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Some questions about the Otter

Controlling your aircraft, using the autopilot etc.

Some questions about the Otter

Postby MIG29pilot » Thu Apr 09, 2020 12:35 am

Hello everyone!
I have been flying the Twin Otter about recently, and I had two questions. First, where are the fuel switches? There is a step in the checklist and tutorials for setting them to normal, but it doesn't show where they are, and I can't find any switch that registers as them having been flipped. Secondly, where can I find information on using the autopilot system and the GPS unit?
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Re: Some questions about the Otter

Postby tdammers » Thu Apr 09, 2020 7:05 am

You're looking for the rightmost levers in the overhead throttle quadrant. From left to right, the levers in that section are throttle (2x), prop pitch (2x), fuel (2x). You also need the fuel pumps running to start the engines.

The autopilot is quite rudimentary, and should be mostly self-explanatory, but here goes.

There are 2 separate controllers: a vertical one, and a lateral one. Each has its own master switch (the bigger "ALT" and "HDG" buttons), which has to be "on" for it to do anything at all, and on top of that, the autopilot master ("AP") also has to be "on". Both controllers have several modes.

For the vertical controller:

- No submode selected means maintain current altitude
- ALT climbs/descends to the altitude selected in the altitude selector dial at a fixed climb/descent rate of 500 fpm
- VS holds the selected vertical speed
- SPD holds the selected airspeed ("speed with pitch" - there is no autothrottle in the DHC6)
- GS arms the ILS glideslope capture; the previously selected vertical mode remains active until the glideslope is captured

Note that, unlike the autopilot systems in a typical airliner, the VS and SPD modes will not disengage and level off when the target altitude is reached, so for example setting target altitude to 7000 ft, target speed to 100 KIAS, and hitting the "SPD" button, will make the aircraft climb through 7000 ft and further.

For the lateral controller:

- No submodes selected means keep wings level
- HDG captures and holds the heading selected with the heading bug
- NAV arms VOR/LOC capture; the previously selected mode remains active until the radial/localizer is captured, after which the NAV mode activates and the previous mode disengages

The fun part about this autopilot is that you can engage/disengage lateral and vertical modes independently, so you can for example enable vertical A/P, but hand-control lateral movements (bank/turn). This is great for sightseeing or flying low in mountainous terrain.

The GPS unit isn't great; it emulates a Garmin 196, with the added twist that, unlike the real thing, the GPS unit can override the NAV1 localizer signal; you can use this feature to make the DHC6 follow a GPS flight plan. Here's how that goes:

- Enter a route into the route manager (Autopilot/Route Manager) and activate it.
- In the GPS menu (Equipment / GPS), select "Leg" mode, and check the "Slave NAV1" box. The NAV1 instrument will now follow the route manager's GPS-based signal rather than the VOR/LOC radial from NAV1.
- Line up, align the heading bug with the runway heading, take off, stabilize, turn autopilot on. Set ALT mode to climb to your cleared altitude, HDG mode to HDG, then arm NAV. If you're not on a suitable heading to intercept the fake NAV1 signal, adjust the heading bug accordingly. Once the autopilot captures the fake radial, the aircraft will keep following the flight plan as long as NAV mode remains active.
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Re: Some questions about the Otter

Postby D-ECHO » Thu Apr 09, 2020 7:20 am

@tdammers: Great write-up, especially about the AP, would you mind adding this to http://wiki.flightgear.org/De_Havilland ... e_aircraft ? ;)
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Re: Some questions about the Otter

Postby tdammers » Thu Apr 09, 2020 8:34 am

Don't have an account, and not going to make one over plain HTTP. Wiki doesn't seem to work over HTTPS.
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Re: Some questions about the Otter

Postby D-ECHO » Thu Apr 09, 2020 8:49 am

May I add your text then? will of course mention you in the summary
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Re: Some questions about the Otter

Postby tdammers » Thu Apr 09, 2020 10:41 am

Sure, go ahead.
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Re: Some questions about the Otter

