Is the NASA's X-57 or the Opener BlackFly using a fan, a prop, or a prop/fan and what is the difference visually/physically?

NASA X-57 Maxwell, courtesy of Wikipedia

Opener BlackFly
Opener BlackFly v3 courtesy of the Opener web site

I understand a prop is like an "airscrew", screwing itself forward through the air and taking the airplane with it. It's exit velocity is related to the inlet velocity.

I understand a fan, is used for higher speed applications, and pushes the air back, it's exit velocity being unrelated to the aircraft's speed.

I understand a prop fan is a combination of both.

What is the difference physically between a 2 bladed prop, and a 2 bladed fan?

For NASA's X-57, and other distributed propulsion aircraft, use the prop/fan to accelerate air over the wing.

I understand Illium's jet uses a ducted fan and has roughly 20 blades.

It seem's NASA's X-57 and the blackfly, are just using a 2 bladed props, or is it a fan, or a prop/fan? How can you tell what it is?

What is the difference physically between a 2 bladed prop, and a 2 bladed fan, and a 2 bladed prop/fan, all unducted?


1 Answer 1


These are all propellers. The visual difference is

  • number of blades (prop: few, fan: many)
  • aspect ratio of the blades (prop: slender, fan: stubby)
  • and, as a consequence of those two points, a much higher solidity of the fan.

The physical difference is a much lower disk loading and a higher efficiency for propellers as compared to fans.

I understand a fan [...] pushes the air back, it's exit velocity being unrelated to the aircraft's speed.

Almost. While the entry speed of a propeller equals flight speed, the same would be true for a fan if it weren't surrounded by a shroud. Instead, the shroud of a fan engine accelerates the inflow at low flight speed and decelerates it at high flight speed. This greatly reduces the flow speed variation at the fan face with flight speed so that the exit flow of a shrouded fan also varies much less with flight speed than the exit speed of an unshrouded propeller.

  • $\begingroup$ One thing that I don't see mentioned anywhere, is that, there is only a certain range of Hp a blade can take. I think even propellers are rated by Hp/blade. So if your blades can only take 100Hp, and you have a 500 HP engine, you would need 5 of those blades. Would also appreciate a comment on my question of how the blackfly flies below 28mph with 77% less wing area... $\endgroup$
    – Fred
    Sep 14, 2021 at 20:15
  • $\begingroup$ @Fred As I understand it, the thrust of the blackly is sufficient to let it hover. In the transition the propellers greatly accelerate the flow speed over the wing so less surface is required for the same lift. $\endgroup$ Sep 14, 2021 at 21:42
  • $\begingroup$ Sorry, I meant power off. Does that mean the props are still producing enough thrust even with no power just after powering down in a demonstrated power off stall below 28mph, or are the props producing enough thrust in autorotation, or other? $\endgroup$
    – Fred
    Sep 14, 2021 at 22:25
  • 1
    $\begingroup$ @Fred With true power off the props will cause drag and will not help with lift production. The reason for super-low stall speeds with power off is usually to be found in marketing, not physics. $\endgroup$ Sep 15, 2021 at 5:35
  • $\begingroup$ So you don't think it could float down to the ground under 28mph in autoration? Sort of like the AN-2 with full stick back. I've taken a stab at it here: aviation.stackexchange.com/q/88949/45806 $\endgroup$
    – Fred
    Sep 15, 2021 at 15:23

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