Advantages of a rectangular cross-sectionEdit
The flatbow is a superior bow design for almost all materials because the stress is more evenly spread out than with rounded limb sections. A bow limb is essentially a flexed beam undergoing bending, and in any flexed beam the farther from the neutral axis (line in the middle of the flexing beam which is not under tension or compression: see diagram in Bending article) the more stress there is within the material. When a limb is rounded, as in a longbow, some material "sticks out" farther from the neutral axis, and thus is put under greater stress. In a flatbow, the flat belly and back ensures that all of the most highly strained material is a uniform distance from the neutral axis, spreading the load over a wider limb, minimizing stress and making weaker woods far less likely to fail (break or become permanently bent and lose the springiness needed in a bow). Only particularly resilient timbers can make an effective and powerful wooden longbow.
In most parts of the world, common hardwoods may be used to create excellent bows. Suitable and easily-available timbers include elm (used in ancient Europe, as evidenced by bows pulled from European bogs), maple, sycamore, hazel, and ash. The flatbow design also lends itself to very dense, high strength woods such as hickory and especially osage orange (a wood favored by many Native American tribes for bow making). Good quality yew wood is still much more expensive and difficult to find than woods suitable for flatbows, and beginning bowyers are strongly recommended to start with a flatbow made from easily-available wood. Because yew, the wood of choice for European longbows, is light, resilient, and has exceptional compressive strength, the rounded design can be used to produce a smooth shooting, efficient, powerful bow. This is economical of wood and of the bowyer's time.
Disadvantages of a rectangular cross-sectionEdit
Compared to a narrow, rounded longbow design, the bowyer needs to start with a wider stave, take more time to achieve an approximately-rectangular cross-section, and may need to cut through growth rings on the back of the bow.
Flatbows were used by Native American tribes such as the Hupa, Karok, and Wampanoag, prehistoric ancient Europeans, some Inuit tribes, Finno-Ugric nations and a number of other pre-gunpowder societies for hunting and warfare because, unlike longbows, good flatbows can be made from a wide variety of timbers. Flatbows fell from favour in Europe after the Mesolithic, replaced with yew longbows. The trade of yew wood for English longbows was such that it depleted the stocks of yew over a huge area. Flatbows are currently used by the paleolithic Sentinelese tribes of the Andaman Islands. Flatbow survived in cold areas, such as Finland, where yew does not grow naturally due to climate. The traditional Finnish flatbow is made either from ash, or as birch/pine laminate glued together with fish or hide glue. The yew was known as an imported material for bows in Finland, but it was considered not suitable for serious use, because it is fragile in cold temperatures and the season for hunting for furs is in January and February, when the furs are at their best.
An American Flatbow made out of ashThe American Flatbow (AFB), also known as the American longbow, was developed in the 1930s. It resulted from scientific investigation into the best cross-sectional shape for a bow limb. This research was expected to explain why the English longbow's D-section was superior to all other extant designs. Instead, it showed that the best cross-section was a simple rectangle. The AFB was developed by applying these research findings to the English longbow. The result was a more efficient and stable bow which can be made from more common woods. Because of its coincidental resemblance to some native North American bows, the AFB is also known as the "semi-Indian" bow.
The AFB was popularised by Howard Hill and quickly displaced the English longbow as the preferred bow for target shooting. The modern Olympic-style recurve bow is a development of the AFB, usually made mainly of fiberglass rather than wood, with a built-up handle section and recurved limbs like those of the traditional Asiatic composite bow.