Some thoughts about creating hydrofoil sections

[matt_burrows]

Having not had any offers for hydrofoil mandrels on my other posts I've been asking around about how best to create a hydrofoil in my garage. Fortunately I have a extremely well respected model aircraft designer/maker/flyer sitting behind me in the office check out Zephyr High Altitude Pseudo Satellite for more on what we are creating. I thought it would be helpful to put down here what I learn and hopefully it will be useful to others who want to follow on. One post per enlightenment I hope.

[matt_burrows]

Creating the shape faithfully can be done in foam. The quickest way is to laser cut a fairly large number of sections. draw round them onto any-old foam form wickes and roughly band saw to the line. Build up the foam foil by sticking them with a wooden template between each block of foam. Then you can use sand paper to gently rub the foam down to the wood. This is good for creating airplane wings and should work for hydrofoils. do not press on the sandpaper as you will create local hollows
Steve reports that this method can create a foamfoil quicker than he can set up his hot wire system.



 

[matt_burrows]

The other way is to hot wire cut. There are plenty of instructables for creating the hot wire cutter but I have made mine out of a H shape wooden jig, nichrome wire and a controllable power supply, all readily obtainable from your local secondary school science prep-room.  Its possible to hot wire up to a length of 1m. In this case you need only two templates for each end of the foam. The MOST important thing is to get the right dry foam. Blue/pink foam form the building supplier is best as it has very few impurities. impurities catch the wire and drag the wire out of shape.  And the other REALLY important thing is to thoroughly dry the foam for between 2 and 4 weeks before cutting. elevate the foam on wooden chocks to allow air to circulate and dry the foam. If its wet the wire will drag and loose heat quickly.

[matt_burrows]

the leading edge cannot be created accurately in a band saw or a hot wire. The thing to do here is sand it back using a carefully profiled sanding block, that conforms to one side of the leading edge. Use it sand back the foam on both sides of the leading edge.

[matt_burrows]

Sanding foam.

only push, using no downward pressure. Pulling will create hollows.

[Clive Everest]

Hi Matt,
I am not aware of any suitable moulds around.
Most recent new builds have purchased T foil rudders from Demon.
I have had some success using hot wire cut Styrofoam.
Often with sections purchased from Foamwings.
Styrofoam is a bit soft as a core but can be used.
If used as a blank you can reinforce it by putting a wooden stringer at max thickness.
One limitation is that the required skin thickness is often greater than the leading edge radius.

We have also taken glass moulds off the Styrofoam, and have also used the other half of the cut as a female mould.

I have contemplated trying to take plaster moulds. They feel like a lot less work than glass ones.

I would also consider 3d printing either a mould or a blank to take the mould from.
3dprintuk charge 9p / cc. A blank would not cost that much.

I would not use the H105 section. It is designed to operate at higher Reynolds numbers and the design is dominated by a desire to avoid cavitation. This is not an issue for a fully foiling Cherub and won’t be for a N12 rudder.
My preferred choice of section is the DAE41 section for high lift foils.
This was developed for human powered aircraft. They operate at similar Reynolds numbers to us.
The section is capable of generating high lift at low drag and is ideal for a Cherub lift foil, either as the main foil when fully flying or for the rudder T which we load heavily.


If you think that you need to generate down force from your rudder foil in extreme conditions the drag does rise rapidly with this section.
It is probable that this is such a small percentage of your racing time as not to matter.
You can solve this by reducing the camber but it will in turn compromise the max lift you can generate.

For symmetric sections my preferred section is the S8035. This was developed for RC stunt aircraft.
It has high stall and low drag at low Reynolds numbers.

Do you still own a Cherub?

Clive

[matt_burrows]

okay, so the foil is too small to externally wrap enough carbon fibre and retain the shape. so take an impression off the foam foil. I like the idea of plaster of paris, but worth experimenting first as the exothermic heat threatens the foam.

[roland_trim]

Agree = in the recent foils we've been making the ammount of core is next to nothing.

You are welcome to head over to the sticky weekend and have a look at the female tool set I have? Makes a T-foil...