Roll damping

[flying.toaster]

Err no trolling intended here so please kick me if I go over the top with this question but ... Isn't something a tiny bit odd with the roll control of the PFM ?

 

I mean the real thing has

A/ Fly by wire with augmentation in roll according to the flight manual (double checked various translations)

B/ LOTS of roll authority, courtesy of a rolling tail + 3/4 span flaperons

 

Yet below 600Km/h indicated it has a serious issue with roll stability... And all those huge control surfaces does not seem to be able to stop the massive inertia of the beast (even lightly fueled and with no stores). I have seen airliners having a crispier roll behavior ...

 

Not having flown the real thing my opinion may be put in doubt (I am OK with that too), but watching the video from Flanker 1.0, you can see that Anatoly Kvotchur does not get especially busy with the stick when making a 45 degrees bank turn. And he nails the bank angle too.

 

Dunno I think I noticed the wild dutch roll tendencies decrease a bit with the last release (but I may be delusional on that one) so I guess tweeking a bit the FCS gains in roll at lower dynamic pressures may help a bit (high speed behavior is, once more, nice enough).

 

Any thoughts on that or first hand experience with FBW fighters is welcome.

 

Don't get me wrong, the FDM is beautifully executed, and it's a very nice touch to have to work the rudder in a turn, but I can't imagine the russians (and their excellent control systems theorists) having a fighter with that kind of low speed behavior, if anything because it can all to easily drag you from stall to spin, or pilot induced oscillations ...

 

Just my two cents :joystick:

[LJQCN101]

Strongly second this. I'm aware that at low speeds and high AOAs, dutch roll does aggravate. According to my countless tests on the dutch roll mode at low speeds, with the activation of FCS (Flight Mode & Takeoff-Landing Mode), we can notice a slight enhancement at the dampening of yawing motion. However, I cannot see any changes regarding the rolling motion whether FCS is on or off. Frequent pilot-induced roll oscillation is still expected. As a typical 4th-gen FBW fighter, I don't think such control quality is acceptable by the FCS control law designer or the air force. Anyways, two of my friends just got contact with in-duty Su-27 pilots, hope he can bring us some good info.

 

(One of them is working in a flight test center and can easily acquire feedback from the pilot. I've shown him a series of control & stability tests of DCS Su-27, and he seems a little bit upset about the rather high longitudinal dampening ratio under Takeoff-Landing Mode, poor lateral-directional stability and high lateral dampening ratio, not to mention the departure susceptibility. He also holds the point that an aircraft with such quality all together as I listed above won't even be approved to be put into production.)

Edited November 19, 2014 by LJQCN101

[flying.toaster]

(One of them is working in a flight test center and can easily acquire feedback from the pilot. I've shown him a series of control & stability tests of DCS Su-27, and he seems a little bit upset about the rather high longitudinal dampening ratio under Takeoff-Landing Mode, poor lateral-directional stability and high lateral dampening ratio, not to mention the departure susceptibility. He also holds the point that an aircraft with such quality all together as I listed above won't even be approved to be put into production.)

 

Yeah it definitely looks like that. As it stands it would surely rate poorly in the Cooper-Harper scale

Guess it's looking more and more like some tweaking is needed after all. Let's hope ED has some time for the fix.

Just for me to get it, what you mean by lateral dampening is slow onset of roll rate or large stabilising moment (read positive dihedral)?

Thanks for the insight

[theropod]

no more dutch roll on 1.2.12:)

when acs on position, yawning no more affects on rolling but if you turn off asc,yawning affects on rolling as expected.

thank you ed,roll problem is done:)

Edited December 3, 2014 by theropod

[archer86]

no more dutch roll on 1.2.12:)

when acs on position, no more yaw affect when rolling but if you turn off asc,yawning affect on rolling as expected.

thank you ed,roll problem is done:)

 

Yes,I've just noticed that one!

[LJQCN101]

Just for me to get it, what you mean by lateral dampening is slow onset of roll rate or large stabilising moment (read positive dihedral)?

Thanks for the insight

 

Maybe he means the strong tendency for the aircraft to zero out any bank angle. This is very obvious if you take a test on the spiral mode. When you bank the aircraft, a serious amount of side slip will build up, causing the aircraft return to zero bank angle. However if you manually apply rudder to counter any induced side slip, the aircraft will then perfectly stay on its current bank angle.

 

Roll mode: you won't see obvious roll oscillation after rapid rolling, but it will take some time to completely zero out roll rate. Also, control response in lateral channel is not good enough (low on frequency).

 

Dutch-roll mode: improved at low altitudes in DCS 1.2.12.:thumbup:

Edited December 5, 2014 by LJQCN101

[Weta43]

The slow onset of roll rate is mentioned in a report I've read by a USN pilot that's flown the plane.