What's with the slow stabilizer lately?

[jackmckay]

Flankers strong airframe holds together two extremely powerful engines (in its class) and thus has lot of hydraulic power available from it's sources. Max angular speed (rate of elevator rotation) is constant and only diminished by hydraulic-aerodynamic pressure battle results on elevator surfaces. Wont broke off either in case of over-deflection because it has been balanced in wind tunnel tests to feather itself on no input and also must sustain very high loads for a very long time. Elevator axle is last to broke off from plane (same as undercarriage) so when looking all aspects of plane systems Flanker should deflect its surfaces safe and crazy - in flight.

 

 

 

Just to add one important issue that sneaked past this topic: su27 has unstable design meaning that its cg is close to aerodinamic friction center, meaning that flanker is damn agile with less hydraulic power needed to make pitch deflection. Thats why it can do a cobra in first place. Roll angle depends on inertia force ruled by horizontal mass distribution, meaning single engined rolls faster. Developers have screwed it by reducing elevator deflection angle. Really screwed to make it easy target for everyone else. Its like horse with suspended legs goin for a race. Tha is damn inhuman an biased. And it makes me so pissed.

[SinusoidDelta]

Just to add one important issue that sneaked past this topic: su27 has unstable design meaning that its cg is close to aerodinamic friction center, meaning that flanker is damn agile with less hydraulic power needed to make pitch deflection. Thats why it can do a cobra in first place. Roll angle depends on inertia force ruled by horizontal mass distribution, meaning single engined rolls faster. Developers have screwed it by reducing elevator deflection angle. Really screwed to make it easy target for everyone else. Its like horse with suspended legs goin for a race. Tha is damn inhuman an biased. And it makes me so pissed.

 

The CG and aerodynamic center being located so far aft makes the Su-27 unstable in the pitch axis. That large pitching moment being generated must be countered via control surfaces to simply keep the nose from climbing out of confrol (aka statically stable). I don’t think using deflection rates on the ground make an accurate reference point as there is no airflow, thus no airspeed input nor aerodynamic force applied.

 

Is the flankers pitch or roll rate lacking in DCS? Is ITR lower than expected? That’s the resultant of this deflection rate concern, correct?

[LJQCN101]

Su33 in game moves its stabilizer much quicker than Su27: https://forums.eagle.ru/showthread.php?t=194015

 

Both Su27 and Su33 in game behaves like a static stable aircraft below 22-24deg AOA (with S Pressed). Static stability became neutral when approaching 24deg AOA, and became negative when above. The Cm curve just feels like this:

 

Edited October 9, 2017 by LJQCN101

[jackmckay]

Switching to your post.

[Fox One]

Both Su27 and Su33 in game behaves like a static stable aircraft below 22-24deg AOA (with S Pressed).

 

If that is true then I don't think the simulator is correct.

 

The standard MiG-29 has negative static stability in 14-25deg AOA range (source, practical aerodynamics manual). That's why it has a system called APUS that commands an electro-hydro-mechanical device (called ARM-150) that is included in the pitch control system. This system in the 14 to 25.6deg AOA range will deflect the stabilizer to oppose any uncommanded change in AOA, making the aircraft appear to the pilot as a normal aircraft with positive static stability.

 

The real Su-27 in the small AOA range it has from the beginning even less positive static stability than MiG-29, it is close to neutral, this is common knowledge. Should I believe the Su-27 becomes statically unstable in pitch at an AOA greater than MiG-29? Not bloody likely :D

 

It is worth noting from the real Su-27 flight manual that in direct control mode (corresponding with pressing S in simulator) it is restricted to an AOA of no more than 10deg. Knowing this I would expect that just a few deg above the 10 deg AOA the aircraft will become neutral then unstable and pitch control will become very difficult or close to impossible.

 

I suspect the Su-27 and possibly 33 too was deliberately made in simulator to have a higher positive static stability margin than the real aircraft. Of course this is just speculation. They have probably done that "for gameplay" purposes - if with S pressed at 12-13deg AOA pitch control would routinely be lost in simulator many people would not like it. Probably the developers are fearing too much the criticism of people with limited knowledge that are probably the majority of FC3 customers.

