Roll rate to slow. (1.5.4.)

[Holbeach]

From the manual, pretty much all Mig-21 manoeuvres are performed using the rudder.

 

It also states that rolls in excess of 90 deg ps are only permitted for 3 - 4 secs, due to the onset of inertia roll which will become difficult to recover from.

 

..

[SNAFU]

Ed Rasimus wrote in Palace Cobra that in the early versions of the F-4, the pilots were strictly told not to use the ailerons to roll, but only rudders. So they only used the stick for pitch control. Adverse yaw effects were so strong when moving ailerons a bit too fast, that some pilots new to the F-4 lost control and crashed.

 

So it all depends on the airframe and the situation.

[Frederf]

An interesting report on loss of control issues from inertial-stability coupling on similar (or more extreme) aircraft of similar design: http://www.nasa.gov/centers/dryden/pdf/88484main_H-2106.pdf

 

This (http://uotechnology.edu.iq/tec_magaz/volume272009/No.7,2009/researches/Text_11.pdf ) has reference to an aerodynamic report I believe

 

"Tехническое описание

КНИГА 1

Летно-технические характеристики"

Books 2-5 may prove the useful if they are found.

 

Inertial roll coupling seems to be similar to dutch roll but a pitch-yaw coupling in the rolling state where the natural frequency stability response in either pitch or yaw (or both) coincides with the frequency of the exchange of these axes by rolls through the quarter circle. It's an instability of pitch and yaw, the loss of roll control is somewhat secondary.

 

It should be tested to see if the ailerons will be unable to arrest this inertial rolling behavior at supersonic speeds (as the manual suggests) as it is certainly quickly ceasing at moderate to high subsonic.

Edited July 13, 2016 by Frederf

[gavagai]

You didn't actually call a pure linkage to hydraulically boosted control system "modern" in comparison to interpreted control surface commands in a FBW aircraft, did you?

 

Oh, you did.

You're saying I called the Mig21 modern and I think it's controls are like an F16's? You are a comedian.:doh:

 

No, I was only responding to the remark about what contemporary military pilots describe, and that Rodd's report was the same as what I've heard from military pilots.

 

You can also look in this same thread and see that I said the Mig21 required rudder for coordinated turns. Am I contradicting myself or did I not say what you think I said?;)

 

Errr, I shouldn't fall into the game of trying to score rhetorical points, because that is what makes these threads such a bore. I'll wait to see what Leatherneck's mig21 pilot says about the new roll behavior.:beer:

[Frederf]

The more I look into this the more I'm not sure. It's not necessarily that the roll rate is too slow but Bad Things should be happening that I can't manage to force to happen.

 

Aileron authority is perhaps a bit too high, especially supersonic. There's no real consequence from holding in maximum aileron indefinitely. You get up to ~130 dps and it just safely plateaus even if you upset the apple cart with beta or alpha pulses.

 

At M1.9 normal (non-inertial coupled) rolls are achieved at about 75dps, at current below the concern for instigating a,b coupled departures.

 

Roll-inertia coupling auto-rotations of the low-alpha type are explained excellently here: http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA319980

 

It may be the case that since the MiG-21 has reversible rudder controls and the rudder pedal forces will be severe during the inertia rolling condition. The resulting pedal movement allowed because these pressures are so high allows the oscillation to continue. However in DCS our legs are unnaturally strong and responsive providing for static rudder deflection in the face of these enormous pedal forces and provides superhuman canceling of the oscillatory effects.

 

This could also explain the unnatural yaw stability exhibited, being critically or over-damped when 3-5 lightly damped oscillations are expected for a sizeable yaw perturbation.

 

My current impression of the roll rate situation is that the MiG-21 should be able to approach high roll rates but encounter particularly nasty departures before those limits are achieved which we aren't seeing now, especially in the supersonic condition.

 

The air show demonstration is possibly deliberately departing in roll but recovering with pilot input reversed or neutral aileron, thus exploiting the departure to exceed the normal limit of roll rate. The concern in DCS is that this super-90 roll rate is departing in a very canned, brief, and safe way.

[CoBlue]

About AoA dependence & roll rate.

Look at https://youtu.be/0-cEdwgYdAw?t=5m12s, the AoA is on the left side of the cockpit, he starts the roll in the yellow band of >20 AoA & makes a pretty fast roll. If you watch the video further you'll see the same. It's not AoA dependent.

 

All of you calming you can achieve fast instantaneous rolls like in the OP videos, please post a track, tacview or video.

