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JoeSambor wrote:

Just for fun sometimes we used to grip the sidestick very tightly while hydraulic power was applied...you can actually see the flight control surfaces move in response to the pulse in your hand. True!

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_Viper_ wrote:

Sorry to bother You guys again but can any one of you tell or estimate how large these control surface movements really are? I mean is it able see or notice when the plane is flying level? Of course I forgot to ask this last time but what can you do. Embarassed

Depends on what the plane is trying to do. Just staying at level flight, the movements are going to be minimal, and probably not noticeable. On the ground during trim checks, you are able to see the movement very clearly as your going through the entire "trim envelope".

Nobody is going to see the rate gyro-commanded control surface movement of an F-16 in flight, even if you're flying formation with the jet. The movements are that small and the rate gyro package is so sensitive that the counteractive surface movement is made before the human eye can see the airplane move. It takes very little movement of a control surface on just about any airplane to effect a big movement of the airplane.

Now, when a pilot slams the stick in one direction or another, you can very easily see the control surface move. This occurs a split-second before the plane makes a huge movement in whichever axis is commanded.

Ach! Many thanks SixerViper for your informative answer. It really explained the whole idea of the gyro-commanded control surfaces Applause

I mean particularly when Gums mentioned that control surfaces are moving whole time; you just can't see it when flying in formation or level or even not touching the stick etc.

When the jet is in the air the FLCS controls the surfaces and filters pilots commands as all other guys mentioned before.The only case where the actual input of the pilot goes directly to surfaces is when the jet is on the ground!The horizontai tail moves due to the nosewheel/jet bumping (up /down) during taxi and because the FLCS thinks ,even on the ground, that because the nose of the jet is moving up/down it must compensate this tendency by moving the horizontai tail! Not Worthy

Elite Joined: May 11, 2007 Posts: 666 Status: Offline

I found when flying if I left my hand on the stick, as you would normally do, the aircraft would bob up and down. (also called porpoising)
The controls are so sensitive they react before you can adjust. So I was told to hold the stick with my fingers on the top just to keep out any other movement. I did and it worked.
The point is the F-16 is very sensitive but if you want to yank her around then you have better feel as if you were flying a non-fly-by-wire plane.
Kinda like the force feed back effect you get from some joy sticks on computer games.
By far the most fun you can have with out without clothes on!
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Veteran Joined: May 24, 2007 Posts: 379 Status: Offline

velos35,
There is a flight case where pilot command goes directly to the surfaces. It is called pitch over-ride, used to recover from a deep stall. Deep stall occurs at a very high AOA (~60 deg) where the horizontal tail effectiveness is essentially zero. If not corrected, the airplane will maintain 60 deg AOA all the way to ground impact. If deep stall occurs, the pilot selects pitch over-ride and is given direct control of tail position. He can then use pitch rocking (like a rocking chair) to try to break out of the deep stall.

Veteran Joined: May 25, 2004 Posts: 385 Status: Offline

While the side stick controller is sensitive, it takes a great deal of force to get to a 9G turn. I was once told that some pilots have difficulty reaching 9G because of this, but have no personal, or first hand knowledge that confirms this. What I do know is that sitting in the cockpit during FLCS checkout procedures, where force is applied to the SSC to measure surface deflection during various flight configurations, you better be in reasonable shape, as it takes time to do it and you have to apply a lot of force to get maximum deflection. No wonder at least two SSC's have broken clean off in flight (as of 1985), but that is another story.

Veteran Joined: May 24, 2007 Posts: 379 Status: Offline

Control surface deflection on the ground has NO relation to control surface deflection required to pull 9g in the air. Every flight condition (mach, altitude) has a different tail deflection for 9g. But the stick force for 9g is constant and it isn't all that difficult to apply it. As I recall, a tremendous pull of 17 lb is required and my grandmother could do it if she were still alive. And from flight test results, it can be applied in 0.1 seconds. Maybe those two SSC broke off because the pilots were pulling far harder than necessary.

Elite Joined: May 16, 2003 Posts: 797 Status: Offline

Salute all!

So I need john-boy to dig up some of the early GD "Technical Description" pubs that I had to use to plan and conduct the initial academic training stuff for the first training squadron back in 1979.

My Flight Control document shows 8 gee command at 31 pounds of force on the stick, with a breakout force of 1.75 pounds. If you figure 1 gee normal flying around plus the 8 gee command you get 9 gees .... duuuuh. So it comes out about the basic USAF spec of 4 pounds per gee that the old planes had to meet using bob-weights and dampers and such.

