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5 Answers

L/Dmax vs. minimum endurance speed

Asked by: 2267 views Aerodynamics

I do believe this statement (which I see all the time in various publications) is wrong: "when flying more slowly than minimum drag speed (LD/MAX) the airplane will exhibit a characteristic known as speed instability..." I think it should read "...more slowly than minimum endurance speed...", correct?  Or am I missing something? Think about it.  We fly below L/Dmax all the time on approach.  My Arrow's best glide is 105 mph, I fly the approach at 85-90 all the time, no "speed instability" there!

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5 Answers

  1. Kris Kortokrax on Jun 19, 2015

    You don’t say why you think it should be “minimum endurance speed” instead of LD/MAX.

    You also don’t say what you believe the term “speed instability” means. Your example indicates that you haven’t read the definition in the Airplane Flying Handbook.

    It states that “speed instability” is “A condition in the region of reverse command where a disturbance that causes the airspeed to decrease causes total drag to increase, which in turn, causes the airspeed to decrease further.”

    You are not flying below maximum endurance speed at 85-90. The Arrow II POH lists 71 MPH CAS as the flaps up, gear up stall speed. Bill Kerschner suggests using 1.2 * power off stall speed as an approximation of minimum endurance speed when one is not published. For the Arrow II, that would make endurance speed 85 MPH CAS.

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  2. Joe Platt on Jun 20, 2015

    Don’t we mean MAXIMUM endurance speed?

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  3. Kris Kortokrax on Jun 20, 2015

    Of course. I quoted his use of minimum, then used the term maximum in my answer, but inadvertently type minimum at the end after looking back at the original post.

    For minimum endurance, all we would need to do is pull the mixture.

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  4. Robert Andreas on Jun 20, 2015

    Sorry guys I meant max endurance. Anyway when you are below l/d max you are most definitely not on the back side of the power curve, (region of reverse command) I see this published a lot and it is wrong. The back side begins at max endurance or min sink speed.

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  5. Kris Kortokrax on Jun 26, 2015

    You never did provide any references to support your belief. I’ll provide them for you.

    On page 10-11 of the Pilot’s Handbook of Aeronautical Knowledge there is a picture of the power curve. It shows a dividing line at the point depicting the minimum power required to sustain level flight. This statement occurs in the second to last paragraph on page 10-10. It also states that this point represents maximum endurance speed. This statement is not universally true. On page 32 of Aerodynamics for Naval Aviators, it states that for jet aircraft, maximum endurance occurs at L/D Max.

    If I were describing the situation, I would say that for propeller airplanes, the front side and back side of the power curve are divided by the point showing the lowest power setting that will sustain level flight. It makes sense to define the power curve in terms of power. Then, go on to state that for propeller airplanes, maximum endurance occurs at the lowest point on the power required curve.

    It doesn’t make a lot of sense to bring minimum sink speed into the discussion. That speed is used in a power off flight regime. For gliders, it is located by finding the lowest sink rate on the polar for a given glider. We also find minimum sink speeds for helicopters in autorotation. It would be of most use when trying to maximize time in the air after a power failure over water to deploy floats and don life preservers. Otherwise max range would be more useful.

    Regarding your statement that one is not on the back side of the power curve when below L/D Max, that would only be true for the small area of the power curve between L/D Max and the lowest point on the power curve. Once below that point, one would be below L/D Max and would be in the region of reverse command.

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