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I'm in the process of designing a flying wing. I'm trying to keep it a small as possible, so the lift coefficient is very crucial for me. I'm currently looking at some airfoils but most of them deliver the good Cl at AOA larger than 0.

My questions:

  • When deciding a lift coefficient --let's assume 0.5,-- should I choose an airfoil that provides me with a Cl of 0.5 at 0 AOA?

  • When the flying wing is cruising, the lift force needs to equal the weight, but does the flying wing cruise at 0 AOA or can it cruise at 3° of AOA so it can it can keep its altitude?

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    $\begingroup$ The angle of attack also determines the drag amount. In cruise thrust must equal drag (from the whole aircraft, not only the wing). So there is a compromise to be found, possibly having the best L/D ratio in cruise, which determines the angle of incidence. See What is the reason of having an Angle of Incidence on an airplane?, in particular this answer. $\endgroup$ – mins Feb 20 '17 at 7:19
  • $\begingroup$ Good question. Don't forget to consider the pitching moment (CM) at your design AOA. Otherwise you may have to employ significant elevator control to keep it trimmed. $\endgroup$ – Gürkan Çetin Feb 20 '17 at 19:48
  • $\begingroup$ This book chapter may also help: Wing Design $\endgroup$ – mins Feb 20 '17 at 20:48
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Do not look at the angle of attack. Instead, focus on ratios like best lift-to-drag ratio. The angle of attack will be a consequence and not a design parameter.

First you need to determine what your flying wing will be good at. Next, determine which propulsion (jet or propeller) it will use. Then you can work on the layout (aspect ratio, wing sweep) and when that is decided, only then you are able to determine the optimum lift coefficient.

The process to determine the optimum lift coefficient is discussed here, here and here.

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When deciding a lift coefficient --let's assume 0.5,-- should I choose an airfoil that provides me with a Cl of 0.5 at 0 AOA?

No. You should find an airfoil which provided minimum drag at this flying CL. At which AoA the airfoil should operate at a given CL is determined almost completely by camber (2D case)

When the flying wing is cruising, the lift force needs to equal the weight, but does the flying wing cruise at 0 AOA or can it cruise at 3° of AOA so it can keep its altitude?

it can cruise at any pitch attitude given that this attitude does not stall the wing.

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The answer is not only one: minimum drag at cruise speed, minimum sink, maximum L/D.....as you know, designing a flying machine is the ultimate expression of positive compromise. You are not mentioning at all which planiform the wing will have, its area, wingspan, wing chord (min.& max.), eventual dihedral and/or anhedral, sweep on leading and trailing edges...not enough data given for a decent answer. More information is required before recommending one of more airfoils.

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