C172 AP - When did the C172 Trim wheel get a MOTOR?

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OK… back to some pretty Basic stuff.

There is Real World, and there is the SIM’s World which is trying to Simulate the Real World, the best it can, given certain limitations. Over the year of FS development, many of those limitations have been overcome.

I am working on a “Better” KAP140 AP for the C172 and other small GA planes, and have hit some obvious issue, as far as a correct simulation is concerned.

Lets not jump the gun, ( this issue is most likely Historically the result of Spring Centering Joysticks & Yokes )— so before getting into how the AP is operating INCORRECTLY in MSFS, it would be better to get a clearer understanding how it operates in the Real World.

Let just deal first with Vertical Control.

There are two basic ways this is done:-
(1) Control the vertical Trim, with a Motorized trim wheel (Airlines and large GA planes)
(2) Control the Yoke, with a servo, applying pressure to the yoke, (and NOT altering the trim with the AP) (Small GA planes)
The C172 should be using method #2 … (It should NOT have a motorized Trim Wheel)

So, The AP should NOT be altering the trim … only the Pilot can do that, by physically moving the trim wheel.

When the AP is turned on, it decides how much pressure to apply to the Yoke, to obtain the desired VS, and start to push or pull on the yoke.
If it required too much force, it disengages.
If the force required is within its limits, it applies that force, and if that force is not minimal, it indicated on the AP, that the plane is out of trim, and it is up to the Pilot to mechanically trim the aircraft, so that the AP’s force is reduced, and it stop indicating an “out of trim” condition.

So why, in MSFS, is the C172 AP modeled like an Airline AP, and not a small GA AP.

Because it has always been done that way … probably initially, because of the issue of the Joystick/Yoke, being spring loaded to fixed center position.

One obvious result of this, is if the AP goes “CRAZY” it can unrealistically spin the Trim wheel to its limits !!!

So – now my 2 question are:
(1) Is the above analysis correct ?
(2) IF so, is there a better way to Simulate a GA AP, given the limitations of Fixed center Controllers.

Hint: Just because the controller is sprung to center, does NOT mean that the AP software has to take that input directly — it could be OFFSET in the Software to “simulate” that the Centered Joystick was not actually at it’s spring loaded center. (and that offsetting software could be inhibited for Force Feedback Joysticks)

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The Garmin GFC 500 for a 172 has an optional pitch-trim servo that adds automatic trim and manual electric trim.

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I’d probably lean towards the sping centered home flight controls making that a thing for the average home sim GA AP systems. But as @MalexPrime mentioned it does exist for the 172’s (I had no idea, cool to know), I’d guess just not as common.

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I don’t know if your analysis is correct, but if they start to offset the ‘center’ position you will likley see the same bugs => aircraft goes crazy when enganging/disengaging the AP.

So i don’t think this will solve any problems or is a better solution in regards to the ‘plane goes crazy’ problem.

As long as they have implement bugs (which will happen in development), either way could send your airplane crazy.

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As @MalexPrime pointed out, the Garmin GFC 500 brings full electric auto and manual trim to the C172. There have been a variety of aftermarket STCs over the years for various GA aircraft that can add electric trim to aircraft which were originally equipped only with manual trim.

An airliner autopilot does not use trim as a primary pitch control. The elevator servo is the primary pitch change mechanism. When a pitch change is commanded, the elevator servo firsts deflects the elevator into the airstream, which increases the electrical current draw on the servo motor since it has to work harder to overcome air resistance. At the same time, the autopilot will actuate the pitch trim to reduce the elevator servo current draw back to zero. This process is repeated until the required pitch angle is achieved. In this system, when pitch is in trim, the elevators will always be faired with the stabilizer in a neutral position.

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The KAP 140 Two-axis autopilot also has a pitch trim servo option, which can also be used any time as an electric trim with an optional button which gets installed on the yoke.

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But with the KAP140, does the AP spin the trim wheel ???

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BTW – as far as the AP is concerned as standard, at least for the KAP140 in the C172, it can get into a CRAZY, unstable state … so that when you turn it on, the Planes goes to a crazy, unstable state.

I think this can also happen in RW, but if the RW AP gets into this state ( gyros toppled ? ), then you cannot turn the AP on, and you see warning indicators to the left on the display.

This is NOT simulated in the MSFS KAP140, ,

I really have little idea what is really going on , but the AP can get into a state, where turning it on when the plane is in steady level flight, the AP will instantly put the plane into an unstable state.
Cycling the power to the AP, and having it Re-Init , does not fix the issue.
You either have to re-load the Plane (which re-loads the AP) , or stop using the AP fro that flight.

A lot going on both on the surface, and probably deep down in the Sims kernel …

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Yes it does. With the option installed, it has two servos for pitch control - elevator servo and elev trim servo. You can manually override from the yoke and with the AP disconnected you can always use it for manual trim.

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Yes, that can happen IRL as well, especially in strong winds.
I am not sure that modeling that in the sim was intentional!
I have seen 172 models with even a disclaimer in the cockpit about the ap - don’t remember the exact wording but the message was, be careful as strong winds can cause autopilot upset.

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Yes, it has SERVOs, but are you 100% sure that there is a servo/motor that actually turns the trim wheel, and is not just acting on some form of “Push rod” system

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The real KAP140 uses the turn coordinator for roll information and an accelerometer (combined with a barometric pressure sensor) for pitch. The turn coordinator rate gyro is electric, and starts spinning up as soon as the battery switch is turned on. You probably could not engage the autopilot on the ground until the turn coordinator is fully up to speed, but unless the bearings are shot, that doesn’t take long. It’s possible for any autopilot to get into an unstable state, but the KAP140 (at least the real one) is pretty basic, and there is not a lot that could go wrong with it. With the sim version, who knows?

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Speaking of which, is there a command to reset the trim to neutral?

Whenever I’m using the AP and need to take manual control I always have to look down and adjust the trim back to the centerline.

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It has to, otherwise AP trim and the trim wheel would be out of sync.

E.g. the trim wheel indicating neutral while the AP has trimmed the AP fully nose up.

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Nono I’m not aware of any rod installation, it’s just an electric motor that turns the wheel.
Even the SOP in a suspected runaway trim situation or upset is for the pilot to grab and stop the wheel manually.

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Exactly … If the AP has adjusted the attitude, so the plane is no longer trimmed, there is force on the trim servo… and an indication on the KAP140 display, tat shows that the plane is not trimmed. Its up to the PILOT to move the trim wheel (or use electric trim) to re-trim the plane, so when the AP is turned off, the plane is in a trimmed condition, now that the AP is not applying torque with its servo motors.

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That is NOT when you have the auto trim option installed.
That is only when you have a two-axis autopilot without the option installed.

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OK, so the ASOBO KAP140 systems are assume to have the full Opts installed …
That makes more sense to me now … Thanks

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Now that’s what I call simulator fidelity! :wink:

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But there is still this “run away” condition, that appears to affect Both Pitch and ROLL, so that even if you manually trim the plane, as soon as you turn on the AP, the plane goes into a steep bank, and may pitch up or down. – and not even re-cycling the Power to the ASOBO AP, will re-init it … It permanently FUBAR, till you get a new Plane ( reload plane)

maybe just re-loading the KAP140 with reset it … should try that – once I figure out how to make it go Crazy on demand !!!