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Autopilot problem in Twin Otter


nuke

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Hi,

I am quite often having an annoying problem getting the autopilot (A/P) in the Twin Otter v.1.11 to maintain the correct vertical speed (V/S) during climbout and before the final approach.

This is what happens:

- At ground I turn on the A/P and enter e.g. 6000 ft as commanded altitude and 1000 ft as V/S. I then turn off the A/P using the ON/OFF button.

- I handfly out with a clean configuration at 90 knots and approx. 1000 ft V/S.

- At 1000 ft AGL I turn on the A/P and select HDG and PUSH ALT.

- The A/P will not be able to maintain the commanded V/S of 1000 ft. The airplane levels out and sometimes goes into a dive before it start to climb again.

- After the level off/dive the V/S will be +500 ft no matter what I set as desired V/S in the A/P panel.

- During approach the A/P will occasionaly not be able hold my altitude (fly level). I will get a V/S of -100 to -300 ft.

- If I turn off the A/P I have also noticed that I cannot trim the airplane to fly level when this happens, i.e. there is no more trim available. I then have to constantly pull the joystick a little backwards to maintain the altitude.

Other times this problem does not occur. I have no idea why this happens and how to fix it. I have tried to remove and re-install the package. Any ideas?

Regards,

Knut

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Trim the aircraft before engaging the a/p.

If you are not in trim - and flap changes DO change the trim on this aircraft significantly (as do power changes) - then as the FS autopilot erroneously uses the elevator trim to control vertical velocity upon engagement the aircraft first has to adjust the out-of-trim before setting the correct vertical speed.

If you are running out of trim then this is a pure and simple indication that your CG is configured incorrectly, or that your speed is incorrect for the current control settings, payload and fuel. Correct the fault using the payload manager, setting the fuel so that CG is within limits, adjust aircraft configuration so that speed and flap settings are suitable or add thrust. Porpoising on approach is either a sign of low airspeed or some serious convection activity, for which the a/p is not designed, so check weather for turbulence and gusts, ensure you are landing into wind or within the acceptable crosswind component, or disengage the a/p and hand-fly the approach.

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Thank you, Snave,

for pinpointing the cause of the problem. It truly is a CG problem. I just loaded up the Twotter putting pax/cargo in the aft of the aircraft, and I now have a decent CG and no trim/auto pilot problems :lol:

However, I find this strange. If I load the aircraft distributing pax/cargo/fuel evenly throughout the aircraft using the Twotter Load app, I get these CG problems, i.e. CG too much forward.

In addition, how can I check that my CG is within the limits when using Twotter Load?

Regards,

Knut

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Not in front of my FSX at the moment, but you can check the CG profile from within the sim itself by simply going to the loadout menu and seeing where the `x` lies. That's the same for all FS aircraft.

I'm no Twotter expert IRL but if I recall correctly there is a requirement to load the aircraft carefully there too, so this might be said to be `as real as it gets`!

http://www.diverdriver.com/Jump_Aircraft/Twin_Otter/twin_otter.htm

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Not in front of my FSX at the moment, but you can check the CG profile from within the sim itself by simply going to the loadout menu and seeing where the `x` lies. That's the same for all FS aircraft.

I'm no Twotter expert IRL but if I recall correctly there is a requirement to load the aircraft carefully there too, so this might be said to be `as real as it gets`!

Oops, I've been flying MSFS since v.5.0, but forgot about that CG diagram in the loadout screen. Thanks again, Snave.

Yes, these airplane are sensitive to where you put the weight. I talked to my brother today regarding this. He is flying the DH-8 IRL. He said that they often cover up the first rows in the plane to force pax more aft if the airplane is not going to be fully loaded.

Rgds, Knut

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  • 2 weeks later...

I found the reason for my weight & balance problems in the Twotter. The aircraft.cfg file is FUBAR! Brobably as a result of the TwotterLoad program!

