View Full Version : Fan blades bending

profqwerty - n/a
25-Nov-07, 04:27 PM
On a 1100mm duct, 5z blades, I'm assuming the blades are going to bend forwards a fair bit as the thrust increases (although would be slightly countered by the centrifugal force).

What kind of distance would I expect the tips to move by? I have duct structure about 5cm from the blade - is this too close? I can move it.

Also do the blades wip backwards when the engine is slowed down?



tonybroad - n/a
25-Nov-07, 09:21 PM
I have scuff marks on my 920mm duct from last year a good inch or so forward of the blades - as for whipping back i don't know

my guess is given the blades are loaded up even from tickover and the rate od deceleration is relativley gradual if there was any i don't think it would be much


jon_curtis - n/a
26-Nov-07, 05:00 PM
blades can wip backwards with say a miss fire! rapid engine stop etc.

i would have 2 inchs forward and a good inch behind (at the tips)

Nick Long - n/a
26-Nov-07, 05:18 PM
You should be able to get an estimate with a simple experiment. Take the expected thrust, divide it by the number of blades, and pull on one blade with that force. If you pull at about 3/4 of the way out to the tip, and pull perpendicular to the blade rather than in line with the craft it would be a fair approximation to the actual load when running.

Then you can let go and see how far it snaps back.

Otherwise you could run it in a darkened room with a strobe light. I'm sure Jon Pert would volunteer to help.


profqwerty - n/a
26-Nov-07, 06:46 PM
Ahh this is going to be an issue. I kind of scrimped and found the easiest way to make the duct fit the fan was to put a thin rod across the wide part and tighten nuts on it.

I will have to move it forwards - it's about 5cm away. I'm glad i asked - if the blades had hit it at high power (which they would) i shudder to think what would happen http://hovercraft.org.uk/images/icons/smiley_icons/icon_eek.gif

jon_curtis - n/a
26-Nov-07, 10:05 PM
you should not need to pull the duct into shape with tie rods!

profqwerty - n/a
27-Nov-07, 09:27 AM
yeah I figured it wasn't cricket. But I thought - It's only JUST slightly oval shaped. pulled the sides closer together a fraction and it fitted fine.

To be honest, I actually put that in before putting the flow straightners in, so they might be strong enough to hold it in shape now. I'll have to see this weekend http://hovercraft.org.uk/images/icons/smiley_icons/icon_rolleyes.gif

Nick Long - n/a
27-Nov-07, 12:46 PM
If that doesn't work you could put the tie rod back in, then fibreglass a ridge or a flange around the outside, then take the rod out again.

With a flange you can make the duct very rigid indeed.


jon_curtis - n/a
27-Nov-07, 05:32 PM
ingoring my pratish clothing in this pic, you can clearly see the strenghtening ridge on this duct!

the norm is to push a mdf disk into the duct, then glass the ridge and pop out the disk! said disk with hole bang in centre can also be handy for getting the fan hub central!

i used a bit of old rope, and glassed that in place, you can use all sorts to get the ridge shape, even a bit of hose pipe lol! the rope i used was good as it soaked up the resin!

with the disk method, it is possible to get a duct round to within a 1 to 2mm


profqwerty - n/a
27-Nov-07, 08:18 PM
I just checked the duct/fan again. It seems that with the tie rod removed, the duct doesn't scrape on the fan at all anyway!

<font size="1">I'll admit it - I HATE fibreglass[/COLOR]

Are those dual fuel tanks? presuming one for each carb?

How much fuel would you expect to use in one race? I was going to put 2x 10L - one for each engine - both under the seat.

sixpackpert - n/a
28-Nov-07, 07:08 AM
If both your engines are 2 stroke then why not just have 1 fuel tank?

A 20l tank will be enough for racing.

profqwerty - n/a
28-Nov-07, 08:38 AM
The lift EC25PS is 40:1, the 447 is 50:1, so i figured might aswell. I might change the 447 at some point.

Plus the 10L tanks can be bought and would fit snugly; the 20L ones seem to be big and cube not long a rectangle.

But if 20L is enough then that's good:)

sixpackpert - n/a
28-Nov-07, 08:50 AM
We use approx 7-9 litres per race. Moto (integrated) Hull with a 503 engine. You may want to get a larger tank for the thrust engine, especially if you may upgrade to a bigger engine later.

sixpackpert - n/a
28-Nov-07, 12:00 PM
Just another quick thought, if you are coming racing then you will have to do a floatation test, engines off, to check you don't sink.

If I remember correctly you have mounted your lift engine in the plenum, which can fill up when floating. http://hovercraft.org.uk/images/icons/smiley_icons/icon_confused.gif Make sure you have a nice long tow rope just in case. http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif

Just my 0.02p.

profqwerty - n/a
28-Nov-07, 04:50 PM
ahaha yes well remembered. Unfortunately I found that it did not fit particularly well (although only just!). And as was pointed out if i hit something the engine would be knocked smashing the fan.

The result was I pulled the frame apart and turned it over the right way up http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

so it *should* be OK now!

sixpackpert - n/a
28-Nov-07, 05:01 PM
The result was I pulled the frame apart and turned it over the right way up http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

I think that could be the best decision you've made http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif