Here's a question from an aviation enthusiast who's still learning about older planes:



In pictures of some older GA planes, like the Cessna 140, I see a small, cylindrical doodad mounted horizontally on the left side of the fuselage, just aft of the cowling. It's kind of narrow in the middle and wider at each end, so it actually looks a bit like an air horn -- although I can't imagine what use a plane would have for a horn.



It can't be a stall warning horn, can it? I can't imagine how a stall warning horn that wasn't integrated into the wing would work.



So what is that thing? My best guess is that it's a combination pitot tube and static air sensor, with the pitot tube opening up front and the static air at the back. But I really have no idea, and I'm curious.



(For what it's worth, one of these gadgets is visible on the C-140 on page 39 of the latest issue of Plane & Pilot. I believe I've seen it on planes pictured in Flying, too -- I think I've even seen two of them, mounted one beneath the other -- but I'm not sure what issue of Flying it was in.)