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Mark Harkin, CC BY 2.0, via Wikimedia Commons

I've seen extra tail surfaces that look like additional vertical stabilizers on the C208 Amphibian. Why is there a need for this? I can't imagine that the floats reduce vertical stab effectiveness.

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Well, it's the same reason that the Challenger ultralight demonstrates extreme "adverse yaw" in response to aileron inputs when flown with the doors on, but less so when flow with the doors off. The floats-- like the doors-- increase the surface area in front of the CG, which has an effect similar to decreasing the size of the vertical fin (which, naturally, is behind the CG.) "Directional" stability is decreased.

Just like if you took a weathervane and added enough surface area in front of the pivot point, it would stop working.

It's not uncommon for aircraft on floats to have modifications to increase the effective area of the vertical fin, usually by adding a small ventral fin below the fuselage.

Which begs the question-- Why did the Wright brothers take steps to increase the amount of vertical surface area forward of the CG on some of their aircraft?

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  • $\begingroup$ That makes a lot of sense. Thanks! $\endgroup$
    – MD88Fan
    Commented May 4, 2021 at 14:02
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    $\begingroup$ When I was a bush pilot one of the minor side jobs besides hauling fishermen and hunters around in a C-185 was doing float ratings in the air service's Fleet 90 Canuck. The factory float installation did not include any additional fin area and the Canuck already had a smallish fin/rudder. On floats the thing had very mild yaw stability and would slither all over the place when maneuvering and you had to be on your feet constantly like flying a glider. I usually had to give a student taking a float rating about 2-3 hours of dual just flying around getting the hang of the yaw behaviour. $\endgroup$
    – John K
    Commented May 4, 2021 at 21:42
  • $\begingroup$ @JohnK: ...How did that installation get FAA approval? $\endgroup$
    – Vikki
    Commented Dec 31, 2021 at 16:38
  • $\begingroup$ It was certified about 1948. $\endgroup$
    – John K
    Commented Dec 31, 2021 at 17:52

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