Su-27 Flight Model Discussion

[blackbelter]

The recent update eliminated the right turning tendency on the ground. Good job. But Su-27 now has a tendency to return to level flight when I try to turn. Is it supposed to do so?

[wilsonov]

I find the new su27 throttle response (as it corresponds to ground taxi speed) is completely different than other aircraft......at complete idle it rolls to a stop, cracked open to first throttle detent and it speeds up to 60-80+ km/hr

[Shepski]

I find the new su27 throttle response (as it corresponds to ground taxi speed) is completely different than other aircraft......at complete idle it rolls to a stop, cracked open to first throttle detent and it speeds up to 60-80+ km/hr

Agreed. Once rolling idle thrust should be enough to keep it going. It also slows very quickly when turning while taxiing.

[Evilducky]

Actually, as long as you are conscious you can usually recover. Deploying the air brake helps a lot. It's when you completely black out and loose consciousness then you can't recover (understandably so :)).

 

I tried everything. I was at about 6000 meters when I flipped it over and nothing would work to recover it. Flaps air breaks... all failed. Eventually my engines shut down and it was game over from there.

[Python]

Just had a play with the SU-27, upon pressing the S key in level flight it immediately nose dives and loses control, I appreciate it probably isn't a wise thing to do but I'm just playing around. Is this behavior normal or am I missing something? I managed to do a couple of maneuvers that somewhat resembled a cobra, but I think this will take lots of practice (and actually reading the manual!) The aircraft completely departed and refused to recover several times after attempting it.

[amazingme]

You actually CAN recover after the inverted departure, as long as the pilot is still conscious and you have enough space between you and the deck :)

You have to use the S key to allow for higher angles of attack and use the trim up/dn as you try to balance it out of the inverted departure.

Try, for example, @ 5000m alt and with low speed to prevent BO fron over Gs.

Success!

[Ironhand]

Just had a play with the SU-27, upon pressing the S key in level flight it immediately nose dives and loses control, I appreciate it probably isn't a wise thing to do but I'm just playing around. Is this behavior normal or am I missing something?

The thing to keep in mind is that, in essence, there are two control systems on this aircraft. There's the mechanical control system and, also, the FBW control system that overlays it. The purpose of the latter is to help you better control the aircraft in all situations.

 

That being said, when you take direct control of the aircraft (press the "S" key) you are turning off the "computerized" control which alters your control imputs to keep you out of trouble. So, when you take direct control, the trim setting you get is the mechanical one you dialed in, not the altered setting you just turned off. When flying at higher speeds, hit the "S" key and your trim goes to just about full nose down which is what you actually have it set at. Not the FBW version of it. And over you go.

[karambiatos]

Anyone run into a thing, where if you are on low fuel and low weapons, the nose will keep pitching up and you cannot trim forward enough to level it out, you reach the forward gimbals limit.

[Ironhand]

Anyone run into a thing, where if you are on low fuel and low weapons, the nose will keep pitching up and you cannot trim forward enough to level it out, you reach the forward gimbals limit.

Not so far. Can you provide a track of it happening?

[Kuky]

Anyone run into a thing, where if you are on low fuel and low weapons, the nose will keep pitching up and you cannot trim forward enough to level it out, you reach the forward gimbals limit.

 

Not so far. Can you provide a track of it happening?

 

I noticed that too, when very light the lift force is greater and even max trim down still leaves some need to push stick forward tad more.

 

Now about that FBW controls... I don't understand why the computer would need to adjust trim? I understand that computer adjusts pilots physical controls to not put aircraft into bad flight parameters, but using trim, and literally flying hands free (physical stick centered) why would the computer need to adjust there? Trimming is not the same as yanking the stick all the way, which without computer input, would make you into bad position... Trim is adjusted slowly and once set (with or without computer input) pilot's hands are off the stick... so it doesn't make any sense for need from computer to further adjust Trim?

 

Basically what I'm saying is that I am not of the opinion that turning FBW off (pressing S key) should not suddenly push the nose down, instead plane should just keep flying level as it was flying leved to begin with. :huh:

 

EDIT: 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.

Edited January 27, 2015 by Kuky

[Ironhand]

I noticed that too, when very light the lift force is greater and even max trim down still leaves some need to push stick forward tad more.

 

Now about that FBW controls... I don't understand why the computer would need to adjust trim? I understand that computer adjusts pilots physical controls to not put aircraft into bad flight parameters, but using trim, and literally flying hands free (physical stick centered) why would the computer need to adjust there? Trimming is not the same as yanking the stick all the way, which without computer input, would make you into bad position... Trim is adjusted slowly and once set (with or without computer input) pilot's hands are off the stick... so it doesn't make any sense for need from computer to further adjust Trim?

 

Basically what I'm saying is that I am not of the opinion that turning FBW off (pressing S key) should not suddenly push the nose down, instead plane should just keep flying level as it was flying leved to begin with. :huh:

 

EDIT: 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.

The problem is a bug, I believe. With the alpha and G-channels turned off, trim behaves as it should. In fact, at high speed (eg: 1150k/h) you need a bit of "up"trim for level hands-off flight. With those channels turned on, you need to input severe nose "down" trim on the mechanical system (which the computer then reinterprets) to do the same.

[Ironhand]

FBW Trim "Bug"

 

EDIT: Disregard this post. Something odd was happening on my end while flying. The tracks do not reflect what I saw while flying.

