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E-foil Foil Wing Design Discussion

Posted: Nov 15 2017 7:30pm
by Max Maker
Please point out if the following logic is right or wrong.

I am not convinced that E-foils should use the same wing geometry that Kiteboards, Wakeboards or Surfboards use. In those three cases, the propelling force acts high above the board. This generates a moment that wants to push the nose down into the water. Therfore, the back wing is inverted and creates downward lift. This counteracts the moment generated by the Kite, Cable or Sail. Remember, they are acting on the exactly oposite end as the motor.

On an E-foil, the propulsion happens very close to the wing, the generated moment is much smaller here and in the opposite direction. So I belive that the back wing is not optimal. I saw a low of videos where the surfer has to lean forward a lot to keep the nose down. So the surfer is acting against the back wing which can only cause drag and wasted energy. The Wings should rather be like on an airplane and going both in the same direction.

I hope this is uplifting news. (Sorry for the pun.)

I think we could push the e-foils development much further if we could create the optimal wing first. An open source design would be amazing, so people could compare their results. Unfortunatley, I don´t know much about flow analysis.

Re: E-foil Foil Wing Design Discussion

Posted: May 13 2018 9:11am
by CobraJet
I just moved onto a lake that requires no gas engines.

It will be very interesting to see your results.

Re: E-foil Foil Wing Design Discussion

Posted: May 13 2018 11:26am
by LockH
Hehe... Had to look up watt yer tapping about. As seen here?:
http://efoil.builders/about

Re: E-foil Foil Wing Design Discussion

Posted: May 13 2018 11:46am
by Chalo
Max Maker wrote:
Nov 15 2017 7:30pm
The Wings should rather be like on an airplane and going both in the same direction.
An airplane's horizontal stabilizer is not set at the same angle of incidence as the wing. It is designed to cause a downward force on the aircraft's tail. But it is often able to be trimmed to different angles for different loading conditions.

I think you're right that the angle of incidence of the rear foil should change if the propulsion exerts a much smaller pitching moment than usual. But that's not the same as having the front and rear foils set to the same angle of incidence.

Image

Re: E-foil Foil Wing Design Discussion

Posted: Oct 13 2018 1:03pm
by classicalgas
RE: Both wings lifting. *Some* airplanes generate lift at both ends, using a small wing ahead of the main wing ("canard") and no tail surface, and they are efficient designs, but tht's not typical. Canard layouts depend on stalling the front, smaller wing, before the main wing for dynamic stability, so the front wing runs at a less efficient, higher angle of attack than the main wing.

That's arguably better than running the tail at at negative lift, in order to keep the main wing at a lift generating angle of attack.

Re: E-foil Foil Wing Design Discussion

Posted: Oct 13 2018 1:21pm
by Voltron
From my fast sailing experience, when you're not driving the sails hard enough you can feel a flutter in the rudder from the disturbed flow off the daggerboards ( or keel, or centerboard, whatevs) hitting the rudder, so keeping your rear foil out of the wake of the first one is a consideration and intentionally trimming the rig to where the rudders are running at a slightly higher incidence angle is normal than the forward foil(s). It creates way more effective lift. You could feel it on my hydrofoil trimaran too... if things were set so the tail foil wasn't lifting too it was really really hard to lift off and get foiling...

One gigantic difference between water based foilers and airplanes is that with aircraft, its essential that it wants to naturally dive a little, otherwise it can lock into stalls and never recover. On the water, you pretty much always want to go up. You might limit the lift a little when you're really cooking, but its a whole different thing if you stall the tail a little even if you wipe out compared to control recovery in the air.

Re: E-foil Foil Wing Design Discussion

Posted: Oct 13 2018 1:40pm
by Voltron
Its great seeing the mixing of ideas that sends you off on a google search for foilers without the drive force acting from high above the water and you find something like this,,,lol


Re: E-foil Foil Wing Design Discussion

Posted: Nov 25 2018 1:52am
by classicalgas
"...On the water, you pretty much always want to go up. "...Don't you get "porpoising" if a foil breaks the surface?

Somewhere I've seen surface skimming linkages that auto trim the front fol to prevent that.

Re: E-foil Foil Wing Design Discussion

Posted: Nov 25 2018 5:12pm
by Voltron
You definitely don't want to be breaking the surface...I did that on my front foils right into a wave at top speed and went farther underwater than I ever have in a boat.

What I meant was, if you stall the tail in a boat, it just noses up, loses speed, and settles back down into the water as you get things going again. You're also not usually foiling in shallow water where settling will crash you or kill a bunch of passengers.

With aircraft if you stall the tail you can go into deep stalls that can be unrecoverable, esp while coming in for landing... It just can't be allowed to happen or terrible things ensue, where with a recreational boat you don't need to design the foils around positive passive stall recovery.

And yes, surface sensing wands that are mechanicaly linked to the foils are a common control method. Moth class boats also have a linkage in the rudder so when you twist the tiller it moves the trim tab on that foil while the wand does the front.