[jackmckay]

Then why is it named Digital Combat "Simulator" if it doesn't "simulate" things right? Maybe it should be renamed to:

"Digital Combat with Artificially Balanced Flight Physics"? :D

[Weta43]

Suspicion != fact

[mytai01]

You should understand that this is computer controlled. The rate of and deflection angle of stabilator is determined based on airspeed. The system is designed to allow maximum SAFE AOA and G. So, on the ground/zero airspeed there is no danger and maximum rate/angles are allowed...not in the air...

[rami80]

Suspicion != fact

 

We have people here giving us informed opinions... It would be nice if someone from the ED team responded at the same level, not like this.

 

All I'm saying is elaborate; we really want to know more.

[Weta43]

I suspect the Su-27 and possibly 33 too was deliberately made in simulator to have a higher positive static stability margin than the real aircraft. Of course this is just speculation

 

^^ this: Suspicion != fact

 

E.D. Have repeatedly said they just make the FM to reflect real world data as well as they can, and it's up to the players to use practice, curves or helpers to deal with that.

 

Fox_One has made complete guesses (perhaps they feel they're 'educated' guesses, but they're still guesses) about what they believe the static stability AOA threshold is, then made comments based on the guess.

 

The fact that someone suspects something, doesn't make that thing fact...

[Fox One]

You should understand that this is computer controlled. The rate of and deflection angle of stabilator is determined based on airspeed. The system is designed to allow maximum SAFE AOA and G. So, on the ground/zero airspeed there is no danger and maximum rate/angles are allowed...not in the air...

 

Perhaps you should do the effort to read the description of the system from real aircraft flight manual. The "maximum SAFE AOA and G" you are talking about is taken care of by the G/AOA limiter. This limiter works with anticipation, it uses G rate and AOA rate as inputs: the faster the G or the AOA is increasing, the lower the value of G or AOA at which the limiter will engage and reject the stick forward. The "anticipation" function is obviously tuned such that it will keep the current values below the admissible level.

 

Here are some videos with aircraft IN THE AIR where a very fast deflection rate of the stabilizer is visible.

 

Trying to compare the deflection rate of the stabilizer visible in videos above with the simulator is truly ridiculous. It is visible from the Moon and with the naked eye the much faster deflection rate available in the real aircraft. Here I am, having to try to demonstrate the obvious...

 

Contrary to what you think, here is why a fast deflection rate is necessary IN THE AIR. This aircraft is a long and very heavy object. That's why it has a huge angular inertia. In the videos above the pilot is making a moderate, but fast stick movement. The system understands that a response is needed ASAP, so it makes a very fast and quite large deflection of the stabilizer, the necessary pitch rate is developed quickly, then the system reacts to the developed pitch rate by returning fast the stabilizer to a position close to the initial one.

[Weta43]

Just to be sure, though I'm sure you do both already realise, the elevators you're watching in the game aren't really making the plane pitch. As such the question of how fast they appear to move is essentially cosmetic.

If you're concerned that the aircraft's performance has been impacted by a patch (which is the initial post's premise), rather than looking at the rate at which the stabalisers appear to move, perhaps look at the actual performance of the aircraft.

 

(also - Fox_one - that font you've taken to using is styley, but how about bumping up the point number so those of us on the threshold of glasses don't have to zoom in on your posts :-) )

[Fox One]

Just to be sure, though I'm sure you do both already realise, the elevators you're watching in the game aren't really making the plane pitch. As such the question of how fast they appear to move is essentially cosmetic.

If you're concerned that the aircraft's performance has been impacted by a patch (which is the initial post's premise), rather than looking at the rate at which the stabalisers appear to move, perhaps look at the actual performance of the aircraft.

