Autothrottle regulator not working correctly in aoa mode

I have found a problem with the autothrottle.

At mode 1 it looks like it´s working correctly but in mode 2 and 3 it´s not.

In mode 2 and 3 it´s supposed to regulate the throttle so the aircraft is at an angle of attack 12 or 15,5deg.

Engage the autothrottle and increase AoA to more then 12deg in mode 2.

The throttle will at this point be in max throttle without the ab.

Decrease the AoA and when it hits 12deg the throttle will instantanious go to min throttle and from there start building up throttle response again.

Same thing with mode 3 and 15,5 deg, when it comes down to 15,5 it will go to min throttle.

This makes the aircraft loose altitude rather quickly.

First of all it should gently reduce throttle response when coming down to setpoint AoA.

Then in the SFI manual part 1 capt 1 it also states that the throttle have higher authority to speed increase than speed decrease meaning it should be slower at decreaseing throttle responce than increasing.

Ref: Swedish SFI part 1 chap 1 tab 10 page 3.(manual page 165)

It should also increase the engine rpm by 1,5% when the gear is lowered when in autothrottle. I cannot see that this is happening.

/illern

i'd suggest that you don't fly like this: ~

if you don't want that continuous up and down, try to hold always 12 degrees, not when the engine comes pull and then when you have like 10 degress push the stick a little...

If done correctly, the engine is staying about the same rpm and you always have 12 degrees aoa

It also has a emergency-mode if the there is positive G >1 so if you are pulling back on the stick you are probably engaging this mode.

It also has a emergency-mode if the there is positive G >1 so if you are pulling back on the stick you are probably engaging this mode.

I´m in 1G and no more. I´m doing this really gently.

Also with more tests if you gently holds it at 12deg AoA the trottle will switch between Min/max several times in a second. You can both see it and hear it since it will go below flight idle and you can hear the click when it is below FI.

Make sure the aircraft is not heavily loaded on approach

So I made a video of the problem instead of trying to describe it.

A 30s clip when I have engaged the autothrottle at 500m.

The AC is clean with no external load.

Gear down and now I´m easing it up to 12deg AoA.

When I hit 12 deg the throttle goes bananas. That is not a correct behaviour of a regulator.

The throttle move between min/max position several times in an instant.

Hope the video works, I´m not used to making them.

/illern

That is very evidently a bug in the code. Reading my code the throttle is limited to not move faster than 1/4 of the range per second (1/8 when slowing down) so there's some garbage here. Will look into it. Thanks for the video! Did this happen after you respawned? If you close DCS down and restart it and go directly to this, does it happen too?

Can confirm this happens on a new viggen on a fresh start of DCS.

That is very evidently a bug in the code. Reading my code the throttle is limited to not move faster than 1/4 of the range per second (1/8 when slowing down) so there's some garbage here. Will look into it. Thanks for the video! Did this happen after you respawned? If you close DCS down and restart it and go directly to this, does it happen too?

Yes it happens at every spawn.

Not to hijack -- but sort of tangentially related: at 24% fuel, with 1x BK-90 (so within weight params), I engage AFK, with gear down, a15.5 pressed and lit. The thing never stays at altitude, it just drops. Am I doing something wrong? I've held altitude and landed it in AFK mode 2 successfully several times, so I feel like I know what kind of 'ease' I should control the aircraft with.

I see this, AFK can start to vibrate the throttle multiple times a second to idle and you get lots of clicking. It also seems to cause a sink rate on approach under certain conditions which in the final phase can mean you drop.

On 2/4/2017 at 9:28 PM, illern said:

So I made a video of the problem instead of trying to describe it.

A 30s clip when I have engaged the autothrottle at 500m.

The AC is clean with no external load.

Gear down and now I´m easing it up to 12deg AoA.

When I hit 12 deg the throttle goes bananas. That is not a correct behaviour of a regulator.

The throttle move between min/max position several times in an instant.

 

Hope the video works, I´m not used to making them.

 

 

/illern

Unfortunately the video is not available anymore, so I'm not sure how it looked like when it was reported. I just tested this and did not observe the throttle going bananas.

It did not manage to find a fixed throttle position to hold 12° though, meaning it is continiously moving around to keep the aircraft at 12°. It does suddenly warp into position from time to time, but it's not what I would refer to as "going bananas".

So, in other words: The AFK behaviour for holdign 12° AOA could use some more fine tuning, but it's a rather subtle misbehaviour.

6 hours ago, QuiGon said:

The AFK behaviour for holdign 12° AOA could use some more fine tuning, but it's a rather subtle misbehaviour.

I often notice the AFK surges and wanes when I'm on approach. I haven't figured out the conditions that cause it. I have to keep adjusting the trim to compensate because the flight path marker oscillates with the throttle.