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From: Lorn H Olsen <lorn@dynacomm.us>
Subject: Re: Lancair gear hydraulic fix

= = = = = 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