Ekranoplans and ground effect

[aztek]

hi im building a model (R/C) ekranoplan at school and i would welcome any help with my design, I plan to have three motors and a tail fin for steering and propulsion. [IMG][/IMG]
P.S:images of different Ekranoplans
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[kach22i]

Cool, I would love to see one of those things flying, I've seen many videos on Google and Youtube of similar efforts.

Try looking in this forum too:
http://www.rcgroups.com/forums/showthread.php?t=819297

[daiquiri]

Originally Posted by aztek

hi im building a model (R/C) ekranoplan at school and i would welcome any help with my design, I plan to have three motors and a tail fin for steering and propulsion.

Ekranoplans (or WIGs, as they are often called) are great machines! An example of an intelligent use of knowledge about fluid dynamics, imho.
You could read first this excellent review of WIG-related issues (thanks to Australian government ):
http://www.dsto.defence.gov.au/publi...TO-GD-0201.pdf

Try to understand well issues related to WIG's longitudinal stability. Use an airfoil with the biggest wing chord you can. It will lead you to a very low AR wing which is ok - high AR wings are not very sensible to ground effect. A reverse-delta wing planforms (triangle tip pointing backwards), positive dihedral angle and airfoils with S-shape camber give an inherently stable platform.
A negative dihedral would increase the sensivity to ground effect and the maneuvrability of the vehicle, but would also result in a less stable platform and could touch the ground during turns.

Some more readings here:
http://www.se-technology.com/wig/index.php (go to "theory" section)
http://www.seaircraft.com/index.htm
http://www.aerospaceweb.org/question...cs/q0130.shtml
And then there is also an RC forum about WIGs:
http://www.rcgroups.com/forums/showthread.php?t=665918

Finaly, you can find an excellent applet in this site:
http://www.mh-aerotools.de/airfoils/javafoil.htm
which allows you to design and/or analyze the airfoils with or without the ground effect. You can find the instructions about how to include the ground-proximity effect here:
http://www.mh-aerotools.de/airfoils/jf_wig.htm

When you have some drawings to show, don't hesitate!

[clmanges]

Such an outrageous vehicle . . . it certainly earned its nickname of "sea monster." Very impressive. Good luck with the project!

Just curious, what kind of finished size or scale do you have in mind for the model?

[aztek]

Hi
thanx to evryone for their posts,
Ive changed my design slightly and will trty to get some images uploaded this week, Ive had coments made that i would need aerolons as well? any coments on this ?

PS:clmanges: about A3 size.

[clmanges]

PS:clmanges: about A3 size.

Sorry, I'm not familiar with this term . . . how about total length?

[daiquiri]

Originally Posted by aztek

Hi
thanx to evryone for their posts,
Ive changed my design slightly and will trty to get some images uploaded this week, Ive had coments made that i would need aerolons as well? any coments on this ?

You are talking about aelerons or ailerons, I presume.
Aelerons are movable horizontal control surfaces at the outer part (towards wing tips) of the wing's trailing edge. In case of conventional airplanes they are used to control airplane's roll angle during turns, during flight with asymetrical thrust (one engine inop, for example) or with lateral wind.
By controling the roll angle, the pilot controls turning radius. Basicaly, in case of conventional airplanes, aelerons are the main control surfaces used to turn the airplane. The vertical control surface (rudder) is used mainly to control yaw angle during turns.

In case of WIGs, things work a bit differently. Remember that as the height from the ground decreases, the airfoil Cl increaeses and Cd decreases.
So, imagine you have a WIG flying in ground effect (IGE) and imagine you try to turn it (to the left, for example) by using aelerons only.
Following things would happen:
- You lower the right aeleron and rise the left aeleron, with intent to turn the WIG.
- This causes the left wing tip to start lowering, and the right tip to start going up.
- Since the left wing is closer to the ground, it's Cl increases and Cd decreases. The contrary happens to the right wing.
- Moreover, a further drag increase of the right-wing drag comes from aeleron's induced drag which will be much higher (though this can be offset to some extent by using a Frise-type aeleron).
- So a system of forces arise which counteract the action of ailerons.
Result of all this: the WIG rolls to some extent but turns with difficulty (compared to an airplane).

So the aelerons are generally not the main directional control surfaces for WIGs flying IGE.
If your WIG is intended to fly IGE only then you'll have to use rudder for this purpose and you can expect quite a large turning radius.

But, if you make a WIG capable of getting out of ground effect (OGE) for a short time, then you can benefit the use of aelerons. That's because during the OGE flight your WIG becomes a conventional airplane (though a very inefficient one), and you can turn your airplane with aelerons in a much more efficient way.
The backdraw is the power reqired to fly it OGE, which will be much higher than that required by an IGE plane, together with an increased complexity and weight.

So it all depends on this basic choice, which is up to you. To be or not to be (...able to fly OGE) - that is the question.

[aztek]

'PS:clmanges: about A3 size.
Sorry, I'm not familiar with this term . . . how about total length?'

A3 paper - 300mm by 400mm about.

