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= = = = = LNC2 landing gear hydraulic lock up problem. = = = = =
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 pump used is the OilDyne 108 series http://www.parker.com/literature/Literature%20Files/euro_cylinder/v4/108_1301-uk.pdf - see pg 9 for internals.
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) Replace both pressure switches with SPDT and rewire the gear 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.
2) Add an accumulator to each HI and LO side to absorb the temperature induced volume/pressure increase.
I began to like #2. 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.
My first proposed electrical solution can overcome the leak problem but is a more involved installation.
If anyone wants more details, feel free to email me Wolfgang@MiCom.net
Wolfgang Franke
--
Lorn H. 'Feathers' Olsen, MAA, ASMEL, ASES, Comm, Inst
DynaComm, Corp., 248-345-0500, mailto:lorn@dynacomm.us
LNC2, FB90/92, O-320-D1F, 1,635 hrs, N31161, Y47, SE Michigan
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