This is from the aircraft.cfg for the Aerosoft Twin Otter 100 Wheels, which is not flyable, i.e. CG too much forward:

[WEIGHT_AND_BALANCE]

max_gross_weight = 11600

empty_weight = 6500

reference_datum_position = 0, 0, 0

empty_weight_CG_position = 0.009, 0, 0

max_number_of_stations = 24

station_load.0 =170, 9.0, -1.4, -3.0

station_load.1 =145, 9.0, 1.4, -3.0

station_load.2 =150, 15.0, 0.0, -4.0 //300 lb maximum

station_load.3 =170, 15.0, 0.0, -4.0 //300 lb maximum

station_load.4 =170, 15.0, 0.0, -4.0 //300 lb maximum

station_load.5 =145, 15.0, 0.0, -4.0 //300 lb maximum

station_load.6 =170, 15.0, 0.0, -4.0 //300 lb maximum

station_load.7 =145, 15.0, 0.0, -4.0 //300 lb maximum

station_load.8 =0, 15.0, 0.0, -4.0 //300 lb maximum

station_load.9 =0, 15.0, 0.0, -4.0 //300 lb maximum

station_load.10 =170, 15.0, 0.0, -4.0 //300 lb maximum

station_load.11 =170, 15.0, 0.0, -4.0 //300 lb maximum

station_load.12 =170, 15.0, 0.0, -4.0 //300 lb maximum

station_load.13 =170, 15.0, 0.0, -4.0 //300 lb maximum

station_load.14 =0, 15.0, 0.0, -4.0 //300 lb maximum

station_load.15 =145, 15.0, 0.0, -4.0 //300 lb maximum

station_load.16 =0, 15.0, 0.0, -4.0 //300 lb maximum

station_load.17 =170, 15.0, 0.0, -4.0 //300 lb maximum

station_load.18 =80, 15.0, 0.0, -4.0 //300 lb maximum

station_load.19 =170, 15.0, 0.0, -4.0 //300 lb maximum

station_load.20 =0, 15.0, 0.0, -4.0 //300 lb maximum

station_load.21 =170, 15.0, 0.0, -4.0 //300 lb maximum

station_load.22 =170, 15.0, 0.0, -4.0 //300 lb maximum

station_load.23 =500, -13.0, 0, -3.0 //500 lb maximum

As you can see, all pax stations have the same position reference as station load 2 (front baggage) (..., 15.0, 0.0, -4.0), with the result that all pax and cargo is loaded into the position of the forward baggage compartment... I suspect that TwotterLoad has messed up this file.

This is from the aircraft.cfg of the Aerosoft Twin Otter 300 modern, which I have not modified with TwotterLoad:

station_load.0 =170, 9.0, -1.4, -3.0

station_load.1 =170, 9.0, 1.4, -3.0

station_load.2 =200, 15.0, 0.0, -4.0 //300 lb maximum

station_load.3 =0, 15.0, 0.0, -4.0 //300 lb maximum

station_load.4 =0, 5.2, -1.8, -3.0

station_load.5 =0, 5.2, 0.6, -3.0

station_load.6 =170, 5.2, 1.8, -3.0

station_load.7 =145, 2.8, -1.8, -3.0 //250 lb maximum

station_load.8 =170, 2.8, 0.6, -3.0 //250 lb maximum

station_load.9 =0, 2.8, 1.8, -3.0 //250 lb maximum

station_load.10 =170, 0.6, -1.8, -3.0 //250 lb maximum

station_load.11 =80, 0.6, 0.6, -3.0 //250 lb maximum

station_load.12 =170, 0.6, 1.8, -3.0 //250 lb maximum

station_load.13 =145, -1.9, -1.8, -3.0 //250 lb maximum

station_load.14 =145, -1.9, 0.6, -3.0 //250 lb maximum

station_load.15 =0, -1.9, 1.8, -3.0 //250 lb maximum

station_load.16 =0, -4.2, -1.8, -3.0 //250 lb maximum

station_load.17 =0, -4.2, 0.6, -3.0 //250 lb maximum

station_load.18 =0, -4.2, 1.8, -3.0 //250 lb maximum

station_load.19 =0, -6.5, -1.8, -3.0 //250 lb maximum

station_load.20 =0, -6.5, 0.6, -3.0 //250 lb maximum

station_load.21 =0, -6.5, 1.8, -3.0 //250 lb maximum

station_load.22 =0, -10.2, -1.3, -3.0 //250 lb maximum

station_load.23 =300, -13.0, 0, -3.0 //500 lb maximum

Anybody else that have experienced this problem, and is there a solution?

