Cirrus SR22 Trim shoots to 100 nose up

[eslader 1,823]

In a similar vein, I discovered that the de-icing boots on the TBM just clip through the ice layer without having any effect on the ice itself. That led me to conclude that icing might be modeled, but de-icing in many planes is not, or at least not completely. 

 

[kerosene31 358]

I had this happen with the 172 this weekend.  Was climbing past 6000 for 7000.  The weather was warm, but nothing so hot that it should cause major issues at this alt (80 degrees f on the ground, not exactly sweltering).  Suddenly the plane pitches up and stalls.  Honestly, I don't think the AP is at fault.  The plane loses power and the AP just stalls.  I disconnect the AP but I couldn't even hand fly the plane to get it to climb anymore, which is why I don't think it is an AP issue.  I had the engine leaned as best I could tell.  I went back down to 5000 feet and everything was fine.  

I looked on my wings and didn't see any icing, and certainly was not in icing conditions (the real weather in the northeast US has been warm and dry, not even cloudy).  It wasn't like it was super hot where you'd struggle to climb.  

I still don't know what caused it.  A random pocket of weird weather (I have hit weird weather shifts with the default weather).  Icing that shouldn't be there but is?  Mixture issues?  I'm trying to lean the aircraft based on keeping it a little rich of peak.  I played around with the mixture when this happened and could not get any more power out of the engine.  The fact that I was able to drop down and the problem went away leads me to believe it was icing, but again I went to the external view and didn't see anything, and I certainly don't think I was in icing conditions (I'm regretting pulling my air conditioners out of the windows in my area lately).  

I'm in the steam gauge version so there's no real way to check the weather (not sure if developer mode would have helped).  

Edited September 28, 2020 by kerosene31

[eslader 1,823]

53 minutes ago, kerosene31 said:

The fact that I was able to drop down and the problem went away leads me to believe it was icing

Dropping down also puts you in higher density air, so it could still have been the mixture. Especially if the mixture control isn't adjusting the mixture the way it should be.

 

 

[kerosene31 358]

16 minutes ago, eslader said:

Dropping down also puts you in higher density air, so it could still have been the mixture. Especially if the mixture control isn't adjusting the mixture the way it should be.

 

 

Yep, I've wondered if the mixture isn't working the way it is supposed to.  The EGT gauge doesn't seem to react the way it should (it seems to barely move), and nothing seems to work in any way I've seen other sims work.  Usually I just keep the RPMs as high as they go and that seems to work, except of course for the fuel flow being off.  The other option is leaning until the engine starts to sputter, but that seems like much lower RPMs.  

If I'm not leaning properly though I'd expect it to happen every time, and it doesn't.  I've had other flights at higher altitudes and not run into the same issues.  Maybe a combo of weather and mixture?  

Edited September 28, 2020 by kerosene31

[eslader 1,823]

Temps are all over the map in a lot of the engines I've messed with. It was amusing when I loaded up the TBM in -20 weather and the ITT was reading in the -50's before startup. Must be one of those refrigerated turbines.

 

[pstrub 998]

2 hours ago, kerosene31 said:

I had this happen with the 172 this weekend.  Was climbing past 6000 for 7000.  The weather was warm, but nothing so hot that it should cause major issues at this alt (80 degrees f on the ground, not exactly sweltering).  Suddenly the plane pitches up and stalls.  Honestly, I don't think the AP is at fault.  The plane loses power and the AP just stalls.  I disconnect the AP but I couldn't even hand fly the plane to get it to climb anymore, which is why I don't think it is an AP issue.  I had the engine leaned as best I could tell.  I went back down to 5000 feet and everything was fine.  

[...]  

I'm in the steam gauge version so there's no real way to check the weather (not sure if developer mode would have helped).  

This happened to me too a couple of times. What was the vertical speed you set in the AP?

I came to realise that it definitely was a user error in the nose-up situations I experienced with the C172. At or near sea level, I set the climb rate to +500 fpm, kept leaning the mixture, and at some point the plane slowed down and went nose up. 

This makes sense insofar that the engine loses power with increasing altitude, even when leaned correctly. At some point the engine is unable to power the climb at the rate set in the AP, the aircraft slows down, and with it the VS goes down too. AP tries to counteract this by increasing trimming higher, which further slows down the aircraft - a positive feedback loop.

I do agree there are instances when the AP is messing up and acts weirdly, but to my experience this isn't one of them. It's exactly how I'd expect a simple control loop to respond with a given VS and running into the climb rate limit of the plane at that altitude. This could be alleviated if the AP adjusted the trim to keep a user-defined certain airspeed, but I haven't seen a mode like this anywhere.

 

[kerosene31 358]

2 minutes ago, pstrub said:

This happened to me too a couple of times. What was the vertical speed you set in the AP?

I came to realise that it definitely was a user error in the nose-up situations I experienced with the C172. At or near sea level, I set the climb rate to +500 fpm, kept leaning the mixture, and at some point the plane slowed down and went nose up. 

