Rudder use in an F-16

Always wondered why the F-16 has a tailhook, or how big a bigmouth F-16's mouth really is ? Find it out here !
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Libelle

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Unread post24 Jan 2005, 04:45

I was recently told that rudder is rarely used by an F-16 pilot, that turns are made soley with aileron. Is this true? Does a plane like the F-16 not suffer from adverse yaw, or is rudder coupled to the use of ailerons? In other words, does the F-16 make a coordinated turn with no rudder?
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Cylon

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Unread post24 Jan 2005, 05:17

Yes and yes...

More appropriatly, Most swept wing jets don't use rudder for turns. The amount of drag on the up wing is generally not enough to cause a noticable adverse yaw. The F-16 does have automatic rudder compensation, but even other jets like the (A)T-38 don't use rudder to coordinate turns.

There are a few of us that use rudder during BFM. Some dudes fly with thier "feet on the floor" which I can't do...Let the battle begin.

Cylon
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LinkF16SimDude

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Unread post24 Jan 2005, 05:21

Don't forget the odd crosswind landing if the winds are stiff enough. Could also be used to move around in a formation (like the T-birds do in the Diamond) and is definately used in the very rare occurance of a spin.
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IDCrewDawg

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Unread post24 Jan 2005, 10:57

The rudder is also autmatically employed during use of the gun to compensate for the yaw caused by the gun when it fires. The employment of the rudder is automatic as required by the flight control computer and imputs of the pilot on the flight stick. The rudder can be used as required by the pilot as well.
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_Viper_

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Unread post24 Nov 2007, 15:51

Cylon wrote:Yes and yes...

More appropriatly, Most swept wing jets don't use rudder for turns. The amount of drag on the up wing is generally not enough to cause a noticable adverse yaw. The F-16 does have automatic rudder compensation, but even other jets like the (A)T-38 don't use rudder to coordinate turns.

Cylon

Hmm that's interesting. When it comes to yaw corrections is this ARI (Aileron Rudder Interconnect) just a like backup system?

Thanks for the answers.
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Raptor_One

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Unread post24 Nov 2007, 20:52

The F-16 has an ARI. Not sure if it's a mechanical system or not though. I have an old -1 manual for a Block 25 that I can reference when I get home and give you a more formal answer. Someone who knows about these types of systems by heart will probably respond to you before then, however.
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guy@rdaf.dk

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Unread post24 Nov 2007, 23:29

Hi LinkF16SimDude

I don’t know anybody that uses the rudder during a cross wind landing. If you do so you will probably scare yourself as the wheels touch down and the ARI disengages. The nose will slice and you might depart the runway. The proper technique is to land in a crab and not use the rudder. After touchdown you use aileron into wind and rudder to keep you on the centerline. If that’s what you where referring to in your post then please disregard this post.

Guy
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der03301

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Unread post25 Nov 2007, 04:43

Raptor_One wrote:The F-16 has an ARI. Not sure if it's a mechanical system or not though. I have an old -1 manual for a Block 25 that I can reference when I get home and give you a more formal answer. Someone who knows about these types of systems by heart will probably respond to you before then, however.




It does not have an ARI like that in the F-15. The DFLCC schedules all coordination between the ailerons and rudder. All electrical.

The horizontal stab anhedral design almost allows the F-16 to get by without a rudder...
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SixerViper

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Unread post26 Nov 2007, 01:47

Aerodynamics 101: The rudder is primarily used to counteract adverse yaw when the ailerons are used. Adverse yaw is the drag imposed on the wing of the downward-deflecting aileron by the increased lift from that wing. This drag tends to make the nose of the airplane move away from the direction of bank. So, you use the rudder ("step on the ball") in a turn to keep it coordinated. This phenomenon is more noticeable in straight-winged airplanes than in swept-wingers. Most fighters from the Century Series forward have not required the use of rudder when turning.

The F-16s ARI, while it may be of some benefit when turning, is primarily there to counteract the yaw from firing the gun.

