Analog and Digital flight control systems

The laws of aerodynamics are analog.
Digitalising the imputs from stick/ yoke and pedals, than processing them with the flight control computer and than converting them back in analog performance and movements of rudders and/ or flaps/ slats is a detour.
For example, the aerodynamic proccesses during ground-effect and stall are difficult to 'translate' into a digital proces.

Why does block 15/ 20/30 Vipers use an analog FLCS and the block 40/ 50/ 60 Vipers use a digital FLCS? And what is the difference (pros and cons) between those 2?

Can somebody shed a 'cybernetical light' on this matter?
Over.

I think you may be misunderstanding the concepts behind "flight dynamics and control". Specifically, I think you may not be aware of more general control theory concepts. I'm not the best person to explain them to you, but you might want to get your hands on an electrical engineering text book or something along those lines. Read up on things like digital to analogue conversion, analogue to digital, etc. Then you might want to start looking into things like proportional (P) control, proportional-integral (PI) control, and proportional-integral-derivative (PID) control. The knowledge you seek is more complicated than you might think. Anyway, translating a signal from the pilot's stick and rudder pedals into movements of the various control surfaces can be done in a variety of ways. You can use purely mechanical linkages, a mix of hydraulics and mechanical linkages (hydromechanical), a mix of hydromechanical and electrical signals to move the control surface actuators, or you can rely purely on electrical signals for control surface actuation.

The F-16 relies on electrical signals entirely. Electrical signals can be dealt with in a number of ways. They can be interpreted by a digital computer after some sort of analog to digital conversion and then converted back into 'analogue' control surface movements via some sort of digital to analogue conversion. This may be a terrible explanation for a digital FCS, but that's my understanding of it. I was never big on electrical engineering. Anyway, if you're using a digital computer/proccesor/whatever to implement your flight control laws, I think you will have more control over how they're implemented. If you use analogue circuitry however, you're relying completely on the physical logic of the circuitry to implement your control laws. In other words, you're using things like capacitors, resistors, diodes, etc. to interpret your input and give you the desired output. If you need to make changes to how your control laws are being interpreted, you'd have to go in and change the physical circuitry. With a digital system however, you generally only have to make changes to the device's firmware. Sort of like upgrading your motherboard's bios as opposed to redesigning your motherboard (or perhaps just making some small wiring changes).

Again, this explanation might be severly flawed. Someone please correct me if that's the case.

In any digital control system, the proper ratio of step size (command interval) to system time constant makes the resulting output indistinguishable from a "smooth" analog system.

Your basic premise that "reality" is an analog process is correct. However, an analog computer means a panel of resisters, capacitors, wires, etc... Reprogramming consists of replacing hardware parts. On the other hand, digital is all done by software, is much easier to maintain, way more flexible in terms of reprogramming, and is much more capable and robust. Digital flight controls also make it infinitely easier to vary feedback gains as a function of flight condition.

I have a masters degree in aero engineering with an emphasis in flight controls. Trying to explain sample rate, how the process is translated into the "w" plane via transforms, would take a semester class. Suffice it to say that every modern military aircraft has digital flight controls.

The early blocks had analog because that was state of the art at the time. Starting with the C-model (Block 30, contrary to your statement) the flight controls went digital.

Block 40 and on, actually.

Best Regards,

JoeSambor wrote:Block 40 and on, actually.

Best Regards,

I'm going to back Roscoe on this one. Digital FLCS entered service on Block 30s from everything I've read. I could give you references if you want.

Block 25's are C models as well..

EBJet wrote:Block 25's are C models as well..

Yes, but those didn't get the digital FLCS.

I have been working F-16s since 1982. I have worked Block 10, 15, 25, 30, 40, 50, 20, and now MLU. You just have to believe me when I tell you that Digital FLCS started with Block 40.

Not to dump on Roscoe, but even the most qualified among us have difficulty remembering stuff from time to time, unless they are exposed to it on a daily basis. When I was in F-16 Product Support Engineering I backed up the FLCS guy. I spoke daily with the FLCS engineers. I have access to every tech order for every Air Force. I know which F-16s have analog flight controls and which ones don't.

