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Author Topic: Underslung rudders.....  (Read 346 times)
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« on: November 11, 2019, 12:27:02 PM »

At our local indoor meet and greet and flying session yesterday, I had a spectator ask me why some of my models had the rudder (vertical stab) on upside down.  I have been using the underslung fin on my LPP's for a while now and like that arrangement.  I felt kind of lame though, as when asked the question, I had no real good answer, other than on the LPP, I wasn't using tip plates on the horizontal stab, and wanted an easy way to mount the fin as far aft as possible.

So that question got me thinking.  Aerodynamically, is there any advantage/disadvantage to the underslung vertical fin?

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« Reply #1 on: November 11, 2019, 02:43:27 PM »

I was intrigued by your question so I did some internet searching... which lead me straight back to HPA  Grin

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« Reply #2 on: November 11, 2019, 10:48:45 PM »

The vertical distance of the force produced by the fin/rudder from the axis of the model induces a rolling moment. A tall vertical conventional fin will induce a correcting rolling moment if the model experience yaw. It helps the dihedral to some extent ( whilst the fin area itself fights against the wings dihedral effect). The BAC TSR2 was a full.sized plane with this feature.

Unfortunately, when you deflect the rudder the rolling effect tends to roll the plane put of the turn. You win some and you loose some.

Putting the fin below the fuselage reverses both effects.

A low fin has the advatage of not reducing effectiveness at high angles of attack as the wake from.the wing goes over the top of it.

So it is quite good aerodynamically but undercarrage considerations prevent its use on full sized aeroplanes. .some float planes have featured low set fins and rudders. Lots.of full sized planes have single and twin ventral fins to fix directional stability issues found in flight test. It seems that full sized designers often put roo small a fin on their planes.

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