Su-27 in 1.5.5

It may depend on the Su27 variant and whether the ACS was upgraded.

Not sure if this has been discussed already, but from what I've gathered, doesn't the real life ACS in the Su-27 is only calibrated for one weight configuration? So it doesn't care if you're max weight and 1.2M, it'll still let you pull over G and break your wings?

The over-G warning system, better known as nagging Nadia (probably part of the Ekran system), is calibrated for an all-up weight of 21,400Kg so it'll give you over-G audible warnings based on what it thinks is an over-G condition for that specific weight.

When you look at the possible configurations that give you that all-up weight:

1) 2 x R-27ER, 2 x R-73, ~35% fuel.

2) 4 x R-73, ~56% fuel.

I'd say that the calibration is fairly obviously aimed at an early-model Su-27 that has PVO (air defence forces) missions in mind. I'm starting to wonder if the Su-27S version that we have in DCS, designed for the VVS (frontal aviation, has A-2-G capability that the standard model doesn't have) might have had a different calibration, or even a basic variable one.

The AOA & G limiter in our DCS Su-27 is designed to keep you from over-stressing the airframe and as YoYo pointed out has a certain maximum stress value in mind. It is the case that it will save you at any weight and at any speed unless you're heavy, right in the middle of the 'mach notch' on the max-G diagram and sharply corner the stick (i.e. max deflection in pitch & roll).

Different issue:

Apologies in advance for a slight topic hijack, I've discovered another autopilot oddity. I was doing a speed test at 10,000m. When I accelerated past about M2.3, the AP produced a very slow but diverging pitch oscillation. Vertical velocity started oscillating from -6 to +6 which grew to roughly -10 +10 by the time I was passing M2.5 at which point I turned off the AP. Not sure if this was a one-off or a small issue with the new autopilot.

Fantastic update. САЛ on hud for jammer is really cool. Autopilot is great at slow speeds (400 >).

Sent from my SM-T231 using Tapatalk

I'm not sure if I'm imagining things, but does anyone else feel like there is less roll rate available than before at slow speeds under high angle of attack?

I'm not questioning realism here (I have zero references for that lol) just whether there is some implicit or explicit change.

I'm not sure if I'm imagining things, but does anyone else feel like there is less roll rate available than before at slow speeds under high angle of attack?

 

I'm not questioning realism here (I have zero references for that lol) just whether there is some implicit or explicit change.

I think there's less roll rate in general than before, I think it's attributed to the AoA and G limiter though, possibly to reduce stress on the plane.

Autopilot "return to level flight" is out of whack now. It puts the plane in roughly 250 meter/minute nose down attitude now. It was perfectly level before this update.

That's weird. I just tested a quick Free Flight and it worked really well.

Different altitudes, speeds, diving, inverted flight…

It failed if I tried to recover while climbing at a 90° pitch angle, but I think it's one of that autopilot mode limitation (it might written somewhere in the Su-27K manual).

Even so, when the Flanker finally stalled and dived it managed to get it to a leveled flight after a few seconds.

Did you check your DCS installation with a "repair mode"? (no idea what specific issue you might have)

Autopilot "return to level flight" is out of whack now. It puts the plane in roughly 250 meter/minute nose down attitude now. It was perfectly level before this update.

Checked over the weekend and I get the same behavior. The AP light show that wings level is engaged and, then, the 2nd light for Baro Hold comes on once the aircraft's within parameters but the altitude never stabilizes and you continue to descend. Disconnecting the AP and immediately re-engaging Baro Hold itself instantly corrects the problem and the altitude starts to stabilize. So there seems to be an issue of some sort.

I'm not sure if I'm imagining things, but does anyone else feel like there is less roll rate available than before at slow speeds under high angle of attack?

 

I'm not questioning realism here (I have zero references for that lol) just whether there is some implicit or explicit change.

Yes there's significantly less roll rate now at high AOA, but only if you're using the stick for roll. The cross-coupling between the rudder & flapperons has been significantly reduced. Roll at high AOA is much more effectively generated by the rudders than the flappersons, hence with reduced cross-coupling we now have reduced roll effectiveness when using only the flappersons to command the roll.

At high AOA if you add in manual rudder it rolls just fine, though at very high AOA (>25) you need to have a very steady touch on the controls.

Rudder should give you more roll than ailerons, but not as much as maximum roll (typically less than half).

Autopilot "return to level flight" is out of whack now. It puts the plane in roughly 250 meter/minute nose down attitude now. It was perfectly level before this update.

Confirmed. That started to happen a couple updates ago and still persists; constant nose-down after engaging level autopilot.

canopy still stays transparent as always :D

Rudder should give you more roll than ailerons, but not as much as maximum roll (typically less than half).

