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Concept airfoil design- des it work? - 7/21/2008 2:30:26 PM   
Yishht87


 

Posts: 96
Joined: 4/5/2008
From: Geneva, SWITZERLAND
Status: offline 		This is an airfoil one of my scratch built airplanes ended up having. When I tested the wing for lift, the 16.5 cm chord and 74 cm span wing gave 1.2-1.3 kg of lift at about 15-25 miles an hour. Now for a wing so short in span and so thin in chord- that is a pretty good lift factor. So my two wings combined give about 2.5-2.7 kg of lift at 20-25 miles an hour.

Here are two diagrams I drew trying to figure out the airstream behavior on this wing. Which one of them looks more close to how the airstream actually behaves around that kind of an airfoil?

If any of you know of any free or demo online/downloadable programs for DRWAING your OWN airfoils and testing them in a virtual WIND TUNNEL, please post a link to that software or at least give the name of the program. Thank you very much.

Yishht87, cheers.

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RE: Concept airfoil design- des it work? - 7/21/2008 3:02:21 PM   
Paper Cub


 

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Joined: 3/13/2008
From: Livingston, TX, USA
Status: offline 		IMHO diagram #2 is a good guess. It would be interesting to see what would happen if the little notch were on the upper side. I suspect it would result in a wing with improved stall characteristics. Interesting post.

< Message edited by Paper Cub -- 7/21/2008 3:06:38 PM >

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RE: Concept airfoil design- des it work? - 7/21/2008 6:26:37 PM   
lnewqban



Posts: 17
Joined: 4/9/2007
From: West Palm Beach, FL, USA
Status: offline 		Yishht87:

Try this software:

http://www.grc.nasa.gov/WWW/K-12/FoilSim/index.html

Not exactly what you want, but very interesting to learn from, indeed.

It would be good for you to study about Airfoil Pressure Distributions at:

http://www.desktopaero.com/appliedaero/airfoils1/airfoilpressures.html

Besides, find an interactive java diagram at:
http://www.desktopaero.com/appliedaero/airfoils1/geometryandcp.html

Diagram #2 is closer to reality, I believe, since the little bump of #1 cannot deflect air as shown.
Such a bump can induce some turbulent flow only, which could help the lift only if used on the upper surface (it is called turbulator).

Regards!!

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RE: Concept airfoil design- des it work? - 7/22/2008 4:44:02 AM   
BMatthews



Posts: 8888
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		Well first off both your diagrams are just you trying to imagine how the air flows. Your streamlines have little to do with reality I'm afraid. I would suggest you don't even bother to try since the air will flow how it wants to flow and you can't alter it other than by studying pressure variations.

There's a slick tool for doing this called Xfoil. You can download the command line versions for free (google for 'xfoil download" or get it when you pay for the unlock code for Profili2. Having used the Profili2 option I can heartily recomend paying the pittance that he's asking in order to have a really nice all in one airfoil analysis and drawing tool. It makes accessing the Xfoil tasks simplicity itself. Meanwhile I've tried to read about how to use the command line version and after a couple of tries didn't bother.

In checking my newly unlocked version of Profili2 I just learned that you don't even need to measure and produce a table of coordinates. The latest version will produce it's own coordinates from a nice raster scan. So if you draw up the airfoil as accurately as you can with a very thin line you can then scan your drawing to an image file and import it. Profili will produce a coordinate table and you can then request a lift drag polar chart. From comparing this to other polars you can determine how your own airfoil stacks up against the competition. Or if you can do it in CAD and then export it as a gif or jpg or bmp you can just open the file with Profili to import it. VERY slick.

I seem to be trumpeting Profili all the time to the point it seems like I was the guy that wrote it. But really I'm just another happy and very enthusiastic customer. This is just a great program that really deserves our attention. He's turned it from just another airfoil drawing program into a full on wing tool.


