lml@lancaironline.net Mailing List Archive

Nice job with the Legacy canopy seal pump / pressure switch Leighton. I changed mine too. Like you, I found the factory supplied pressure switch was set at around 20 psi....way too high for the Legacy canopy. I reset it to around 4 psi but then found it was not stable in flight. The pump would oscillate on and off rapidly plus it would change pressure dependent on temperature in the location where it's mounted. When set that low, the pressure switch seems to not have enough dead band (hysteresis) so it continuously cycles on and off. I ended up discarding the factory pressure switch and designed a new circuit using an inexpensive silicon differential pressure sensor from Freescale Semiconductor along with a comparator circuit to provide about a 30% dead band around 4 psi.

I had fun with the circuit but the important point here is that the factory pressure switch is 1) set way too high and 2) seem to not work well when reset lower (may be unique to my switch on a sample of one). Perhaps some of you have seen the Lancair Legacy brochure with Carl Lewis' N14CL in flight (Josh Burngardt flying) with the moon in the background. If you look closely at the canopy, it seems to be pushed up in the front showing a gap. I think this gap may be caused by 20 psi in the canopy seal. Just a guess but high pressure in the seal may stress the Legacy canopy latch / hinge mechanism.

John Richardson

Legacy N16DX

----- Original Message -----

From: marv@lancair.net

To: lml@lancaironline.net

Sent: Friday, January 27, 2012 4:59 AM

Subject: [LML] Re: Canopy seal pump

Posted for "Leighton Mangels" <leighton@teleport.com>:

All the seal pump pressure switches I've seen come set at 20psi, which is way
too high.  I reset them on the bench to about 6 psi which is more than
adequate.  We use a small 3- way solenoid valve in lieu of the brass toggle
dump valve.  If the pump switch is turned off the seal then automatically
deflates.  If the master is turned off it deflates. On the Legacys we put a
small micro switch at the bottom of the inside canopy latch lever so that if
the canopy is unlatched the seal automatically deflates.  In other words, if
the power is turned off to the pump the seal deflates. That way there's no way
to open or close the canopy with the seal inflated.  And it eliminates the
toggle valve from the panel, or wherever you choose to put it.  I'll attach a
pic of the pump assembly with all the components I use.  The part with the
yellow top is the solenoid valve.

Leighton Mangels
leighton@teleport.com
503-647-5779

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