Su27 inverted depart is realistic?

[Kwiatek]

Can someone explain me why in DCS Su27 after pass negative AoA got steady inverted depart?

 

As i know most planes can't keep such position in inverted or normal flight. Plane if stall and dont have slip input drop its nose byself. It could be in normal or inverted fly. After all planes would drop its nose - thats how planes are constructed. Or if it would be slip input plane eventually would got spin (normal or inverted).

 

But from some unkown for me reason Su27 in DCS when got inverted depart it could stay it until crash? No inverted spin just depart like a stone. Why plane can't drop its nose byself in such configuration?

 

It looks really strange to me.

[Shein]

Pretty sure most fighters are tail heavy, at least the fly by wire ones are. Could be wrong about the 27, not sure to be honest but that's my speculation.

[Kwiatek]

Well i tried similar things with F-15 but it behave like other clasical planes and can't get such inverted depart like with Su27.

[Kwiatek]

i think with last today update something change in Su27 behaviour. Now with negative G apply planes dont want to go inverted depart like before. Mostly after pilot recovery from blackout plane stabilise with nose down postion. I think now is much better and easy to recover.

[LJQCN101]

Maybe its because that the aircraft doesn't have any negative AoA/G limiter and if you push too hard, it will lose all pitch-up control effectiveness at around -25..-30 AoA due to some kind of flight characteristics. More discussion here: http://forums.eagle.ru/showthread.php?t=134028&page=7

[Weta43]

Great - thanks !

I spent half an hour today looking for that image on the forum to post it here.

Don't know why I couldn't find it...

[Squale-BAF-]

My opinion is that it looks more like a "scripted" behaviour than a physical result,.... An PFM model should be based only on physics !

Edited December 4, 2014 by Squale-BAF-

[Weta43]

It's a graph showing data for an F-16.

You think F-16 are scripted and should use real physics ?

[Chappie]

Pretty sure most fighters are tail heavy, at least the fly by wire ones are. Could be wrong about the 27, not sure to be honest but that's my speculation.

That would imply a deep stall in which rocking the aircraft (think F-16) would be required correct?

[Chappie]

Maybe its because that the aircraft doesn't have any negative AoA/G limiter and if you push too hard, it will lose all pitch-up control effectiveness at around -25..-30 AoA due to some kind of flight characteristics. More discussion here: http://forums.eagle.ru/showthread.php?t=134028&page=7

Here is the problem and why it feels scripted. The aircraft when stable about its lateral and longitude axies will suddenly and without explanation nose hard over on its own once the limiter is disabled.

 

The Su-27 has fly-by-wire and operational limits applied for safety. Disabling the limiter does not imply disabling the flight system (Fly-By-Wire). Thus I should be able to disable the limiter and continue to fly within limits.

 

This is not what happens and makes doing so extremely hazardous. Way I see it I am free to operate the aircraft up to and beyond the limits as I desire knowing beyond some bad can happen but that bad thing happens when limots are exceeded and not during straight and level flight.

[Robin_Hood]

Here is the problem and why it feels scripted. The aircraft when stable about its lateral and longitude axies will suddenly and without explanation nose hard over on its own once the limiter is disabled.

 

That is because your aircraft is heavily trimmed nose-down. What I noticed is that in direct link mode, pitch input are much more sensitive and therefore so are trim inputs. Also, stability is very different in direct link mode: keeping the trim neutral, there is a slight nose-up tendency at low speeds and a slight nose-down tendency at high speeds, while around 700 km/h or so it is relatively neutral.

So the thing is if you are above 400 km/h in normal mode, you are heavily trimmed nose-down, so that when you switch to direct mode it is exactly as if you suddenly and violently pushed the stick forward.

 

The bottom line is, if you want to go in direct mode in a stabilized flight, reset the trims first. This being said, I do not know why the stability is so different, but I assume the artificial traditional stability being to be deactivated in direct link mode.

