C208 autopilot expectations

[billkirk888 28]

I'm wondering if I am doing something wrong from a procedural standpoint, or if it is an autopilot bug.  When flying the C208, I hand fly takoff, get the plane trimmed with a climb of about 700-900fpm, get the flaps retracted, then engage autopilot.  I have heading mode selected with the current heading set, with either FLC or VS selected.  For FLC, I've tried various speed settings anywhere from 95kts to 125kts.  For VS, I've tried +700-900fpm.  Each time I engage autopilot, the plane wants to take a nose dive.  I man-handle it until the autopilot "catches up" with my settings, then things get under control - sort of.  It still wants to porpoise up and down until reaching my selected cruise level unless I intervene.  If I disengage the autopilot, re-engage with all the same settings, sometimes that gets it under control as well.  If anything, the AP should level off initially until the actual airspeed reaches what you have selected, not take a -2000ft nose dive (FLC mode).   Occasionally instead of the nose dive, it will do the opposite and pitch up drastically until the plane stalls.

I have better results if I simply hit autopilot after take off without trimming the plane for level flight (or slight climb) first.  It still takes a little manual assistance before it flies the correct pitch for climb based on my speed or VS setting however less than if I were to trim it first.  It seems as if autopilot isn't taking the adjusted trim setting into account prior to making adjustments for the climb pitch.  Once AP "settles down" or catches up, it flies as it should.

I'm still playing with my procedure to see if I can get it to behave better, but I'm thinking the autopilot itself is the culprit.  It seems like it only happens on initial AP engage.  Once it is under control, it does what I tell it.  I can use FLC or VS for my decent without any erratic behavior. 

[BusheFlyer 87]

I have been flying the caravan a lot... about 74 hours logged according to my log book. Mainly because of FSEconomy.  The autopilot is way too aggressive in pretty much anything it does.. therefore I only use it occasionally to hold altitude or in heading mode. I take off, trim to climb around 110kts, and follow the SID in the climb or the desired heading. Once I have reached altitude and am on track I typically engage hdg mode or nav mode and alt. Then enable autopilot. It then works fine, it holds altitude and tracks fine.

I absolutely do not trust it on flying procedures, so at the descent I usually take back control and hand fly the procedure.

All in all, it's a fun aircraft to fly IMHO. And until the various issues with the AP are ironed out it's not really a problem.

[Virtual-Chris 682]

My process is similar except I don’t try to trim it. I just hit the AP once I’ve got it in a steady climb (usually around 800-1200 FPS) and I get the exact same behaviour... I hit AP, take pressure off the yoke... dive!  W T F?! ! Pull back up... then the AP engages. It’s frightening and dangerous. It’s like there’s a 10 second delay before the AP actually kicks in. 

Edited October 6, 2020 by Virtual-Chris

[BusheFlyer 87]

7 minutes ago, Virtual-Chris said:

My process is similar except I don’t try to trim it. I just hit the AP once I’ve got it in a steady climb (usually around 800-1200 FPS) and I get the exact same behaviour... I hit AP, take pressure off the yoke... dive!  W T F?! ! Pull back up... then the AP engages. It’s frightening and dangerous. It’s like there’s a 10 second delay before the AP actually kicks in. 

Well, that sounds like half the problem.. if you don't trim it then the moment you engage AP and release the yoke the nose is going to drop rapidly. The trim wheel is just that a wheel.. which the autopilot will take over from the position you left it. It will start to trim nose up when engaged but that takes time which explains what you are seeing happening.

The same thing would happen in the real aircraft.

Edited October 6, 2020 by BusheFlyer

[Virtual-Chris 682]

3 minutes ago, BusheFlyer said:

Well, that sounds like half the problem.. if you don't trim it then the moment you engage AP and release the yoke the nose is going to drop rapidly. The trim wheel is just that a wheel.. which the autopilot will take over from the position you left it. It will start to trim nose up when engaged but that takes time which explains what you are seeing happening.

Makes sense. I would have expected it to trim a lot faster. But that doesn’t explain the issue the OP is having who DOES trim the climb but gets the same behaviour. 

[Virtual-Chris 682]

Maybe related... one thing I notice is that when the plane is climbing or descending on AP, the trim indicator on the wheel (or in HUD using external view) is dead center. In other words it’s not showing the trim used by the AP. It makes me think the user set trim is decoupled and different than the AP trim which might explain why trimming the climb before activating AP makes no difference vs not trimming... the AP ignores the user trim setting and does its own thing. Just a thought. 

[BusheFlyer 87]

I should test it again myself, I pretty much gave up on using the autopilot except the way I described. I fly only GA types as a rule so I basically don't use the autopilot hardly at all.. just really for those long legs in the caravan or the kingair. 

Edited October 6, 2020 by BusheFlyer

[edpatino 1,964]

I don't know if this would help. but I have my elevators assigned to a button on my joystick and during the initial climb-out I control the VS using the elevators, up and down, depending on the VS speed to keep it under control at about 700-900 fpm. Once VS is under control above 500 ft AGL, I engage the AP and set the current HDG, and then set the VS to the required value (800-900 fpm). There's certainly a light nose dive after selecting VS, but it can be controlled quickly after selecting the required rate of ascent.

Cheers, Ed

 

[billkirk888 28]

1 hour ago, edpatino said:

I don't know if this would help. but I have my elevators assigned to a button on my joystick and during the initial climb-out I control the VS using the elevators, up and down, depending on the VS speed to keep it under control at about 700-900 fpm. Once VS is under control above 500 ft AGL, I engage the AP and set the current HDG, and then set the VS to the required value (800-900 fpm). There's certainly a light nose dive after selecting VS, but it can be controlled quickly after selecting the required rate of ascent.

Cheers, Ed

 

Pretty similar to how I am doing it, only controlling the elevator with my joystick axis until AP decides to behave.  The other person mentioned physical trim setting and AP trim setting possibly being separate - that makes allot of sense.  I only assumed the trim wheel moved while being directed by the autopilot.  In fact, I thought I noticed it once out of the corner of my eye and didn't give it another thought.  It was probably me just imagining it did.  It also brought a symptom to mind I forgot to mention - when disengaging autopilot, say wings and attitude level, just prior to making my base turn to the arrival runway, the nose always drops slightly.  Most likely reverting back to where my manual trim setting is. 

 

My passengers really must be wondering where I received my pilot's license.... (after they make good use of their barf bags)

[BusheFlyer 87]

I have been playing with the AP climbing out on both FLC and VS, it functions exactly as I expected it to.. if you leave trim alone and just pull the aircraft with the yoke at your chosen speed then enable either mode.. the nose will instantly drop as expected until the aircraft trims itself.. it will then porpoise around whilst trimming out. If you however establish the climb at say 110kts trim it stable at that speed turning on AP will hold it as expected.

The aircraft has only 1 elevator trim.. the thing you are describing is in a real aircraft the autopilot will be adusting the trim of the aircraft using servos.. the secondary effect is the trim wheel in the cockpit will also turn because it's linked by cables and pulleys. If you observe in MSFS you will see the wheel moving as the autopilot makes adjustments. The problem is with most simulator setups is you have a joystick axis (most likely a wheel) that only moves when you manually touch it. It does not move in sequence with whats happen on the aircraft unlike the real one.

So if you disable the autopilot and touch the trim wheel it will cause the sim aircraft to suddenly react to whatever your trim was at prior.