Welcome, Guest. Please login or register.
Did you miss your activation email?

News:
Precision Simulator update 10.132 (14 February 2021) is now available.
Aerowinx NG FMC and More is now released.

Author Topic: Rudder and elevator trim during power failure  (Read 434 times)

Blake H

  • Join date: May 2011
  • Posts: 559
    • http://www.747flightsensation.com.au
Rudder and elevator trim during power failure
« on: Wed, 27 Jan 2021 06:42 »
Question about the Rudder and Elevator trim indicator during a power failure.

If you have a total power failure in Aerowinx, why does the rudder and elevator go to the OFF position? It's a 28 DC motor. It should fail in its current indication state? There is no spring to pull it into the OFF position of the tape.

Am I missing something?

Hardy Heinlin

  • Moderator
  • Join date: May 2009
  • Posts: 12310
    • Aerowinx
Re: Rudder and elevator trim during power failure
« Reply #1 on: Wed, 27 Jan 2021 07:05 »
You mean the rudder and stab trim? There's no elevator trim on the 747.

Yes, there are springs on the indicator tapes.* It snaps back to the OFF flag like any other vital instrument so that you see the indication is invalid.

An invalid trim indication doesn't necessarily mean that the trim control has failed. Indication is one thing. Control is the other. E.g. when all engine indications are blank, it doesn't mean all engines are out.


Regards,

|-|ardy


* After power loss, capacitors provide some current to the motor to drive the tape back to the OFF position.
« Last edit: Thu, 28 Jan 2021 01:21 by Hardy Heinlin »

farrokh747

  • Join date: May 2009
  • Location: Mumbai, India
  • Posts: 919
    • http://www.farrokhchothia.com/
Re: Rudder and elevator trim during power failure
« Reply #2 on: Wed, 27 Jan 2021 07:47 »
the stab and rudder trim indicator system/s are powered with 26 V AC, 400 Hz via a synchro/xmtr/recvr loop between the sensors on the surfaces/actuators  - As hardy says, the tapes in the indicators are spring loaded to snap to OFF when unpowered....

The Rudder trim actuator has a built in synchro xmtr that drives the rudder trim ind tape

I believe there are similar synchro xmtrs attached to the stab for the stab trim tape

cheers,

fc




Blake H

  • Join date: May 2011
  • Posts: 559
    • http://www.747flightsensation.com.au
Re: Rudder and elevator trim during power failure
« Reply #3 on: Wed, 27 Jan 2021 10:40 »
Interesting. Seems like the 737 Rudder trim indicator doesn't have a spring to OFF. I wonder why?

Roddez

  • Join date: Jul 2009
  • Location: Sydney
  • Posts: 134
    • http://www.simulatorsolutions.com.au
Re: Rudder and elevator trim during power failure
« Reply #4 on: Wed, 27 Jan 2021 22:14 »
Just to clarify here, there are no springs involved, just some very cool electronics and capacitors.

When the Stab or Rudder Trim indicators loose power, the circuits sense this loss and drive the tape to the OFF position.  During our testing, we have managed to remove power in such a way as to leave the indicator tapes in random positions.

Cheers,

Rod.

Hardy Heinlin

  • Moderator
  • Join date: May 2009
  • Posts: 12310
    • Aerowinx
Re: Rudder and elevator trim during power failure
« Reply #5 on: Thu, 28 Jan 2021 01:17 »
Capacitors? Cool indeed.

The engineering books just say the tape returns to the OFF position when power is lost.


Cheers,

|-|ardy


P.S.: I thought it incorporated a spring because I noticed, when the tape has hit the OFF stop, it oscillated a couple of times back and forth. If this remaining vibration comes from the servo, maybe the oscillation is the effect of an electric coil-capacitor oscillation? :-) (The oscillation is also modelled in PSX, but you need a high and stable frame rate to see it. It's very fast and short.)
« Last edit: Thu, 28 Jan 2021 02:20 by Hardy Heinlin »

Jeroen Hoppenbrouwers

  • Moderator
  • Join date: May 2009
  • Location: KTMB
  • Posts: 4166
  • Hoppie designs avionics equipment for airliners.
    • http://www.hoppie.nl/
Re: Rudder and elevator trim during power failure
« Reply #6 on: Thu, 28 Jan 2021 10:06 »
Aircraft electronics must by DO-160 survive a certain time without power or with bad power. In practice you want to double that time, given the uggeley things that happen during engine start. We ended up incorporating big-ass supercaps in the 1.0 F range and special circuitry to keep the thing humming along for seconds after a power cut and then commit clean suicide instead of losing half its circuitry and then the power comes back so you are left with inop boxes...


Hoppie

Hardy Heinlin

  • Moderator
  • Join date: May 2009
  • Posts: 12310
    • Aerowinx
Re: Rudder and elevator trim during power failure
« Reply #7 on: Thu, 28 Jan 2021 10:14 »
(It's certainly a fact that the servo drives the tape back with the power from the capacitors. Nevertheless, there may be a flexible mechanism incorporated just to dampen the impact of the tape at the outer edge. Maybe that's the cause of that oscillation.)

farrokh747

  • Join date: May 2009
  • Location: Mumbai, India
  • Posts: 919
    • http://www.farrokhchothia.com/
Re: Rudder and elevator trim during power failure
« Reply #8 on: Thu, 28 Jan 2021 14:34 »
Just to clarify here, there are no springs involved, just some very cool electronics and capacitors.

When the Stab or Rudder Trim indicators loose power, the circuits sense this loss and drive the tape to the OFF position.  During our testing, we have managed to remove power in such a way as to leave the indicator tapes in random positions.

Cheers,

Rod.

thanks rod - i assumed its a mechanical thing..... 

fc