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I am currently 3 years into my trebuchet. Little bit about the machine. Stands 10'6" to the pivot axle with an 11' throwing arm. Using a weight basket that is 6' tall and mounted 2' from the pivot axle and it currently has a static weight of 1900 pounds but go up to 5000 lbs. Sling is 10' long. My throwing arm is balanced almost perfectly so my cg of the arm is only about 1' on the long arm side in relation to the axle. I have been struggling to figure out the release angle. I use www.virtualtrebuchet.com and I should be around 33 degrees on the release. In my understanding, the release angle is when the arm is at a particular height that the pumpkin is released so it has that particular angle in relation to a horizontal plane. So how can I easily figure out the correct angle of my release pin to have the proper release angle for the pumpkin? Or do I just need to practice, practice, practice? Hope I'm not confusing with my question.

So as you say, the release angle is the angle your projectile should travel along relative to horizontal. Generally 35-40 degrees are the angles you want to shoot for, though there are differing schools of thought on that one. 33 degrees sounds a tad low but it can indeed work better depending on the configuration.

To get your machine to release at such an angle is really only something that can be found via trial-and-error. Video footage shot directly from the side is extremely helpful. For a hinged counterweight trebuchet (HCW - sounds like this is what you have), you ideally want the arm and hanger to both "stall" in the vertical position. Stall is seen only via video, and will show as several frames in a row where the object in mind does not move (much at least). As long as your arm isn't too heavy relative to your counterweight and projectile (sounds like you are OK on the last 2 at least, assuming this is a ~10-pound projectile we are talking about), a HCW arm will tend to stall fairly close to vertical. There are things that affect this, but it ought to be close.

Your sling wants to release at the arm stall, and be roughly 35-45 degrees behind the arm/above horizontal at that time (as a general rule of thumb). Tweaking your release pin angle will tweak the final trajectory but typically has only a minimal effect on the sling location at that time. To get your sling length right for the release timing: short slings will get into position earlier, long slings will take longer to get there. A sling roughly 80% of the length of the long arm (10'6" in your case I think?) is a good starting point, it sounds like you might be too long at 10' though without a video it can be hard to tell. Pin angle should start at 45 degrees (measured as the angle between the axis of the arm and the axis of the pin), and be adjusted accordingly per above.

Hopefully that helps some? Feel free to ask for clarification if something seems confusing! And feel free to post pictures/video (or links to such) as it can help give some better guidance (the above are general rules of thumb).

That's a pretty big machine to be learning on. But release angle is governed by two factors: the position of the arm and the angle of the pin in relation to the arm. Add the angle of the sling to that.

OK, what we try to do is to get the sling at right angles to the desired launch angle at the desired arm position and adjust the pin angle so that release occurs at that point. Think of the sling as a pendulum that's attached to the beam tip. You adjust sling length to speed its rotation up (shorter) or slow it down (longer). Arm speed and degrees the arm rotates from triggering to sling release vary from one machine to the next so sling length varies, too. Arm speed is largely governed by the mass of the arm and payload and the weight of the counterweight: more CW or less projectile weight or a lighter arm give more speed and require a shorter sling. And if the arm is slow the sling needs to be longer.

Just a note on your particular treb: from the given dimensions it looks as if your arm will be cocked nearly vertical. I use this configuration a lot and find that a longer sling works best, something like 115 % of long arm length.

Thomas wrote:Just a note on your particular treb: from the given dimensions it looks as if your arm will be cocked nearly vertical. I use this configuration a lot and find that a longer sling works best, something like 115 % of long arm length.

I guess that depends on how much clearance is under the axle, but you make a good point.

A picture would be a huge help!

Sounds like a cool project! Pictures would be great.

Tried posting a photo, Matt, got a message "It was not possible to determine the dimensions of the image". Whatever that means.

Thomas wrote:Tried posting a photo, Matt, got a message "It was not possible to determine the dimensions of the image". Whatever that means.

That's odd. I assume its a normal image file (JPG, PNG, GIF, etc), which normally work fine. Can you try emailing it to me and I'll do some tests to see if it is a server or config issue? info @ this site's name DOT com.

I may just be doing it wrong. Long time since I posted pics to one of these forums.