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Modelhovercraft - n/a
7-Jan-08, 06:17 AM
on a ducted fan how close does the propeller or axle fan have to be to the side of the duct?



What happens if the prop is very small in the duct?

Iceman - n/a
7-Jan-08, 07:47 AM
For a prop, tip clearance is probably irrelevent as the air flow from a prop does not need a duct to contain it. I would imagine that a duct would be detrimental to a prop's performance.



For a fan, the efficiency increases with lower tip clearances. A gap of about 5-10mm is usually a good gap to avoid blade/duct contact without any noticeable drop in performance.

Ian Brooks - n/a
7-Jan-08, 09:59 PM
An improperly designed duct will reduce the performance of a prop significantly. Mine is is 70% of the expected output. (And a new duct is in the works!!!!)



Also - props normally have the tips washed out so that it doesn't contribute the the flow, in order to reduce tip vortices, so arn't suitable for operation in a duct anyway, unless you are happy to accept the loss for cosmetic reasons - which I can see might make sense for your purpose.



Ian

jon_curtis - n/a
8-Jan-08, 06:25 PM
an oversized prop cut down, so there is less washout to the tip will be more efficent, than a small prop in the same duct! calculating this is tricky blade theory calculations, its all to do with how the air breaks away from the end of a prop (or not in a ducted setup)and interacts with the wall of the duct, so more of the lenght of the blade does work! might of been up for it 15 years ago but not now lol!

your obviously a bit crazy enough to chuck some cash at this, would be interested if you do any experiments as to the effectiveness of a cut down prop in a duct! thrust of open prop and ducted prop can be measured with scales/spring scales etc.



as ian says, a prop and the duct have to be designed specifically! ducted props look a little more like fans anyhow http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

the mesh thats required infront of a ducted prop and also rudders behind can all effect effiency in hovercraft! generally if you look at ducted fans/props on airships for example they dont have guards front and back!

Iceman - n/a
8-Jan-08, 06:33 PM
In a test I did a few years ago with a duted fan on a purpose built test rig, the addition of a guard on the front of the duct had no effect on the static thrust whatsoever!

jon_curtis - n/a
8-Jan-08, 06:40 PM
fair enough david



i know the same could not be said for a prop in a cage and out of a cage!



i would say the difference of the guard, was perhaps too small to measure?

Iceman - n/a
8-Jan-08, 07:10 PM
I am sure there was some difference but yes, far too small to measure, the load cell we were using was measuring in increments of 100g. It was quite a still day and yet the wind had more effect than the guard.



Unfortunately other events prevented me from testing more ideas.

jon_curtis - n/a
8-Jan-08, 07:13 PM
its all the little inefficencies in a thrust system that build up to have a greater effect at the end of the day i suppose!

Iceman - n/a
8-Jan-08, 07:25 PM
Too be totally honest, in my experience, you can throw as much theory as you like at these things, but you won't effect performance.



The basic rules I apply are, keep it simple, clean and light. And for racing, make sure it is driveable.