3
$\begingroup$

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

$\endgroup$
  • $\begingroup$ Isn't this just a ultra high bypass turbofan without the turbo(turbine)? $\endgroup$ – user3528438 Oct 23 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 at 20:42
4
$\begingroup$

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 that jets already use ducts because the size of their fans are limited by ground clearance and keeping the fan blades subsonic.

$\endgroup$
  • $\begingroup$ 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 at 20:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.