In the past i had read (mostly on the internet and other forums) and also heard from my professors that ducted fans are generally ineffective above around 100 kts since the duct then creates more drag than the thrust benefit it provides (at lower speeds).
However since i read about the Astafan, i've gone underground and dug up some work done by Hamilton Standard (as a contractor for NASA) on a concept called the Q-Fan in the early 70's which shows that ducted fans are better than or on par with standard props (and better for noise) so based on that i have a few questions:
History is littered with many ducted fan examples but we still don't see any flying around
Photo © Martin Stephen
Photo © Doug Green
Photo © Sergey Ryabtsev - Russian AviaPhoto Team
Photo © Ted Miley
1. Why are ducted fans not used on current general aviation (pistons) aircraft?
I'm guessing that the added weight and complexity of the structure needed to support the duct imposes a larger fuel penalty than the gains offered by it, i just want to confirm that this is indeed the case.
So in case that ducted fans are not efficient why aren't we seeing advanced props on GA pistons. What i mean by this is most piston engine GA's i see mostly have nearly "rectangular blades". Now i understand that props with larger "spans" are better but they are limited either by:
- a. Wave drag on the tips
- b. Ground Clearance of the props
I've noticed that the new Cirruses(Cirrii?) and the Cessna Corvallis have varying chord props but
2. Why is that we don't see scimitar shaped (a la A400M) or some other fancy props to reduce both wave drag and induced drag (may be raked prop tips since "proplets" are obviously impossible) or something more efficient on GA pistons?
3. What about props on larger jets? Now I understand that fans on turbofans serve to compress air while also pushing it out the back but when talking about open rotors etc. why is that we see no duct? Kinda like this
Photo © Mescheryakov Dmitriy - Aviator Project
Photo © Andrei Nesvetaev
I guess what I'm trying to ask is why don't we see ducted turboprops? Is the reason the same as why ducted props aren't seen on pistons (i.e. weight penalty offsets drag benefits)