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RE: Concept airfoil design- des it work? - 7/24/2008 7:27:23 AM   
Yishht87


 

Posts: 96
Joined: 4/5/2008
From: Geneva, SWITZERLAND
Status: offline 		Thank you Bmatthews for the profili2 and xfoil but I am not going to pay for them unfortunately- I am sure no matter how good the programs are real life situations always exceed computer dynamics- because that software works on an algorythm which in many cases is not so perfect in real life. So I used my creativity and took a flat piece of plastic and glues a small cardboard and tape ridhge on the bottom. What happened was close to diagram 1 although it was diagram 2 all the same. The ridge acted as a flap, thus making all the airflow turbulent behind the ridge. I noticed that the boundary layer never detached from the wing- and that is good since that is the crucial part for producing lift. All the flow above the flap was slightly deflected upwards but all the same it was still attached to the bouydary layer.

The conclusion being diagram 2 is underdone and diagram 1 is overdone. What I got was much closer to diagram 2. My conclusion is that the ridge will not severely act on lift production, although it will induce airflow transition from laminar to turbulent a centimeter or two earlier that the natural transition. The wing will produce slightly less lift than the other one (by about 20-30 grams because lift production relies 80 percent on the upper part of the wing which is perfect) but that will not severely affect the flight considering plane weight being about 2.2 kg. Some trimming will do the job.

How did I do my test? I used a tap and some cold water. You all know hydrodynamics are the same as aerodynamics.

Do you think my conclusion is right?

Yishht87.

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RE: Concept airfoil design- des it work? - 7/24/2008 4:51:23 PM   
Mike Connor



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From: Tulsa, OK, USA
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quote:

ORIGINAL: Yishht87

You all know hydrodynamics are the same as aerodynamics.

Do you think my conclusion is right?

Yishht87.

Due to compressibility, I would think hydrodynamics and aerodynamics would be very different.

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RE: Concept airfoil design- des it work? - 7/24/2008 6:37:08 PM   
Yishht87


 

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Joined: 4/5/2008
From: Geneva, SWITZERLAND
Status: offline 		Not really. Put a plane under water andit will "fly". The flow is the same fella- not exact but a particle in the air would act similarly to a water particle.

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RE: Concept airfoil design- des it work? - 7/24/2008 8:25:50 PM   
Allfat


 

Posts: 156
Joined: 4/22/2008
From: Portland, OR, USA
Status: offline 		The difference between water and air is pretty much only the density. Other than that, they act very similar.

This being said, to do a test on an airfoil using water, you can use a lot lower velocities of water because the density is a lot higher. This results in the same reynolds number.

Yissht, you may have done the experiment, but it is likely not valid because you did not take into account that the water has a much higher density than air. This will drastically effect the reynolds number and how the airfoil reacts with the environment. To figure out what velocity the water should be at to compare to air, you need to know the velocity you want the airfoil to travel at through the air. Then calculate the reynolds number over the airfoil. Then use that calculated reynolds number to calculate the velocity of water that is needed to obtain the same reynolds number.

Then you need to regulate this velocity and put it over your airfoil and see how it reacts. This will be a valid test to see how your airfoil performs in air using water.

This is a very good method to use when evaluating high speed planes because it is really hard for the average joe to make a wind tunnel that will produce very high speed air. But if you use water, it can move at a much slower speed that is much easier to obtain.

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RE: Concept airfoil design- des it work? - 7/25/2008 12:16:50 PM   
lnewqban



Posts: 17
Joined: 4/9/2007
From: West Palm Beach, FL, USA
Status: offline 		Yishht87:

Unfortunately, equivalent modeling and testing for liquids and gases are not that simple.

Liquids and gases have different fluid properties, which include density, specific volume, specific gravity, viscosity, kinematic viscosity, changes in viscosity with temperature and speed, vapor pressure, surface tension, compressibility, shear resistance, speed of sound, etc.

