Question: A question about "minimum power" airspeed had me wondering, why not think of "maximum Lift/Drag ratio" as *maximum Lift/Thrust ratio*.

Gliders would be included, as they use gravity for propulsion.

It seems that the condition of maintaining linear flight using the least amount of fuel (altitude) *per unit time* is V min sink.

So what is Vbg? It seems to be the maximum *Speed/Drag* ratio, but it *burns more fuel per unit time than V min sink*.

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> Power = Thrust x Airspeed
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Would it be more accurate to describe Vbg as maximum *Speed/Drag* or *Power/Drag* (Thrust = Drag at steady state) ratio?

Looking at the **units** for Lift and Drag uncovers something troubling:

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> Lift is a *Force* = ma = mv/s that is constant regardless of speed.
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> Drag = Thrust = mv/s varies with speed and Angle of Attack
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> "Glider thrust" = fuel burn = mg×delta h/s = mv$^2$/s.
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You see, the units of lift and the units of fuel burn do not cancel to form a proper ratio (no units).

Lift × Speed = mv/s × v = mv$^2$/s. Now one can divide this by **fuel burn** to get a meaningful ratio.

So "Lift/Drag" appears to be a bit of a curiosity$^1$. Here, slower may be better.

$^1$ analysis of the Clark Y [airfoil](http://airfoiltools.com/airfoil/details?airfoil=clarky-il/) show *similar L/D ratio* between AOA of +3 and +7 degrees.

Answer: 
This answer is for gliders, or airplanes with failed engines or engines producing zero thrust. To adapt for a powered airplane with operating engine, in a descending glide under partial power, simply substitute "Drag-Thrust" for "Drag", throughout the answer.

The speed for best glide ratio (in still air) is the speed for maximum L/D.

For reasonably flat glide angles it is easy to show that Lift is only slightly less than Weight, and nearly all the variation in the L/D ratio, as airspeed is varied, is due to changes in Drag. A graph of L/D versus airspeed will have a rather broad "plateau" where we can vary the airspeed above or below the optimum for best L/D, without making a huge change in L/D. By the logic above, we are also not making a huge change in D itself. Therefore it's very clear that if we are trying to maximize Airspeed/Drag, this will certainly come at a higher speed than the speed for maximum 1/D (i.e. minimum Drag), which is very nearly the same as the speed for maximum L/D.

Therefore the speed for best glide ratio (in still air), which is also the speed for maximum L/D, and is also very nearly the speed for maximum 1/D, *cannot* also be the speed for maximum Speed/Drag. The speed for best glide ratio (in still air) is *always* slower than the speed for maximum Speed/Drag.

(It's not obvious that the quantity "Speed/Drag" should have any particular practical significance to a glider pilot, or to a power pilot for that matter, but that need not prevent us from talking about it.)

PS as you read this answer, please don't confuse Lift and Drag with their respective *coefficients*. While it happens to be true that the ratio of L/D is also the ratio of the Lift coefficient to the Drag coefficient, in most applications we can't simply interchange Lift and Drag with their respective coefficients, and most of the statements about Drag in this answer would *not* also be true of the Drag *coefficient*.
One way to resolve the issue of units is to consider "glider thrust" to be the sin of the angle of descent × the weight vector. Now there are all force units as mv/s or simply pounds.

It then becomes clear that, allowing for minor differences in lift requirements for angles of descent, modeling follows reality in that:

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> Best speed for maximum distance glide, *after satisfying the lift requirement*, will occur at Vbg.
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A steeper *angle of descent* is used at V min sink (the plane does not glide as far). This resolves as higher AoA and lower speed keep one in the air longer, but more drag is used for lift, and less on airspeed.

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> Lift is similar, gravitational "push" is greater, therefor L/D is less.
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A steeper angle of descent at lower AoA produces the same result: a shorter glide path, will more drag than Vbg from greater speed.

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> sin angle of descent x weight translates as "Drag"
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[![enter image description here](https://i.stack.imgur.com/TcETv.jpg)](https://i.stack.imgur.com/TcETv.jpg)