The anhedral of the horizontal tail is there to help keep it in clean air at high AOAs. Look at the F-4's tail for an extreme example of that. Ain't nothing gonna ruin a pilot's day quicker than losing smooth airflow over his horizontal tail at high AOAs. It's the main reason that T-tailed airplanes have a stick shaker installed in them.
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Unread post26 Nov 2007, 05:43

Careful with the terminology - a turn is a pitch command maneuver. The ARI is active in rolls, not turns. The confusion is understandable, since you have to roll into a turn. SixerViper is correct about the ARI being used to counteract yaw from ailerons, but there at least two other sources of yaw in a roll. The ARI tries to remove ALL sources of yaw in a roll. The other two sources are the up aileron deflecting air onto the side of the vertical tail in a helix flow and the anhedral tails generating a strong side force from their up and down deflection. How? Say the left tail is trailing edge down and the right tail is trailing edge up. Due to anhedral, the left tail force is up and to the left, while the force on the right tail is down and to the left. The left acting force on both tails is a strong yaw on the airplane. By the way, if this yaw acts in the same direction as the roll, it is called proverse yaw. Generally, subsonic, the horizontal tail yaw is adverse and supersonic, it is proverse.

The ARI is not used to correct yaw in gunfiring. The ARI is a set of control laws driven by roll commanded aileron angle, angle of attack, and mach number. The rudder is used in gunfire compensation, as are the ailerons, but the gunfire compensation control laws are entirely separate from the ARI.

Tail anhedral does help to keep the tails in clean air, but it also provides a significant amount of directional stability to assist the vertical tail in keeping the pointy end in front. They are especially helpful at supersonic conditions when the VT becomes less effective due to tail twist.
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Unread post27 Nov 2007, 02:14

John,

A very nice summary of ARI. The gun compensation is performed by Gun Compensation Circuitry inside the FLCC. A discrete is sent from the Gun Control Unit and the computer computes rudder (and flaperon) deflection based upon current gains.

I remember working a jet at base X with a faulty GCU which arbitrarily decided to send the discrete to the FLCC for no particular reason. It made it somewhat interesting for the pilot to have a faulty box kick in yaw (and a little roll) when he thought he was going straight and level.
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asiatrails

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Unread post27 Nov 2007, 03:53

Cylon wrote:Yes and yes...

More appropriatly, Most swept wing jets don't use rudder for turns. The amount of drag on the up wing is generally not enough to cause a noticable adverse yaw. The F-16 does have automatic rudder compensation, but even other jets like the (A)T-38 don't use rudder to coordinate turns.

There are a few of us that use rudder during BFM. Some dudes fly with thier "feet on the floor" which I can't do...Let the battle begin.

Cylon


Cylon, Me too, I was always taught feet on the pedals but take care using rudder in high alpha maneuvers as interesting things can happen.
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Weasel_Keeper

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Unread post27 Nov 2007, 10:00

It's things like this that keep me coming back to this site, and I appreciate all the answers. Hell, I'm a Crew Chief, I don't know about this stuff...and I never think to ask my pilot because it never comes to mind. You tell ME if it's working or not...heh.

What I do find interesting as a Crew Chief...and don't shoot me...a flight simmer is that I use a lot of rudder while flying sims. Even in THE USAF F-16 sim after testing for engine runs and my instructor says go ahead and have some fun, I take off, fly around, and use the rudder to sharpen my turn. It seems to work in the F-16 simulator, is it not realistic?

BTW just for bragging, AS a Crew Chief I have had 3 successful landings out of 4 in the sim (and I could have walked away from the not so successful landing...just broke my gear on a bounce). ;)
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LinkF16SimDude

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Unread post27 Nov 2007, 17:03

Well I can sure see the tried-n-true "the sim ain't the jet" statement comin'. :wink: Personally I never used rudder to sharpen a turn. Just a little more bank and a little more pull (while keepin' the vector centered on the ZPL, natch..). I never got the chance to try it for real. :x :wink:

It's kinda tough to know if you're skiddin' or not when the sim doesn't move, I'll give ya that. If your sim is the model I'm thinkin' of, I can't recall whether we installed a functional skid ball on that or not. If the skid ball works you'll see the ball slide around in the window as ya step on the rudder. And also, if the winds are calm the flight path marker will deflect from the center of the HUD with rudder usage. Just some things to notice next time you're in there.

And while we're braggin', and not that anyone gives a rat's keister, but....about 3000 simulated landings (including touch-n-go's) for me. Some greased, some not so much. I lost count at some point. Doin' a preflight/postflight with 5 or 6 touches each, 5 days a week for 12 years will tend to jack your numbers up.
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Unread post27 Nov 2007, 22:33

Well, I'm hoping not to hear about the "sim ain't the jet" thing. I was just curious because the official sim is the only way I'll ever get a chance to "fly". I give all the Falcon 4 guys all kind of grief when they talk about that anyway. ;)
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