Airfoils

jander14indoor
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Airfoils

Postby jander14indoor » August 6th, 2009, 3:20 pm

Since we have a dedicated board for WS now, I thought it might help to organize by kicking off topics around the typical WS questions. Content of string to be driven by subject line (I hope).

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Re: Airfoils

Postby blue cobra » October 18th, 2009, 1:42 pm

Would there be a point to experimenting with airfoil shapes other than simplex/logarithmic spiral?
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Re: Airfoils

Postby jander14indoor » October 19th, 2009, 3:30 am

Small amounts after you have the important stuff done. The ratio of height of curve to length can be handy for tuning to short vs tall site, I've bee told by folks I trust. Fairing the leading and trailing edge to reduce drag is good. Beyond that little data.

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Re: Airfoils

Postby jcollier » October 24th, 2009, 6:55 pm

Hi, everyone. I was on the old board as jacdad, but could not get back with that for some reason. At any rate, this topic is one that has intrigued me. My son will be flying WS planes again, and actually has 3 good wings already, but the simplex or curvature of the airfoil for different heights is what I'm suggesting he works on as well as experimenting with props a lot more. One of his wings had a lot of curvature, flew very slowly, and did not climb out of our gym like the other wings often did, but he got within an eyelash of 3 min. with a 22 ft. gym. I would have thought, logically, that the more curvature, the more lift and drag. What is the reason for the less-curved airfoil getting so much more height? I know they have less drag, but what else is involved in this? Thanks.

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Re: Airfoils

Postby jander14indoor » October 25th, 2009, 5:36 pm

Lift vs drag, there's the rub. If you are flying at minimum weight, (and why aren't you?) you need the same lift, period, to fly the plane. A high lift/high drag wing will fly at the same exact lift, but possibly more drag than the lower lift, lower drag wing. Thus the lower drag wing will fly longer, or higher.

Trick is to get the same lift for that high lift wing, it is probably at a lower angle of attack OR slower. Lower angle of attack probably doesn't help against the low drag wing, but slower MIGHT. This is where experimentation in trim conditions comes in.

Unfortunately I'm not good enough to lay out those equations for you, and I doubt there's good enough data on our wings to do it analytically anyway, so you have to experiment. This is why this is SUCH a good event for test and eval.

Jeff Anderson
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Re: Airfoils

Postby waawamellon » November 12th, 2009, 3:10 pm

Oh yeah air foils... those are the thingys on the wings that give them that sort of tear drop shape! because they make higher air preasure under and lower air preasure on top.

Am i wrong?
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Re: Airfoils

Postby blue cobra » November 12th, 2009, 3:47 pm

waawamellon wrote:Oh yeah air foils... those are the thingys on the wings that give them that sort of tear drop shape! because they make higher air preasure under and lower air preasure on top.

Am i wrong?

Indeed, the airfoil is the shape of the wing that causes lift due to lower air pressure on top than on the bottom, using Bernoulli's principle. The ribs are shaped to create this.

There are many different types of airfoils, but WS planes usually use simplex airfoils.
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Re: Airfoils

Postby carneyf1d » November 12th, 2009, 6:02 pm

Airfoils are quite complicated. The reason airfoils provide lift actually comes from viscous effects of air. A circulation of air is created around an airfoil and as a result, the lower surface of an airfoil has a lower velocity, and the upper surface has an increase in velocity. Bernoulli's equation relates velocity, density, and pressure. 1/2 * density * velocity ^2 + pressure = constant along the airfoil. Therefore when velocity increases, pressure decreases and vice versa. Since there is a pressure differential, the wing will lift.
Airfoils without camber, also called symmetrical, have a lift coefficient related to 2* pi * angle of attack. Angle of attack is in radians, or (degrees * pi / 180). Although at angles of attack greater than 10 degrees, the wing undergoes stall, where there becomes a separation of airflow, leading to a lot of drag and little lift (aka stalling out.)
Cambered airfoils on the other hand have a special property of having a negative angle of attack when there is zero lift. So if you have a cambered airfoil at -1 degree, youll get 0 lift, where as symmetrical airfoils all have 0 degrees angle of attack as zero lift. This zero lift angle is known as alpha_L=0 . The lifting coefficient of a cambered airfoil is 2* pi * (alpha - alpha_L=0).
Thus cambered airfoils will have higher lifting coefficients than symmetrical airfoils at the same angles of attack, making cambered foils better for this competition.
As to whether you should use simplex airfoils or your own design of camber is undetermined. Ill get back to you guys later with that info.

