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The Very Best Speed to Fly

Gemini Sparkle

Key Takeaways:

  • The article explores optimal aircraft cruising speed, moving beyond maximizing "gas mileage" (specific range) to incorporate the inherent value of speed, leading to a "figure of merit" like "speed times mpg."
  • It introduces the "Carson speed," derived from CAFE races, which identifies the speed maximizing this speed-efficiency product (e.g., speed times L/D ratio), effectively finding the "least wasteful way of wasting" an aircraft's excess power for higher velocity.
  • The concept is extended to a universal efficiency framework, based on Gabrielli and von Kármán's work and further developed by Carson, allowing for the comparison of any conveyance against a theoretical performance limit and serving as a preliminary design tool.
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I’ve noticed lately that I fly more and more slowly and pay more and more attention to fuel flow. Clearly this behavior is related to the rising cost of fuel; as it goes up, the best speed to fly goes down. But that relationship – speed divided by fuel flow – must be corrected by a scaling effect related to one’s impatience, anxiety regarding disposable income, age, number of children in college and so on. No doubt one could construct a long equation, loaded with fractional exponents whose values would vary from day to day, and whose solution is the reading on my airspeed indicator.

At a first glance you might suppose that by slowing down I was doing nothing more complicated than trying to maximize my airplane’s specific range, or what in common parlance is called its gas mileage. If that is the case I’m not doing a very good job of it, because that number, which is about 25 nautical miles per gallon for Melmoth 2 at 12,000 feet – I usually cruise as high as I can without needing oxygen – is achieved at an indicated airspeed of 100 knots. I’m going slower, but I’m not going that slow.

Peter Garrison

Peter Garrison taught himself to use a slide rule and tin snips, built an airplane in his backyard, and flew it to Japan. He began contributing to FLYING in 1968, and he continues to share his columns, ""Technicalities"" and ""Aftermath,"" with FLYING readers.

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