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Direct control mode pitch down tendency - possible cause


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So I finally got more time to fly Su-27 with PFM

One of most annoying things I’ve noticed (as well as other community members) is crazy pitch down tendency when switching to direct control mode. I did some reading of forum posts (not everything) but I did not find satisfying explanation of this behavior.

 

So decided to make small investigation as such behavior would make aircraft extremely dangerous to fly in real life and seemed like modeling fault.

 

So I did read FCS description at: http://www.digitalcombatsimulator.com/en/products/flanker/?PAGEN_2=2

 

We do have block diagram of FCS and trimming law scheduling so it should be possible to find out what is going on here.

 

 

Longitudinal_channel_EN.png

Fig38_EN.png

 

 

Let’s assume level flight condition at constant speed without any disturbances causing intervention of FCS. In that case:

· control stick inputs (ST) = const.

· rate of pitch (ωz)= 0

· g force (ny) =1.

 

Since rate of pitch = 0 then pitch damper output = 0

Since rate of pitch = 0 , and g force =1 then pitch autostabilizer output = 0

So stabilizer deflection (ϕ stab) depends on calculated Klong for given Pdyn and Pstat. (trimming law?)

 

Now I’ve checked how much is stabilizer deflected actually (in external view) Its quite easy to do that because you have deflection angle markings on fuselage. For level flight deflection angle is from ~-2 deg. (pitch down at small IAS) to ~+2 deg. (pitch up at high IAS) – nothing unusual here.

 

Switching to DIRECT CONTROL EMERGENCY mode disables Klong calculation and enables Klong man instead (from the diagram it appears to be constant but it is named man like manual??).

 

Since we assumed that in our flight condition pitch damper output =0 and pitch autostabilizer output = 0 then (for level flight)deflection of the stabilizer should change proportionally to the change of Klong - (from Klong calc to K long man) when switching to direct control mode.

In one special case when Klong calc = Klong man - the stabilizer deflection should not change ergo we should continue more or less level flight – so let’s find those conditions!

 

The problem is that every time you engage DIRECT CONTROL EMERGENCY mode the stabilizer sets itself at about -5 deg. (pitch down) position in flight causing immediate flip over the airplane nose, blackout and crash. Stabilizer deflection seems to correspond with actual stick position being trimmed nose down by trimming law before we engaged direct control mode.

 

This looks like huge design fault (I do not believe it is truth here) or more probably some error with FCS modeling in DCS. Since trimming law is not working below 230 km/h only, ergo: in basically whole flight envelope you have big mismatch in stabilizer deflection (for level flight) for given stick position between FCS modes! Once again I think this is wrong. How it should be working then?

 

I can only suspect that:

1. Assuming Klong man value is constant then

Klong man is wrong and it should be bigger to at least partially compensate trimming law and associated ST position to reduce nose down tendency when switching to DIRECT CONTROL EMERGENCY mode – the drawback of this approach would be that at some speed you would have pitch down tendency (to lesser extend then now) and sometimes pitch up tendency - not perfect but still better than current behavior and still dangerous in real life.

 

2. Klong man value is manually set by pilot based on IAS or Mach prior to engaging DIRECT CONTROL EMERGENCY mode to compensate ST position forced by trimming law – something like additional counter trim to trimming law which in my opinion is missing now in DCS Su-27 PFM

 

My Russian language knowledge is pretty much nonexistent and I have no access to Su-27 flight manual but let’s take a look at panel where you flip the switch to enable direct control mode

 

DCS_Su_27.png

 

 

b2ytt9wt1There is some button (knob?) and indicator next to mentioned switch (Mach number?)– maybe there is no fault per se in FSC modeling but we are missing some features actually? Maybe all we need to have added is possibility to manually set Klong man ? according to Mach number? before we engage Direct control mode.

[sIGPIC][/sIGPIC]

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My Russian language knowledge is pretty much nonexistent and I have no access to Su-27 flight manual but let’s take a look at panel where you flip the switch to enable direct control mode

 

DCS_Su_27.png

 

 

b2ytt9wt1There is some button (knob?) and indicator next to mentioned switch (Mach number?)– maybe there is no fault per se in FSC modeling but we are missing some features actually? Maybe all we need to have added is possibility to manually set Klong man ? according to Mach number? before we engage Direct control mode.

Somewhere in this forum there's a post buried in a thread that addressed those controls. I did a quick search but couldn't find it. If memory serves, it was a new addition to an older thread.

YouTube Channel: https://www.youtube.com/channel/UCU1...CR6IZ7crfdZxDg

 

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Heres the thread: http://forums.eagle.ru/showthread.php?t=120007

 

Translation and explanation for: КШ (Контроль Жесткие)

Proportional Control, belongs to the FCS system which stands for linking the movement of the stick and rudder directly to the control surfaces and providing the adjustment of the movement intensity, known as rigit link system.

 

The switch above with the red cover stands for:

КШ РУЧН (Контроль Жесткие Ручное)

Manual Proportional Control, belongs to the FCS system and if activated bypasses the FCS and rigit link system turning the fly-by-wire system completly off.

 

Now to your question. The left orange button reads Нажать Повернуть (Push to turn), if you push and turn you control the number of the right field which can have several positions: 0.5 - 1 - 1.5 - 2.

This number stands for the sensitivity of the Flight Control System. If you have selected 2 (0.5) and you move your stick the computer translates that into a heavy (small) movement of the control system.

 

Be aware that this knob only is functional if the СДУ (FCS) fails or you manually disengage it with the switch under the red cover.

 

Hope that answered your question, feel free to ask more :D

 

PS:

СДУ (Система Дистанционного Управления)

Flight Control System (FCS), this system controls all control surfaces of the aircraft to maintain a steady optimal flight in all situations, it is connected to the pilot inputs and converts them into the best movements of the control surfaces in order to achieve what the pilot wants.

