Is the F-16 unstable?

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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Chrisse

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Unread post27 Apr 2004, 20:22

Hello,

I have read an article on a site on the web that the F-16 Falcon I unstable. But I can´t find out if that is true. I think that it isn´t, but I´m not sure

I´m glad if someone can help me with that :D
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IDCrewDawg

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Unread post27 Apr 2004, 20:46

Chrisse wrote:I have read an article on a site on the web that the F-16 Falcon is unstable. But I can´t find out if that is true. I think that it isn´t, but I´m not sure


I wouldn't say unstable, however during the inital stages of flight testing the viper (correct me if I am wrong fellas) the viper was tested with out the ventrals, and because it was without those, it tended to slip during turns. Which could be viewd as "unstable". I know that I have seen the ventral explode from improper repair and the pilot had no indication that he was missing parts of his airframe, thus giving me doubt to what I had just told you.

I doub't that the viper has ever been unstable as you might think, now however the Night Hawk is fairly unstable as airplanes go, but because of the advancement of flight control computers it flies very smooth and stable.
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TomaTo

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Unread post27 Apr 2004, 21:12

almost all modern fighters are designed from start to be un-stable, because this makes you do better maneuvers.

However, if it hadn't been for the advent of the Fly-by-wire system, none of these planes couldn't be flown. Onboard computers make tiny adjustments to ailerons, flaperons and so on, automatically and all the time.

So, if you take this in mind, yes the Viper is unstable, but the computers make it stable to fly.
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greg

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Unread post27 Apr 2004, 21:16

The F-16 has "relaxed static stability". That means that F-16 is designed to be very close to unstable as you said.The F-16 has the FLCS (Flight Control System) that protects the jet by placing limits on what the flight controls will let the pilot do (maintaining G's). As can you understand, the pilot many times makes mistakes in the stick.The FLCS is correcting this mistakes. This system helps the plane to be very close to "unstable". No, the F-16 is not unstable. Many times the F-16 comes to stall conditions.Stall is very low AOA, and its easy to recover if you have a good altitude.Deep stall is another thing. Deep stall is a more complicated situation that F-16 comes.This matters when we are saying: "It is failling like a rock!!". Also and in this case, the F-16 can be recovered, but with some procedures like overriding FLCS because of its mistakes during the stall... Keep in mind one thing... the F-16 is a great plane...
It doens't matter how many G's you are taking,but it matters if you are taking them rapidly
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marsu

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Unread post27 Apr 2004, 22:56

greg,

TomaTo is actually right. The F-16 is inherently unstable by design to improve maneuverability. It is only through the Flight Control System (which continuously generates minor but necessary corrective control commands) that the F-16 is able to achieve controlled flight.

It is not that the pilot makes mistakes (the idea alone borders on blasphemy :-) ), it's just that only a computer can react fast enough to keep the F-16 under control. Even when the pilot isn;t doing anything, the computers are working hard, moving control surface continuously, to keep the aircraft straight and level.

there's actually another thread in this forum that covers the FBW system:

http://www.f-16.net/f-16_forum_viewtopi ... ywire.html

marsu
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Unread post28 Apr 2004, 21:44

Simply put, without electricty or hydraulic power the jet will not fly. This is the BIG reason the viper has it's EPU to provide emergency power (both electricty and hydraulic) in the event the primary systems or the engine fails.

Someone help me here, but I believe the F-16 will go nose up and swap ends if all the surfaces are at neutral due to the fact the CG is slightly behind the CL? (opposite of "conventional" aircraft)
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FlightTestJim

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Unread post28 Apr 2004, 22:12

The F-16 is "aerodynamically" unstable in the pitch axis during subsonic flight. Joe Bill Dryden wrote a good article a while ago in Code 1 on this.

"This (the F-16) is the first operational aircraft intentionally designed to have a negative static margin. In subsonic flight, the F-16 is negatively stable (read, unstable) in pitch. "

http://www.codeonemagazine.com/archives ... /f16_aero/
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lamoey

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Unread post28 Apr 2004, 22:31

The F-16 was the first operational fighter that was made unstable on purpose, were the FLCS maintains the stability, and not the airframe design. I have not heard that it will flip backwards if all control surfaces (CS) stick to neutral, but it does make sense as it then makes the plane faster to react in the pitch axis. I have heard that after the initial command to the FLCS to start a pitch change, the horizontal stabilizer (HS) moves up in the normal way to start the change in pitch attitude, but to give the pilot the amount pitch change, or G, he is requesting through the pressure he puts on the side stick controller, the HS may actually move all the opposite way to prevent the plain from toppling over backwards.

The F-16 can still be controlled if up to 3 of the 5 control surfaces (CS) have failed and gone to a neutral position, but not any 3, obviously. If the FLCS senses a problem in one of the four redundant channels for any given CS the pilot is then advised to “ARM” that CS. The given CS will continue to move as usual, but if a second channel fails for the same CS that CS will move to a neutral, mid position and stick there, come hell or high water…
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Gums

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Unread post29 Apr 2004, 03:24

Super post.

PLZ refer to the referenced posts above for more.

The relaxed static stability was the result of having the mean aerodynamic center of pressure very close to the center of gravity. If not pulling with some increased angle of attack, it was clearly 'unstable'. Only the FLCS control laws and resulting control surface movements kept the pointy end forward.

Some neat things.........

- the horizontal stabs would initially move leading edge down when initiating a hard turn, then go leading edge up to keep the AOA under control. This is how one could get into a 'deep stall', hard as it was. even with full leading edge up, the tail couldn't get the nose down. Hard to get into, but hard to get out of until we gt the 'pitch override' switch and the 'big tail'.

