Analog vs. Digital FLCS

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Patriot

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Unread post17 Oct 2019, 14:41

I dont know whether someone asked/explained this before but, how does an analog and digital fly-by-wire differ in terms of physical components and functionality looking from a technician and pilot POV?

All I know is that all blocks up to block 30/32 included have analog one, blocks 40/42 and later ones have the digital.
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basher54321

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Unread post17 Oct 2019, 17:14

Some info here on functionality:

I don't believe any analog airplanes were ever converted to digital controls. The first digital airplanes were Block 40, as already stated. The original digital control laws were identical to the latest (Block 30?) analog control laws. USAF insisted there be no changes at that time, so they could identify any flying quality changes as being due to switching from analog to digital computers, not control law changes. I don't have any knowledge about more recent control law changes, but I suspect there have been some. viewtopic.php?f=21&t=29015&start=15
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Unread post20 Oct 2019, 18:13

I was initial cadre on Block 40/42 and LANTIRN. It's been awhile, but as I recall we had two Block 30s at Edwards that had their FLCS systems converted from analog to digital for DT&E reasons. Again, if I recall correctly, the stated reason for going from analog to digital was the LANTIRN TFR requirement. In order for the LANTIRN TFR pod to be able to interface with the aircraft's FLCS, both needed to be digital. The pod was digital, and the aircraft was not. The pod needed to work with both the F-16C and the F-15E, so it had a digital system which enabled it to work with both. Since GD was already making major changes to the F-16C to accommodate increased weight, it was decided that it would be a good time to also change out the FLCS to a digital one.

The change to the digital system was supposed to be transparent, with no change to the flying characteristics. While there were no discernable differences in 95% of the envelope, I did find that it handled a bit differently (worse) at high AOA during rolling maneuvers in air-to-air.
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rheonomic

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Unread post20 Oct 2019, 21:39

AFTI F-16 also was modified analog jet that had a prototype DFLCS. VISTA similarly was a block 30 which I think was updated to include the base DFLCS plus the Calspan stuff.

I don't think the CLAWs changed much if any for the analog to digital jump (might have been some gain tweaking). Block 60 is probably another story.
Meteor wrote:While there were no discernable differences in 95% of the envelope, I did find that it handled a bit differently (worse) at high AOA during rolling maneuvers in air-to-air.

Interesting, but not surprising.
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Unread post21 Oct 2019, 03:24

All the answers above agree with my memory of the change from analog to digital. I have a few more thoughts on the topic. The analog system on early F-16s not only had analog computers but also analog sensors input into the computer and analog outputs to the flight control actuators. The sensors were for linear accelerations, angular rates, AoA, airspeed, altitude, stick and rudder inputs, and control surface positions. The analog outputs to the control surface actuators sent signals to the integrated servo valves on the actuators. In the computer, analog signals were processed as DC voltages continuously, not so many samples per second as in digital.

With the switch to digital computers, the inputs and outputs remained analog, so A to D conversion was done on input signals and D to A conversion was done on output signals. Thus the entire system is hybrid analog - digital.
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Unread post23 Oct 2019, 11:05

Great info guys! Thank you.
Does anyone maybe have any actual pics of the FLCS components? The FLCS computer itself, AOA limiter, the cables, the servos? If not pics maybe some schematics showing how the system is distributed arround the airframe?
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rheonomic

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Unread post24 Oct 2019, 03:09

"You could do that, but it would be wrong."
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Unread post03 Nov 2020, 01:23

Meteor wrote:I was initial cadre on Block 40/42 and LANTIRN. It's been awhile, but as I recall we had two Block 30s at Edwards that had their FLCS systems converted from analog to digital for DT&E reasons. Again, if I recall correctly, the stated reason for going from analog to digital was the LANTIRN TFR requirement. In order for the LANTIRN TFR pod to be able to interface with the aircraft's FLCS, both needed to be digital. The pod was digital, and the aircraft was not. The pod needed to work with both the F-16C and the F-15E, so it had a digital system which enabled it to work with both. Since GD was already making major changes to the F-16C to accommodate increased weight, it was decided that it would be a good time to also change out the FLCS to a digital one.

The change to the digital system was supposed to be transparent, with no change to the flying characteristics. While there were no discernable differences in 95% of the envelope, I did find that it handled a bit differently (worse) at high AOA during rolling maneuvers in air-to-air.


My airplane 83-1176 (5D-3) was the DFLCS testbed aircraft a Block 25. I actually wrote the first Digital Flight Control troubleshooting white pages, called " The Lazy Mans guide to Digital Flight Controls".
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Unread post03 Nov 2020, 05:11

From a pilot perspective, I never noticed the difference between my background in digital FBW Hornets and later on flying the analog FBW block 15's. In fact, once the jets were in gear up/flaps up configuration, I thought they flew largely the same aside from the increased engine response/thrust of the Viper comparatively. I'm sure a test dude could tear that assessment apart, but from a simple man's perspective, there was basically no difference........there were some distinct differences in FCS/FCNS control laws, but nothing specific to digital vs analog as best as I could tell.
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Unread post05 Nov 2020, 14:00

35_aoa wrote:From a pilot perspective, I never noticed the difference between my background in digital FBW Hornets and later on flying the analog FBW block 15's. In fact, once the jets were in gear up/flaps up configuration, I thought they flew largely the same aside from the increased engine response/thrust of the Viper comparatively. I'm sure a test dude could tear that assessment apart, but from a simple man's perspective, there was basically no difference........there were some distinct differences in FCS/FCNS control laws, but nothing specific to digital vs analog as best as I could tell.


I completely agree, from the pilots seat of the pants flying there is no way you can tell the difference. I do believe that the sensor tracking is better with the digital, but the F-111 proved you don't need digital flight controls to shack the target... :2c:
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35_aoa

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Unread post06 Nov 2020, 08:01

magnum4469 wrote:
35_aoa wrote:From a pilot perspective, I never noticed the difference between my background in digital FBW Hornets and later on flying the analog FBW block 15's. In fact, once the jets were in gear up/flaps up configuration, I thought they flew largely the same aside from the increased engine response/thrust of the Viper comparatively. I'm sure a test dude could tear that assessment apart, but from a simple man's perspective, there was basically no difference........there were some distinct differences in FCS/FCNS control laws, but nothing specific to digital vs analog as best as I could tell.


I completely agree, from the pilots seat of the pants flying there is no way you can tell the difference. I do believe that the sensor tracking is better with the digital, but the F-111 proved you don't need digital flight controls to shack the target... :2c:


Hah, yeah. We actually weren't allowed to do any air-to-ground deliveries of any kind in our Vipers.

Also, oops, I meant to write "FLCS".....FCNS is a super hornet and growler thing that is unrelated.
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Unread post13 Nov 2020, 15:43

When I was the wing weapons and tactics officer at Cannon (late 1998 to summer of 1999) we had one Block 40 squadron, two Block 30 squadrons and one Singaporean Block 52 squadron. I flew primarily with the Block 40 squadron; but, since the ops officer of one of the Block 30 squadrons was an old friend, I got a lot of Block 30 time in addition. I never flew with the Sings. It was not, however, unusual for me to fly a digital and analog FLCS in the same day. There is a subtle but noticeable difference in aircraft 'feel' at lower speeds and lower g, which, in the end, amounted to about the square root of zero.

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