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.
| Distance | ||||
| Arrow | 0 | 10 | 20 | 30 |
| 269 straight | 337.4 | 327.7 | 315.4 | 301.1 |
| 269 helical | 337.1 | 326.8 | 313.3 | 297.4 |
| 508 straight | 253.9 | 249.2 | 244.2 | 238.1 |
| 508 helical | 253.8 | 248.7 | 242.8 | 236.4 |
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
- Heavy vs. Light Arrows: Downrange Speed and Power
- Arrow Kinetic Energy and Momentum: what they mean to the archer
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{ 8 comments… read them below or add one }
All my arrows are around 400 gr., but I do have one that is 370 grains. I use the Arizona Easy Fletch which is about 6 degree helical. What are lite arrows and what are heavy arrows? Since I didn’t know because I just got into archery, I called Carbon Express, I just purchased 12 CX arrows and needed to know what is a good weight for these arrows. What I was told by staff member, he has always used a 370 grain arrow and had just return from a trip to Africa 2/3 years earlier, that 370 grain arrow, shooting nine animals with eight pass through, that’s very good to me, but again just about all my arrows are around 400 grains. Also, I’m getting from this weight 280 fps.
Rudolph, “light” vs. “heavy” is certainly subjective and really depends on your individual setup. Generally speaking, in my mind, “light” is something around 5-6 grains of arrow weight per pound of draw weight (70 lb. bow, 350-420 grains) and heavy would be more like 7-10 grains per pound of draw weight. Of course, some people will shoot even much higher than that!
For my personal hunting setup, I use 470-490 grain arrows and shooting a Bowtech Destroyer at 67 lbs. This gives me an arrow speed of just over 300 fps and is a good balance of speed vs. weight for me. I like the heavier, stronger arrows for better penetration potential when hunting elk. If I were to only hunt deer and smaller animals, I would probably go with a lighter arrow.
I love these physics lessons!
I too was especially interested to see the difference between velocities of straight vs. helical, especially with the heavy arrows:
” With the 508 grain arrow the loss is 6.2% versus 6.9%..”
At roughly .6% difference in speed at 30 yards tells me that the helical arrow took less than one percent of the available energy to start the spinning and keep it spinning. The heavier arrow has more rotational inertia, which means that once the arrow is spinning, it takes comparatively less energy to keep it spinning, which I think completes the explanation above about lower velocities having less resistance. This is also true in flesh! Dr. Ed Ashby demonstrated this with a physical constant relating to surface tension of water, I believe. Thus it’s better for penetration purposes to have a heavier arrow going slower rather than a lighter, faster arrow, if both have the same KE. So, to me, a big fat heavy arrow & broadhead, spinning nicely, is the optimal choice.. ahh, but then you best get your visual distance estimation on the money! I’m quite certain that the recent speed craze in archery comes from that exactly, the need for speed is a crutch to help minimize poor distance calculations.
All that said, i like the FOBS for this exact reason–it lets me shoot a heavy arrow & a big (165 grain) broadhead. I’m still tuning this setup out past 30 yards, but I have a feeling this is my long-distance answer since the field tips fly awesome out to 60 yards.
It would be interesting to know exactly how far down range an arrow reaches it’s maximum spin rate under different configurations. Unfortunately this is beyond my current analysis equipments abilities! I am sure that a heavier arrow would take longer to spin up, but how does this affect the overall energy consumption? Fun stuff for thought…Maybe someday when I get a really nice high-speed camera it will be interesting to look at.
I too love the FOBs and they tend to be my choice when shooting fixed blade broadheads, especially in windy conditions. I might have to do some more similar testing with helical vs. FOB someday.
Hi Michael,
I’m just starting in archery and find these articles very interesting as I enjoy tyring to understand the physics behind it.
I’m just wandering if using an aerofoil rather than helical (but flat) fletch would still produce a spin but with lower drag and therefore improved velocity at the longer distance?
Richard,
There is a vane that uses the airfoil principle, the Aerovane II (http://firenock.com/main.php?page=Aerovane2) which uses aerodynamical lift to spin it. It’s an interesting idea and I did some testing with them that you can see here: http://archeryreport.com/2009/10/fletching-review-speed-drop/
They performed well, but not noticeably better than other 2″, high-profile vanes. Certainly interesting and perhaps in the future we will see a more measurable gain in performance.
In your report you didn’t state how much helical you used. That is an extremely important detail. You also didn’t say what type of fletch you where using or the configuration that they where in(3×120, 4×90, ect). All of those factors play a huge role in the results of the test. I prefer a 4×90 configuration with 5″ shield cut feathers and 12-14 degrees of helical. This arrow would hit it’s mark if the archer was having a seizure.
Jeremiah, if you read the first article in the series (linked at the top of the article) you will find answers to your questions
Helical cannot be measured in “degrees” but rather is measured in revolutions per distance and is mostly determined by your fletching jig/clamp. Jigs such as the Bitzenberger that have a clamp adjustment do have some flexibility in alignment.