Contact   Imprint   Advertising   Guidelines

Question for you hydro, aerodynamiclly informed guys?

A forum dedicated to Hydrofoil riders
User avatar
Peter_Frank
Very Frequent Poster
Posts: 12710
Joined: Wed Oct 16, 2002 1:00 am
Brand Affiliation: None
Location: Denmark
Has thanked: 993 times
Been thanked: 1177 times

Re: Question for you hydro, aerodynamiclly informed guys?

Postby Peter_Frank » Sat Oct 25, 2014 12:07 am

Hi GP

I politely disagree.

Of course the CoG is high above the wing yes, and you also got the vector component from the kite (lines and direction).

But IMO we can quite easily find an angle where the hydrofoil feels very neutral :thumb:

As the kite is always in the same angle typically, both in height and L/D (forward vector).

Yes, accelerating and decelerating will influence, but this is natural and easy to compensate in the small amount of time when this happens.
The best example I can give is when going far downwind, and looping the kite forward and backward all the time.
Here you get a huge amount of accelerations in a short time, both ways.
And this is quite easy to compensate so you ride pretty level - does not take long till one masters this.
An extreme example though, as when riding with the kite parked or small sinus'es - the acceleration we get is almost nothing.

If your foil tend to seek "up" when you go faster - have you tried shimming the stabilizer a bit up, and see how it actually works when riding at different speeds ?

Mine is neutral - so I dont need to put more weight on my front nor my rear foot, when going faster or slower :naughty:

And if we look away from the acceleration issues (meaning force vector is changed a lot high above the wing) , I can not see ANY theoretical reason why it would not work exactly as gliders - they dont search up nor down, no matter what speed you experience, when trimmed correctly for speed flight.

It also coincide with the experiences I have out there, that I can ride at different speeds without having to move my feet positions, nor change weight distribution :D

But of course, very different kite "positions" can slightly change the trim, yes.

8) PF

User avatar
flying grandpa
Medium Poster
Posts: 199
Joined: Tue Mar 11, 2014 10:02 am
Local Beach: Siemiany,
Favorite Beaches: Stegna, Orle, Karwia
Style: hydrofoil long distance, slalom, waves
Gear: ZEEKO, Takuma, Sonic, Kestrel
Brand Affiliation: NONE
Location: Poland
Has thanked: 15 times
Been thanked: 14 times

Re: Question for you hydro, aerodynamiclly informed guys?

Postby flying grandpa » Sun Oct 26, 2014 9:12 am

Peter_Frank wrote:Hi GP
Mine is neutral - so I dont need to put more weight on my front nor my rear foot, when going faster or slower :naughty:
Hi, Peter,

You're so good at foiling now, that you don't even consciously notice your control inputs. :thumb:
Most of it is done by muscle memory automatically.
But - remember about newcomers and what was going on under your feet few month ago.
Was not so easy. Also nowadays, when conditions are harder (chop, waves, gusts) some positive amount of stability is useful.
I'm rather slalom/wave maniac than speed, so I prefer to have decent amount of decalage (your term relative AoA may be a bit misleading).
Try to shimm the stabilizer a bit down when you ride waves(You made a nice wideo of it :allbegood: ).
It will give you more stability to stay longer above water-also in very turbulent place where the wave breaks.

So, JS concept -
"the speed-modulating character of most airplanes, which results from a horizontal stabilizer with lesser AOA than the main foil and from the corresponding relatively forward CG, is a hindrance to a foil board's speed and pitch stability"
- leaded us to a very intersting discussion. :thumb:
Good for us

GP

User avatar
JS
Very Frequent Poster
Posts: 918
Joined: Wed Aug 03, 2005 6:25 pm
Brand Affiliation: None
Location: Vancouver
Has thanked: 0
Been thanked: 1 time

Re: Question for you hydro, aerodynamiclly informed guys?

Postby JS » Sun Oct 26, 2014 4:40 pm

flying grandpa wrote:Also nowadays, when conditions are harder (chop, waves, gusts) some positive amount of stability is useful.
I don't really follow what you mean by "positive amount of stability". What is effectively "speed stability" for an airplane translates to "pitch instability" for a foil board.

User avatar
Peter_Frank
Very Frequent Poster
Posts: 12710
Joined: Wed Oct 16, 2002 1:00 am
Brand Affiliation: None
Location: Denmark
Has thanked: 993 times
Been thanked: 1177 times

Re: Question for you hydro, aerodynamiclly informed guys?

