is there a difference between the speed at which you can pump a foil and the speed at which it stalls? I have a RRD blast1150 foil, and I find its pump speed really high, but the stall speed is not as high.
Yes there is. Stall is per definition the slowest speed you can maintain flight at. Since we arent foiling anywhere near the speed of light, mass and thus load on the foil os constant and minimum speed means maximum angle of attack. This is very high drag. Every foil has a single optimal angle of attack where the lift to drag ratio, and thus efficiency, is at its maximum and since we are shite energy sources thus is where you want to be pumping. The speed at which the foil operates at this optimum angle of attack depends on the load it is lifting, heavier load equals higher speed to hit optimum lift to drag. In all cases, this optimum efficiency speed is well above stall speed.
You’ll feel a speed where you know it’s not worth your effort to keep pumping, but you’ll still have a long glide before stalling and dropping off.
I wonder for foils how close optimum pump speed is to top speed. Is there a speed that is too fast to pump?
According to this efficiency is closer to stall speed? Surprising to me
Great explanation silas! The interesting thing happening during pumping is the significant load difference between the pushing and unweighting phases. This means the foil’s peak efficiency is oscillating as well.
Yeah, need to revise my statement about what speed you want to be pumping at a bit. While having the foil work at its best efficiency point is ideal if you can sustain the required energy input indefinitely, drag is proportional to velocity squared so as you go faster the energy required to maintain that speed increases dramatically. The lift to drag ratio is a smooth curve so it flattens out around the optimum point, giving some room to play with to find what works best from a limited energy perspective.
I think the question then becomes are you pumping for time or distance? Have heard other folks, maybe KdW, compare going for time to be like min sink on a glider, lower speed than optimum efficiency but still well above stall, while max distance would be like max glide and pretty much at the optimimum L/D.
Good point Omen, I actually suspect that the tap tap tap style short small pumps that guys like Oskar favor once they are up to speed work well in some part because those load and efficency oscillations are minimized. It is quite a non-linear system so it definitely doesn’t average out to zero over a pump cycle.
Thanks Silas, the constantly changing load and speed of hydrofoils causing the fuselage to operate through various angles of attack is under analyzed IMO. Thinner, more aerodynamic fuselages make a big difference in increasing the efficient operating range of foils.
Matt, optimum pump speed is surely mid speed range?
Near stall speed you can’t dig it out of the hole.
Near top speed, no need to pump just glide.
Mid speed range, pump to prevent speed dropping further.
Yes exactly, but I guess there must be some optimum that is maybe not linear.
Like on a planing boat there is the non linear efficiency curve where just above planing speed is most fuel efficient and just below is very inefficient, and efficiency also drops off quickly if you increase speed.
Ion foiling I wonder if there is something similar where it is not mid curve but possibly closer to stall speed for very cambered foil sections and closer to top speed for more symmetrical foil sections.
Some foils have their maximum pump efficiency close to their stall speed, some mid speed, some top speed. Cabrinha H series pump best close to their top end, as do the eagle/army HA I believe. Kujira v1’s pump best towards mid-low of their speed range. I think the progression has the best pumping close to the low end (?), it makes the proning much less energy consuming as the cadence can be very low.
I think it’s around the mid speed point as a rule of thumb. If you know your top and bottom speed its around half way between.
The foils you’re comparing are fast vs slow foils. PP is said to be very slow all round, so still the mid point
It could be, not sure how accurate that rule is compared to the reality. Hopefully a foil designer can chip in to clarify.
In paragliding/flying aircraft there are 4 main speed regimes in the speed polar curve :
- stall speed ~25 km/h
- min sink speed ~32 km/h
- max glide speed ~36 km/h
- max speed ~50 km/h
When you are climbing weak thermal in the air, you fly at the min sink speed, so you capitalize as much as possible from the lift from the air. I suspect the minimum energy expense per pump is achieved at the minimum sink speed of the foil. Now can that min sink speed be moved in the polar depending on the foil section I am not sure.
Yes agree an oversimplification.
@foilstate nice one, the difference between min sink and max glide I think already gives a sense that it is likely one or the other between those, and both are closer to the stall than to top speed for most (all?) foil sections.
I suppose efficiency isn’t accurate then as a target, there are two options:
If you want to glide the furthest, then you need the Max Glide, this is for speed distance records, downwind racing?
If you don’t need to get anywhere (like waiting for a wave, pump time records), then you want the Min Sink, which is lower. This matches what the Axis record guy said, remain just above your stall speed.
In any case I don’t think it is mid way.
I’d be very interested to hear from someone designing foils on how to get a rough estimate on the actual speeds attached to foils.
I believe you are correct. With wings there are so many different speeds, there’s like half a dozen or more named points just around take off and stall speed. So it’s more complex than just half way