 

In Su-27 cockpit, with aircraft stationary on the runway I press rctrl+enter. Then make a fast, full pull of the stick. The stick in the cockpit is moving slowly. The controls indicator is moving slowly. I look over the shoulder, the stabilizer moves slowly. So what you are saying is that actually there is no relation between them?

 

I am looking at the performance of the aircraft, that's how I noticed this the first time, then I looked at the aircraft from outside and noticed the slow moving stabilizer. In the low to medium speed range, when making a moderate to large, but quick stick movement in pitch, the aircraft is simply taking too long to respond. And of course it does that - what the simulator is seeing is that I am pulling the stick slowly.

 

(also - Fox_one - that font you've taken to using is styley, but how about bumping up the point number so those of us on the threshold of glasses don't have to zoom in on your posts :-) )

 

I wasn't aware of that, sorry. On my monitor in 3 different browsers with 100% zoom set, the text in my previous post appears normal in size (it looks identical with other people's posts). Maybe there is something I am not understanding correctly. I am writing this post with forum's default settings, how does this looks like?

[Ironhand]

...

 

 

 

I wasn't aware of that, sorry. On my monitor in 3 different browsers with 100% zoom set, the text in my previous post appears normal in size (it looks identical with other people's posts). Maybe there is something I am not understanding correctly. I am writing this post with forum's default settings, how does this looks like?

The font you've been using looks identical to every other post in this thread on my end.

[Weta43]

(...) I am looking at the performance of the aircraft, that's how I noticed this the first time(...)QUOTE]

 

:-)

OK - like I said, just making sure - the discussion seemed to be very focused on external measurements of elevator deflection.

 

re. the font.

Does it not look like this for you :

[jackmckay]

Flanker control surfaces move too slowly compared to 1)hydraulic power available regarding percentage of engine power fraction assigned to hydraulic pumps 2)has relatively more unstable design compared to other designs as featured in cobra maneuvers 3)inboard limiters/safety systems are designed to protect PILOT from hurting ITSELF 1'st then airframe that has reserve safety margins above pilots physical limitations 4)its a damn fighter and has to be agile as possible 5)holds significant number of world records and is proven to be one of the best (if not the best) WWR combat airplane in the WORLD! This is pure and unbiased truth. ED should look at these facts and reconsider its policy about flight performance of Flanker. NHF

[Esac_mirmidon]

Please could you elaborate more?

 

1: How much hydraulic power is available in the Su-27? What meassurement units?

 

2: How much is the fraction of the engine power assigned to hydro pumps? Units?

 

3: How much unstable in % is the Su-27? Positive, negative? What axis?

 

Thanks

[jackmckay]

The hydraulic pumps are engine driven pumps (via a gearbox). Fraction is noted in percentage of engine power available and is usually less than 5% but depends on design requirements. Average pump looks like this:

 

 

And if you want to know nominal pressures - check the in-cockpit gauges man or ask these guys: http://www.salut.ru/index.php (1st and 2nd question above have the same scope).

 

Look man, lets be real, this plane must be very agile because it

 

1) has best max instantaneous turn rate (SL)º/s - 28,0(8,0g)SL in its class.

2) can do Cobra and

3) has 2x12.3T(vs Eagles 10.5T) static thrust engines

.. and that means LOTS of power available even more(!) than any other plane in DCS except Su33. Put an average high efficiency hydraulic pump and you can make a Vegas Hotel water fountain type with hydraulic oil from power available from those two engines. Anyway Flanker is younger plane (than Eagle for example) so that means younger and more efficient pumps. You want me to calculate pressure drops in hydraulic piping? What are we developing here?

 

Next, Su27 is up to 5% MAC unstable (limit α<24°).

