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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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    $\begingroup$ related: How much thrust is gained from placing a propeller in a duct? and Why don't drones use ducted fans? $\endgroup$
    – Manu H
    Aug 3, 2020 at 17:54
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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, 2020 at 20:09
  • $\begingroup$ RPMs for models are typically 3x higher than full-scale aircraft, allowing props to hang in there with all their benefits at higher airspeeds. Few models are flown at 30,000 feet or at airspeeds greater than 200 knots. $\endgroup$ Mar 20, 2023 at 8:00

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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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Like turbo fans, the idea of EDFs is to have higher air speed in a smaller dimeter compared to a propeller. The problem is you need 4x the air speed when halving the diameter to produce the same thrust. EDFs are typically even smaller diameter than half a propeller. To get the same thrust the rotational speed of the EDF is higher than a typical electric motor so gearing up would have to be achieved. Then the top speed of the fan would be supersonic which introduces stresses on the blade and vibration and noise. So steel blades would be needed. The upside is the max speed would be a lot higher than a prop because the pitch x rpm would be faster than any propeller.
The result would be an RC that would not have prop stall at take off and a higher top speed than a prop can achieve.

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