Can some real F-16 maintenance guys back me up on this?

Best Regards,

Joe,

You know what... it seems that you are right. The DFCS was came online with the Block 40/42. Were Block 30s ever upgraded with the DFCS though? Perhaps that's what I'm remembering.

ive always worked block 40's with digital, but when i went to augment the reserves for a few weeks and their block 30's they had analog controls.

As Roscoe said, the original question requires the understanding of what "analog" is and what "digital" is.

In general, an analog signal is infinitely variable from a specified minimum to maximum value. For example, say an analog sensor measures tempature. Say its lowest value is 0 degrees and its highest is 100 degrees. Say that at the lowest value it has an output voltage of 0V and at its maximum it outputs 10V. What makes it analog is that it can output 2.5v, or 3.25v or 1.0000001v. There is an infinite number of values that could be output.

With a digital signal, everything breaks down eventually to 0s and 1s. When you talk about digital signals the first things that generally come up are resolution and sampling rate.

Resolution is usually defined in bits. A bit can be either a 0 or a 1, right? So say you have a 1 bit digital signal. It can either be a 0 or a 1. In a linear ADC, what would happen if you took the example above, you would assign a 0 to any value less than Max-Min / n^2, in this case 5. 5v and up would be a value of 1.

A 1 bit digital signal is pretty crappy for measuring tempature then. So say instead you have a 12 bit signal. That gives you 4096 possible values for the signal, from 0000 0000 0000 to 1111 1111 1111. Not quite "infinite" like an analog signal, but much better, no? Ok so given this, what is the smallest possible change that could be detected? Well, now instead of 5v or 50 degrees, it's down to 0.0244v or 0.244 degrees. Is that enough fidelity? Well, that's what electrical engineers get paid a lot of cash to decide.

The other big quality (and yes there are others, but I'm not writing a dissertation here) is sampling rate. How often do you need to see changes in this data? Is once a second enough? 10 times? 100? CD audio is sampled at 44,100 times per second for 2 channels (R and L) and it uses a 16 bit resolution. That is a pretty big chunk of data (about 11 megabytes per second), but human ears are pretty sensitive...

So why bother converting signals to digital in the first place? As Roscoe said, because it's the language of computers. Once you have the signal digitized, you can feed it into a processor and "do it in software". There is extra effort up front, but once you can tweak code instead of swapping out boards, life becomes much easier when it comes time to make changes.

cchief16 wrote:ive always worked block 40's with digital, but when i went to augment the reserves for a few weeks and their block 30's they had analog controls.

Do they still have Block 30s with small mouth (or normal shock if you prefer) inlets operating? Block 30s were a strange mix and I wouldn't be surprised if some actually had DFCS. Why not? There were two different types of inlets on them.

No Block 30s were ever produced with, or modified to, the DFLCS configuration, with the possible exception of test birds at Edwards. I think there may be one or two but I would have to defer to someone who has worked them at Edwards. I think there was at least one Block 25 aircraft that was modded to Block 40 configuration.

Customers have asked if it is possible to upgrade to DFLCS, and of course anything is possible, if you throw enough $$$$ at the problem. Nobody has thrown enough at that particular one yet.

Best Regards,

JoeSambor wrote:No Block 30s were ever produced with, or modified to, the DFLCS configuration, with the possible exception of test birds at Edwards. I think there may be one or two but I would have to defer to someone who has worked them at Edwards. I think there was at least one Block 25 aircraft that was modded to Block 40 configuration.

Customers have asked if it is possible to upgrade to DFLCS, and of course anything is possible, if you throw enough $$$$ at the problem. Nobody has thrown enough at that particular one yet.

Best Regards,

That's gotta be it... the Edwards test birds!!! I could have sworn reading some technical reprots or thesis papers (perhaps coming out of the USAF's Insititute of Technology?) that focused on features of a Block 30's DFLCS. I tried to look for them, but I have so many tech reports on my computer just about the F-16. I didn't feel like going through all of them. What you say about the test birds makes perfect sense now.