During that first year we had GD folks measure actual stick commands by we dweebs and saw 150-200 pounds of back stick commands. Hell, we couldn't tell when we had commanded max pitch. The original stick didn't have a discernible "stop" to it. So Block 1A or 5 had the one-eighth inch movement. Only thing we pilots noticed was a smoother formation flying feel. We still pulled as hard as we could when we wanted max pitch command.

I disagree about seeing the control surfaces moving when taxiing, especially on a rough surface. Suckers were jittering all the time. In-flight, hardly noticeable, but I guarantee they were moving ever so slightly.

**************

If we could have some $$$$$, ol' Gums could have a few of those old GD documents converted to PDF for all to see. I am not gonna let those things ouuta my nicotine-stained hands, but will be there when they are digitized. It's a personal, historical thing, don't ya see?

I have the Flight Control, Propulsion, and Stability& Control "Technical Description" documents.

John-boy can go to the LM library in Ft Worth and get ALL OF THE SUCKERS!!!!!

If the SR-71 tech order is on the net, then I think we oughtta have the original Viper stuff there as well, huh?

Gums sends ....

Veteran Joined: May 24, 2007 Posts: 379 Status: Offline

Gums, I sure could be wrong about that 17 lb figure, might be confusing it with the max roll command force. I'll check it out with my sources. I'm pretty sure it is less than 31 lb, but I have been wrong before. Its been 21 years since I worked F-16, so the memory cells may have faded a bit. Speaking of the old Technical Description Docs, guess who co-wrote the Structural Description Doc?? Wish I still had a copy.

Wish I could go the library at LM, but I've been retired several years, so maybe one of my young pups could dig them up. Those guys are now managers and directors, hard to believe.

Hope you have a good Christmas!

velos35 wrote:

The only case where the actual input of the pilot goes directly to surfaces is when the jet is on the ground!The horizontai tail moves due to the nosewheel/jet bumping (up /down) during taxi and because the FLCS thinks ,even on the ground, that because the nose of the jet is moving up/down it must compensate this tendency by moving the horizontai tail! Not Worthy

This isn't correct, but it's going the right way. The horiz. tail moves as the airplane is taxiing because the pitch rate sensor gyro senses ACTUAL NOSE MOVEMENT up and down and directs the tail to correct for it. That movement comes from the nosewheel riding up and down imperfections in the pavement. The flaperon movement is less noticeable due to the fact that the flaps are down when the gear is down, but it's there.

It's been a couple of years since I've worked this jet, but if my two brain cells remember right, the stick's force sensor only recognizes 25 lbs of pull in any direction, regardless of how much force the pilot puts on it. Whatever G-load is commanded is determined by AOA, current g-loading, and what the pilot wants, cross-referenced against what the FLCS OFP will allow in that particular circumstance.

Elite Joined: May 16, 2003 Posts: 797 Status: Offline

Salute!

Yeah, Sixer, you can pull or yank as hard as you want, but once past 34 pounds in pitch and 17 pounds in roll, you are getting all she can give you.

Those numbers are for original Bk1 jets we checked out in in 1979.

When holding the LEF failure jet level, they figured I was using about 15-16 pounds of roll command of the 17 pounds the FLCS would accept. O.K., as I made it.

John-boy!!! I may have an "inside" source and shall try to get with him before he retires this next 5 or 6 months. I ask yiou to do the same.

Gums sends ...

Veteran Joined: May 24, 2007 Posts: 379 Status: Offline

Will do, Gums. What all do you want?

Rolling down the taxiway, pitch rate may have a small input into tail movement, but the normal accelerometer (nz) effect is much stronger. At taxi conditions the main gear struts are solid, so the only spring is tire deflection. That makes for a stiff ride and causes the accelerometer to feel a few tenths of a g (+/-).

The pitch command gradient in the F-16 FLCS goes up to a little more than 10 (g's incremental) at roughly 32 lbs. There are limits downstream of the gradient, though, that cap the command at 8.3 (or less, depending). That value (8.3) falls out of the gradient at roughly 23.9 lbs. There is also the aformentioned 1.75 lb 'software' breakout prior to the gradient, so that would put the actual (pilot applied) force for max command at about 25.6. (I believe there may also be a mechanical breakout force on the stick itself, but I don't remember exactly - in any case it would be relatively small.)
I don't remember what the max force value the stick actually senses is - it may correspond roughly to that 32 lb point in the gradient (as I'm pretty sure the F-16 was never intended to be an 11+g airplane). Doesn't really matter, as the bottom line is that any force over 26 lbs is extraneous.
I'm fairly certain that these number haven't changed recently, if ever. I compared some older (15+ years) and newer diagrams, and there was no difference.

JohnW: I'm not sure where you might have got 17 lbs from - maybe from external stores? Assuming 17 lbs is before the breakout (15.25 lbs to the gradient), you get just about 5 g's, which would be ballpark...

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