wings profils

[tkettlepoint]

RH I think you are thinking too much again..hehehehe no wind there?

Terrie
www.jellyfishboards.com

[zfennell]

I tried running an Eppler 817 against a Speer H105 in XFLR5, the wing used exactly the same plan form, just the airfoil profiles varied. The analysis uses a constant weight loading and varies the angle of attack (alpha is independent variable). The flying speed and lift to drag ratio are calculated as dependant variables.

The conclusion is that the flying speed varies between the two for the same angle of attack (to be expected with different lift graphs) - but that the lift to drag ratio for a given velocity is pretty much identical (see the same colour blocks).

The conclusion to the conclusion is that there is no magic profile and no free lunch

Lift to drag graph -Eppler 817 vs H105 .JPG

thanks Phezulu,
the comparisons in XFLR are interesting.

T. Speer has also compared the same profiles where he describes his motivation for developing the H105 profile.
( http://www.tspeer.com/Hydrofoils/h105/h105.htm)
his description is more complete.
But to paraphrase, Speer believes the eppler 817 was designed for high speed craft with greater wing loading at high reynolds number. In comparison, kite or moth hydrofoils have relatively low lift requirements at low reynolds numbers ( in the neighborhood of 1 million). This provided some opportunity ( or constraints) while defining the H105 profile.

i believe he was attempting to make the profile more forgiving with respect to cavitation by minimising the peak low pressure magnitudes.

he also seems to delay laminar separation , presumably to allow a more graceful transition to a turbulent boundary layer (instead of separation). The low reynolds number for a submerged foils makes laminar flow a real possibility.

as you have mentioned, i doubt peak performance is any higher than the EP817.
but he does make a good case that the foil is more forgiving by avoiding the drastic lows associated with off -design performance from cavitation or laminar separation.

regards,
-bill

[Bradn]

if someone is interested in their own profile designs, T.Speer was helping/teaching someone in a forum about using xfoil and what to look for. should only take a quick google to dig up

[flying grandpa]

I'm a fan of the inverted profile on the rear. At first I couldn't understand why foils need so much stabiliser wing area, then I realised that it's because the force input - the kite line attachment at the harness - is very high above the foil and tends to pull the foil over forward and you need a down force at the tail of the fuselage to counter this. It's analogous to having a plane with the propeller mounted 3 or 4 fuselage lengths above the wing axis. You can correct for it to some extent with where you place your COG, but only to a degree

That may work a bit diffrently.
On a foil you just ride a very small airplane flying in water.
As the cargo (your weight) is 15 times heavier than the weight of the plane (hydrofoil with the board),
it is quite easy to control the movement of CoG and in fact,
that is the way we control the pitch of our ride - moving our CoG back or forward.

The same way we can counterreact our driving force (kite pull) that is comparatively quite small.
So we change our COG position a bit, even without moving our feet.

An inverted profile just add more AoA difference between front and back foil-(french foilers call it "decalage angle"?).
This way the plane you ride in H2O is more stable.

I attach a clip "Stab" here[youtube]https://www.youtube.com/watch?v=3VOnuArhvys[/youtube].

The difference in plane behaviour with:
small decalage angle - first plane setting and first flight and
big decalage angle - second plane and second flight
is a simple example of paper plane flight, where I changed the position of flaps.
We can see, that the plane on the first flight had small decalage angle and just dive with decent speed.

Plane on the second flight had more stability, so during the initial dive it starts to rise the nose and go up,
untill it looses speed and starts to dive again.

The first setup is for speed, but lacks stability,
the second is for free/wave riders - gives more stability, but at a cost of a bit lower Cl/Cd, that will steel some speed.

But if you put too little (or negative) "decalage angle", it will be not easy to control the foil ride.
On the other hand, if you put to much of "decalage angle", your foil will become prone to cangarooing.

Just my 2 cents.

Oooops!!! It looks I need some help, as my clip does not show on the screen and I don't know how to fix it..

[Phezulu1]

Hi Grandpa

I like the paper plane - I agree with you about our COG control for our pitch, I even made a very simplistic spread sheet that demonstrated it. My thinking was more that the conception that the stab can be symmetric profile and just trailing along behind without any forces on it is probably flawed. I found that with one of my own foils with a fairly small stab, the faster I went, the more my back foot had to work to keep the tail of the board down, when I did my own numbers (admittedly very crude) I found that the stab actually needed to be working pretty hard pulling downward to get a reasonable balance of smallish COG shifts. Net sort of forces were about 1.5W up on main wing and about .5W down on stab (W being my weight).

I'm very interested in how the Gong foil (new post below) with lift profiles on both wings will go (this seems similar to a Canard concept which can be quite unstable in pitch), but certainly could be lower drag with small wetted area with both wings contributing lift.

I'm not an aerodynamics expert, so you could say I was just taking a stab in the dark....