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Bert Pieke

Autopilot - IAS mode

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Does anyone have a stable IAS climb or descent? Mine oscillates, swinging between 0 fpm and 1000 fpm..

Not sure what I could be doing wrong.. looking for suggestions ;)

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I have the same effect but after some time it stabilize more or less.

I have another problem with the autopilot. If I engage the autopilot with "HDG" and "ALT" selected it does not hold the altitude. It seems it holds an arbitrary climb or sink rate or maybe pitch angle.

I must deselect ALT and select it again then it works.

It seems the preselected ALT mode is not recognized if the autopilot is switched on.

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Bert, I have had the same issues with mine too. I think I saw a comment from Finn somewhere that there might be a bug he is going to work on.

Nachtmond, I also noticed that about the HDG & ALT that it seems not to hold the altitude very well but goes to some climb or sink rate (I have not found a way to control that yet). I just assumed I was expecting too much from the A/P. I will try your idea about deselect ALT and then back ON again and see if that helps me. Thanks for that.

Jerry

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The oscillation occurs because, for IAS, the AP is using FSX raw pitch up/down control instead of a true PID. Also the complete AP gauge is processed 4 times per second, which I believe don't have the same precision as the standard 18 times per second (18HZ).

Tom

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Which makes the IAS mode pretty much unusable... luckily, the Pitch mode works better :)

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If used properly, then it's definitly not unusefull.

Like the real thing You must trim the elevator to obtain the wanted speed and then engage IAS mode.

MS flight simulator has always given people the false impression that the AP makes an aircraft fly like running on rails. The same physics applies to the AP as to a pilot controlling it.

But let's see if we can tweak it a bit more.

Finn

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If used properly, then it's definitly not unusefull.

Like the real thing You must trim the elevator to obtain the wanted speed and then engage IAS mode.

MS flight simulator has always given people the false impression that the AP makes an aircraft fly like running on rails. The same physics applies to the AP as to a pilot controlling it.

But let's see if we can tweak it a bit more.

Finn

Finn,

The oscillation has nothing to do with trimming before engaging. It happens because a pitch_inc, pitch_dec is used as a trim control, which applies fixed amount of pitch on each call, instead of using a PID controller like in the real thing. Plus the fact that the logical code is executed 4 times/sec instead of the standard 18 times/sec.

And because autopilots use PID controllers, they are -generally speaking- more efficient than pilots when it comes to keeping an aircraft in a safe and comfortable flight envelope. Without an autopilot, for example, wouldn't be possible to make an aircraft fly smooth and steady at 35,000 ft in a long haul trip.

Tom

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Finn,

The oscillation has nothing to do with trimming before engaging. It happens because a pitch_inc, pitch_dec is used as a trim control, which applies fixed amount of pitch on each call, instead of using a PID controller like in the real thing. Plus the fact that the logical code is executed 4 times/sec instead of the standard 18 times/sec.

And because autopilots use PID controllers, they are -generally speaking- more efficient than pilots when it comes to keeping an aircraft in a safe and comfortable flight envelope. Without an autopilot, for example, wouldn't be possible to make an aircraft fly smooth and steady at 35,000 ft in a long haul trip.

Tom

I had a terrible time yesterday trying to trim the plane manually. It is very sensitive using ther FSX trim. If there is any way to assign thru FSUIPC the PID from the plane itself instead to use FSX one ? Also, using IAS mode for altitude changes is difficult at time even with the plane trimmed. Thanks.. Carlos

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Unfortunately FSUIPC won't make things better in this case. It's a programming code issue.

The best solution would be to write a PID routine, which is not difficult per se but it may take some time to obtain the proper figures.

OTOH, I don't think it is working so bad as to need an urgent update.

