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Old Jan 06, 2012, 05:20 PM
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Canada, QC, Montreal
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Originally Posted by RdsG View Post
I have to admit that I didn't understand really the principle of wing chord dominated ground effect (as it turned out lately). I thought that ram air is always needed to produce ground effect, but it's not true!
From what I understood of all I read, ground effect always comes from the effect of ground on the airflow around the wing:
- In span dominated ground effect, ground limit the diameter of the vortice since the vortice cannot go below the ground. Less room to move pressure from under wing to above wing equal less pressure escaping at the wing tip = more lift.
- In chord dominated ground effect, the ground create friction and slow down the air below the wing while the air above the wing still have to travel the same profile at the same speed: more difference in air speed = more difference in pressure (Bernouilli principle) = more lift.
- In Ram effect, the ground limit the space below the wing so AoA will create a larger surface where the air "enter" below the wing than where it "exit". If this slow down the air, it will create more pressure = more lift, but it's not always the case. For example, in a symetrical wing at 0 degree AoA, ground creates a RAM effect but that increase average speed below the wing vs. above the wing, which increase downforce, not lift.

To best understand lift, even in ground effect, you should think about air speed rather than pressure. Air speed lowers pressure and pressure difference creates lift. And, contrary to what intuition tells us, a restriction increase air speed, therefore lower pressure, and create downforce, that why in some configuration, ram effect creates downforce if it's dominant vs. air friction on ground which lower air speed and create lift.

I hope this is clear but I'm afraid it's just more confusing than before.
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Old Jan 06, 2012, 05:31 PM
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Originally Posted by sdfgeoff View Post
I have a little theory about single wing elkranoplans (...) it also affects the maximum range of the center of gravity.
I'm afraid you took a wrong shortcut in your theory. The "range" of center of gravity, or I would rather call it the sensitivity to Cg position depends on the wing(s) configuration (wing profile, number and position). A flat symetrical wing still have a reversed Cp travel whether the AoA is 0 or close to stall, so is still more tolerant to Cg position, a lifting wing has a regular Cp travel whatever the AoA and requires more stabilization.

OTOH, a wing closer to the ground has a stronger ground effect which makes it more stable so somewhat more tolerant to Cg position (as long as you stay close to the ground) but using this to move the Cg where it shouldn't be for proper ground effect stability leads to catastrophic reaction when something makes you leave the close proximity to the ground (think of ground effect cars or boats, ground effect downforce reacts the same as ground effect lift).

So, at the end, you better be careful with Cg position because you need to be stable even when not too close to the ground.
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Old Jan 06, 2012, 08:53 PM
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Old Jan 07, 2012, 08:07 PM
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Sometimes I wonder about aerodynamic theories. After all remember the aerodynamics say the bumble bee can't fly!

In my humble opinion lift is all about pressure differential. High below low above!

In my humble opinion lift is all about pushing air down to stay up!

Guess what, the pressure differential happens because we pushing air down. Ok faster air causes a pressure drop, suction, but the part of airfoil aft of max thickness throws air downwards by coanda effect. Even at 0 degrees AOA that happens on a cambered airfoil.

GE increases pressure differential so we need to throw less air down, less AOA so our lift vector tilts forward, less induced drag.

I'm not too sure of the votices being thrown out. The increased press differential would cause the high pressure air to want to flow to the low pressure side more strongly. Well if GE does throw the vortices outward then maybe not as far as we think.

What about circulation? Air is pushed up at the LE and pushed down at the TE but IGE it can't be pushed down too far. Stagnation below the LE pushes the air coming at the LE up more strongly increasing up wash, sometimes causing separation because of adverse pressure gradient. The big negative of vortex action is the increased down wash caused by altered airflow, thus increasing induced drag further. But IGE that down wash is much less.

I haven't given span vs chord dominated GE much thought as I feel that they both wrk together and the AR determines which is dominant.

This reverse CP travel. I think it's just that the circulation, up wash at LE, down wash at TE is trying to tip the wing upward therefore the lift is felt at approx 1/4 chord instead of 1/2 chord. In a cambered airfoil the frontal area presents more of the top surface forward of max thickness than the bottom surface causing airflow to push down on top surface because of its downward sloping angle pushing the LE down giving the airfoil a negative pitching moment that as camber increases overcomes the tilt up moment caused by the circulation.

So here we are accelerating a symmetrical wing and the circulation pushes the LE up because the lift acting at 1/4 chord pushes the forward 1/2 more than the aft 1/2. But as it starts to climb it slows and the upward force reduces causing the LE to drop. So we say the CP has moved back. On a 'self incidence' (NACA1412(?)) airfoil that the camber produces just enough neg pitching moment to cancel this pos moment. A more cambered wing as it accelerates causes more pitch down because it behaves as if it wants to follow the camber line as i mentioned earlier. So do we call that 'normal' CP travel? I'm not sure. But I guess the camber dominates that normal push up acting at 1/4 chord.

Regarding CG for a safe WIG it needs to be stable OGE. IGE the CP moves back making it more stable so to err on the side of caution would mean as CP moves forward as it climbs to OGE it should not go out of CG envelope for safe flight.

Well folks that's my 2 cents for what it's worth.

Cheers
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Old Jan 10, 2012, 07:33 AM
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hi all,

has anyone tried KF airfoils on their WIG models?

cheers

armour
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Old Jan 12, 2012, 01:58 PM
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Originally Posted by captarmour View Post
Sometimes I wonder about aerodynamic theories. After all remember the aerodynamics say the bumble bee can't fly!
The old myth won't die
http://www.sciencenews.org/view/gene..._the_Bumblebee

... and aerodynamics say the wig will fly ...

Luc
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Old Jan 14, 2012, 06:49 AM
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Egyesült Királyság, Skócia, Cumbernauld
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Hi everyone,

Im Attila from Hungary, but now I living in Scotland.
Here is my first ultra-lightweight, simple one line drawing WIG without RC actually.

I made a little DIY video:
WIG DIY (ground effect vehicle) (3 min 4 sec)


Cheers!
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Old Jan 14, 2012, 06:53 AM
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Hungary
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Welcome to the forum!

Üdv nálunk!
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Old Jan 14, 2012, 06:54 AM
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Originally Posted by captarmour View Post
hi all,

has anyone tried KF airfoils on their WIG models?

cheers

armour
Hmm.. I guess you would be the first
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Old Jan 14, 2012, 05:05 PM
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Winterthur ZH, Switzerland
Joined Mar 2009
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hi Attila

I did something similar some time ago! was the "initial spark" of getting involved to the matter

As much as I heard about Kf airfoils I think it would be worth a try. I heard there isn't a big difference to normal profiles (I'm not really up to date, that's what I read some time ago). I think I should try to build a lippisch with a kf airfoil, to be honest the only thing that kept me from building a lippisch was the thought to build a proper profile lol
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Old Jan 19, 2012, 02:27 PM
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I've just tried both a cambered wing and a KF and it is not working. As soon as she picks up speed she wants to dart off to one side or the other. Used to do the same thing with the flat airfoil but I thought it was tip stall but I'm not sure anymore. One thing I know is it was way too nose heavy when I tried OGE. Could be due to too much lift in the back fom the KF.
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Old Jan 21, 2012, 10:14 PM
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Winterthur ZH, Switzerland
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I probably try the Kfm airfoils on a Lippisch type wig. I wonder if it works.


On friday I presented my matura paper (final paper in school) about wig craft and my models, it was a straight A . The main focus was on theory & my models, but (as stated a few pages behind) I made a wind tunnel experiment, I just wondered if I could find something out there. But I have never made experiments like that before, had no idea how to test relevant things and I had relatively few time, so this rather amateur experiment came out. But nevertheless, I want to share the diagram below. But I have to say it's only good for a lala-comparison of the 0.25 / 0.33 / 0.5 AR's of a bixel wig. the numbers may not be 100% correct due to the rather amateur wind tunnel and my methods (), but there was rather clearly visible that the 0.5 wing produces the most lift.

If I find some time, I try to make a flying height (relative to wing chord) - airspeed graph as you Gabriel asked for some time ago. But again, I wouldn't rely on my results, for more professional results I should try to get access to a more professional wind tunnel and plan my experiments more in detail and more accurate.
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Old Jan 22, 2012, 10:20 AM
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Deland, FL
Joined Jun 2002
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Have you guys seen this before? It is a Korean prototype design with a single aft wing, a long forebody, and a moveable canard with twin motors mounted to the canard.

http://www.youtube.com/watch?feature...&v=gyLwK7Akx2I

Although it is sized to be a person-carrying vehicle, it is actually mostly lightweight or foamy construction and actually is remote controlled. That is why it flies so slowly. The same sized vehicle would have to be heavier and fly faster to actually carry a person.

I find it interesting as myself and some friends were discussing whether we could bolt a set of wings onto a kayak sized boat and make a truly personal sized WIG vehicle for zooming round the St. john's river here in FL. The concept would be much like those hovercraft designs that use bolt-on fabric wings but at the bare minimum.

This Korean design seems like it could be adapted to a kayak/canoe type hull.

The design home page is here:

http://triton.naoe.t.u-tokyo.ac.jp/a...ist/WISES.html
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Old Jan 22, 2012, 10:32 AM
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Old Jan 22, 2012, 12:03 PM
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Hungary
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On friday I presented my matura paper (final paper in school) about wig craft and my models, it was a straight A .
Congratulations!

Quote:
I find it interesting as myself and some friends were discussing whether we could bolt a set of wings onto a kayak sized boat and make a truly personal sized WIG vehicle for zooming round the St. john's river here in FL. The concept would be much like those hovercraft designs that use bolt-on fabric wings but at the bare minimum.
I do not recommend this canard design, because it's naturally unstable/semi-stable. Kaien is a computer stabilized WIG, equipped with numerous sensors, and it's still hard to fly according to its designer/builders.

Gabriel
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