Manual trim in F-16

Hello!

Falcon 4.0 AF manual wrotes that F-16 doesn't require much trim because the FLCS does it for the pilot. But it also says that trimming is required occasionally.

I would like to know what are the occasionally situations when the FLCS can't do it's job

Having conducted a lot of test missions on the F-16 during my days as a flight test engineer, (575 to be exact) and majority were in loads testing. I would say that the time you need to manual trim is when you have an asymmetric loading on the aircraft, because you either took off that way, or you fired missiles or dropped bombs to get yourself asymmetrical.

Cheers,

ViperDude

When equiped with a targeting pod you need to trim manually as well.

I am not a pilot and never really understood what "trim" meant and how it is carried out. Would it be possible to get a plain language definition please.

A perfectly trimmed airplane requires no control input to keep it pointed straight and level. Without the ability to "trim up" an airplane the pilot would have to maintain constant pressure on the controls, which can get very fatiguing.

Additionally, say you want to climb for 10 minutes. Do you want to maintain back pressure on the controls for 10 straight minutes (while also doing everything else you need to do)? No. So what you can do is "dial in" some pitch up trim until you get the climb rate you want. Then the aircraft will maintain that attitude until the trim is "dialed out" at the end of the climb.

For the "how", it depends. Most single engine general aviation aircraft only have pitch trim, and it's accomplished by moving "trim tabs", which are basically miniature control surfaces that "fly" the elevator in the direction desired. See http://www.americanflyers.net/aviationlibrary/pilots_handbook/images/chapter_1_img_32.jpg for a picture.

As to why the F-16 doesn't usually require much trimming (and real Viper pilots feel free to point out any rectal-cranial inversions here) is because the flight control computer sits between the pilot's controls and the flight control surfaces. The computer constantly keeps the jet's pointy end going forward when there is no input from the pilot. When the pilot yanks on his stick (heh heh) the computer decides which control surfaces to move and how much, assuming of course the computer agrees with the pilot's input.

And the are also some Fly-By-Wire passanger jets (like A320, B777) but I'm not quite sure how much they require trimming.

Additionally, the FCLS can fail as well due to whichever inflicted type of damage. You state that you fly on Falcon 4.0. Have a round trough tripple A without trying to properly egress safely. You will find the trimming functions usefull quite quickly

IIRC, the pitch trim drive on the Viper was wicked fast when using the trim switch on the SSC. Ya only had to give it a few quick taps up or down to get a response. Roll trim didnt' drive nearly as fast. For finer adjustment you could use the trim wheels on the FLCS panel.

As for the commercial jets, they're on AP from about 1500' AGL after takeoff and throughout the flight until just before touchdown. The surface that gets the most attention by far is the horizontal stabilizer. The AP will make constant minuscule tweaks to the stab trim to stay in cruise attitude and larger changes when flap settings or airspeed changes. Not too much aileron trim is required except for an engine-out condition or the odd fuel imbalance that isn't corrected either manually or via a CG compensation system.

AFAIK, depends on the aircraft. Like some guys already said here, asymmetrical weapons load will affect the trim. On the other hand, some aircraft just need trim, this could be caused by rig of the ISA, when you rig the ISA ( and by association the flight control surface) this is done with a range messurements (for example 1 and 1/2 inches to 1 inch) not a specific number, so left or right control surfaces might be off by several fractions of inch, enough (belive or not) to cause a slow small movement in any of the aircraft axis. Also, some aircraft airframe have twisted over time ( whether just normal wear or faulty maintenance) and this causes the aircraft to require trim as well.

Viper trim ain't like "normal" trim

Salute

OK, let's review the bidding.

Normal jets/planes would "trim" by positioning trim tabs or the actual control surfaces to supply a relatively constant aero force for a specific speed. So, for example, climbing you would trim back and the plane would try to hold a relatively constant AoA. If you pulled power back, the nose would lower, push it up and the nose would rise. A good pilot in the old days would soon have a small hole on the thumb of his gloves. The deal was to reduce stick pressure.

