Lift

[Demon AESOp]

So I am teaching theory of flight at a ground school, and as I am reading through FTGU studying the subject a bit, I have come accross somthing that has always bugged me about this subject, and have never had answered to my satisfaction. Maybe someone here has some thoughts on the matter an can help me out.

20 years ago, when I first learned about Theory of flight, and the source of lift, It seemed that bernoulli's (spelling?) theorem, and the small amount of airflow striking the bottom of the wing was enough to explain why planes go up.

Now as I am reading FTGU (and learned on my last ground school), it tells me that there is this downwash effect. Airflow being forced downward over the trailing edge of the wing creates an oposite force on the wing, a'la Newtons third law, causing the wing to go up. There is even a nice diagram showing this. FTGU seems to imply that this is the principle source of lift.

Now, my problem. What about the upwash? Its in the diagram. Should it not force the wing down, and cancel out the downwash?

One explanation that has been presented to me, is the the physics make the effect of the downwash more pronounced than the upwash, creating a net surplace of upwards force on the wing. At first glance this seems reasonable. In fact it is almost vital to the explanation. If this upwash had been considdered initially, it would be obvious that the explanation would not hold with discussion of this. FTGU does not considde this upwas at all.

So, what do you think. Personally, I think that this was someones pet theory that seemed to make it through into groundschoolhistory, but that it had not been thought comlpetely through. Like I said, 20 years ago, Bernoulli seemed to explain it all quite well.



M Schnabel
Wahunga!

 

[wridon]

You may want to do a little surfing: check out NASA web site and search for references to Bernoulli. There is a neat little model that NASA came up with from actual wind tunnel testing. This is an interactive model that lets you change Camber and Angle of attack and see the effect they have on airflow over and under the wing.

The old theory also states that the air has to go faster over the top because it has further to go and it must get to the back at the same time as the air going under. (Why, we don't know!)

In the model you will see that in fact the air going over is accellerated and the air going under is slowed down giving a net effect of air going over at about three times the speed of air going under. (no the molecules of air do not arrive at the back at the same time)


I'll see if I can find the direct link to the simulator and post it here later.

 

[wridon]

http://www.lerc.nasa.gov/WWW/K-12/aerosim/...plet/vj402.html

This should be the link. It is referred to as: FoilSimApplet

If this does not work for you search: Bernoulli, simulator, lift

Scroll through until you find reference to the Applet. It's kind of interesting.

 

[Demon AESOp]

Thanks for the link, Ill check it out. It may be useful for teaching lift at ground school. Unfortunately this does not solve my dilema.

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The old theory also states that the air has to go faster over the top because it has further to go and it must get to the back at the same time as the air going under. (Why, we don't know!)
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What you wrote here is actually fairly simple to answer. The air gets to the back at the same time simply because it is the wing that is actually moving, and not the air itself. When you are discribing bernoullies theorem it is just simpler to tallk as if the air is moving. :blink:

Thanks again.

 

[wridon]

Check out the model and you will see that the air does not get to the back at the same time. That is one of the major points that is proven.

 

[Demon AESOp]

<_< All those little lines representing the air flow seem to line up all the way across, whether they are above the airfoil or below. According to the theory, planes should not fly if they did not.

 

[wridon]

Use the controls to move the probe up and down(above and below) the wing and forward and aft. you will be able to see the difference in velocity. The illustration (ie. lines moving in diagram) is not representative of the actual results. These results are from actual testing in a wind tunnel.

There is another model that I looked at some time ago where the graphics were representative of the results. I might just see if I can find that for you.