Re: air pockets

Date:         08 Sep 97 02:03:51 
From:         faurecm@halcyon.com (C. Marin Faure)
Organization: Northwest Nexus Inc.
References:   1
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In article <airliners.1997.2018@ohare.Chicago.COM>, lowwol@pacific.net.sg wrote:

> can anyone pls explain to me what airpocket is?  how it happens and
> also, when can it occur.

The explanation is very simple.  There is no such thing as an air pocket.
This would have to be a vacuum or a place where there "wasn't any air"
(which is what early airline passengers assumed the bumps were when they
flew through turbulence).  What many people still call "air pockets" are
simply the physical result of the airplane moving through air that is
travelling vertically at different rates.  So when the plane moves from
stable air into a downdraft, the plane feels to the passengers as though
it's suddenly falling, thus the "air pocket" theory.  But all that's
happened is the plane is now flying through a mass of air that's going
down, so the plane goes down with it.  It's still flying, however. In fact
outside of the initial change from stable to rising or falling air, the
wing has no idea which way the mass of air it's flying through is going.
A wing trimmed to fly at 200 mph in stable air will fly at 200 mph in
descending air (assuming the entire air mass is descending at a constant
rate) and it will fly at 200 mph in ascending air.  The bumps passengers
feel as the plane flies through turbulence are no different than the bumps
they would feel if they were crossing a tubulent river in a canoe.  The
only difference is that you can't see the rising and falling air so you
can't anticipate what's going to happen, where on a river you can see
water that's being humped up by a boulder or rolling over itself
downstream of a ledge so you have some idea of what's going to happen
before you get to it.

Two more examples of how air affects, or doesn't affect, an airplane.  An
airplane trimmed for a specific airspeed flies at that speed regardless of
whether it's heading into the wind or flying with it.  Headwinds and
tailwinds only affect the plane's speed over the ground, not through the
air.  And drift, the speed of wind across the earth's surface, affects all
airplanes flying in that wind equally.  In other words, if you flew a
Piper Cub at 70 mph on a heading of due west in a 30 mph crosswind coming
from due north, at the end of an hour you would be 70 miles west of your
starting point and 30 miles south.  If you flew a 747 at 600 mph along the
same route in the same wind, at the end of the hour you would be 600 miles
west of your starting point and 30 miles south.  And if you released a
balloon from the same starting point in the same wind, after an hour it
would be zero miles west (or east) of the starting point and 30 miles
south of it.

C. Marin Faure
  author, Flying A Floatplane