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Wing rudders movement landing config & negative trim during acceleration.


outbaxx

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When I’m on ground the rudders doesn’t move as I would expect them to. It well might be my joystick as I’ve built my own so I wonder if anyone else see the same as me?
If I move my joystick full aft and back to center, the outer rudders go up and down but doesn’t always follow the joystick movement, they can move full fwd or aft and fluctuate. They do this until I’m airborne and I’m wondering if it’s just an animation problem or if it’s how the joystick movement is actually output on the control surfaces?
 
When I rotate for take off I always have to fight the aircraft for not over rotating and I wonder if it’s that the rudders doesn’t move with my joystick and kind of get stuck in aft position? I don’t get this nose up behavior when I’m airborne.
 
/F

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23 hours ago, outbaxx said:

When I’m on ground the rudders doesn’t move as I would expect them to. It well might be my joystick as I’ve built my own so I wonder if anyone else see the same as me?
If I move my joystick full aft and back to center, the outer rudders go up and down but doesn’t always follow the joystick movement, they can move full fwd or aft and fluctuate. They do this until I’m airborne and I’m wondering if it’s just an animation problem or if it’s how the joystick movement is actually output on the control surfaces?
 
When I rotate for take off I always have to fight the aircraft for not over rotating and I wonder if it’s that the rudders doesn’t move with my joystick and kind of get stuck in aft position? I don’t get this nose up behavior when I’m airborne.
 
/F

I think turning off SPAK would make them follow your commands at any time. With SPAK active, it tries to stabilize.

To avoid over-rotation, use 6° Nos upp and SPAK active as a good & healthy take-off trim. In fact values of 5° to 6° should help for a smooth rotation (for most load-outs this is a very good value, but depends on load-out AND density altitude).

 

edit: sorry, I was on aileron, but oubaxx talked about rudder. Changed my reply accordingly. Sorry!


Edited by TOViper
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AFAIK rudders are bound to the stick while on ground to make takeoffs easier as per the real aircraft.

Cheers!

Edit: Just realised we're talking about different things. Ailerons, rudders, apples and pears. 😊 

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Edited by MAXsenna
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I think turning off SPAK would make them follow your commands at any time. With SPAK active, it tries to stabilize.

To avoid over-rotation, use 6° Nos upp and SPAK active as a good & healthy take-off trim. In fact values of 5° to 6° should help for a smooth rotation (for most load-outs this is a very good value, but depends on load-out AND density altitude).
 
edit: sorry, I was on aileron, but oubaxx talked about rudder. Changed my reply accordingly. Sorry!

I think this video shows better what I mean:
http://


Trim is set to zero here.
First I move stick forward and back to center, the aft and back to center.
The outer wing rudder is not drooped as the inner, they both should be, but the outer returns to level when the stick is centered.
It behaves like this some time after take off too and then they both droop and behave as expected.

The outer wing rudders should follow the inner wing rudders movement, but they don’t on ground and some time after take off.
The autopilot can steer the outer rudder up to 10.4 degrees from the inner rudders but that it does this on ground doesn’t seem right. Never seen this on a live video. If you look at a live video of the Viggen take off you see that the outer and inner rudders are drooped and “connected” until the stick is pulled aft.
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3 hours ago, outbaxx said:


I think this video shows better what I mean:
http://

 


Trim is set to zero here.
First I move stick forward and back to center, the aft and back to center.
The outer wing rudder is not drooped as the inner, they both should be, but the outer returns to level when the stick is centered.
It behaves like this some time after take off too and then they both droop and behave as expected.

The outer wing rudders should follow the inner wing rudders movement, but they don’t on ground and some time after take off.
The autopilot can steer the outer rudder up to 10.4 degrees from the inner rudders but that it does this on ground doesn’t seem right. Never seen this on a live video. If you look at a live video of the Viggen take off you see that the outer and inner rudders are drooped and “connected” until the stick is pulled aft.

Yep, we have this effect since the DCS Viggen came out. Does it? I always wondered what this is all about ...

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I said that the difference between inner & outer wing rudder disappeared soon after take off.
I did a flight today and the outer rudders move up during deceleration and down when accelerating.
-Is there some kind of auto trim logic in SPAK when in landing config?

