DarkFire

Outstanding work Ironhand, wish I still had some rep to give :notworthy: And let this be a lesson in how to user-test the DCS flight models: recording verifiable, repeatable, observable phenomena is the essence of the scientific method. Everything else is mere opinion and / or conjecture. So, theory predicts the changes in CG with fuel depletion. Computational analysis verifies this. The £1K question then is: can this be verified by experiment? Does this happen in the real Su-27? It'd be great to have an answer to that.

Get well soon Cap, donation made.

Well that's damned hard work to keep it there for the entire fuel load duration, but good fun! I'm going to play around with this over the weekend & see what the permissible ambient temperature limits are that make this possible.

So it does exist! Thanks for the confirmation YoYo.

If I remember correctly it's 30 degrees look down & 60 degrees look up. Pages 50-52 of the flight manual.

Interesting! Looking forwards to having a look at your track if it's not too huge to upload. I've done a couple of tests now in level flight at a couple of different altitudes (~2,750m, 5,000m & 7,500m) and so far I've not come across anything that I can definitively say was outside of the experimental uncertainty produced by my lack of ability to maintain dead neutral pitch trim. I'll see if I can put some time in over the weekend to test very high altitudes where any variance will hopefully be amplified by the very low IAS.

Suggestion: "Flight Test" mode. This idea came from some Su-27 flight model testing that a couple of us are doing. TacView is great for observing the behaviour of a given aircraft under all sorts of conditions but sometimes the sample rate seems to be a bit low to capture fast-time transients. Would it be possible to have some sort of "flight test" mode which would increase the sample rate 2x or even 5x but limit the total recording time?

I actually like Tacview for this sort of thing. I think the readings are relatively accurate. The only issue is the relatively low (for this sort of testing) sample rate, but I guess it has to be otherwise .acmi files would be gigantic for missions of any sort of real duration. Hmm, I might suggest a "test" mode for TacView which could be 2x or even 5x sample rate but limited in track length.

Ah I see! Slightly different flight regime to what I was testing :) Reading up about the data export abilities of the in-game LUA engine, I'm not sure that what we want to achieve will be possible. It's obviously a very powerful tool but there's no obvious functions for accessing the guts of the flight models. In fairness this may be an entirely deliberate step by ED to help prevent others from simply copying their R&D work on flight models. It looks like it is possible to output information such as pitch angle and what I think is the total velocity vector of a given unit but I'm not sure that data would help to asses potential anomalies in CG / CP behaviour. Unless I've missed something or there are undocumented lua functions I think getting that sort of data is probably only possible at the development / code testing level.

Unfortunately it is possible to 'run out' of trim authority in the Su-27 / 33. Depending on conditions you should only see this at relatively high subsonic speeds (>0.85M) and at fairly low altitude (<500m). The only solution is to engage the ACS, either radar or barometric altitude hold or in the case of the Su-33 you could use terrain avoidance mode.

30 seconds? Damn, that must have resulted in the world's biggest spreadsheet :shocking: I need to have a look at the in-game lua engine documentation to see if it's possible to script data output to a log file. In terms of relatively high AOA flight, I remember there being a region of stability up at around 13-14,000m at what used to be cruise throttle (87% RPM I think) which would settle at IIRC about 240 Km/h IAS and around 8 degrees AOA. The problem will be getting up there before tank 4 starts to empty. It'll be interesting to try though. Might give this a bash later. Edited to add: Of course it'll be possible. Air start... :doh:

Damn, that must have been a real "Oh... crap" moment for the pilot!

