As a followup to Why do turbofan blade tips exceed the speed of sounds while prop tips should not my question is: why have rings on propellers not been used to overcome this problem?
It seems from this site that they are produced at least for maritime use.
According to this site and this site it seems to have advantages and experiments have already been done as I can see from the picture. (I do not know how reliable it could be though)
Propeller rings (ducted propeller) have been used un aircraft, notably in some early VTOL designs, like the Bell X-22, XV-11 Marvel and also the RFB Fantrainer.
Bell X-22. Image from diseno-art.com
They do have some advantages like,
Better noise reduction and safety compared to 'open' propeller.
For the same thrust, the propeller can be made smaller.
The duct can be designed such that it provides a limited amount of thrust vectoring, useful if we are using it for STOL.
However, they are not widely used because of various other reasons like,
For effecient operation, the tip clearance should be very small. Any increase in tip clearance leads to rapid degradation in effeciency.
The duct and its support structures increase the weight.
The duct increases the drag at cruise speeds (as drag increases as square of velocity).
If we decide to use a lower sized propeller as a result of using the duct, the effeciency suffers- as we are actually accelerating a lower mass of air more (rather than accelerating a higher mass of air less, which is more effecient).
The design of the duct is difficult- the aiflow into the duct comes not only from the front, but is also sucked from the sides from the sides (i.e. the capture area is greater than the duct frontal area). If not designed properly, the flow may seperate, leading to loss of effeciency.
However, ducted fans are used in a number of small UAVs (in RC model jet aircraft, for example, in lieu of miniature jets), electric aircraft like the Airbus E-fan and also some dirigibles.
In case you are asking about connecting the propeller tips like in the maritime propller in the link, the resulting stresses at high speed would tear the whole assembly apart, causing more problems than it solves.
