*


All times are UTC + 1 hour



Post new topic Reply to topic
 [ 18 posts ]  Go to page Previous  1, 2
Author Message
 Post subject: Re: Foil plans?
PostPosted: Sun Mar 18, 2012 3:55 pm 
Offline
Medium Poster

Joined: Thu Aug 04, 2011 4:21 pm
Posts: 64
Location: Ottawa, Canada
Anhedral slows down the outward flow of water which effectively gives you longer span without the drag of more span.

It also makes the foil less stable.

Dihedral makes the foil more stable but encourages spanwise flow outward.

Same as an aircraft I presume.

If you want a better understanding here is the short version but google 'wing tip vortices' for the extended explanation..

Basically under the foil the water is being compressed and on top it is lower pressure. The water doesn't like to be compressed or reduced in pressure so it tries to flow from the higher to the lower pressure.

When you are moving along, the only way it can do that is to move sideways out towards the tip. When it gets to the tip it tries to flow around the tip to fill the low pressure on top. It ends up curling around somewhat and on an aircraft you can see this if you have a day where it is cool and moist. The extreme low pressure on the wingtip or around flaps sucks the high pressure air which is somewhat warmer from being compressed and when the two temperatures collide the warm moist air hits the cold mixes and the moisture condenses causing a long stringy curling vortice to be visible.

This curling air is called a wingtip vortice and is pure drag, useless wasted energy.

The amount of air that makes it around the wingtip compared to the span of the wing is one of the main determinations of the efficiency of the wing. That is why sailplanes have long skinny wings.

However long skinny wings are fragile and make a lot of drag at high speeds and can twist easily which can be disastrous and therefore require extra weight in structure reinforcement.

Another solution is to put upward pointing or downward pointing winglets to capture that spanwise air/water flow. Downwards is better for low speed but gives you less wiggle room on landing if the wingtip drops. In a 747 size aircraft, the efficiency gains might be 5%. It also makes the controls feel tighter. (more powerful)

Also sometimes wing fences are put on wings (typically on top) to stop the spanwise flow of air to give enhanced short field takeoff performance.

There is a whole science to aerodynamics and whatever the name is of the water equivalent.


Top
Profile
 Post subject: Re: Foil plans?
PostPosted: Sun Mar 18, 2012 7:36 pm 
Offline
Frequent Poster

Joined: Mon Sep 02, 2002 1:00 am
Posts: 477
Location: Teahupo'o, Tahiti
SaulOhio wrote:
Why does everyone build foils with anhedral instead of dihedral?


A lot of the stability/tracking comes from the centre upright going into fuselage. Adding anhedral and dihedral to the foils adds some more stability. Preference is usually to have the front foil with an anhedral angle and the rear foil with dihedral. This helps take the rear foil out of the turbulence from the front foil and fuselage. This was just a mock-up final will have rear foil with dihedral mounted on the top. I've some some foils with both foils with anhedral angles they also seem to work.


Top
Profile
 Post subject: Re: Foil plans?
PostPosted: Tue Mar 27, 2012 1:28 am 
Offline
Very Frequent Poster
User avatar

Joined: Sun Sep 04, 2011 5:37 pm
Posts: 1778
flyyboyy wrote:
...
There is a whole science to aerodynamics and whatever the name is of the water equivalent.


It's called : Hydrodynamics or Fluid Dynamics

http://en.wikipedia.org/wiki/Fluid_dynamics

Secondly -- You seam to have a Really Good understanding of this subject there -- flyyboyy-- !!
I'm gonna assume you do stuff that Flies ??

As i was reading ; i kept waiting for an unexplained opening in your statement ...
There Wasn't any ; Ya explained each item rather Well !!!!!

Bille


Top
Profile
 Post subject: Re: Foil plans?
PostPosted: Tue Mar 27, 2012 2:24 am 
Offline
Medium Poster

Joined: Thu Aug 04, 2011 4:21 pm
Posts: 64
Location: Ottawa, Canada
Thanks Billie,

Hydrodynamics is somewhat different due to the density but a lot similar to aerodynamics. There is cavitation which is a bit like stalling I suppose.

