Su-27 Flight Model Discussion

[Andrei]

Thanks, much appreciated.

 

Alright, the answer came fast, but brief :)

http://forums.eagle.ru/showpost.php?p=2314338&postcount=369

 

Translating it more-or-less directly he said: "Naturally, momentum curves change". Based on my original description of the problem and assumption above, it appears that I was correct in saying that it depends on the trans- and supersonic flight modes when aerodynamic focus of the aircraft changing and thus changing the longitudinal stability.

 

Well, if someone can put it better, do go ahead :)

[DarkFire]

Alright, the answer came fast, but brief :)

http://forums.eagle.ru/showpost.php?p=2314338&postcount=369

 

Translating it more-or-less directly he said: "Naturally, momentum curves change". Based on my original description of the problem and assumption above, it appears that I was correct in saying that it depends on the trans- and supersonic flight modes when aerodynamic focus of the aircraft changing and thus changing the longitudinal stability.

 

Well, if someone can put it better, do go ahead :)

 

Very interesting, thanks. Something else I'll have to train with I guess...

[jackmckay]

There is no way that pilot would blackout in half second to second of inverted Gs (-5 to -10G max) at ASC disengage, no way, human can withstand +60Gs for few seconds!

[Ironhand]

There is no way that pilot would blackout in half second to second of inverted Gs (-5 to -10G max) at ASC disengage, no way, human can withstand +60Gs for few seconds!

It does seem much too rapid in onset, although there has been little actual study on negative G effects. High negative Gs 1) increase blood pressure in the neck and chest causing the heart rate to slow to the point where it might be several seconds between beats and 2) keeps blood from leaving the head so that the oxygen to the brain isn't replenished. So LOC is indeed possible. It just seems to happen too rapidly in the sim.

[IonicRipper]

Yesterday i was flying the Sukhoi in MP. Stayling as low as possible to avoid radars. I later decided to climb above the clouds but just as i got over the cloud layer, the low speed caused the plane to depart. At first the nose pitched up uncontrollably and then pitched down and back up again etc... Even when pitched down and gaining speed it would pitch up uncontrollably again. I took all sorts of different maneuvers with ACS off in order to regain control several minutes later. I wish it happened in SP so i could've saved the track. Does this sound like normal behavior for the Flanker?

[Zeke]

Yesterday i was flying the Sukhoi in MP. Stayling as low as possible to avoid radars. I later decided to climb above the clouds but just as i got over the cloud layer, the low speed caused the plane to depart. At first the nose pitched up uncontrollably and then pitched down and back up again etc... Even when pitched down and gaining speed it would pitch up uncontrollably again. I took all sorts of different maneuvers with ACS off in order to regain control several minutes later. I wish it happened in SP so i could've saved the track. Does this sound like normal behavior for the Flanker?

 

Multiplayer Tracks are saved in your multiplayer folder in the tracks folder in your saved games folder. At least mine are. They are saved automatically.

[GGTharos]

There enough other physiological effects which make the representation 'fair'. Problems include headache, mental confusion etc, all within just a few seconds of exposure of up to -3g in real subjects. As a result, tolerance is marked at -4g for unprotected persons (and there's no such protection in modern fighters. You just don't pull -g's).

 

In real life, aerobatic pilots have lost consciousness with -2 to -2.5g.

 

Some people lost consciousness AFTER completing the maneuver, and in particular, possible when transitioning from -g to +g.

 

More importantly, and I don't think it's modeled in the sim - negative g's over an extended period ( ... single digit number of seconds) will instantly destroy your positive g tolerance - pretty much for the reasons you stated.

 

In summary, this is a very complex subject and the in-game representation is not entirely unfair.

 

It does seem much too rapid in onset, although there has been little actual study on negative G effects. High negative Gs 1) increase blood pressure in the neck and chest causing the heart rate to slow to the point where it might be several seconds between beats and 2) keeps blood from leaving the head so that the oxygen to the brain isn't replenished. So LOC is indeed possible. It just seems to happen too rapidly in the sim.

[GGTharos]

Sorry but your conclusion is incorrect. The best you can do with +g's in the 'reserve oxygen' in the brain, and even that duration goes down at you increase g's.

 

Negative g's at -10 would probably flat out turn you into a vegetable. There are very, very few people who can withstand -5g for 5sec out there, and even they will probably suffer immediate mental dysfunction, lower eyelids covering their eyes, burst capillaries and at minimum a fierce headache during the maneuver, and they will be left with a less painful but very uncomfortable headache for a day to a day and a half from it.

 

In addition, no one trains for -g's as far as fighter aircraft are concerned.

 

If you're pulling -g's in a fighter, outside of a very small number of exceptions you're basically doing it wrong ;)

 

The in-game representation can't give you a head-ache, cause you mental dysfunction, or any of these other effects ... but it can 'knock the pilot out', which is fair because that's exactly the practical effect you'll get when pulling so many -gs.

 

There is no way that pilot would blackout in half second to second of inverted Gs (-5 to -10G max) at ASC disengage, no way, human can withstand +60Gs for few seconds!

