Readying and Firing the Second Onager

[ChromeFang]

Cool

[lukassnakeman]

man fuck onagers, trebuchet is the only bay 2 go

[TonyAirlines]

I'm building one of these for a physics project. What kind of cord should I use for the Torsion Spring?

[imgunby]

I've had excellent success using 25lb fishing line for these engines, but it depends on what size skein you need to make, and how strong your frame and arm are. At full power, this engine can throw the hex nut projectile somewhere around 75 feet. Good luck on your project.

[TonyAirlines]

It's just a tad bigger than the one shown in the video. We're supposed to have a wooden arm, and I'm not using balsa wood. Same as yours, minus the wind-up system you have. (Ours will have 2 hooks attached to each other at the end.)

[imgunby]

Go for the fishing line then, and I'd suggest 15-20 windings for the skein. Good call on not using balsa... it would fail the first time you tried to use the engine. I've got a bunch of pics up on flickr showing various parts of the engines in close-up. As far as the arm goes, you might want to glue thin strips of wood together, then shape the arm from that; It will be under a lot of strain when it hits the upper cross-piece :-)

[TonyAirlines]

Good idea. 2 questions about the arm: How would I go about attaching it to the tied-up fishing line and 2: Our teacher said we need some sort of holding dish for the projectile, as opposed to your sling you have for yours. I'd imagine I can simply drill through the arm near the top and place some sort of projectile-holding thingy there.

[imgunby]

The arm doesn't get attached to the skein, rather it is just held in place by the tension of the skein passing on either side. I generally neck-down the arm where the skein passes over it to help prevent it from sliding out or otherwise shifting around too much, but as long as the tension is even, it isn't much of an isssue. As to the projectile, I don't really like the cup as it doesn't generate anywhere near the distance as with a sling and pouch, and is more medieval than ancient.

[golfer15963]

your awsome!!!!!!!!!!

[TheDarkGuardian]

What does making the arm longer do? Does it allow the projectile to shoot a higher target, or does it make it hit the target harder?

Please explain. Thanks.

[imgunby]

The longer arm results in a higher speed at the tip when firing, which results in more energy being transferred to the projectile, which ultimately results in shooting further and with more force. This particular engine, at full power, was able to shoot the 1/4 inch hex nut projectile nearly 75 feet, but I'll have to try the one with the shorter arm to see how much affect the length has.

[harrygamer]

did it break or something?

[imgunby]

No, the "thwack" you can hear is the projectile bouncing off the wall. The padding used on the cross-piece is four layers of heavy canvas under three layers of leather, so at lower powers, the arm doesn't really make any noise when it strikes.

[harrygamer]

wow thats not i mean. i mean the end of the stick

[imgunby]

No, nothing broke. The throwing arm is 5 ply's of one sixteenth inch thick walnut glued together, then wound with wood glue soaked linen bookbinders thread. I believe the twenty five pound fishing line used in the skein will start breaking strands before the arm is under enough tension to get damaged during firing.