Comment Book Archive: Year 2000

Stall is when the flow over the upper surface no longer follows the curve of the airfoil. It separates and creates turbulence. A stalled wing can still produce lift, but not as much and at much higher drag. Drag is counteracted by thrust from the engin es, so higher drag means higher fuel consumption, which means higher cost to the operator. Stalled wings are also are more difficult to control.

Aerodynamic lift force is proportional to the local density of air, the velocity squared, the wing area, and the lift coefficient of the wing. The lift coefficient is a constant for any given wing, but changes if you modify the curvature of the airfoil or add flaps or winglets or other high-lift devices. As long as the wing isn't stalled, the lift coefficient is essentially a linear function of angle of attack. But the value of the lift coefficient (and therefore the total lift) when the angle of attack is zero (level flight) is not necessarily zero. This value depends on the airfoil curvature of the wing.

The camber line is defined as the curve halfway between the upper and lower curves of an airfoil. So at any point on the camber line, the upper surface and the lower surface are the same distance away. When the upper surface is curved and the lower surface is flat or less curved, the camber line is curved and we say it has positive camber. For a symmetric tear-drop-shaped airfoil (common for aerobatic airplanes), the camber line is flat. The amount of camber (how curved it is and in which direction) is what determines the lift coefficient at zero angle of attack, not the relative length of the upper and lower surfaces. A flat camber line has zero lift at zero angle of attack, but an airfoil with positive camber has a small amount of lift at zero angle of attack, though usually not enough to get off the ground.

But even if a wing has camber and flies at zero angle of attack, it still produces downwash, which is not shown in the "wrong" diagram on your web page. Downwash happens for any heavier-than-air vehicle producing lift. It's simple Newtonian physics. The mass of air being accelerated downward equals the lift force upward, which holds the aircraft from falling to the ground due to gravitational attraction. The difference between the higher pressure of air on the lower surface and the lower pressure of air on the upper surface multiplied by the wing area also equals the lift force upward. And that pressure difference can be created by both Bernoulli's effect (camber) and by tilting the wing at some angle of attack.

Here's the angle of attack perspective. A molecule of air far above or far below the wing will be undisturbed, so imagine these molecules as like a fixed ceiling and floor. The air above the wing has more space above the trailing edge and below the ceiling than above the leading edge, so it expands, lowering the pressure. Conversely, the air below the wing has less space between the trailing edge and the floor than below the leading edge, so the pressure is increased.

Lift force can also be downward. If the wing is at a negative angle of attack (more so than compensated for by any positive camber), the lift force is also negative. For an aerobatic plane flying upside down, the lift force is "down" to the pilot, but the weight to be offset is pulling "up," so they balance. To do this, the pilot points the nose "below" the horizon to fly at a negative angle of attack. The same maneuver could be seen from an upright nearby plane as a plane flying at a positive angle of attack with lift upward, weight downward, landing gear on top and an upside-down pilot. It's all a matter of perspective.

FOILSIM II
http://www.grc.nasa.gov/Other_Groups/K-12/airplane/foil2.html

BERNOULLI VERSUS NEWTON
http://www.grc.nasa.gov/Other_Groups/K-12/airplane/bernnew.html

AIRFOIL EXPLANATIONS

Here's a thought. If airfoils work like balloons, if they work by static forces only, then a helicopter could still fly if its rotor was placed in a box. After all, if we place a balloon into a large box, the balloon does not push downwards on the interior of the box as the balloon rises. If the box has low weight, then the balloon can lift the entire box upwards! So, if a helicopter attains lift only because of pressure fields and not because it flings air downwards, then we can place a helicopter into a sealed box, and the whole box will rise into the air! There will be no downdraft coming from the sealed box as it hovers in the air! If the box was fairly thick (but light), the helicopter would be totally silent. Now wouldn't THAT make a great toy, if you built a small one.

