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hydrofoil vs. aerofoil

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Kevin Brooker
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Re: hydrofoil vs. aerofoil

Postby Kevin Brooker » Tue Nov 29, 2016 12:06 am

Tom Spear has written many papers and they are all really good at explaining why different foils work better then others. The Reynolds number is a way to scale foils to work in different mediums like air and water. Aerofoils have limits because water is s much denser than air and the water is not compressible while air and other gasses are. If you had a foil good for water it will behave very differently if you sailed it in a sea of oil (less dense) or mercury (significantly more dense). An airfoil will behave differently in an atmosphere of hydrogen and bromine compared to the mixture we have here on earth. To obtain similar performance we can use Reynolds numbers to find a foil which works in one medium and scale it to work in another. The density of the medium is a parameter in the Re formula.

To measure performance (whatever the parameter we want to study) it is imperative to be able to alter the foil configuration and set up on the go. As the thread has shown, performance has many requirements for each user. Pure speed is easy to establish but at what cost to the rider? If the run is 500 meters we can put up with a lot of discomfort for the short duration. If we are trying to go fast all day a foil which is user friendly might be slower over 500 meters but is much less demanding to use and will be fast over 100 km.

What if we had a foil which was fast and easy to use but could be changed to be really fast at the expense of usability For racing, the fast and easy could get us through most of the race until we need to break free of the mob or in a sprint to the finish. Just as in many forms of human powered racing (the wind powers the kite but the human is the interface between the foil and kite) there are time when we must push to the limit and recover to do it again. Variable geometry can do this.

Changing how the foil sits in the water might change the way the human feels the stress of using it. What if we can rotate the foil with relation to the board. The lift vector will change and so will the experience. Reflexing the foil at speed will alter the drag which will change how the rider needs to balance everything.

When I fly different sailplanes the experience is different when flying a 15 meter flapped ship and a 13.5 meter ultralight. When the flaps are reflexed on the 15 m ship you can feel the sailplane accelerate and penetrate the air. When slowing to thermal dropping the flaps changes the handling and the ship climbs better in the high wingloading and slow speed environment then if I dropped flaps and intended to go fast interthermal..

If my kite foil can adapt to hanging speed (wind ) I should have a ride that is more fun. The fun will be based upon the workload. Some gliders are fast as hell but take a lot of work to fly well. There are ships which are easy to fly and are lower performance but the poimt to point speed is higher because of the lower effort required to fly the ship well. A Foil boards should be the same and I think being able to make changes on the go are realistic and not complicated.

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Re: hydrofoil vs. aerofoil

Postby skyte » Tue Nov 29, 2016 12:30 am

I would agree that there is potential for gaining benefits from fine tuning on the fly. I think it might be more complicated than just changing the AoA of the main wing though. I think you might have to change the AoA of the rear wing too and possibly things like mast position and fuselage length to keep everything balanced.

I am a VERY inexperienced foiler so I am really just guessing based on trying to understand the design but it looks like each foil has a balance of lots of parameters and a small change to one parameter can mess up the balance quite considerably.

It's like balancing an aeroplane where all the passengers sit almost 2 wingspans above the cockpit! :)

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Re: hydrofoil vs. aerofoil

Postby ronnie » Tue Nov 29, 2016 12:47 am

Kevin Brooker wrote:Tom Spear has written many papers and they are all really good at explaining why different foils work better then others. The Reynolds number is a way to scale foils to work in different mediums like air and water. Aerofoils have limits because water is s much denser than air and the water is not compressible while air and other gasses are. If you had a foil good for water it will behave very differently if you sailed it in a sea of oil (less dense) or mercury (significantly more dense). An airfoil will behave differently in an atmosphere of hydrogen and bromine compared to the mixture we have here on earth. To obtain similar performance we can use Reynolds numbers to find a foil which works in one medium and scale it to work in another. The density of the medium is a parameter in the Re formula.

To measure performance (whatever the parameter we want to study) it is imperative to be able to alter the foil configuration and set up on the go. As the thread has shown, performance has many requirements for each user. Pure speed is easy to establish but at what cost to the rider? If the run is 500 meters we can put up with a lot of discomfort for the short duration. If we are trying to go fast all day a foil which is user friendly might be slower over 500 meters but is much less demanding to use and will be fast over 100 km.

What if we had a foil which was fast and easy to use but could be changed to be really fast at the expense of usability For racing, the fast and easy could get us through most of the race until we need to break free of the mob or in a sprint to the finish. Just as in many forms of human powered racing (the wind powers the kite but the human is the interface between the foil and kite) there are time when we must push to the limit and recover to do it again. Variable geometry can do this.

