Perhaps the semantics are incorrect, but I maintain that having more stab lift than your body weight can counteract while positioned correctly over the aerodynamic center creates a very unstable system. More stab lift isn’t always more stable in a foil system that includes the rider. I suppose it’s because a lighter rider in the same position as a heavier rider will effectively move the COG back, making your statement true. Still, I maintain that it’s important to bring the rider as counterweight and their position into the discussion otherwise we are left with “more stab lift=more stability” which will not help a rider figure out how to get more stable in my opinion.
I asked AI why Centre of Mass ahead of Lift is good for staboility.
What happens if the foil pitches up?
Suppose a wave or disturbance causes the foil to rotate nose-up.
- The angle of attack increases.
- The foil generates more lift.
- That lift still acts near the centre of lift.
Because the lift acts behind the centre of mass, the increased lift creates a nose-down restoring torque about the centre of mass.
This restoring torque tends to bring the hydrofoil back to its original angle.
Now suppose a wave causes the hydrofoil to pitch nose-down.
What happens?
- Angle of attack decreases.
- Lift decreases.
- Since the lift acts behind the centre of mass, the reduced lift creates less upward force behind the CoM.
The result is a nose-up restoring moment, which tends to rotate the foil back to its original angle.
yes, that’s a good explanation.
imagine a ball sitting on the very tip of a pitched roof. if it falls slightly to one side, it will accelerate away down the slope. In contrast if a ball sits in a valley, it will tend to find its way back to the bottom of the valley even if a disturbance pushes it off center.
You could think of stability as the slope of the valley walls or the roof walls. A roof has negative pitch and is unstable. A valley has positive slope and is stable. A valley with steeper walls is more stable than a valley with slightly sloped walls.
That’s the closest analogy I could think of.
The description above explains why a disturbance creates a restorative force. That is the definition of a stable system.
Here are the outputs from the modelling of a foil that I’ve been riding recently assuming a 100 kg foiling weight (rider+gear). Fz is lift and downforce for the wing and stab, in this case a 777cm2 wing and 110cm2 stabilizer. My are the moments: the main wing, fuse and mast create negative pitching moment mostly from drag but also from the Cm of the wing section. The elevator (stab) creates positive moment to counter the drag/Cm induced negative moment.
this is at around 22 knots (my top speed while parawinging the other day):
Elevator:
Fz= -120.419 lbf
My= +217.160 lbf.ft
MainWing:
Fz= 340.672 340.957 lbf
My= -120.555 lbf.ft
Mast:
My= -10.940 lbf.ft
Fuse estimated: My= - 13 lbf.ft
Summed My: +73 ft lbs or a 4 inch forward offset of CG to balance it out
And at a more normal 14 knot cruising speed:
Elevator:
Fz= -42.318 lbf
My= +76.573 lbf.ft
MainWing:
Fz= 262.881 lbf
My= -61.061 lbf.ft
Mast:
My= -4.839 -4.839 lbf.ft
Fuse estimated: My= - 4 lbf.ft
Summed My: +6 ft lbs or a 0.4 inch forward offset of CG to balance it out
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amazing to see the numbers, thank you!
The front wing has to generate 340lbf of lift due to the tail force and the system drag!
That’s 50% more lift than strictly necessary (likely in the neighborhood of 50% more (edit - apparently not 50% more) drag than necessary), all in the name of system stability
It’s actually only about a 10% delta with or without a stab, at the higher speeds most of the drag is viscous so adding a bunch of load doesn’t increase drag as much as you might think. The extra load also helps keep the main wing inside it’s drag bucket up to higher speeds so that helps reduce the delta at the top end. this is a drag polar of the same foil with (solid) and without (dashed) the stabilizer:
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Does this modelling sofware tell you what the stall AOA for a typical front wing is?
And for same normal setup…at what front wing AOA would the tail start working in positive lift (if it happens).
And third, in some old Naish CFD pics they showed the front wing at liftoff (producing positive lift pretty much all over the span) and then at cruising speed with the positive lift concentrated on the inner third of the span,near the fuse, and the rest of the wing to the tips working in neutral due to twist.
At what AOA would the tips start working in negative lift?.I remember doing dives in my hangglider and watching the tips compress down into the antidive sticks.
I always wonder if a foil’s top controllable speed is limited by this effect (if it does happen).
The software does not model stall directly. It gives a good idea of the low end range but can’t tell you exactly when stall will occur
I don’t think a bit of negative lift at the tips would cause a loss of control but too much would add drag and that could contribute to a foil getting pitchy.
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I’d like to know the effect of stab size / angle has on pumping efficiency. My intuition is that a bigger stab will help pumping by giving you a bigger surface to push against but I really don’t know if that’s true.
I’ve heard people say that it is EASIER to pump with a bigger tail. That’s a totally fair point and I generally agree with it. However, it is super important to detail what the word EASY means. I think there are two conflicting definitions.
- EASY = lower effort or less energy input needed to keep it going
- EASY = more forgiving and easier to keep it going without making a mistake
I think a bigger tail wing can make it EASIER (2) as in more forgiving, but it also absolutely makes it HARDER (1) as it will increase the drag and therefore reduce the efficiency.
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I agree, added stab size/ angle will make it simpler to pump, but less stab is more efficient. the push to ever tinier stabilizers on pump foil setups demonstrates this I think. Compared to other forms, pump foilers can use tiny stabilizers because they operate in a narrow speed range
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…good discussion