The Viper doesn't work that way.

The FLCS attempts to maintain the jet at a commanded gee or commanded roll rate. Airspeed is not a factor - only the rates. With gear down, the FLCS adds in some pitch rate and AoA bias to help, but the basic op-amps for pitch were still designed for a gee command.

ROSCOE Alert. ROSCOE alert. Help me, plz.

The roll rate trim still seemed sensitive to asymmetric loads more than I would have thought, but the Viper was less sensitive to uncommanded roll when dropping a big egg off of one side than anything I ever flew and dropped bombs from.

OTOH, when my LEF folded up, the FLCS tried to help me and I did not go into an uncontrolled roll or yaw. I still had to trim all the way and hold fifteen or sixteen pounds of pressure, but I am fairly sure the FLCS control laws were not designed to work properly with such a drastic "modification" to the aero properties the jet and I were experiencing.

The pitch trim control "wheel" was neat, as you could roll the sucker forward and trim the jet for zero gee. Let go of the stick and you had a good ballistic flight path when "extending" outta a fight.

Secondly, I would demo the pitch trim to student studly on first hop. Roll the wheel all the way aft and the jet trimmed for about three and a half gees. Do a neat loop and just sit back and watch Mr. FLCS do his thing. Up and AoA increases to hold the gee command. Aero forces decrease and soon AoA limiter reaches twenty-five degrees, gee decreases, over the top and down the backside. AoA decreases soon as trim command achieved and out the bottom at three and a half geees. Neat, huh?

The rudder trim was a pure angle of deflection - not a yaw rate or anything. It was almost like every plane I ever flew from Luscombs, Aeronicas, Tweets, Sluf's, etc.

So that's my story and I'm stickin' to it.

Gums sends ...

All the jets are torqued in one way or another nowadays. I'm always on the rudder trim after takeoff. Fortunately its a one time deal per flight. A couple of the jets have some airspeed issues as well. Slow subsonic roll trim is one way and high subsonic requires trim the other way and above the Mach the trim needs to go the other way yet again. I've never flown a Viper with zero manual trim inputs.

Guysmiley wrote:The computer constantly keeps the jet's pointy end going forward when there is no input from the pilot. When the pilot yanks on his stick (heh heh) the computer decides which control surfaces to move and how much, assuming of course the computer agrees with the pilot's input.

Does that mean no matter where the F-16's nose is pointing or even if the plane is upside-down and/or descending the FLCS tries to keep it going forward

Hold on SnakeHandler. Do you mean the torque caused by the turbofan engine?
http://www.aerospaceweb.org/question/dy ... 015a.shtml
The last sentece says that

"Even the rotating fans and turbines in modern jet engines will do so, but these effects are usually so small that they are difficult to notice. In addition, modern fly-by-wire control systems probably correct for these effects automatically. "

Is this an issue in Viper? Of course torque would be nothing compared to WW2-era fighters (P-51, Spitfire, Fw-190)

Perverted, oops, I mean "inverted" flight

The Viper trim is limited to about a negative gee and a half. You can roll inverted and trim "forward" and the sucker will hold level flight, inverted until you get real slow, no stick commands required.

With gear down, you can't hold one gee inverted below about a hundred and fifty knots or so, trim or no trim. It's in the FLCS control algorithm, so look it up.

Folks have to understand that this machine "ain't like any machine" you ever flew before unless you came from Nighthawks, Raptors, Lightning Deuces, the Space Shuttle, and prolly some stuff at Groom Lake.

Our biggest fear with the two-elltees checking out was that they would forget how most planes actually flew. Apparently, they did O.K. after leaving the Viper and flying "conventional" jets. Whew.

Gums sends ...

Thanks guys, now I have an idea what we are talking about.

Does that mean no matter where the F-16's nose is pointing or even if the plane is upside-down and/or descending the FLCS tries to keep it going forward

No, I mean forward as in "into the wind", 1g, no roll rate. But Gums and Snakehandler are much better equipped to explain it.