OR... How Do Arrows Fly Straight?
I've been reading a lot about people's preferences for one style of fletching vs. the other and thought I might post some general info that might be of interest.
Note- I am not declaring a knowldege of all things aero, but I have a pretty thorough knowledge base in the field. If I forgot something or have overly simplified something, please let me know. This is a short and sweet version of a bigger paper I wrote a while back that cuts the really boring stuff out.
Many people know that fletching or "fins" make an arrow or rocket fly straight, but here is a little more background knowledge that might be found interesting-
Bernoulli's Principle is often described as how an airplane's wing creates lift. It's the idea that a vacuum is created above the wing since air traveling over the curved surface of the wing must travel faster than the air on the bottom or flat side ofthe wing. (easily proven with a straw and a piece of paper and someone to blow through the straw)
Bernoulli's is only part of what allows airplanes to fly. Another part (of many) of the equation are the simple effects of Angle of Attack or AoA.
The word "PLANE" in Airplane has been found to originate with the definition of "level". This can be interpreted as "angle".
Ever heard the boating phrase "up on plane"? The angle of the hull at a certain speed will force the boat up onto the top of the water. Boat hulls are designed to "plane" at certain speeds when loaded within certain weight tolerances. The exact same thing happens with an aircraft's wings. At a certain speed, the angle of attack of the oncoming air to the wings will force the plane upwards, overcoming gravity. AoA lifting forces are aided by Bernoulli's findings. Thats why airplanes land and take off with the nose up higher than the tail. At slower speeds, the AoA needs to be much higher than during faster flight.
Kites use the same idea- they "plane" in the wind. The weight of the kite's tail helps keeps the AoA angle up. Your hand out of your car window on the highway makes a good example (fun until a bee hits it). If your a boat or river fisherman, think of a "side planer" by Luhr Jensen.
By the way... Airplanes fly with the help of the TAILPLANE, or "TAILFEATHERS", or simply as the TAIL. The tail is what forces the wings to hold a certain AoA using a reference angle called "Decalage". The tail stays in back of the plane during flight because it causes drag further away from the center of gravity on the aircraft. The tail usually has a longer moment or lever arm action than the nose.
Center of gravity is the point where we take the measure to find out FOC on our arrows or the "balance point". These points can be called a fulcrum. The want is to have a long moment so you can use small tailfeathers, therefore keeping the tail light. But to be effective, you need to have strength (read as arrow spine) in the fuselage (shaft) and stronger usually means more material and an overall gain in weight. Increasing FOC makes a longer tail moment and decreases the need for larger fletching.
(Sorry to you flying wing lovers out there, I wont get into tail-less aircraft stability in this post since an arrow is actually a wingless airplane with a tail)
How does all of this make it's way into archery? The "tail" of an arrow is the fletching. An arrow just has non existant wings. Vane or feather offset angle (AoA) causes spin. Spin is a side effect of drag. The higher the AoA, the higher the drag, the more stability. But, the more stability, the faster the arrow decelerates after launch.
Spinning forms stability in its own. Since most fletching force the arrow to spin, the arrow will tend to ignore small attempts to change it's path. (Think of a football or a bullet. )
A much more fun experiment would be to take a wheel off of a bicycle and have someone spin the wheel while you hold the axle. Try to rotate the wheel. It will resist small attempts to turn it, but allow some heavier attempts to turn it. (These same forces help you stay upright on a moving bicycle so you can enjoy the scenery.)
So the trick is, to have a strong, light tail that is the most effective for the task at hand. If an airplane needs more stability, it will get a bigger vertical fin or horizontal stab, but will have to add more noseweight to compensate for the increase in tail weight in order to hold the same FOC. An airplane designer can also change the rake angle or thickness of the fin or stab to play with drag properties and therefore stability. Stability in Arrows can be simplified to meaning drag at the rear of the shaft.
More drag = more noise. Sucks, but it's the truth. Noise is caused by vibration or flutter. The faster something travels, the stronger it needs to be to keep from fluttering.
