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n4gix

Runaway trim when AP is selected.

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Hi,Does anyone know why the trim runs away all the way to + or - 81 as soon as you press AP?I want to set runway heading before take off and activate AP and Heading. In all aircraft the trim runs away as soon as you press the AP button. Searching on the net gives me the idea that this is normal? in FS9.Does anyone recognise this and is there a cure?Yes! I started FS9 clean. Default Cesnna on Meigs!Roelof

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Greetings Roelof,Autopilots don't work on the ground. They try to climb and roll the aircraft, but it just won't roll and climb because the wheels are firmly planted on terra firma. The AP gets angry. The AP applies more and more aileron trim, and more and more elevator trim, until they are maximised or minimised, but the darn aircraft still won't roll and climb.FS9 is a flight simulator not a game. Consequently consumers must learn how to use the systems it simulates realistically. The avionics systems in FS9 don't have a video game cheat mode.Both in real life and in FS9 autopilots differ in their complexity and operating capabilities. Some developers install the wrong kind in the wrong cockpit environment but the consumer must learn how each type works and operate it accordingly as soon as they encounter each type. Limited function APs that do not allow predial can be identified by dialling a value and then switching the AP OFF with the avionics master switch ON. Then switch autopilot ON again. If the predialled selection is not retained the AP does not accept predial for that mode. Some APs will accept predial for all modes, some for some modes, and some for no modes at all. The consumer must learn to use all three types when they first encounter them.If consumers believe the aircraft whose operation they are simulating should have an AP that allows predial of specific target values the consumer can substitute the name of an AP gauge with those attributes in the panel.cfg, in place of the one installed by the developer. Of course installation of an AP lacking those capabilities may have been entirely correct.The problem is also compounded by developers who assign non zero default climb rates for use as soon as an AP or VSI mode is engaged. All real autopilots have a default climb/descent rate of zero. Microsoft have fixed the bug that rejected this value and FS9 can accept the real world value of zero.So the rules are;IF a) The AP is a limited function AP that does not allow pre dialling for a particular mode AND:( The consumer wishes to retain bogus AP default rates associated with that mode for use by the AI engine during airport simulation THEN c) the consumer must calculate or obtain from the documentation the V speed and VSI required to achieve that V speed for any phase of flight, before invoking AP for use in *any mode during climb* including HDG hold/seek modes.ELSEd) In the aircraft.cfg edit the bogus VSI default rate to match the real life value which is always zero.Be that as it may what has been described is also commonly the simple consequence of pilot error.Whether or not the AP being simulated accepts predial for a given mode the consumer should never turn the autopilot ON until;1) after they are airborne2) in the correct post take off gear, slat, flap and cowl flap configuration for the current phase of flight3) trimmed for the target V speed for the current phase of flight4) close to achieving the target V speed for the current phase of flight4) have looked up or calculated and have entered, or are ready to enter, into the AP window the VSI or PITCH required to deliver that target V speed for the current phase of flight.Just turning an autopilot ON before any of the above are true may cause the trim to run up or down to a value that prohibits the aircraft from achieving the target V speed. Turning an ALT or VSI mode ON as well almost certainly will.Whether trim maximises depends on how badly the flight dynamics replicate reality and how out of trim and badly configured and diverging from the V speed target cumulative pilot error caused the aircraft to be when the AP (or predialled mode) was turned on.

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>Greetings Roelof,>>Autopilots don't work on the ground. They try to climb and>roll the aircraft, but it just won't roll and climb because>the wheels are firmly planted on terra firma. The AP gets>angry. The AP applies more and more aileron trim, and more and>more elevator trim, until they are maximised or minimised, but>the darn aircraft still won't roll and climb.Thanks. Very comprhensive! I figured something like that.I was just wondering what people are doing for auto take off etc.Thanks a lot.Roelof

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>Thanks. Very comprhensive! I figured something like that.>I was just wondering what people are doing for auto take off>etc.They don't, because there is no such thing as "auto take off"! ;)

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