Misleading and Incomplete Information

The airflow diagram commonly seen with many explanations of lift can be confusing because it's the wing that moves through the air, not the other way around. Looking for lift this way is like trying to understand a ceiling fan while sitting on the blade as it spins around. Which will probably only make you dizzy. What you'll feel is wind in your face, and you'll see the room spinning around you. But if you stand underneath you'd see the fan go around and feel a downdraft on the top of your head.

Like the little man on the fan, a pilot travelling with the wing sees airflow, while an observer on the ground sees the wing moving. Your typical airflow diagram (below) shows air flowing around a wing. This is how it looks from the point of view of the wing which sees air moving around it. That is, if a wing could see and air were visible.

This is misleading because in this case the thrust is on the air rather than on the wing, which isn't how planes usually do it. The air's front to back movement is from thrust, the up and down movement is from the wing. Lift is from forcing air down, not back.

If we switch this around and show the wing moving as observed by the air we get a different picture. The wing goes forward, the air isn't flowing backward it goes up and down.

But even this is an incomplete picture. The second problem with typical airflow diagrams is they often show a narrow flow of air above and below. Yet the wing affects a lot more air than that, and a lot farther and for longer. The vortex extends well above, below, behind and in front of the wing.

You need to pull back to a wider angle to fit it all in. A better picture of airflow might be something like the following. (Note: These diagrams are rough approximations done just to make the point. Don't take these to the bank.)

Then again, we have the same problem as in the first diagram. We have the thrust on the air moving it backward rather than on the wing moving it forward. Not only that, but the view from the wing would also show the ground, and an observer on the ground moving backwards with the air. So we revise that to the following diagram.

This is somewhat better, yet can still be misleading as the viewer might infer the air is moving from one end of the frame to the other when it's not. The ground, an observer on the ground and the air doesn't flow backward. The wing is moving from end to end, not the air. The actual movement of air is like the diagram below.

The red arrows show how the air moves from the time it first encounters the wing to when the wing is about to exit the frame. What the unmoving observer on the ground sees, if air were visible, is pretty much all downward motion. That's why it's called downwash and not backwash. The opposite of down is up, so the reaction to downwash is lift. I also tossed in a rough approximation of the bound vortex which is the driving force behind it all.

Just to add a little more which might interest, you can't get lift without drag, the reaction of the air on the wing. Think of it this way, drag is forcing the wing back, lift is drag forcing the wing up. Since everything starts with the wing going forward, the reaction is back, or drag. The angle of attack redirects most of the drag upwards, changing it to lift.

Another way to do it would be to flap the wings like a bird. This is not very practical for airplanes which are a lot bigger and heavier and don't have feathers. But then, birds don't have propellers or jet engines. Birds use wings both for lift and thrust. It's a whole different animal, and planes aren't animals at all.

copyright Terry Colon, 2009

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