This page, like many others, is incomplete. Little editing has been done, but the minimal content has been written. Tuning tips for specific devices will be added and expanded on soon. Details for tuning whippers will be added soon. Please fix any errors and typos if you seen them.
A great alternate source for tuning information is Ripcord's Site. Specifically see the "Tuning" page and the "Stuff" page.
Sub pages
The Tuning Process
The first step in the tuning process, which often takes place before design, is determining the optimum tuning. This is often mixed with tuning the design before construction to limit the needed adjustments after construction. Tuning the design through tuning simulators is often useful.
The Optimum Tuning
The optimum tuning is goal worked toward throughout the rest of the tuning process, and usually involves getting the maximum possible energy into the projectile is the best possible angle. To do this as much kinetic and potential energy must be removed from the device as possible. If all the energy were this would be the optimum tuning and design (for the amount of energy), and then the projectile is released no parts would be moving (no kinetic energy left), or would start moving (no potential energy left). This can be impossible to achieve, but the goal is still to get as close to possible to it.
Counterweight Stall
The counterweight stall is the a trebuchets counterweight stops during the throw. Sometimes there is no stall. Optimally there should be a CW stall, and it should occur when the CW is at its lowest (potential energy is all used). If the Counterweight stalls, its kinetic energy has been transfered to the arm. If it happens at the lowest point then there is no other energy available from the counterweight.
Arm Stall
When the arm stalls, or stops, it has no kinetic energy. The goal with the arm stall is to have it occur as the right time which is when the potential energy has all been release into it. This means in the case of a trebuchet the arm stall would optimally occur when the CW is at its lowest point, at the same time as the counterweight stall. In the case of all hinged counterweight trebuchets this means the arm and hanger need to be aligned vertically and completely stopped (stalled) at release for mathematically optimal performance.
The Sling
Once an optimal angle is calculated for the sling at release (45 degrees backwards roughly, see sling length below), the sling length needs to be adjusted so it gets there at the right time (universal stall) and the pin adjusted so it releases there.
Energy Use
As stated before, optimally all energy should be used. Removal of kinetic energy has been discussed, but removal of potential energy for non trebuchets has not. Removing all possible potential energy from torsion devices means that the arm has finished rotating.
Making Adjustments
The tuning process consists of making adjustments to the following factors to progress toward the optimal tuning, or release conditions of having used all the energy as descried above. This is usually done by observing a throw and determining what about the throw could be improved, then relating the desired improvements to possible changes. Of the possible changes usally only ones designed to be adjusted are considered. Once the correct change is determined a guess or calculation is made about the amount of change needed, and it is applied to the machine. It is often very hard to observe what is non-optimal because the throw happens so quickly so a video camera is often used. On site frame by frame analysis can be done with most modern digital cameras at can usually determine what is wrong with some good interpretation, however smaller devices have shorter and proportionally faster movements which can be a problem especially with torsion devices.
Sling Length
The projectile when released from the sling (assuming the arm tip is stopped) will travel at an angle perpendicular to the sling. Without aerodynamic drag, the optimum angle for the projectile would always be 45 degrees, but with drag it is slightly lower. Longer slings will throw lower and shorter sling will throw higher assuming release is not caused by the arm stopping. If the case where the arm stall causes release the sling needs to be shortened to release sooner.
At Rest Sling Position
The sling can hang vertically or rest in a trough or on the ground. If resting on the ground, a combination of sling length and arm top height usually set the angle of the sling, but in some cases it can be adjusted. Currently the effects are not well documented.
Sling roll and Spin
The distance between the sling two sling attachments to the arm causes sling roll adding spin to throws causing lift through the magnus effect. Too much sling roll can cause the projectile to fall out of the pouch or add enough spin to make it explode. Adding spin takes energy, so it may be harmful to range. The benefits of this spin may out weigh the costs sometimes, but it is hard to tell. It is probably safe to assume if your onager is not stalling, which is usual, that adding this spin will come at not very much of a cost.
Firing Pin Angle
The pin angle is probably the simplest. Bent it more forward to delay release and throw lower, and backward to release sooner and throw higher. Pin angle is usually measured as the displacement from inline with the arm where forward is in the throwing direction with the pin end of the arm pointing up.