[Nate--IRL--]

There are no limiters in any mode for negative Pitch IIRC - the limiter is the pilot.

 

Nate

[GGTharos]

You are actually disabling part of the FCS. The 'S' key puts you in 'direct control', which removes ratio modifiers for pilot stick input, so deflection of controls is based purely on stick position. Also, the trim control law that make the aircraft behave like it has positive stability instead of neutral is removed.

 

The Su-27 has fly-by-wire and operational limits applied for safety. Disabling the limiter does not imply disabling the flight system (Fly-By-Wire).

[Esac_mirmidon]

But in the real Flanker there are two switches:

 

The AoA limiter that it´s possible to turn ON OFF

 

The AFCS Direct Control Mode, to turn ON OFF again.

 

So in real life it´s possible to turn OFF the AoA + G limiter without any effect over the FBW or AFCS. But in DCS pushing "S" turns OFF both AoA+G limiter and activates the Director Control Mode.

 

Edited December 10, 2014 by Esac_mirmidon

[Chappie]

Su27 inverted depart is realistic?

 

But in the real Flanker there are two switches:

 

The AoA limiter that it´s possible to turn ON OFF

 

The AFCS Direct Control Mode, to turn ON OFF again.

 

So in real life it´s possible to turn OFF the AoA + G limiter without any effect over the FBW or AFCS. But in DCS pushing "S" turns OFF both AoA+G limiter and activates the Director Control Mode.

 

 

 

Good find and why so many have stated it is flat out wrong in the ED implementation. On any fighter aircraft that supports it, the pilot can disable the limiter without disabling the FCS. In fact, the manufacturer typically doesn't want the FCS disabled period and to only allow the pilot to exceed limits as they desire.

 

F/A-18 pilots disable the limiter and yank the aircraft off the runway during demonstrations. They reenable it because flying around without it on is dangerous but flown at or below limit they are fine.

 

ED's implementation is wrong. Flanker pilots doing Cobras are not disabling their FCS but its limits so they can yank the stick past AoA limit and Cobra. Those pilots need the FCS to control the aircraft as it is designed unstable and without such the aircraft is gonna flip over or do something crazy.

 

As far as I am concerned, we are past the novelty of inverted deep stalls in the Su-27. This behavior is flat out wrong and should already be considered a reported bug.

 

Matter of fact, when the F-16 is in a deep stall, what is disabled to get out? The FCS? No, the AoA limiter so you can rock the plane past the lit and get out.

Edited December 11, 2014 by Chappie

[Esac_mirmidon]

Well, your conclusions are a little beyond my knowledge.

 

Everything i have tested about entering inverted departure match 100% with the SK manual ways to do so. I have listed this ways in a previous post ( link).

 

So i can´t find anything strange in how the Su-27 reacts in this situation.

 

The AoA + G limiter is a "mechanical" system that reacts taking in count some parameters, making the stick harder to move ( more kg ) when the plane is moving near the AoA+G limits. If you try to push or pull beyond the limit the stick reacts with a violent shake forcing to move it to the center.

 

It´s possible to fly without this limiter enabled but the manual says that no benefit in any maneuver or flight parameters could be extracted because turning it off. I assume that this limiter is not linked to the SDU and both could be working independently, but is only a guess. I´m not sure about this.

 

But the SDU is different. The manual says it´s forbidden to take off with any of the four SDU channel warning lights ON. The pilot must abort any flight and return to base with any SDU channel failure. The Direct Control Mode is a safe measure to be able to control the plane to a certain degree with a lot of limitations in handling.

 

There is no reason to change to Direct Control Mode if the SDU is working and again i´m assuming that when you turn off the AoA limiter this has no effect over the SDU or FBW. But is only a assumption.

 

Regarding the inverted depart i don´t find anything strange. A vertical tail slide over 75º cause it, flying inverted at low speeds cause it and pushing the stick forward all the way at low speeds also cause it, like the manual says.