 

Something has been wrong with the way trim works with the alpha and G-channels engaged. Tracks attached.

 

In track 1, both alpha and G-channels are engaged and an inordinate amount of nose "down" trim has to be entered to maintain level flight. The FBW system is fighting the mechanical system (as depicted by the distance traveled by the stick). At high speed, the FBW system has the elevators in a neutral position but the stick (trim) is almost full nose "down".

 

In track 3, I turn off the FBW taking direct control (S key). Both the alpha and G-channels are disengaged. At the same high speed, I need slight nose "up" trim to maintain level hands free flight.

 

In track 2, I lower my landing gear to disengage only the G-channel. Unfortunately, this also alters the wing shape because the forward slats are lowered. I eventually enter an shallow dive to help drive my speed up. There is still significant nose "down" trim required to maintain hands free flight but the elevators are actually in a nose "up" trim position.

 

Obviously this is not adding up. In at least two of these scenarios (tracks 1 & 3), the elevators should be in the same position. Yet they are not. Track 2, G-channel disengaged, mimics the elevator setting of using only the mechanical system--not exactly as speeds were different--but in the same direction (nose "up" trim).

 

 

Rich

Edited January 29, 2015 by Ironhand

[karambiatos]

I can confirm, while in direct control the 27 is more stable in a straight level flight, requiring minimal trim input to keep it straight.

[Python]

So to confirm, should it nose dive upon pressing S (with no control input) Or is that a bug?

[OnlyforDCS]

Nice find. This can't be working as intended, can it?

[DarkFire]

So to confirm, should it nose dive upon pressing S (with no control input) Or is that a bug?

 

As I understand it this is not a bug but is dependant on airspeed. I believe that the 'natural' tendency of the aircraft is to pitch down upon decoupling of the FCS though the strength of the downward pitch tendency appears to depend on airspeed. Decoupling the FCS at very high speed is... unwise... :cry:

[DarkFire]

Envelope limits

 

So, I decided to see what the Su-27 could do. With standard meteorological conditions, 100% fuel, empty gun pad and no weapons I took it up to 14,000m, headed out to sea for about 200km, did a 180 turn and hit maximum afterburner. I managed to get a hairs' width under 2.4M and in a zoom climb achieved just over 81,000'

 

Great fun! Without weapons this bird is amazing to fly. Which again brings me back to the nagging feeling that AA missiles are generating huge amounts of parasitic drag.

[Nedum]

Why should the "S" key took the computer input off? Are the russian engineers all nuts? The "S" key should only raise the limits for the Pilots inputs. NO Human of these world could handle a modern jet without any computer help.

So I would bet all of my bucks, that this is a design bug off the DCS Su27.

If not, all russian Pilot are completely nuts to do a Cobra with the real Su27.

And why should the trim switch if only the outer limits of the stick inputs are wider (like in the real Su27)?

In the real Su27 you must toggle 2 keys to do a Cobra and I hardly doubt that the real Su27 gets any key to kill the pilots. Pilots are VERY expensive and good Pilots are priceless. So why give them keys to kill themself in a stress situation?

That would be like a gamble with the pilots live!

Never ever in my live would I believe this... never!

[Shepski]

It's a bug... airplanes aren't designed to kill pilots. Pilots kill pilots.

[archer86]

It's a bug... airplanes aren't designed to kill pilots. Pilots kill pilots.

 

I think so!

 

On reality, while your speed is almost 1000kmh, aircraft's elevator contols wont be able to answer your commands easily or suddenly because of mass friction force.

 

For me ,Dcs su-27 elevator controls are too sensitive at higher speeds and this causes buggy pilot deaths.

[Kuky]

I think so!

 

On reality, while your speed is almost 1000kmh, aircraft's elevator contols wont be able to answer your commands easily or suddenly because of mass friction force.

 

For me ,Dcs su-27 elevator controls are too sensitive at higher speeds and this causes buggy pilot deaths.

that isnotcorrect though. In modern fast jets elevators are epecifically designed to be able to move even at high speeds... and that is for them to be hinged and rotating around their center (whole wing moves).

[GGTharos]

Kuky at high CAS you're in trouble. In the F-15 is fair to destroy the ailerons of you use them above 800-850CAS. And if you want to took at all the ailerons WILL deflect. Yes the elevators might take it, but your seriously in the danger zone.

[archer86]

that isnotcorrect though. In modern fast jets elevators are epecifically designed to be able to move even at high speeds... and that is for them to be hinged and rotating around their center (whole wing moves).

 

yes but there should be latency on controls because of mass friction.

[Kuky]

Kuky at high CAS you're in trouble. In the F-15 is fair to destroy the ailerons of you use them above 800-850CAS. And if you want to took at all the ailerons WILL deflect. Yes the elevators might take it, but your seriously in the danger zone.

 

Yes, I know, ailerons are different from elevators (with old aircraft elevators that were only part of the wing, would block at high speed as they were too hard to move... which is why they thought about making whole wing move (and that solves it)... so at any speed the elevator is still movable.

[Kuky]

yes but there should be latency on controls because of mass friction.

 

That friction you talk about pilot doesn't feel as it doesn't transferr to stick. The air friction will only transferr into higher drag, and puts more pressure force on the wing, but doesn't block it from moving, as both part of the elevator are working oposite each other (as whole wing is elevator and rotates pretty close to its center where its hinged to the fuselage)