Sudden deep ventilation ?

[revhed]

Just one more thing: using CFD sotware or building a "soft" aluminium front wing, you will notice that the resulting shape of the wing under load is a "seagull" shape.

Sorry to post so much, snowing hard here and my workshop is cold.......
But this is to good.....
Real life proof of the computer modeling from A K.

before, very low A R 4mm aircraft alu wings from 2009

After MANY hours, F wing bent during and due to flight to GULL shape, interesting?

And this F wing is a prized possession, unfortunately not mine.

GULL WING FRONT.jpg (8.92 KiB) Viewed 1510 times

And often when I see birds glide I see this beautiful form.
Do we learn something?

R H

[tkettlepoint]

I thought the same about the profile of the wing so I designed mine that way.. Mine isn't 100% the same but close

terrie

[Peter_Frank]

Great to hear different suggestions and experiences.

Apart from some impliyng it could be good if true, as then they have an excuse when learning ha haa - it seems to be happening quite a lot ?

It seems we agree that it is a violent and fast separation thus loss of lift, we experience, not a max speed only, but also at medium speed.

Will try to summarize (without too much explanation), what the reasons I noted and these added are, in no particular order.


Causes for sudden loss of lift when foiling medium or fast and not close to the surface:

1. Underwater bubbles/whirls.
2. Delayed surface ventilation.
3. Hitting seagrass.
4. Hitting a fish.
5. Parasitic drag from seams and bolts, causing ventilation/separation.
6. Cavitation at a certain speed for a certain design.
7. Design of the wing profile, and overall design.
8. LE too sharp or too blunt (almost same as #6)
9. Turbulent flow transition.
10. Soft mast twisting (and maybe wrong profile).
11. Wing and LE imperfections/scratches.
12. Mast surface ventilation travelling down to the wing.
13. Non gull shaped wings.


I particulary noted RH saying that it could be the "environmental issues" on a given day, as he has sometimes seen many experience this phenomenon the very same day at a spot ?
Which heels to #1 in that case.

#9 laminar/turbulent flow transition, well, then one could try to sand down the "perfect" surface with wet paper no 600 or 1200 to make it matte, and see what that does (havent done this myself yet).

#13 I must say I personally dont believe one inch whatsoever regarding this very topic.
And have never seen any useable similarities from birds being usable in glider design as such.
Even though I love watching and following seagulls and bird of preys in their fantastic flight

#10 seems to be a known issue !
Not the issue in my case I think, as my mast and wings are unbelievably stiff, and it is not extremely thick or bad profiled either.

#3 and 4, most are saying it would definitely feel different and you will know if you hit seagrass or a fish, so in my case not that (maybe not belonging to the list if one can feel this right away ?)

#5 is interesting of course, if the seams/bolts could be the reason - although I find it hard to believe that this turbulence could move out over the wings so drastically ?
One could glue the wings and bolts and polish to get super smooth junctions, to see if it cures the phenomenon.
I would think it would lower the drag yes, but not fully convinced it is the cause for the sudden full loss of lift.... But could be wrong.

#6, 7 and 8 are somewhat the same, and of course extremely important no doubt.
Its just, having tried a few quite different hydrofoils and my different wings and profiles, both cambered and very low cambered and very different area - they still occasionally does this at speeds much lower than where the WR challengers are fighting the cavitation issue, so I am not so sure really that it is #6 cavitation... But could be wrong.

#12 is interesting, eventhough the mast is behind the wing, so seems unlikely it influences ?
Of course it could lead to erratic behaviour and loss of control when hitting the stabilizer, agree.
It just, it does not feel like that, in my case at least.... But could be wrong.

#2 delayed surface ventilation.
It seems noone really commented this one - and I dont think this is what happens either.
But my thoughts were, that you get a locally very small ventilation on part of the wing at an early stage of your "run", maybe the tips giving small vortices and keeping this air for a long time, untill suddenly it "breaks" as it transforms over the wing because of the low pressure at higher speeds ?
We know that going at higher lift when carving, all of our different wings can generate these visible small air vortices following for the full turn - this is also known and can be seen quite easy (looks good in fact although drag).
But they usually go away again when at lower AOA on the straights after the turns.
Besides, it does not feel like air - but then again, when travelling relatively fast, your wing could get a small local ventilation from a whirl from even small surface chop waves, which follows for a loooong time when you go ahead foiling deeper - so you dont mentally connect the issues of sudden drop of lift with the earlier incidence (as you can not feel anything)

It could also be a mix of several of above causing it, and most likely different of us might experience different reasons of the 13 we have noted.


Its just, I have a strong feel that many are dealing with the very same problem - and we dont have any idea yet as to what it is ?

Now it becomes REALLY interesting

PF

PS: Yes it is winter here, but air and water is around 34 to 36F both now so not warm but still okay rideable of course, as not ice.
The reason I dont give more details Bill, is because it seems to be a more common issue than what I first thought, and because I have experienced it on very different hydrofoils/wings.
Always on halfwind or broad/deep reaches, going medium or really fast (relative to the top speed of the given wing though) as I recall.
And also because it is winter so not out as much because it gets dark before you are home from work - so weekends only or taking off work early on good days.

[ronnie]

Great to hear different suggestions and experiences.

