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Saab Climbout on Autopilot oscillates/porpoises


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Didn't see this mentioned in posts from the past few months. After take-off in the LES Saab 340A, I hit gear up, then after 500' AGL, I engage A/P, then click CLIMB(H), then click on NAV. The climb rate oscillates from plus 2000 fpm to minus 2000 fpm or more in either direction. I don't know if I'm going to crash like a lawn dart, or climb like an F-16 and then stall out. It oscillates back and forth and I don't know how to overcome it or stabilize it. It does not happen every time, but often enough to be quite annoying. Is there a tip or trick someone can share? I've only owned this plane for 2 weeks, so it could be pilot error! Thanks.

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2 hours ago, Goran_M said:

This usually happens when the airspeed is too high.  Are you getting close to the barber pole speed?

I did notice that, but when I initially engaged the AP and hit CLIMB(H), the airspeed was still in the 150-160 range and gaining.

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Are you remembering to set your altitude target beforehand? Sometimes I'll forget to reset it after approach and it'll be very low. Then the AP will level out and it will start to accelerate like crazy.

Also you might want to consider engaging the flight director and climb/nav modes and hand-fly it prior to engaging the AP. If you enable the AP when you're already following the FD cue you'll get smoother and more predictable actions from it. If there's a big difference then when you enable the AP you can get significant pitch moments as it tries to sort the situation out.

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I have an issue with the oscillation as well and I won't start a different post.

I'm flying level at FL170, the A/P is keeping track and altitude just fine, then there's about a 30 degree turn and the A/P loses maybe 100-150 ft of altitude during the turn. Speed is around 200 kts IAS, 45kts below the barber pole (shouldn't be too fast). The A/P compensates heavily for the altitude loss, starting off with VS +/- 2000 feet, then increasing the oscillations more and more until VS is +/- 6000ft up and down. This sort of dynamically unstable behaviour would end with a stall. Tried reducing power just in case but that had no effect. Had to disengage the A/P, manually level the aircraft and only then re-engage the A/P and the aircraft would continue flying level. 

That's probably not normal behaviour, is it?

I have screenshots of the plane at the extremes of the oscillation and logfiles, should those be necessary for investigation of the matter.

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This has happened to me before when after takeoff I forget to disengage AutoCoursen and CTOT.  The "150-160" range and gaining into a progressive overspeed porpoise is a tell-tale sign.  Not saying this is what happened, but this is the first thing I would check on myself.

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I've done numerous flights in the last couple of days and for me having Autocoarsen and CTOT off doesn't seem to have a specific effect. About every other flight the progressive oscillations occur - usually after the aircraft has passed a turn of more than 30 degrees or so. I always keep my speed around 40-50 kts below the barber pole, so it shouldn't be overspeed induced. It seems there are issues with damping the oscillations whenever VS goes over 1500 fpm during a turn. It can sort of stabilise when the VS is smaller but anything over that and it keeps progressing until pitch is going up and down over 20 degrees and there is no other way than to disengage the A/P, manually stabilise and re-engage the A/P. 

 

Edited by Matt_thePilot
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  • 1 month later...
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4 hours ago, Matt_thePilot said:

That is great news, really looking forward to that update to get that Saab back in the air!
Any chance this update will also include the option to use RXP GTN 750 with this beautiful bird? The default 530 is such a pain to use and ability to pair the 750 with it would be amazing!

Definitely not in this update.  We will evaluate for v2.0  (need to see some real world examples to justify).

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On 12/26/2019 at 2:52 PM, Burning Bridges said:

Good to hear that you are making the update and that it will adress the autopilot.

I hope it will not take forever though. After 200+ hours with your Saab 340 I am getting close to giving in and buying a Carenado.

No don't get the Carenado one. I have both, just recently got the LES Saab and this one is 1000 better and has more character ;).

Regarding the next update of this wonderful aircraft, will it use the experimental flight model of xplane 11.40, which will anyway be the standard with 11.50?
 

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On 12/27/2019 at 12:52 AM, Burning Bridges said:

Good to hear that you are making the update and that it will adress the autopilot.

I hope it will not take forever though. After 200+ hours with your Saab 340 I am getting close to giving in and buying a Carenado.

I have some of carenados planes ,but without Thrandas input they are just pretty planes, and if you missed the discuss about the breakup, carenado just want output and he couldnt keep up with there demands, I hope it doesnt cause a drop in quality.

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On 12/31/2019 at 9:44 AM, janis22 said:

Regarding the next update of this wonderful aircraft, will it use the experimental flight model of xplane 11.40, which will anyway be the standard with 11.50?
 

Yes.

 

Update:  I should clarify this.  We will not be releasing production versions of the Saab that "require" the experimental flight model.  Austin will always be adding things to the experimental flight model and we may not be aware of them or have tested with them.  So, due to the potential instability of the experimental flight model we will not recommend (nor support) you use it with the Saab, but that is your choice.  

Of course, when the features in the experimental flight model get moved into the base sim we will test and release updates as necessary.

Edited by JGregory
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