How does a vibrator work in an altimeter??

[SuperTed]

How does a vibrator work in an altimeter to prevent stiction? I know that static pressure can somehow 'stick' to the capsule but how exactly does it stick, and how much of an error can it produce? What kind of altimeter is such a devise found in, a servo or sensitive alt?

Thanks,

ST.

[ShyTorque]

SuperTed,

It has got nothng to do with static pressure or the capsule.

A vibrator in an altimeter is there to overcome stiction / friction within the mechanism, i.e. cogs and a suitable system of levers and pulleys etc between the capsule and the indicating needles.

Without it an altimeter needle may lag / jump and operate in "steps". A vibrator just smooths out the movement of the needle. Note that it isn't often necessary to fit one to helicopters or piston engine aircraft that vibrate through other means.

[SuperTed]

Oh right, Thanks ShyTorque!

How come then a vibrator isn't fitted to other instruments that contain gears/cogs to reduce friction and lag?

[IMA Conehead]

Super ted
It really depends on the type of instrument and the method it employs to achieve the desired result.
The altimeter uses pure reaction of a sealed capsule to changes in static pressure surrounding it, this is coupled via sector gear / cog / lever arrangments and the lengths of the levers are altered for calibration purposes. Also coupled to the system is a datum shift to account for baro correction. As you can understand all this wizardry attached to a capsule will cause a degree of load which will be smoothed by the use of a vibrator.
Other instruments use a pedulous weight to assist the capsules reaction ie. IVSI., or they may employ more dynamic means as in an eddy current drag cup which is used in tachometer type instruments.

The variety is endless but the requirements will generally be dictated by the type of sensing element.


Regards Conehead

[ 13 January 2002: Message edited by: IMA Conehead ]</p>

[ShyTorque]

SuperTed,

I suppose cost comes into it somewhere as well but a sticking altimeter in a descent is the one that is going to allow you to fly into the ground....That's why it's more important for it to work smoothly and accurately without lag.

[RatherBeFlying]

In gliders, the vibrator is replaced by tapping with a finger -- a good thing to do before deciding to do that last circle before heading back to the field. Much easier than having to setup your very first outlanding at 500 feet. :)

[Shore Guy]

In one of my first (in person) introductions to the great dry Britsh wit, I was in Sim 1, Day 1 on the BA-146 at Hatfield. Instructor in the back, using his pointer to go over all the instruments, switches, indicators (retired BA fellow as I recall).

He pointed at a red flag in the standby attitude indicator, and said "It's time to get out the flight attendants emergency checklist - it seems our vibrator is inoperative".

[Shore Guy]

Ooops, meant to say standby altimeter in previous post....

In reference to the original post, I believe that there are not vibrators on servoed altimeters, but there are one or more failure flags (CADC, power). Vibrators on (straight) barometric altimeters to aleviate the previously listed friction/lag problems