fm and scas logic w.ip. FMC, SAS, CAS, and FM

[NeedzWD40]

Note: I'm going to work from the AH-64A -10 (TM 1-1520-238-10) which is listed as Distribution A (https://www.abebooks.com/Operators-Manual-Helicopter-Attack-AH-64A-Apache/30838982215/bd); the present document I have is dated August 31, 1994. I understand that the AH-64D likely has many changes from the A model, but this is only as a general reference in an attempt to understand the DCS AH-64D SCAS/SAS/FMC system.

Presently, the SCAS/SAS/FMC on the DCS AH-64D provides SAS, CAS, and various active and passive hold modes. For example, heading hold applied at low speeds/hover when pedal and/or force trim release is not pressed. This system is ostensibly to allow for reduced pilot workload throughout the flight regime, given the overall complexity of the aircraft, employing weapons, operating sensors, and the massive array of other potential taskings. However, within the module, the capabilities of the SCAS/SAS seem to be diminished; that is to say, when the FMC functions regarding the various channels are shut off (pitch, roll, yaw, and collective), the aircraft's handling is not noticeably altered.

The experiment conducted was a hot start on a pad at Kobuleti airfield, 20C with no wind. An FCR was equipped, 50% fuel, two rocket pods with M274, two empty M299 racks, and an empty gun magazine. The aircraft was taken off into a hover with force trim utilized to stabilize within ground effect. The controls were then released to observe behavior, whereupon the aircraft began to oscillate and pendulum a small amount, then a large amount. Controls were arrested and the aircraft was again pushed into a hover, then the A/C UTIL page was brought up and the FMC YAW, PITCH, ROLL, and COLL channels were commanded off. A stable hover was again commanded, followed by a controls release to observe. As with the test with FMC engaged, the aircraft began to oscillate and more rapidly fall out of control. The aircraft was arrested and then landed.

According to the AH-64A -10, the Digital Automatic Stabilization Equipment (DASE) is described as: "The DASE augments stability and enhances maneuverability of the helicopter. DASE includes and/ or controls the following: stability and command augmentation in pitch, roll, and yaw; attitude hold; heading hold; hover augmentation; turn coordination; and the BUCS." This is controlled via a panel in the pilot's cockpit with 5 switches and a BUCS test switch. Paragraph C describes "Automatic Stabilization" in the following:

Quote

The DASE has a stability augmentation system (SAS) and a command augmentation system (CAS). The SAS reduces pilot workload by dampening airframe movement caused by external forces such as in air turbulence and weapons recoil. The CAS augments helicopter response by mechanical control inputs and commands to the longitudinal (pitch), lateral (roll), and directional (yaw) flight control servoactuators. CAS signals are generated by movement of crew station flight controls which are sent to the DASE computer. The DASE computer sums the SAS/CAS information with inputs from the heading and attitude reference set (HARS) and the air data sensor system (ADSS). The DASE computer provides positioning commands to a two-stage electrohydraulic SAS servo valve on the primary side of the longitudinal, lateral, and directional flight control servoactuators. The position of the SAS servo valve determines the amount and direction of movement of the SAS actuators. The position of each of the SAS actuators is transmitted to the DASE computer by the LVDTs. The motion of each SAS actuator is summed with the mechanical input to each flight control servo, but the SAS actuator authority is limited to 10% bidirectional motion in all axes except the longitudinal where the authority is 10% aft and 20% forward. The DASE is engaged through the pilot ASE control panel.

Given this information, I have the following questions:

- Does shutting off FMC channels in the DCS AH-64D have any present effect beyond disabling various hold modes? That is to say, are there any SAS or CAS operations being performed when the FMC channels are shut off?

- Does the SAS attempt to keep the aircraft on a particular heading, bank, or pitch, or does it only act as a damper for various external influences up to a certain percentage?

- How much stability should be expected from the SAS outside of any hold modes? ie in level flight, there is no perceptible difference from FMC channels on and FMC channels off. Should any stabilization occur in forward flight by the FMC, or is it strictly in the hover regime? Is it only for commanded holds such as attitude or altitude?

Continued:

A second experiment was conducted with the Mi-24P module, for lack of a comparable platform within the game. There is of course a marked contrast between the aircraft in a variety of ways, from the weight to design purpose, to capabilities. For our purposes, it is only to contrast a SAS on a somewhat similar helicopter.

