Duct Myths

First, let us dispel a couple of myths: 

Myth 1: A ducted fan is less efficient than a free propeller.

This is a meaningless statement, since it does nothing to compare apples with apples. Whether ducted or free, the basic rule is that a propeller with low disk loading (thrust or power vs. disk area) will be more efficient that one with high disk loading. Small diameter, high disk loading ducted fans are often conceived to allow the use of a high rpm engine running a direct drive propeller. While these highly loaded fans (if properly designed) will be more efficient than a free propeller of the same diameter, they typically won't match the efficiency of a larger free propeller (of much lower disk loading), leading to the myth that ducted fans are “less efficient.” 

It has been amply demonstrated, both theoretically and empirically, that a ducted fan (shrouded propeller) system, properly designed for a specific operating condition, will always outperform (propulsively) a free propeller of the same diameter (that is, it will produce more thrust for the same power input) at that operating condition. While this fact is always true (at least at subsonic speeds), it neglects the drag of the duct itself, which at high speeds can easily overcome propulsive benefits. 

Myth 2: Ducted fans are efficient only at high speeds (where turbofans operate efficiently).

In fact the opposite is true. As speeds increase, high mass flow becomes easier to attain as a result of the speed itself, making large disk-areas (and ducts) less important or effective. At the same time, however, duct drag rises with the square of the speed. 

Why then do turbofan engines always place the compressor in a duct? For a very different reason: to slow the airflow so that the compressor can operate subsonically even when the aircraft is close to sonic speeds—or even supersonic! The compressor duct of a turbofan engine actually lowers the propulsive efficiency of the compressor (slows the airstream) in order to avoid a transonic drag rise. 

The UDF (unducted fan) seeks to avoid this tradeoff by doing away with the duct altogether and designing the compressor (propeller) with exotic plan forms and airfoils to operate efficiently in the transonic realm. This approach has its own problems, of course, and the ducted, high bypass turbofan is still the most efficient high-speed engine we have. Just this does not apply to us Hovercraft builder.

Myth 3: The duct is dead weight and just increases drag of craft

Just opposite can be the case if the duct is a fix component of your craft and provides strength to the hull as well as skin of the craft. In a first aspect everybody sees the duct as a attached unit to the aft end of the Hovercraft craft - the engine located in front of it and the duct producing high drag at the low craft speed. If you try to rethink this aspect you will get to a design as the  Donar  where the duct is a part of the craft and ad's not only a high amount of safety in aspect of blade failure but as well aerodynamic streamline appearance and strength to the craft. Or as we use in the  Alpha-II  the duct is integrated in the lower hull of the craft, provides the positive aspect of a self draining cockpit, reduced noise because of it's high length of more than 6', the internal skin to the craft, a perfect location to integrate  the controll surfaces to make this craft better maneuverable without aft buckets - and a aerodynamic efficient thrust force. And all over all there is a lot of wasted space in the aft section of a Hovercraft.

Duct Physics Without Numbers

We begin by looking at a ducted fan operating statically (zero free-stream velocity or airspeed).

 A further important factor is "diffuser ratio", being the ratio of exit_area/disk_area. Theoretically, static thrust increases with diffuser ratio: as the induced airflow is slowed by the expanding duct aft of the propeller disk, pressures increase on the inner  duct wall, thus contributing to thrust. In practice, diffuser ratio is strictly limited by the requirement to avoid separation.  In summary, ducted fans can produce more thrust than a free propeller of the same diameter for the following reasons: 

• The duct extends the reach of the propulsion system radially to work on a larger mass of air (by analogy,  “non-planar” propulsion system) and thus the system takes on the characteristics of a free propeller of larger diameter. 
• If clearances between the propeller tips and duct wall are kept small compared to tip chord, the presence of the duct wall will maintain pressures on the blade towards the tip, improving blade L/D. 

 Since the total thrust of a ducted fan/shrouded propeller is the sum of pressures on the propeller and pressures on the duct, to  increase thrust one increases the net propulsive pressures on the duct and/or on the propeller. To increase net pressures on the propeller for a given power input, one increases the blade L/D by keeping tip clearances small. To increase net pressure  on the duct, one optimizes the duct geometry for a specific airspeed. 
 
 

Hope you enjoy the script 
Michael    4wings.com