No Su-27 auto trim, can it really be true?

[Hummingbird]

Can it really be true that the Su-27 doesn't feature any auto trim system like that on basically every western fighter since the late 70's?

 

I mean it's one of the more simple things to implement in a FCS and it definitely reduces pilot load a lot.

 

I know that manual trimming is mentioned in a manual, but does that mean that auto trimming does not exist in the Su-27 FCS? And if not has it been implemented in the new models of the Su-27?

[Steel Jaw]

More troublesome for me is the sub standard RWR, which is a shame since I really like the Flanker, going all the way back to Flanker 1.0 circa 1996.

[Hummingbird]

I understand your worries, but can we please stay on topic :D

[mvsgas]

Can it really be true that the Su-27 doesn't feature any auto trim system like that on basically every western fighter since the late 70's?

 

I mean it's one of the more simple things to implement in a FCS and it definitely reduces pilot load a lot.

 

I know that manual trimming is mentioned in a manual, but does that mean that auto trimming does not exist in the Su-27 FCS? And if not has it been implemented in the new models of the Su-27?

 

I know for a fact that the F-16 did not have/had auto trim. The F-117 did not have auto trim neither. So what western Fighters have auto trim? F-15, 18?

Edited March 19, 2015 by mvsgas

[Hummingbird]

I know for a fact that the F-16 did not have/had auto trim. The F-117 did not have auto trim neither. So what western Fighters have auto trim? F-15, 18?

 

F-14, -15, -16 & -18 all feature auto trim in the pitch axis. Can't speak of the F-117 but I'd very much assume it had it too considering the computer power needed just to keep that thing in the air.

 

Regarding F-16 auto trim:

Yes that's a common misconception. One reason is Viper pilots used to misunderstand the question coming from flyers of previous Falcon versions where you never had to trim at all. Of course the pilots' answer was that no, you do have to trim. Another reason is that depending on AOA, there might be a slight pitch up rate. You can trim that out but its usually very small so you can also not trim it out. IRL some aircraft might also require some (yaw?) trim due to airframe fatigue etc (or so I hear). Plus, trimming for asymmetry is not required only when dropping bombs, you also need to trim for example when you have a pod loaded (HTS/TGP). And when you have an asymmetry you do have to trim for speed but only in the roll axis. Some pilots might also prefer to trim out the backpressure needed during final approach.

 

The point is, trimming is necessary both in BMS and IRL, but you do not need to trim in pitch for speed. The FLCS manages that aspect of trimming on its own, and only that aspect. You do have to trim in roll for speed when there is an asymmetry for example. The thing to keep in mind is that numbers don't lie. As I said before, in BMS you do not fly a simulation of the real FLCS, you fly the real FLCS. Whatever behavior you see in BMS happens in the real thing. Our pilots attest to that as well.

Edited March 19, 2015 by Hummingbird

[mvsgas]

Trim for the F-16.

The aircraft can be trimmed about all three axes. With pitch

trim centered in cruise gains and no input to the stick, the

aircraft attempts to maintain 1g flight regardless of flight

condition unless AOA exceeds 15 degrees. Full noseup/full

nosedown trim corresponds to +3.4g or -1.4g in cruise gains

NOTE

Airspeed must be closely monitored because

there is little aerodynamic indication of large

changes in airspeed. Cues which normally in

dicate airspeed changes, such as stick move

ment or trim changes, are absent.

Above 15 degrees, the FLCS commands an increasing nose

down pitch attitude as a warning of decreasing airspeed. A

specific force applied to the stick commands a specific g

increment from the trim condition. Moving the PITCH TRIM

wheel changes the hands-off trim condition.

In takeoff and landing gains, zero pitch trim commands zero

pitch rate until 10 degrees AOA. A slight amount of noseup

trim is required to zero stick forces during an 11-13 degree

AOA approach.

When properly trimmed and no command is applied to the

stick, the aircraft attempts to maintain zero roll rate. Moving

the ROLL TRIM wheel changes the hands-off trim condition.

