F/A-18 Rudder Question
 

Wondering if any Hornet jockeys out there can answer this one. I have noticed through photographs that the F/A-18 can simultaneously deflect it's dual rudders inwards towards the centreline of the aircraft. This is usually seen as the jet sits on aircraft carrier catapults prior to launch. As there is no yaw advantage in either direction going to result, what is the aerodynamic benefit of doing so?
Likewise, at what point does the conventional use of the rudders return and how does a pilot control yaw with both rudders canted inwards?

There's an obvious answer to all this but can anyone explain the design better?

cheers

Todd.

The fins on the Hornet are canted outwards to keep them away from the strong vortices generated by the forward fuselage at high AOA, this turns out to have an unexpected benefit. By toeing the rudders in for catapult launches, they produce downforce and nose up pitching moment. The toe-in was added during flight test to address high nosewheel-liftoff speeds.

I figured that the outwards can't of the fins was was a player in all this. I assume then that the rudders fall back into streamline after launch and resume their normal duty.

What about landings, do the rudders play a role there ?

Vic

They are used all the time. During landing/take-off below 8 degrees AOA toe-in is used, above 8 degrees toe-out. On the F-18E, the airbrake has been removed and toe-in is used, in conjunction with the other control surfaces, to add drag. Toe-in is also used during normal flight for stability and lift.

It's just another control surface used along with the rest.

am aware that the F18 uses toe in on landing in order to generate drag; this means the engine can be run t a higher RPM and is thus more responsive if a go around is reqired after missed cable.

When they got rid of the airbrake, as is stated above, rudder deflection was used as a replacement. apparently the FBW system simulates the performance of the old aircraft's airbrake by use of various control surfaces. very clever!

Not sure about TO though, could be to aid some form of flow control; I am aware that at one point the 18 had severe vortex bursting problems early in its life; apparently at high alpha the extremely thin (and flexible) tail surfaces were fatiguing after only ten hours!

mbga,

That's why those little LEX fence 'vortex bursters' were fitted - significantly reduced the fatigue on the V stabs.

victor,

If you look how far back the mainwheels are (when it's on the ground) and consider how much mass is fwd of the mainwheels, you can see that it needs a lot of downforce at the back to rotate the nose on take off. In some configs you needed up to full back stick to get the nose up on take off (from a runway that is, I have no carrier experience). Rudder toe-in, as ORAC says, helped a little to get the nose up.

MT

Thanks to all for the responses. Answers all my questions in detail. Much appreciated.

cheers
Vic