Recalibrate Stall Warning Horn. Any Ideas?

[akviperdriver]

Guys,

I have been reading through the posts here for at least the last six months and haven't come across this thread yet...

Does anyone know how to recalibrate the stall warning horn? I read the post about the stall warning horn failure, and eventual fix after a throrough cleaning (congrats!), but my issue is slightly different. The horn in by '52 170B comes on too early. Early to the point that I just want to disconnect it because it is not providing me useful information.

My horn comes on around 50mph +/- 2mph (indicated) the best I can tell. My wing has the sportsman STOL package, with drooped tips, and VGs on the wing and horizontal stabilizer. It doesn't stall anywhere close to 50mph. Based on the typical weights (200lbs under gross) I fly, my best guess is about 40mph +/- 2mph. I've spent about 1.5 hours at 3k' msl, flying off the bottom of my 40mph airspeed indicator, both power on, power off, and will all flap settings. For example:1) Power off glide, yoke burried in lap full aft, the nose doesn't even bobble, the A/S reads/barely tickles the 40mph mark; cross checked with GPS ground speed (both directions to account for wind) at an average of 41 mph (400 fpm VVI, correcting for the glide angle in the ground speed, I figure about 44mph). 2)Power on (24" squared) 20 flaps, the nose starts to bobble at a rediculously high attitude. At the nose bobble, I modulate power to hold VVI at ~0 with the yoke burried in lap and my airspeed is no longer even tickling. GPS shows an average 36mph maintaining the ~0 VVI bobble. Yes, I realize I am well into the backside of the power curve. 3)Now Power off, 20 flaps. Nose will no longer bobble with the yoke in full aft. A/S still doesn't tickle, and the GPS (6 runs) showed 38mph.

So, yeah no, this is not an advertisement for the Sportsman STOL with VGs. And I do realize that at any glide airspeed that I maintain with the yoke buried in my lap will not give me a reserve to flare with... But having a stall warning horn come on 20% too high makes me wish I had a horn silencer! When I practice my short field landings, I like to fly 1.1Vs (vs. the more commonly accepted 1.3Vs) for more precise control. And when flying in Alaska, a lot about being able to land anywhere you want to land, is about being able to land exactly where you want to.

Any ideas, or known approved solutions?

Thanks in advance,

Chris

As a side note, in my professional line of work, my emergency parachute decends at 1000fpm. You bet I told my wife about the neat safety feature in having a plane that (not that I intend to fly single engine, night, IFR) even in the worst case, you can keep the wings level, pull the yoke all the way back, and there should be no problem walking away from it.

[Bruce Fenstermacher]

I'm glad you asked for known APPROVED solutions. Because if the STCs for the mods you have don't have an approved method to make adjustments then I doubt there is one. I also will bet that the modifications don't OFFICIALLY change the stall speed though we all know they do.

[canav8]

If I remember correctly, the sportsman STOL kit has you cut out the old mount for the original stall horn and move it higher on the wing and then rivet a doubler in the back and then you reinstall the horn. It is moved up so the rear holes of the doubler are forward of the original mounting holes. There is a video on youtube that shows what I am talking about. Here,
http://www.youtube.com/watch?v=h1tfjmTpU0g This is part of their install instructions.
Best of luck, Doug

[futr_alaskaflyer]

http://www.cessna170.org/forums/viewtop ... =30#p58369

[akviperdriver]

Richard,

Thanks! That is exactly what I was looking for! I've got some time to work on it prior to the spring reopening of flying season.

And a very Merry Christmas to you!

Chris

[jrenwick]

The video linked above doesn't show where the switch gets remounted on the STOL cuff, but I think it has to be lower than its position on the original leading edge, not higher. That would tip its rear end upward, requiring the additional clearance on the upper side of the hole it used to occupy in the original leading edge. (The added doubler looks like it's only there to reinforce the wing structure that was cut away when the switch was removed.)

The stall occurs when AoA reaches the "critical angle," which was established when the wing was designed, increased when the STOL cuff was added, and increased again with the VG modification.

For a given AoA, there's a point (actually a spanwise line) on the leading edge, called the stagnation point. Air hitting the leading edge above this point goes over the top of the wing. Air hitting below this point goes below. The stall warning vane needs to be positioned slightly higher on the leading edge than the stagnation point at the critical AoA.

The engineers of the original Cessna wing knew where this was, but on your twice-modified wing, it can only be determined by experimentation. The existing slots in the switch mounting plate might not be long enough to allow you to move the switch down the distance it needs to go, so you might have to lengthen the slots, make a new mounting plate, or drill new holes in the leading edge.

[alaskan99669]

John is right. That is my video of the Sportsman install. The Sportsman kit comes with a template telling you where to mount the Stall Warning Switch to the new leading edge, but it is not real clear on the exact location of the doubler. One would assume you simply cut out a hole centered around the 4 screws left by the old location. This new larger hole in your original leading edge is required to accommodate the whole body of the stall warning switch, not just the vane that is exposed to air flow. So if you did what I originally did by centering my cut-out around the original screw holes you will find the Stall Warning Switch will not fit properly in your new leading edge as identified by the template. The correct fix at this point is to modify the location of the doubler. Another fix would be to modify the position of the switch by moving it further up the new leading edge, but that would cause your stall warning horn to sound too early.

So you may find that even if you modify the mounting screw holes to slide the front of the stall warning switch further down your new leading edge, the rear of the switch may contact the top of your old leading edge limiting how far you can slide it down.