Rotary knob for heater control?

mburch · #1 11-23-2008, 06:30 PM

Hi all,

Instead of using the standard Bowden cable to open and close the heat box valve, I have been daydreaming about cobbling together an automotive-style rotary heat valve control knob... similar to the example shown in this photo of a Cirrus panel.

Problem is, I have no idea how Cirrus does this. It would be super easy to whip up a circuit to drive a hefty RC airplane servo based on a potentiometer position, but I am reluctant to go with an all-electric solution (what do you if there's a fire and you can't close the hole in the firewall because your little servo is melted or without power?). I know in automobiles they typically use vacuum for this application, but that's obviously out in my all-electric airplane. It seems like there ought to be somebody out there who makes a purely manual rotary-to-linear mechanism, but an evening of googling has not turned one up. I suppose that with enough effort I could throw together some kind of rack-and-pinion thingy, but I'm hoping that one already exists for me to just buy. Anybody have any ideas?

thanks,
mcb

AME at Large · #2 11-23-2008, 07:27 PM

Have no idea how Cirrus does it either but what you are describing does exist and it's been around since the early sixties!!
The device you are looking for uses a Bowden cable and has a kind of gear drive to it behind the panel. It does take up a bit more space behind the panel than a push/pull type cable control though. The knob on the front can be any knob that will fit and is secured with a setscrew.
If I recall it's a Cessna 320 cockpit heat control. Checked Google for it and only came up with a few cockpit photos on Airliners.net. It's the knob to the right of the flap handle just above the push/pull heat and defrost controls. I recall removing one to put in an electric rheostat type control as a mod for the heat control by C and D Associates. The C and D website does not have the part number of the original control only the electric replacement.
So it is possible to do it the way you want a little searching for the parts is required. That is half the fun of homebuilding, taking an idea and making it work.
Good luck.
Phil

flyeyes · #3 11-23-2008, 07:33 PM

Quote:

Originally Posted by mburch

Hi all,

Problem is, I have no idea how Cirrus does this. (snip) Anybody have any ideas?

thanks,
mcb

Does this help?

JHines · #4 11-23-2008, 07:48 PM

Actually, for automotive applications at least some of the knob-type controls use a direct mechanical connection to a cable. For example, the "temperature" (heater valve) vs. the mode selector doors. My first check would be the auto parts stores. Gotta be WAY cheaper than an "aircraft" version.

This is the only thing I could find on short notice, it's for a '91 Toyota Celica GT but you get the point. A quick trip to the junkyard should get you what you want.

http://www.toyotanation.com/forum/sh...d.php?t=261412

mburch · #5 11-23-2008, 07:51 PM

Quote:

Originally Posted by flyeyes

Does this help?

Oh, very interesting... just a bellcrank attached to the cable, eh? Hmm...

thanks,
mcb

Aerosport1 · #6 11-23-2008, 08:05 PM

Try this page. I was looking for the same type cable.

www.rustrepair.com
parts #54-059

Geoff

kevinh · #7 11-23-2008, 08:48 PM

Quote:

Originally Posted by Aerosport1

www.rustrepair.com
parts #54-059

Wow! That part looks awesome!

AME at Large · #8 11-23-2008, 09:05 PM

Yep that auto part # 54-059 looks pretty darn close to the control I was referring to earlier. You got to wonder where Cessna got them from.
As you can see it will take up some space behind the panel.
Good luck however you do it.
Phil

mburch · #9 11-23-2008, 09:23 PM

Wow, no kidding! The one Geoff posted is cheap enough that I might call and order one tomorrow just to see how it works. Very interesting...

mcb

rwhittier · #10 11-23-2008, 10:44 PM

Quote:

Originally Posted by mburch

Wow, no kidding! The one Geoff posted is cheap enough that I might call and order one tomorrow just to see how it works. Very interesting...

mcb

That Cirrus heater knob is not all that good. Sure, it looks good, but its hard to modulate. Full on or full off is easy, anything in the middle is hard to set and it will creep from there. Its a fairly common complaint.

I like that auto part # 54-059 - it looks like just the ticket...