Postby Peter_Frank » Mon Oct 27, 2014 9:49 am

JS wrote:
flying grandpa wrote:Also nowadays, when conditions are harder (chop, waves, gusts) some positive amount of stability is useful.
I don't really follow what you mean by "positive amount of stability". What is effectively "speed stability" for an airplane translates to "pitch instability" for a foil board.
Exactly my point, as you write JS.

As said, the classic flying stability where you slow down when pitching up, and reverse - which favors a negative lifting tailplane and forward CoG so it is self stable, is IMO of no use on a hydrofoil, and as you say, will actually lead to pitch instability instead :wink:

I think that relatively neutral trim is the way all hydrofoils are, maybe slightly to one side only.

As you can not make a hydrofoil self stabilizing (to my knowledge).


What you can do, is you can make it pitch insensitive, so great for beginners :thumb:

Having a low AR wing, so the overall lift coefficient is "dulled" down to change very little with big AOA changes (because the lift evens out at the tips), is one of the major keys for beginner foils.

Then one can also increase the stabilizer and/or distance, to get it less pitch sensitive (requiring more force to pitch = easy to ride but not as lively)

The very reason why high AR wings are harder to ride, is because they are so pitch sensitive.
Whether cambered or not, is not that important.
The cambered one a bit more sensitive at high lift though, because of the higher Cm.
But to some extent the "ease" of riding slower compensates a lot for this, so I would call it equal.

A bit off topic though.

My point is, that I can not see why negative tail lift should be more stable ?
I was slightly in doubt at first myself, as it is on your backbone - but it seems clear that it gives hydrofoil pitch instability so almost reversed from classic "beginner" flying.

Almost, as in flying we most often go for overall neutral trim instead, for the same reason as on a hydrofoil - works so much better when going slow/fast and agility and ease :naughty:

8) PF

User avatar
flying grandpa
Medium Poster
Posts: 199
Joined: Tue Mar 11, 2014 10:02 am
Local Beach: Siemiany,
Favorite Beaches: Stegna, Orle, Karwia
Style: hydrofoil long distance, slalom, waves
Gear: ZEEKO, Takuma, Sonic, Kestrel
Brand Affiliation: NONE
Location: Poland
Has thanked: 15 times
Been thanked: 14 times

Re: Question for you hydro, aerodynamiclly informed guys?

Postby flying grandpa » Mon Oct 27, 2014 12:06 pm

JS wrote: I don't really follow what you mean by "positive amount of stability".
Peter_Frank wrote: As you can not make a hydrofoil self stabilizing (to my knowledge).
When riding high, I like to kick a bit with my front foot.
The foil starts to go down and accelarates a little, then rises its nose up and starts to slow down.
When I kick repetedly, then from a side view the board is doing a sinusoid -like path.
I use this action sometimes to reduce my muscle tension.

But the feeling itself is also amazing - flying up and down, up and down. :cool2:
This path of my foil is due to positive stability I put into the trim through decent amount of decalage.
I know, that you can do the same through conscious inputs even if you trim neutrally, but it is so much harder to do!
When you look after next wave to turn, positive amount of stability adds a lot to your control in ever changing water level at a shorebrake.
Peter_Frank wrote:My point is, that I can not see why negative tail lift should be more stable ?
IMO majority of foilers most of the time ride with positive AoA=positive lift on rear wing.
And it happens even if they have a decent 3 deg of decalage angle.
Tony Heineken and other racers on a downwind course have negative lift on their rear foils probably.
Hope this put some light on my standpoint.

GP

revhed
Very Frequent Poster
Posts: 1373
Joined: Sun Feb 24, 2008 6:15 pm
Local Beach: france
Gear: kites
Location: France
Has thanked: 106 times
Been thanked: 34 times

Re: Question for you hydro, aerodynamiclly informed guys?

Postby revhed » Mon Oct 27, 2014 1:44 pm

flying grandpa wrote:
JS wrote:


IMO majority of foilers most of the time ride with positive AoA=positive lift on rear wing.
And it happens even if they have a decent 3 deg of decalage angle.

GP
I ALWAYS have thought that for quite some time now! :thumb:
I wonder why there are still others who think not?
Maybe someone out there can explain if they disagree.
R H


Return to “Hydrofoil”

Who is online

Users browsing this forum: Brent NKB, Peter_Frank and 59 guests