Re: Yes, they look beautiful
> That's rubbish, it's the Venturi effect that keeps a plane up, whereby air moving quickly over the top of the wing exhibits less pressure than the slower air moving under the wing, thus sucking the plane upwards. Nothing to do with pushing air downwards (like a rocket)
Actually it has _everything_ to do with pushing air downwards. A plane's wing is an air pump that pushes air downwards. Without doing that it would fall from the sky (and sometimes that happens). It is just that the plane is flying along faster than the air is going down so it streams in a down wash behind the aircraft.
A helicopter rotor blade is exactly like a wing, except it moves through the air, pumping it downwards, without the fuselage needing to move forward. This can easily be seen in any photograph of a helicopter flying low over the sea, or over long grass or crops. An agricultural aircraft, flying low, causes the same effect on crops.
The amount and downward speed of the air that is required can be directly calculated from the mass of the aircraft and gravitational acceleration (ie the weight), just as it is calculated for a rocket exhaust, or the efflux from a VTOL aircraft.
Think of it this way. Normal air pressure over a particular area of land has a certain value (average is 14.7 lb per sq in or so at sea level). This is caused by the weight of all the air over that area. ie the column of air over 1 sq inch of ground from the ground up to space weighs around 14.7 lb (depending on weather, altitude of ground, etc). If a plane flies into the volume of air over a particular area (much more than 1 sq in) then the weight of that plane adds to the weight of all that air. This would increase the pressure at ground level over that area (granted by a small amount). The only way to increase the pressure between the aircraft and the ground over that area is for the plane to pump air from above it to below it.
A hovering helicopter close to the ground does this very well over quite a small area and the air pressure increase is easily measured (and is equal to the weight of the helicopter divided by the area covered by its downwash). A 747 does exactly the same except the area covered is very large and often well behind the current position of the plane, but at the end of a runway when it is landing it is _very_ noticeable.
The pressure distribution above, below and around the wing is part of the means of pumping the air downwards. Air at some distance above the wing 'falls' into the lower pressure caused by the alleged 'venturi effect' while the higher pressure created under the wing pushes air below that downwards.
Gravity applies a force of mass x 32 ft/sec/sec. The plane opposes that by accelerating air downwards, taking a particular mass of air in each second that is, in effect, stationary and accelerating it in a downwards direction (and also in unwanted lateral and rotational directions) until the mass of air in each second x acceleration applied matches the force of gravity pulling at the aircraft (more if it is climbing).
Biting the hand that feeds IT