 

The 21 is slow now when it comes to turns in a dog fight, the instantaneous roll rate to 90° is very slow, at 6-900km/h. Making it not as agile & maneuverable, compered to previews FM, it's a huge difference.

 

Yes the 21 was not build to be a dog-fighter, but it was a good & agile dog-fighter anyway! read the article

Mig-21and fighter maneuverability in today's terms:

[GGTharos]

I would suggest using real evaluation documents like the HAVE DOUGHNUT reports, not an article presenting their own interpretation of a few choice pieces of information gleaned from those documents.

 

No opinions should be based on that article.

 

As for the video, I see that he is very clearly driving AoA to zero or near zero before performing 90+ degree rolls, conforming to the basic flight instruction of 'manipulate one axis at a time'.

Edited July 14, 2016 by GGTharos

[Frederf]

Attached is a TacView file of a battery of roll tests. Three regimes are flown in order, supersonic (M1.45), high subsonic (600 km/h, M0.75), and low speed (400 km/h, M0.4). Each regime is demonstrated:

 

Standard: A nominal G and AOA loading roll producing a median roll rate

Loaded: A high G and AOA loading roll producing a reduced roll rate

Bunted: A low/slightly-negative G and AOA loading roll producing an elevated roll rate

A continuous roll of three turns of the standard type

 

[table=head]Regime|Roll|Roll Rate

Supersonic|Standard|135

--|Loaded|85

--|Bunted|210

Subsonic, high|Standard|100

--|Loaded|70

--|Bunted|170

Subsonic, low|Standard|70

--|Loaded|50

--|Bunted|155[/table]

 

These are rather loosely-controlled tests but show qualitatively the relative roll rates in the regimes and vertical loads. The high subsonic demonstration in particular would show increased roll at altitudes where 700-800 km/h could be reached at 0.8M or below. Under ideal conditions and inputs the peak roll rate can exceed 300 dps.

 

What should be noted is that positive vertical load arranges the inertial and aerodynamic axes to apply a counter-rotation effect, negative load an arrangement of a pro-rotation effect, and neutral load produces a median roll rate neither hindered nor aided by inertial-aero coupling effects.

 

P.S. the three-turn standard rolls showed about a 10 dps elevation over the single-turn roll rate in all cases.

MiG-21_Triple_Roll_Test.zip

Edited July 14, 2016 by Frederf

[Wizard_03]

Attached is a TacView file of a battery of roll tests. Three regimes are flown in order, supersonic (M1.45), high subsonic (600 km/h, M0.75), and low speed (400 km/h, M0.4). Each regime is demonstrated:

 

Standard: A nominal G and AOA loading roll producing a median roll rate

Loaded: A high G and AOA loading roll producing a reduced roll rate

Bunted: A low/slightly-negative G and AOA loading roll producing an elevated roll rate

A continuous roll of three turns of the standard type

 

[table=head]Regime|Roll|Roll Rate

Supersonic|Standard|135

--|Loaded|85

--|Bunted|210

Subsonic, high|Standard|100

--|Loaded|70

--|Bunted|170

Subsonic, low|Standard|70

--|Loaded|50

--|Bunted|155[/table]

 

These are rather loosely-controlled tests but show qualitatively the relative roll rates in the regimes and vertical loads. The high subsonic demonstration in particular would show increased roll at altitudes where 700-800 km/h could be reached at 0.8M or below. Under ideal conditions and inputs the peak roll rate can exceed 300 dps.

 

What should be noted is that positive vertical load arranges the inertial and aerodynamic axes to apply a counter-rotation effect, negative load an arrangement of a pro-rotation effect, and neutral load produces a median roll rate neither hindered nor aided by inertial-aero coupling effects.

 

P.S. the three-turn standard rolls showed about a 10 dps elevation over the single-turn roll rate in all cases.

 

Are you using the rudder or not?

[CoBlue]

As for the video, I see that he is very clearly driving AoA to zero or near zero before performing 90+ degree rolls, conforming to the basic flight instruction of 'manipulate one axis at a time'.

Really? then I suggest you re-watch the video, coz you are wrong!

You can see the time-stamps on the pics.

[GGTharos]

Yeah, I watched the video and I'm still right.

 

These instruments lag. You can very clearly see that he stops the nose before initiating larger rolls, and very soon after this the AoA drops to nearly zero.

 

At 8:32 you can clearly see that he stops the nose and drops AoA to make his rapid roll, then maintains high AoA during a much slower roll into the turn.