Regards,

Knut

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Guest Hodge001

Hi Knut

Purely out of interest I decided to try out the Twin Otter Load Manager on all the versions installed (after making backups just in case things went pear shaped), and I have to say I can not repeat what you are seeing on your system, I changed the loadouts on all the variants and then checked that the aircraft,cfg file had indeed been accessed by checking the date and time stamp, and then opened the file and found that the station point locations remained exactly as they were before running the load manager, the weight had changed in line with my adjustments, but not the station locations

Just a though, but are you running any other programs that could possibly add to or subtract from the initial weight of the aircraft, I was thinking of maybe something like FSPassemgers or a similar program that MAY be modifying the weight distribution of the Twin Otter, which could possibly cause an issue like the one you are experiencing, as I say it is just a though, and it is nothing against FSPassengers, which I actually purchased last week but have yet to try it out, as I need to find out exactly what if anything it does change in the aircraft,cfg file before using it on the payware aircraft I have installed.

Not much help I know, but that is what I found when I ran the test on my system.

Jim Hodkinson

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Same as Jim, changed all model loadouts, one at a time, and got modified .cfg as expected in every case.

Perhaps, as I so often mention, this might be a good point to tell us your system specs and what addons you have in FSX, and where it is installed...

Not sure this is a conflict, it could simply be improper operation of the OS. Depending...

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Hi, Jim and Snave,

Thank you for showing interest in my strange problem.

I am not using any programs that could have changed the weight & balance part of the cfg file. I used to use FSP in FS9 (it's great fun!), but have not tried it for FSX and the Twotter.

I re-installed the Twotter yesterday and got fresh aircraft.cfg files. Today I tried the TwotterLoad on the 100 Wheels model, and this is what happened:

Before TwotterLoad:

station_load.0 =170, 9.0, -1.4, -3.0

station_load.1 =170, 9.0, 1.4, -3.0

station_load.2 =200, 15.0, 0.0, -4.0 //300 lb maximum

station_load.3 =0, 15.0, 0.0, -4.0 //300 lb maximum

station_load.4 =0, 5.2, -1.8, -3.0

station_load.5 =0, 5.2, 0.6, -3.0

After TwotterLoad:

station_load.0 =170, 9.0, -1.4, -3.0

station_load.1 =170, 9.0, 1.4, -3.0

station_load.2 =10, 15.0, 0.0, -4.0 //300 lb maximum

station_load.3 =0, 15.0, 0.0, -4.0 //300 lb maximum

station_load.4 =0, 15.0, 0.0, -4.0 //300 lb maximum

station_load.5 =0, 5.2, -1.8, -3.0

I am showing the first 6 stations as the rest of the stations did not change. As you can see, station 4 just inherited the position of station 2 or 3. It seems that different station positions are changed to 15.0, 0.0, -4.0 in an unknown pattern every time I use TwotterLoad. As you saw from my previous post all pax stations had this position in the old cfg file.

I am running Windows 7 64-bit version, Norwegian language. I have FSX with the Acceleration Expansion Pack, i.e. with SP2. FSX is installed in C:\FSX\. The Twotter is v. 1.11. I have one freeware add-on to the Twotter: Widerøe textures for the 300 modern (DHC6_Wideroe_BFJ.exe). The exe file installs textures for the 300 modern, but you have to change the aircraft.cfg manually.

Regards,

Knut

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Guest Hodge001

Hi Knut

Now this is getting a little more than odd, you have to take into account that I have never actually used the Twin Otter Load Manager before yesterday as I was quite happy with using the FSX loading screen to set up Twin Otter, mainly because when I got the Twin it was the only way to do it as there was no Load Manager with the initial release, but after using the load manager today to see what my aircraft.cfg file returned using a new loadout I now find that my station positions are misplaced in the same manner as yours, so it would seem that you may have a point regarding a possible problem with the Load Manager, as to will the Load Manager be revisited by the designer, to be honest I would imagine that an issue like this would not be at the top of a list of things to do at this stage of the products cycle, and at the end of the day it is not really a show stopper, but this is only my opinion, yours may and probably does differ

For myself I have returned my aircraft.cfg files back to the original load stations and will continue to load the Twin Otter as I have always done, but at least you know that somebody else can now get similar results to what you have.