As with the AH-64D, the yaw, pitch, and roll SAS channels were ensured on in the same environmental conditions, with the exception of the collective/altitude channel. The aircraft was lifted off into a hover and stabilized with the force trim release. Once the controls were released, the aircraft maintained a generally accurate pitch, bank, and heading, while the aircraft slowly drifted in one direction. A correction was applied and the aircraft stabilized, followed by shutting off the pitch, roll, and yaw SAS channels, then the controls were again released. The airframe remains stable for a few seconds, then begins to oscillate and pitch up, followed by a retaking of the controls and stabilizing again.

From this experience, the SAS system on the Mi-24 appears intended to keep the aircraft at a commanded heading, bank, and pitch, and of course in various flight regimes. Given this, these are my questions:

- The Mi-24 in general seems to be a far more stable gun platform in the game, with a decidedly powerful SAS that keeps the aircraft well in constraints requested by the crew. Is the more unstable SAS of the AH-64D intended to open up more maneuverability than the more rigid one on the Mi-24?

- As with the AH-64D, does shutting off all the SAS channels remove all damping and potentially CAS?

A control test was done with the UH-1H, which while lacking a true SAS, does have at least some damping and resistance thanks to the control bar (as intended). The UH-1H, when performing a similar test as with the AH-64D, does not exhibit the same violent oscillations, though it obviously eventually will oscillate out of control. This was merely done to have an aircraft tested in a similar state as the previous two while lacking the same control system. An attempt was made with the SA342L module, but there are idiosyncrasies with the track that prevent it from working as desired.

Final notes: This is not to establish whether or not the DCS AH-64D's FM, SAS, and/or SCAS are flawed, inaccurate, improper, or poorly modeled; this is to better understand the underlying aircraft design and intent, so as to properly convey what needs to be learned and taught with the module itself.

Tracks have been attached for review per the above testing.

Mi-24P SAS vs NO SAS.trkAH-64D FMC vs NO FMC.trkUH-1H CONTROL.trk

Edited January 18 by NineLine
Added link

[NineLine]

I have restored this thread, please note I have added a valid link to the mentioned manual this is the proper way to share something available to be shared, legally.

On that the thread has been tagged that the FM and SCAS are WIP, so most likely this will not get looked at by the team, but you may share your thoughts here as things develop.

Thanks

[NeedzWD40]

13 minutes ago, NineLine said:

I have restored this thread, please note I have added a valid link to the mentioned manual this is the proper way to share something available to be shared, legally.

On that the thread has been tagged that the FM and SCAS are WIP, so most likely this will not get looked at by the team, but you may share your thoughts here as things develop.

Thanks

Thanks, NineLine. As noted, the purpose of the thread isn't to cast shade on the current state of the WIP FM and SCAS, but to have a dialogue on what we should focus on learning now and what we can expect in the future.

[bradmick]

12 hours ago, NeedzWD40 said:

- Does shutting off FMC channels in the DCS AH-64D have any present effect beyond disabling various hold modes? That is to say, are there any SAS or CAS operations being performed when the FMC channels are shut off?

- Does the SAS attempt to keep the aircraft on a particular heading, bank, or pitch, or does it only act as a damper for various external influences up to a certain percentage?

- How much stability should be expected from the SAS outside of any hold modes? ie in level flight, there is no perceptible difference from FMC channels on and FMC channels off. Should any stabilization occur in forward flight by the FMC, or is it strictly in the hover regime? Is it only for commanded holds such as attitude or altitude?

Shutting off the FMC completely disables the SCAS for the helicopter, you get only the control authority granted by the push/pull rods connected to the flight controls with the added bonus of there being a significant lag.

 

When the hold modes are off, you have Pitch, Yaw, Roll and Collective SAS which provides rate damping, atmospheric upset damping and turn coordination above 40 knots. The CAS is providing the instant response to input, without the CAS the helicopter has a delay when inputs are made. It flies like a giant TH-67 (Bell 206). There's a pretty appreciable lag with the FMC out.

 

The SAS makes for an insanely stable platform. If you trim the helicopter correctly, and without hold modes, and assuming the winds are light, the helicopter will hold very steady. The SAS does a really great job. I can easily go long stretches without the hold modes on with finger tip pressure on the controls, this is at a hover and in forward flight. With the hold modes on, I can remove my hands entirely from the flight controls and she'll fly herself for a really long time without me having to touch the flight controls at all. I've demonstrated this more times than I can count.

 

Another thing about the SAS and the hold modes is that there is no "hunting" or "seeking" when the hold modes are turned on. Even if you have say 2 to 3 knots of velocity at a hover and you turn on the attitude hold, the system will smoothly apply an input opposite of the velocity vector and stop dead center at 0. It won't overshoot at all. In doing that though, you've probably robbed the flight control of all of it's SAS authority and will get a SAS SATURATE message in short order. In forward flight the capture for altitude and attitude are immediate and there's no drift either. They are insanely steady.

