Su-27 Flight Model Discussion

[OB1]

Is loss of elevator authority normal ?

 

Had an interesting experience last night landing the Su-27 after an online sortie, Details as follows:

 

Was running of fumes got the fuel 500 warning 3 minutes out from runway so I was light.

Some ordinance still attached, asymmetrical load, 4xR73 wing tips, 2xR27ER centre line, and 1xR-27ET.

Flaps extended and gear down, there was a moderate crosswind.

Just prior to touch down I was dead on 220km/h at 12.7 AoA very shallow approach almost touch down, I throttled back hoping to drop on the runway.

 

Just prior to touch down AoA increased to 15.1 and speed dropped to 210, at this point I lost total pitch control. AoA limiter was enabled. I had stick full forward with no response.

Rear wheels touched down at 206km/h with an AoA of 16.1. The nose still did not come down and rear wheels lifted off again causing the aircraft to hover along the runway until finally the speed dropped to 170km/h and it just fell out of the sky but still with no pitch control and nose high, at this point AoA of 21.1.

 

Nose came down when only after the complete stall and slammed down. No damage and kind of fun but does that all seem right ?

 

The track file is a tad large so I hope this is enough information to form an opinion.

[Sgt_Baker]

Another point is that my pilot remained in full +ve G-LOC despite there being no obvious +ve G on the aircraft during its inverted "shuttlecock" descent. Had the vigour of the initial uncontrolled tumble killed him?

 

Clarification: I zoomed to the stall point with heavy weapons and fuel. Having seen video of the Su-27 at high alpha, I attempted a high-AoA wingover to bring the target to my lift vector, and SNAP. Full stall (my fault) with six or seven spins before blackout. By the time I looked at F2, I'd been in G-LOC for perhaps 15s? Already sailing Inverted Shuttlecock.

[DarkFire]

I suppose it's possible that the FM models conditions whereby such low airflow over the elevators that they no longer provide directional control for the aircraft...? But were that the case I can't see why the aircraft should have remained in a nose-high attitude.

 

So to answer your question - no, that doesn't seem right.

 

What was the pitch trim setting at the point of crossing the runway threshold? I ask because I've occasionally seen what I can only describe as anomalous trim behaviour on occasion.

[OB1]

I trimmed out well prior, approx 350+ km/h for no input. To stop the aircraft spearing into the ground prior to crossing the piano keys I was approx half stick to stop it ditching. So as you can imagine when I finally wanted to lower the nose I was having to push forward through center pos all the way max pitch down with no effect.

 

Edit: checked track trim was for about 400 but still nosing down slightly.

Edited January 13, 2015 by OB1

[karambiatos]

if your engine RPM drops, i suppose your hydraulics would stop working

Edited January 14, 2015 by karambiatos

[esb77]

Seems odd, but I normally get wheels down with less AoA at 235 to 275 km/h, mostly out of habit from bringing back Su-25s with a lot of ordnance still on the wings.

 

I haven't pushed the Flanker's low speed envelope yet, I remember the Grach behaving nicely down to about 180 km/h when light, but as a matter of general practice I look at that sort of speed when landing as just asking for trouble.

 

What were you doing with throttle and airbrake? I normally come in with the throttle at roughly 80% so I have thrust for a wave off if I need it and use the airbrake to bleed speed. Both tend to exert a nose down moment on the plane once the rear wheels are down, so it tends to go nose down as soon as you reduce pitch up input on the stick.

 

Wheelbrakes will also get the nose down, but that's not recommended unless you have a light touch and the brakes are bound to an axis.

 

 

The gliding along above the runway sounds like ground effect lift keeping you up.

 

Loss of pitch control I'm not sure what the cause would be. The AoA would put the tail very low, so maybe there was some sort of ground interference with the normal aerodynamics? Center of gravity changes from being so light also might have contributed.

[GGTharos]

Did you run out of fuel by any chance?

 

I find this a bit unlikely right now, but if you ran out of fuel early enough and the engines spooled down, you'd lose hydraulics, which means kiss your steering good-bye. For most of these fighters, the manual says to eject under those circumstances.

 

If this was not the case, then could you try to re-create it? There is till a possibility that there's some sort of augmentation system bug.

 

I trimmed out well prior, approx 350+ km/h for no input. To stop the aircraft spearing into the ground prior to crossing the piano keys I was approx half stick to stop it ditching. So as you can imagine when I finally wanted to lower the nose I was having to push forward through center pos all the way max pitch down with no effect.

 

Edit: checked track trim was for about 400 but still nosing down slightly.

[OB1]

At work so cant respond at length atm, no air break deployed and I had fuel as I taxied to ramp after.

[OB1]

Ok I had a quick go at replicating the feel of what I experienced and I got some results.

 

I exaggerated some of the initial input to ~force the aircraft into the situation but I got close enough to what I experienced before.

