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Su-27 autopilot bugs?


PawlaczGMD

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I noticed the autopilot on the SU-27 goes into heavy shaking/oscillation after a few seconds, making it useless or at least silly and immersion breaking. Also, when disengaging AP with the same key you switch it on (say "A"), weird things happen to my bank trim,violently banking to one side, only to be fixed by trim reset. Resetting AP with alt-9 seems to switch it off fine.

Are these bugs or am I doing something wrong here?

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Everyone experiences this, usually at high altitudes. I think the Flanker irl has this characteristic but it's something to with the sim reading the jets ground speed rather than air speed and it believes it's going too slow at higher altitudes. Whatever it is exactly, we all have it and to stay in level flight you just need to be going above x speed at x alt.

 

As I remember it you wanna get yourself trimmed and be above 1100kph groundspeed to be safe from the oscillation if you're going above the contrails. Above 9000 metres you'll need at least low stage burner.


Edited by Mamba
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Everyone experiences this, usually at high altitudes. I think the Flanker irl has this characteristic but it's something to with the sim reading the jets ground speed rather than air speed and it believes it's going too slow at higher altitudes. Whatever it is exactly, we all have it and to stay in level flight you just need to be going above x speed at x alt.

 

 

I don't think it's realistic for the autopilot to complete mess up the trim so you can't even go back to neutral bank anymore, and the plane continuously rotates to one side or the other.

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I don't think it's realistic for the autopilot to complete mess up the trim so you can't even go back to neutral bank anymore, and the plane continuously rotates to one side or the other.

 

I've found that it doesn't if you do autopilot reset instead of disengage... what is supposed to be the difference anyway?

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I've found that it doesn't if you do autopilot reset instead of disengage... what is supposed to be the difference anyway?

 

When you engage AP in conditions that will cause the jet to oscillate, the jet adjusts trim to help correct. Presumably disengage keeps the last held trim adjustments, while reset also wipes them.

 

I always used the alt-9 combo due to this.

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There's a known bug with the Su-27 autopilot. The problem is this: for it to produce stable flight the autopilot must be engaged at an indicated air speed (TAS is irrelevant as is ground speed) of at least 550-560 Km/h, otherwise it exhibits the oscillations described. Altitude is also irrelevant, the IAS is the limiting factor. The reason most people don't notice the oscillation problem at low to medium altitudes is that the IAS is that at most commonly used altitudes IAS is well above the minimum permitted value.

 

Note that the described oscillations will also start if your IAS falls below 550-560 Km/h, even if your IAS was above that when the autopilot was engaged.

 

What this means in practice is that using the autopilot limits you to flight at or below 10,000m altitude. At any altitude above this, unless you're in afterburner your IAS will always be below the required 550-560 Km/h minimum.

 

This is definitely a bug and is not real Su-27 behaviour. The real Su-27 has an autopilot mode that can pilot the aircraft through a landing pattern and in to final approach, where your IAS will be 240-300 Km/h (depending on weight and other things). The DCS Su-27 autopilot can't do this and will exhibit the oscillations as described, which at that sort of air speed and very low level can be fatal.

 

With regards to the autopilot, what you need to remember is that unlike some aircraft the automatic control system of the Su-27 operates by adjusting trim settings within the flight control system, rather than directly adjusting control surface position.

 

Disengaging the autopilot does exactly that - it simply turns the system off.

 

The "ACS reset" function is slightly different. While one of the autopilot functions is turned on, if control surface deflection exceeds 20% of maximum value, or if the aircraft exceeds 80 degrees of bank, the ACS will drop in to manual mode. As an example, imagine that you're motoring along in altitude hold mode when you suddenly notice an incoming missile. You grab the stick to carry out an emergency max-rate turn to avoid the missile. Under these circumstances the ACS will drop back to manual mode, but the ACS actually works using the trimmer system so giving it a manual input while it's in automatic mode (i.e. autopilot turned on) causes the system problems.

 

You can try this for yourself: fly around in alt hold mode, do a snap roll to drop the system back to manual mode and the ACS reset warning light will probably illuminate. Try re-engaging the ACS and see what happens :) Even if you don't re-engage the ACS you'll likely notice that the controls have become quite sensitive and twitchy.

