Airfoils

calgoddard
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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

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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Re: Airfoils

Postby carneyf1d » December 5th, 2009, 10:58 pm

get some really dense and hard wood, and carefully cut out a rib template. you can find rib simplex foils online at indoornews.com. Use this to cut a ribs upper surface, slide it down a bit and make another cut. you have to use sheets of balsa to do this.

Ribs should be medium density.

Wing ribs around anywhere from 3-6 % simplex. Stab ribs are a bit smaller around 2-5%.

Tissue tubes: get a drill bit around the size of your spar, roll a small strip of paper around it in a circle once. glue the paper with a small amount of CyA, and then make one more wrap. Then cut the excess off.

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Re: Airfoils

Postby gh » December 6th, 2009, 3:10 pm

By the way, the percentage for the shape of the rib is the ratio of the thickness of the airfoil to the chord. "Simplex" is the name given to the shape resulting from plotting the log function. The percentage can apply to any airfoil.

An interesting property of the "simplex" airfoil (and the reason why we use it) is that you can produce a rib of say, 5%, cut it in half, and the half that contains the previous leading edge will still be 5%. This would clearly not work if you had, say, an elliptical-shaped airfoil.

That's why you can use a simplex template to slice ribs of any chord to that percentage, instead of needing a special template for e.g. 8cm wing ribs or 6cm stab ribs.

But anyways, my point was that it's not important "what simplex" the ribs should be, but what percent thickness they should be (generally higher for low ceilings and vice versa), which is more significant. That they have the simplex shape is a matter of convenience in construction, not an issue of aerodynamic performance.
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Re: Airfoils

Postby blue cobra » December 7th, 2009, 1:34 pm

A lower simplex makes your plane fly higher. So would having a relatively high simplex on the tail give it less "real lift" (as in vertical distance), which could make it easier to trim?
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Re: Airfoils

Postby erbach » March 19th, 2010, 8:48 am

>How do you slice ribs?

The easiest way is to create a template of the desired shape from either thin balsa (about 1/16 inch) or matboard. The edge should be sanded to ensure a smooth curve, because the slightest kink will be transferred to all ribs. Once the shape is right, glue a thin wire (about .020 inch) along the edge. When this is done, sand the edge of the profile with very fine sandpaper to remove any glue lumps. Then use this profile to cut ribs carefully with half a double-edge razor blade, or an exacto knife with a pointed blade.

> Should ribs be low or high density?
Doesn't matter much. Make them from the same wood as your spars.

>Approximately what simplex should the ribs be? And the ribs on the tail?
The exact profile is not very critical for SO purposes. But common would be a thickness of 4-6% compared to the chord, with a high point about 40% of the way back. An ordinary French curve can be used to obtain a reasonable shape.

How do you roll tissue tubes?
> It takes a little practice. One way is to start with a strip of straight wire of the desired diameter, say 1/16 inch. Make some thinned glue. (I use Duco thinned 50-50 with acetone.) Cut a strip of japanese tissue the needed shape; say 3/8 inch by about 2 inches long. Gently spread a drop or two of the glue on about 2/3 of the strip, and wrap it around the wire wet end in. Then wait a bit until the glue is half dry. With care the rolled tube can be slid off the end of the wire. This is where the practice is needed. If you wait too long, the tube will stick on the wire. If not long enough, the tube layers will slide. Depending on how much glue you use, about 2-3 minutes is likely to be about right, but you can test gently to find the moment. Once the tube is free, let it dry. Then cut strip wood (say 1/16th square) and gently sand off the corners to make the wing post. If you check every few strokes, you can make the post just small enough to have a snug fit.


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