 

But let me explain Cobra for you so you get the point what are we dealing with:

 

The key to the Cobra maneuver are four characteristics of the aircraft:�

- High pitch agility: A huge so called tailplane 'volume' (relative area and arm), with the help of LEX vortex lift mechanism and longitudinal instability that allows dynamic pitch angles and α in excess of 100°. F-14/15/18/M2000 planes cannot reach these angles;

�- Lateral stability: every plane is to a lesser or greater extent dynamically unstable directionally at α of about 20-40º, where the airflow begins to separate. Su-27 rolls-off here at 40° α. The essence of the Cobra figure is to quickly pass unstable α region (while the large inertia of the aircraft prevents the potential roll/yaw-off) and come into benign region of completely separated air​flow;

�- Longitudinal stability at 100°+ α: because of the high pitch rate, the aircraft passes the point of max trimmed α (that is about 50-60º α), but the tailplane size and deflection pitch aircraft back to the initial α.

The aircraft was designed with over-dimensioned tailplane (as a backup) because the static longitudinal instability was still unproved.

- Stable engine operation at 100° α / less than 200 km/h IAS (earlier generation turbofans has about 20º α limit);

 

Anyway, �Su-27 set new standards in fighter design and paved the way for super and hyper (up to �120º α) maneuverability (dynamic entrance into supercritical α - flight mode that permits a decrease of airspace needed for turn by 2 times .. and to achieve this (aside from other aerodynamic features and advancements) plane needs LOTS of hydraulic power available and it DOES.

 

What is your standpoint about this issue here anyway? :music_whistling: Do you think it should be less agile and why?

[SinusoidDelta]

The Flanker hydraulics operate at 3,000 psi, same as the eagle. I don’t see the relevance of that though. I honestly lost the point of this thread somewhere. What is your stance? What is the performance issue you’re trying to illustrate?

[Esac_mirmidon]

Im trying to understand the same.

[jackmckay]

300kg/cm3 is not 3000psi its 4267psi!

 

Learn to convert units first then place trolly argument please.

 

For a plane that has max instantaneous turn rate (28º/s) tail control surfaces moves too slow. Period. Its cripplled or can we say "balanced". Period. ED should fix that.

[Esac_mirmidon]

So you are arguing that ED is limiting the Su-27 agility for balance purposes?

[SinusoidDelta]

300kg/cm3 is not 3000psi its 4267psi!

 

Learn to convert units first then place trolly argument please.

 

For a plane that has max instantaneous turn rate (28º/s) tail control surfaces moves too slow. Period. Its cripplled or can we say "balanced". Period. ED should fix that.

 

I'm not trying to make trolly arguments. Not everyone is your enemy. I'm actually trying to understand the problem here ok? Making blatantly insulting comments like "learn how to xyz before" are very immature and contribute nothing towards any possible solution.

 

The axial piston pump you are showing is a fixed displacement pump. Commonly a variable displacement type is used, which is nearly identical except flow and pressure is regulated by changing the swash plate angle. I'm not sure which the Flanker uses but I'd lean toward variable pump.

 

The 210kg/cm^2 = 3000 psi figure I referenced from the Su-27 Warbird Series book by Yefim Gordon. (Note that hydraulic pressure is measure in force/area i.e., unit^2 not unit^3, maybe a just a typo on your part.). Regardless, I checked the the Su-27SK FM it seems Gordon's figure is low but the pumps do not operate at a static 300 kg/cm^2 :

 

So, working pressure range of 260-300 with flight controls operational as low as 100kg/cm^2 (In imperial units, this is 3698 - 3982 psi / 1422psi lower limit).

 

Now I don't mean this as in insult, I respect your dedication here, but all of the above hydraulic data is superfluous.

 

You say the max ITR is 28deg/s. I assume that means your concern then is that 28deg/s ITR is not achievable in DCS. Yet I haven't seen a single estimate as to what ITR we are seeing in DCS from anyone in this thread (please note that I'm not singling you out here).

 

Has anyone attempted to measure ITR in DCS? Wouldn't you agree that's the most logical starting point?

Edited October 20, 2017 by SinusoidDelta

[Esac_mirmidon]

I will post tacview ITR as soon as possible.

[SinusoidDelta]

I will post tacview ITR as soon as possible.

 

That would be much appreciated. Post the tacview and I’ll make some plots. And if you could, test with both full and 50% internal fuel.