Jim Hodkinson

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Jim,

I'm using Win XP SP3 32-bit. Are you using the same OS as Knut? I wonder whether this could be another one of those `permissions` issues that seem to afflict write-backs to FSX? Interesting though, that the obvious cause is not the case as there is a known problem with FSX when it's installed in the default Program Files folder in Vista and Win 7, but Knut has his installed in a direct folder, as do I.

If not, then I'm at a loss as to the cause, as clearly something is being written back to the .cfg, just not to the right location.

IIRC the Load Manager was written by Hans Hartmann who is still active in the community so it might be a quick fix.

Meantime, when I get some time at the weekend I'll run the LM through each of the aircraft variants and see if I can get a similar result.

Final thought: Knut, by any chance have YOU modified the load through the default FS loadout configurator? I know I haven't...

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Guest Hodge001

Hi

That is what has me more than a little confused, I am running Vista 64Bit with UAC turned off since I installed Vista, also with full read / write permissions granted to anybody who cares to use my system, in addition my FSX installation is on a dedicated drive outside any programs folder the full path is..

D:/Microsoft Games/Microsoft Flight Simulator X/

This drive is not even scanned by my security software.

The load stations of the Twin Otters SEEM to be moved back one row, I say seems as a I have found the first load station for each row of seating, as in seat row 2A and so on has that first seat location transferred from the preceding row, so seat row 2A’s fore and aft location is the same as the row 1B and 1C fore and aft location after running the Twin Otters Load Manager.

The main thing that has me confused is why this did not show in the aircraft.cfg file after the first run, what stopped the error from being introduced then rather than on a later run of the Load Manager.

Man you have to love computers.

Jim Hodkinson

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Yeah, it's a strange one for sure.

Let me check my XP/dedicated FS hard drive installation this evening, with a few sundry changes across all the different aircraft and see if there is a common error.

Like you, I don't see this as a `permission` issue, it makes no sense that it would allow one change, but not another. It may be a code error that no-one has picked up before, but it has to repeatable to be one of those.

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Hi again,

Yes, this is really an odd problem. 20 pax in the front baggage compartment must be hard to trim...

I never used the FS loadout screen to change pax/cargo before, but for now I think that I have to scrap the TwotterLoad. I may contact Hans Hartmann to see if he can look into this, but I do not think Aerosoft will release a patch for this.

I also found another bug: If you enter 0 % fuel in the front tank (left tank in the FS loadout screen) the AFT fuel meter will show 0 % in the Twotter.

Regards,

Knut

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Nuke,

...that last isn't a bug, it's a developer choice. The problem with offsets for fuel load in FSX is that lateral-offset effects are drastically exaggerated - I fly 172's and even with only me in the plane and a substantial tankful in `my` wing tank, considerably less in the offside and no aileron trim, the difference is easily manageable. Try that with ANY plane in FS and you're inviting a crash, much less the 172!

Some developers get round the problem by placing all fuel tanks on the centreline, with no deviation from the centreline when seen in Plan View (from above, looking down)but you can also simply borrow the `fore` and `aft` tanks and just call them what you want - FS doesn't care what the names are! So even though the Twotter fuel tank says it's the `left` it isn't to Flight Sim. And that way the offcentre trim trap is avoided.

Hope that's clear?

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Hi,

I was a little unclear in my previous post. My point was that it seems that the aft fuel qty gauge shows the content of the forward tank (left tank in FS fuel screen), while the front fuel qty gauge shows the content of the aft tank. I concluded this from the tank position references in the aircraft.cfg:

[FUEL]

leftmain= 3.3, 0, -6.5, 184.063 , 1 //-0.5, 0, -2.5, 184.063 , 1

rightmain= -4.5, 0, -6.5, 201.463, 1 //-6.2, 0, -2.5, 201.463 , 1

center1= -2, -29.4, 4, 0.00 , 0

center2= -2, 29.4, 4, 0.00 , 0

This is not of importance as both tanks should be approx. evenly filled.

I found a set of "best practice" tips for loading the Twin Otter in another forum (http://www.pprune.org/archive/index.php/t-188742.html):

1) Put two pilots and their kit in the two pilot seats, assume a maximum weight of 400 pounds for all that.