 

When the system has dropped back to manual mode due to a drastic manual command input, to restore the proper function of the system it needs to be reset, hence the reset command being different from the ACS on/off command.

 

Pages 44-45 of the Su-27 flight manual give a description.

 

Hope this helps...

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Yeah, until ED tweaks that AP / PFM behavior some more, the "reset trim" function is your friend when coming out of AP.. :) I believe that the PFM / FBW / AP modelling is not yet final.

 

Here's an excerpt from a PM discussion I had with Yo-Yo on that recently, and he said this:

 

"Originally Posted by Yo-Yo

Yes, but as Su-27 is still beta and any minor FM change requires FBW and AP tweaking we postponed the final tweaking for final."

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So what is the autopilot disengage/autopilot off button that's on the su-27s joystick called in the options or what is it's default binding, since i can't get it to appear.

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So what is the autopilot disengage/autopilot off button that's on the su-27s joystick called in the options or what is it's default binding, since i can't get it to appear.

 

Not sure if this helps....

 

EDIT: Forgot to mention that the "Autopilot On" trigger switch when holding it temporarily disables the AP, so you can make some attitude/altitude changes without having to turn the AP completely off, and releasing it turns the AP back on. This works in DCS as well, and is mapped to the trigger by default.

 

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Edited by rrohde

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So, while slightly off-topic, the Su-27 does have a brake lever on the stick and no toe brakes? Just thinking of the discussion here: http://forums.eagle.ru/showthread.php?t=168234&page=3

 

Seems like my memory served me right, at least for the original Flanker.

 

Might be just a "holding brake" instead for spooling up the engine...?!

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Yeah, until ED tweaks that AP / PFM behavior some more, the "reset trim" function is your friend when coming out of AP.. :) I believe that the PFM / FBW / AP modelling is not yet final.

 

Here's an excerpt from a PM discussion I had with Yo-Yo on that recently, and he said this:

 

"Originally Posted by Yo-Yo

Yes, but as Su-27 is still beta and any minor FM change requires FBW and AP tweaking we postponed the final tweaking for final."

 

That's very good to hear. Hopefully once the ED team have finished with the initial 2.5 roll out they'll have some time to sort out smaller issues like the Su-27 ACS tweaks.

System Spec: Cooler Master Cosmos C700P Black Edition case. | AMD 5950X CPU | MSI RTX-3090 GPU | 32GB HyperX Predator PC4000 RAM | | TM Warthog stick & throttle | TrackIR 5 | Samsung 980 Pro NVMe 4 SSD 1TB (boot) | Samsung 870 QVO SSD 4TB (games) | Windows 10 Pro 64-bit.

 

Personal wish list: DCS: Su-27SM & DCS: Avro Vulcan.

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"Originally Posted by Yo-Yo

Yes, but as Su-27 is still beta and any minor FM change requires FBW and AP tweaking we postponed the final tweaking for final."

 

Thanks for the info!

I don't know how many full time devs ED have, but with all the ongoing projects they must have some serious to-do-lists (2.5 merge, F/A-18C, Caucasus map upgrade, MiG-29 and Su-33 PFMs/models, the announced naval operations, NTTR, other modules/models/…, unannounced projects (?), etc.).

 

The Su-27 is still very slowly evolving (wings structural resistance, etc.) but I'd rather see them working relatively slowly but accurately than getting a hasty unpolished content.

Even if it's awfully frustrating (cockpit pilot still not wearing a G suit, canopy texture, ACS adjustments, overwing vapor, no multiplayer datalink, etc.).

 

If that can help, the AP is very stable in straight flight with an IAS above 500 km/h (520), and from what I've tested the speed can now be reduced around 400 km/h or lower once the AP is engaged (iirc, it wasn't possible in a previous version)…

 

Also, we still don't have the landing AP.

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Also, we still don't have the landing AP.

 

*cough* I once used the follow route AP to let it land my plane.. was oscillating like crazy but touched down without breaking anything! I'd say that counts as landing AP? hehe

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