In reality, Newton's laws insist that a helicopter can do no such thing. Helicopters don't fly by bouyancy, they fly because they launch a stream of air downwards, and they are lifted by the reaction force. Bernoulli's equation is important because it describes how the deflected air creates a pressure difference on the wing's surfaces. But without deflection, the circulation pattern vanishes, and without circulation, the upper and lower flow velocities become the same.

Hi Mike! In a "2D" situation the air can only move up and down or forwards and back, it does not move along the wing. If you place a crossection of a wing into a narrow wind tunnel, and the ends of the wing are against the walls of the tunnel, that's a "2D" situation. If you place an entire model airplane into a wind tunnel, and the tips of the wings are far from the walls, that is a "3D" situation. > Also, I have extreme difficulties believing a "wing"
> launches packets of air downwards

My explanation is here
Also look here for photographs of the downwards-moving air.

> What is your definition of flight? Does a hot air balloon fly?

In physics we have forces caused by F=MA, but we also have "contact forces". When a rocket hovers above the earth, it is flying because of F=MA, since the rocket accelerates exhaust parcels downwards and is lifted upwards as a result. When a ship floats on the ocean, no mass is accelerated, and the ship stays up because of contact forces between the ship, water, and earth. The same is true of a balloon: it hovers because of contact forces between the balloon, the air, and the earth.

I've also heard that it applies to "ground effect" flight, where the aircraft is less than one wingspan from the ground. In this situation the field of pressure/flow supposedly causes most of the lift, and the surface of the earth can "feel" the pressure- footprint of the aircraft.

> What happens when we, say, cut a finite bit of span out of
>infinite wing, is that air flows around the ends, n'est-ce pas?

But other things also occur: the air was flowing downwards at the time it left the trailing edge of the wing, and because of inertia it keeps going down. Air did try to flow around the ends of the wing, and therefore after the wing has passed, the air is left with a structured flow: moving down in the center, and upwards at the edges, but with an overall downwards motion. It forms a pair of huge vortices.

> There is no reason for this trailing vortex to do anything but
> trail straight back, no up, no down.

The reason it moves down is inertia, and experiments show that the air behind an aircraft is moving downwards. When an aircraft flys horizontally over fog, it throws air downwards, which punches a slot in the fog. See this famous GIF IMAGE. Also see the index of fluid dynamics photos

> Do you not see that the wing CANNOT force air downwards. There's
> air already there. If air were forced down it would pop back up, like a roostertail behind a boat

In two dimensions, there is no wake vortex. In two dimensions, a vortex is locked to the airfoil and extends to infinity (or extends to the distant ground.) In two dimensions, an aircraft can stay up because it makes contact with the ground via a field of dynamic pressure. But in 3D an aircraft does not do this. Instead it stays up by launching pieces of air downwards.

Here's a crude animation of moving chunks of air.

Why does smoke 'ring?' http://amasci.com/wing/smring.html

But why did the cloud form? To quote:

"If that explanation is not simple enough for your students, just present the facts: when the temperature drops below the dew-point temperature, there is a net condensation and a cloud forms." This explanation is incomplete and ignores that the fact that the "Dew Point" in a volume of air is also dependant upon pressure. As in the case of the summit cloud on Mt. Rainer...these are caused by the due point being reached due to the reduction of air pressure as the wind blows over the summit (Bernoullis Principle). Notice the cloud only over the summit not in air at the same temperature(elevation).

Also I found a somewhat erraneous sentence:

I do find one fault in the explanation given in the linked e-mail and again in BAD CHEMISTRY. The authors state that the ice skate rides on an area of about 11 cm by 3-4 mm. That would be the total length of the blade. The ice skate is curved (that's how you can turn it) and the contact area between the blade and the ice is much smaller. If we make the wild assumption that the actual contact area is 2 cm long by 3 mm wide, that gives an area or .6 cm^2 and a resulting pressure (80 kg skater) of 133 atm. That would raise the melting point of the ice a little over 1 degree C.