Reynolds number is the ratio inertial forces / viscous forces in a fluid.
Inertial forces are proportional to shape, velocity and density.
Viscous forces are proportional to absolute viscosity.

All fluids switch the way they react to a disturbing surface at a range of Reynolds number.
In other words, approximately at Re=500,000 (this post has been edited to corrected this value), for an open flow around an airfoil, any fluid suffers a transition between laminar flow (viscous forces are dominant) and turbulent flow (inertial forces are dominant).

The absolute viscosity ratio water/air is 66.
However, the density ratio water/air is 775.

An in-house test of an airfoil may give you close results for moderate angles of attack and velocities, regardless of using water or air, adjusting the velocity of the water stream to obtain a Re number equivalent to air, as explained above.
However, I believe that more rigorous and exact testing would require a stream of air.

With all respect to your statement, and declaring myself an ignorant in aerodynamic tests in water tunnels, I do not know of manufacturers of airplanes testing their machines in the ocean.
Instead, they all use very expensive tests with air.

I would rather travel in a commercial aircraft that has been tested and certified in a wind tunnel!!

Regards!!



< Message edited by lnewqban -- 7/26/2008 5:02:49 AM >

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RE: Concept airfoil design- des it work? - 7/25/2008 5:44:47 PM   
Allfat


 

Posts: 156
Joined: 4/22/2008
From: Portland, OR, USA
Status: offline 		lnewqban,

The Reynolds number for the transition from laminar to turbulent that you posted (2100-4000) is valid for pipes and enclosed flow. This is not the situation we see in an airfoil and the transition occurs somewhere in the vicinity of a reynolds number of 500,000. This is of course assuming a perfectly smooth surface, which is not really valid because no such thing exists. So in reality, the transition will be before 500,000, but the point is it is a lot higher over a plate or an airfoil than flowing through a tube.

That being said, the water test is actually very much so valid, if you do it correctly. Now, I do not believe that the OP did the test correctly, so his results are most likely wrong. But if he takes the time to set up the experiment, then you can get very close to how it would perform in air. Using water you will be able to compare different airfoils very easily as well. The numbers might not be exact, but all the relationships will be present. And even though this may not yeild exact and perfect numbers, it would be more than adequate for the world of model airplanes.

Now if you really want to have some fun, get some glycerin and do the tests in that. The viscosity is huge and you can really slow the flow down and see what is going on. One cool trick is to take a syringe filled with some colored glycerin and inject it into the flow of glycerin and you can see the streamlines around the airfoil.

These small models can do things that real airplanes can not, so rigorous testing of an airfoil that you will simply put on a model is pointless to me. Unless you have a friend with a big windtunnel, then give him a call. But I would not pay to use a windtunnel or build a windtunnel or anything like that. Seems like a waste of time to me, that is less time you have for flying!

Point is, a test with water is valid if used correctly, and a test using a windtunnel would be ideal, but not absolutely necessary for what you are looking for.

< Message edited by Allfat -- 7/25/2008 5:45:18 PM >

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RE: Concept airfoil design- des it work? - 7/25/2008 10:01:02 PM   
mjfrederick


 

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Status: offline 		The OP did the test under a running faucet... I promise it wasn't done correctly.

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RE: Concept airfoil design- des it work? - 7/25/2008 10:13:38 PM   
Allfat


 

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quote:

ORIGINAL: mjfrederick

The OP did the test under a running faucet... I promise it wasn't done correctly.

Ha, I did not catch that part. Yeah, that is definitely not a valid test and all results should be thrown away and forgotten.

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RE: Concept airfoil design- des it work? - 7/25/2008 10:20:48 PM   
mjfrederick


 

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quote:

Ha, I did not catch that part. Yeah, that is definitely not a valid test and all results should be thrown away and forgotten.


Yeah, considering the turbulence just from coming out the faucet... geez.

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RE: Concept airfoil design- des it work? - 7/25/2008 10:52:36 PM