The first article in this series, Helical vs. Straight Fletch: Accuracy and Repeatability, dealt with the performance of helical vs. straight fletchings at the terminal end, the target. For this article I’ll take a look at what happens between when the arrow leaves the bow and at impact. One argument for using straight fletchings is that they are theoretically faster; read on to see if this theory meets reality.
Theory behind the speed difference between helical and straight fletchings
In this world we are bound by certain laws of physics that govern how things work. For the most part these laws are very well understood and easy to predict. One such law is the law of conservation of energy. In its basic form, it states that the total amount of energy in a system is constant. For this experiment, our system consists of the arrow at the moment it leaves the bow string, and the air surrounding it throughout its flight.
According to conservation of energy, the energy in the arrow and its surroundings must remain constant. The instant that an arrow leaves the bowstring it begins slowing down because it is pushing against the air and imparting some of its energy to the air. Energy is also lost if the arrow leaves the bow imperfectly and flexes/vibrates, and this happens with every arrow.
Also, if there is any offset or helical to the arrow’s fletchings, the arrow will begin to spin. This is just like blowing on a pinwheel, the angle of the fletchings imparting a force against the fletching and pushing it into a circular motion. Of course this takes energy, just like the energy of a person blowing on the pinwheel. The only source of this energy is from the arrow itself and if energy is used to spin the arrow, that energy is no longer available to keep the arrow at it’s initial velocity.
One other important point is that it takes more energy to accelerate/decelerate a heavier object than a lighter one. Try pushing a car and see how fast you can accelerate it versus pushing a bicycle. The same should apply to arrows: a heavier arrow should take more energy to spin than a lighter arrow.
To sum up the theory, the higher the offset or the stronger the helical on the fletchings, the faster the arrow will spin, the greater the energy loss and the greater it’s deceleration.
Testing the difference in deceleration of helical and straight fletchings
This testing will use my Elite Envy bow and four different arrows. The same arrows that were used in the accuracy testing, Carbon Revolution Speed Pro Max shafts with Blazer vanes fletched both straight and with a helical. In addition, two more of the exact same shafts with be used along with the addition of a 1516 aluminum shaft and point inserted inside of the carbon shaft for additional weight. Arrow weights for the two different configurations are 269 and 508 grains with the FOC being nearly identical.
All four arrows where shot at point blank, 10, 20 and 30 yards through an Easton Pro Chronograph a minimum of 9 times each, with the high and low readings thrown out and the remaining results averaged.
The first observation that can be readily seen is that the heavier arrows, even though they start out slower, maintain a higher percent of their initial velocity. This is agreement with the testing done in the Heavy vs. Light Arrows: Downrange Speed and Power article and is to be expected. At 30 yards the lighter arrows have shed an average of about 11% of their velocity and the heavier arrows have lost about 6.5%.
Secondly, the helical arrows do show a difference in velocity that is more apparent the further the arrow is from the bow. At the 30 yard mark the 269 grain arrow with straight fletch has lost 10.8% of its velocity and the helical fletchings have lost 11.8%, a difference of 3.7 fps. With the 508 grain arrow the loss is 6.2% versus 6.9% and 1.7 fps. While not a huge amount of difference, it is there!
At point blank ranges the initial velocities are nearly identical, with perhaps a very tiny edge to the straight fletchings. This is to be expected because the fletchings have not had time to begin having much of an effect on the spin of the arrows. As the distance grows, the effect of the energy loss to spinning the arrows becomes more apparent.
What is also interesting is that the helical fletching has less of an effect on the energy loss of the heavier arrows. I believe this is because of two things: the heavier arrows take more energy to spin-up and the helical can impart only so much force to create spin, and also because the heavier arrows have a lower velocity, thus less flow and resistance against the fletching.
Conclusions of helical vs. straight fletchings
I had surmised that straight fletchings would be faster but I had never done extensive testing on this until now. It’s nice to see some actual numbers and better understand the effects the fletching style has on speed, especially downrange. Of course this will vary depending on the amount of helical as well as the size/type of the fletching involved, but I believe this is a good baseline for most shooters.
So how does this affect me and how I will fletch my arrows? Combined with the results from the Helical vs. Straight Fletch: Accuracy and Repeatability article, I am inclined to be on the side of accuracy and stick with helical fletchings. While having a little extra speed can certainly be nice for kinetic energy, momentum and distance judging, I am not willing to sacrifice accuracy for a few fps.
For the future I will definitely continue this testing with broadheads, where little differences tend to be more magnified than with field points.
Other posts you may enjoy:
- Helical vs. Straight Fletch: Accuracy and Repeatability
- Heavy vs. Light Arrows: Downrange Speed and Power Part III
- Arrow Penetration Testing: Real Bows, Real Arrows, Real Results…Part II
- Arrow Kinetic Energy and Momentum: what they mean to the archer
- Heavy vs. Light Arrows: Downrange Speed and Power