Stall warning system on Seneca

skydriverdc6

skydriverdc6

Well-Known Member
I was wondering if anyone could explain why the Seneca/Seminole has two stall warning lift detectors on the left wing? one is outboard and one is inboard. I cant remember the reasoning behind this, and the POH simply states that they are there, but not why they are there. I believe its got something to do with one being at higher angle of attack and detecting the stall sooner as the disturbed airflow moves inward? so outboard detector "detects" stall first to give a pre-warning? or am I totally off?
 
skydriverdc6 said:
I was wondering if anyone could explain why the Seneca/Seminole has two stall warning lift detectors on the left wing? one is outboard and one is inboard. I cant remember the reasoning behind this, and the POH simply states that they are there, but not why they are there. I believe its got something to do with one being at higher angle of attack and detecting the stall sooner as the disturbed airflow moves inward? so outboard detector "detects" stall first to give a pre-warning? or am I totally off?
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Usually the stall progresses from the inside to the outside on rectangle planform wings.
 
skydriverdc6 said:
I was wondering if anyone could explain why the Seneca/Seminole has two stall warning lift detectors on the left wing? one is outboard and one is inboard. I cant remember the reasoning behind this, and the POH simply states that they are there, but not why they are there. I believe its got something to do with one being at higher angle of attack and detecting the stall sooner as the disturbed airflow moves inward? so outboard detector "detects" stall first to give a pre-warning? or am I totally off?
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Maybe I'm way off base...it's been a few years but were they for different flap settings?
 
Yep, flap settings. The inboard vane is for flaps 25 and 40, the outboard is for everything else.
 
I guess I wasnt clear, but when I said why are they there, I meant aerodynamically what is the benefit of having two? I realize in the POH it says outboard is for 0 to 10 degrees of flaps, and inboard is for 25 to 40 degrees, but whats the benefit of that?
 
Since you changed the chamber and chord of the wing, your CoA now is different. The change might be so great in this aircraft where it requires two stall horns.

Just a guess.
 
Ah, I did misunderstand you.

My guess is that the stagnation point that corresponds with the critical angle of attack changes with flap deployment.

That's just a guess, though.
 
skydriverdc6 said:
I guess I wasnt clear, but when I said why are they there, I meant aerodynamically what is the benefit of having two? I realize in the POH it says outboard is for 0 to 10 degrees of flaps, and inboard is for 25 to 40 degrees, but whats the benefit of that?
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Theres no aerodynamic benefit from having any stall warning horns. They are there to warn the pilot of stall.

Having two devices allows for more accurate stall warnings. If the flaps are down, one of the two stall indicator devises is inaccurate. When flaps are up, the other one is inaccurate. Based on the position of the flaps, the system uses the one indicator that is tuned correctly.
 
skydriverdc6 said:
I meant aerodynamically what is the benefit of having two?
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A flapped wing stalls at a lower AOA than an unflapped wing. That alone may suggest a reason for having two stall warning detectors. However, the airflow around any particular, real world airplane can only be determined through flight and/or wind-tunnel tests, and the actual airflow at the locations of the stall warning vanes may not conform to this simple theory for a variety of reasons.
 
how about because Piper determined 2 were necessary?

kinda like on my MEI checkride "why is there a t-tail on the Seminole?" my response "because piper determined it was necessary and put it there"

at least they build safe aircraft and they actually design the systems to help the pilot out. even though their POH's are pure crap
 
i like your previous technical response and it makes sense to me. but no one ever explained that to me.
 
tgrayson said:
Typically for marketing purposes. They look cool, but don't provide any benefit for light twins.
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Except for when the window falls out...


A coworker was doing training in the nole and the emergency exit window was just replaced the day before. Her flight was the first one with the new window. On departure out of Boeing Field the window fell off and sailed into the Puget Sound. It was later determined that it was improperly installed but wasn't visible in the normal walk around. If it wasn't a T-tail there would be a definite dent in the horizontal stabilizer.
 
BuickCFI said:
why is there a t-tail on the Seminole?
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When an engine goes out there is less negative lift on the tail due to less accelerated slipstream. By having a T-tail design, this completely takes the horizontal stabilizer (or stabilitor in the case of the Seminole) out of this accelerated slipstream and reduces the nose down pitch effect that happens when an engine loss occurs.
 
RPJ said:
When an engine goes out there is less negative lift on the tail due to less accelerated slipstream. By having a T-tail design, this completely takes the horizontal stabilizer (or stabilitor in the case of the Seminole) out of this accelerated slipstream and reduces the nose down pitch effect that happens when an engine loss occurs.
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that makes sense! i know that they did engineer a lot of bad things out of the seminole
 
RPJ said:
By having a T-tail design, this completely takes the horizontal stabilizer .
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That may be, but it's not why they have a T-tail. The nosedown effect can be fixed by some backpressure on the yoke. A T-tail is often chosen for jets so they can put the engines on the tail; props choose T-tails to look like jets. :)

T-Tails have some theoretical advantages; the horizontal tailplane can be smaller when it resides outside the propeller slipstream and wing downwash. So can the vertical stabilizer, because the horizontal stabilizer acts like an endplate, making the vertical stabilizer more effective. And, again in theory, effectiveness in a spin is increased. (We see how well that worked on the Tomahawk.)

On the flip side, the physical structures needed to support a T-tail are more complex and heavier, tending to offset the advantages. And in props, having the slipstream impinge on the horizontal stabilizer is often useful to piloting technique, so the lack of it is annoying.
 
Personally I dislike the T-tail. It makes the Seminole a lot more prone to dropping onto the runway in the flare and tends to jerk itself into the air more than one might expect since there is no slipstream from the props helping keep its effectiveness. Quite honestly it's easier to fly a King Air (from a non-systematic point of view) than a Seminole.

Edit: King Air C90 with conventional tail. I've never flown an F90 or any other T-tail King Air so I wouldn't know about those.
 
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