Ducted fans versus rotor blades

[machineman9]

I have a question, and I wonder if anyone knows the general assumptions regarding it.

What are the pros/cons of ducted fan blades For example, on a jetpack compared to traditional rotor blades, such as used on helicopters.


Ducted fans seem to be a lot smaller in general, but I've heard that they are less powerful. That said, their cowling helps to contain the thrust so it can have more control in that sense.

Do ducted fans generally 'push'? It seems that rotor blades generally 'cut the air' (downforce as a resultant) but the ducted fans force the air down instead of working off the pressure difference.

Ducted seems safer, as they're enclosed and more rigid (no drooping rotors) but provide less lift (possibly because it can't cut the air due to the cowling)


Does anyone have more experience on this? What sort of applications suit each design, and is there a general reason why one would choose ducted over rotors, and which is generally the 'best'?


Cheers

[skoker]

Ducted fans do a better job of stream lining air and pushing it in a downward motion, creating more lift at less power with less turbulent air.  Helicopters can't do this because their blades flex and move so much that it would be impossible to create a reliable cowling for them.  The gap between the cowling and the blade to be effective needs to be as small as possible and the relative unpredictability of the blade makes this hard to achieve.

[machineman9]

Indeed... Some helicopters have been able to cut their own nose off because of rotor warp 

Why not produce a rotor half the size and make it rigid? Do ducts become impractical after a certain size?

[Xpand]

Well, you can't put a duct on a large rotor, but you can put a duct in 2 or more smaller rotors. That's why there are twin-rotors, tri-rotors, quad-rotors with ducted fans. Only that technology has only been applied to UAVs (Military or Civilian)...
For some reason the realiability of a rotor system is still superior due to the direct connection between the rotor and the engine.
Though this:
http://flyawaysimulation.com/media/imag ... vrocar.jpg
and this:
http://hackedgadgets.com/wp-content/_flying_car.jpg

Are good examples of successful (well mostly) applications of the ducted fan to large size VTOL vehicles...

[Jayhawk Jake]

http://en.wikipedia.org/wiki/Ducted_fan

This explains it well enough

In practical use, there are a few advantages.  They are much quieter than unshrouded blades, and can produce more thrust by use of a nozzle.  However, in a helicopter it's not about thrust, it's more about lift, and a ducted fan produces less lift.  Also, to produce enough lift and thrust out of a single duct for a helicopter would take a very large duct, which is heavy and draggy.  That's why any ducted fan VTOL aircraft has multiple engines.  Look at the old concepts for the V-22 style transport, there is one with 4 ducted fans as opposed to 2 rotors.

[jetprop]

pffff.
Use good-old fashioned fart power...

[machineman9]

pffff.
Use good-old fashioned fart power...

I'm sure there's a film where they use the power of trumps to make it into space 


By inspection it does look to me like the ducted systems push air through, and rotors slice the air instead. But a higher RPM is generally needed by ducts. It'd be interesting to play around with them... I have some designs in the work for university 

[Jayhawk Jake]

pffff.
Use good-old fashioned fart power...

I'm sure there's a film where they use the power of trumps to make it into space