Maximum roll trim authority is approximately one-fifth of

maximum stick command of cruise gains. However, precise

trimming is difficult using the stick TRIM button. Roll trim

requirements may change with stores, particularly at supersonic speeds. For asymmetric configurations (asymmetrical

stores or rudder mistrim), roll retrimming may be required as

flight conditions change. Roll trim inputs also command rudder deflection through the ARI.

The ARI switches out with wheel spinup upon landing. Likewise, the ARI switches in following takeoff as the wheels spin

down. This switching may cause abrupt rudder inputs to occur

if roll (due to asymmetries or crosswind) is being input via

the stick or trim.

Rudder trim inputs command rudder deflection. Rudder trim

is required with asymmetrical configurations and frequently

during supersonic flight, especially with stores. Maximum

trim authority is 12 degrees.

 

As you can see, you do have to make some pitch trim input among others, depending on the conditions, AOA, g's etc. Taken form F-16-1 manual page 6-2. F-117 had the same FLCS as the F-16A

Edited March 19, 2015 by mvsgas

[Hummingbird]

But as you can read it does have auto trim (in pitch):

 

"Airspeed must be closely monitored because

there is little aerodynamic indication of large

changes in airspeed. Cues which normally in

dicate airspeed changes, such as stick move

ment or trim changes, are absent.

 

Above 15 degrees, the FLCS commands an increasing nose

down pitch attitude as a warning of decreasing airspeed."

 

However trim is obviously still needed sometimes due to various factors, but not because of changes in airspeed (to a certain limit of course), the FLCS takes care of that aspect. It's the exact same deal in the F-15, as you can experience ingame.

Edited March 19, 2015 by Hummingbird

[mvsgas]

Ok, so we agree that trimming is required under certain flight conditions on western aircraft, right?

Now going back on the original post about the SU-27, did ED finalize the flight model or is it still working progress?

[OnlyforDCS]

The FM is WIP and we all know that. However that has nothing to do with Hummingbird's point.

 

The Su27 does not have the pitch correction found on most western military aircraft. (At least the early Su27 version that we have in the game doesn't). The latest versions probably feature more advanced FBW systems.

[mvsgas]

But as you can read it does have auto trim (in pitch):

 

"Airspeed must be closely monitored because

there is little aerodynamic indication of large

changes in airspeed. Cues which normally in

dicate airspeed changes, such as stick move

ment or trim changes, are absent.

 

Above 15 degrees, the FLCS commands an increasing nose

down pitch attitude as a warning of decreasing airspeed."

 

However trim is obviously still needed sometimes due to various factors, but not because of changes in airspeed (to a certain limit of course), the FLCS takes care of that aspect. It's the exact same deal in the F-15, as you can experience ingame.

So, I tried it in game:

F-15

No weapons, 100% fuel, not chaff/flare, 0% gun.

Took off,, climb to 10K feet (MSL) speed was 350 knots, RPM at about 85%. Seem trim at level flight. Raised throttle to 90% RPM, nose raised and started to climb.

 

Su-27

No weapons, Gun 0%, No chaff/flare. Fuel 80%

Took off, climb to 2500 meters, aircraft was less stable in speed, moving from 760 to 780. at 780 nose would be about 6 degrees up as indicated by HUD at 760, nose would be at about -2 degrees down. maintained this cycle, to me trimmed. Throttle was at 80%. Raise throttle to 90 accelerated and nose climb, got as high as 50 degress but once aircraft decelerated, it went down to about -6 and it started the same cycle.

 

To me it seems speed varies more in the Su-27, but both react to speed by raising the nose. So how are their trim different in game? Su-27 seem less stable, but is the flight model finish? Is it suppose to be less stable?

[mvsgas]

The FM is WIP and we all know that. However that has nothing to do with Hummingbird's point.

 

The Su27 does not have the pitch correction found on most western military aircraft. (At least the early Su27 version that we have in the game doesn't). The latest versions probably feature more advanced FBW systems.

 

Ok, flight model is not finish, never mind. moving on.

[EngineerFalcon]

More troublesome for me is the sub standard RWR, which is a shame since I really like the Flanker, going all the way back to Flanker 1.0 circa 1996.

 

Call me weird, but I find the Flanker's rwr to be much easier to use. The American TADS has that nasty feature of making icons move dynamically with your craft - but as soon as it updates icons rubber-band back to their actual position, and things get really confusing.

