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Why are there two sets of hinge lines on the rudders of the Grumman C-2?

enter image description here
A photo from the related ASE question Why does the C-2 left inboard stab not have a rudder?

Why would two hinge lines provide more control authority than just using the forward hinge line and deleting the rear one?

And why was it thought worth the extra complexity and weight to use this feature on this aircraft when so few other aircraft have been designed with this feature?

What other aircraft have been designed with this feature?

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    $\begingroup$ So, could we arguably say that this aircraft actually has 10 rudders? (!) (See the photo linked in question-- ) $\endgroup$
    – quiet flyer
    Commented Mar 26, 2021 at 22:58

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The double hinged rudder is also a Dehavilland Canada feature on most of its larger designs like the Caribou/Buffalo/Dash 7/Dash 8, where maximum yaw power for a given surface area was a design priority (especially if there are height or other limitations that prevent you from adding more area by going up; in the C-2's case, they also added more surfaces as well to limit height, an obvious issue on a carrier, and the same reason the Constellation had 3 tails).

It improves yaw power for a given overall surface area by allowing the rudder to operate at a larger net angle before significant flow separation, by allowing the airflow to make 2 smaller changes in direction vs one larger change in direction for the same overall displacement.

It's not quite as complicated as it looks. The trailing edge segment is effectively a giant anti-servo tab and is simply geared to the vertical fin by a push rod. The actuation system only has to drive the forward segment. The extra parts and complexity are in the second set of hinges and the actuating rod. In the C-2 pics you can see the gearing rod attachments in the little blisters. On the DeHavilland Canada products, the gearing rod is hidden at the base of the rudder.

enter image description here

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  • $\begingroup$ You might want to add that tail height on carrier airplanes is restricted to the height of internal hangars, so the added complexity is the result of strict height limits. $\endgroup$
    – Peter Kämpf
    Commented Mar 28, 2021 at 13:53
  • $\begingroup$ Added some text thanks. Have you ever seen that sort of thing done on a non-slotted plain wing flap? $\endgroup$
    – John K
    Commented Mar 28, 2021 at 14:31
  • $\begingroup$ The first that comes to mind is the SB-11, but here only the rearmost part is hinged; the rest is sliding out of the wing cavity behind the spar. Since it slides on curved rails, it works like a Fowler flap, increasing camber and chord. Double-hinged flaps without slots are to my knowledge only used on rudders, like on the C-17 or the examples you list. $\endgroup$
    – Peter Kämpf
    Commented Mar 28, 2021 at 19:20
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2 hinge lines will give the rudder more authority because it will direct the air more aggressively. We can think of rudders as vertical and two way extending flaps. It's like the difference between a fowler flap and a split flap. fowler will create a curvature with 2 or more components while split flap only hinges down. Fowler flaps increase the Cl of a wing more than split flap while using low AoA's this makes sense because a vertical stabilizer will not have high AoA.

The fact that C-2 is a naval cargo aircraft means it will be heavy and it will need lots of control authority but it also needs to land on a carrier so it should be small.

One way of increasing yaw control authority is to increase the distance between the empennage and CG hence increasing torque caused by aerodynamic force, the other way is to make the vertical stabilizer and the rudder bigger.

I think the fact that the plane should be controllable while remaining compact, made the designers chose the second option.

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    $\begingroup$ Both answers are very good, I took the liberty of editing this: it is not just shortness, but overall compactness in all dimensions that is crucial in navy planes. Hence the double hinges and many small rudders instead of one larger, or one "normal" further back. (Didn't wanna make yet another answer to "compete" here :) $\endgroup$
    – Jpe61
    Commented Mar 27, 2021 at 8:31
  • $\begingroup$ A Fowler flap will also extend the wing chord. $\endgroup$
    – Peter Kämpf
    Commented Mar 28, 2021 at 13:56
  • $\begingroup$ Yeah of course, I was only talking about the curvature. Even if we neglect the extension and only focus on the curvature it comes on the same thing. If we compare cl vs alpha of a cambered and a symmetrical airfoil we can see that the cambered one at 0 AoA has almost the same effect with the symmetrical at 5⁰. So in this case we can say that increasing the camber is definitely better than just using a split surface. $\endgroup$
    – Kozakov
    Commented Mar 28, 2021 at 16:42

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