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Would best glide ratio speed vary with total load in aircraft?

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Aerodynamics

The POH has a stated best glide speed.... best glide ratio. Important for that hope-never-have-to estimate as to how far I can glide power off from the AGL I`m at.

On the face of it without digging into arcane areodynamics I`m too lazy for, it might seem that not only should the glide ratio be better with the plane lightly loaded, but that possibly the best glide speed, too, might be different at light load.

 

In my LSA there is a very significant difference in weight when flying solo with my 140 pounds and 1/3rd of a tank fuel vs flying at legal MTOW. I`m willing to wager big dollars that my glide ratio is much better at light load, but not convinced the best glide speed changes.

And if best glide speed changes which direction would it change? Higher or lower at light load?

Alex

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2 Answers



  1. Jonathan Seitz on Jul 01, 2014

    Glide speed, or any speed for that matter, is always calculated at max certificated gross weight. It goes down if the aircraft is lower than max gross.

    In short, it follows the following formula:

    Vl/d x sqrt(actual gross weight / max gross weight)

    For example:
    Glide speed is 105 knots
    Gross Weight is 1700 lbs, and Max Gross is 2100 lbs.
    Glide speed for this weight is actually 105 x sqrt(1700/2100) = 105 x sqrt(0.8095) = 105 x 0.8997 = 94.48 knots

    +4 Votes Thumb up 4 Votes Thumb down 0 Votes



  2. Brian on Jul 02, 2014

    “it might seem that not only should the glide ratio be better with the plane lightly loaded”

    Speed will vary as noted above. Ratio will not vary. The heavier airplane will glide just as far as the light airplane, but it will arrive at the scene sooner due to it’s higher glide speed.

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