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Author Topic: Reynolds Number Calculator  (Read 4017 times)
Howard Rush
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« on: December 04, 2009, 03:10:52 AM »

I needed to know Reynolds number for Gary James's CFD analysis, so I made me a Reynolds number calculator.  I also included Reynolds number / SLSD Reynolds number, which I think is an interesting thing to measure. Inputs are line length, lap time, chord, temperature and air pressure.  It's an Excel spreadsheet.  Send me a stamped, self-addressed email message, and I'll send you a copy. 
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Dean Pappas
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« Reply #1 on: December 04, 2009, 11:57:04 AM »

Hi Howard,
You've gotten a hold of a CFD analysis package? How costly is it? Where can I get it?
Thanks,
 Dean P.
deanf3af2b@pappasfamily.net
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Howard Rush
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« Reply #2 on: December 04, 2009, 01:03:18 PM »

It's Gary James.  According to him, he's not as costly as he used to be. 

Gary uses Profili, http://www.profili2.com/ (I hope that's the real site, not the impostor).  It's cheap.  It contains Xfoil, http://web.mit.edu/drela/Public/web/xfoil/
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Impacts zijn oranje,
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Dean Pappas
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« Reply #3 on: December 05, 2009, 11:03:57 AM »

Is that Gary that is not as costly as he used to be or Profili?
I find that my price has gone down! It's hard to justify top dollar when you're falling apart. Embarrassed
Thanks for the links, Howard.
Dean P.
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Dean Pappas
Igor Burger
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« Reply #4 on: December 05, 2009, 11:08:44 AM »

Great, if it calculates also elevator, it will be nice to have :-)
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Howard Rush
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« Reply #5 on: December 05, 2009, 03:53:52 PM »

It's Gary that's not as costly as he used to be.  Profili is dirt cheap.
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Gary James
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« Reply #6 on: December 10, 2009, 06:49:20 AM »

 Rant Nope, I'm not costly at all anymore... my employer saw fit to declare bankruptcy, use the bankruptcy court system to cut my pay 40% and to freeze my retirement pension without providing any significant alternatives.  We've had 7 suicides.  Any you thought being an airline captain was glamorous.  At least I have a job, even if it's not what I signed up for. Rant
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Gary James
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« Reply #7 on: December 10, 2009, 06:55:39 AM »

Hi Howard,
You've gotten a hold of a CFD analysis package? How costly is it? Where can I get it?
Thanks,
 Dean P.
deanf3af2b@pappasfamily.net

A 2D airfoil analysis package is dirt cheap, free in fact.  Profili is a GUI for the Xfoil 2D single element airfoil design and analysis system.

http://www.profili2.com/eng/default.htm

There's also XFLR5   http://xflr5.sourceforge.net/xflr5.htm

A 3-D CFD package is available from AeroLogic.  It is based on the NASA Pmarc-12 CFD code and is "reasonably" accurate at predicting forces that are "pressure based" but not all that accurate (most CFD's aren't ) at predicting friction-based forces.  The code is rather expensive unless you happen to have a continuing need for it.  Also, making the CFD 3D model is not a trivial task and I don't recommend that you try it unless you already have some experience in this field.

http://www.aerologic.com/



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Gary James
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« Reply #8 on: December 10, 2009, 04:54:19 PM »

Thanks Gary,
I sympathize with you: I've been keeping the wolf from the door since Februarty with 2 and sometimes 3 small consulting jobs at a time.
Folks are only willing to pay a little when they know you are in a tough spot.
Dean Pappas
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« Reply #9 on: December 10, 2009, 06:22:53 PM »

Let me ask a dumb aerodynamics question (it was related to the drag issue on Igor's airfoil with flaps).

Does the coefficient of drag cd include induced drag?

I've been reading my copy of Theory of Wing Sections, and quite honestly I am not sure if it does or not. I thought that cd would. So I had thought that the drag would simply scale as a function of cd/cl * the lift for a given AOA.

Have pity on a mere physicist!
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« Reply #10 on: December 10, 2009, 09:49:28 PM »

Hi Alan,
If you are looking at the plots for an airfoil: the ones with Cl and Cd on them, that is only for the profile drag of the section!

