The feathers, of course, are simply for air friction, to keep the rear end of the shaft in the line of progress of the point, and to give rotation or stability to the arrow while in flight. This is invariably accomplished by placing three feathers from the same wing on an arrow. Their warped contours act as revolving planes and establish an axial rotation. Some aboriginal arrows have a spiral arrangement of feathers to assist this motion. But this is an unnecessary exaggeration and retards the velocity and striking force of the arrow, as is shown in the following experiment.
Experiment in feather friction.—A 436-grain target arrow with the regulation feathering 2½ inches long cut in a parabolic curve ½ inch high, has a friction area of approximately 1 square inch to each feather, or three feathers on an arrow present a friction surface of 3 square inches. The diameter of the shaft is 5/16 of an inch, and the head or pile is shown in plate 10, fig. 2, and plate 13, fig. 4. This typical English target arrow, shot from a 50-pound bow at the paraffin block, penetrated the following distances:
At 20 yards, penetration 1 1/16 inches
At 30 yards, penetration 1 inch
At 40 yards, penetration 15/16 inch
At 50 yards, penetration 14/16 inch
The same shaft feathered with heavy feathers 1 inch wide and 3 inches long, giving a total area of 9 square inches, shot at the paraffin block with the same bow, are the following penetration:
20 yards, penetration 13/16 inch
30 yards, penetration 11/16 inch
40 yards, penetration 9/16 inch
50 yards, penetration 7/16 inch
The loss of penetration evidently amounts to almost 50 per cent and is due to the increased air friction with diminished velocity.
These feathers had a perceptible spiral arrangement, and visibly slowed the flight of the arrow to a speed of approximately 110 feet per second. Such arrows are useful for killing small game at short distances, because they are very accurate in their flight and soon lose their speed after striking the ground, grass, or brush, owing to the friction presented to the air. Maurice Thompson has named this type of arrow (pl. 12, fig. 10) a "fluflu" because a Florida Indian friend had a favorite arrow of this sort which made this characteristic sound during flight.
The speed of rotation given an arrow varies according to the size and concavity of the feathers. It is more rapid in target arrows than in heavy shafts, for heavy heads require more feather surface to turn them than do cylindrical points.
It is obvious that to deliver the greatest blow the arrow should strike with its long axis in the line of direct motion. An arrow wavering or tumbling from improper feathering loses in velocity and striking force. An arrow should have air resistance in proper proportion to its weight. Excessive resistance means loss of striking force.