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I have been working on a way to close the inner gear doors when the
landing gear is extended. I have come up with a solution which I would
like to propose to the group for feedback.
The heart of the system is a solenoid operated 4-way directional control
valve that reverses the flow to the door cylinders at the correct times.
A single valve controls both doors. An example of this valve type can be
found at:
http://www.hydraforce.com/Solenoid/Sol-pdf/1-400-1.pdf
When the gear switch is set to the down position, the gear and doors
extend together. After both down switches close, the solenoid is
energized, closing the doors. When the gear switch is moved to the up
position, the valve remains energized while the doors open and the gear
retract. When both gear legs contact their up switches, the solenoid is
de-energized, and the doors close.
Key Assumption:
During the initial retract phase, the doors are moving down at the same
time the gear is moving up. Since the door cylinders are smaller
diameter, the doors will move faster than the gear. Additionally, the
doors are moving with gravity, while the gear is moving against it.
Hydraulic fluid will take the path of least resistance, feeding the
doors first, and only moving the gear when the door cylinders hit their
stops and pressure builds up.
Emergency Gear Extension:
Since the valve allows fluid flow in both directions at all times, the
dump valve should work as before.
Disadvantage:
Since the down switches are wired in series, a separate pair of switches
must be used for the gear indicator lights.
Solenoid Valve Failure:
Normal flight operation - If the solenoid fails for any reason, the
doors will not open unexpectedly at cruise speeds.
Ground operation - As soon as the master is shut off, the valve will
de-energize, allowing the doors to open, and dumping all pressure in the
system.
Alternate wiring method:
The schematic shows a second method where the solenoid is energized
during flight and de-energized once the gear is extended.
Advantage:
The standard microswitches can be utilized, with the NC pin going to the
solenoid, and the NO pin going to the green indicator light.
Solenoid Valve Failure:
Normal flight operation - Fluid flow will reverse, causing the doors to
open.
Ground operations - The doors remain closed when the master is shut off,
and system pressure is retained. (Also showcases the fact that I went to
all this effort!).
Summary:
This design eliminates the two sequence valves, and adds a single 4-way
valve plus two extra microswitches.
Method 1 prevents the doors from opening unexpectedly in flight, but
also guarantees that there is no hydraulic pressure during ground
handling operations. Is that a problem?
With method 2, a failed valve would cause the doors to open at cruise
speed or above, (although any loss of hydraulic pressure would have the
same result). What would happen in that case? Also, there is the
additional current load of the solenoid, possibly shortening battery
life should the alternator fail.
Let me know what you think,
Adam Molny
Legacy #151
Wings and H.stab attached, vertical closed
Sequenced gear doors p1.jpg
Sequenced gear doors p2.jpg
Sequenced gear doors p3.jpg
Sequenced gear doors p4.jpg
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