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Part 1 of 4: How Wind Affects Arrow Flight

|  May 11th, 2011 0

I was asked a question the other day by an experienced bowhunter from back East. He asked, “If you have a wind from left to right, will the wind push the entire arrow right? Or will it just push the back to the right and cause the arrow to go left?”

If you want to bowhunt in the north country or in the West, you need to learn how the wind affects your shooting. The author spends a great deal of time hunting above timberline where wind is a given. This is his 2010 high-country mule deer.

I was asked a question the other day by an experienced bowhunter from back East. He asked, "If you have a wind from left to right, will the wind push the entire arrow right? Or will it just push the back to the right and cause the arrow to go left?"

Just last fall, I wrote a column that lightly touched on this very complicated subject. Well, the readers of Petersen’s BOWHUNTING tend to be hardcore bowhunters who crave in-depth information. So, at the risk of being redundant, I’m going to devote this column — and the following three — to hunting in the wind.

In this first column, we’ll cover the basics of how the wind affects an arrow’s flight. In the next two columns, we’ll talk about the finer points of building a hunting arrow designed to buck the wind. In the fourth column, we’ll discuss how to hold steady, aim and shoot in the wind.

First of all, to answer the original question: In a lateral (sideways) wind, the nock end of an arrow will tail off to the downwind side in flight, because as soon as the arrow is released, it will line up with the airstream it is moving through.

If the air is perfectly calm as the arrow is released, the airstream the arrow encounters is aligned in the direction the arrow was aimed. However, if there is a prevailing wind from the left side, the airstream the arrow experiences will be lined up slightly to the left side of the target. So, the arrow will be pointed slightly to the left of the target as it flies. The slower the arrow’s velocity and the stronger the prevailing wind, the more sideways the arrow’s flight will appear.

However, the fact that the arrow is pointed upwind does not mean the arrow will travel upwind. The arrow will always drift downwind on its way to the target. To better understand this, watch an airplane or helicopter fly in a straight line on a windy day. The aircraft will actually be pointing upwind of its true direction of travel.

If an arrow is shot into a 10 mph crosswind and the arrow is allowed to travel indefinitely before it hits the ground, eventually the arrow will be traveling both forward and sideways at the full crosswind speed of 10 mph (or 14.5 fps).

If that arrow is moving at 300 fps, it will travel 300 feet in one second. Assuming it is moving laterally at 14.5 fps, the arrow would drift 14.5 feet sideways while covering that 300 feet forward.

As we all know, an arrow shot from a bow will not miss its intended target by 14 feet when shot into a 10 mph crosswind at 100 yards. The reason this does not happen is because it takes a significant amount of time for the arrow to accelerate to maximum lateral (sideways) velocity.

Once free of the bow, an arrow shot into a crosswind is accelerated sideways by that wind. The rate of sideways acceleration on any given arrow is determined primarily by two factors; the arrow’s drag characteristics and the weight of the arrow. (Think of a lead bullet vs. a feather. The weight-to-surface area ratio is the determining factor.)

I used to think sideways acceleration was determined by the arrow’s profile (the amount of surface area the sideways wind "sees"). I was enlightened by engineer Thomas Liston, author of The Physical Laws of Archery. He explained the only forces an arrow sees (besides gravity) are aligned on the point of the arrow. So, once the arrow has aligned with the airflow it is experiencing, it no longer has any sideways forces. However, the air mass it is in moves laterally in relation to the target.

So, it is not the sideways profile of the arrow, but rather the drag of the arrow that determines how fast that air mass is going to push the arrow sideways. If you allow two arrows to fall until they reached their maximum speed (terminal velocity), the one going the fastest is the one having the best drag characteristics. It is also the arrow that will be least deflected by a sideways wind.

The other factor that determines how far an arrow is going to drift at a given distance and lateral wind speed is how long that arrow is in the air. And that is determined by initial arrow velocity and the rate of deceleration (wind drag). To minimize wind drift, we want an arrow that not only comes out of the bow at a high velocity, but one that maintains its velocity downrange. The best way to keep an arrow flying fast is to reduce surface area and any turbulence-producing surfaces

So, the take home message is this: In order to minimize wind drift we want a low-profile, small-diameter arrow with low-profile, short fletchings and a low-profile, short broadhead. We also want a short arrow.

The arrow needs to be relatively heavy as well, though this is a balancing act between speed and weight.

I realize this column has been a little dry, but put all four wind columns in a drawer somewhere, and if you ever come out West to hunt, pull them out and read them again before your trip. You’ll thank me later.

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