what is the best size for a prop for a hexacopter  having motor to motor as 60cm ie 600mm

should i settle for 10inch or 12 inch?

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    how much will your aircraft weigh? Is it for fixed wing, or rotor craft? How many props do you plan to spin?

    These are important starting points. You should start with a weight range, then look at motor/prop pairing.

    Once the general weight range is known, and the number of props are decided, only then do you want to look at specific propulsion solutions.

    And when you do, you should look at finding the most efficient motor/prop combination, delivering the correct thrust for energy cost at the required RPMs... this determines the blade length, not the geometry of your aircraft (except that if you know you are building something small, you obviously cannot have props that are too large to fit)

    And *after* that, you look at the distance between props to see if it is workable for you. NASA has some good (old) documents describing the efficiency losses when your props are too close tip-to-tip...

    As a good rule of thumb, you'll have much better efficiency if you place one radius between the tips of your props. If your prop is 10", then having 5" between the closest point between two props is a good starting point. Everything in multi-rotors is a compromise. Reducing the distance between props means less weight (less structure), more maneuverability (props move through less of an arc when rotating the airframe around an axis), a smaller airframe (easier to transport), but it also means that the air turbulence makes the props work harder (increased resistance) to degraded thrust (disturbed air) which means slightly less weight carrying capacity and lower flight times (more energy use)

    It is all a trade off.... but here is a secret of efficient design: everything starts with the weight budget. Chose your motor/prop/RPM based on the target weight of your aircraft, and you can design an efficient system. 

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