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6 Answers

Stalling

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Aerodynamics

Why do we have Vs when we can stall at any airspeed?

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6 Answers



  1. Russ Roslewski on Aug 15, 2017

    Yes, you can “stall at any airspeed”, as CFIs (myself included) are known to say, but Vs is determined in a given set of conditions – level, unaccelerated flight, max gross weight, etc.

    What would you propose as an alternative? You have to have a starting point of some sort. “Level, unaccelerated flight at max gross weight” seems to be the most reasonable way to compare airplanes and set other performance parameters (like approach speed). Sure, a Boeing 747 can be stalled at 150 knots, and so can a Cessna 172. But that hardly means that they’re in any way similar airplanes.

    Besides, the “you can stall at any airspeed” is too oversimplified for my taste anyway. Yes, theoretically you can stall a Cessna 172 at 250 knots, but more likely the wings would depart the aircraft first. And yes, you can even make a Cessna 172 not stall until you’re down at 20 knots, but to do so would require a parabolic flight path where you’re only experiencing a fraction of a g. For that matter, any airplane pushed over into zero g flight will not stall (no weight means no lift is required). But that’s hardly useful information for performance calculations.

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  2. RickS on Aug 15, 2017

    Stick and Rudder by Wolfgang Langeweische. Best book you will ever read for explaining AoA, and by extension, stalling.

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  3. Mark Kolber on Aug 16, 2017

    Why do we have Vs when we can stall at any airspeed?

    Vs (no flaps) and Vs(full flap landing configuration) at max gross weight are shown on the airspeed indicator for a general reference point (stall speeds in various banks are listed in the POH and you can calculate more of them too, but I am assuming you are asking about an in-flight quick reference).

    I’m curious what you would prefer. None shown on the ASI? An infinite number of them shown on the ASI? A selection shown on the ASI (if so, which ones)?

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  4. KDS on Aug 17, 2017

    John, that’s actually a good question. As others have stated, it’s a reference point used as part of the certification process when the airplane is designed. There are many such numbers in aircraft. A good example is Vmca for multiengine airplanes. There are a whole host of conditions that can change the speed at which a multiengine pilot cannot control the direction and Vmca is just calculated under a narrow set of conditions.

    What you asked is key to why angle of attack indicators are in advanced aircraft and installed aftermarket in others. The airspeed indicator alone doesn’t tell the whole story about stalling.

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  5. RickS on Aug 17, 2017

    An interesting aside about AoA indication, even the pros can get vexed. Remember Air France 447? The airplane was stalled for 3.5 minutes until it hit the ocean surface. The accident report found it’s AoA to be 16 degrees, and its deck angle 34 degrees. And it’s rate of decent was just over 11,000/min down. And the pilots couldn’t figure out what was going on…. all their training failed them. And there was no AoA indication in the cockpit.

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  6. Brian on Sep 23, 2017

    True, you can stall at any airspeed. The real question, however, is…

    What stick/yoke position stalls my airfoil, regardless of speed?

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