Tom

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I had a terrible time yesterday trying to trim the plane manually. It is very sensitive using ther FSX trim. If there is any way to assign thru FSUIPC the PID from the plane itself instead to use FSX one ? Also, using IAS mode for altitude changes is difficult at time even with the plane trimmed. Thanks.. Carlos

I can't help wondering if there's something unusual about your settings or controllers. When you say you find the trim sensitive, I assume you mean that you find small changes to the trim result in large changes to the VS? If this is happening to you, something is certainly wrong (or very different). The trim on the Twotter is so fine that I need multiple clicks on the hardware trim button to see much change in VS.

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I can't help wondering if there's something unusual about your settings or controllers. When you say you find the trim sensitive, I assume you mean that you find small changes to the trim result in large changes to the VS? If this is happening to you, something is certainly wrong (or very different). The trim on the Twotter is so fine that I need multiple clicks on the hardware trim button to see much change in VS.

Indeed I am surprised by these comments as I simply can not recreate. Very smooth trim, often need 5 or 6 clicks to even start moving.

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Ditto.

Must be something outside Twox causing this, assuming it's not a loadout issue moving the CG aft.

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Indeed I am surprised by these comments as I simply can not recreate. Very smooth trim, often need 5 or 6 clicks to even start moving.

The main issue is not the trim per se but to descend and ascend using the IAS and subsequent Alt hold.

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I can confirm the IAS hold seems to "chase" the speed with too much trim input. Even if I establish a steady climb at 800fpm hand trimmed, once I hit the IAS the trim starts hunting creating large undulations both on the + and - side of the VSI.

Othe than that, I have 35hours and counting. Ahhh speaking of which, is there a HOBBS meter on the otter?

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In the next update the AP has become a makeover.

IAS should be alot better as well as quite a few enhancements.

There is no Hobbsmeter, since we don´t simulate accumulating wear.

Finn

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Okay, right, I'm totally confused now as to what you guys are doing. There's a lot of mention of trim, and pitch up/down, but no mention of power (throttles) to make the AP work. Surely if you trim the aircraft down at say -800fpm with a speed of say 140kts and hit IAS mode on the AP WITHOUT altering the power, the AP will NOT be able maintain 140kts AND -800fpm? This plane hasn't got auto throttle like a 737 or Avro RJ.

The AP can control IAS and, in a round about way, pitch, but it CANNOT control the power correct? So without YOU controlling the power, the AP is trying to maintain speed and pitch with the existing power output. A bit like a car going up hill, if you don't press the accelerator the car will slow down, or down hill, if you don't lift off the accelerator, or brake, the car will go faster.

Surely it's a balancing act of power to maintain EITHER the IAS or the pitch? And as this aircraft is a turboprop, using the power isn't quite as precise as say a piston engined plane, slight adjustments in the levers will cause "oscillations" in either pitch or speed due to the lag, yes?

I think the AP can control ONE of these options at a time, but not both. I think people are expecting too much of this particular type of autopilot.

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Please wait...

Update 1.02 is just around the corner where we have fixed quite a few bugs on the autopilot.

IAS mode has become a true PID controller and works alot better now (thanks to Joshua from the Airbus Extended project).

But I really cannot emphasise enough the importance of reading the manual (will be changed slightly for 1.02) inorder to understand the modes as well as how the FD bars are working.

Finn

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Here is the text for the 1.02 manual:

COLLINS AP-106 AUTOPILOT

The Twin Otter Extended comes with a Collins AP-106 autopilot and FD-112V Flight Director. The ADI contains the Flight director Bars which indicates required flight path to obtain the desired autopilot functions.

When the Autopilot is engaged but no lateral mode selected, called "Manual" mode. In this mode the Left/Right acting knob on the center of the Yoke will set the desired bank angle.

When the Autopilot is engaged but no vertical mode selected, called "Manual" mode. In this mode the thumbwheel in the center of the Left/Right knob on the center of the Yoke will set desired pitch.

The following lateral modes can be set on the AP-106 autopilot panel:

  • HDG: Sets desired heading to follow.
  • NAV: Sets the autopilot to follow either the radial of a VOR station or will track the localizer of an ILS. The VOR or ILS frequency must be tuned on the GPS NAV radio and the radial /front course must be set on the HSI.