-When I pull full stick aft, in landing config, the inner rudders only go about 5deg over the horizontal plane, I thought that max was 22 degrees for the elevator.

In SFI part2 page 34 it says that up to Mach 0.85 the stability is positive so that during deceleration you have the use aft force on the stick.
- I don’t know any aerodynamics but wouldn’t that mean that during acceleration you would need to push the stick forward?
In DCS I need to pull aft during acceleration.

On page 37 it says that at M0.95 there is a small nose down change in trim that at M1.03 get negligible when SPAK is active.
-In DCS I get a nose down and a nose up trim interference. Is SPAK working as it should?

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2 hours ago, outbaxx said:

In SFI part2 page 34 it says that up to Mach 0.85 the stability is positive so that during deceleration you have the use aft force on the stick.
- I don’t know any aerodynamics but wouldn’t that mean that during acceleration you would need to push the stick forward?

Yes, that is what it should do. If you take-off and level AND trim out at e.g. 350 km/h, THEN accelerate you have to push the stick forward more and more until reaching M0.85.
On the other hand, if you are at trimmed M0.85 and start decelerating, you have to pull the stick back further and further to keep the nose up.

Once above M0.85, the center of lift (C.O.P.) moves backwards. That means that the C.G. lift now "pulls the aircraft upward" at a point far more behind the C.G. (center of gravity) - or other way seen - the C.G. is now more in front of the C.O.P. Because the aircraft was trimmed before, now the nose dives down therefore.

"In DCS I need to pull aft during acceleration." .... are you sure about? This sounds very odd to me... :wassat: Are you sure you are below M0.85 and started with a trimmed aircraft?


Edited by TOViper

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First, small terminology nitpick so we can all agree what we're talking about: in Swedish (and maybe also in German), all control surfaces are "roder", but this is not the case in English. A rudder is the control surface you use for yaw - the thing you move with the pedals. The control surfaces we're talking about here are called elevons (combined elevators and ailerons). I think the misunderstanding was already cleared up in this thread, but just to make sure we're all on the same page 🙂

On 3/31/2022 at 10:09 PM, outbaxx said:

-When I pull full stick aft, in landing config, the inner rudders only go about 5deg over the horizontal plane, I thought that max was 22 degrees for the elevator.

Yes, 22° is the max deflection that can be achieved with a pure pitch up/down movement of the stick, but the missing bit here is that this is both altitude and airspeed dependent. As dynamic pressure increases (that is, you fly faster and/or lower), you get more force per degree of control surface deflection. If the control system didn't compensate for this, the aircraft would be essentially unflyable at high speeds because the stick would be extremely sensitive, and any big movement would rip your wings off. So, the Viggen has two almost literal hydraulic gearboxes (one for pitch and one for roll) that attempt to keep stick force per G the same regardless of airspeed and altitude. The roll gearbox only has two modes, high speed and low speed, and switches between them at 350 km/h IAS. The pitch gearbox though is continuously variable, and at its highest setting (least control surface deflection for a given stick movement), the max pitch-up deflection is only 8.7°. Here's a chart of max elevon deflection by airspeed and altitude, in SPAK mode (from Fpl 37 aerodynamik III):

NvPFCql.png

The line at M 0.93 corresponds to the "series trim" kicking in to correct for transonic and supersonic effects, and since control surface effectiveness is reduced at transonic and supersonic speeds, this also coincides with an expansion of the deflection envelope by about 3.5°.

We're not done yet though, because there's yet another thing that can mess with the elevon deflection, and that's the "droop mechanism". This is used to add a drooping offset (additional pitch down deflection) to the elevons, and it's controlled by the canard flap control mechanism. This is connected via a mechanical wire linkage to the droop controls, so when the canard flaps are deploying the elevons are continuously drooped at the same time. Unfortunately I don't know how big this deflection is, since the SFI doesn't say, but this is probably at least part of the reason why you're seeing a surprisingly small deflection upwards when in landing configuration - you effectively have some invisible pitch down trim already, to compensate for the pitch up trim change caused by the canard flaps.


Edited by renhanxue
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