Air is good if your breathing and bad if
leaking into your brain. OK, I'll go out and rebuild one this am and
report back. Oh, but wait, I guess the rebuild of the leaker will have
an affect on our test.
Larry,
I love spirited discussions leading to one's spirit sipping spirits
during contemplation of the metaphysical. I thought more about all
the problems reported over the years and the possible effect of
air. Let's see if we can work thru this by picking the system
apart. But first, I said you should only introduce air into the system
as a test of your theory, not rebuild a cylinder.
Air in cylinders
4 of the six cylinders, those associated with the main gears and doors,
can be eliminated because they are oriented horizontally with all ports
pointed upward so the air will not remain in those actuators.
The nose gear door cylinder, vertically oriented cannot hold much air in
the bottom after extension because the air would have to fit in a donut
cylinder volume 5/32 high with the inside 1/2" diameter and the outside
5/8 diameter - a volume of .017 cubic inches. This space is inside of
the extension piston stop that allows fluid to be passed in and out of the
cylinder. Upon retraction, even though the piston does not tightly reach
the top, the fluid exit is at the top and air would be expelled. The
.017 ci of air could be trapped at the piston that is pushed in by the hi
pressure retraction activity and the bubble could rise to the bottom of the
piston.
The nose gear cylinder is somewhat vertically oriented and the cylinder
cannot hold any air in the bottom beyond the piston at full extension
since it is stopped tightly against the forward bulkhead (if properly
rigged). However, retraction is limited by the external stop and the
fluid exit is at the aft lower side of the cylinder. I forgot to check
how vertically oriented the cylinder gets on retraction, but let's say the
whole remaining space can retain air. That is a cylinder
approximately 3/4" high with a diameter of 9/8" for a volume of about .75
cubic inches. Of course, at retraction this pocket would not be under
pressure since it is the other side of the piston that is under
pressure. So, it is on extension that some of this pocket could be
compressed although it would seem that some of the air would come out at each
retraction.
Air in lines
There also are short pieces of Easton flexible line that feeds into
1/4" hard Al lines. Let's discuss how much volume these lines can
hold. For the Easton line (I could not find a sample), assume it has an
interior cross section of 1/4". That is 20.37 inches of tube
per 1 cubic inch of fluid volume. The 1/4" hard lines have a wall
thickness of .035, thus the interior diameter is .18" or 39.3 inches of tube
per cubic inch of volume.
Door actuators move about .9 ci of fluid in when extended and .7 ci in
when retracted. The nose gear cylinder (I forgot to measure the length,
say 4.5" of movement) moves about 4.5 ci of fluid in when extended and 4
ci in when retracted.
Since the opening of doors requires fluid passage thru sequence valves,
those lines might not be completely cleared because of the hard line length
may hold more fluid than is transferred to and from the actuator. One
could consider a small bubble remaining in those lines.
The retraction side moves more fluid thru shorter lines so it is less likely
that air is trapped in those lines. The large actuators move much
more fluid and it is hard to believe any air remaining in those lines.
Air Elsewhere
The dump valve body can't hold air once it has been opened.
Perhaps the teed vertical lines leading to the pressure switches retain
air.
Gas Laws
For those that suspect that heated trapped air raises the system pressure
quite a bit should note that when the volume is held constant, the pressure
increases as the temperature ratio, in Kelvin, increases. A rise
from 50F to 80F is 299K/283K or approximately a 5% increase - Hardly
accounting for a rise of several hundred psi. I await an engineer to
evaluate effects of the differing expansion factors for aluminum, Easton
lines, cylinders and the fluid itself.
On the converse, if the air is compressed (so far this looks like a
problem that is possible solely on the down side at the nose gear
actuator), I would finally have to agree that more volume of fluid under
pressure from the air would have to pass through the system
leak to activate the pressure switch than that from incompressible fluid on
its own. Maybe. That these air pockets may work themselves out so
that the chirp rate increases is interesting.
So I can see where an accumulator could delay pump activation. No
problem as long as other parts of the system are working properly. It is
hard to see where any air bubbles could influence the up side of the system
other than those trapped in the sequence valve circuits.
----
As to some of the other contentions.................
You said: "You're an electrics guy. Does that little chirp every 20
seconds not bother you just a little? You know, arcing and corrosion in
the relays, airborne fire, alternator shorting, etc?"
You are correct, the wiring is correct. Thus, arcing and corrosion
are not a problem - intermittent relays are operating intermittently
. Airborne fire and alternator shorting are not a problem from the pump
blips for the small percentage of block time that the gear is down,
nor is anything else associated with the hydro-electric gear
system. However, "etc." does indeed bother me, although it is not
related to this situation.
One of the risks you may add is that the accumulator on the high pressure
side may lock up the low pressure side if you have a failure like that
sometimes described by other Lancairites and not yet fully understood.
This is the case where opening the dump valve does not resolve the peculiarly
built system pressure present on both sides and that trips both
pressure switches. There may be an electrical solution -
perhaps bring duplicate wires from the pressure switches forward to
secret over ride switches operable by the captain. Forced activation of
the pump may free the lockup if it is the case of a stuck shuttle
valve. Of course the captain's hearing has to be good enough
to perform an "undo" if the pump is straining as the captain becomes the
acting pressure switch.
Good Luck,
Scott
Krueger AKA Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL
(KARR)
Darwinian culling phrase: Watch
This!