Just trying to understand the use of shims with the tail. Is this generally correct (for tail top-mounted)?
NEGATIVE (Eg -.5 degree)= less lift, more speed as less drag, less stability, looser turns.
POSITIVE (Eg +.5 degree)= more lift, less speed as more drag, more stability, stiffer turns.
I know it is a complex area but just after a simple, generic answer. Cheers
It's like the elevator on an airplane? I have actually never shimmed stabs, but if I did, I would read up on the effects of adjusting the elevator.
www.grc.nasa.gov/www/k-12/airplane/elv.html
My current understanding of tail shimming is that it counteracts the downwards pitching caused by the drag of the foil and distance from center of drag to center of gravity of the foil + rider.
What I've found is it should be adjusted when the drag force or lever length changes. This could be speed, wing size, mast length, board thickness, or rider height.
Tail can be adjusted using fuselage length, tail size/design, tail angle, or a combination.
Thrust or from a sail above the center of gravity can probably increase the pitching too. I haven't found much need to adjust for it though.
Select to expand quotemikey100 said..
Just trying to understand the use of shims with the tail. Is this generally correct (for tail top-mounted)?
NEGATIVE (Eg -.5 degree)= less lift, more speed as less drag, less stability, looser turns.
POSITIVE (Eg +.5 degree)= more lift, less speed as more drag, more stability, stiffer turns.
I know it is a complex area but just after a simple, generic answer. Cheers
The stabiliser doesn't create lift. It affects how the front / rear foot balance changes with speed. Also, I wouldn't worry about the drag from increasing the shim angle as that's a small effect compared to the drag from the mast, wings, fuselage and mast. You're likely to go faster if you have a balanced setup regardless of adding a small amount of extra drag from the stabiliser.
To answer your question (if negative means less stabiliser angle of attack):
NEGATIVE (Eg -.5 degree)= reduces the tendency of the foil to become front footed as speed increases
POSITIVE (Eg +.5 degree)= reduces the tendency of the foil to become rear footed as speed increases
I think the stabiliser should be setup to minimise the front / rear balance change as speed changes (using the adjustments above). Once you've go the foil balanced reasonably independent of speed you can then adjust the foil track position and foot straps to get your preferred front and rear foot pressure.
I haven't noticed any difference in stability from changing stabiliser angle with shims. Larger area stabilisers feel more pitch stable than smaller ones and generally require less shimming as they create more downforce at smaller angles of attack.
Also, I've never noticed any difference in turning ability with stabiliser angle - only with stabiliser size.
After lots of searching, I found this-kdfoils.myshopify.com/blogs/foil-info/hydrofoil-balance-tuning
In a nutshell..
Re: tail plane-
*diving at high speed = too little angle
*breaching at high speed = too much angle. .by angle I mean negative angle (front pointing down) Re: mast position-
*digging rail in turns = nose heavy = mast too far back.
*breaching in turns = tail heavy = mast too far forwards.
Another excellent one. www.realwatersports.com/blogs/news/how-to-use-tail-wing-shims-on-your-hydro-foil
Select to expand quotemikey100 said..
I keep finding gems.
It's worth noting that this + & - nomenclature is opposite the Armstrong plus and minus shims
Select to expand quoteradair said..mikey100 said..
I keep finding gems.
It's worth noting that this + & - nomenclature is opposite the Armstrong plus and minus shims
I think it will always be kind of arbitrary. Depends on a lot of factors, are you looking at the foil wit the board laying on the shore with the foil up, or thinking about it like its under your board as you're riding, is your stab top mounted or bottom mounted, etc.
I think its easiest to think in terms of increasing the difference in angle between the stab and frontwing or decreasing it. Increasing = power further forward, decreasing = power further back.
Anyhow, I think stab shimming shouldnt be needed by most, I would only really worry about it if racing. Dabbled a lot with it in windfoiling because back then the foils weren't as good as they are now (and I was into racing). For the average freerider it would be best to just buy a good (recent) foil with a matching brand board and be done with it.
Select to expand quoteWhiteofHeart said..radair said..mikey100 said..
I keep finding gems.
It's worth noting that this + & - nomenclature is opposite the Armstrong plus and minus shims
I think it will always be kind of arbitrary. Depends on a lot of factors, are you looking at the foil wit the board laying on the shore with the foil up, or thinking about it like its under your board as you're riding, is your stab top mounted or bottom mounted, etc.
I think its easiest to think in terms of increasing the difference in angle between the stab and frontwing or decreasing it. Increasing = power further forward, decreasing = power further back.
Anyhow, I think stab shimming shouldnt be needed by most, I would only really worry about it if racing. Dabbled a lot with it in windfoiling because back then the foils weren't as good as they are now (and I was into racing). For the average freerider it would be best to just buy a good (recent) foil with a matching brand board and be done with it.
It's only kind of arbitrary because foilers botched aircraft design terminology. A foil should be looked at like a plane. If you take the front wing and increase the angle of attack wrt the freestream, that's what we call a positive angle. The same rotation should be called positive for the tail.
The confusion came when some brands wanted to call a positive angle whenever "lift" increases, so when you add negative angle to the tail.
But Armstrong's nomenclature is in line with ac terminology.
