Foil Physics Question RE: AR and drag

[PonoBill]

Yeah, aerodynamics is still an intense area of study, hydrodynamics not so much. In many ways it's simpler, the long and complex term in the lift formula that deals with compressive effects and Mach number resolve to very close to 1 in water, so it's generally ignored. But the density is 1000 times greater, so parasitic (form) drag doesn't decrease so drastically as a percentage of total drag.

Probably the simplest way to look at aspect ratio is that most of the lift force is generated in the first third of the wing--the rest of the wing is just along for the ride, though it has to be there to minimize turbulence. Fat leading edges are, of course, very draggy so thinning out the wing reduces drag but means the wing has to be longer to generate a similar lift. A high aspect wing with thin leading edges can more easily be accelerated to enough velocity to provide the lift you need. A stubby wing with a fat leading edge can generate more lift for a given area because the foil is more extreme, giving a higher foil factor, but it's going to be draggier. In simplest terms, the lift is roughly surface area times foil factor times velocity squared. 

You have to go faster to get a 999 wing to lift you, but it's easier to go that fast. You don't need to go nearly as fast to lift off with a 1020, but it's harder to go fast enough. Velocity is the kicker in all these circumstances--it's the only squared term. Double the speed and you get four times the lift.

High aspect wings have a problem turning in part because of the differential speed of the fluid across the wing. Turn a sailplane too tightly and the wingtip at the outside of the turning arc is going a lot faster than the wingtip on the inside. All pilots turn by rolling the wings with ailerons and turning with the elevator and perhaps a little bit of rudder. We don't have those, we skid into turns. A tight turn with an HA foil can stall the inside wing while you're increasing the lift of the outside wing. That helps bank the wing into the turn, but makes it hard to recover.

[Dontsink]

I would not say we skid into turns, we bank into them with weightshift.Like a hangglider.

Since we do not have aileron induced adverse yaw we do not even need a rudder or a true vertical stab.Even stab winglets are optional, i cut mine off for a looser feel.

[JohnnyTsunami]

But the density is 1000 times greater, so parasitic (form) drag doesn't decrease so drastically as a percentage of total drag.

Thanks, that's the answer I was looking for. Based on everyones views it seems like we wingers will never get above to the right side of the graph and so a HA small foil will be faster.

[PonoBill]

Most of the wing foilers and surf foilers I watch torque their back leg to initiate and hold a turn--I know I do. That's a skid. It's a little harder to see with a wing than in foil surfing because we torque against the wing, but in surf foiling, that counter-rotation of the upper body is unmissable, especially in the really hard turns. Banking is an aftereffect of shifting weight inward to stay over the center of lift. Rolling inward can generate some turning force from the lift of the front wing, so it helps, but if all you do to initiate a turn is roll inwards then it's going to be a very long turn. We don't have any element of the equipment we use that duplicates the effect of an elevator in a banked turn. But we do have a vertical stabilizer, it's actually the largest aero element--the mast.

The exception is Eddie Ogata, and I don't know what the heck he's doing or how it works.

[Dontsink]

The mast is very close to the front wing, so it is not a very effective rudder ,even though it has lots of surface.If you put huge feathers near the tip of an arrow, it will not fly straight.

And you do not need an elevator input to pitch the nose up in the turn.Just weight shift back...
Aa for pivoting and yaw input  i will agree that they are used, just like in normal surfing.Very little pivot/none in speed carves and lots of it in reentries and slow speeds.
I still think that roll is the primary control input in turns, and then yaw and pitch adjustments are required.

 Pretty much all the top riders use a very strong offset stance to get increased roll command.
Mike Pedigo even built his own asym surffoil board,to allow more offset in a narrow build.

[MikeLima]

Pono, in your equation above, I assume foil factor means thickness or some other value based on foil section?

And concerning turning, a while back there was a discussion on the progression project about initiating turns. I don’t remember exactly what was said, but it made me watch closely, and it looks like many are yawing the board out(which can happen quickly), so their body is leaning in, and then roll the board under them through the turn. It reminds me of pushing motorcycle handles in the opposite direction that you want to go, so you can initiate the lean.

[Dontsink]

Pono, in your equation above, I assume foil factor means thickness or some other value based on foil section?

And concerning turning, a while back there was a discussion on the progression project about initiating turns. I don’t remember exactly what was said, but it made me watch closely, and it looks like many are yawing the board out(which can happen quickly), so their body is leaning in, and then roll the board under them through the turn. It reminds me of pushing motorcycle handles in the opposite direction that you want to go, so you can initiate the lean.

I took a clip and slowed it down showing that Erik Antonson pre-turn move:

https://youtu.be/oUettZThZMM

I think he got the idea from watching KDmaui vids.

[MikeLima]

Exactly, and yes that’s who he was talking to.

[PonoBill]

Pono, in your equation above, I assume foil factor means thickness or some other value based on foil section?

Foil factor is either a complex calculated value or an empirical value taken from the NACA specifications of a particular foil shape. Depending on who is writing the formula it may also be called the foil coefficient, or in combination with the area, the lift coefficient.

That's an interesting video to look at in slow motion. The simple truth is that no one really knows in absolute terms how our foils work and how we control them, there isn't any supporting research, all the serious stuff was done 50 to 100 years ago, and I don't see anyone instrumenting foil boards or working in a tank to see how they are controlled.

In the boat world, foils are not controlled the way we do them. Pitch and height off the water is controlled by a wand that actuates the stabilizer in the simpler rigs like moths, or by some other surface sensing mechanism that actuates control surfaces. Turning is by a rudder, so there's not much help there, other than observing that hydrofoil boats are turned by what in aerodynamic terms would be called skidding or slipping, meaning turning with a rudder only. Banking is an artifact of the turning motion changing the relative lift of the inner and outer wings.

[PonoBill]

The front wheel on a motorcycle is a different set of forces. The wheels are big gyroscopes. Twisting the gyroscope of the front wheel precesses the gyro and applies a force perpendicular to the force applied, which makes the bike lean. You can do that consciously if you're trying to go fast, or unconsciously if you're five years old and learning to ride without training wheels.

[Dontsink]

I think our foils are closest to a Hang Glider in terms of control.They are very similar but with lots of quirky differences,like the weight is on top,the fluid density,the tail and god knows what else.

The preturn thingy is just a way to move the CG inside the turn quicker IMHO.We are very high from our support point so any serious lean  requires a long lateral displacement that takes time.So Mr. Antonson twitches the board to the outside of the turn first ,creating that separation in a blink.
Motorbikes ,Race Skiers....they do the same.

[Hdip]

Isn’t the outside turn to inside turn just a Tom curren double bottom turn that has been used forever in surfing?

[surfcowboy]

Hdip, exactly. That's what these guys are saying. This IS how you turn.

It's just way more obvious now with the distance of the mast. In surfing it's just an ankle wobble for most of us so it goes unnoticed. This is my assumption at least.