The "airplane-type" design is basically a "solid pendulum."  Although 
the period can be made
very long, the sensitivity becomes correspondingly low.  I don't 
recommend this design.

Cheers,
John

At 02:44 PM 10/27/2007, you wrote:
>In a message dated 27/10/2007 22:23:56 GMT Daylight Time, 
>tchannel1@............ writes:
>Hi Folks, Sometime in the past I saw a web site describing different 
>types of pendulums used in sensors.  I don't remember their names, but
>
>1.  one was a long vertical rod, with a center pivot, it contained a 
>mass at the top of the rod and a mass at the bottom of the rod.
>
>My question has to do with 1.  I thought this pendulum (positioned 
>like an airplane propeller) was able to adjust it's period by moving 
>the top and or bottom mass, up or down the rod?
>I made a simply mock up of this illustration, but was not successful 
>in any way. Can one adjust this arrangement to result in an extended 
>period? Does anyone know of this web site or one similar?
>
>Hi Ted,
>
>     There are several possible systems, but you need to suspend the 
> pendulum near the middle, a small height above the centre of 
> gravity. This gives a high moment of inertia due to the masses at 
> the ends and the small restoring force due to 2x mass operating 
> just below the suspension axis. It should work fine. You can also 
> vary the position of one of the masses.
>
>     Regards,
>
>     Chris Chapman


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