What defines low end/low speed foiling in a wing?

[Strekke]

What are the most important characteristics that define low end on a foil?

Is low end = how early you get up on the foil at low speeds? What types of foilwings get you going in the least amount of wind? Does it depend on the surface area of the front wing? or the angle of attack?

[Lokihel]

To get going at lower speeds, you obvious need more surface area on your wing.


You also want to be increasing your maximum coefficient of lift (and push back the maximum angle of attack before stalling) by increasing your Reynolds number by increasing your chord.
You can also use a different airfoil profile, but that does not make a huge difference when you're talking about crawling speeds.


So more chord is the easiest way to get going sooner, but will be costly in top speed.

[Peter_Frank]

IMO the following 3 aspects gives you the lowest possible foiling speeds:

More surface area.
Higher cambered profile (and/or thicker).

They are by far the most important factors, but also higher AR will get you going at even lower speeds because of more overall lift, as there are not the same huge loss at the tips (creating votex'es and drag but giving a softer stall).

Notice that too much rearward sweep in a wing, will also be at a little low end cost, loss of overall lift but more stable.

AOA is irrelevant here, as you control this with your feet

PF

[evan]

Do not confuse low-end with low foiling speeds. I even doubt that most aluminium low-speed beginner foils have a better low-end as the highly efficient racefoils because foiling in marginal winds is all about efficiency and apparent wind. Getting up on the foil is not that big of a limiting factor as the higher drag of the beginner-friendly foils that need more power to maintain a steady speed.

The high-performance racfoils are build for top speed and minimum drag, that is why the wings are a bit smaller in surface area. If someone built a racefoil with double-sized super high-aspect frontwing we would have a low-end winner!

[stevez]

As for the thread title questions, here are two possible definitions of the "low end":
1. The lowest minimum foiling speed
2. The minimum amount of power required to get going and stay foiling - i.e. being able to foil in the lowest wind for a given kite size.
To me the second is more useful - a foil that gets going in low wind will generally also have a low minimum foiling speed, but not necessarily vice versa.

[Mossy 757]

When you're describing performance of any kind of foil, you're basically optimizing L/D ratio within a given performance envelope. This is what we're seeing with SUP Foils...they operate in a very narrow speed window to provide a steady, stable lifting platform. All the math is calculated backwards from an approximation of riding speed on the face of a wave.

With a kite it's more dynamic, not only are we operating across a larger speed range because of a more dynamic power source (kite versus wave) but we're not putting 100% of the wing loading on the foil at all times depending on kite angle relative to the horizon...our power vector moves up and down and accelerates, something for which SUP wave foils don't account.

My point in referencing an SUP wave foil is that they're basically in the process of discover the answer to the question, "how can you get a foil to provide stable lift for one rider at a minimal speed?" When you add a kite to this equation, you can reduce the wing size to account for the higher possible speeds as well as well as the component of vertical lift, but ultimately to optimize around a performance envelope you'd have to define your parameters. Race foils are arguably some of the most efficient, but they account for the fact that the rider will be going 25+ knots at all times, so while they technically have great efficiency in low wind speeds, you can't cruise around nice and slow on them and expect that performance to exist below 10 knots board speed.

[Regis-de-giens]

Interesting topic !
For that analysis, IMO analogy can be done with a low-wind kite (except weight maybe) since the HF wing has the same role as kite but in water instead of air. Hence first first parameter , far from others , is the wing area ... so to optimize low end, get a SUP foil wing ...

Then in a second order I would add the lift philisophy of the rear wing.
(hence AoA difference between front and rear wing). E.g. positive lift rear wing (lift/lift configuration like Gong foils but which is rare acttually) should be better for low end for a same front wing area but not stable at high speed. So installing a little angle plate on the rear wing could improve low end but not usable at normal/high speeds.

I also think that a higher camber should help.

Aspect Ratio ... impact is not obvious IMO , it is more a matter of accessibility and stability.

Obviouslly having a large volume board is better but out of topic.

[TomW]

Let me add another aspect, what about roll stability at low speed? If you are moving really slow, I've noticed this even at my 10 sessions noob stage, that at lower speed I have less roll and yaw stability. so I might want higher stability at low speed.
Would I want a bigger span for roll stability?

[Peter_Frank]

Let me add another aspect, what about roll stability at low speed? If you are moving really slow, I've noticed this even at my 10 sessions noob stage, that at lower speed I have less roll and yaw stability. so I might want higher stability at low speed.
Would I want a bigger span for roll stability?

Yes, higher span gives higher roll stability.

BUT, it will most likely not help you, as if you ride so slow that it becomes "twitchy", then a higher AR wing (= higher span) will stall easier and more rapidly leaving to a disaster, compared to your forgiving wing you got now.

More area plus more span gives you more roll stability at lower speeds, but will of course ride really slow and no acceleration nor any fun left

PF

[palmbeacher]

More area plus more span gives you more roll stability at lower speeds, but will of course ride really slow and no acceleration nor any fun left

PF

I have a MHL 150 (their biggest wing) and can do 20mph on it easily, but I can also ride it at +/- 5mph. There is a lot of fun in jibing and surfing the waves.