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Yasim - Rudder behavior, possible bug?

Good sims require good FDMs (the "thing" that makes an aircraft behave like an aircraft).

Yasim - Rudder behavior, possible bug?

Postby kaklik » Mon Jan 13, 2020 9:15 pm

Hello,

I tried to make a new aircraft model in Yasim, but in the middle of work, I noticed a weird behavior of yawing in full rudder deflection - the yaw movement stops, after a particular angle similarly as in case the nose of aircraft will be towed by a rope.

To eliminate possible trouble with my model I made test flights with several Yasim aircraft, but all behave similarly. The trouble is clearly noticeable even in the case of the new version (5.0) of ASK21 glider in the Yasim model variant, but it is not present in the JSBsim model variant. The problem is clearly indicated by the Turn indicator instrument.

The expected behavior is the ability to make a full circle rudder-only turn (uncoordinated obviously), instead of this, the aircraft stops turning at certain heading.

I am not the first one noticed this, there are multiple posts mentioning similar troubles:
viewtopic.php?f=49&t=27963&p=272270&hilit=rudder+yasim#p272270
viewtopic.php?f=6&t=6014
kaklik
 
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Re: Yasim - Rudder behavior, possible bug?

Postby danielHL » Tue Jan 14, 2020 11:06 pm

Hi,

I can't reproduce this. I tried the fgaddon ASK21 yasim and the Citation II and they both let me fly uncoordinated turns with only the rudder. I didn't use full rudder deflections, though. I apply right rudder, the aircraft yaws into a sideslip, begins to roll a bit and starts to turn. One difference to turning with ailerons only is that in order to get enough rate of turn you have to keep the rudder applied, at least a bit. Whereas with ailerons you apply them until the desired bank angle is reached and reset them to neutral, the turn continues. But this seems like correct aerodynamic behavior to me.

Do you have a specific yasim fdm with which you can't turn with the rudder at all?
danielHL / GVA0387 / D-FMPW / (UA83DAN)
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Re: Yasim - Rudder behavior, possible bug?

Postby kaklik » Mon Jan 20, 2020 10:56 pm

I have found that the described behavior could be corrected by increasing the flap lift coefficient and decreasing the drag coefficient below the value of the lift coefficient.

My previous test with ASK21 seems to be insufficient and there is actually a turning produced by rudder alone. Although there is a significant discrepancy between Yasim and JSBsim models.

The rudder configuration of my model which I now corrected to acceptable behavior is on this line.
kaklik
 
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Re: Yasim - Rudder behavior, possible bug?

Postby danielHL » Tue Jan 21, 2020 1:53 am

So this is a problem of a specific yasim configuration file and not a general behavior - that's reassuring.

I've had a short look at your model. Is this supposed to be a model helicopter? Mass of 25kg and two rudders as well as rotorgear? I honestly don't know anything about helicopters but if that's what you want to do forget everything I said about yaw/roll/turn inter-relation.
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