What does "best L/D" mean? Is it meaningless without context? Do you need to know "best for what?"
Or, is it synonymous with max L/D?
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Sign up to join this communityWhat does "best L/D" mean? Is it meaningless without context? Do you need to know "best for what?"
Or, is it synonymous with max L/D?
Best $L/D$ is short hand for the airspeed where you have the best ratio of Lift to Drag, and is the same as $\textrm{max}\ L/D$.
If you graph Airspeed -vs- the $L/D$ ratio, it will look something like this:
The peak point of the graph would be the "best" $L/D$ and represents the airspeed where the airplane is the most aerodynamically efficient. For unpowered flight, it is the airspeed which will result in the best glide ratio.
What is "best" depends entirely on what you try to achieve. Without context it is meaningless.
With respect to L/D curves, I can imagine you want to:
"Best" is different for most of the situations above, and mostly engine performance curves will play an important role as well to find the optimum.
However there are two points on the L/D curve that are important:
$\left.\frac{L}{D}\right|_{\textrm{max}}$ results in the lowest energy required per distance travelled and, disregarding propulsion efficiency, will give the most economical fuel consumption for long range travel. This will also give the best glide distance in unpowered flight.
$\left.\frac{L^3}{D^2}\right|_{\textrm{max}}$ results in the lowest power required and, again disregarding propulsion efficiency, will give the lowest possible fuel flow. It will also result in the lowest sink rate in unpowered flight.
Edit:
From googling around a bit it seems that in most situations when people talk about best L/D they seem to intend max L/D