If efficiency of converting battery power to lift or thrust is paramount, why don't drones use shrouded or ducted fans, instead of unshrouded propellers?
This is the only image I could find to clarify what I mean by "ducted fan":
Image source (public domain)
My thoughts:
The reason not many drones use ducted fans is efficiency. Ducted fans are great for fast forward flight but not for static lift.
Explanation: Yes, the 'duct' reduces losses at the blade tips, However, adds significant losses at the intake lip and exit where the adjacent air is pulled into the flow. For ducted fans installed in aircraft, These losses are reduced at high forward velocities and increased at low forward velocity. For drones, there is situation with perpendicular intake flow, when drone is travelling sideways and fan pointing up, there is even greater losses at front intake lip.
The most efficient blade design for static lift is to have the blade as long as possible (like a helicopter) and travelling as slow as possible. Least efficient to have short blades travelling at high velocity.
It is easier to imagine what is going on if you increase the viscosity and imagine the fluid is water instead of air. Very large blades would move water only a small about, but over a large area. however, very tiny blades would be require to move vast amounts of water through a small area to produce the same thrust. moving that much water increases drag as the blades have to suck in water at the intake and the exiting water would be slowed very quickly by the surrounding water at the exit.
They are used - have been for almost 2 decades. Check out Avid Aersospaces T-hawk.
AVID, in subcontract with Honeywell, assisted in the design of T-Hawk, a ducted-fan micro air vehicle (MAV) as part of an accelerated DARPA project. T-Hawk is an unmanned micro air vehicle that provides real-time situational awareness in critical situations with over 30,000 hours total flight hours. (At the link)
The Ducted Fan has a narrow field of speeds where its efficiency is higher than an open propeller, or a turbine, see image, as elements in DF design, as distance of blade tip to duct and others, it was discussed in Aviation Stack Exchange, may change results, this is better tested experimentally, according to the desired use for machine.
Cons
Using ducts, weight will increase than advantage of getting smooth airflow to blades. Using only propellers there will be more maneuverability for sudden change in direction.
Pros
Using ducted fans, undisturbed airflow during windy or gust situations
The efficiency of the ducted fan can be increased by the shape of the duct. If the airflow is compressed, i.e. the outer airflow section is thinner of the inner section, the fluid is compressed (more compression means more speed in the airflow) and that's what gives most of the trust in normal propellers even if not ducted, if you note in small propellers airplanes, typically twin engines, there's a small porthole and another smaller one in the back of the engines, that's where the compressed airflow comes out, more or less similar design happens in jet engines I mean bigger airliners, not rockets. I was looking into building a ducted fan drone, but none of the commercially available ducted fans seems to follow this design.