Trebuchet Specific
Arm Ratio
The arm ratio is the ratio of the length of the long arm, to the length of the short arm. Generally if the trebuchet passes through the optimum stall position, but the counterweight and arm do not stall the arm ratio is too low. If the arm stalls too early it may sometimes be caused but too high of arm ratio. Many other factors effect arm stalls so they muct be accounted for before accessing the arm ratio. A general rule, known as Phssthpok's Rule, for arm ratios is that they should be roughly 1/20 of the mass ratio.1 In the traditional setup an increased arm ratio results in less counterweight drop, however several of the newer designs overcome this problem, including the whipper which gains drop distance as the arm ratio increases, and the King Arthur which always has the same drop length.
Mass Ratio
The mass ratio is the ratio of the counterweight mass to the mass of the projectile. Phssthpok's Rule (see arm ratio) can also be applied to calculate a rough mass ratio given an arm ratio, but the mass ratio is usually on the order of 100:1. With much higher mass ratios it often becomes impossible to build an efficient trebuchet, and therefor the rule of diminishing returns applies to counterweight mass with relation to range.
Hanger Length
Hanger length is the distance from the hanger axle in the short arm, to the center of mass of the counterweight. Fixed counterweight trebuchets effectively have a hanger length of zero. For a standard hinged counterweight trebuchet the hanger length adjusts the stall point, where a longer hanger causes a later more vertical stall. This means that a standard HCW should usually have the longest hanger possible. For whippers and King Arthurs having the maximum hanger length is even more important because the hanger length adds to the counterweight drop distance, and therefor total energy.
Cocking Angle
For most trebuchet designs the higher the cocking angle (more rotation added in the cocking process) the better because it adds more energy. The arm length usually limits the cocking angle, but in the case of KAs Whippers and some F-series designs the cocking angle is limited by other factors.
Prop Angle
The prop angle refers to the angle between the short arm and the hanger in HCW trebuchets if the counterweight is propped (not hanging freely). There are a few methods of propping
that are different. First a rigid prop sets a minimum angle between the hanger and short arm. Rigid props increase the counterweight drop but effect stall timing. A rigid prop usually causes the stall to happen earlier, and thus being less efficient, but the extra energy can make up for it. There is also a free swinging released prop such as used in the King Arthur design. It's effects vary greatly depending on how it is used.
King Arthurs
The optimum release for a King Arthur is quite achievable, especially if it has a floating axle or wheels and a light frame. This optimum tuning is a release with arm and hanger aligned vertically and stopped with the sling releasing at the optimum angle (see sling length on this page). In the case where the axle or frame can roll, it should also be stopped. King Arthur trebuchets are usually tuned through the adjustment of three things, sling length, pin angle, and secondary trigger timing. Making the arm ratio adjustable can easily be done through multiple hanger axle holes in the hanger and short arm. The adjustment process is as follows;
- adjust the secondary trigger so the counterweight stall is in the correct location. A shorter line causes an earlier secondary release and a more forward stall. If the arm ratio is too far off a stall may not be achievable at the time of arm and hanger alignment. In this case adjust the arm ratio is possible. It the arm and counterweight continue to rotate right through the alignment a higher arm ratio is needed.
- tune the sling length to get the proper angle at stall and alignment. This may require adjusting the pin forward. See sling length for details. If this messes up the tuning of step one, return to step one.
- adjust the pin to release at the stall.
Torsion Specific
Arm Length
Many torsion devices have arms that are too short. Optimal arm length is hard to determine, but if projectile mass does not effect performance the bundle is likely limited by rope recovery speed and can not turn faster. One solution to this is a longer arm. Another solution may be to remove some of the rope in the bundle if it is not near its rated load. Your arm is also too short if there is no arm stall; however, arm stall can be very hard to see on torsion devices; Video footage is usually required. If the arm is over stalling and getting pulled backwards, or stalling much before the arm stop, the arm may be too long, but this is rarely seen. Another possible solution to this could be to increase the arm mass.
Arm Diameter in Bundle
This effects some of the leverage profiles. It has not been tested much but generally smaller provides more speed when speed is limited by rope recovery rates, and larger provides more torque.
Angle of Arm Rotation
The angle or arm rotation is usually near 90 degrees however larger angles of rotation have been tried with success. Some problem with more rotation include frame orientation, sling position, tuning and construction. More rotation also causes torque to drop off more at then which can be countered with longer bundles.
Onagers
Rope Bundle
Tension
The bundle is usually not tuned, but its tension is often changed. A higher bundle tension and therefor torque causes a faster arm rotation, usually resulting in lower throws. If this causes throws to be too low the firing pin can be adjusted, but more range can be gained by lengthening the sling and possibly the arm.
Length
Longer bundles provide for a more consistent acceleration curve.
Material
Best choices are synthetics, which have varying properties. Different bundles have different needs depending on length diameter and size. Popular choices are polyester and nylon.