 

Really? then I suggest you re-watch the video, coz you are wrong!

You can see the time-stamps on the pics.

[Frederf]

The bunt at 8:32 is much more subtle than at 6:41 but is being done in both cases.

 

Are you using the rudder or not?

Not as in the standard and loaded cases the ARI (the MiG does have ARI right?) seems to be more capable of coordinating the turn than I am. Ball deflection was minimal.

 

However in the negative load cases rudder has a drastic improvement first in the normal direction to begin the nose moving as well as opposite direction to act as a third aileron to hasten the roll. I did not use this technique during the provided track however.

 

I could follow up with a high subsonic test using various rudder techniques in both the standard and negative load cases.

[Wizard_03]

Idk if it has ARI I thought SAU was the only flight control assist and the rudder doesn't seem to move on its own in game when I do aileron rolls so I don't think it actually does, but idk.

 

Anyhow reason I ask because I think the roll rate in the video is achievable with our FM as long as you use the rudder, now if you are supposed to use the rudder or not IRL is the issue. Which from the previous few posts I get the impression you do.

[CoBlue]

I can still not do instantaneous fast rolls like here: https://youtu.be/0-cEdwgYdAw?t=9m8s with current FM.

With the previous FM like in 2.0 I still can.

[Vitormouraa]

That's a MiG-21 Lancer, not a Bis, can we compare them? I don't think so.

[Wizard_03]

I can still not do instantaneous fast rolls like here: https://youtu.be/0-cEdwgYdAw?t=9m8s with current FM.

With the previous FM like in 2.0 I still can.

 

Yeah first off it is a lancer so no idea what if any modifications to the FCS have been made.

 

Second off the pilot in the video is extremely smooth however just eye balling it (since the actual roll rate he gets is not available) I can make our Mig do about the same thing if I'm unloaded and going around M 0.8 and pointed down with AoA around >10

 

But again he's a pro I'm not. So If I can get anywhere close to the video and without more information I'd say the FM is pretty spot on, don't really see what the issue is.

Edited July 15, 2016 by Wizard_03

[GGTharos]

I believe the LanceR upgrade was applied to MiG-21bis, so the airframes would be quite comparable, being the same.

[cro_mig_21]

I believe the LanceR upgrade was applied to MiG-21bis, so the airframes would be quite comparable, being the same.

It was actually applied to the MF versions

[Rainbowgeorge]

Ive just found out that the roll rate is much faster if you dont pull your stick enteireley, so maybe the axis is bugged?

[grunf]

https://leatherneck-sim.mantishub.io/view.php?id=375

[CoBlue]

https://leatherneck-sim.mantishub.io/view.php?id=375

"Delfin887 can you please comment if rudder is used in fast rolls?

Thank you"

 

"No.

Set desired pitch, wait a moment to stabilize.

Stick to neutral, rudder neutral, roll without applying +/- pitch.

 

With each AoA pilot adds when roll, the more irregular roll will be + it gets worse with the rolling speed (this is due to increased difference in local AoA on each wing). At small AoA it is hardly noticeable.

Needles to say, at near-critical AoA roll can cause one wing to stall."

 

(For you that don't know. Delfin887 is an active Mig-21 pilot, he's the coder of 21's FM & has been with LN from the start.)

[CoBlue]

Roll rate is slow again.......we are back to post 1.

[Big Nuts]

God isnt it just an alpha issue? If you are pulling g's you are loading the aircraft, when you try to roll you can see there will be resistance. If i want to roll the 21 i just bunt, unload the aircraft, and roll. That's surely how it's done?

How aware are you of your g load? Your trim? In a simulator these things can become more subtle, look down the nose of the aircraft and see what it's doing.

Edited October 5, 2016 by Big Nuts

[GGTharos]

^^^^

 

Actually, neither of the two posts above provide any proof/measurement/science at all regarding how the aircraft performs.

 

'Roll is slow again' is meaningless without providing the sort of information that Frederf did.

[CoBlue]

God isnt it just an alpha issue? I.

Nope, flying with ≃0 AoA.

 

^^^^

Actually, neither of the two posts above provide any proof/measurement/science at all regarding how the aircraft performs.

'Roll is slow again' is meaningless without providing the sort of information that Frederf did.

Delfin887 acknowledged the issue before & it was fixed, now it's back again after the latest patch.

It's exactly the same issue as before, hence my reference to post nr:1.