[Fox One]

Regarding inverted departure, it is interesting that the real aircraft is restricted at below M 0.85 to -2G. With a little but important exception - at below 300 Km/h it is restricted to only -0.5G. The same manual says the cause of this restriction - avoiding entering into a spin...

 

Also note that the HUD AoA indicator that on real aircraft appears only in "Takeoff" and "Landing" display modes - on the AoA scale below "zero AoA" mark there is also a mark for min AoA. It looks like for the real aircraft negative AoA at low speeds are a firm no go.

[rami80]

Not to sound like I'm not happy with the Su-27 module (I'm really enjoying it) but it seems like the S key is just implemented so you can do "out of envelope maneuvers" such as the cobra. The cobra requires turning off more than one switch,not the limiter only, as per the following account of an F-18 pilot :

 

We discussed the Cobra in our flight brief. It went something like this:

Me: "I would like you to show me the Cobra maneuver."

Kvochur: "Yes, sure."

Me: "What entry airspeed and altitude should we use?"

Kvochur extended the fingers of both hands in a calming gesture and said: "I show you."

That was it. I looked toward our interpreter for a read on this physical punctuation, but she returned the internationally universal shoulder shrug. So I took his brevity as an admonition against prying too much into this top-secret aerobatic phenomenon and made a mental note to be prepared to absorb as much technical data as I could during his demo.

When I was ready for the demo, he had me turn off the angle of attack limiter and another fly-by-wire switch that was never explained completely to me. Then he said, "I do one. We do one. You do one." The setup was 350 kilometers per hour (approximately 190 knots) in straight and level flight. He pulled the stick all the way back, and the airplane pitched nose up past vertical. In a little more than one second, we were more than 90 degrees nose up after the stick pull. He recovered the plane back to straight and level, and the maneuver was complete. The speed was about 90 knots as the nose approached the horizon. During the "We do one," I was again surprised at the non-aggressive control inputs. He used large pedal displacements during the first half of the pitch-up then transitioned to differential throttle control to keep the roll and yaw minimized. For the nose-down recovery, the stick was moved well forward but not all the way. Differential throttle that gave way to rudder-pedal activity essentially kept us wings-level throughout. My turn: I replicated what I had just ridden through, and the results were the same. I didn’t get past 90 degrees nose up like Kvochur, but I was awed anyway. Despite the radical attitude change, the entire maneuver was completed under 3G. Throughout this seemingly suicidal contortion, the engines never complained, and there was no implication of an impending loss of control.

Fun complete, I switched the limiter and fly-by-wire switches back to their normal positions, and we headed back.

[archer86]

maybe, they should add other switches for use seperately , like F-15 CAS channels.

is it possible?

[rami80]

It would be a nice feature to add but I don't think ED will work on it unless there is an ASM Flanker in the works as according to the flight manual it isn't really useful.

 

It´s possible to fly without this limiter enabled but the manual says that no benefit in any maneuver or flight parameters could be extracted because turning it off.

[archer86]

It would be a nice feature to add but I don't think ED will work on it unless there is an ASM Flanker in the works as according to the flight manual it isn't really useful.

 

But this fucntions are about Flight Control Systems.

we have an example like FC3 F-15C.

if this is true ,PFM has responsibilty about those functions

[karambiatos]

maybe, they should add other switches for use seperately , like F-15 CAS channels.

is it possible?

the buttons that are labeled 1-4 are the stabilization channels, i think, itd be really nice if we can turn them on or off.

[Esac_mirmidon]

This buttons are the four AFCS channels and they only lit on when a failure in this channel is detected.

 

To turn ON-OFF the SDU system you must use this other switch

 

[Fox One]

^^^ The switch is not for turning on/off both SAU and SDU. Notice the label says СВЯЗЬ САУ-СДУ. СВЯЗЬ means connection, communication. It is for emergency disconnecting autopilot from flight control system if the autopilot starts going crazy.

[archer86]

 

But I guess those switches , same line with AoA limiter are about FCS ,right?