Lancair 320/360 hydraulic system questions and
fixes
There have been instances of gear operation failure in the
Lancer 320/360. I have looked into this, spoken with Lorn Olsen-LNC2 owner,
Brian Anderson-product manager at OilDyne for the 108 pump,
Ross-320/360 specialist at Lancair, Christopher Zavatson-Lancair owner,
Randy Stuart-Lancair owner and believe I have the answers.
Gear operation failure is usually due to both HI/UP and LO/DN
hydraulic pressures being above their respective pressure switch settings
preventing operation of the pump. This happens because hydraulic fluid gets
trapped in the lines and with a temperature rise, pressures exceed normal
conditions.
Earlier models were 108AM/19-CLB3VT-11-08, Lancair # 637715
and later models are 108AMS32-CZZ3V-14-08 Lancair # 642666. The pump has check
valves that operate with a 1:5 pressure ratio. The ZZ in the newer pump is
still an LB circuit but with a slightly modified back pressure circuit.
Looking at the pressure port side of the pump housing, the left port is labeled
"U" and had the higher pressure output. The right hand one is labeled "D" and
has the lower pressure output. The back pressure circuit is powered by pressure
from the "D" port connected to the rod side of the cylinders as shown on
page 9 of the product literature link above.
This is problem # 1. This is backwards
from the way the 320/360 needs it ! The Lancair 320/360 requires "U"
pressure on the rod side of the cylinders to raise the gear using the higher
pressure. For the back pressure valve to operate properly, the spool valve needs
to be flipped around. This is why there have been reports of the spool being in
backwards.
With this corrected, everything works and all is right with
the world.
You can check your own system. You will need pressure gages in
the HI and LO side to do this.
Put the plane up on jacks.
Power the gear up.
With the dump valve, induce a leak from HI side to LO side.
Note the pressure on the LO side stays at zero.
Power the gear down.
With the dump valve, induce a leak from HI side to LO
side.
Note the pressure on the HI side rises a bit and then goes
back to zero.
- - - This is because the pressure has to leak back past the
pump gears before it gets to the pressure relief valve.
This is expected operation when the spool valve is in properly
and stays where it was last pressured.
Most of the time things work all right but there continue to
be reports of gear actuation failure.
Problem #2 is that vibration and pump internal leakage
can move the spool valve off it's position of all to one side or the other while
the pump is not running. When this happens, the check valves close and fluid is
trapped in both the HI and LO sides of the system. Temperature increase,
cylinder leakage and dump valve leakage can all cause both sides to
exceed pressure switch settings. (If there is leakage, the pump runs and can
building pressure in both sides) The thermostatic relief valve settings in
the pump will prevent line pressures from exceeding about 1500 psi.
So, the fix is to prevent the non-action side pressure from
increasing above it's pressure switch setting due to heat soak or leakage
and bring it down if it does.
This can be accomplished in two ways.
1 ) 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.
- - - I consider this solution really just an engineering
exercise.
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 until 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.
2 ) Electric only = = = This will address actuation
failure in either direction. Rewire the system so the pump will run in the
direction set by the gear switch even though both pressure switches are open
(high pressure). This will build up pressure
inside the pump causing the shuttle to move to one side and open the other
side's check valve releasing the undesired pressure.
I have designed a circuit board that has 2
relays and can be wired into the pressure switches of the existing system.
It will be about 2" x 2" x 1". If I get at least 5 requests, I will make a batch
of units for those interested.
The electric only solution can overcome the
leak problem and is my desired installation.
Wolfgang Franke
-- Lorn H. 'Feathers' Olsen, MAA, ASMEL, ASES, Comm, Inst LNC2, FB90/92, O-320-D1F, 1,635 hrs, N31161, Y47, SE Michigan
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