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Thread: Best Glide Speed

  1. #11
    Senior Member Delta Whisky's Avatar
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    Default Re: Best Glide Speed

    Jim - the point at which the line is tangent to the curve is the max L/D speed. The minimum sink speed is at the bottom of the curve. The issue is: which one if the best glide speed? Some say it is the speed that gives the max time to stay aloft (which is min sink; max time to evaluate a landing spot and prepare for the event), others say the speed at which you glide the farthest (max L/D; max distance to hunt more landing spots?). Tomato, tamato maybe?

  2. #12
    Senior Member PapuaPilot's Avatar
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    Default Re: Best Glide Speed

    Best glide speed is defined as the speed that gets you the most distance. This is where the line intersects the Speed/Rate Of Descent curve. The lowest point of the curve is called minimum sink rate, which will keep you in the air a little longer but you won't glide as far. On the graph that DW made best glide is 54 and min sink is about 44.

    It's not one or the other. If you want the greatest distance you should use best glide, and if you want max time aloft you should use the min. sink rate. Glider pilots have a good handle on this, often they want max time aloft, unless they are trying to do a x-country flight.

    https://generalaviationnews.com/2016...0of%20altitude.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  3. #13
    Senior Member jiott's Avatar
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    Default Re: Best Glide Speed

    OK, I needed some education and you guys straightened me out, I agree with you 100%. I was thinking only sink rate and forgetting about distance which is the most important.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  4. #14
    Senior Member Delta Whisky's Avatar
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    Default Re: Best Glide Speed

    I owe jiott an apology and PapuaPilot a note of thanks for the clarification. PapuaPilot clarified the relationship between the regulated world in which we fly and my response to jiott which was directed to the real world question which wasn't his question. (Real world meaning: best glide speed for what purpose? Are you trying to stay up or go far?)

    Past that - I would add a cautionary note to all those measuring their plane's glide performance: do your very best to run them at the same weight. That way, when you are finished, you can adjust your critical speeds to fly (minimum sink and max L/D speeds) by the aircraft's "new" weight. The formula is new speed = measured speed times the square root of the new weight divided by the old weight. In my case my best L/D occurs at 61 mph at a weight of 1081. If I head off on a trip at max gross (1320 in my case) and the good ol engine becomes silent, my max L/D speed becomes 68 mph and my minimum sink speed becomes 54 mph. Another way of saying this - if you run your test glides at various weights, you are really collecting data for a variety of graphs. Note though - the glide ratio stays the same (at the various weights). That could also be one of the may possible reasons we have differences between our planes' measured performances.

    I'm not satisfied with the quality of my test points and plan to continue testing into the winter months when the air becomes more stable.

  5. #15

    Default Re: Best Glide Speed

    Did some engine off testing in my S7 with the Airmaster prop. Wanted to know exactly how it would perform with the motor actually off. With the prop not feathered at 60mph and 1225lbs the lowest sink rate was 500fpm. Feathering the prop gave me 400fpm and took 12 seconds. Unfeathering to start took 12 seconds plus about 8 seconds for the 912is computer to test and be ready. The Airmaster prop gives me 25 miles at 10000ft. to find someplace at sea level. It is a nice thing to have.
    Bob

  6. #16

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    Default Re: Best Glide Speed

    Best glide speed is best L/D speed. One should not care how long it takes them to land but how far you can go to get to a good landing spot. The tangent line is the best L/D for a no wind condition. For a head wind you need to be a little faster. If you study the curve you will see that being slightly on the fast side of Best L/D does not effect you much. You are better off being on the fast side rather than then "the back side of the curve".

    Flew sailplanes for 25 years.

    Allan

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