Postby SurferTim » Thu Apr 09, 2020 10:46 am

That is a great article! One correction,
In the ALT mode, it climbs at up to 1200 fpm, seemingly throttle setting dependent.
Has a glitch when engaging. It tries to get a zero rate of climb initially, causing some wild gyrations if you are climbing at a high power setting and rate of climb.
I get level or near zero rate of climb before engaging it.
Fly low, fly slow, land on a dime. Twin Otter DHC6-300+. https://github.com/SurferTim/dhc6p
PirateAir videos at https://www.youtube.com/user/SurferTim850
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Re: Some questions about the Otter

Postby D-ECHO » Thu Apr 09, 2020 12:10 pm

Done :) Thank you very much!
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Re: Some questions about the Otter

Postby tdammers » Thu Apr 09, 2020 12:55 pm

SurferTim wrote in Thu Apr 09, 2020 10:46 am:That is a great article! One correction,
In the ALT mode, it climbs at up to 1200 fpm, seemingly throttle setting dependent.
Has a glitch when engaging. It tries to get a zero rate of climb initially, causing some wild gyrations if you are climbing at a high power setting and rate of climb.
I get level or near zero rate of climb before engaging it.


I have seen that in your 300+ model, but not the stock one.
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Re: Some questions about the Otter

Postby SurferTim » Thu Apr 09, 2020 2:09 pm

I don't mean to disagree, but I see it in both if YASIM. That I did not edit until I found how to. I had to tame it down a bit in mine.

Edit: If yours climbs at 500fpm in ALT mode, we are using two different autopilots.
I did not start with the FGAddon version. I started with the aircraft hangar version here:
http://mirrors.ibiblio.org/flightgear/ftp/Aircraft/
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PirateAir videos at https://www.youtube.com/user/SurferTim850
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Re: Some questions about the Otter

Postby WoodSTokk » Thu Apr 09, 2020 3:51 pm

SurferTim wrote in Thu Apr 09, 2020 2:09 pm:I did not start with the FGAddon version. I started with the aircraft hangar version here:
http://mirrors.ibiblio.org/flightgear/ftp/Aircraft/


That should be the same. There is a script running that check the FGAddon-SVN (I think hourly).
If there are changes inside an aircraft, it pulls the aircraft and pack it into a zip-file and store it on the hangar.
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Re: Some questions about the Otter

Postby legoboyvdlp » Thu Apr 09, 2020 9:11 pm

Sorry, that link is from October 2018 - the one that is synced is this one:
http://mirrors.ibiblio.org/flightgear/f ... aft-trunk/

However it looks like there were no changes after that date in the dhc6 until your version was unloaded, so it should be fine :)
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Re: Some questions about the Otter

Postby MIG29pilot » Fri Apr 10, 2020 4:13 am

@tdammers, thanks for the help!
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Re: Some questions about the Otter

Postby AzulProfundo » Mon May 11, 2020 2:41 pm

Hi,

There is a knob between the two fuel pumps switches that must be set to "Normal" during startup. Is this some kind of cross feed between the tanks (if turned to the left or right positions)? When the airplane is loaded, there is some imbalance between the tanks...

Another thing: the checklist asks for a "full throttle" during takeoff. However, when doing that, it is easy to get warnings for excessive rpm and speed for the flaps, a few seconds after takeoff. So, as soon as I takeoff, should I reduce throttle a bit and retract the flaps to zero? Or perhaps I am doing a too shallow climb for the Twin Otter (around 1500 fpm, but it can handle 2500 fpm...)
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Re: Some questions about the Otter

Postby SurferTim » Sat May 16, 2020 9:56 am

I use the "fuel truck" to balance the tanks. Check your fuel. The fuel switch determines which tank is being used. I start with 840 lbs in each tank.

I don't use full throttle for takeoff at sea level on my version. I use about 50 psi torque (about 7/8 throttle) for takeoff, but as soon as I get a positive rate of climb, I throttle back to 40 psi and flaps up. That 50 psi may throw a copilot message on the stock version.

Edit: Actually, I just realized the fuel levels should probably be saved between sessions. If you want it fueled, fuel it. The "fuel truck" makes it entertaining and easy.
Fly low, fly slow, land on a dime. Twin Otter DHC6-300+. https://github.com/SurferTim/dhc6p
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