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5 Answers

Slow flight and the target airspeed

Asked by: 1117 views Commercial Pilot, Flight Instructor, General Aviation, Private Pilot, Student Pilot

good morning,

I recently had a instructional flight with my instructor, and we performed both dirty and clean config. for slow flight.

Let us say Vso for CE172S is 40kts, and in order to do the dirty config, we need to dump all flaps and maintain slow speed possible.

Now the question is that Vso does not change regardless of altitude and when I was maintaining the consistent altitude, airspeed dropped below 40kts and aircraft was still flying without stalling; ASI indicating around 30kts.

Does anyone know how this is possible?


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5 Answers

  1. Mark Kolber on May 04, 2016

    What weight is published stall speed based on? Assuming all other things are equal, what happens to stall speed as that weight changes?

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  2. John D Collins on May 04, 2016


    Did you forget the answer to your question “Why do we correct airspeed for installation error?” You need to apply the knowledge in that answer to your current question.

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  3. Best Answer

    Brian on May 06, 2016

    Weight and CAS aside, while not irrelevant, don’t really answer this gentleman’s question. The published stall speeds are for power off conditions. Since slow flight is typically performed with power it should be expected that the aircraft will stall below published stall speeds for two reasons:

    1) The thrust component carries a portion of lift where lift is defined as all upward acting forces. If the thrust component is larger than its’ angle can be increased, despite lifts decrease as explained next, allowing for a proportionate increase in the vertical component. Despite much literature fed to pilots, lift does not disappear when Clmax is reached; It merely degrades exponentially much in the same way it rises. In other words, the presence of thrust allows you to fly an aircraft beyond Clmax. Google Air France Flight 447…if they only had more thrust 🙁

    Want an extreme example? Imagine an aircraft with a one to one thrust to weight ratio. It would have a power off stall speed of whatever Clmax allowed. However, with power it could fly at zero knots pointed straight vertically, well beyond Clmax.

    2) In most tractor type (engine in front) aircraft the slipstream contributes to an increase in airflow velocity over the root portion of the wing. This allows a portion of the wing to maintain a lower AoA to a higher relative pitch attitude than powered off flight. The end result, as with scenario one, is a lower speed before stall is incurred.

    I do hope this wan’t too confusing, it’s a bit late but I couldn’t resist posting the moment I saw this thread! I’ll clarify in the morning if you have questions Steve. Goodnight!

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  4. Steve on May 06, 2016

    Sir, Thank you very much for you help.
    When I get reply like this long, I always wonder where you got this information from suh as books or any credible sources, so it can be of good reference for myself, as well as students.

    The book I recently started reading was Aerodynamics for Naval Aviator.

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  5. Brian on May 16, 2016

    Not a problem Steve. AFNA is a great source, though it’s quite dry to read simply page by page. I would only recommend doing that for Chapter 4 on Stability & Control. I don’t have the book handy so my apologies if the Chapter number is wrong, the title isn’t.

    Answer #1 above is just vector analysis, I cannot recall a source that expressely discusses it. Maybe An Illustrated Guide to Aerodynamics by H.C. “Skip” Smith would be worth looking into.

    AFNA mentions somewhere about how thrust accounts for a portion of lift and in the same paragraph discusses how this effect is small below 15 degrees of pitch. Going on further to explain how, for conditions with less than 15 degrees of pitch, the forces can be considered equal.

    To give a number to that, in a 172 at 10 degrees nose up pitch the thrust accounts for something like 7 pounds if memory serves. I haven’t run the numbers in many years. That’s a trig problem if you feel so inclined.

    CL Curve: This is discussed extensively in AFNA and The Illustrated Guide to Aerodynamics. However, where I probably learned the most about it was John D. Anderson’s book Aircraft Performance & Design. He goes on to discuss how the Cl curve drops off and later, at extremely high angles, can sometimes exceed the first Cl max value quite significantly. However, at those angles the drag would be too much to overcome so it is not feasible for flight.

    Comment 2 above is also discussed in AFNA under power effects in the various subsections of Chapter 4 and I believe in a few other locations. However, it’s also a bit of a common sense one, move air faster over a surface than it’s Cl can be less since the airstreams velocity is higher. A basic lift formula of Lift = Speed * Cl where lift remains constant, equal to weight, can help you mentally analyze that situation.

    —Book Suggestions—

    If you’re really into this sort of thing, and it seems you are from the posts I see you make on here, then you should get your hands on a copy of The Illustrated Guide to Aerodynamics. It’s an easy read as far as this topic goes and retains a high degree of accuracy. Far simpler to read than AFNA.

    Another series you might wish to look into is Flightwise Volume 1 & 2 by Chris Carpenter. They are deep intellectual reads, however they avoid the analytical approach common in most books on this subject. This is helpful if you don’t have a degree in mathematics 🙂

    If you prefer the analytical approach check out John D. Anderson’s books or, if you don’t mind the budget expense, get a copy of Aircraft Design: A Conceptual Approach by Daniel Raymer or Mechanics of Flight by Warren Phillips.

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