On Thu, 26 Feb 2004 12:34:18 +0000 (UTC),
(Paul Tomblin) wrote:

big snip

Actually, reading that again, it's possible that what is happening isn't
that it comes on at a lower speed if you have power on, but that the
propellor slipstream holds the gear up against the springs if you have
power on.

little snip

A crude "pitot" head (located within the propeller slipstream)
provides the combined airspeed/prop stream pressure input to the
"super high tech auto extension device"-which consists of a crude
diaphram balanced against a pair of ground adjustable springs attached
to a parking brake valve plumbed between the "up" and "down" hydraulic
lines.

The diaphram sees lower airspeed + higher slipstream the same as it
sees higher airspeed + lower slipstream. If the combined "sense"
pressure is higher than the spring pressure, the gear stay up. If the
sense pressure drops below the spring pressure, the mechanism trips,
and the park brake valve opens, allowing hydraulic fluid trapped in
the "up" line to flow into the "down" line. The mains fall out via
gravity, the nose does the same with a spring assist.

As others have mentioned, the trapped "up" pressure is what holds the
gear in the wells, there are no mechanical up-locks. There are
strictly mechanical spring-loaded down-locks that hold the drag/side
braces in an over-center position. With normal or emergency extension,
these mechanical downlocks are all that is holding the gear down,
there is no "down" pressure in the system with the gear
down-and-locked. The switches that turn the green lights on (and turn
the electro-hydraulic pump off) are driven directly off of these
mechanical down-locks.

Turning the auto extension "off" mechanically pins the trip linkage
into the park brake valve closed position, irrespective of "sensed"
diaphram pressure.

The SB to remove the auto extension feature removes the diaphram from
the loop, and the valve stays in the closed position during normal
operation. The lever that useta be used for auto extension, is now
used to mechanically open the valve to allow emergency extension if
desired.

The really, really neat part is adjusting the balance springs standing
on your head in the back of the airplane while the test pilot
repeatedly accelerates and decelerates (airspeed pressure), and varies
power setting (prop stream pressure) until you get the durn thing
adjusted so it works...

Hope some of this helps;

TC

P.S. the "smallest" hydraulic gear airplane that I am aware of with a
totally independent back-up emergency extension system is the Cheyenne
II XL. It has a secondary set of pneumatic actuators that un-up-lock
and assist/extend the gear pneumatically using 4 cute little CO2
bottles (Mr. Laird very likely has a similiar bottle)