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75% Power vs. Maximum Range

Asked by: 2952 views Student Pilot

The performance specifications for a Cessna 172 N with 40 gallons usable fuel indicate at 75% Power at 8,000 feet the range is 485 NM and 4.1 hours with a 45 minute reserve.

Then it indicates Maximum Range at 10,000 feet with 40 gallons usable fuel is 575 NM and 5.7 hours with a 45 minute reserve.

That's a significant difference between 75% power and maximum range numbers: 90 NM and 1.6 hours more. I understand fuel consumption would be less at 10,000 feet and I understand how to obtain 75% power, but what I don't understand is how would I acheive Maximum Range? What is Maximum range? What power setting would I use?

Thank you

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1 Answers

  1. John D. Collins on Oct 31, 2012

    Technically, maximum range in still air occurs at the speed where the lift to drag ratio is at a maximum. This speed is the same as the best glide speed and can be found in the emergency section of your POH. It varies with weight by the square root of the ratio of the actual weight to the gross weight (or other weight specified for best glide). This is an indicated speed and does not vary with altitude. However, we rarely use this speed for stretching to the maximum range because it is too slow and we bought or rent an airplane to go fast.

    Another speed is often used as a compromise, one that maximizes the fuel flow per knot. It is called the Carson Speed. It is related to the best range speed by a factor of 1.316 times the best range speed. Carson also relates the best endurance speed by the same factor (1.316) divided into the best range speed.

    So the speeds can be determined as follows:

    Best range = best glide

    Carson Speed = best glide * 1.316

    Best endurance speed = best glide / 1.316

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