On the GPS pressing the CDI button You can select whether NAV should foloow a tuned VOR station or a GPS flightplan. VLOC means that the AP will follow a VOR or Localizer and GPS means that the AP will follow the flight plan loaded in the GPS. The GPS will indicate what course the HSI should be set to whenever the GPS flight plan changes to the next waypoint. "NAV ARM" and "NAV CAP" lights will indicate whether the AP has armed or has captured the desired radial or localizer.

  • APPR: Approach mode will arm and track both localizer and glideslope of an ILS once they are captured. "NAV ARM" and "NAV CAP" as well as "GS ARM" and "GS CAP" lights will indicate whether the AP has armed or has captured the desired localizer and glideslope.
  • B/C: Will make the AP follow the back course of a localizer.

The following vertical modes can be selected on the AP-106 autopilot panel:

  • ALT: Will hold the altitude present when the ALT button was pressed.
  • IAS: Will use pitch to hold the indicated airspeed present when the IAS button was pressed. Increasing power will increase rate of climb, while decreasing power will decrease rate of climb. IAS mode can be used for both climb and descend.

The "TRIM IN MOTION" light will lit whenever the elevator trim is altered by the AP. There are also two lights "TRIM UP" and "TRIM DN" on the main AP panel.

FD-112V FLIGHT DIRECTOR

The Flight director will indicate the computed commands from the autopilot through two bars on the ADI.

The vertical bar indicate required roll inorder to obtain the selected lateral mode. The horizontal bar will indicate required pitch to obtain the selected vertical mode.

The FD turns on if either the autopilot has been tengaged or a FD mode has been selected (HDG,NAV, B/C,APPR,ALT or IAS).

Basicly the FD bars will give the pilot cues on how to fly the aircraft the way the AP would have done inorder to get to the set lateral and vertical mode i.e if the vertical bar is displaced to the left, it means the pilot should roll the aircraft further left. If the horizontal bar is displaced above the aircraft symbol then the pilot shoul increase pitch.

Perfectly flown the horizontal and vertical bar should form a cross at the middle of the aircraft symbol in the ADI.

Note that in IAS mode the horizontal bar indicates whether You fly too slow or too fast rather than required pitch.

5506L Altitude Alerter from United Instruments

The AP-106 is also connected to United Instruments 5506L altitude alerter. The alerter can be set to a desired altitude and will give an audible warning when within 1000 ft of the set value. A light will be lit when between 1000 and 200 ft from the set value. There are 3 buttons and lights associated with the altitude alerter and AP.

  • MDA: Is used during a coupled ILS approach, where the altitude alerter, after having captured both localizer and glideslope, should be set to the minimum descend altitude and the MDA button pressed. Once the aircraft descends to the set MDA, the AP will level out and maintain the set altitude in order to prevent from descending below MDA. For landing, once the runway is insight and you are cleared for landing, the AP should be turned off.
  • G/A: The Go around button is used when executing a Go Around. Pressing the G/A button will cancel all active Flight director modes (HDG,NAV,B/C,APPR,ALT,IAS) and set pitch a 8° climb and roll to wings level.
  • ALT ALRT: Will automatically level out the aircraft once it has reached the set altitude when the AP is in either "Manual" or IAS mode.

When NAV mode is used for tracking a VOR radial and the signals disappears when near vertical to the station, then "DED REC" mode will be indicated for "Deduced reckoning". In this mode the AP memorizes the heading present when the signal was lost and will maintain it until past the station where the signal will be received again.

Note that using the autopilot to aquire an exact altitude, as set on the Altitude Alerter, the ALT button should not be used. The ALT button simply levels the aircraft at the altitude present when the ALT button was pressed.

To aquire a selected altitude on the Altitude Alerter use either IAS mode or Autopilot manual vertical mode (Autopilot on, but with IAS,ALT and APPR unselected).

Finn

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Thanks Finn. I have yet to update from the initial release. Thanks for the response. I am having a blast with the new otter. Well done guys! Very satisfied customer.

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