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How does the flyboard air get so much thrust when it can't have more than an 6" disc area (if that) on each of its four lift engines?

Isn't the thrust of a rotor = CT * p * Area * tipSpeed^2?
(CT - coefficient of thrust, p = air density, tip speed = linear speed at tip of rotor).

Shouldn't this limit the thrust such small rotors can create without the tips exceeding mach (and causing all the associated problems with that)?

How much thrust could each of these really be generating? I'm assuming their ducted fans, but still shouldn't be enough, should it?

It's also clearly not ground effect as it reaches heights well above where that would be relevant (claimed height of 1000 feet, though the videos tend to show only up to around 50-75ft).

Flyboard air

Source: http://www.guinnessworldrecords.com/news/2016/4/confirmed-franky-zapata-sets-new-farthest-hoverboard-flight-record-in-france-427011

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    $\begingroup$ Flyboard is turbine-powered, based on what I've seen and read. $\endgroup$
    – egid
    May 29, 2017 at 17:13
  • $\begingroup$ A ducted fan is not limited by Mach like an open propeller. $\endgroup$
    – TomMcW
    May 29, 2017 at 17:38
  • $\begingroup$ Maybe you should rewrite your question, asking for purely technical data, e.g. what is the theoretical maximum thrust that can be generated by a rotor with n blades of size m at p rpm? And let the Guinness stuff alone (as we don't have the full description of the propulsion system). $\endgroup$
    – mins
    May 29, 2017 at 17:40
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    $\begingroup$ Right -- I think that's the main misconception. There are variants of the Flyboard that have been referred to in various news stories, but the present one that I think is being referred does, as egid says, run on kerosene-powered turbine engines, which are relatively common. Asking for what the powerplants are, maybe the rationale for using turbines vs. ducted fans/open rotors would be good (theverge.com/2016/4/15/11439798/…). $\endgroup$
    – Marius
    May 29, 2017 at 18:35

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