[masalai]

http://en.wikipedia.org/wiki/Paper_size - achieved by entering "A3" in a search engine
ISO paper sizes (plus rounded inch values) Format A series B series C series
Size mm × mm in × in mm × mm in × in mm × mm in × in
0 841 × 1189 33.1 × 46.8 1000 × 1414 39.4 × 55.7 917 × 1297 36.1 × 51.1
1 594 × 841 23.4 × 33.1 707 × 1000 17.8 × 39.4 648 × 917 25.5 × 36.1
2 420 × 594 16.5 × 23.4 500 × 707 19.7 × 27.8 458 × 648 18.0 × 25.5
3 297 × 420 11.7 × 16.5 353 × 500 13.9 × 19.7 324 × 458 12.8 × 18.0
4 210 × 297 8.3 × 11.7 250 × 353 9.8 × 13.9 228 × 324 9.0 × 12.8
5 148 × 210 5.8 × 8.3 176 × 250 6.9 × 9.8 162 × 229 6.4 × 9.0
6 105 × 148 4.1 × 5.8 125 × 176 4.9 × 6.9 114 × 162 4.5 × 6.4
7 74 × 105 2.9 × 4.1 88 × 125 3.5 × 4.9 81 × 114.9 3.2 × 4.5
8 52 × 74 2.0 × 2.9 62 × 88 2.4 × 3.5 57 × 81 2.2 × 3.2
9 37 × 52 1.5 × 2.0 44 × 62 1.7 × 2.4 40 × 57 1.6 × 2.2
10 26 × 37 1.0 × 1.5 31 × 44 1.2 × 1.7 28 × 40 1.1 × 1.6

The tolerances specified in the standard are

* ±1.5 mm (0.06 in) for dimensions up to 150 mm (5.9 in),
* ±2 mm (0.08 in) for lengths in the range 150 to 600 mm (5.9 to 23.6 in) and
* ±3 mm (0.12 in) for any dimension above 600 mm (23.6 in).

[aztek]

Originally Posted by daiquiri

You are talking about aelerons or ailerons, I presume.
Aelerons are movable horizontal control surfaces at the outer part (towards wing tips) of the wing's trailing edge. In case of conventional airplanes they are used to control airplane's roll angle during turns, during flight with asymetrical thrust (one engine inop, for example) or with lateral wind.
By controling the roll angle, the pilot controls turning radius. Basicaly, in case of conventional airplanes, aelerons are the main control surfaces used to turn the airplane. The vertical control surface (rudder) is used mainly to control yaw angle during turns.

In case of WIGs, things work a bit differently. Remember that as the height from the ground decreases, the airfoil Cl increaeses and Cd decreases.
So, imagine you have a WIG flying in ground effect (IGE) and imagine you try to turn it (to the left, for example) by using aelerons only.
Following things would happen:
- You lower the right aeleron and rise the left aeleron, with intent to turn the WIG.
- This causes the left wing tip to start lowering, and the right tip to start going up.
- Since the left wing is closer to the ground, it's Cl increases and Cd decreases. The contrary happens to the right wing.
- Moreover, a further drag increase of the right-wing drag comes from aeleron's induced drag which will be much higher (though this can be offset to some extent by using a Frise-type aeleron).
- So a system of forces arise which counteract the action of ailerons.
Result of all this: the WIG rolls to some extent but turns with difficulty (compared to an airplane).

So the aelerons are generally not the main directional control surfaces for WIGs flying IGE.
If your WIG is intended to fly IGE only then you'll have to use rudder for this purpose and you can expect quite a large turning radius.

But, if you make a WIG capable of getting out of ground effect (OGE) for a short time, then you can benefit the use of aelerons. That's because during the OGE flight your WIG becomes a conventional airplane (though a very inefficient one), and you can turn your airplane with aelerons in a much more efficient way.
The backdraw is the power reqired to fly it OGE, which will be much higher than that required by an IGE plane, together with an increased complexity and weight.

So it all depends on this basic choice, which is up to you. To be or not to be (...able to fly OGE) - that is the question.

ok so none of them then will a twin tail rudder be enough to turn it?

[daiquiri]

Originally Posted by aztek

ok so none of them then will a twin tail rudder be enough to turn it?

It will, like any rudder.
If you give your wing a small positive dihedral, the effect will be augmented (and the lateral stability will benefit, too).

If you are willing to complicate your design a little bit, you could even think of adding controllable spoilers or aerodynamic brakes at the wing tips. During a turn, the inward spoiler (or brake) could pop out ad create an additional drag at the inward wind, tightening the turn radius.
It's an idea, don't know if it has already been tried on some existing design. Could be a touch of originality for your model.

[aztek]

hi
thanx daiquiri i understood the just.

PS: please slow up with the technical language im 15 and not very familiar with boats or planes.

[daiquiri]

Originally Posted by aztek

hi
PS: please slow up with the technical language im 15 and not very familiar with boats or planes.

Ooops... Ok, sorry - that was a missing info.
Think pink - I'm using technical words to force you to search around for their meaning and explanations. In that way you'll be learning more and more and will eventually become the chief aerodynamic engineer in your school.

[aztek]

thanx

[aztek]

Hi
Slight change in design I hope to put a turned up edge to create lift.
Aztek