Edited April 2, 2015 by GGTharos

[WildBillKelsoe]

Guys I just read the inverted departure but I'm not sure if this is it:

 

I am flying, I engage direct control (the checkered yellow black cover switch), then I'm instantly in G-LOC, with the aircraft upside down, sinking very rapidly, with zero forward thrust.

 

I'm no real life pilot, but from what I know the Su-27 is FBW (like viper, A320)... but she's also very unstable (wing design, lack of thrust vector nozzles, you name it).. that switch cancels the FBW (sort of alpha protect Airbus folks here?) and enables very much the direct control of all pitch surfaces (pretty much the high AoA situation, without being in high AoA)...

 

because the stability is FUBAR without FBW, this switch is not normally used, and if so, it occurs when gear down (again sort of landing gains in the viper?)

 

Therefore, in my normal, trimmed, (can't remember speed but immediately following takeoff), I engage that switch ( lets call it the damn switch cuz I'm pissed at the output in F2 I see), then almost immediately, I'm in a half cuban curve-less invert, and plunking down at very high VVI, let alone blacked out)...

[GGTharos]

I am flying, I engage direct control (the checkered yellow black cover switch), then I'm instantly in G-LOC, with the aircraft upside down, sinking very rapidly, with zero forward thrust.

 

Correct. If you don't understand what ASC does and how and why it operates, leave it alone.

 

I'm no real life pilot, but from what I know the Su-27 is FBW (like viper, A320)...

No, it's not 'FBW' like either of those. It's also not unstable.

 

that switch cancels the FBW (sort of alpha protect Airbus folks here?) and enables very much the direct control of all pitch surfaces (pretty much the high AoA situation, without being in high AoA)...

It gives you direct control of the flight control surfaces versus generating an artificial positive stability feel.

 

because the stability is FUBAR without FBW, this switch is not normally used, and if so, it occurs when gear down (again sort of landing gains in the viper?)

It's not FUBAR. It's different. It's still stability, but you go from positive stability to neutral stability, IIRC.

 

You have to understand what each of those means, because the aircraft will act differently but predictably in both cases.

 

Therefore, in my normal, trimmed, (can't remember speed but immediately following takeoff), I engage that switch ( lets call it the damn switch cuz I'm pissed at the output in F2 I see), then almost immediately, I'm in a half cuban curve-less invert, and plunking down at very high VVI, let alone blacked out)...

Because ASC sets the controls to a specific neutral position ... don't want to 'flip' if you push the switch without doing anything else? Fly the correct speed ( about 600kph IIRC).

 

Having air all that, Yo-Yo is still working on things regarding the aircraft, so there will still be a few corrections, but not necessarily to this.

[DarkFire]

Guys I just read the inverted departure but I'm not sure if this is it:

 

I am flying, I engage direct control (the checkered yellow black cover switch), then I'm instantly in G-LOC, with the aircraft upside down, sinking very rapidly, with zero forward thrust.

 

I'm no real life pilot, but from what I know the Su-27 is FBW (like viper, A320)... but she's also very unstable (wing design, lack of thrust vector nozzles, you name it).. that switch cancels the FBW (sort of alpha protect Airbus folks here?) and enables very much the direct control of all pitch surfaces (pretty much the high AoA situation, without being in high AoA)...

 

because the stability is FUBAR without FBW, this switch is not normally used, and if so, it occurs when gear down (again sort of landing gains in the viper?)

 

Therefore, in my normal, trimmed, (can't remember speed but immediately following takeoff), I engage that switch ( lets call it the damn switch cuz I'm pissed at the output in F2 I see), then almost immediately, I'm in a half cuban curve-less invert, and plunking down at very high VVI, let alone blacked out)...

 

Having experimented with turning off the FCS, it appears to help prevent the instant departure if you're at a reasonable speed (say 500 - 750 Kmh TAS) and pull a small amount of +g at the moment when you disconnect the FCS. This appears to help counter the instant huge nose-down that I think causes the blackout and -G lock. That being said, "S" is still the damned death switch :(

[Ironhand]

There enough other physiological effects which make the representation 'fair'. Problems include headache, mental confusion etc, all within just a few seconds of exposure of up to -3g in real subjects. As a result, tolerance is marked at -4g for unprotected persons (and there's no such protection in modern fighters. You just don't pull -g's).

 

In real life, aerobatic pilots have lost consciousness with -2 to -2.5g.

 

Some people lost consciousness AFTER completing the maneuver, and in particular, possible when transitioning from -g to +g.

 

More importantly, and I don't think it's modeled in the sim - negative g's over an extended period ( ... single digit number of seconds) will instantly destroy your positive g tolerance - pretty much for the reasons you stated.

 

In summary, this is a very complex subject and the in-game representation is not entirely unfair.

:) Seems reasonable when put that way. It occurs to me that there should be an "Operational Limitations" section in our manual similar to the real world manual. Though most of us would probably not read it (like the rest of what we already have).... Just a thought, though.

[GGTharos]

I agree with respects to training/educational material :)

[Ironhand]

...

 

Therefore, in my normal, trimmed, (can't remember speed but immediately following takeoff), I engage that switch ( lets call it the damn switch cuz I'm pissed at the output in F2 I see), then almost immediately, I'm in a half cuban curve-less invert, and plunking down at very high VVI, let alone blacked out)...

