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I'm fascinated by how ducted fans have the ability to be more efficient than open propellers of equal diameter in vertical flight through the use of tight tolerances, lightweight duct material, proper duct lip shape, and other factors. However, when learning that these benefits don't translate over into horizontal flight due to the duct's induced drag and pitching moment, I'm wondering if we'll ever see ducted fans in future aircraft.

I'm aware of the NASA ducted fan manned-aircraft project that failed in the 60's, but today I found out about AgustaWestland's Project Zero and noticed that they tilt the ducts and include fixed wings to provide sufficient lift. How would an aircraft like this compare efficiency-wise to something like a similarly sized two rotor helicopter?

AgustaWestland Project ZeroPhoto

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    $\begingroup$ Isn't this just a ultra high bypass turbofan without the turbo(turbine)? $\endgroup$ Oct 23, 2019 at 13:56
  • $\begingroup$ @user3528438, not really; there is still a big difference between a short shroud like this and a long duct of a turbofan. $\endgroup$
    – Jan Hudec
    Oct 25, 2019 at 20:42

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Fan shrouds are a fix for a problem that can normally be designed around in other, more efficient ways. They do reduce tip loses, but usually the tip loses are smaller than the weight and drag penalty of the shroud.

The fundamentals of prop/fan/rotor design is that thrust is proportional to the increased momentum of the air flow, but the power required to get that thrust is proportional to the added kinetic energy i.e. the ''square'' of the speed increase.

Therefore small, highly loaded fans will always be inefficient compared to a traditional helicopter rotor. The fact that they suffer from large tip losses only makes things worse.

So no, we won't see lots more ducts on future aircraft - except for on jets, which use ducts because the size of their fans are limited by ground clearance and mitigating the losses from shock-waves when the fan blade tips go faster than sound.

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    $\begingroup$ Note that turbofans generally don't keep their fan blades subsonic. The tip speed goes up to around M1.5. The real reason for the duct is that the air exchanges speed for pressure in the inlet and flows through the duct at about the same speed independent of speed of the aircraft, so the fixed blades can do adequate work over large speed range. But it requires not just shroud, but duct of some minimal length. $\endgroup$
    – Jan Hudec
    Oct 25, 2019 at 20:40
  • $\begingroup$ Would the tip speed be kept subsonic relative to the air around the fan, whose velocity would be significant but subsonic relative to the shroud, or would the tip speed be supersonic within the surrounding air? $\endgroup$
    – supercat
    May 27, 2023 at 14:16
  • $\begingroup$ @JanHudec - thanks, I've corrected myself. I saw a good video on supersonic compressor blade tips recently but annoyingly can't find it now! $\endgroup$ May 28, 2023 at 19:20
  • $\begingroup$ @supercat - air in the engine is usually referred to as 'slowed down' relative to the air passing the aircraft (i.e. from the aircraft's point of reference). Of course, from the ground, the air is accelerated. Either way, the fan blades are moving at right angles to the aircraft's motion, and the fan-blade tips are supersonic relative to the rest of the aircraft. What matters is that there are shockwaves inside the compressor. $\endgroup$ May 30, 2023 at 13:13
  • $\begingroup$ @RobinBennett: Would the perpendicular component of the velocity of any part of the blade surface, other than a narrow leading edge, be anywhere close to supersonic, or would the angle of attack near the leading edge be shallow enough, and the curve designed in such a way that the perpendicular component would always be subsonic? $\endgroup$
    – supercat
    May 30, 2023 at 15:22

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