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-   -   737-300 Stab trim wheel revolutions -- how many (https://www.pprune.org/tech-log/303968-737-300-stab-trim-wheel-revolutions-how-many.html)

piesupper 10th Dec 2007 20:06

737-300 Stab trim wheel revolutions -- how many?
Firstly apologies to all if this is inappropriate for this forum. I'm sure the mods will shunt it accordingly.
I'm extending a 3-d cockpit model of a 737-300 for FlightGear www.flightgear.org Because we'd like maximium accuracy (we pride ourselves that this is a sim, not a game!) I thought I'd ask here.
How many complete turns of the stab trim wheels are required to go from full nose-down to full nose-up trim?
Any input will be gratefully received.
FlightGear is an Open Source collaborative simulator project freely available for download at www.flightgear.org. With nearly 10 years of development now, it is a realistic and much cheaper alternative to Microsoft FS or X-Plane. It will run on Windows, Macs and of course all flavours of Unix/Linux. Although lacking some of the "eye-candy" of the commercial competitors, we are told by those who should know that the Flight Dynamics Models are much more realistic. We are working to extend the range of aircraft and scenery that is freely available (no expensive add-ons) Input and criticism from real-life pilots is especially welcome.
Thanks folks.

piesupper 3rd Jan 2008 02:37

Shameless Bump
Shameless bump back up to the top

Nobody can give me a hint?
For now its been set to four complete revolutions to go from full nose-up to nose-down trim.
Now somebody please tell me if this is any way close to realistic.

Happy New Year

rmm 3rd Jan 2008 06:10

We have to hand cycle them through the full range of travel after a cable change. I haven't done one for sometime but you'd be looking in the range of 30 to 45 full turns.

dolly737 3rd Jan 2008 10:38

I only know the number of rev's on the -700: It's 268 (thwo-hundred-sixty-eight)!

aces low 3rd Jan 2008 10:50

The range of the trimmer depends upon whether you are using the auto trim feature or the manual handles. Full range only available in manual. I am afraid I have no idea how many revs the wheel will turn.

piesupper 5th Jan 2008 21:44

Thanks for this guys, I didn't expect to be right , I also didn't expect to be wrong by one or perhaps two orders of magnitude!
We're dealing with manual trim only at this time. The auto-trim feature may come later when someone with more autopilot modelling skillz than me looks at it.

I'll set it to 40 for the full range for now.

Sorry for the late response, its been a helluva good new year :-)

frontlefthamster 5th Jan 2008 21:50

We're dealing with manual trim only at this time. The auto-trim feature may come later
Oh dear... Someone help him out...

piesupper 5th Jan 2008 21:55

Yes - please help

The only trim I have any real experience of is winding (usually the wrong way) that second hand car window handle in the roof of the Cherokee.

So what obvious ( to you lot) blooper have I made?

frontlefthamster 5th Jan 2008 22:13

Manual trim is hardly ever used - it means getting the handles out and winding the bl**dy thing (and the handles will have your leg off if you're not careful). I have used it in testing and following failure, but I'm in the minority. :suspect:

Auto-trim is to do with ensuring that the pitch force per G meets certification requirements especially in the climb after take-off, and is not under the pilot's control at all. :cool:

Electric trim, using the pickle switch on the control wheel, is the normal trimming mode. :ok:

Either way, I'm afraid that 'how many revolutions' is firmly in anorak territory for me, but doubtless someone with time on their hands will be along. :(

Tech Log The very best in practical technical discussion on the web

Hmmm, well. :ugh:

criticism from real-life pilots is especially welcome
Glad to oblige. :zzz:

NSEU 7th Jan 2008 03:31

"So what obvious ( to you lot) blooper have I made?"

None. If the 737-300 is anything like the NG series, stabiliser trim is a complex beast indeed.

The A/P trims at a different rate to manual electric trim.

Trimming speeds vary, dependent on flap position.

There are different trim limits depending on trim mode (auto/manual) and flap position.

There is interaction between control column movement (and manual electric trim) and autostab trim operation.

There are position feedback circuits to different autopilot computers. e.g. The A/P knows how fast the trim drive motor is running. The A/P also has links to the elevator tab servo valve and the mach trim actuator.
There are messes of electrical relays (these will have to be linked to the electrical busses in proper simulations).

There is a trim sensor monitor in the A/P which, during dual autoland, "compares the Neutral Shift expected value from stabilizer position and Mach Trim feedback with the actual Neutral Shift Sensor input. If the difference is more than 0.5 degrees, the autopilot will disengage."
Etc, etc...

Then there is the matter of "Speed Trim" (controlled by the A/P computer, but only when the A/P is DISengaged). This uses CAS, Mach, Intertial Vertical Speed, Roll Angle and AOA, Radio Altimeter, N1 speeds, flap position....)

It's painfully obvious that no one has a 100% understanding of these systems (you realise this when sim programmers ask embarrassing questions :} )

Regards (and good luck with the programming).


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