- The aileron-rudder-interconnect was so good that in a high AOA condition, the rudder would deflect a whole lot when you rolled, even though you had your feet on the floor versus the rudder pedals. Used to make student studly look back and watch this.....

Not so neat..........

If all hydraulics were lost, or the computers shut down, control surfaces would seek a 'neutral' point and the actuactors would lock when some 'finger' doofers reached a detent/lock. The jet would then normally go nose down, not due to CG being ahead of MaC, but just the basic 'neutral' position. I don't recall any Viper going nose up when the computers shut down. Some electrical system mods kept the FLCS running no matter what, and later problems resulted in more benign degradation of control authority. So we only lost one troop before the problem was fixed. Others had the 22 negative gee experience, but bailed before it was too late - not Jay Moats, though.............

later,
Gums
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"God in your guts, good men at your back, wings that stay on - and Tally Ho!"
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lamoey

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Unread post29 Apr 2004, 18:02

A few comments to Gums exelent post:

Gums wrote:the horizontal stabs would initially...

I guess the 30% increase in tail surface area introduced on the block 15 came as a result of the problem recovering from too high AOA.

Gums wrote:If all hydraulics were lost...

I am sure it has happened, and will in the future, but with all the redundancy built in to the viper's hydraulic and electrical supply to the FLCS systems is hard to imagine them all failing at the same time.

There are two independent hydraulic systems that run from pumps connected to the engine gearbox, so unless the engine totally freezes, the oil will keep flowing. Then there is the EPU that can keep the oil flowing as well in one of the systems, for a few minutes at least, if all ells fail.

On the electrical system there is four independent sources, the main generator, the main aircraft battery, the EPU and finally, if all these have failed, each of the four independent channels of the FLCS has its own battery that would run converters to generate the necessary 28V/400hz for the FLCS.

I know there where modifications made to the electrical power system for the FLCS after I left the job as FLCS technician in 1985, so I can't speak for these.
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Unread post30 Apr 2004, 04:02

I guess the 30% increase in tail surface area introduced on the block 15 came as a result of the problem recovering from too high AOA.


Actually the increased tail surface was introduced with the addition of the ability to carry stores (sensors) on the inlet (5L/5R), to compensate for the forward shift in both CP and CG when pods were carried on the inlet. The added effectiveness in pitch authority was just a nice by-product.
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Gums

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Unread post30 Apr 2004, 04:53

Salute!

TNX for nice words.

As far as original design decision for the 'big tail', all I remember is that we didn't have to manually transfer fuel forward anymore. Initially, we would run the gas forward to help keep us outta the dreaded deep stall. Was very, very hard to get into one, but a few folks managed to do it.

Secondly, we lost two birds within a short period at Hill due to FLCS computer shutdown. First was USAF's first fatality, as Jay was very low when he punched. He ejected when the negative gees got down to minus 3, GASP!

Second one, the pilot actually saw the lights come on while looking down at FLCS panel. Before the jet went ballistic, he noted that the EPU 'run' light was illuminated. Then, a troop at Eglin noted the EPU run light come on when not commanded and pulled throttle back, returned to base.. Saved his butt, too.

Despite all the power sources, what was happening was that the EPU's permanent magnet DC generator was hitting the FLCS computers with too many volts!!! The voltage regulator was not capable of handling anything above 37 volts or so, if I recall the number. Instead of switching to the small batteries, the COMPUTERS SHUTDOWN to 'save' themselves!!! When GD briefed the Wing that day, our Wing CO came outta the seat and almost strangled the GD computer geek.

To get us flying again quickly, the computer power supplies were modified to NEVER shutdown and to also accept power directly from the main aircraft battery. So until the entire power supply design was changed, we lost at least one other bird the same way. But this time, the computers didn't shutdown, they gradually melted, heh heh. The pilot rode thru a few wahoo's extending over 4 or 5 minutes, maybe longer, before the jet became uncontrollable, but the FLCS degraded fairly slowly. Second part of the mod was to work on the bleed air valve that was failing and turning the EPU turbine (remember, bleed air if motor running, hydrazine when not).

out,
Gums
Viper pilot '79
"God in your guts, good men at your back, wings that stay on - and Tally Ho!"
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f100pw229

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Unread post01 May 2004, 18:20

Great posts! Great forum. :D One of the reasons the F-16 is the best plane!

At the USAF Academy [didn't go there, not in the AF yet, but during an open-house seminar thingy] I visited the aero department and they have a simulator. I was flying our favorite bird @ about 700kts @ 40,000' when the instructor turned off the FCS, and of course it just flipped out :P Then, he restarted the flight program that starts you in final approach. I landed it pretty well, then getting out of the seat he noticed he never turned the FCS back on!

Now, I know it was just a flight sim, so it ain't all that accurate. However, it was just the FCS system that was turned off, so, all power was available - would one be able to control the plane?

When I was landing, it was real jumpy. Could a pilot still keep her airborne at low speeds like this? I didn't think it was possible.

Thanks.
Happy landings -- Jeff - AIM <jeff16falcn>
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lamoey

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Unread post05 May 2004, 21:15

If the FCS was shut down none of the commands you send with the side stick controller would ever reach the controll surfaces, so you would still be up there going straight forward into space... (not taking the natural instability into account). I would say that this is a potential error with the simulator software, or a nice sim controler telling you what anybody in your position would like to hear. 8)
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