I am a pro pilot. (retired) I started out teaching flying many moons ago. However I have been an aviation junkie since my dad took me up when I was ten or younger. My brain is airplane wired. Even when I am driving the car. I caught myself putting down flaps (pulling the emergency brake on LOL) one day after a long period of intense instructing.

My hobbies have been wind/water.
Windsurfing/KS/canoeing/catamaran/monohull/ model gliders, helicopters, fixed wing/hang gliding paragliding/fighter kites The only thing I haven't flown is a gyrocopter.


Top
Profile
 Post subject: Re: Foil plans?
PostPosted: Tue Mar 27, 2012 4:38 pm 
Offline
Very Frequent Poster
User avatar

Joined: Sun Sep 04, 2011 5:37 pm
Posts: 1778
JMF wrote:
...
Still some tweaking to be done but once I've finished T-bar I can go to town on the foils.
...

What's the AOA, (angle of attack ) on your front and rear foils going to be ?

SaulOhio wrote:
...
Now I am working with a CNC machine. A friend of mine got me in touch with a guy who uses one for woodworking.

This way I could design the foils any way I wanted on my computer.
...

What is the Cad-Cam system operations that the CNC machine uses ; any idea ?

A foam cutter in Vegas has a system called BOB-CAD, which i purchased for this purpose;
havent learned it yet.

There are plans on the internet for building the actual machine for a home made cutter used
on foam or a medium density partial board.

For most of my hand shaped mail-blanks i "ALWAYS" finish with Epoxy, ((Well Worth the expense !! )) Then
apply 4 coats of wax and PVA. With an Epoxy finish on the mail blank; if Ya miss a spot with the PVA, Ya
probably won't loose the mail blank when making a female mold from it because the Shore Hardness of both
surfaces is the same.

Bille


Top
Profile
 Post subject: Re: Foil plans?
PostPosted: Tue Mar 27, 2012 5:38 pm 
Offline
Frequent Poster

Joined: Sun Feb 12, 2006 8:16 pm
Posts: 410
Location: Kettle Point Ontario Canada
Billie What i have been told is to stay away from BOB-CAM. not a very good program for the G Codes.

Mach is alot cheeper and works... I am in the mids of building a 5x10 CNC router for our rough shape of the water boards and shape the snowboards. Going to go back to night classes again for update Solid works too... Not sure if I am too old to learn it but what the hell. I will give it a shot.

Once i get my CNC built I think I am going to trackle a foil board now too LOL just something different .

No worries about the beer I might need a hand someday , with something

Terrie


Top
Profile
 Post subject: Re: Foil plans?
PostPosted: Sun Jun 23, 2013 9:24 pm 
Offline
Very Frequent Poster
User avatar

Joined: Wed Aug 24, 2011 11:39 am
Posts: 1068
"SaulOhio

Why does everyone build foils with anhedral instead of dihedral?"




Because the wings actlike an anchor in the water against the pull of the kite. Anhedral is more efficient than dihedral.


Top
Profile
 Post subject: Re: Foil plans?
PostPosted: Mon Jun 24, 2013 4:35 pm 
Offline
Medium Poster
User avatar

Joined: Tue Jul 19, 2011 5:26 pm
Posts: 196
Location: Langebaan (Cape Town)
Hawaiis, at first I could not really understand what you meant in your answer, but thinking it through it totally makes sense. The anhedral tends to add a better hold capability at speeds basically under stall speed.

But I have been putting a lot of thought into why anhedral, and I am quite sure that the biggest reason is for the control/dynamics, so that a yaw input gets translated to a roll, even if only slightly. I've got two foils at the moment, one with anhedral and one totally flat and the differences are less than I expected.


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic
 [ 18 posts ]  Go to page Previous  1, 2

All times are UTC + 1 hour


Who is online

Users browsing this forum: No registered users and 3 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group