Ducted fan, contra-rotating or stators?

[vonsteimel]

I was wondering what is likely to be most efficient in a ducted fan. The design of the duct is long, approx 4x the diameter. This is to be used to create thrust, not as a compressor

The option of placing 2 to 4 contra rotating fans/props or...

placing 2 to 3 fan/props rotating the same direction and using stators in between. Each fan/prop would have a different pitch and increasing number of blades as seen on a turbine compressor.

Any info will be appriciated.

 

[rb1957]

intuitively, counter-rotating props will produce more thrust (i think stators remove/reduce swirl without producing thrust) but they'll be heavier and way more complex.

stators have the advantage of providing a really usefull support to the cowling. i guess you could possibly make the stators variable pitch, but i suspect the improved efficiency will be offset by the increased complexity and weight.

 

[MikeHalloran]

Efficiency is irrelevant if the weight of the added complexity makes the vehicle too heavy to fly.



Mike Halloran
Pembroke Pines, FL, USA

 

[KENAT]

Can you give some more information on your application as that might help.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[MikeHalloran]

Right, he didn't actually say it had to fly.
On a boat, weight would be less important.


Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

The main purpose of counter-rotation is to counter balance gyroscopic effects. Axial compressor jet engines may have counter-rotating sections but they still use stator guide vanes.

 

[rb1957]

counter-rotating props are flyable (Tu?? Bear, Gannett, ...) and i don't think that putting them in a duct makes them unflyable. certainly it'd be a fantastically complicated piece of machinery ... multiple concentric shafts.

 

[Dan320]

First of all, calculate how large blade area is needed for the given power. Then find out if one such fan stage will fit in the given diameter duct. If it doesn't fit, add stages to suit. Counterrotating will add a lot of complexity. If the fan(s) are driven by a motor or engine inside the duct, stators may cancel some torque.
It's not homework, this?

 

[rb1957]

but multiple stages will increase the power from a given diameter (i think it's used when the tip speed limits the prop diameter, but you need to provide more power, eg the Tu95 Bear LR bomber)

 

[vonsteimel]

This is for an R/C aircraft. I've splash molded an el cheapo fuselage and am redesigning the plane.

By design, the overall geometry limits the diameter of the duct. The length of the duct runs nearly the entire length of the fuselage (like seen in a F-86 Sabre, Mig-17 & Mig-21).

It is powered by electric motor(s). I’ve upgraded the power pack and motor(s) providing a substantial increase in power/weight ratio and have plenty of wiggle room as far as gross weight is concerned.

Contra-Rotating is simple with these electric motors and will weigh no more than the alternative setup of having them rotate in the same direction.

The power-pack will provide more endurance than I need and the motors can produce more power than a single fan/prop can absorb.

I was looking for advice on which would provide me with the most thrust for the given power.


Concerning the comments about the stators, If I am going to put several fan/props inline, all rotating the same direction there will have to be stators to redirect the airflow off of the leading fan/props blades to provide an optimal angle of attack for the next fan/prop.

When it comes to ducted fans, I’m lost. This is just something I took up in my spare time for fun. As far as calculations go, it’s just a shot in the dark. I was just wondering what your opinions were on the best way to go as far as contra-rotating or single direction rotation with stators? I have the option to add multiple stages. I’m looking for recommendation on what will give me the best thrust/weight ratio.
Thanks,

CS

 

[rb1957]

that helps alot ... it won't be traditional contra-rotating props with a common shaft, but rather mulitple shafts. they'll also be fixed pitch props.

my 2c, don't over-fuss the "ducted" part of the problem, just think of it as a prop with a geometry restriction. a key design point is accurately locating the center of the duct. this'll allow you to seal the tip of the baldes against the duct, i suspect that paddle tip blades should work well.

if weight isn't a problem, then i'd use more powered props. but since you're going to be creating shaft supports in the duct, why not make them stators ?

good luck with this !