up for it with stronger springs. We’ll need to study how much Drag and Rotational Force can be obtained from multiple vanes.
I made a few iterations of the Nerf head to test in SOLIDWORKS Flow Simulation, compare results and see what configuration might be best. I stuck to one angle of attack and checked the results for more vanes. Afterall, more surface area = more normal and force = more spin. I do have to be careful about drag and I must test if more vanes cause a discernible amount of drag.
These are the two contenders:
The dart heads are hollow to match the weight to the original dart head. It may affect the inertia but I will consider it a neglectable detail for this test. I may be wrong to do so but the interest is spin vs drag right now.
The average speed of a Nerf dart is around 16 m/s. I run an external Flow study with Air at ambient temperature to find out the output Torque on the Z axis (spin) and Force in the Z direction (drag) for both configurations. I used Standard SOLIDWORKS Materials, ABS for the head and Flexible Polyurethane Foam for the body.
A local mesh was added to the Flow study to refine meshing to level 2 at the Fluid/Solid Boundary around the complex geometry of the head.
The output Torque around the Z axis was obtained by using Surface Goal Torque (Z) with all the outer faces of the dart.
The Drag force was obtained with Surface Goal Force (Z) for the same faces used in the Torque Surface Goal.
The global Drag force Global Goal Force (Z) will be used as convergence criteria only.
You can download the Solidworks 2017 Flow simulation file here (Note: you must have Flow Simulation to view the study):
The negative Drag force corresponds to the direction of drag force correctly, as does the negative torque in respect to the right hand rule. According to the results the drag in the Z direction is 0.0119 N for 3 vanes and 0.0125 N for 4 vanes. This means 3 vanes give 4.8% less drag than 4 vanes.
The output Torque around the Z axis for 3 vanes is 0.0250 Nmm and 0.0461 Nmm for 4 vanes. This means 3 vanes give 45.7% less Torque than 4 vanes.
So 3 vanes give 4.8% less drag but 45.7% less Torque?!! I’m sticking to 4 vanes.
I am starting to regret not doing a 5 vane study. It may have added some serious Torque but it does seem it could be a bit too crowded for the print.
I printed these heads using a Stratasys Fortus 250M with ABS P430 and 0.007 layer thickness, layers were parallel to the flat side of the head. The print wasn’t too bad but I did have to file some areas at the vane tips for a satisfactory finish.
To mount the head I had to cut off the rubber off the dart and carefully cut away the remaining stem piece. The stem has a tendency to come off with some of the dart’s foam, this may contribute to disbalancing the spin. I’m going to consider it negligible. The “negligibles” are starting to pile up…
After a bit of super glue, the darts pass an airtight test and they are ready for action!!!
I was able to get a visual cue of the rotation obtained with the help of a high-speed camera we had laying around.
Just look at that spin! That was a lot better than I expected.
After shooting some of these darts I realized some flew consistently straight while others consistently wobbled off. This may be due to the surgery process to exchange heads, I strongly believe this is linked to that “negligible” dart foam. You can see a wobble pretty clearly on the second take.
Personally, I think the spin has improved the accuracy significantly as long as I use the good darts. I may need to put some time in refining the manufacturing process to get more good darts but overall, I would call this test a success!