Changing how the foil sits in the water might change the way the human feels the stress of using it. What if we can rotate the foil with relation to the board. The lift vector will change and so will the experience. Reflexing the foil at speed will alter the drag which will change how the rider needs to balance everything.

When I fly different sailplanes the experience is different when flying a 15 meter flapped ship and a 13.5 meter ultralight. When the flaps are reflexed on the 15 m ship you can feel the sailplane accelerate and penetrate the air. When slowing to thermal dropping the flaps changes the handling and the ship climbs better in the high wingloading and slow speed environment then if I dropped flaps and intended to go fast interthermal..

If my kite foil can adapt to hanging speed (wind ) I should have a ride that is more fun. The fun will be based upon the workload. Some gliders are fast as hell but take a lot of work to fly well. There are ships which are easy to fly and are lower performance but the poimt to point speed is higher because of the lower effort required to fly the ship well. A Foil boards should be the same and I think being able to make changes on the go are realistic and not complicated.
This is a hydrofoil where the AOA is changed relative to the mast.
http://www.glidefree.com.au/Operating%2 ... s%20R4.pdf

If you can change the AOA of the one wing, you may only need one wing, which will have a centre of lift at the one position. No fuselage, no second wing, just one wing.

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Re: hydrofoil vs. aerofoil

Postby Kevin Brooker » Tue Nov 29, 2016 1:27 am

Ronnie. Thanks for the link. The system looks interesting.

The ability to change the foil along the span is also interesting. Interference drag near the mast is different from the drag near the tips. Changing the fpoil long the span might yield very good results; it might not. The ability to alter the foil profile along the span exists and is worth exploring. Using a single wing is also an interesting design idea and there will need to be some washout to balance the tucking tendency of most single plane set ups. The main wing and tailplane does this as does a canard. Eliminating one foil and the associated drag will be good only of the single plane has the ability to compensate efficiently.

Changing one aspect , AoA for example, will have an effect on other things but these can a;ll be balanced.

As the speed increases the induced drag of lift goes down and the parasitic drag of moving through the medium goes up. At some point they change over so reducing the biggest offender will yeild the biggest gain in speed. Most foils used on boats have a wand and it will be beneficial to reduce externalities and have the changes be in direct control of the pilot. Having all control will be stressful but having control over one or two parameters of the wing;s interaction with the water will be doable. The system needs to be simple and intuitive.

Thanks for kicking around ideas and please keep 'em coming.

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Re: hydrofoil vs. aerofoil

Postby ronnie » Tue Nov 29, 2016 11:48 am

Kevin Brooker wrote:Ronnie. Thanks for the link. The system looks interesting.

The ability to change the foil along the span is also interesting. Interference drag near the mast is different from the drag near the tips. Changing the fpoil long the span might yield very good results; it might not. The ability to alter the foil profile along the span exists and is worth exploring. Using a single wing is also an interesting design idea and there will need to be some washout to balance the tucking tendency of most single plane set ups. The main wing and tailplane does this as does a canard. Eliminating one foil and the associated drag will be good only of the single plane has the ability to compensate efficiently.

Changing one aspect , AoA for example, will have an effect on other things but these can a;ll be balanced.

As the speed increases the induced drag of lift goes down and the parasitic drag of moving through the medium goes up. At some point they change over so reducing the biggest offender will yeild the biggest gain in speed. Most foils used on boats have a wand and it will be beneficial to reduce externalities and have the changes be in direct control of the pilot. Having all control will be stressful but having control over one or two parameters of the wing;s interaction with the water will be doable. The system needs to be simple and intuitive.

Thanks for kicking around ideas and please keep 'em coming.
This is a sketch of one way to alter AOA. The centre of lift would be slightly behind the hinge on the wing. The reason for a torsion bar spring is that it is linear in operation and it can occupy a very narrow space. The rod could be so thin that it operates much like a cable, as it is always under tension.
One Wing Foil.jpg

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Re: hydrofoil vs. aerofoil

Postby tahoedirk » Wed Nov 30, 2016 3:32 am

Nice chat guys,
I realize more than ever that the more I learn the less I know. My resources have been airfoiltools.com and my DLG building experience and whatever else I can find . It has taken an unprecedented amount of work to build what I have and a great deal of time to learn to ride them. I am so frustrated at times I want to quit trying and be a consumer, I have too many foils already. Some production hydrofoils ride so well nowadays .

I love the idea of being able to adjust on the fly, but only AOA? When I ride in a airplane glued to the window watching the mechanics of the wing in action it is marvelous. To bring these functions to a hydrofoil sounds like a daunting task. This summer I spent a lot of time adding just the slightest bit of camber to my main wing. After much effort I nearly ruined my well tuned foil . Many hours of further " refining" and days later I think it was working better, no camber in wingtips. Without scientific controls , I can actually hardly remember. There are so many variables I barely know what my next move will be, so I just keep trying.

