Tuning

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 two most common ways to tune a machine after it is built is to adjust the sling length and the pin angle. The pin angle is adjusted to change the initial trajectory angle of the projectile and the sling is used to adjust when in the motion of the arm the projectile reaches the proper trajectory.

The Optimum Tuning

The optimum tuning is a goal worked toward throughout machine operation and involves converting the maximum possible potential energy from the power source into kinetic energy of the projectile at the best possible release angle. The kinetic energy of your power source is lost in three ways, friction, in leaving it in the motion of the machine, and sending the projectile the wrong direction. Tuning is typically in reference to the latter two sources of loss, friction typically being taken care during construction and through routine maintenance.

Kinetic energy that is left in a machine is easy to spot. Everything that is moving immediately after the projectile is released is lost energy. A throwing are continuing to rotate, a swinging counterweight, the rocking of a frame, these are all examples of energy that didn't go into your projectile. A perfectly tuned machine will have to moving parts at release. Although this is not possible (try to make your sling stay up in the air after it has released your projectile) a finely tuned machine will move surprisingly little. Leaving kinetic energy behind tends to be more of a concern to a trebuchet operator than a torsion operator since the trebuchet typically has more moving parts.

Release angle is important for both the trebuchet and torsion machines. What good is it to get all of the energy into your projectile just to have it fly straight up in the air and then smash your machine to pieces upon its return to earth? Or fly backwards, or straight into the ground one foot in front of your machine? Unlike trying to turn all of the potential energy into kinetic energy, it is possible to achieve a perfect release angle. Plus release angle tends to be easier to adjust.

Counterweight Stall

The counterweight stall is when 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 usually 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. In general a longer sling causes a slower angular velocity and a shorter sling causes a higher angular velocity. In simpler terms a longer sling will make the projectile come around slower while a short sling will make it come around faster. Therefore, if your sling gets to a 45 degree angle before the arm is vertical the sling should be lengthened. If your arm is in the vertical position and your sling is level with the ground, it did not come around fast enough and the sling should be shortened. In 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 two sling attachments to the arm (the pin and the permanent side) 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 used to adjust the angle of release for your projectile. Pin angle is typically measured from inline with the length of the arm (pointing straight away from the arm would be 0 degrees, or open, perpendicular to the arm would be 90 degrees or closed) If your projectile flies to high/releases to early, close the pin, if it releases low/late open the pin. For poorly optimized sling length, negative pin angles or pin angles of greater than 90 degrees may be needed. These out of the ordinary sling angles can be used if you have a sling that is difficult to adjust and you want to throw a single object whose weight is significantly different than you standard projectile.

In general, a pin angle of 45 degrees is preferred.

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;

  1. 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.
  2. 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.
  3. 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.

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