Why the squat switch on the stall horn of the PA44?

I'm not sure what type of PA44 ya'll are flying, but most seminols have a stall tab that sits in the "stalling" position when on the ground, hence the horn would always be going off. Once enough air is moving across the wing (before rotation as your stall speed is below your rotation speed hopefully) the tab lifts up due to the airflow. At this point is disconnects from the contact and even with the WOW switch off you won't get the horn. During a stall as the AOA increases there is less airflow over the tab and eventually, just prior to the stall the tab drops back down to where it sits on the ground and hits the contact and hence you get a stall horn.
 
BobDDuck said:
I'm not sure what type of PA44 ya'll are flying, but most seminols have a stall tab that sits in the "stalling" position when on the ground, hence the horn would always be going off. Once enough air is moving across the wing (before rotation as your stall speed is below your rotation speed hopefully) the tab lifts up due to the airflow. At this point is disconnects from the contact and even with the WOW switch off you won't get the horn. During a stall as the AOA increases there is less airflow over the tab and eventually, just prior to the stall the tab drops back down to where it sits on the ground and hits the contact and hence you get a stall horn.
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Say again over...

I don't recall the stall tab wab (lift detector) working any different than a warrior or an arrow, ie high AoA causes the airflow to lift the tab up and make the connection. BUT, it would be something that simple that I overlooked all those years.
 
I'd always had been told, and told all my students (and kind of remember reading somwhere) that the tab on the Seminole is backwards from other light AC due to reasons I can't remember. Hence the WOW switch is needed where as it isn't on other aircraft. I could be wrong though. It always amazes me how often that happens.
 
BobDDuck said:
I'd always had been told, and told all my students (and kind of remember reading somwhere) that the tab on the Seminole is backwards from other light AC due to reasons I can't remember. Hence the WOW switch is needed where as it isn't on other aircraft. I could be wrong though. It always amazes me how often that happens.
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Haha, ain't it true about being wrong when you think you are right, it is a hard feeling to overcome. This is the first I have ever heard of the stall warning working "backwards". Easy enough to test, next time one is up on jacks for a gear swing flip the battery switch on and see if the horn goes off.
 
Regretably (or maybe not) I haven't been near a seminole in over a year. But someone should check it out.
 
My guess: The flap position switch may be wired in series with the stall horn switches. The flap position switch is disabled, along with the left/right throttle position switches when on the ground via the squat switch (Flap & throttle position switches activate gear horn in certain configurations.) As a consequence, the stall horn switches are disabled.

Again, just a guess after looking at some simplified Seminole diagrams.
 
I happened to fly a seminole with the WOW sensor broken. The propwash is why they disabled it on the ground. During taxi, the horn would go off intermittently. The real problem came when takeoff power was applied. The horn came on right away and didn't turn off until about 100kts. Freaky feeling to continue a takeoff with that thing blaring in your ear.
 
launchpad said:
I happened to fly a seminole with the WOW sensor broken. The propwash is why they disabled it on the ground. During taxi, the horn would go off intermittently. The real problem came when takeoff power was applied. The horn came on right away and didn't turn off until about 100kts. Freaky feeling to continue a takeoff with that thing blaring in your ear.
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Yeah, I'm pretty sure that's the reason. I'm sure some people at UND wouldn't like this, but I could have a buddy push in the right squat switch and then I could go press the stall warning horns up to see if they operate like the warrior and the arrow. Im sure they do given their position.
 
The "stall-tab backwards on the seminole" was the original explanation given to me by my MEI while working on my Multi. I've discussed this idea with many people and it just doesn't make sense. In a normal flight situation, the center of pressure on the leading edge of the wing (the point where the air separates to go above/below the wing) is above the stall tab, so the air going above the wing doesn't come into contact with it and the air going below the wing, on the way down, creates a downward force on the stall tab, keeping it in the downward position. As you increase your angle of attack, that point where the air separates moves lower on the leading edge of the wing. The point where the air separates at the stall tab is almost the critical angle of attack. As you continue to increase pitch, the point where the air separates moves below the stall tab. Therefore, the air going below the wing no longer contacts the stall tab and instead, the air going above the wing now pushes the stall tab up on the way up, therefore activating the stall horn.

The only way to make the opposite true would be to place the stall tab so far up on the wing (literally on top of the wing) so that when the airplane stalls and the air going above the wing is no longer hugging the surface but breaking away (we've all seen the illustrations), the stall tab is no longer being pushed back by the relative wind and is allowed to fall to the forward position, indicating the stall.

The stall tabs on the seminole are in the same place (on the leading edge) as most any other piper (if not all) with the normal stall-warning system. Just not possible.

And as far as the prop-wash theory... Now I'm no aeronautical engineer, but I would think that the stall-tabs are pretty darn far out on the wing to be affected to that extent by the propwash. However this is the argument that makes the most sense to me out of all that I've heard so far on this subject.

I called a piper representative three times about three months ago and left the question on his voicemail. No return phone call. Thanks Piper.


CFI, CFII, MEI
 
FlyEDouglas said:
Thank you tgray. Just didn't feel like looking that one up.
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I only nitpicked because someone else in another thread asked if the CP was located around the stall warning vane. Got me thinking that perhaps this is a common thought. That particular idea would make it difficult to communicate what the CP actually was.
 
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