Yep, roll authority using ailerons alone at AOA >20 degrees is roughly 25% of level flight full roll authority. Using rudders adds roughly another 25 - 30% at high AOA so the total feels like a bit over half, maybe 55-60%.

Good stuff. I actually wonder about using ailerons at high AoA - in most aircraft I believe that it is not recommended because you can immediately stall out one wing. Of course by high AoA here I mean 'on the edge' :D

Good stuff. I actually wonder about using ailerons at high AoA - in most aircraft I believe that it is not recommended because you can immediately stall out one wing. Of course by high AoA here I mean 'on the edge' :D

Presumably modern digital FCS would know about that problem and automatically avoid using the ailerons for rolls at high AoA. Dunno if the same can be said about the Flanker's more primitive FCS though (doesn't it still use valves?).

Related: would the same problem apply to the stabilators?

Horizontal stabs provide some lift, but moving them won't stall your wings.

I think the Su-27 do have aileron-rudder interconnect system as per described in the manual, but is doing its job ineffectively?

In a F-16 for example (which is also equipped with ARI), rudder will deflect much more than aileron to eliminate any side slip build up:

The flanker in DCS has an aileron limiter that restricts lateral stick movement at high AoA. It's up to the pilot to make sure she stays coordinated though ;)

Modern FBW aircraft have control laws that limit aileron movement and replace it with rudder as alpha increases, this can also be seen in the F-15 at high AoA: Even small stick inputs command full rudder and practically zero aileron (unless the gear is down)

Good stuff. I actually wonder about using ailerons at high AoA - in most aircraft I believe that it is not recommended because you can immediately stall out one wing. Of course by high AoA here I mean 'on the edge' :D

Yep, works as per spec. I tried this to see if it's modelled, and indeed it is. I tried flying low altitude (~300m) circuits at between 20 and 25 degrees AOA at around 240 Km/h - essentially balancing the aircraft on the engine thrust together with a little bit of lift from the wings. Using only the flapperons I maintained 45 degree bank turns around the circuit. Several times with no additional command input the roll increased quickly to 60 degrees and beyond, always in to the direction of the turn.

At the time I was wondering what was happening and it only occurred to me later that the 'inside' or lower wing was probably stalling due to the high AOA combined with low airspeed and use of the flapperons.

PFM in action. Yoyo did an amazing job with the Su-27 :thumbup:

The flanker in DCS has an aileron limiter that restricts lateral stick movement at high AoA. It's up to the pilot to make sure she stays coordinated though ;)

 

Modern FBW aircraft have control laws that limit aileron movement and replace it with rudder as alpha increases, this can also be seen in the F-15 at high AoA: Even small stick inputs command full rudder and practically zero aileron (unless the gear is down)

Just took a look at the DCS manual, the Su-27 roll-yaw crossfeed system do take AOA into consideration:

It seems to do the same as the F16's ARI system as both of the systems run as a function of AOA, dynamic pressure and static pressure, since you want different rudder deflection at different AOA/Speed/Altitudes, which is a common sense in flight control design. (More deflection at high AOA, low speeds and high alts.)

Good stuff. I actually wonder about using ailerons at high AoA - in most aircraft I believe that it is not recommended because you can immediately stall out one wing. Of course by high AoA here I mean 'on the edge' :D

I beg to differ. Rolling the aircraft at high AOA will create large sideslip due to kinematic coupling between AOA and sideslip, hence should be countered by rudder. But without deflecting the ailerons, you won't have much sideslip buildup, and you create sideslip with the rudder alone? You know that sideslip at high AOA is bad for stabs nose-down ability and departure resistance of the aircraft. So basically you don't want to use either of them alone.

eg. Rudder input signal at high AOA is nulled out in a F-16, and is crossfeeded to roll axis in a F-18 (the rudder pedal-to-rolling surface).

Currently the DCS Su-27 presents a very strong lateral-directional dynamic stability and you won't even enter into a spin. The aircraft will not punish if exceeding AOA limit or creating large sideslip at high AOA.

For ailerons, you want to use it as much as possible to the extent that the rudder is enable to counter all the adverse sideslip. FCS limiting roll rate is largely due to inertia-coupling considerations and the rudder's insufficiency to counter adverse sideslip.

The effect of inertia-coupling in real life Su-27SK/J-11A according to the manual (chinese version):

When manoeuvring at V < 400 km/h on the limiter, increasing roll rate will increase AOA immediately, even exceeding maximum allowed AOA.

Regarding the lateral stick limiter: Actually, lateral stick movement limiter will limit stick travel to 1/3 full range only above 25 AOA, which opposes the current DCS simulation of KF-5 FBW system (which is the same as J-11A). In addition to that, one division of PLAAF who still operates the real Su-27SK (the oldest original Russian version) actually tried DCS Su-27, and reported a lack of roll manoeuvrability at high alpha regions in the simulation.