[Hummingbird]

Ok, so we agree that trimming is required under certain flight conditions on western aircraft, right?

 

Yes ofcourse, every aircraft does, but that's not the point. The point is that pretty much every western fighter aircraft since the early 80's has featured auto trim in pitch, which means that the FCS automatically adjusts pitch in relation to speed, and this to avoid excessive nose up attitude with increases in speed.

 

The F-15 features auto trim ingame as well, whilst the Su-27 does not. Question is wether this can really be the case in reality, and if so then wether or not this has since changed with subsequent models of the Su-27?

[GGTharos]

It is the case in reality. It's design philosophy.

 

As for later models, who knows?

 

Question is wether this can really be the case in reality, and if so then wether or not this has since changed with subsequent models of the Su-27?

[Hummingbird]

It is the case in reality. It's design philosophy.

 

Alright, although I'm wondering what philosophy that might be.

 

Seems odd that the Russian wouldn't want to reduce the workload of their pilots.

 

As for later models, who knows?

 

Well I'd assume such a thing wouldn't really be a secret and that one could ask actual Su-27 pilots :) I mean we somehow got a hold of the manual :P

 

Also there are two captured Sukhoi's in the US AFAIK.

[esb77]

There is auto pitch trim in the Su-27 flight model.

 

However, it is not designed to create or maintain level flight without pilot input.

 

It is designed to make the Su-27 behave like the vast majority of aircraft that pitch up with speed increases and pitch down with speed decreases. In a high performance aircraft with a wide speed range that behavior can produce a lot of pitch under the right (or wrong depending on how you think of it) conditions.

 

The early models of the Mig-29 and Su-27 were known for imposing a higher pilot workload than the contemporary NATO counterparts. You have to pay attention to the fundamentals of good piloting from when you start rolling to when you stop rolling. They're also very responsive planes, sometimes more than Western counterparts, so it's easy to get in very deep trouble very quickly if you have a brief lapse in attention.

 

If you find that stressful, well, that's what vodka is for when you come off duty.

 

Parts of it may be that the FM is still WIP, but these planes are more demanding than some.

 

Think of it as an opportunity to improve your airmanship. After all, if the plane could fly itself what use would the pilot be?

 

I don't know what the thinking was at Sukhoi when they were designing this, but one feature was commonality in the pilot interface across different planes. Mig-29s, Su-27s, Su-25s, and probably others have very similar cockpit designs as a deliberate feature to facilitate rotating pilots into different aircraft. It's possible that they decided to carry this philosophy over into handling characteristics as well.

Edited March 20, 2015 by esb77

[21stCenturyPilot]

Russian computer technology has been behind the west so while the F-16 had digital FBW back in the 70s the Russians weren't up to the same level, so the Flanker and Fulcrum original FLCS is not as sophisticated. I read somewhere the Flanker and Fulcrum are not easy to fly and impose high cockpit workload. I've seen the Su-27 Flanker B perform the cobra at an airshow and do it's routine and it seemed very smooth and stable, but I don't like the so-called professional flight model much for the Su-27, if it really flew that bad I think they would've crashed half of them by now. It's not a design philosophy, it's a technological limit of Russian computer technology of the time, now with the Su-35 it has full digital FBW. Highly agile unstable jets like the F-16 and Eurofighter Typhoon need digital FBW to fly, the Su-35 has been made more unstable and agile than the Su-27 which the digital FBW system allows. It's much better. The design philosophy is to make the aircraft as easy to fly as possible so the pilot can concentrate on the enemy.

[mvsgas]

Yes ofcourse, every aircraft does, but that's not the point. The point is that pretty much every western fighter aircraft since the early 80's has featured auto trim in pitch, which means that the FCS automatically adjusts pitch in relation to speed, and this to avoid excessive nose up attitude with increases in speed.

 

The F-15 features auto trim ingame as well, whilst the Su-27 does not. Question is wether this can really be the case in reality, and if so then wether or not this has since changed with subsequent models of the Su-27?

