= = = = = LNC2 landing gear hydraulic lock up problem. Type 1
& Type 2= = = = =
This problem has been lamented on for quite a while by my
friend Lorn Olsen and recently I finally got into the details
involved.
It appears that when the system is heat soaked, both sides end
up at pressures above the pressure switch settings and consequently the pump can
not run in either direction rendering the gear cycle inoperable. This can happen
after a long cruise at very low temperatures and a descent to warmer conditions.
The only thing left at that point is to open the dump valve equalizing the HI
and LO side pressures so gravity and springs can lower the gear. Additionally,
this could prevent gear retraction after takeoff if the pressures are still
high.
The Lancair part number is 6422666 and the OilDyne number is
108AMS32-CZZ-3V-14-08
I have talked with the project manager of the 108 series at
OilDyne and found that the only non standard part is
the ZZ which indicates a modified LB circuit where the back pressure valve
has been modified to be bi-directional and it's restrictor has been removed. The
check valves are standard and still have a 1:5 actuation pressure
ratio.
Heat soak causes the hydraulic volume to increase noticeably
increasing the line pressure. This pressure will build up because the check
valves in the pump prevent any fluid flow back into the pump when there is no
power to the pump. There is a shuttle valve in the pump that opens the
non-action side check valve to open by pressure from the action side but
that pressure is from the pump side of the check valve. This shuttle
valve is spring loaded to center so without the pump running, neither check
valve is open.
A heat soak of as little of 40 degrees can cause this
problem.
So, the fix is to prevent the non-action side pressure from
increasing above the pressure switch setting due to heat soak and / or bring it
down if it does.
This can be accomplished in two ways.
1 ) Electric only = = = Rewire the system so the pump will run
action side even though it's pressure switch is satisfied AND the
non-action side switch is also satisfied (and it shouldn't be). This will build
up pressure upstream (pump side) of the action side check valve causing
the shuttle to move off center and the non-action side check valve to
open releasing the non-action side pressure.
I have designed a circuit board that has 4 relays that can be
wired into the system using the existing switches and wiring. The pressure
switches, gear switch and pump solenoids will be connected to it. It will be
about 3" x 4" x 1". If I get at least 5 requests, I will make a batch of units
for those interested.
2 ) Hydraulic only = = = Add an accumulator to each HI and LO
side to absorb the temperature induced volume/pressure increase.
I initially thought 5 cu.in. units (Parker #
AD007B25T9A1) with a precharge of about 100-200 psi would work even though they
were larger than needed.
Upon further examination, I considered a length of flexible
(expandable) hose added to the system since that would be much easier to
install. I just needed to figure out the required length.
After rounding up expansion numbers from AeroQuip on their 303-5 and 303-8
hose and other details;
303-5 ID=1/4"
303-5 hose expansion = 0.094cc / in at 1000
psi
303-5 hose expansion = 0.123cc / in at 2000
psi
303-5 hose expansion = 0.149cc / in at 3000
psi
303-8 ID = 13/32"
303-8 hose expansion = 0.100cc / in at 600
psi
303-8 hose expansion = 0.137cc / in at 1250
psi
303-8 hose expansion = 0.171cc / in at 2000
psi
Hydraulic fluid volume in the LNC2 is about 27 cu
in ; 6.4 cu in in the
lines, ; 20.6 cu in the actuators with door
actuators
Hydraulic fluid thermal expansion coefficient = .0005 /
deg.F
Aluminum thermal expansion coefficient = .0000123 / deg.F
(negligible for this purpose)
Modulus of elasticity of hydraulic fluid = 250,000
(compression under pressure - psi per percent reduction in volume -
negligible for this purpose)
I calculated that about 8ft of 303-5 with AN4 fittings
would provide the desired volume expansion from heat soak.
Fluid in the non-action side of the system = 3.2 cu
in.
8ft of 303-5 line = 4.71 cu in
Fluid in the non-action side of the system with added
line = 7.9 cu in (ignoring volume of removed line segment)
Extrapolated volume increase in the added line at 300
psi = 0.065cc / in
Volume increase = 0.065 / 2.54^3 x 96 = 0.381 cu
in
Delta T allowance = 0.381 / (7.9 x 0.0005) = 96.5 degF with
300 psi increase
Replace an existing piece of line with a 3 loop coil
under each seat pan, one plumbed into the HI side and the other plumbed into the
LO side and bleed the system.
There are other details but these are the basics.
These are initial calculations and have not yet been
tested.
CAUTION
If there is a leak in a cylinder or the dump valve, there will
still be a problem that can render the gear inoperative leaving
only emergency dump valve use.
The action side can leak into the non-action side,
the pump will turn on to bring the action side pressure back up, the leak will
continue untill both sides are above the pressure switch setting. Should heat
soak come into play after that, the pressures can continue up to 1500 psi,
the thermostatic relief valve setting of the pump.
The first proposed electrical only solution can overcome
the leak problem but is a bit more involved installation.
Wolfgang Franke