 

I would also say, that the 64A DASE is not the FMC. I never had the pleasure of flying A models, but I know enough A model dudes that have spoken at length about how much better the FMC is. They rarely used the holds in the A model because the DASE was so bad. That's anectodal, but they were flying A models before I was born and close to as long as I'd been alive...so i'm inclined to believe them. Either way, I wouldn't use the A Model as a point of reference because there were significant changes/differences to the brains behind the flight controls.

Edited January 19 by bradmick

[placsea]

20 minutes ago, bradmick said:

Shutting off the FMC completely disables the SCAS for the helicopter, you get only the control authority granted by the push/pull rods connected to the flight controls with the added bonus of there being a significant lag.

 

When the hold modes are off, you have Pitch, Yaw, Roll and Collective SAS which provides rate damping, atmospheric upset damping and turn coordination above 40 knots. The CAS is providing the instant response to input, without the CAS the helicopter has a delay when inputs are made. It flies like a giant TH-67 (Bell 206). There's a pretty appreciable lag with the FMC out.

 

The SAS makes for an insanely stable platform. If you trim the helicopter correctly, and without hold modes, and assuming the winds are light, the helicopter will hold very steady. The SAS does a really great job. I can easily go long stretches without the hold modes on with finger tip pressure on the controls, this is at a hover and in forward flight. With the hold modes on, I can remove my hands entirely from the flight controls and she'll fly herself for a really long time without me having to touch the flight controls at all. I've demonstrated this more times than I can count.

 

Another thing about the SAS and the hold modes is that there is no "hunting" or "seeking" when the hold modes are turned on. Even if you have say 2 to 3 knots of velocity at a hover and you turn on the attitude hold, the system will smoothly apply an input opposite of the velocity vector and stop dead center at 0. It won't overshoot at all. In doing that though, you've probably robbed the flight control of all of it's SAS authority and will get a SAS SATURATE message in short order. In forward flight the capture for altitude and attitude are immediate and there's no drift either. They are insanely steady.

Hey Brad,

In your updated force trim usage video you don't recommend holding the ftr at a hover because you loose the benefits of the scas and I definitely concur with that. My question is should we expect any major changes to the handling and feel at low speeds with the ftr depressed or is it about where it should be?

[bradmick]

No, there is no change in feel, because the CAS is still active, you are the SAS when the force trim is held interrupted. The aircraft still responds the same thanks to the CAS, that’s its purpose. The only difference is you have to do the turn coord and atmospheric upset damping. 

[placsea]

Ah right, its just a case Ive gotten so accustomed to what the sas was doing for me and then struggling when it was absent. Thank you for explaining.

[deloy]

Referring to OP, the point is that there is no difference in handling when FTR is pressed vs when FMC modes are off, so apparently there is no CAS active when FTR is pressed. There is no additional lag compared between FTR  pressed when FMC is off. As per Brad’s description the lag should be noticeable when FMC is off . This isn’t the case now in our Apache  . I guess that’s why they say it’s still being improved and not in final state.

If you see control positions indicator you clearly see CAS working when SAS is on, as clearly described in the manual . However this is not the case when either FTR is depressed or FMC off , which is relevant in this discussion. 

cheers 

[bradmick]

What I said was with the ford trim pressed, SAS is off, because the SAS servo (the servo on top of the main servo) in the flight control itself is being commanded to center. You absolutely have CAS with the force trim held. You do not have CAS or SAS with the FMC off, which is when you get the input lag. You don’t get the input lag with the force trim held because you still have CAS. Thinking it like this:

Level 0 is no FMC, which means no SCAS (purely mechanical input only) and force trim mag brakes. In this level there is significant control input delay.

Level 1 is FMC on and the force trim held which gives you CAS. This level is mechanical inputs augmented by digital inputs to remove input lag. This also disables all hold modes until the force trim switch is released. 

Level 2 is FMC on and the force trim not held which gives you SCAS (SAS + CAS). This level is mechanical inputs augmented by digital inputs to remove input lag. You get atmospheric upset damping, heading hold, and rate damping. 

Level 3 is FMC on, the force trim not held which gives you SCAS (SAS + CAS) and a hold mode enables. This level is mechanical inputs augmented by digital inputs to remove control lag. This also gives you attitude(position, velocity, attitude), altitude (radar/barometric), heading hold/turn coordination.