 

The RL aircraft (F-16 and F-117 in this case, the only two I have worked on) Try to maintained 1g ( as I posted before) here someone explains it better. I lack eloquence and the ability to fully convey coherent thoughts :).

http://www.f-16.net/forum/viewtopic.php?t=11691

The FLCS attempts to maintain the jet at a commanded gee or commanded roll rate. Airspeed is not a factor - only the rates. With gear down, the FLCS adds in some pitch rate and AoA bias to help, but the basic op-amps for pitch were still designed for a gee command.

 

This has also been cover extensively in other forums as well.

Edited March 20, 2015 by mvsgas

[GGTharos]

Yes, it is a design philosophy and not a technological limitation. The F-15C trims to 1g on the same level of technology.

 

It's not a design philosophy, it's a technological limit of Russian computer technology of the time

[Ironhand]

Yes, it is a design philosophy and not a technological limitation.

To amplify GG's statement, if you look at the DCS manual for our Su-27, you'll see that the basic underlying FBW system is the usual trajectory control system you expect to see--a sort of "point and that's where it'll go" system. But, if you keep reading the subsequent paragraphs, you'll see that the Flanker version which DCS models has an airspeed trimming law that's added.

 

I explained what it does in another thread: "This law causes the leading edge of the horizontal stabilizer to deflect down as airspeed increases and up as it decreases. In other words, it'll raise the nose as speed increases and drop the nose as it decreases..." Someone, somewhere along the line decided that adding this law was a good idea because it would make the Flanker safer and/or easier to learn to fly.

 

Many pilots learning to fly this aircraft would already have experience flying conventional aircraft. And that's what the airspeed trimming law mimics--that characteristic of conventional aircraft. Hold the stick position, increase the throttle and the nose rises. Decrease the throttle and the nose drops.

 

So, in that sense, I suppose it might make it easier for someone entering the cockpit to understand what the aircraft would do next. The other thing this law does, is drop the nose as airspeed decreases making it harder to unwittingly stall the aircraft. So I suppose that would be a benefit, too. What it does do with certainly, however, is force whoever is in the cockpit to pay attention to the aircraft. Every change you make gets a response. :)

[mvsgas]

Do we have a link to download the DCS SU-27 manual? I have it, but can't remember where I got it.

[Ironhand]

Do we have a link to download the DCS SU-27 manual? I have it, but can't remember where I got it.

Did a quick search and came up with this: DCS Su-27 Flight Manual. Don't remember where mine came from either. :)

[21stCenturyPilot]

It's interesting.

I've looked at the manual and it does seem to me to have some conflicting info.

 

This is what it says in my DCS Su-27 manual.

 

The fly-by-wire system is a trajectory control system. In other words, if the control stick remains in the same position, the system will hold the set flight path. This is why the reduction of airspeed (for example) and resulting decreased lift and increasing angle of attack, will lead to the aircraft attempting to hold the initial flight path and prevent the aircraft from descending. This will result in the airspeed stability degradation up to neutral stability.

The longitudinal control system includes an airspeed trimming law that generates a signal proportional to the ram air. When the indicated airspeed is increased, the FCS causes the stabilizers deflection (up to 5 degrees) nose down. When decelerating, it causes the nose to come up. This imitates the airspeed stability of the aircraft, which is neutral in the presence of a g loading feedback signal. The airspeed stability imitation allows the pilot to use the control stick like he or she would with a stable aircraft.

 

The Flanker was designed and built at the end part of the cold war and maybe the designers didn't think of making it fly at 1.0 G like US jets. Or maybe the Russian technology of the time was not capable of doing this. I also think there is probably some disinfo floating around about flight characteristics. I got the DCS World, FC3, SU-27, F-15C, Su-25T manuals in a folder when I installed DCS FC3.

 

Anyway it's really a great flight sim game.

Edited March 20, 2015 by 21stCenturyPilot

[GGTharos]

The Flanker was designed and built at the end part of the cold war and maybe the designers didn't think of making it fly at 1.0 G like US jets.

 

They did, pilots had a lot of input into this.

 

Or maybe the Russian technology of the time was not capable of doing this.

 

It's capable of completely changing the stability behavior and you think it couldn't trim to 1.0g?

[21stCenturyPilot]

That would be great if it could trim to 1.0 G.

And limit negative AoA to prevent the forward stick of death move. lol.