Drelinger's Research

All true flutes work basically the same. The blown air directed from the lips is divided and oscillates as it strikes the far edge of the blow hole. The molecules of the air column within the flute’s tube are then set into a vigorous back and forth motion sounding a fundamental note. 
 

In transverse flutes this occurs when blown across a hole cut in the side wall of the tube. One end of the transverse flute’s tube is stopped (closed) creating a virtual baffle.  Also the inside tube wall immediately below the blow hole provides baffling critical to its function.  A baffle holds air, like the bottle in figure one.  When blowing into a baffled structure whether a bottle or a transverse flute, the jet stream and resulting note will be stable as expressed by straight lines originating from the lips in figure one and figure two.
 

In vertical flutes, more commonly known as the end blown flute, sound begins the same as air is blown across one end of an open tube as illustrated in figure three.   However, the vertical flute is essentially unbaffled as its tube is open throughout. This unbaffled acoustical environment allows the jet stream and resultant note to become intermittenly unstable as expressed by tear dropped shaped lines in figure three.

Pictured here is a drawing excerpted from a vertical
flute patent issued in the late 19th century.  Even
though its headjoint is stopped at both ends “a-a”,
its blow hole is located in the same position “c”
(unbaffled) as an end blown flute and would
exhibit jet stream instability if actually built.

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