The penetration issue is a complex multi-parameter problem. FlyingSticks approach is use a physics based model and trim the various coefficients until the model output matches the known empirical data. See the FlyingSticks eBooklet - Penetration for more details.
The Standard Penetration Model that has a minimal set of independent variables and is base on a standard understanding of penetration.
Optionally, an additional Ashby Overlay Model may be applied. This adjusts the Standard Model's result by a factor derived from a model based on Dr Ed Ashby's published field penetration data.
FlyingSticks penetration model is most accurate for large game using 50-80 lbf bows, heavy arrows with efficient broadheads and perfect flight characteristics. For lighter kit and smaller game it is believed the model scales well, but the results may not have been confirmed with field data.
The power to which the impact speed is raised. A value of 1.0 implies a momentum proportionality and 2.0 implies an energy proportionality. The default is 1.35 that has been found to best fit the available data.
When ever the Speed Exponent is changed, the Scale Factor needs to be adjusted to bring the penetration back to sensible value. Defaults to 1.0.
The ribs are a major obstacle for an arrow. Bone requires significantly more energy than other tissue to penetrate. The rib width and spacing determines the probability of a rib strike. For larger game, FlyingSticks assumes a rib strike.
The mass of the arrow as currently calculated in the Arrow>Assembly, panel.
These are the vital arrow dynamic attributes for penetration.
Resets the Speed Exponent and Scale Factor to defaults, 1.35 and 1.0 respectively.
Loads the calibration parameters with preset values and changes the button to "Uncheck". A second check will re-stall the parameters to their original values. Mimic the operation of the Action>Penetration_Calibration_Check menu action.
The action should see the "Likely Penetration" change to 16.5" (~420 mm) and the Ashby enable but the contributions all to 0.0%.
This process can change many arrow, bow and target parameters, so be careful as this may cause confusion, especially if the kit is saved in this state. Parameters that are set to specific values in calibrating the Standard Model scaling includer: Arrow mass: 825 gr, Shaft Diameter: 9 mm, Point: Standard Broadhead, Point Mass: 200 gr, Tip Design: Tanto, Edge Bevel: Double, Slickness: Standard, Sharpness: Honed, Shaft Taper: Straight, Mechanical Advantage: 2.25, FoC: 15%, Ferule to Shaft Ratio: 1.05:1, Mass Threshold: none.
To set the arrow's mass and FoC combination, the shaft's weight (gr/in) and insert mass are adjusted.
To set the impact velocity, the wind speed is set to zero and the bow's draw force is adjusted.
There are a multitude of secondary parameters that are not adjusted and can lead to small discrepancies.
This action calculates a new Scale Factor (b) and can be used after some adjustment such as to the Speed Exponent. The process involved making temporary changes as per Check button above to create the standard conditions for a one point calibration.
This model is an overlay optionally applied to the Standard Penetration Model.
Check the checkbox to enable. When disabled the related fields will be greyed out and disabled, but are still calculated so as to show possible advantages.
Ashby Factor Components are tabulated and each contribution is discussed in more detail below. The contribution columns show the impact of the Ashby overlay on the standard penetration model. The contribution is often negative as the Ashby calibration setup is already very efficient, so deviations are likely to reduce penetration. These are updated even if the overlay is not being applied.
The calculated mechanical advantage of a broadhead. This is a measure of the likely blade penetration efficiency, favoring blades set with acute cutting angle with a minimum number of blades. Change this parameter in the Hunting>Broadhead panel.
While the needle is best for penetration in soft tissue, its very likely to bend on striking bone, statistically reducing it probability of achieving its potential penetration. So, by avoiding damage, the average penetration can be greatly improved. The high percentage contribution of the Tanto tip is a measure of how prone the needle tip is to damage on thick bone.Change this parameter in the Hunting>Broadhead panel.
The importance of tip design increases with the size of the game. The calculations here apply a correction proportional to the square of the rib thickness.
The Tip Design parameter is different to others in that it is a precursor to mechanical integrity on impact. Not in the list is the shaft's integrity. Breaks can occur at stress points that are equally performance compromising.
Ashby has convincingly show a single bevel is superior in all respects, however the magnitude of the benefit might be overstated. Change this parameter in the Hunting>Broadhead panel.
Shows the currently selected level of "slickness".
Slickness is a fuzzy measure of the cleanliness of the design in moving through bone and soft tissue. A clean design is one that looks like a classic jet fighter - smooth, sleek and without protrusions or rough surfaces. The calculations assume slickness is less important when encountering thinner bone. Change this parameter in the Hunting>Broadhead panel.
High FoC can significantly improve penetration over a normal FoC with medium mechanical advantage broadheads. This believed to be due to the reduced shaft bending on impact and hence reduced angle of entry relative to the flight path, and hence the reduced friction on the shaft.
Needless to say, the cutting edge should be razor sharp for maximum efficiency. Serrated edges to be avoided due to fibrous material being collected and dragged through the cut.
Ideally the broadhead's ferule diameter should be at least 5% greater than the shaft diameter to achieve a ~10% gain in penetration. These parameters may be adjusted in the Arrow>Shaft, Arrow>Point and / or Hunting>Broadhead panels.
Shows the currently selected shaft longitudinal profile.
Ashby's work tends to indicate there is a critical mass threshold below which the probability of perpetration through rib bone is much reduced despite other factors being ok. See the "Penetration vs Arrow Mass by Bow Draw" predefined plot to get a feel for the effect.
The Factor is the multiplier for the penetration calculated by the standard model.
The Change field is the percentage change in penetration due to the application of the Ashby penetration model.
When these are set and Ashby enabled, the effect should be minimal - i.e. the Net Ashby Factor 1.0 and the Change 0.0%.
Summary of penetration achieved.
The likely penetration is that calculated using the above equations and assumes rib bone strike is possible. In reality it is a difficult concept to express as a single number. For example with a needle tip broadhead the probability of the tip bending on rib contact is high, leading to very low penetration, but if the rib is missed the penetration will be higher than other tip types. The "Likely" figure could be taken as a "Minimum" figure if no bone other than rib is struck.
The Max field is the penetration likely without striking rib or other bone. It can be noted that point tip designs that are most effecting in penetrating bone are not necessarily the best in soft tissue.
The required penetration field is the penetration required for a certain kill of the current game target. This can be modified here or in the Hunting>Game panel.