 

First one where I almost went swimming I lost pitch control completely, I added power to try recover which I did for a second then went vertical full pitch down and no response. Basically I stalled out then was able to recover, again only temporary as I lost pitch control again leading up to when I was able to pitch down.

 

Then after that I twitched down on the stick for a split second then full pitch up but continued to invert, then went strait to depart. This particular bit is not what I want to talk about as it is different to the positive AoA unresponsive stabilators. Because the stabilators could still aid in the rotation when progressing through negative AoA, you just don't have enough authority to bring it back. Where as the positive AoA situation I experienced you loose complete use of your stabilators until you reach stall.

 

Note AoA limiter was always on I never touched it, Flaps were down. Inherantly very unstable at low speeds.

 

Does anyone know what exactly is going on under these conditions ?

 

TacView and Track attached. Notice stick pos when watching in-game track.

Tacview-20150114-154643.txt.zip

El2Test.trk

Edited January 14, 2015 by OB1

[Ironhand]

There are two sections. In the first, up to and including the vertical climb, everything seemed normal. The thrust component (your engines), because of the high pitch angle, was acting as the lift component. Judging from both your airspeed and your alpha, you shouldn't have had pitch authority.

 

The problem is in part 2. After the vertical climb on mostly, if not entirely, thrust, there was a point where your alpha and airspeed indicated that you should have been flying with some limited pitch control and you didn't have it. If you had been in level flight at the time, you would have. I've been in similar situations in level flight and have had it. So there's an issue somewhere, in my opinion.

 

EDIT: BTW, my comments are made based entirely on the TRK file. I didn't have time to view the TacView file this morning.

Edited January 14, 2015 by Ironhand

[Ironhand]

Random air show routines? That sounds like a very useful training tool, where is it to be found?

Not random... :) Radom. The 2013 Radom Air Show. That's the one I quote/unquote reproduced in the sim. I used it because there are several YouTube videos of the Su-27's routine available, each from a different viewpoint in the crowd. So it's a lot easier to figure out what's actually happening.

 

BTW, viewed the track. Looked like you had fun trying to make it happen. :)

 

Another point is that my pilot remained in full +ve G-LOC despite there being no obvious +ve G on the aircraft during its inverted "shuttlecock" descent. Had the vigour of the initial uncontrolled tumble killed him?

 

Clarification: I zoomed to the stall point with heavy weapons and fuel. Having seen video of the Su-27 at high alpha, I attempted a high-AoA wingover to bring the target to my lift vector, and SNAP. Full stall (my fault) with six or seven spins before blackout. By the time I looked at F2, I'd been in G-LOC for perhaps 15s? Already sailing Inverted Shuttlecock.

Since you actually regained control for a very short time, death is not in the picture. Besides, when you die, you exit the sim (I believe).

 

I'll have to try repeating what you did and see what happens. Just not sure when that'll be. I'm going to be out of town all of next week and won't have much sim time available between now and then either.

 

 

Rich

[sniperwolfpk5]

I think he means ACS, not ASC--Automatic Control System or the system primarily involved in the various autopilot routines.

 

DC, I assume, is Direct Control which, in the sim, is switched on/off with the "S" key. (Sound is the "S" key with one of the modifiers (Ctrl, Shift, etc)). When you have direct control of the aircraft, the Flight Control System is turned off and you have, well, direct control of the aircraft's inherent instability. FWIW, I posted a short video tutorial on YouTube this morning that talks about it's use in performing the cobra.

 

Rich

 

By the way i watched your cobra maneuvering video on Lockonfiles site. Nice video with good explanation.

[OB1]

Yeah that's pretty much my thoughts on it as well, the first was too forced.

 

-----

 

Ok the exact situation I first started this topic is here in TacView. Actually compressed quite good.

 

Skip strait to the end where the landing is.

 

Note at exactly +1:09:34 is where I lost pitch control, I have the stick full pitch down the entire time from that point through the first rear wheel touch and bounce until it stalled and fell onto the runway.

Tacview-20150113-212547.txt.zip

[Ironhand]

...

 

Note at exactly +1:09:34 is where I lost pitch control, I have the stick full pitch down the entire time from that point through the first rear wheel touch and bounce until it stalled and fell onto the runway.

OK. Something weird happens as a result of the bounce. Prior to that, things seem more or less normal.The numbers make sense. But, once you bounce, your flight path is, for all practical purposes level (just before and after 1:09:40) even though your airspeed and alpha can't sustain that. Then you finally stall. It's almost as if the bounce added energy than you shouldn't have had.

[GGTharos]

Does it look like he's stalled in the wing ground effect region? That's a pretty dangerous place to be, because you're 'flying' at lesser speed than your stall speed. AFAIK he should still have pitch authority though because those control surfaces are not washed out.

 

I think we need Yo-Yo to have a look, but for that, a short track showing only this issue is required.