"High speed, low drag" is a common phrase associated with modern sleek aircraft. Problem is, higher speed and lower drag can mean less stability. Our arrows dont have computers onboard (yet) to make minute changes to AOA or airfoil thickness in flight to compensate for different speeds like modern jet fighters, so they are stuck working great at only one speed- launch. To get vanes or any fletching for that matter, to work great at many different speeds, we will need offset (AoA), airfoil and camber changing capabilities in flight. What works at one speed might not work well at another. It's a fact of life that an arrow is going to change speed constantly during it's flight.
Blazers work well because they were designed to create more drag than standard vanes due to the rake angle mixed with the plastic thickness. For their weight, they create more drag than standard vanes. They are also stiffer, which makes them work great in a Whisker Biscuit style rest. Stiffness also allows them to vibrate and flutter less at high speeds like the older vane styles do. Vibration causes more drag and noise and is very hard to design a stability formula for, since there are so many variables.
At today's speeds Blazers work great. At slower speeds, and as the arrow decelerates, they might not work so great. Because of this, I have never told anyone with a slow bow to use Blazers (until I do some lower speed tests). With the increase in drag, and smaller size and weight, they allow for higher FOC on arrows than regular shaped vanes but...many shooters end up using lighter arrow points for greater speed. (Lighter points equals lower FOC, so its a vicious cycle. )
Bi-Delta vanes are a neat item as well. These vanes utilize several drag creating features while keeping the stiffness up. I think these vanes deserve more credit than they get. I describe them as a 6 fletch arrow using only 3 fletchings. They even sell a synthetic feather version that might be pretty cool.
I shot Bi-Delta's in the early 90s. I have not tried their fake feathers yet. Not sure they would work better than real feathers or regular vanes though. Bi-Delta makes vanes in several sizes and shapes. They might do even better now that average arrow speeds are higher than the early 90s. My plan is to test them on the new headhunter style turkey broadheads this summer.
FOBs are a neat breed on their own. I like their modern spin on aerodynamics, but am not impressed with their weight and the overall drag that they create. The outer circle does some amazing things for stability, but I'm not sold yet on the weight so far out from the arrow. My mind says it will create more stability, but I need to do a few tests. In aeronautics, weight further out from the axis is good for some things and bad for others.
Having a larger frontal area than most regular fletched arrows, and around the same wetted area will make for some neat drag numbers. I have not played with them yet. Hopefully I will this summer. I'm really looking forward to testing them.
If you are interested- one of the neat things about encircling the fins is that it usually makes using a smaller vertical fin and sometimes save weight. (engineering nightmare though- involves stiffness and weight comparisons)
So far, to my testing, feathers work well because they work good at many different speeds. This is better than plastic vanes and their 1 speed. At launch, with the higher speeds, they compress, dropping overall wing chord a little and create less drag. Once launched and in flight, they spring outwards and expand a little, and loosen up, causing more overall drag and therefore more stability at the slower speeds. That is from my theories and testing so far. They are light weight, yet have good surface area and thickness. They seem to work good, and are fairly durable.
I have a set of aluminums that I fletched back in 92 that I still use today. A buddy of mine is using feathers on his aluminums right now during winter league that are so badly beaten and fluffed out that they look like a dead crow on the side of the highway. He's getting groups that are just as good as the rest of us.
I've seen people get good results with cutting bad parts out of their plastic vanes, but I've heard them scream like a banshee as they fly. Feathers just seem to get more of a SHHHH! sound as they age. By the way- I've seen crappy feathers in use and can tell you- there are some feathers that should not be used as fletching.
Watch out for moths! That seems to be the number one durablity issue of feather fletched arrows around my house.
Shooting in wet weather with feathers does kinda suck. To their credit, I've shot wet feathers with good results. Dry them a little by waving the arrow in the air, and fluff them up a bit. They still work OK. I've seen people Saran Wrap their fletchings before heading out in the rain.
I like feathers. I use feathers on my hunting compound and recurves. I'm shooting vanes on my target compound. Not sure why. Indoors might be the place they work best!
BTW- One of the coolest things you can do with feathers is to kill a turkey with an arrow fletched with turkey feathers from a previous kill.
I need to rewrite my paper when I get some good info on FOBs.
Note- I know I left some things out, simplified others and just plain forgot about even more things, but this is a simplified version of a long drawn out paper.
Hopefully someone enjoys all this besides me!
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