Su-27 Flight Model Discussion

...wait, I think I get it now... at slow speed the computer can allow for more direct control surface deflection, while at higher speeds the computer reduces them (same stick deflection but at higher speed the actual control surface deflection is less than stick deflection) So as you turn off the computer, suddenly control surface deflection increases, causing the already set Trim to be too high, hence the nose suddenly goes down.

I want to revisit this, since I didn't have much time to post my reply yesterday. It happens just as you describe and that is how it should be. However...

I think there is still a problem--two problems, really. 1) There is a logic law added to the real world system that causes the aircraft to imitate how a conventionally speed stable aircraft will behave with speed changes. That control law changes the speed stability of the aircraft from neutral to positive. As speed increases, the nose pitches up. As it decreases, the nose pitches down. In the sim, you end up with your nose trimmed down to the max allowed by the mechanical system itself at high speeds. Ram air has no mitigating effect on the logic law. So you have a law which causes you to add the maximum amount of trim, then other laws which take that requested mechanical trim and reduce it a mere fraction of what you're asking for at high speeds. This just seems wrong. It may be as it really is but it just seems wrong to me.

2) If how it's modeled is actually correct, then there is still an issue. With the FBW engaged, take your sped up to about 1150 k/h and trim for level flight (pull up the flight control indicator so you can view your trim). Hands off stick. Your trim is full nose DOWN. You are in level flight without touching the stick. Go to external view (F2) and look where the elevators are. They are at 0* or, perhaps the smallest fraction of 1*.

Now, starting from a slow airspeed, do the same after pressing the "S" key to disconnect the FBW system. With your airspeed at 1150 k/h and trimmed for level flight, you actually have a few degrees of nose UP trim dialed in. And that is what shows in the external view as well.

Both elevator positions can't be correct. One, the other, or both are wrong.

Rich

Very funny this Flanker FM (real or not real)

I just tried this also, granted I did it in latest Tester's version, not 1.2.14, and I get weird result also

With FBW On at around 900Km/h IAS I need to be fully trimmed down to fly level hands free. Watching in externals and I see elevator pretty much at neutral position... whaaaat?

With FBW Off at around 870Km/h IAS (I spent some fuel in process so little less speed was probably due to less weight) I managed to fly straight and level, WITHOUT any trim needed!

This doesn't seem right indeed.

Please do not replace facts with your consideration. If you have your own (of course, the most real!) diagram for Su-27 FBW, we would have fun looking at it.

 

P.S. Be careful with all your $$ savings... somebody can accept the bet. :D

So you say that the "lost trim behavior" after hitting the "S" key it's like in the real Su27?

Have you a document that you can show, that will show that this behavior is the real thing?

It's hard to believe that such a "kill switch" is build in in a plane!

 

Both elevator positions can't be correct. One, the other, or both are wrong.

 

So there's a problem somewhere.

I think I 'get' what Yo-Yo is hinting at. The FBW simulates positive stability in the aircraft. So at high speed the trim tabs would be where you would expect them to be in an airframe with positive stability.

When you turn off the FBW system, the aircraft is definitely no longer inherently stable, and the trim tabs would reflect that as the aircraft would no longer want to pitch up at high speeds.

Its definitely a strange system when compared to what we have in the western aircraft, but if thats how it works in RL then Im more than willing to accept that. I quite like how the Su27 handles, even with all the trim work it requires.

Yes, and it's the way artificial stability works.

I understand and can accept that part. The issue remains that, for the same altitude, weight and airspeed, you have the elevator itself in 2 different positions depending on whether you are using the FBW or have direct control. This cannot be correct. Regardless of how you get there, if the weight, altitude and airspeed are the same, the elevator positions would have to be the same as well if level flight is to be maintained. IT can't be 0* in one situation and 3* nose up in the other.

@Yo-Yo With all due respect sir, did you try to dogfight against another Su-27 pilot which disables (all the time) the FBW system when turning?