 

PPS:

You need to trim the Su-27 to neutral prior to engaging "DIRECT CONTROL EMERGENCY" then you won't pitch down and everything is fine. Thats because the trimming for a steady flight in the Su-27 is nose down and if you engage that emergency mode your nose down input (from trimming) gets translated in a huge sudden pitch down movement.


Edited by Zabuza
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So I finally got more time to fly Su-27 with PFM

One of most annoying things I’ve noticed (as well as other community members) is crazy pitch down tendency when switching to direct control mode. I did some reading of forum posts (not everything) but I did not find satisfying explanation of this behavior.

 

So decided to make small investigation as such behavior would make aircraft extremely dangerous to fly in real life and seemed like modeling fault.

 

So I did read FCS description at: http://www.digitalcombatsimulator.com/en/products/flanker/?PAGEN_2=2

 

We do have block diagram of FCS and trimming law scheduling so it should be possible to find out what is going on here.

 

 

Longitudinal_channel_EN.png

Fig38_EN.png

 

 

Let’s assume level flight condition at constant speed without any disturbances causing intervention of FCS. In that case:

· control stick inputs (ST) = const.

· rate of pitch (ωz)= 0

· g force (ny) =1.

 

Since rate of pitch = 0 then pitch damper output = 0

Since rate of pitch = 0 , and g force =1 then pitch autostabilizer output = 0

So stabilizer deflection (ϕ stab) depends on calculated Klong for given Pdyn and Pstat. (trimming law?)

 

Now I’ve checked how much is stabilizer deflected actually (in external view) Its quite easy to do that because you have deflection angle markings on fuselage. For level flight deflection angle is from ~-2 deg. (pitch down at small IAS) to ~+2 deg. (pitch up at high IAS) – nothing unusual here.

 

Switching to DIRECT CONTROL EMERGENCY mode disables Klong calculation and enables Klong man instead (from the diagram it appears to be constant but it is named man like manual??).

 

Since we assumed that in our flight condition pitch damper output =0 and pitch autostabilizer output = 0 then (for level flight)deflection of the stabilizer should change proportionally to the change of Klong - (from Klong calc to K long man) when switching to direct control mode.

In one special case when Klong calc = Klong man - the stabilizer deflection should not change ergo we should continue more or less level flight – so let’s find those conditions!

 

The problem is that every time you engage DIRECT CONTROL EMERGENCY mode the stabilizer sets itself at about -5 deg. (pitch down) position in flight causing immediate flip over the airplane nose, blackout and crash. Stabilizer deflection seems to correspond with actual stick position being trimmed nose down by trimming law before we engaged direct control mode.

 

This looks like huge design fault (I do not believe it is truth here) or more probably some error with FCS modeling in DCS. Since trimming law is not working below 230 km/h only, ergo: in basically whole flight envelope you have big mismatch in stabilizer deflection (for level flight) for given stick position between FCS modes! Once again I think this is wrong. How it should be working then?

 

I can only suspect that:

1. Assuming Klong man value is constant then

Klong man is wrong and it should be bigger to at least partially compensate trimming law and associated ST position to reduce nose down tendency when switching to DIRECT CONTROL EMERGENCY mode – the drawback of this approach would be that at some speed you would have pitch down tendency (to lesser extend then now) and sometimes pitch up tendency - not perfect but still better than current behavior and still dangerous in real life.

 

2. Klong man value is manually set by pilot based on IAS or Mach prior to engaging DIRECT CONTROL EMERGENCY mode to compensate ST position forced by trimming law – something like additional counter trim to trimming law which in my opinion is missing now in DCS Su-27 PFM

 

My Russian language knowledge is pretty much nonexistent and I have no access to Su-27 flight manual but let’s take a look at panel where you flip the switch to enable direct control mode

 

DCS_Su_27.png

 

 

b2ytt9wt1There is some button (knob?) and indicator next to mentioned switch (Mach number?)– maybe there is no fault per se in FSC modeling but we are missing some features actually? Maybe all we need to have added is possibility to manually set Klong man ? according to Mach number? before we engage Direct control mode.

 

Your reasonings have only one but profound mistake: first of all, let's note that for neutral stable plane (it's not very important if it's slightly positive or negative stable) balance stab trim position is near 0 (again - it could be +- near zero depending of initial pitching moment of the airframe).

Imagine that there is no artificial stability signal depending on IAS. As IAS increases and the stick is in its initial (central) position, FBW will maintain 1 g flight keeping the stab at its balance position.

If we write down the whole math for this kind of control we can see that the resulting g-load will be considered proportional to the stick movent because of g-load feedback loop (if the gain of this loop is sufficient). THis is what we use to call AUTOTRIM. And this is what we used to have for SFM.

But in the real Su-27 a special signal is used to have artificial stability or necessity to move the stick forward at higher IAS to imitate conventional stable plane.

THis signal is mixed to the whole sum of control inputs, so moving stick foward you compensate this bias.

Then you can trim the stick to the new balance position WITH THE SIMPLE SEPARATE ARTIFICIAL LOAD DEVICE.

What will happen if you trim the plane at high speed (stick forward), move your hands away from the stick and then switch to direct?

 

As the stab position is Phi_stab = K_long*ST and ST corresponds to Phi_stab 5 degrees to dive the plane gets strong pitch down moment almost in all range of possible AoAs (because of neutral stability).

 

And this is the effect you see in DCS.

 

And once again - the problem is in stick trim for high speed flight. In real life this transition is limited up to only 600 kph to let the pilot enough time to pull the stick immediately as he feels tendency to unloaded flight.

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

There is nothing so hurtful for the brain as splinters of broken rose-coloured spectacles.

Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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