So I saw the episode of Mythbusters the other day where they built a jetpack/personal flying machine.
1 comment that stuck with me was that cowlings (enclosure for the propellers) were supposed to increase thrust through the propellers. Anybody have experience with this? Would it make sense to add cowlings around the propellers of my hex for increased thrust or would the additional weight offset the gains in the scales these multicopters are created at?
Apparently it has to be quite a tight fit to create suction.
Thanks in advance.
Replies
The primary efficiency-related purpose of a duct/shroud/ring around a propeller is to fix a problem created by abnormally high disk loading (trying to get too much thrust out of a prop that is too small).
All wings, whether fixed or rotary, generate thrust by accelerating air. A natural by-product of this is the creation of a low pressure zone above the wing and a high pressure zone below. The pressurized air seeks a way to return to equilibrium, and fortunately, most of it goes in the desired thrust vector direction. However, the air near the tip of the wing finds a path of least resistance by merely wrapping around the tip toward the low-pressure zone, creating a vortex field or "tornado" like effect. These wingtip vortexes can often contain (waste) a lot of energy, and the higher the upper and lower pressure differences, the worse they get.
You've probably seen the little fins sticking up on the wing tips of some larger aircraft - these are there to mitigate this undesirable vortex phenomenon and help the efficiency of the wing. On a rotary wing (propeller), you really have only two options - you can make the blade longer, thereby accelerating more air and producing the same thrust (F=MA) with less pressure differential, or place a solid, fixed barrier near the blade tip in the form of a ring/tube/cowl/duct. Naturally, the dimensions and contours of the barrier are as much a subject of engineering analysis as the design of the blade itself.
It is interesting to note that quite a number of aeronautical techies get wrapped up in the whole duct-design-thing and start evangelizing their use as if they were a panacea of efficiency. In every case, their data compare an already overloaded propeller with the same prop using the magical shroud. Viola! They see tremendous results in fixing a mistake they should not have made in the first place. Having been thoroughly researched by literally hundreds of scientists since the 1930's, the properties of ducted propellers are well understood. Assuredly, if there were such great advantages in using them, we'd almost never see a "naked" prop on any scale of aircraft.
However, there are indeed proper applications for a shroud, the most notable being one of dimensional constraint. If for some reason you must generate lots of thrust from a short prop, such as with an autogyro (gyroplane) where you obviously don't want the prop to collide with the main rotor, then having a duct is a good idea. The Carter Copter and the high-speed-record-setting Sikorsky helicopter both come to mind as appropriate applications of duct technology. If you need a compact form factor for some reason, a duct might be the answer. If you want to reduce the prop strike dangers, it's hard to ignore the potential safety benefits of some sort of barrier, efficiency considerations aside (the shrouded tail rotors on some helicopters comes to mind - they are a significant ground ops hazard).
On a small-scale quad, there is no point in applying a duct for efficiency - there are too many other areas of design attention which would yield greater results. The collision ring bumper is a good idea, especially if you plan on flying around people or pets. But don't expect any great efficiency gains.
It is hard to answer to your question. There are advantages and disadvantages.
http://en.wikipedia.org/wiki/Ducted_fan