I think so. Just when things are already too complicated, the Su-30K shouldn't be confused with the Su-27K, also sometimes known as the Su-33! :huh:

Hmm, I tried this again and I think that my manual trim control isn't sufficiently accurate to be able to differentiate between divergent vertical velocity changes due to non-neutral trim and any changes that are occurring due to shifts in CG due to fuel depletion. However small a nudge I give the trim adjustment it eventually enters a runaway divergence from neutral. There were very subtle hints at possible trim changes at roughly between 7900 - 7750 Kg of fuel loaded and maybe another at some point between 7,500 and 7,300 Kg fuel but again I can't trim accurately enough to tell if these are simply symptomatic of non-neutral trim or subtle CG changes. I would imagine that the way to do it would be to use alt hold and then watch for changes in the trim position commanded by the ACS, but without having the sort of access to the FM that YoYo would have, I'm not sure there's any way for us to get that data. I tried watching the control monitor graphic but the changes in ACS-commanded trim position are just too small to see on that small graphic. Anyway, the relevant track is attached. Edited to add: when testing, are we using a full gunpad? IIRC that would be an additional 147Kg of ammunition that should be forwards of the CG. Not sure if it makes a difference here in the Su-27 under these test conditions, but given that the potential trim anomaly happens during consumption of a similarly small amount of fuel it makes sense to standardise our tests. I had a full ammunition load for the gun in the attached test track. Su-27 Fuelk Test 1 Track.zip

There's another trim oddity which normally manifests at low altitude. The trim behaviour normal up to about 780 Km/h. As IAS increases through 800 the trim effect inverts a little and the nose starts to dip but then the effect reverses again to normal behaviour as IAS increases through 890 Km/h. What sort of altitude are you testing at? I always put the pitch trim anomaly at 800-875 Km/h IAS down to an anomaly in the control laws applied to pitch trim, but It could conceivably be be that the two effects are in fact linked, depending on what IAS full military thrust produces at a given altitude. Edited to add: just gave this a try. Standard DCS day. Flew at 5,000 m altitude. 100% dry RPM gave me 1150 Km/h TAS and 890 Km/h IAS. If I've understood the drain sequence correctly tank 4 should start to drain at 7,680 Kg total fuel. I didn't observe any anomalous changes in trim, at least within the limits of my ability to manually maintain zero trim.

That makes complete sense. Wing tanks + centre tank means no chance in CG as the fuel is depleted.

Great job even doing that much :thumbup: Ironhand - thanks for the clarification. Interesting that the AB fuel drain regime is different. This is pure supposition on my part but I would suggest that maybe the fuel drain is from tank 2 / feeder tank only, as that has the least probability of introducing any weight distribution imbalances by being rapidly drained. Similarly I would suggest that fuel dump may be sourced from tank 4 as it's the closest to the dump nozzle. This calls in to question something else that I had believed: I'd always thought that the "standard" 60%-of-maximum fuel load for the Flanker was comprised of tanks 1 & 4, but I don't see how this could be true given the flow diagram, unless tanks 1 & 4 feed directly in to the cache tank and not in to tank 2, but this layout seems unnecessarily complicated to me in terms of plumbing. Unless filling tanks 2 & 4 would introduce an unacceptable shift in the CG maybe,

Ah OK, sorry I missed that. So must be something that's dependent on in-helmet volume. I'm sort of guessing here but to me that suggests it's something to do with the aircraft in question rather than the general game.

Very useful, thanks for posting that. IIRC the capacity of the cache tank is 500 Kg.

Hmm OK, let's see if we can narrow down the cause. When the beeping happens, does it appear that the sound is coming from the game, or more specifically from the aircraft? Try changing individual volume controls to see of that narrows down the source.

Does your team training mission have any LUA scripting in it? I wonder if some sort of scripting bug is causing the LUA run time engine to bug out. Not sure how it notifies you of any issues in game, but it could potentially be the source of the beeping.

And here we go again with the conspiracy theories :thumbdown: ED strive for realism. There is no "balance" in DCS. There is no national bias. There is reality, or as close to it as can be achieved. All missiles in DCS suffer from excessive drag which results in unrealistic range. SARH missiles have additional issues with the way in which expendables are modelled. This is all well known & is not a conspiracy. /thread.

Duh, my bad and yes I was thinking of the Su-27UBM :doh: thanks for the correction.

Hmm most odd. Something else that occured to me: are you running a game overlay such as Teamspeak or Discord? If so, try disabling the overlay & see if that helps.

Well spotted, I hadn't noticed that. Helipad for visiting senior officers maybe? I can imagine a scenario whereby a senior officer would be deployed to the launch complex in times of heightened alert posture to take command in case of actual war.