Ballpark Length for Shrouded Propeller

[wybenga]

Hi, I'm interested in exploring a shrouded propeller for a quad rotor helicopter. Before I get to far into the process I'm wondering if there is a general rule for duct length versus propeller diameter. In other words, if I have a 10 in propeller how far (approx) past the prop should the duct go to help eliminate tip vortices and how far above the prop should the duct go to realize thrust gains due to the pressure drop across the inlet.

Thanks for your help.

 

[plasgears]

It would be best to couple the duct length with fan chord. My sense tells me that 1.5 chord lengths fore and aft of the fan should be a good starting point.

 

[GregLocock]

People seem to us a lot less than that... 3/4 of the fan diameter for example.

BUT that may explain the poor FoM they get

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[rb1957]

a ducted rotor for a helicopter ?
wouldn't that create a very large surface
ie, lots of drag ?

 

[MikeHalloran]

I think the bigger issue is that the duct and the structure to support it adds weight that probably can't be offset by the thrust gained from presence of the duct.

If the rotors blow through a structure that's already present for other reasons, as in fenestron tail rotors, then the structure specifically associated with the duct is not such a big problem.





Mike Halloran
Pembroke Pines, FL, USA

 

[rb1957]

you're right about tail rotors, but i think the OP is talking about a helicopter with 4 (quad) lifting rotors

 

[der8110]

The OP also must be talking about a model - 10 in propeller.

Even with four of them this is not going to be lifting a person and range/duration or even forward speed are not an issue.

I am curious to see how the RC servos will be connected to control it.

Also heard Moller is getting close to "flying" their similar machine.

 

[rb1957]

i guess each fan would be independently controlled, in order to tilt the rotor plane, or else tilting rotors ...

 

[MikeHalloran]

Yeah, but "independently controlled", how? Rotor speed differential seems slow, complex and heavy. Rotor pitch differential could be done with stable technology, perhaps not at model scale. Tilting the rotors, or rotor cans, may have a chance.



Mike Halloran
Pembroke Pines, FL, USA

 

[wybenga]

The system uses differential fan rotation rates to control direction and rotation. It's a similar idea to the draganflyer or the microdrones.

By the way, thanks for the help so far. As an aside, I also contacted Mr. Marc de Piolenc who has written a book on the topic. He said that his simplified model using a duct length of one prop diameter.