2) Split the baggage compartment loads equally between the nose and the rear compartment until the nose compartment reaches its limit (typically about 275 pounds if you have radar installed, 300 pounds if no radar), then put the rest of the load in the rear compartment.

3) Assume normal human behaviour of the passengers as follows:

a ) they will occupy all the window seats first, before they begin to sit three abreast.

b ) they will distribute themselves evenly forward and aft of row 3/4.

Regards,

Knut

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Knut, I think you have hit on something here.

I reinstalled the Twotter 1.11 ASC, and after using the Twotterload utility I am now seeing the same as you - co-incident station loads, where in the default loadouts the configuration settings seem to show a physically correct relationship between the the Stations and their relative positions.

I guess I must have run Twotterload before so didn't notice the change. Which at least confirms that this is a consistent, persistent change. I wonder whether it might be deliberate?

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  • Aerosoft

Knut, I think you have hit on something here.

I reinstalled the Twotter 1.11 ASC, and after using the Twotterload utility I am now seeing the same as you - co-incident station loads, where in the default loadouts the configuration settings seem to show a physically correct relationship between the the Stations and their relative positions.

I guess I must have run Twotterload before so didn't notice the change. Which at least confirms that this is a consistent, persistent change. I wonder whether it might be deliberate?

I'll have this checked out next week guys, indeed something weird going on.

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I'll have this checked out next week guys, indeed something weird going on.

This is great, Mathijs! To be honest, I find the task of using different weight loadouts in the plane a vital part of my simming experience. Sometimes I fly a ferry flight (empty) and other times the plane is fully loaded. Or anywhere inbetween. Makes a big difference.

I made a spreadsheet that gives me random number of pax between 0 and 19, random baggage weight per pax (15 - 21 kg) and random other cargo (limited to 800 lbs - pax baggage). And then the TwotterLoad comes in very handy so that I don't have to do the loading in the FS default screen.

So I really hope you can fix this and also the fuel gauge issue.

Regards,

Knut

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  • 2 weeks later...

Hi,

I have a related question on the Autopilot I hope you can answer.

I'm not too familiar with it in the Twotter and don't know how to program it. Can I set all the parameters from with in the VC?

I hope you can help me out and provide me with a quick tutorial on the Twin Otter AP to get me going.

Thanks in advance, any help is much appreciated :-)

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Guest Hodge001

Hi Richard, and welcome to the forums.

It depends on which variant of the Twin Otter as to how much navigational information can be accomplished from within the cockpit, if you had the modern variant in mind then yes with the KLN 90B navigational equipment you can plan out your entire trip, if however you are talking about what for a better description is the vanilla variant, and possibly the most common as far as navigational equipment goes, then your flight planning will have to be made pre-flight as this variant is equipped with the basic Maule autopilot and the DME gauge, along with the Course, Heading and Glide Slope facilities of the HSI, as is the modern variant.

As for the usage of the KLN90B, to post a how to description or a tutorial for this unit would take up a lot of forum space (and involve more typing than I am used to), and as it would only duplicate what is included in the Twin Otter manual anyway, it would be far better to go through the included manual, you do not have to try and take in all the functions of the unit at once, just take it one function at a time if you wish, and then try what you have gleaned from it in the sim itself, and then go back to the manual and take on another function.

Turning to the Maule autopilot unit, well this is a pretty basic but serviceable unit that is perfectly suited to the Twin Otter, and as for using the functions, if you have used any other autopilot in the sim then you will not have any problems with the Maule unit, as it does what it says on the can so to speak.

I realise that this is probably not the type of reply that you were after, but I hope some of the above is of some use.

Jim Hodkinson

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Jim, he was asking about the autopilot, not the nav system... you've probably gone a bit too far there!

Richard, as Jim says it's the default Maule autopilot. Full operating instructions are included in the Learning Center of FSX, which is why they are not repeated in the Twotter manual.

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Guest Hodge001

Hi Simon

Sorry about that, it is not the first time that it has been said that I have gone to far, ask my Ex-Wife.

Jim Hodkinson

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Hi Simon

Sorry about that, it is not the first time that it has been said that I have gone to far, ask my Ex-Wife.

Jim Hodkinson

:groan_s: :band_s::D

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