[Ironhand]

Does it look like he's stalled in the wing ground effect region? That's a pretty dangerous place to be, because you're 'flying' at lesser speed than your stall speed. AFAIK he should still have pitch authority though because those control surfaces are not washed out.

 

I think we need Yo-Yo to have a look, but for that, a short track showing only this issue is required.

Come to think of it, based on the TacView file, he should still have had some pitch authority at the point he says he lost it completely (1:09:34). Right then he was at 202 k/h IAS with an acceptable AoA. That's slow but his controls shouldn't have been washed out based on the AoA he was flying. He was sinking, of course, with those numbers. Hit the runway and bounced. After that the numbers didn't make sense at all for about 3 or 4 seconds.

 

I won't have an opportunity to try it myself anytime soon. OB1, would you be able to give it another try? Make sure you make the attempt with the gear extended. That's supposed to change how the FBW works and, so, there should be changes in its modeling.

 

EDIT: GG, he didn't actually "stall" until after the bounce when the numbers weren't making any sense in terms of what the aircraft was doing.

Edited January 14, 2015 by Ironhand

[Ironhand]

By the way i watched your cobra maneuvering video on Lockonfiles site. Nice video with good explanation.

:) Thanks! I enjoyed making it, too.

[DarkFire]

Not random... :) Radom. The 2013 Radom Air Show. That's the one I quote/unquote reproduced in the sim. I used it because there are several YouTube videos of the Su-27's routine available, each from a different viewpoint in the crowd. So it's a lot easier to figure out what's actually happening.

 

BTW, viewed the track. Looked like you had fun trying to make it happen. :)

 

Rich

 

Ah I see, sorry I misread what you typed. That sounds like a very good idea, thanks for the tip :thumbup:

[OB1]

Cool, i'll try some extensive testing soon I hope, work is hectic at the moment so I can't do much for the moment. Weekend looks promising for a bash.

[GGTharos]

Yep, that's why I asked what I asked: A bounce can send you high enough to exit the WIGE (or not), but you'll lose airspeed because of the attitude, and either stall out of the WIGE or out of it. This is a very real result of poor landing technique.

 

That's not to say that there isn't something potentially wrong with the FM, just saying ... it might be exactly right.

 

EDIT: GG, he didn't actually "stall" until after the bounce when the numbers weren't making any sense in terms of what the aircraft was doing.

[OB1]

Yep, that's why I asked what I asked: A bounce can send you high enough to exit the WIGE (or not), but you'll lose airspeed because of the attitude, and either stall out of the WIGE or out of it. This is a very real result of poor landing technique.

 

That's not to say that there isn't something potentially wrong with the FM, just saying ... it might be exactly right.

 

Not sure what to think of this, but shouldn't I have been able to pitch the nose down to plant the front on the runway ? Sitting in the pit at the time felt like the aircraft lifted off gently after touchdown and not so much of a bounce. The nose just wanted to climb no matter how much I tried to bring it down.

 

Is ground effect even modeled for fixed wing in DCS ?

[Ironhand]

...That sounds like a very good idea, thanks for the tip :thumbup:

Give it a try. If nothing else, you'll have a lot of fun while learning a lot.

 

The sequence (which I screwed up in my video) is takeoff and pull up immediately into 1.5 loops. Fly down to the opposite end of the runway and pull into a tight level left turn. Upon completion immediately pull up into a tight climbing left turn. As you come over the top, roll upright and descend toward the other end of the runway. Once there pull up into a tight, climbing right turn. As you come over the top, roll out and fly down to the opposite end of the field. Extend and come around for an inverted pass over the runway.

 

Roll upright and pull up into another climbing right hand turn. Roll out and fly back down the runway. At the other end pull up into an Immelman. Fly back down to the other end of the field and pull up. Climb. Tail slide. Recover and fly to the other end of the field and repeat the tail slide there. Back to the center of the field and perform a tight 360* turn. Upon completion, extend and return for a slow speed, high AoA pass. Regain speed and extend. Return with gear lowered. Nearly land, then go to full burner, pull up into a climb while performing an aileron roll. Retract gear. Extend. Return for a pass along the runway. Pull up into a climbing left turn. At the top, roll to the right and pull perpendicular to the runway. Perform an aileron roll. Extend. Return and land.

[GGTharos]

Not sure what to think of this, but shouldn't I have been able to pitch the nose down to plant the front on the runway ? Sitting in the pit at the time felt like the aircraft lifted off gently after touchdown and not so much of a bounce. The nose just wanted to climb no matter how much I tried to bring it down.

 

The way you describe it sounds like a problem.

 

Is ground effect even modeled for fixed wing in DCS ?

 

It is modeled period :)

[bolek]

Here is my entry :-). Very simple: fully loaded plane, push the stick all the way forward. No recovery.

departure2.trk

[tagge]

I flew my first test flight in SU-27 the other day. If you mean "upside-down shuttlecock of death", then yes; managed that on first flight.

 

Right on! That goes right on my tag! :lol: