stall warning lift switch

[Steve Gruber]

hello fellow flyers, i recently discovered that my stall lift switch wasn't working. we took the switch apart and were able to get it to work somewhat but the travel range to activate the internal switch wasn't consistent. we found the part for anywhere from $500-$1700..amazing.....either way my mechanic had a used one that we installed and seems to be working. my questions are...
1. there is a travel range for installation bases on slotted mounting holes in the switch which allows the location on the lift switch to travel up and down before you tighten the screws setting the position, does this position matter or affect the stall speed activation of the switch?
2. this switch on the leading edge of the wing is not a sealed system and will allow water inside the wing when flying in rain, is this really the design and is it ok?

thanks for any replies.
steve

[Tom Veatch]

Yes, the vertical position of the switch affects the speed (actually angle of attack) at which the switch is actuated.

The airflow stagnation point on the leading edge of the wing marks the point at which the flow separates. Air above the stagnation point goes over the top of the wing and air below the stagnation point goes over the bottom of the wing. The switch trips (moves upward) when the stagnation point moves below the switch and air moving up over the top of the wing lifts the switch. As angle of attack increases, the stagnation point moves down. At a particular angle of attack, the wing stalls. At the stalling angle of attack, the stagnation point is at a particular location on the wing leading edge. The stall warning switch should be slightly above that point.

If the switch is too high, the stagnation point will be below the switch and actuate the switch at too low an angle of attack (too high an airspeed). If the switch is too low, the stagnation point may still be above the switch when the wing stalls and switch will never be actuated.

[Steve Gruber]

great reply and i agree with all the concepts....do you happen to know where the proper set point is, since it's supposed to be a stall warning indicator should it activate a few knots prior to the buffeting, i understand it is an angle but only have instruments to tell me the number of knots before buffet after indication.

is it supposed to be adjustable of is there a proper position?

also any thoughts on the water tight concept or lack of?
thx

[Joseph Munoz]

I just discovered my stall switch doesnt work either. I hear replacements are hard to come by or very expensive. Any ideas on where to start with a fix? Is it usually the stall switch on the wing or is it the electronics behind the panel that usually goes?

[9089P]

The failure point can be anywhere in the system but the switch in the wing is by far the most expensive issue. If you determine it is the switch that is not working you can try this.

Remove the switch from the wing, first marking its position so you can re-install at the same location, then go to Radioshack or another electronics store and by a large spray can of electronic cleaner. With the little flexible nozzle start giving the inside of the switch a bath by sticking the nozzle into all the little holes. The switch is riveted together and no intended to be serviceable. Do this periodically over several days and then retest the switch. On two different pipers I got my switch back to working condition.

Don

[Steve Gruber]

there aren't any complicated electronics, it's just a normally open switch that closed when lifted by stall angle of attack and closes the circuit and buzzes the horn or in my case lights the light...we tested with a meter and had intermittent closure and hence the bad stall lift switch...webco quoted $500 but the other place i don't remember was $1700 making webco seem like a good deal in comparison....but it's a simple switch if you can work on it....

[Francois Marquis]

I had mine not working last year. I saw the prices and that alone gave me enough motivation to understand the issue at hand.

That switch being an SPDT, the other side was working ok. I used a deliberate amount of a good quality electronic contact cleaner sprayed in the blade opening while actuating it and dripping the solvent. It worked for me. I think that residues present in rain did contaminate the contacts and that was the reason of its failure.

Please note that breaking the rivets and opening the microswitch is cause for rendering it unairworthy.

It still works fine to this day. As usual, YMMV !

Francois Marquis
PA-30 C-FTWK

Edit: oops, I did not realize Don had replied the very same thing, just confirming that it also worked for me.

[Joseph Munoz]

Thanks guys. I will give it a try!

[Steve Gruber]

Great stuff..what a valuable and effective forum we have....can anyone comment on the rain factor getting water into the wing...is it just the way it is and perhaps infrequent enough to not matter

[BobPicker]

I just completed our annual where we checked out the stall warning system. Our wing switch was not working, but after liberal application of electronic cleaner it was functioning very well. But the other part of the system that can lead to failure of the system to flash the stall light is the flasher unit that is under the dash. It is a long black box mounted on the upper side of a phenolic resin board at the forward edge of the breaker mounting strip. It is a bi-metallic switch that after so many cycles finally breaks. Ours was bad. There is an article from the Comanche Flyer (December 2011) that covers a fix including part numbers, etc. You just need to find a dealer in your area. Mine cost just over $50 with shipping. The fix replaces the bi-metallic switch with a solid state electronic flasher unit from Motogadget. Think of Moto Guzzi motorcycles. Note that a picture of the single stall flasher unit is not shown in the article, but the black box looks exactly like the one shown in the article, just mounted differently. I removed the old switch and with some work to get solder to attach to the end terminals, it worked great. My biggest problem was getting a good solder joint due to the material of the original switch terminal ends.

[Steve Gruber]

wow, thanks for the addition info, i never checked but just thought other end was just a light bulb, i'll look for the flaser, thanks again