size of lifting T foil.

[phil_kirk]

Following this year's nationals it is clear that boats are sailing with a range of sizes, sections and aspect ratios of t foil. 

The event results clearly don't show a trend (eg. bigger is better) so I think there is still some evolution to occur or there are other factors at play eg. depends on hull shape, crew weight and even balance between upwind and down wind speed.

Dave and I were certainly able to overload or stall e numbers skinny lifting foil causing the transom to sink and the boat to slow when twinning upwind.
So I think we need a larger and stiffer foil.  Sailing with Sarah we have never loaded the foil as much but probably could load it more upwind.

anyone had similar thoughts?

 

[simon_jones]

We have been talking about replacing Eleanor's foils. We also find that it's easy to stall out the lifting foil. I don't know if bigger is better, had Paul C left the rest of us behind we would be in a better position to know what's right. We seem to put too much on at times causing it to slow the boat right down.

[phil_kirk]

Paul thinks that Marmites foil is a bit too big.

Obviously too big means draggy in the down wind conditions when we use less of the lift or in very light winds when the foil is not working very well. 

However from about 7 knots of wind speed upwards they start to work. So there is only a small band at low wind speeds when we will be racing but where the foil wont be effective.

The span of the foil was originally set at 0.9m to match the width of the transom to catch the upwash of the wake leaving the transom however we set or foils at the bottom of the rudder where this effect is much less.

E-Numbers lifting foil is 1m span but only 10cm cord at the rudder blade and quite skinny.  It is actually lower in plan area than the first 0.9m span foil.  So overall less draggy for light winds and down wind but less lifty.

I'm thinking of a larger foil at present but obviously don't want to go too large to get the adverse drag penalties when it's not working.

[ade white]

Yes the cherub camp are mostly using the T foil on the base of the rudder. The Nat 12s have a plethora of spans, shapes and angles but use them higher up the foil as do the 14s. My last blade was up the foil but I never reached any conclusions. other than the fact that building is made easy and cost effective by using windsurfing fins. these can be a ready made and cheap alternative to making, though you need a matched pair, but the advantage is you can mess around with different ideas a lot more quickly on an existing blade.
So thats an idea for messin and testin, Just join, cut slots, bog and tape to see what might work best.

[Neil C.]

I was sitting on the beach on my summer holidays a couple of weeks ago. There were a load of kite surfers out on the water, including a couple on foiling boards. I sneaked a quick look at their hydrofoils when they came ashore. They have what looks like a simple model aeroplane on the bottom of a long keel. The striking thing was how much lower aspect ratio their lifting foils are compared to a typical Moth or Cherub t-foil setup. More like a Vulcan bomber than a glider wing.
Just an observation really, I suppose it might be so the foil continues to work at wildly varying angles of attack?

[HighwaytoHelm]

I've been wondering about making delta shaped foils (e.g. vulcan/concorde wing profile) however there would be less lift at low speeds than glider profiled foils. Despite the lower drag coefficient. However it would be interesting to experiment.

[roland_trim]

I think this is where we all need to go - more experiments around the current norm.
Sadly, I'm still struggling making stairs - so no workshop time for me for the forseeable.

[Phil Alderson]

My understanding was that the main benefits from a delta wing planform came when operating at high subsonic, transonic, and supersonic speeds.
There is some re-energisation of the boundry layer due to the tip vortex rolling over the top of the wing, which gives them a very high stall angle. This is useful on high speed aircraft as they need to operate at a high angle of attack at low speed as the wing area needs to be small to allow for high speed flight. The good stall caricteristics may be why they are used on the foiling kites.

It is worth looking at the performance of a  for a few foils shapes to understand the range of angles of attack that are worthwhile. If you look at the drag polar for the H105 here you can see that there is a big increase in drag when the coefficient of lift is around 0.8, this corresponds to about 5 degrees. the foil still seems to be generating lift up to about 10 degrees, so the drag increase is happening well before stall.

The optimum foil size is going to depend on the all up weight, how much the hull drag drops with a reduction in displacement, and how the foil drag increases as you increase the lift. Match this with the expected wind conditions and you will know how big to go. Alternatively just go two boat testing. 

[Tim Noyce]

I think the difference with the kite surferers is that they have so much more power on tap, and so much less weight to deal with. If you look at the foil on the bottom of an 'air chair' they just seem to be very simple punched metal plate, but I guess that this is because they have power to overcome the drag, and also, as they don't race they are not pushing for efficiency. The other thing about their foils is that they are also a lot more likely to actively take off from the water and land in it again (normally having done some crazy aerobatics in the mean time!)

[roland_trim]

I think the difference is force not power. Kites can make a fair pull over a short disatance to get going.
Slight difference, but kites do not actually appear to harness that much energy vs a turbine blade/sail (energy is force vs distance used).

I think the difference with the kite surferers is that they have so much more power on tap, and so much less weight to deal with. If you look at the foil on the bottom of an 'air chair' they just seem to be very simple punched metal plate, but I guess that this is because they have power to overcome the drag, and also, as they don't race they are not pushing for efficiency. The other thing about their foils is that they are also a lot more likely to actively take off from the water and land in it again (normally having done some crazy aerobatics in the mean time!)

[Neil C.]

Some interesting kite hydrofoil wing design information here:

http://www.kitehydrofoil.com/index.php/design/the-wings.html

[phil_kirk]

The aeroplane configuration on the kite surf board foils provides some longitudinal trim stability. A cherub doesn't need this because we are currently planning or skimming.  A moth doesn't use the configuration because they need a rudder as well to provide steering.  The kite surfers only require 1 vertical foil for lateral resistance. So the aeroplane configuration is not relevant for a cherub or moth because both need 2 vertical foils and may as well have a lifting foil on each.

Sucking eggs I know.

There is no 1 optimum lifting foil configuration as it depends on too many variables.  So looking round the boat park for inspiration will only identify the extremes of the design space.