I understand and can accept that part. The issue remains that, for the same altitude, weight and airspeed, you have the elevator itself in 2 different positions depending on whether you are using the FBW or have direct control. This cannot be correct. Regardless of how you get there, if the weight, altitude and airspeed are the same, the elevator positions would have to be the same as well if level flight is to be maintained. IT can't be 0* in one situation and 3* nose up in the other.

How does trim work in the Su27. Does the plane have special trim 'tabs' or does trimming the plane move the elevator directly?

How does trim work in the Su27. Does the plane have special trim 'tabs' or does trimming the plane move the elevator directly?

Trim is accomplished by moving the entire elevator.

I agree, but FM itself does not know anything else except stab angle... though I am going to check myself your observation.

Thank you. Much appreciated.

Reaching "high speed" what Mach number did you get?

I wasn't looking at Mach, unfortunately, though you can check in the tracks attached to my initial posts--I'm at work and can't check myself. Sorry The IAS on the HUD was right around 1150km/hr, IIRC. I did this at high speed because direct control flight is more stable in that region making it easier to see where control surfaces actually are.

I think I 'get' what Yo-Yo is hinting at. The FBW simulates positive stability in the aircraft. So at high speed the trim tabs would be where you would expect them to be in an airframe with positive stability.

 

When you turn off the FBW system, the aircraft is definitely no longer inherently stable, and the trim tabs would reflect that as the aircraft would no longer want to pitch up at high speeds.

 

Its definitely a strange system when compared to what we have in the western aircraft, but if thats how it works in RL then Im more than willing to accept that. I quite like how the Su27 handles, even with all the trim work it requires.

Except that, for a given airspeed, weight and altitude, the actual physical position of whatever surfaces controlled by the trim setting which cause an aircraft to fly a level path--in our case, the elevator itself, would have to be identical. How you get there can be different but what's actually there when you arrive has to be identical. And right now they are different.

Trim is accomplished by moving the entire elevator.

Zero force point of the stick moves too, or just control surfaces?

Can you guys "make" that robot to counteract by itself the trimming push?

Can you guys "make" that robot to counteract by itself the trimming push?

If I'm interpreting the DCS manual correctly, this "trimming law" is added on top of the existing FBW system. In a sense, it's value added. Without this added law, the aircraft is neutrally stable--it goes where you point the nose, even as the airspeed rises or falls, etc. So you don't need anything to counteract the added trimming law. You'd just need a way of turning it off. But it seems that you can't. Unless there's something Yo-Yo isn't telling us. :)

I tried to balance the aircraft at 400, 600, and in supersonic 1300 kph IAS below 2000m. The third case is against all existing rules :) but the plane is stable there so it's not so dangerous if you are fast and coordinated enough.

 

Ufortunately... the stab position was exactly the same with and w/o FBW as it should.

That's too bad...or good. :)

Have you taken a look at these tracks (especially the first and last) in my post here: FBW Trim "Bug"? Wonder why our results differed. Unless I overlooked something simple. It was, after all, early morning while on my first cup of coffee that I was testing this.

I'd like to ask when you are pulling high G, the stick starts jumping back and forth, as if the FBW is grabbing and then letting go of the joystick, is this normal behavior for the su-27?.

track in 1.2.14.36041

FBWJoystickBounce.trk

I do not think that we are using different versions... by the way, I am confident that this issue is impossible because only one parameter can affect FM itself - the stab pitch. So, if you fly at the same configuration at the same IAS and altitude, there must not be any difference.

Anyway, I will try your tracks. :)

Ahh... that's right. You had mentioned that earlier. So I bungled it somehow.

I'd like to ask when you are pulling high G, the stick starts jumping back and forth, as if the FBW is grabbing and then letting go of the joystick, is this normal behavior for the su-27?.

 

 

track in 1.2.14.36041

Haven't noticed this myself but haven't had much stick time with the latest version either. Not even sure that I've pulled any high Gs, since I installed it. Will view the track, when I can, unless someone else beats me to it.

I have seen #1 and #3 tracks - do not see any different stab angles neither at the external view nor directly referring to the angle.

????? Very strange because there was a distinct difference which is why I saved the tracks in the first place. I did not, however, review the tracks before posting them. I'll check them at my earliest opportunity which will probably be in the morning.