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Author Topic: gathering & working airfoil data..  (Read 13561 times)

bomber

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gathering & working airfoil data..
« on: March 21, 2015, 03:36:16 pm »

Following on from completing the weight ballance file, we now need to work on co-efficients for the airfoils...

this is the primary tool for determining the Co-efficients

http://www.mh-aerotools.de/airfoils/javafoil.htm

ok right click 'save file as' the following working out spreadsheet for the Ask-13 which we'll modify.

http://target4today.com/_posted_files_/Cesna_172/Airfoils-Cesna_172.xls
« Last Edit: March 22, 2015, 03:46:34 pm by bomber »
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bomber

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Re: gathering & working airfoil data..
« Reply #1 on: March 22, 2015, 07:10:28 am »

Now using the javafoil app you can define your airfoil co-ords within the geometry tab...



or

you can do a search on this web site
http://airfoiltools.com/airfoiltools/search/index?m%5Bgrp%5D=naca4d&m%5Bsort%5D=1&MAirfoilSearchForm%5BtextSearch%5D=2412&MAirfoilSearchForm%5BmaxThickness%5D=&MAirfoilSearchForm%5BminThickness%5D=&MAirfoilSearchForm%5BmaxCamber%5D=&MAirfoilSearchForm%5BminCamber%5D=&MAirfoilSearchForm%5Bgrp%5D=naca4d&MAirfoilSearchForm%5Bsort%5D=1&yt0=Search

and copy n past the airfoil data (far right link) into the co-ords window of javafoil and then 'update view'



Next we need to work out a reynolds number to calculate the co-efficients at..
cord = 1.66m
cruise speed = 226km/p = 62.7m/s

Reynolds number = 7285740 (use the spreadsheet)

Within the polar tab we need to add the reynolds number and the range of AoA we wish to work in -30 to 30 degs and analyse it.



Going into the reynolds number tab we do a copy n paste the data to the spreedsheet.



The above is the basic procedure for getting the co-efficient of lift for an airfoil shape without any control surface deflection

But we need to perform the same procedure but with control surface deflection...
Assumption - I can't find any angles of deflection for alierons so we'll take a punt at 15 down, 30 up..... flaps are full deflection at 40 degs.

Within the modify tab we adjust the flap deflection (don't get hung up on the word flap) first to -30 [press enter], go back to the polar tab, analyse, copy n paste data to the spreadsheet....



Then back to the modify tab, undo, 15 [press enter] go back to the polar tab, analyse, copy n paste data to the spreadsheet....



HOWEVER

we don't work in co-efficients of lift but instead co-efficients of normal.... so within the spreedsheet we take the Cl's and convert them creating a new table to be used within the JSBsim files...

I've done that and updated the spreedsheet...

http://target4today.com/_posted_files_/Cesna_172/Airfoils-Cesna_172.xls
« Last Edit: March 22, 2015, 03:48:35 pm by bomber »
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bomber

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Re: gathering & working airfoil data..
« Reply #2 on: March 22, 2015, 03:39:55 pm »

So once you've created all that data for wings, h-stabs and v-stabs you copy n paste the tables from -180 degs to 180 degs into the co-efficients file

http://target4today.com/_posted_files_/Cesna_172/co-efficients.xml

And that's the aerodynamics part done with... and now the engine and prop work begins.

Regards

Simon

(p.s. they have me back under a tube train next week, so study the above during the week and we'll start on the engine and prop when I get back from London)
« Last Edit: March 22, 2015, 03:50:58 pm by bomber »
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PytonPago

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Re: gathering & working airfoil data..
« Reply #3 on: March 22, 2015, 04:22:31 pm »

One little question ... it takes the airfoil in what part of the wing ? ... i mean, its not constant along the lenght of the wing, so is it a too big problem ? ... also, what about leading edges and non-basic flaps  ? :




 ... its not criticism, i just have no idea about that stuff.
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bomber

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Re: gathering & working airfoil data..
« Reply #4 on: March 22, 2015, 04:37:32 pm »

I think I understand the first part of the question,  the example is for a cesna 172, which does have the same airfoil along the length of its wing...

If you have a plane with differing airfoils along the wing you just have to perform the task for each airfoil.

Now the flaps issue is a bridge we have to cross when we get to it.... I've not done one,  but it's only a change in airfoil shape per notch of flap,  so should end up being a bit of rinse and repeat, copy n paste.
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PytonPago

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Re: gathering & working airfoil data..
« Reply #5 on: March 22, 2015, 04:58:40 pm »

If you have a plane with differing airfoils along the wing you just have to perform the task for each airfoil.