Kevin , if you have the energy go for it , explore your ideas . I , for one, am listening intently . You might have to quit your job and abandon your family. These subjects have not really been publicly discussed in English anywhere that I have looked . I am so pleased to even read along. I do suggest obtaining a foil and learning to ride first or at least in conjunction with your DIY. I finally broke down this summer and bought a inexpensive spotz2 to scrutinize in private. It is nice, fast, very pitch stable , but IMO not nearly as fun and playful as what I am after.

One of my favorite airplanes right now is a Crack Yak foamy with flat plate wings, no airfoils at all , it flies so well. The age old discussion of whether a wing is creating lift or simply deflecting and diverting air or water is so legitimate.

I really want to make an electric powered foil but I am not sure the power system is in my budget at the moment. It's is hard to invest so much R&D when way more than half my great ideas flop.

Go for it and keep up the chitchat.

thanks , Dirk

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Re: hydrofoil vs. aerofoil

Postby Blackrat » Wed Nov 30, 2016 7:42 am

Changing the AOA of the front wing isn't going to do anything , no different from shifting your weight
However , having a small flap that can be reflexed at high speed or cambered at low speed would be very interesting

I personally would like to have a 'floating ' stab with a gyro on it , if I think of how stable an rc aircraft becomes when FBW (fly by wire) is activated , I can only dream of what it will do to a foil

Maybe my next project after my electro foil .. It's almost done :-)

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Re: hydrofoil vs. aerofoil

Postby Johhnn » Wed Nov 30, 2016 1:06 pm

A long time ago I heard someone talking about some of the promises of composite materials. He suggested that you could make something that, due to the orientation and flexibility of the fibers, would bend in the appropriate way as conditions changed. I'm wondering if it would be possible to have the small flap be thin enough that at high speeds it flexes in such a way that the camber is reduced. With no user input.

One of the first times I flew on a 747, I was seated over a wing. I watched as we went down the runway and slowly the wingtip lifted up. By the time we took off, the wingtip was many feet higher than when the plane was at rest. It was a little unnerving to see all that metal change shape like that and then move in unusual ways during turbulence.

The other thought I had on this stuff is that someone with a 3D printer, CAD software and a water tunnel could turn out many prototypes and test them very rapidly.

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Re: hydrofoil vs. aerofoil

Postby Kevin Brooker » Wed Nov 30, 2016 1:17 pm

The DLG (discus launch glider) experience will be super helpful building a successful foil. With enough power you can fly a barn door. The plates work because the mass being lifted is so small. If planks were so efficient you'd see them on sailplanes and airliners. The takeaway of this for me is there are certain tools which work best for certain applications. I doubt there will ever be a foil which is super entry level friendly and able to win races on the WC.

Right now the focus is on the foils but where are there gains to be made. What about the mast? When the foil is tilted the lift vector resists the pull of the kite . What if the mast was sectioned or configured to provide a lifting force towards the surface. When flying a sailplane and thermaling at a 45 degree angle a bit of top rudder helps relieve the stick forces and stabilizes the turn so airspeed remains more constant and the climb is much more efficient. When tipped at the 45, the rudder still provides yaw control but the vector holds the nose up from the horizon. Most of the mast might be above the water but every little bit helps and what a marketing piece to boot.

With solid state gyros active stabilization is possible and might allow for hyper sensitive rigs which are fast but unridable if the battery goes dead. Battery technology is pretty good today so the weight penalty will be small.

Just shifting the AoA without retrimming to the stab is inefficient. The Angle of Incidence (difference between the main wing and stab) might be the best mechanism for fine tuning. Anytime the main wing is altered the tailplane should be retrimmed and this is where automation might be very helpful.

What on the fly adjustability might bring is the ability for a rider to tune the board as they like it. Most will just leave it alone after getting the rig to where they want it. Adjustabilty might also be nice enabling a rider to figure out what they like and then buy(make) a less expensive and less complicated fixed rig.

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Re: hydrofoil vs. aerofoil

Postby Mossy 757 » Wed Nov 30, 2016 3:11 pm

Johhnn wrote:He suggested that you could make something that, due to the orientation and flexibility of the fibers, would bend in the appropriate way as conditions changed...With no user input.
There was a video put out by an America's Cup team (Ben Ainslie?) saying exactly that...all of their R&D is aimed at getting the right kind of dynamic properties built into their foils so they can flex in the right places at the right time without requiring a control system. The idea being like you said, the foil does one thing during acceleration and something similar but not the same at top speed depending on wing loading.


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