#2 delayed surface ventilation.
It seems noone really commented this one - and I dont think this is what happens either.
But my thoughts were, that you get a locally very small ventilation on part of the wing at an early stage of your "run", maybe the tips giving small vortices and keeping this air for a long time, untill suddenly it "breaks" as it transforms over the wing because of the low pressure at higher speeds ?
We know that going at higher lift when carving, all of our different wings can generate these visible small air vortices following for the full turn - this is also known and can be seen quite easy (looks good in fact although drag).
But they usually go away again when at lower AOA on the straights after the turns.
Besides, it does not feel like air - but then again, when travelling relatively fast, your wing could get a small local ventilation from a whirl from even small surface chop waves, which follows for a loooong time when you go ahead foiling deeper - so you dont mentally connect the issues of sudden drop of lift with the earlier incidence (as you can not feel anything)

PF

PS: Yes it is winter here, but air and water is around 34 to 36F both now so not warm but still okay rideable of course, as not ice.

In this video at 1:24 there seem to be eddies left behind by the mast and air travels down them. Maybe if you had a foil design that could create eddies of a type that could be a path for air down to the rear wing? The sudden change of lift in the rear wing may tilt the front wing down?
Does it feel like you hit something? - or does it feel like the front wing didn't slow down but nose-dived?

[revhed]

13. Non gull shaped wings.
#13 I must say I personally dont believe one inch whatsoever regarding this very topic.
And have never seen any useable similarities from birds being usable in glider design as such.
Even though I love watching and following seagulls and bird of preys in their fantastic flight

Sorry P F,
Never ment to imply that non gull shaped wings relate to your question at all, just replying to A Ks computer model finding, off topic, sorry.
R H

[zfennell]

My apologies for more questions with no contributions.
But a couple of things still don't add up for me.
First.
Gunner and scuba state that their "glitches" caused them to fall to windward.
This was due to the strut losing lateral lift.(either ventilation or separation) ?
Second.
Peter has left me with the impression that his front wing has lost lift causing him to fall forward.
Did I read that correctly?
Third.
Gunnar's vid clearly shows the strut twisting or least showing a change in yaw angle.
What force or moment caused this catastrophic change in AOA of the strut?
1- center of pressure of the front wing?
2- center of pressure of the strut, itself ?
3- center of pressure of tail / vertical stabilizer ?
4- yaw input from rider ?
In other words, is the strut twisting due to unbalanced / unsteady forces at the bottom or top?
Any thoughts would be appreciated .
Bill

[Johnny Rotten]

has anyone considered or experimented with gates on a hydrofoil?

A well placed/designed gate could

a) provide ventillation resistance.
b) limit twist and provide torsional stability.

at the expense of a minor amount of drag.

I'm sure the problem could be solved with VERY thorough design but it would likely require a VERY thorough knowledge of hydro/aerodynamics that is beyond the average hydrofoil maker and it appears to have a factor that is dependent on conditions as well.

Sometimes more "drivable" makes the race car faster.

[plummet]

hmmm.....

If it happens at medium speeds as well as high speeds you would think that would rule out cavitation or laminar/turbulent flow transitions as that should happen at the same speeds each time.

I'm leaning more towards Gunars mast deflection theory.

Here's an od ball thought.

What about a sudden change in current speed/direction in the water?

[Peter_Frank]

hmmm.....

If it happens at medium speeds as well as high speeds you would think that would rule out cavitation or laminar/turbulent flow transitions as that should happen at the same speeds each time.

I'm leaning more towards Gunars mast deflection theory.

Here's an od ball thought.

What about a sudden change in current speed/direction in the water?

Not really, as I might not be able to remember at which speed it is for which wing - so of course a different profile with have a different speed where this happens.

AND, the environment can not be without influence I think, meaning if there are small slalom like whirls (not air) under the surface one day, the transition might simply start at another speed, and not the very same as in smooth water ?

So could be my memory that jogs me a bit... Or not.

Soft mast twisting is as said not the reason in my case, but would be in some it seems yes.

Bill is right, I feel it as a sudden loss of lift and falling forward, whereas the twist seems more irregular and must be two different issues

Change in current could very likely be the cause yes, althoug we ride at deep water closed bay often, without any noticeable "main" current - so I doubt it would generate fast local currents ?

Maybe the flow is also extremely stable when a foot or more under the surface, so my theory about the environment (slalom whirls following the surface small chops in a pattern) is also wrong ?
Or what DO we actually have one or 1½ foot under the surface in 10-15 knots of wind regarding water flow (clean/laminar or S bent sometimes), when no swell only wind chops, and how does it affect the maybe different causes for loss of lift ?

PF

[Andokite]

13. Non gull shaped wings.
#13 I must say I personally dont believe one inch whatsoever regarding this very topic.
And have never seen any useable similarities from birds being usable in glider design as such.
Even though I love watching and following seagulls and bird of preys in their fantastic flight

Sorry P F,
Never ment to imply that non gull shaped wings relate to your question at all, just replying to A Ks compputer model finding, off topic, sorry.
R H

Peter Frank please check my post again.
I stated that a particular shape seems to work well, I never said that the "seagull shape" is the only profile that avoids sudden whiplash.