Way over-engineered pullcopter v3

@ScottA_1512411 - where did those measurements come from? A 56x29 hand seems more like a baby still in the womb, not an 8-year old. I mean, think about it, 29 mm is just over 1 inch in width. I found that a good child-size handle uses the measurements for an average adult female, 171x76.

I have to say that I am pleased that my design managed to accommodate such a tiny handle, and even more surprised that you printed it after seeing it generated. I sure didn't ever check what happens with those parameters.

TPU propellers should work fine if you use a shore hardness above 95a. Even then, the prop blades are thin and would likely still be flexible, so it might require a smooth slow pull to launch it.

Personally I'd worry less about the dog and more about roofs and trees. I have always lost a propeller every time I try to fly this thing.

@Anachronist Definitely good points. This is the site where I found what is supposed to be average hand sizes: https://www.medicalnewstoday.com/articles/average-hand-size#children. I decided to use the difference between your men's and women's size and subtract that again for a child's size. I think it'll be close enough.

I have printed one propeller out of TPU, but it's a toroidal blade I designed using the Ultimate Toroidal Propeller Generator at https://github.com/RaulBejarano/Ultimate-Toroidal-Propeller-Generator. It's a little too light and flimsy, and it doesn't fly nearly as far as your design, which flies like a bat out of hell. (edited)

@ScottA_1512411 - the table in that article has the wrong units. Those numbers are inches, not centimeters. I mean come on, 2.5 cm hand width? That's less than 1 inch! No kid has hands that small. In inches, those numbers make more sense. In fact, look at the original study linked in that article. It shows the correct hand size values in cm. That medicalnewstoday article converted them to inches but neglected to label their table as inches.

I wrote a blog article about hand sizes that might be of interest, here: https://www.nablu.com/2022/03/whose-hands-are-biggest-you-may-be.html

Try printing mine out of TPU. There is a parameter where you can add more blades. Also my blades get thicker toward the center, which may help with stiffness. I wouldn't expect a toroidal prop to perform well, though, They have a lot of drag because there's a region around the tips that contributes more drag than lift. (edited)

@Anachronist I've given up on printing your design from TPU. It's a great design, but I just can't get it to print well. I have a couple of them, and they do fly, but they look terrible. I've printed some of them from PLA Tough, and they work very well. I've also made some 4 bladed toroidal ones from both TPU and PLA (not 3 blades like before), and they work very well. Not surprisingly, the PLA ones perform better.

I'd be happy to share the toroidal one if you'd like. I'm very interested in your comment about the drag since I've had trouble finding any information on the web about the difference in characteristics of the toroidal propellers. (edited)

@ScottA_1512411 - Nice. I saw the remix you uploaded.

My comment about drag is based on what I know about the physics, not any formal analysis. I see two things that increase the drag:

1. The toroidal propellers I have seen don't use an airfoil anywhere in their cross section, but rather the cross section is all flat surfaces. An airfoil shape encourages laminar flow. A flat surface providing lift due to an attack angle causes flow separation at the leading edge, resulting in turbulence along the top surface rather than laminar flow. Any time you induce turbulence, you increase drag.

2. Also, the outer curves contribute almost nothing to lift but they are the fastest-moving parts of the propeller beating the air around, expending energy (more drag) for minimal benefit. With my propeller design, the circular ring also contributes nothing to lift but its shape is such that it has negligible effect on the air.

I've been wanting to design a more "proper" toroidal propeller with an actual airfoil but the transitions would be interesting, transitioning from asymmetric convex at the root to concave cambered mid-span, transitioning to convex symmetric at the outer curves and then back to concave cambered as the curve goes back inward. I'd also need to distort the shape of each ring so that the outer curve disturbs the air less. (edited)

@Anachronist That's extremely interesting. I've learned more about the design of airfoils from reading through your descriptions than anywhere else. In the comments for the Ultimate Toroidal Propeller Generator, I saw that one of the creators was experimenting with an airfoil design for the toroid, but that comment is rather old, and I fear the project may not be being updated anymore.

I noticed on the toroidal design, that it stops spinning and reverses as it falls, and I was wondering if adding an inertial ring would work there, or just mess up the aerodynamics. The toroidal propeller generator also has an offset that I couldn't understand the use of, but it might be intended to reduce drag at the tips.

@ScottA_1512411 - the inertial ring is a balancing act between spinning longer and more weight. I suspect also that maximum flight duration may not necessarily equate to maximum altitude. There are so many variables that can be adjusted with that propeller, experimentally trying to find the optimal combination would involve an unwieldy huge test matrix. The design I ended up with was the result of testing many different configurations, but I have no idea how close it is to being optimal. All I know is that it performs really well.

The blade_hole_offset parameter in that toroidal propeller script just positions the hollow part of the blade inward or outward. Each propeller blade is made by first twisting a solid cylinder around the propeller axis, and then cutting a hole into it by subtracting a smaller twisted cylinder (this is why the cross section isn't an airfoil, it's due to the way it's constructed). You can position the hole inward or outward to make the outer edge of the blade thicker or thinner. Too far outward and the outer edge disappears completely. I don't believe this would affect drag, but it does redistribute the mass. More mass near the center makes the blade stand up better to torque stress from launching, but more mass away from the center gives the propeller more inertia.

@Anachronist I suspected as much. If the blade I made was more than 2mm thick that offset might be of some use, but as it is, it just makes the tips, or the entire blade, disappear.