The induced drag is calculated by figuring out the Cl at which the wing is lifting. That's based on lift, airspeed and wing area. Then that gets plugged into another formula to calculate the induced drag coefficient, and that gets stuffed into yet another to get induced drag. It's not that bad.
It's late and I'll check the book tomorrow, but I'll bet wikipedia has a decent article if you just search on induced drag!
Dean
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« Reply #11 on: December 11, 2009, 01:18:03 AM »

>>>If you are looking at the plots for an airfoil: the ones with Cl and Cd on them, that is only for the profile drag of the section! <<<
means infinit span or aspect ratio

limited span causes spanwise flowing and thus it will reduce the lift and enlarge the drag:

http://www.grc.nasa.gov/WWW/K-12/airplane/downwash.html

http://www.grc.nasa.gov/WWW/K-12/airplane/induced.html

the lift can be calculated also via AoA and so we something called induced AoA = - cl / (pi * AR)

or slope of lift polar is not theoretical 0.11 per 1 deg AoA as on infinite AR, but it is 0.11 * (AR/(AR+2))
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Gary James
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« Reply #12 on: December 11, 2009, 06:26:42 AM »

Let me ask a dumb aerodynamics question (it was related to the drag issue on Igor's airfoil with flaps).

Does the coefficient of drag cd include induced drag?

Alan:  That's a very GOOD question and one for which the answer isn't intuitively obvious.  As best that I can explain it, SOME of the induced drag is included in those plots that you see, but on a real, 3D airplane there will be more than what you see on those plots.  The plots that you see in "Theory of Wing Sections" and from computerized "wind tunnels" are supposed to represent 2 dimensional results as if the wing had infinite span.  

The atmosphere can ONLY act in 2 ways on any wing surface, it can generate frictional forces and/or it can generate pressure forces. (we'll ignore the frictional forces for now). The pressure forces can ONLY act PERPENDICULAR to the surface AT THE POINT where they meet.  So up near the nose of an airfoil the pressure vector is tilted a little bit forward due to the curvature of the section surface and aft of the max thickness point the pressure vector is tilted a little bit aft.  Since you are a physicist, you will easily understand that the pressure vector can be resolved into two orthogonal components, one that is perpendicular to the X-axis (we'll call this lift) and the other along the Y-axis, either pointing a little bit forward or a little bit aft. We'll call that one "induced drag" since it is a force in the 'drag direction', "induced" by the production of lift.  If we integrate all of these little pressure components along the X-axis we come up with the "induced drag" of the section.

NOW, if you change the angle of attack of the section, the magnitude of the pressure forces change, and also, since the chord line of the section is no longer along the X-axis you can see that the vector component of the pressure along the X-axis also increases.  That is, some of the "lift" being produced is actually "tilted back" and is "dragging" the airplane backward.  That's why you see the induced drag increasing as the angle of attack increases.

So, THAT part of the induced drag IS included in those 2D section plots that you see everywhere, BUT on a real, 3D airplane there is some more induced drag that comes about from the fact that real airplanes have a finite span as opposed to an "infinite" span in the 2D data.  On a 3D airplane there will be drag corrections due to span-wise flow, and the energy lost in the tip vortex.  On a low aspect ratio wing, it can be quite significant.  THAT's why you see sailplanes with such high aspect ratios and elegantly shaped wing planforms.

So to make a long story short (too late...) Yes, the plots that you see include "most" of the induced drag, but on a real airplane the induced drag will be somewhat higher than what you see in the 2D plots.

If you remember that a "fluid" can ONLY act on a surface in two ways, friction parallel to the surface and pressure perpendicular to the surface, a lot of the aerodynamic "gobbledy gook" that we aero-types like to throw around will make a lot more sense.  All those formulas are just an easy way to quantify and integrate those forces in different ways.
« Last Edit: December 11, 2009, 07:20:53 AM by Gary James » Logged

Gary James
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« Reply #13 on: January 14, 2010, 03:22:05 PM »

Rant Nope, I'm not costly at all anymore... my employer saw fit to declare bankruptcy, use the bankruptcy court system to cut my pay 40% and to freeze my retirement pension without providing any significant alternatives.  We've had 7 suicides.  Any you thought being an airline captain was glamorous.  At least I have a job, even if it's not what I signed up for. Rant

Been there...I was a former Overhaul AMT for Eastern. Lost everything except my pride in the Strike of `89. We had 5 suicides plus hundreds of divorces nationwide. After waiting 5 yrs. I finally got my back pay and vac. - pennies on the dollar. After 30 yrs. I wonder why I never moved on to another career. Currently with A'A, at least I have a job...True.
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