This makes sense insofar that the engine loses power with increasing altitude, even when leaned correctly. At some point the engine is unable to power the climb at the rate set in the AP, the aircraft slows down, and with it the VS goes down too. AP tries to counteract this by increasing trimming higher, which further slows down the aircraft - a positive feedback loop.

I do agree there are instances when the AP is messing up and acts weirdly, but to my experience this isn't one of them. It's exactly how I'd expect a simple control loop to respond with a given VS and running into the climb rate limit of the plane at that altitude. This could be alleviated if the AP adjusted the trim to keep a user-defined certain airspeed, but I haven't seen a mode like this anywhere.

 

I was at 500 fpm as well, which seems reasonable for a 172.  The other thing is the change seemed rather sudden.  Obviously we're going to see decreased performance as altitude goes up, but I was at an easy 80 knots climbing and looked away for a moment and I was at 40 knots and the stall horn was going off.  

I don't think the AP in a real 172 is sophisticated enough to "fix" this, the problem is the sudden loss of power.  More complex aircraft would have this, but a 172 normally wouldn't I don't think.  I couldn't even get the plane to climb flying by hand after I disconnected the AP and put the trim back to normal.  I tried to level out, and tried various mixture settings with 100% power and couldn't get enough airspeed back.  

It was like at 6000 feet I hit a "wall" of some sort.  I'm going to do some more testing and see if I can figure out anything.  

[pstrub 998]

38 minutes ago, kerosene31 said:

I was at 500 fpm as well, which seems reasonable for a 172.  The other thing is the change seemed rather sudden.  Obviously we're going to see decreased performance as altitude goes up, but I was at an easy 80 knots climbing and looked away for a moment and I was at 40 knots and the stall horn was going off.  

I don't think the AP in a real 172 is sophisticated enough to "fix" this, the problem is the sudden loss of power.  More complex aircraft would have this, but a 172 normally wouldn't I don't think.  I couldn't even get the plane to climb flying by hand after I disconnected the AP and put the trim back to normal.  I tried to level out, and tried various mixture settings with 100% power and couldn't get enough airspeed back.  

It was like at 6000 feet I hit a "wall" of some sort.  I'm going to do some more testing and see if I can figure out anything.  

I've seen reports that the performance may be a bit off at altitude, so maybe it should still be able to keep climbing - I don't have the hands-on experience to know this. 

But the way the AP acts makes sense to me, also the fact that it leads to a runaway trim-up effect once the aircraft can't keep up the necessary climb rate. 

That's why I keep an eye on the airspeed: once it drops below 80 and keeps decreasing, I reduce the climb VS.

So yeah, this may be an issue with leaning and performance at altitude rather than with the AP. Just to be clear: I do think the AP has issues in other situations... it's a bit weird that it happens so quickly, but all in all it makes sense to me. Maybe it could be alleviated a bit by limiting the max trim adjustment per second allowed for the AP, but I don't know...

 

Edited September 28, 2020 by pstrub

[Phantoms 800]

While the lights are bad and can't be fixed because of the plane being encrypted, I still love this little boring (in a good way) aircraft. I do most of my flying in it. Simple to operate, great cruise speed, easy to fly, still looks amazing, good payload, good range, etc.

[BaronKen 147]

6 hours ago, Phantoms said:

While the lights are bad and can't be fixed because of the plane being encrypted, I still love this little boring (in a good way) aircraft. I do most of my flying in it. Simple to operate, great cruise speed, easy to fly, still looks amazing, good payload, good range, etc.

Wish I could wipe off that windshield though... 😉

[devgrp 1,233]

On 9/22/2020 at 11:29 AM, Nyxx said:

SR 22 can fly for 15 hours across country wow!

100 hours, well done, that’s some flying.

Did he say he flew for 15 hrs straight? He could have landed and took a break then started flying again. 

[Nyxx 6,994]

31 minutes ago, devgrp said:

Did he say he flew for 15 hrs straight? He could have landed and took a break then started flying again. 

OMG Behave yourself.

Did he say he did not? ( it was also half joking as it could not fly for 15 hours)

Edited September 29, 2020 by Nyxx

[devgrp 1,233]

1 hour ago, Nyxx said:

OMG Behave yourself.

Did he say he did not? ( it was also half joking as it could not fly for 15 hours)

No you wasn't joking, you were being a smart word not allowed. Probably wanted to show how much of an expert you were 😏

[Nyxx 6,994]

1 hour ago, devgrp said:

No you wasn't joking, you were being a smart word not allowed. Probably wanted to show how much of an expert you were 😏

What rock did you craw out of? adding you to the ignore list. ******* talk about BS.

[devgrp 1,233]

2 hours ago, Nyxx said:

What rock did you craw out of? adding you to the ignore list. ******* talk about BS.

The same rock you crawled from