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Hanging Arm Trebuchet


This trebuchet is portable and very easy to make. There are no hinges or tight tolerances.

The two main parts are a tripod and the swing arm. The tripod is three poles tied together at the apex. The swing arm is tipped with the weight and sling. A rope is tied to the apex and swing arm tips.

It's portable - you can bundle together the tripod legs, along with the swing arm.

To launch, you plant the swing arm slightly into the ground and let it fall toward the tripod. The rope arrests the arm's fall, causing the sling to swing upward.

That's pretty much it. I think this design could be efficient, with barely any friction losses and low final weight velocity. The real kicker is ease of construction, though.

Oh, I think this design follows principles of tensegrity, albeit dynamic rather than static.

#gizmo
Very elegant design. A pleasure just to sit and imagine it working 😀
I've seen some slightly similar fall and flip designs that work really well.
@Brie Mmm Thanks so much! I was inspired by the Walking Arm Trebuchet design, but I thought there must be a way to make it even easier to make.

I kept thinking about having an arm fall onto a short see-saw pivot, but this seemed kind of iffy ... I mean, that's going to put a lot of stress onto the impact point, so the arm will break, right?

But yanking upward from above feels more robust to me. The way I show it tied together, there's no bending impact in the middle of the arm. Like I noted ... tensegrity principles.
Still, for a small scale version, just tying the rope to the arm near the weight should be fine.
@[email protected] I'm curious what's out there. I was inspired by the Walking Arm Trebuchet design, which I noticed via Wikipedia article noting more modern treb designs.

But I know there are just oodles of unusual designs out there. I just have no idea how to search for fall and flip designs.

Obviously, my "hanging arm" design isn't a direct copy of the Walking Arm design, but it is meant to emulate its method. It's a remix, perhaps.
Yeah, my recollection of the video I saw was that it was a walking arm. I really like the compound ones where the pivot point moves during the throw. The dynamic analyses of these are really impressive.
@[email protected] If you find any examples of the moving pivot point, I'd really appreciate it. Like I said, I haven't been able to google search very well with this stuff.

My first thought was to have a series of vertical sheets to approximate a rounded "hill" that the arm rockers over. A rope attached to the weight and the rocker would ensure it rockers rather than sliding. So, instead of falling suddenly on just one pivot point, it rockers over a series of pivot points.

It would actually be easy enough to build. You just cut some rectangular boards with slots. A central "spine", with perpendicular pivots boards. With slots, they just slot into each other. No need for adhesives or brackets or anything. Just slot-slot fit.

But I was daunted by the immense number of variables involved.
AHA! The tripod can be replaced with just a single vertical pole. You drive that pole into the ground. You aim the fall of the swing arm off to the side of the pole.

This makes the hanging arm trebuchet even easier to make, although the off-vertical throw of the swing arm is a bit inelegant.
simulator with at least some documentation, that might aid in modeling this one:
http://www.virtualtrebuchet.com/
Here's a compound aka floating arm treb:
https://tjlaughlin.wordpress.com/portfolio/floating-arm-trebuchet/
some of the math involved in a good analysis of a FAT:
http://www.benchtophybrid.com/How_to_Simulate_a_Trebuchet_Part3.pdf
Last time I was reading a bunch about this I found a neat page about a very careful analysis of position and acceleration of the projectile during launch. Some of the FATs end up with almost two full rotations of the projectile around the axis before launch, and the acceleration is very nonlinear, at points being negative because the sling radius is changing with respect to the arm.
@[email protected] Thanks! I think it can be approximated by assuming a fixed pivot point at the hanging height, and a much larger weight ... but it's not perfect because the simulator assumes the weight starts at zero speed but this has the weight start off going downward pretty fast at that point.

The floating arm trebuchet is an interesting design but it seems a bit challenging to pull off - lots of sliding slots and tracks that need to be precise and rigid and aligned with each other.

My hanging arm trebuchet is actually moves similarly to the floating arm, but without the tracks and sliding.

Instead of vertical slots to guide the weight, the weight falls in an arc that ends nearly vertical at the end before the "action" starts. Simple speed and momentum keep it going along a nearly vertical track.

Instead of horizontal tracks to guide the pivot point, the support line restricts the pivot point to a nearly horizontal arc. Same basic theory, but much easier to implement and without roller friction.
It's a neat design and it'd be fun to make one at some point.
@[email protected] If you didn't notice, I posted a simplified version with just one vertical support pole here: https://pluspora.com/posts/827c78803e3801394179005056264835

It's also possible to configure a hanging arm trebuchet to look and work like a traditional treb - extremely similar to the floating arm trebuchet. Instead of falling from a vertical, the hanging arm starts off at an angle - hanging from the pivot. The tip of the arm is locked in place by a small cup in the ground (like a golf hole). Pulling on a "firing line" pulls the arm out of the hole, letting the tip swing up.

This makes the motion very similar to a floating arm trebuchet. The differences are:

1) The falling weight is not restricted to perfectly vertical motion (but this is not really necessary in the first place)

2) The pivot point swings in a mostly horizontal arc instead of travelling along horizontal tracks

Although actually ... this all goes back to the classic trebuchet design. Often, classic trebuchets were on wheels to let the entire thing roll forward/back during launch. This is geometrically the same as letting the pivot point move forward/back along raised horizontal tracks. Obviously, raised tracks reduces the mass that's moving forward/back.
Oh, here's an explanation of wheels on classic trebuchets:

https://www.pbs.org/wgbh/nova/lostempires/trebuchet/wheels.html

Not only do the wheels make the treb more durable, it adds power to the throw.
So there's an old folks home I walk past every now and again. A few blocks away there's a Cemetery. Today, I finally got out of the house with a friend. While driving by the two locations I got to thinking about the engineering behind building a solar powered trebuchet (inspired by this post) to deliver the deceased from the home to the cemetery, without using fossil fuels, and in the case of COVID decreasing the amount of handling and exposure for EMS, mortuarial service providers, etc.

I won't say y'all are a bad influence on me since I got that covered. But damn - sometimes my settings get stuck on "Monty Python" and just won't stop.
@Richard Healy Transporting bodies from one fixed point to another? You need a combination of a clothesline loop and a slip'n'slide. Pull the body with the wire loop, and use water to lubricate the body bag.
@Isaac Kuo There are several blocks of homes that have to be navigated around and right-of-way + eminent domain probably wouldn't cut it. Silently launching in the dead of night though...
This seems like blimp territory to me. A blimphearse would be amazing.
Blimphearse would totally be traveling in style.