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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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  • $\begingroup$ Can you provide a bit more of context? Where did you find this expression? To me, this ratio is one of the most important ratio when dealing with aircraft performances and you want it to be as large as possible for fuel efficiency. $\endgroup$ – Ludovic C. Apr 29 '14 at 21:07
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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:

Best L/D Graph

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.

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  • $\begingroup$ This answer confirms my previous understanding that "best" L/D is synonymous with max L/D... is that correct? $\endgroup$ – user2168 Apr 29 '14 at 22:20
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    $\begingroup$ @user2168 Yes, I am saying that best/max L/D mean the same thing. :) $\endgroup$ – Lnafziger Apr 29 '14 at 22:24
  • $\begingroup$ @dvnrrs I don't know, I saw it that way and just did it because. That being said, I just copied and pasted it into a text-only program with no issues, and it won't increase site performance because MathJax gets loaded regardless, once it's enabled for the site. $\endgroup$ – Lnafziger Apr 30 '14 at 19:24
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    $\begingroup$ @JanHudec Well, there is no "excess thrust" in unpowered flight. That would be best glide at that point, which makes sense (and is how they refer to it in your link). $\endgroup$ – Lnafziger Apr 30 '14 at 19:38
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    $\begingroup$ @JanHudec Minimum drag is not the same as minimum required power. The latter occurs at a higher CL $\endgroup$ – DeltaLima Apr 30 '14 at 19:56
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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:

  • minimize your fuel burn per unit of time to stay aloft as long as possible
  • minimize your fuel burn per unit of distance travelled to go far as possible
  • climb as fast as possible
  • climb as steep as possible
  • descent slow at idle power
  • glide far without engines
  • maximize your fuel burn to prevent overweight landing in an emergency situation.

"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

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  • $\begingroup$ So, best L/D is not synonymous with max L/D? $\endgroup$ – user2168 Apr 30 '14 at 20:52
  • $\begingroup$ @user2168 In most conversations, especially between pilots, it is max L/D I guess. Within the aerospace engineering community it is not always the case. $\endgroup$ – DeltaLima Apr 30 '14 at 20:55

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