bob mackey wrote:
has anyone experienced a downward blast from a landing aircraft?
Yes, I have. As you can readily convince yourself, the vertical
component of the airflow diminishes to zero at the ground surface.
I did a series of experiments years ago with hang gliders
that showed the vertical motion of the air behind the glider.
It is substantial, and just exactly what you'd expect from
F=ma, Bernoulli, Coanda, Langmuir, and Stokes...
They are all in agreement, and all follow the same rules.
BTW...
Bob,
There is a beach on San Maarten that is under the flight path of
landing jets. Cool video of same can be seen here:
http://www.youtube.com/watch?v=PvNONncieu0
If the entire mass of the 747 was being supported exclusively by
accelerating air downward then then everything under the flight path
would be scoured by hurricane force winds. Try putting your beach chair
under a Delta rocket and light the fuse. You can see be the video there
is only a slight disturbance. Where did all that energy go!
It is true that the vertical component of an air blast diminishes to
zero as it approaches the ground, because it turns into a horizontal
component and diffuses radially. Point a leaf blower straight down and
note that the air flow hits the ground and then travels in a thin
layer radially. The layer "sticks" to the ground because it has a
higher velocity (lower pressure) than the surrounding air. This is
part of the mechanism behind "wind shear".
As for your hang glider, it is a lot closer to a kite or a parachute
than a laminar airfoil wing.
There is an air disturbance in the aircraft's wake, some is vertical,
some is horizontal, some is turbulence and some is heat. The total
energy of the disturbance equals the energy released in burring the
fuel, but not more (level flight).
Here is another way to look at it. For the hose theory to be right then
the airplane must be able to release enough energy to throw air
downward fast enough to lift the mass of the airplane against the
acceleration of gravity. If this were true, then the most efficient way
to accelerate air and throw it downward would be to stand the airplane
on it's tail and let the propeller do the job directly, without minimum
aerodynamic drag from the body of the airplane. There are only a small
handful of fixed wing aircraft that can accelerate straight up so there
must be something else keeping the airplane in the sky. That something
is the difference between the air pressure on the outside of the bottom
of the wing and the air pressure on outside of the top of the wing.
Paul's hose theory for aerodynamic lift is all wet.
If anyone out there that doesn't get this
then I would recommend you seriously reconsider the whole "I want to be
a pilot" thing.
Paul, please tell us you pulled a "Pariah Kerry" and just let a bad
joke go too far. You don't actually believe this, right??? April first
is still some time away.
Regards
Brent Regan
bob mackey wrote:
has anyone experienced a downward blast from a landing aircraft?
Yes, I have. As you can readily convince yourself, the vertical
component of the airflow diminishes to zero at the ground surface.
I did a series of experiments years ago with hang gliders
that showed the vertical motion of the air behind the glider.
It is substantial, and just exactly what you'd expect from
F=ma, Bernoulli, Coanda, Langmuir, and Stokes...
They are all in agreement, and all follow the same rules.
BTW...
If you were flying at 70 Kts (118 Ft/sec) stall at an AOA of
45 degrees (extreme example) and a projected wing / fuselage
area of 100 square feet the you would be "intersecting" 11,800
cubic feet per (896 lbs) second.
A better approximation of the "area" of disturbed air is pi*span^2/4,
or the area of a circle circumscribed around the wingtips.
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