On a side note, this only briefly touches the subject of airfoil theory. If you really wanna find more, visit your local library. In addition, make sure the overall angle of attack takes into account wing aspect ratio and geometric design of the wing (elliptical or square).

good luck

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Re: Airfoils

Postby SMS SO Rules » November 29th, 2009, 6:26 pm

edited - question removed

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Re: Airfoils

Postby talicoa » November 29th, 2009, 8:12 pm

With the scale that Wright Stuff planes are built at, it seems like they would differ greatly from the common wisdom of airplane design. The angle of incidence( angle of the wing chord to the longitudinal axis) that is built into a design of a plane, can generate lift even if the wing has no curvature at all. To build a wing with curvature or camber to it, would seem to require covering on both the top and bottom surfaces to be efficient. This would add a significant amount of weight. If the bottom remains uncovered, then there seems like there would be a lot of drag. Has anyone had good results with a zero camber wing? What we have flown to date seems relatively positive. Though, we haven't had decent props or motors yet to work with. I am just curious if anyone else has put some thought into this.

Thanks,
Tom

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Re: Airfoils

Postby calgoddard » November 30th, 2009, 10:49 pm

Tom -

It is good of you to think outside of the box.

Indoor rubber powered free flight airplanes of the general type flown in Wright Stuff (WS) have been around for decades. The wing covering has changed from tissue to film. All of the experts agree that covering the underside of the main wing is counter-productive, even if it can be done without exceeding the minimum weight of the plane, which would actually be fairly easy with Ultrafilm.

A main wing with no camber would be a disadvantage in WS. Jeff Anderson could elaborate on the theoretical reasons why this is true.

In general, a larger main wing camber is better for low ceiling flying sites and a smaller main wing camber is better for high celing flying sites.

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Re: Airfoils

Postby carneyf1d » December 1st, 2009, 1:10 am

I might be misinterpreting what you said, Tom, but you do not need to cover the underside of a wing to have a cambered airfoil. Simply having curved ribs will create a camber line.
And cambered airfoils have the advantage of having more lift than flat foils for the same angle of attack.
As for the camber of the airfoil:
For simplex foils...the higher the simplex the slower the plane will fly because of an increase in drag. however there will be an increase in lift coefficients, which is good.
For lower simplexes, the plane will fly faster and have less of a lift coefficient.
Use lower simplexes for higher ceilings, and higher simplexes for lower ceilings.

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Re: Airfoils

Postby talicoa » December 1st, 2009, 7:49 pm

Thinking about this with a simple airfoil, the benefit seems to be in part driven by the different velocities of air over the top wing and the slower air under the wing. That's where the comment of putting covering on the bottom of the wing comes from. Keep that air in a straighter line and there is less distance to travel and a lower speed/higher pressure. This is classic Bernoulli. The wright stuff airfoils more closely resemble a more complex wing, like a 747's wing when using leading edge and trailing edge flaps. There is relatively a lot of curvature there, and the lift seems to be developed by two things. One from the redirection of air downward and two from the desire of the air over the top of the wing to want to travel in a straight line and pull away from the wing after the maximum camber causing a lower pressure. Coaching this event surely has made me think. I still am trying to get my head wrapped around the higher simplexes for the smaller gym comments. Hopefully soon, this stuff will start to make more intuitive sense to me so I can more effectively teach this to our team.
Thank you for the comments, this is really a great site.

Tom

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Re: Airfoils

Postby gh » December 1st, 2009, 11:45 pm

talicoa wrote:Thinking about this with a simple airfoil, the benefit seems to be in part driven by the different velocities of air over the top wing and the slower air under the wing. That's where the comment of putting covering on the bottom of the wing comes from. Keep that air in a straighter line and there is less distance to travel and a lower speed/higher pressure. This is classic Bernoulli.
This is a perfectly legitimate thing to bring up, and I used to wonder about it all the time. As it turns out, the real problem here was that the "classic Bernoulli" most of us learn below the uni level isn't really the full explanation for how a wing generates lift. It really doesn't have much of anything to with the shape generating a pressure gradient. A wing just pushes a heck of lot of air downwards when it moves through the air, and the lift on the wing is just the -d(momentum of air pushed)/dt, i.e. conservation of momentum.

Check out this link for a better treatment on the topic: http://www.allstar.fiu.edu/aero/Flightrevisited.pdf
(Also try the original edit, which I think is a more interesting read: http://www.pongo-air.com/lift.html)
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Re: Airfoils

Postby blue cobra » December 5th, 2009, 7:25 pm

How do you slice ribs?

Should ribs be low or high density?

Approximately what simplex should the ribs be? And the ribs on the tail?

How do you roll tissue tubes?
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