Dorsal Fins: Why Do Airplanes Have Them

They're not just some added lateral area—they're a vortex generator.

Peter Garrison

Wednesday, December 18, 2024 at 08:51 AM ET

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Edited By: tccjeremy

The dorsal fin is a curiosity. Some airplanes have them, some don’t. [Getty Images]

Key Takeaways:

Dorsal fins, which began appearing on aircraft around 1936, were often mistakenly thought to primarily enhance directional stability.
Their true and critical function is to prevent "rudder lock," a dangerous condition where the rudder stalls at high sideslip angles, causing it to stick.
Dorsal fins achieve this by acting as vortex generators, maintaining high-energy airflow over the fin and rudder to delay rudder stall, a need that became more pronounced as planes became larger and faster.

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Reluctance to throw useless things away has at least one benefit: After a sufficient passage of time, they become historical artifacts.

I still have 14 single-spaced typewritten pages chronicling the first five months of flying my first homebuilt, Melmoth, in 1973-74. The document, now slightly frayed but bidding fair to defy the silverfish for another century or two, ends with the admission that “it is getting tedious and pointless to record all these trivia.” Rereading it today, I came to the same conclusion well before the final page.