Newton's third law to explain lift

[rcgldr]

Cambered air foils can generate lift at zero "geometrical" angle of lift

airfoils can and do generate lift at negative angles of attack

Yes, but they still produce downwash at those negative angles of attack. On a cambered airfoil, most of the diversion of flow takes place near the peak of the cambered airfoil and the pressure differential decreases with distance aft of the peak. If the trailing edge is shortened somewhat, the diversion still takes place, but now the wing has a negative AOA due to the trailing edge being shortened relative to the leading edge. This is why effective angle of attack is sometimes used to compare the properties of different airfoils (effective AOA being zero at zero lift).

As far as downwash goes, forces don't dissipate. ... the force that gravity exerts on the aircraft ... eventually exerts ... back onto the surface of the earth.

Do you know of a good reference that really explains this.

Consider the Earth's atmosphere, and any objects "supported" by the atmosphere (aircraft in level fliight, balloon hovering, ...) as a closed system. The average force on the surface of the Earth will correspond to the total weight of the atmosphere and any objects "supported" by the atmosphere.

Reduce this to a very large sealed container. The container weighs 50 lbs, the air inside weighs 49lbs, and there's a model aircraft inside that weighs 1 lb. As long as the center of mass of the system is not accelerating vertically, the weight of the system will be 100 lbs, even when the model aircraft is in level flight (doing circles or figure eights) inside the container. The weight of the air and the model result in a pressure differential within the container, lower near the top, higher near the bottom, and the resultant downforce on the container corresponds to the weight of the air and the model aircraft. The model's affect on the air inside increases the pressure differential so that the downforce increases by the weight of the model. The model increases the pressure differential by inducing downwash as it flies (unless in ground or "ceiling" effect mode).

 

[Stan Butchart]

Reduce this to The model's affect on the air inside increases the pressure differential so that the downforce increases by the weight of the model. The model increases the pressure differential by inducing downwash as it flies QUOTE]

I hope that this response does not mark me as overly dense!

I agree with the principle but still missing the mechanism. If you get a chance could you exspand a bit on the last/

For DH
Again density. You are obviously making a point with that vidio which I missed. Could you give a hint? I see a classic stalled upper surface and the lower surface looks like Newtons original thoughts!

 

[DarioC]

"Yes, but they still produce downwash at those negative angles of attack."

Well yeah. You are preaching to the choir here. Why the but? Of course it does. Did I say somewhere that I thought different? That was my whole point.

I suddenly have this impression that what is actually happening at the leading edge is what is giving Stan problems. I do not think the air is doing what he thinks it is doing at the leading edge. If it did, the wing would not work.

I'm getting the impression that he thinks that the wing is sucking up a huge volume of air from FAR below the leading edge of the wing and throwing it over the top and then down with the equivalent amount of energy behind the trailing edge. Same up energy as down energy. That cannot be what is going on at the leading edge.

DC

 

[rcgldr]

Reduce this to The model's affect on the air inside increases the pressure differential so that the downforce increases by the weight of the model. The model increases the pressure differential by inducing downwash as it flies

I agree with the principle but still missing the mechanism.

See my reply to Dario C below about pressure and acceleration of air. The closed system example is one way to help explain Newton's third law and lift.

video ... a classic stalled upper surface and the lower surface looks like Newtons original thoughts!

Even though the upper surface is stalled, it's still producing some downwash.

"Yes, but they still produce downwash at those negative angles of attack." Why the but?

For anyone reading this thread that might think that downwash wouldn't occur in this case.

I'm getting the impression that he thinks that the wing is sucking up a huge volume of air from FAR below the leading edge of the wing and throwing it over the top and then down with the equivalent amount of energy behind the trailing edge.

I've seen this claim made for "2d" flows, but not for a real wing. The fact that pressure is lower above and higher below a wing is going to lower the flow separation point in front of the leading edge as some air will be diverted upwards to the low pressure zone above, but the net effect of a low pressure zone above a wing is that air accelerates towards that low pressure zone from all directions, except upwards through the wing, resulting in a net downwards acceleration of air from above the wing. A higher than ambient pressure below the wing would similarly result in downwards acceleration of air below the wing.

 

[DarioC]

That pulsed smoke video from Cambridge really got my attention, particularly the reduction in velocity of the flow in relationship to how close it is to the bottom surface of the wing.
There are some "strange" things going on there, Chuckle.

OK, so maybe I am interested in some of the details of what is going on--curiosity will get you every time.

Added after looking at the pulsed flow several more times.

It appears that the net velocity on top of the airfoil departing the trailing edge is about the same as the lower pulses in the bottom stack. Due to the slant of the bottom stack still existing at the lowest pulse I would surmise that the air below it is moving even faster. Which means that the top flow at the trailing edge is moving at the same speed as the "ambient" flow in the tunnel, but the bottom flow, near the wing surface, is slower.

That is not at all what I would have expected and the implications are really interesting.

DC

 

[Stan Butchart]

What we are missing in much of this is the effects of circulation.

On the lift from a neg AOA ~ as long as the last about 1/3 of the mean camber line is pos to the direction of motion, there will be some circulation and therefore lift.

Starting at the beginning ~ At the instant that the wing, with AOA, starts to move the displacement flow attempts symetrical distribution where the forward stagnation streamline originates on a level where the aft stagnation would finish on a lin midway through the airfoil profile. At the TE discontinuity, with the formation of the starting vortex forms the aft stagnation streamline moves down to the area of the trailing edge with an circulation adgustment around the entire foil. Given.

We now have a an additional volume at the back of the wing that must come from somewhere. At the "top" of the wing we can see the added volume of flow from circulation and it had to come from somewhere. At the front of the wing, circulation has moved the stagnation streamline and stanation point downward to direct more of the air over the top.
The origin of the fwd stagnation streamline and the tail of the aft stagnation streamline are at a common level. (rough and theo)

The energy of the displacement flow is almost uncomprehensible. It is required to establish the original motion then remains with an object as it moves forwards. Like ocean waves.

Local motions caused by differences in pressure need care in understanding.
For the flow next to the surface, inetially, the flow is accelerated to the velocity of the wing. As it flows along the surface it actually slows down over the top then speeds up.
( The speed relationship with the surface is fine for normal acceleration)

The "inflow" of motion over the top is from the pressure release of the curving fluid boundary (wing surface) and is a far greater gradiant than one caused by the fluid dynamic motion.