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I'm designing an RC aircraft of which the final goal is a crazy top speed. I'll be 3D printing and sanding / finishing the air frame surfaces to reduce drag. In order to achieve maximum top speed, should I use open 5 inch diameter 3 blade props (the ones used in racing drones) with a large pitch and beefy motor, or two (60-80mm) ducted fan (EDF) units? Power draw is not a limiting factor here. Also, should I go for a larger EDF than 80mm? (wingspan is about 70 - 80 cm, total mass of about 700-1000g) . I'm aware that EDFs induce a greater delta V on the air they move, but does this actually mean I can expect a larger top speed?

[edit:] I could also go for a single 100mm unit.

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    $\begingroup$ According to my understanding, the general rule is that, after taking into account pitch and everything else, if you can accommodate a prop with a diameter that is larger than the largest duct you can accommodate, go with the prop. If the largest prop you can accommodate is the same diameter as the largest duct you can accommodate, go with the duct. In other words, only go with the duct if you can't accommodate a prop of larger diameter. A duct doesn't beat our a larger prop, it only helps make up for a smaller one. $\endgroup$ – DKNguyen Aug 3 at 20:09
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A propeller is better. Just look at any pylon race model to see what they use.

DKNguyen put it very nicely:

A duct doesn't beat a larger prop, it only helps make up for a smaller one.

You're right that "EDFs induce a greater delta V on the air they move" but EDF models generally use twice as much power as a similar aircraft with a prop. Moving half as much air twice as fast generates the same thrust but requires twice as much power. This is because momentum is mass x speed, but energy is mass x speed squared.

Ducts (and multi-blade propellers) are used when you don't have space for the right sized prop for the engine power.

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