 ...so a angled wing of a lets say Boing 737 plane would need almost each meter of the lenght to have a new data-set, whyle at physics there is some parameter saying how much of a lenght of the wing uses this or that set ?
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bomber

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Re: gathering & working airfoil data..
« Reply #6 on: March 22, 2015, 05:29:13 pm »

The twisting of the wing along its length is covered in the front end.. The change in airfoil shape is covered here within the co-efficients section and is a matter of dividing each wing into 8 sections and determining the airfoil shape change along its length.
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bomber

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Re: gathering & working airfoil data..
« Reply #7 on: March 23, 2015, 09:37:39 am »

... its not criticism, i just have no idea about that stuff.

Challenge EVERYTHING.....

The assumptions and compromises we are forced into making need to be transparant, understood and agreed... that way when someone takes an existing flight model for modification into a similair plane there's a level of confidence as to the output.
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PytonPago

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Re: gathering & working airfoil data..
« Reply #8 on: March 23, 2015, 02:06:49 pm »

Challenge EVERYTHING.....

  ... i will. :D

  ... i did just thought initially that the calculator maybe could support multiple closed splines, doe i then thought about our lessons from hydrodynamics ( i dont know how its called specifically in technical Eng, in svk its literally taken as liquid-dynamics, cause the pricnipe is in both l and g flow-substances states and were then framed both as liquids for this purpose ) and laminars-Reynolds stuff. Must be interesting math behind the calculator ...
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bomber

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Re: gathering & working airfoil data..
« Reply #9 on: March 23, 2015, 05:05:41 pm »

just a simple engineer here who thinks along the lines of .... a force acting at a point.

I looked at your flap issues and hypothesised if the cessna was equipped with double slotted flap with leading edge slat.
Imported the 2412 data into Autocad and then modifed the wing to match a leading edge slate of -5 degs and double slots of 15 degs with an increase in cord length of 12% and then exported the point data into javafoil.




I've updated the autocad and excel spreadsheet with an extra tab showing the increase in Cl for this hypothesis.



regards

Simon
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bomber

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Re: gathering & working airfoil data..
« Reply #10 on: March 25, 2015, 05:49:08 pm »

Ok have a look at the Co-efficients of normal force for the wing..



I've red circled the 2 areas that don't quite look correct....
Now if you look at the sinusoidal tab in the spreadsheet you can see what the co-efficients would look like if the effects of an airfoil never existed
The flat plate tab shows how even though it's a flat plate the air sticks to it and creates a virtual airfoil, it's not got a great AoA before the airflow breaks away.
The important thing to notice is that once the airfoil has stalled the co-efficient falls to the sinusoidal value, not below it.

So within the areas circled you must copy n paste the sinusoidal values, to get something looking like this.



« Last Edit: March 25, 2015, 05:52:24 pm by bomber »
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bomber

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Re: gathering & working airfoil data..
« Reply #11 on: March 25, 2015, 06:20:27 pm »

A question you're likely to be faced with  (if there's no data)  is just how much control surface deflection should you have, is too much a waste?

And the answer is yes... look at this first image, it's the above image only I've circled the areas of interest... only if you're either above or below the sinusoidal line are you getting bang for bucks.. 



If I say to myself how about having -40 degree upwards deflection, you can see in this image that the co-efficient doesn't go above the sinusoidal line, so as a pilot at the extreme AoA of 30 degree, with your wing well stalled out, you'll have no ability to unstall it and get it back generating useful lift..

« Last Edit: March 25, 2015, 06:25:03 pm by bomber »
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PytonPago

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Re: gathering & working airfoil data..
« Reply #12 on: March 26, 2015, 03:53:14 am »

 ... i noticed, there is quite a hole between the wing and the flap where airflow is partially allowed to get trough without being much obstructed. Doe i found this little digital visualization an noticed how small that effect actually is in a presure profile :

   http://cfd2012.com/types-of-flaps.html

Also nice to see how much a difference are all those profiles.
« Last Edit: March 26, 2015, 03:56:53 am by PytonPago »
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bomber

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Re: gathering & working airfoil data..
« Reply #13 on: March 29, 2015, 09:58:00 am »

yes a nice set of images... enlightening.




This is the first test on the cessna using flightgear as a wind tunnel...  we know that the lift 2 drag ratio for this plane is 9:1, yet the max we get at present is 6.36:1 at 2 degs.... so we need some adjustment here.. but how much, do we lower the drag or increase the lift, or even both ?

What we know is that the cesna 172 stalls at 57mph (flaps up)...and by looking at the graph above the max Cl is at 13 degs.... so if I put the plane back in the wind tunnel at 13 degs with a wind of 57mph the lift generated by the wing has to be equal to the weight of the plane, if not we adjust the modifier until it is.

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bomber

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Re: gathering & working airfoil data..
« Reply #14 on: March 29, 2015, 10:48:36 am »

Some more thought on this....

http://cessna.us/cessna-172/

This says that the zero lift drag is 0.0319, and the lift to drag of 7.5.

We have the zero lift at -2 degs with the drag being 0.039....  so only a small adjustment needed in drag modifier..


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