I should probably double check this before I open my mouth but it'll be tomorrow before I can check anything. So...

 

With the ACS engaged, you get the reinterpreted trim that the FBW system gives you. When you hit the "damn" switch, you turn off the ACS's control and get the full mechanical value that you have dialed in.

 

Because of the airspeed trimming law (providing the feel of a positively stable aircraft), you must add nose down trim as your speed increases. But the amount that you "turn the trim wheel", so to speak, does not correspond directly to the amount of trim sent to the horizontal stabilizer. It's less. But, when to turn off the ACS, you get the full mechanical trim that you've been dialing in. If you're above roughly 350 km/hr IAS when you flick your switch, you'll pitch over unless you've already taken steps to negate it. Setting trim to "0" would help along with a quick, short tug on the stick as you disengage.

[GGTharos]

Actually the ACS is 'dialed in' to trim to neutral IIRC at 600kph (or I'm misrembering and IronHand is correct here about the 350kph :) ) or so. The safety advisory is to keep your fingers off it if you're flying faster. People have killed themselves going 'ASC off' in RL, and they were trained in the whole thing.

 

Come to think of it, the real instruction (and note that unlike what virtual pilots do, it is NOT on the HOTAS) is to leave it alone period. It has almost no application in combat ... and why?

 

Because the Su-27 turns so well it's not even funny. It makes me cry when I see someone who knows how to enter my TC do so with a Su-27, because now I have a very difficult problem and that pilot obviously has a clue.

 

In other words, if you're using ASC off to win engagements, you're doing it wrong, and you're doing it wrong because you don't know BFM.

 

AFAIK that switch is not there for combat, much like the F-16's pitch override switch is not there for combat.

[Ironhand]

Actually the ACS is 'dialed in' to trim to neutral IIRC at 600kph (or I'm misrembering and IronHand is correct here about the 350kph :) ) or so...

I find it a very dicey affair at anything over 400 km/hr IAS. Unless the danger begins to reverse itself as you approach 600... [EDIT] But that doesn't appear to be what you're saying since you say that it's the ACS that's dialed in to neutral trim at that airspeed. In the cockpit, you're still trimmed nose down. [EDIT ENDS]

 

If memory serves, if you go to external view when flying with the ACS engaged and look at the horizontal stabilizer at the airspeeds you're talking about and higher, it's showing very little trim even though in the cockpit you are trimmed very much nose down. So...if you turn off ACS at that point and go to direct mechanical trim from there...

 

That's something that, hopefully, I'll be able to double check tomorrow unless someone beats me to it.

 

[EDIT]

...if you're using ASC off to win engagements, you're doing it wrong, and you're doing it wrong because you don't know BFM.

 

AFAIK that switch is not there for combat, much like the F-16's pitch override switch is not there for combat.

I suspect that when/if the Su-27's damage modeling is fully implemented, using it in combat will be much less appealing.

Edited April 2, 2015 by Ironhand

[Xenovia]

wait what

[GGTharos]

That is quite correct :)

 

I suspect that when/if the Su-27's damage modeling is fully implemented, using it in combat will be much less appealing.

[Svend_Dellepude]

If memory serves, if you go to external view when flying with the ACS engaged and look at the horizontal stabilizer at the airspeeds you're talking about and higher, it's showing very little trim even though in the cockpit you are trimmed very much nose down. So...if you turn off ACS at that point and go to direct mechanical trim from there...

 

This is also true...

[Mizzy]

I can't fly this thing anymore, it's all over the place !!

[Ironhand]

wait what

 

Question? :)

 

 

I can't fly this thing anymore, it's all over the place !!

Unless your joystick settings are screwed up, sure you can. You just can't throw it all over the place like we used to. Also you have to keep trimming. The airspeed trimming law which overlays the FBW system, insists on it.

Edited April 2, 2015 by Ironhand

[Weta43]

I don't know about with a non-FFB stick, but with one you can disengage the ACS without fear at any speed if first you:

 

neutralise the trim,

Let the stick return to neutral

 

The plane won't be flying straight and level at the time, but it also won't change direction when you do so, and you can then control the aircraft as desired.

 

Mizzy - I agree with Ironhand - of course you can :)

You just have to stop expecting it to be what it was with SFM, learn it as a new aircraft, and unlearn some bad habits SFM taught you - then it's a joy - as long as you're present to what you're doing :)

If you get a fright and yank/jab at the controls, it'll bite !

 

(are you still skinning ?)

[WildBillKelsoe]

Thanks for the wealth of information GG, and Ironhand. I know its not to be used in combat (you use sustained and instantaneous turn from your video, Ironhand), but I didn't know that 600 km/h is minimum speed to perform a cobra (which is airshow specific)

[GGTharos]

Maximum speed.

[Kuky]

I disable ASC only when I stall it due to high AoA and too low speed (lets me have better control to recover from the stall). I am now to flying the Flanker trimmed down even before I roll for take off (about 60-70% trim down) and adjust trim more as I gain higher speed (600-800kmph IAS).