REFLECTOR: Back-up gear switch, Implement at your own risk!

[cgputney at peoplepc.com]

Back-up gear switch

Most of you that fly RG aircraft know the old axiom "there are those who have and those who will, land with the gear up" Not wanting to join that group during some distraction, I have emulated the original Piper system on my Velocity. Simulated flights on the ground show that it works. Flight tests to follow.

The implementation is quite simple:

1) Remove original squat switch.

2) Replace it with an adjustable differential pressure switch (Dwyer 1910-5, 1.4-5.5 W.C. $35, one pound, for example. See their web site.)

3) Attach squat switch wires to "common" and "normally open" terminals to retain the original squat switch function. Be sure original wire from gear toggle switch "up" terminal goes to "common" on the differential pressure switch.

4) Add wire from gear toggle switch "down" terminal to "normally open" terminal on differential pressure switch.

5) Plumb pitot pressure tube (formerly going to squat switch) to high pressure fitting on differential pressure switch and add a static pressure tube going to low pressure fitting.

6) I calibrated the trip airspeed by disconnecting the line going to the pitot tube and extending it with 5 feet of poly tubing. Originally I used a bucket of water (3.16 in.=80mph) but found success by watching the airspeed indicator and pressurizing carefully by mouth. First I adjusted the trip point to approximately 85 mph, with electrical power off, using a continuity checker across the switch. Full ground tests followed with AC up on wing and nose jacks.

Results to date:

1)With gear switch down and AS< 85mph, gear is down.

2) With gear switch up and AS< 85mph, gear is down.(squat switch function)

3) With gear switch down and AS >85mph, gear is down.

4) With gear switch up and AS < 85mph, gear is down (new back-up function)

5) With gear switch up and AS > 85mph, gear is up

These switches have a "dead band" wherein trip points with increasing AS may be 85mph, but decreasing AS trip points may be 6 mph lower.

Mount the switch vertically with elastomeric isolators to minimize effects of turbulence.

As you can see, the gear switch can be used as before because this system is transparent.

Conversely, leaving the gear switch up results in an automatic mode!

Manipulating the circuit breaker near the gear switch allows one to control when the gear comes down.

Implement at your own risk. Piper apparently had problems, both technically and with liability. One Piper pilot sued because he failed to reach an intended airport when his engine failed, due to gear down drag.
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