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I am working on a school project (aircraft design).

I would like to know how to calculate the cruising speed of a given airfoil. For example, if I have a NACA 5613-35 airfoil (that is the NACA 4 digit modified series), what is the the cruising speed at which this airfoil has the best aerodynamic properties?

Another way to ask this question: If I want my aircraft to have a cruising speed of 120 knots, how do I choose an airfoil to fit the cruising speed requirement?

I tried looking it up on google and on some other aviation forums, but I did not find the answer.

Thanks in advance and have a great day!

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  • $\begingroup$ You need to go through airfoil tech-sheets. These are available ini some textbooks $\endgroup$
    – skipper44
    Jan 10, 2021 at 12:41
  • $\begingroup$ Correct version: You need to go through airfoil tech-sheets. These are available in some textbooks - AC Kermode's "Mechanics of Flight" has everything you need. Combination with fuselage and tailplane is also to be understood. Also, in aviation, and theory of flight, "the best aerodynamic properties" can have more than one answer - best range, best endurance, high speed, more payload and these often conflict with each other. $\endgroup$
    – skipper44
    Jan 10, 2021 at 12:56

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The cruising speed is only one parameter. For optimum flow conditions, you need to look at the Reynolds number and the lift coefficient at which the airfoil shows its best behavior and then tailor the aircraft to achieve those numbers at its design operating speed. Normally, a higher Reynolds number is better, but at low speed (= low Reynolds numbers) airfoil performance drops off a cliff. It is important to stay above this limit in order to get good performance.

This tailoring is done by selecting the wing area in order to achieve the right wing loading (the ratio of airplane mass to wing area) and the wing's aspect ratio (that is the ratio between span and chord) to arrive at the lowest possible drag. Selecting too high an aspect ratio (wing becomes too slender) will drive up the mass of the wing structure and reduce its Reynolds number. Conversely, picking too little wing span will give you plenty of wing chord, but induced drag will be higher than necessary.

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