Question about airfoil (wing) – aerodynamics

start at edge

Member
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442
I address this question especially (but not exclusively) to someone who has access to a small scale wind tunnel for aerodynamic testing, maybe even his (her) self built version in his (her) own basement or workshop.

An airfoil in a wind tunnel is usually depicted as in the image below:

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… where we have a transparent tunnel (to be able to observe the smoke trails), an airfoil (wing) in the middle, a thin pipe for smoke injection through tiny nozzles (in this case 8 nozzles), with airflow from end “A” of the tunnel to end “B” of the tunnel.
This depiction is a generic one, but my question is about a more specific situation, where a second thin pipe is placed behind the airfoil (wing) and having not only the airflow direction (from A to B) but also the placement of the air pressure source (air pump / propeller / turbine …).

In scenario 1, the air pressure source is placed at end “A”, actually pushing (pressuring) the air into the tunnel, as seen in the image below:

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In scenario 2, the air pressure source is placed at end “B”, actually sucking (vacuuming) the air out of the tunnel, as seen in the image below:

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My question is:
How would the smoke trails (shape) after pipe nr. 2 look like in scenario 1 and how would they look like in scenario 2 ???
A reasonable assumption is that in both scenarios the smoke trails (shape) between end “A” and pipe nr. 1 would be the same, also between pipe nr.1 and pipe nr. 2.
Anyway, logic suggests that between pipe nr.2 and end “B”, the shape of smoke trails should be different, depending on the placement of the air pressure source (scenario 1 or scenario 2).
Any answer will be welcome and appreciated, thank you.
 
Last edited:

Witch Hunt

Active Member
Messages
988
Anyway, logic suggests that between pipe nr.2 and end “B”, the shape of smoke trails should be different, depending on the placement of the air pressure source (scenario 1 or scenario 2).
Any answer will be welcome and appreciated, thank you.

Okay, I am not an aeronautical engineer or a physicist. I am just another anonymous drunk asshole on the internet so what I am gonna say is from a bar stool kind of perspective.

Be that as it may, let me take you back to WW2 and the height of aeronautical engineering. The Americans and Germans were locked in a deadly world-wide conflict and were using every trick in the book to gain the advantage. Every conceivable idea was developed to it's fullest potential. Fuel injection, flush-riveting and streamlining were all improvements either developed or improved due to the war effort. Another cool development? Rear engine design. Case in point...

OIP.Dj90VAbHo12c-Rd5bWLgDwHaD5


Engineers on both sides tried this concept out because they thought along the lines you do, that airflow might be so different that It would improve performance. It didn't. There was no appreciable difference in performance at all. There was absolutely no change in airflow over the foils, wings and canopies. NONE.

In short, once airflow comes over the wing, it is all the same.
 

start at edge

Member
Messages
442
Okay, I am not an aeronautical engineer or a physicist. I am just another anonymous drunk asshole on the internet so what I am gonna say is from a bar stool kind of perspective.

Be that as it may, let me take you back to WW2 and the height of aeronautical engineering. The Americans and Germans were locked in a deadly world-wide conflict and were using every trick in the book to gain the advantage. Every conceivable idea was developed to it's fullest potential. Fuel injection, flush-riveting and streamlining were all improvements either developed or improved due to the war effort. Another cool development? Rear engine design. Case in point...

OIP.Dj90VAbHo12c-Rd5bWLgDwHaD5


Engineers on both sides tried this concept out because they thought along the lines you do, that airflow might be so different that It would improve performance. It didn't. There was no appreciable difference in performance at all. There was absolutely no change in airflow over the foils, wings and canopies. NONE.

In short, once airflow comes over the wing, it is all the same.
Thanks a lot for you reply, that was very kind of you.
Yes, up in the sky, whether you push an airplane or pull it, it makes almost no difference to the airflow above (and below) the wing, that is true. However, common sense and also the so far gathered information (as I am very interested in this subject) tell me that inside a wind tunnel things happen slightly different.
I would be happy to have access to a small scale wind tunnel, to do this testing myself … but I don’t.
A better way to make myself understood is to explain as simple as possible where I am heading, as an image of that would be too large to post here on the site, so here it goes:
If inside a (long) wind tunnel, I have more airfoil profiles placed one behind the other, would I have maximum sum of lift (all airfoils combined) if I place the air pressure device in front of all those or behind them (behind the last one) ??
Without an actual small scale site (tunnel), I can only assume that if the air is pulled from the back (end “B” in the previous images), the airflow would be “straightened” after passing each wing (as there are also disturbances and vortexes created once it leaves the wing surface) and forced to hit the next wing under the same angle it hit the previous one.
I think that an accurate result of this could be much easier obtained if actually testing, rather than doing calculations. That is why an actual tunnel (even a small sized one) would be the best choice.
 

Einstein

Temporal Engineer
Messages
3,906
Thanks a lot for you reply, that was very kind of you.
Yes, up in the sky, whether you push an airplane or pull it, it makes almost no difference to the airflow above (and below) the wing, that is true. However, common sense and also the so far gathered information (as I am very interested in this subject) tell me that inside a wind tunnel things happen slightly different.
I would be happy to have access to a small scale wind tunnel, to do this testing myself … but I don’t.
A better way to make myself understood is to explain as simple as possible where I am heading, as an image of that would be too large to post here on the site, so here it goes:
If inside a (long) wind tunnel, I have more airfoil profiles placed one behind the other, would I have maximum sum of lift (all airfoils combined) if I place the air pressure device in front of all those or behind them (behind the last one) ??
Without an actual small scale site (tunnel), I can only assume that if the air is pulled from the back (end “B” in the previous images), the airflow would be “straightened” after passing each wing (as there are also disturbances and vortexes created once it leaves the wing surface) and forced to hit the next wing under the same angle it hit the previous one.
I think that an accurate result of this could be much easier obtained if actually testing, rather than doing calculations. That is why an actual tunnel (even a small sized one) would be the best choice.

I would think there would be turbulence that would affect the lift of airfoils behind the front wing.
 

start at edge

Member
Messages
442
I would think there would be turbulence that would affect the lift of airfoils behind the front wing.
That is exactly what I was thinking and so I asked myself if those turbulence would be different if the air is pushed from the front or if it is pulled from the back.
 


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