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Question about F-18/F-35C rudders

Old 11-07-2014, 11:08 PM
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xmech2k
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Question Question about F-18/F-35C rudders

Well, I know this is more about full scale planes, but it is the aerodynamics thread, so I thought I'd ask here.

I was watching this really cool video of recent carrier trials of the F-35C:

[media]http://www.youtube.com/watch?v=APuYyfq12ts[/media]

I noticed that the rudders, like on the F-18, both move inboard at some times. It seems to be at low speeds. I'm assuming it's with flaps down. Anyone know the reason for it? I guess they would act as air brakes? But you wouldn't want that while taking off. Do they provide a downward force at the tail?
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Old 11-08-2014, 07:57 AM
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thepiper92
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I would say since the stabilizers aren't vertical, they would act slightly as elevators, although just a bit. Also, they may keep the plane better planted during takeoff, preventing any potential situation of the plane losing footing on the rear wheels. Since takeoff is not by simply the thrusters alone, it needs drag to keep it on track perhaps, including tracking sideways. With the rudders like that, it will not want to turn. I suppose you can compare it to a paper plane, or rc if you can really toss it hard. When you give a paper plane a really hard you may cause it to fly at all. The jet itself is being catapulted to extreme speeds before it is actually pushing itself, so all airflow has to keep it perfectly straight.

You could also look at it like a F1 car, or something. Tons of speed is lost from the rear wing and front wing, but the car would just lift off at some point. The rear wing isnt just downforce, but some side force too.

Just a guess.
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Old 11-08-2014, 09:16 AM
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Thanks for the input Piper. I just did some searching, and Wikipedia to the rescue! Under 'Vertical Stabilizer', it has this to say:
"The F/A-18, F-22 Raptor, and F-35 Lightning II have tailfins that are canted outward, to the point that they have some authority as horizontal control surfaces; both aircraft are designed to deflect their rudders inward during takeoff to increase pitching moment."

So there it is.
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Old 11-12-2014, 02:58 PM
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Originally Posted by thepiper92 View Post
I would say since the stabilizers aren't vertical, they would act slightly as elevators, although just a bit. Also, they may keep the plane better planted during takeoff, preventing any potential situation of the plane losing footing on the rear wheels. Since takeoff is not by simply the thrusters alone, it needs drag to keep it on track perhaps, including tracking sideways. With the rudders like that, it will not want to turn. I suppose you can compare it to a paper plane, or rc if you can really toss it hard. When you give a paper plane a really hard you may cause it to fly at all. The jet itself is being catapulted to extreme speeds before it is actually pushing itself, so all airflow has to keep it perfectly straight.

You could also look at it like a F1 car, or something. Tons of speed is lost from the rear wing and front wing, but the car would just lift off at some point. The rear wing isnt just downforce, but some side force too.

Just a guess.
Good guess, When launched off of a carrier an aircraft is PULLED off he Carrier not pushed. The Catapult attaches to the front landing gear. If you compare it to a paper airplane, you always throw it holding the front. Same concept. Airflow isn't an issue till the aircraft leaves the deck at 180+ MPH.
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Old 11-12-2014, 03:07 PM
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Originally Posted by xmech2k View Post
Thanks for the input Piper. I just did some searching, and Wikipedia to the rescue! Under 'Vertical Stabilizer', it has this to say:
"The F/A-18, F-22 Raptor, and F-35 Lightning II have tailfins that are canted outward, to the point that they have some authority as horizontal control surfaces; both aircraft are designed to deflect their rudders inward during takeoff to increase pitching moment."

So there it is.
IF you wach the horizontal stabilizers when an F-18 is coming in for a landing, they are working hard, almost looks loke they are going crazy trying to keep the anles of the plane level with the moving deck. So any assistance to keeping the aircraft nose up will help.

From what I have read about the F-35C (NAVY Version) is the wing is larger and "alerions" are added to wing tips to assist in landing. I have not researched but I am guessing the F-35A is controlled by stabilators only?
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