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The Antonov An-2 is notable for being able to fly at a very low speed. However, what confuses me is that it has no stall speed:

The An-2 has no stall speed, a fact which is quoted in the operating handbook. A note from the pilot's handbook reads: "If the engine quits in instrument conditions or at night, the pilot should pull the control column full aft and keep the wings level. The leading-edge slats will snap out at about 64 km/h (40 mph) and when the airplane slows to a forward speed of about 40 km/h (25 mph), the airplane will sink at about a parachute descent rate until the aircraft hits the ground."

(From Wikipedia article on An-2)

How is this possible? What makes the airplane have no stall speed?

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    $\begingroup$ Well, that's not the same as saying there is no stall speed. Rather, the manual seems to be saying that the stall speed is very low, and pulling the stick or yoke full aft does not provide sufficient control authority to reach the stall speed. $\endgroup$ – quiet flyer Jun 19 at 23:00
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    $\begingroup$ OK, I guess more recent answer suggests stick full aft does produce sort of a stall, but a nose-high mushing stall not a severe nose-drop stall. $\endgroup$ – quiet flyer Jun 19 at 23:13
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The horizontal tail can't make sufficient down force to slow the plane to get the slatted wings to stalling AOA. With the slat system, the stalling AOA will be perhaps 10 degrees higher than without. With the AN2's large wing area, to get the slatted wings to stalling AOA might require the speed to drop to, say, 20 kt. But the tail simply runs out of down-push before that angle is reached, in this case at 25 kt, so no stall can occur and the airplane settles into a near vertical mush as long as you hold the full up elevator.

Because there is no actual stall break or departure, the published "stall speed" is the lowest speed that airplane can be flown and maintain altitude.

Note that a "parachute descent rate" is about 20-25 fps, or 1200-1500 fpm. That's still coming down pretty fast (you would probably collapse the gear landing normally at that descent rate) and the airplane will be wrecked when it hits the ground, but you'll live.

The interesting thing is that if you do get a slatted wing to actually stall, the departure can be very violent. An acquaintance who flew a Pegazair he built, which has automatic slats, was horsing it around and was able to get an aerodynamic stall by using large elevator pulsations as he was approaching minimum speed, reported that it flipped over violently and his flight bag flew from the baggage well behind him up to around the windshield. It recovered fine however.

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This site states it a bit better:

Stall Speed: 35-40kts. but controlled descents are possible at 25kts. or 30mph.

In reality most aircraft have the ability to glide at some speed. The AN-2 simply has the combination of a quite a good glide ratio and a low stall speed so the aircraft can be settled to the ground in a controlled manner.

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    $\begingroup$ How is a controlled descent possible at below stall speed? $\endgroup$ – vidarlo Jun 19 at 23:07
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    $\begingroup$ All airplanes still produce lift even when stalled; the An-2 produces enough of it to remain in controlled flight even in a stall. $\endgroup$ – Sean Jun 20 at 0:54
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    $\begingroup$ @vidarlo There are various definitions of stall. I'm guessing the definition used here is one where the wings no longer generate sufficient lift to maintain altitude - which is technically stalling the wing. However this does not mean lost of aerodynamic control - which is the other common definition of "stall". Planes can be designed with oversized and/or slotted control surfaces or with delta wings or have vectored thrust to be able to do post-stall maneuvers $\endgroup$ – slebetman Sep 13 at 7:33

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