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Recently I made some new Larp crossbow bolts and at the same time gave my old crossbow an overhaul, tweaking the retaining spring and remounting the arms. I noticed with the additional distance I was getting that I had very poor flight characteristics beyond short range on my old bolts.

As I had to test my new bolts anyway I retested my old ones and both performed very badly. What did strike me however was that running through this might be of help to people who might not know how to check directional stability of a projectile.

This article is written about bolts but applies to all projectiles, just Larp bolts have pretty bad flight characteristics so most other items have fewer issues.

This was a technique I learned probably 20 years ago when I used to build home-made rockets and in the interest of not torching myself with exhaust gasses used this as a simple method to test if what I was planning to do would work.

I'll go through some basic theory first but if people don't want to understand what's happening then just skip to the test section to read through the actual test.

Theory:

This is a bit simplified but covers the basics. An unpowered-projectile moving through the air is effected by two effects, resistance and effort. All parts of the object have their own effort and resistance but they can be centralised into two points:

The centre of effort, which in this case is the same as the centre of gravity (CoG)

The centre of resistance, which in this case is the same as the centre of friction (CoF)

Principles_1Principles

The centre of effort/gravity is the point of action of a momentum acting forward, the centre of friction/resistance is made up from air resistance (in level flight) and is a force acting backwards. The two are separated by a distance.

In short, the centre of effort/gravity pulling forward must always be in front of the centre of friction/resistance, the greater the distance between the two the greater the directional stability. 

Effects_1
Effects

In case A the bolt has a good separation between the centres, in case B they are on top of each other (or as near as dammit). In stage 1 both are flying true but in stage 2 as soon as any misalignment is introduced they act differently. In case A the distance d produces a force couple clockwise which works to remove the misalignment. In case B the distance is effectively 0 so no couple is generated and the bolt tumbles.

Case A has directional stability, B lacks it.

Effects_2
Effects - including a Larp Bolt

Case C introduces as slight variation due to the large heads on Larp bolts - though this is technically true of all projectiles the impact on Larp bolts is significantly more than most other projectiles.

In the above s shows the initial separation of the centres. When a Larp bolt tilts in the air a larger cross-section is present and the air resistance at the front actually increases, pulling the centre of resistance forward along the bolt. This means that if s, the reserve stability, of the bolt isn't large enough to account for this shift then it will quickly become unstable and tumble (or even invert). If the distance is large enough to account for the shift then the bolt will return to true flight.

 

Test:

For the test you'll need the bolts you want to test (though if the construction is identical you'll only need to test a couple) and a 1000-1500mm long length of sewing thread or light cord. Tie a loop with a slipknot into one end of the cord - make sure the size of the loop can pass over the head of the bolt. You'll also need some masking tape or similar.

You'll want to do this test after finishing the bolts, so there are no changes in weight or air-resistance after the test is done.

For this test you'll need to find the centre of gravity of the bolt. This can be done by trying to balance the bolt on your outstretched finger or another straight edge. If you're able to get the bolt to balance level that's great but more likely you'll get close then have two close points with it tipping to either side, in that case the centre of gravity will between those two points. Once you have the centre of gravity position the slipknot on the cord about the bolt at that location and pull it tight, the use a small piece of masking tape to help hold it in position.

Test Setup
Test Setup

The bolt should hang horizontally at the end of the cord. If it doesn't you've probably not correctly positioned the cord on the CoG so you'll need to adjust it until it sits level. If it isn't perfectly level and for some reason you cannot easily move the cord the test will still work so long as the angle of tilt is pretty limited.

Once this is done spin the bolt around your head on the end of the cord.

Spin Test
Spin Test (weeeeeeeeee!)

I'd go for a rate of about 30-60 rpm (so one revolution ever 1-2 seconds) ideally the bolt wants to be moving at about the rate it does leaving the crossbow. The bolt will almost certainly want to fly backwards. With Larp bolts don't worry, they'll always want to do this - with more regular projectiles it's a bad sign. You'll need to start and stop until the bolt starts flying in the correct direction, it takes a bit of practice but isn't too hard. Once that done you want to slightly vary the speed and/or angle of the spin just to add some misalignment in the bolt's flight.

Typically you'll get three results.

FAILS:

Bolt will never fly straight or will only fly backwards - this is bad. The bolt is totally lacking in directional stability.

Bolt flies true but tumbles with very little provocation - not great the bolt lacks a reserve of directional stability

PASS:

Bolt flies true, if misaligned it will quickly right itself to true flight.

 

Corrective action:

If the bolt fails there are two easy solutions, you need to either move the centre of gravity forward or move the centre of resistance back. Both can be easily done but moving the centre of resistance back is the easiest on a finished bolt.

Move the centre of gravity forward - you'll need to add mass at the front of the bolt to move the centre of gravity. Add too much and the bolt will slow down and loose too much range. This is also harder to do to a finished bolt but fine if you're making a prototype.

Move the centre of resistance back - the easiest way to do this is to remove the existing flights and add larger flights or if only using two go to a three flight (Y) configuration. Adding larger flights will slow the bolt down and limit range. Also larger flights have more risk of damage so I always use the smallest flights I can.

Other options are lengthening the bolt, reducing air-resistance at the front or lightening the rear end but all three are generally less ideal the two above.

 

 

Questions:

Q: Why do this test instead of just shooting the bolts?

A: The bow itself might have problems that causes a tumbling flight. Testing this way removes the bow from the equation. It can also show if your bow has problems if this test is done followed by a test firing.

Q: Problems like what?

A: Bows often can make bolts tumble by having uneven string tension (either from manufacture or incorrect drawing) resistance in the bolt runners or poor string release.

Q: If my bolt gets wet will it change the test result?

A: It depends on your design (and you can always test a wet bolt) but normally the soft foam at the front will absorb water which will pull the centre of gravity forward giving more stability.