From: Lyon_Jim@Tandem.Com (Jim Lyon)
Subject: Re: What is Ground Effect?
Followup-To: rec.aviation
Organization: Tandem Computers, Inc
Date: Wed, 2 Sep 1992 00:06:13 GMT

In article <9u3e4a3@Unify.Com> raveling@Unify.com (Paul Raveling) writes:
> OK, anyone got a good aerodynamic explanation of ground effect. 

First, let's define ground effect to be the following two phenomena:

1.  An aircraft flying near the ground needs a smaller angle of attack to
    sustain level flight than an equivalent airplane flying at altitude at
    the same airspeed.

2.  An aircraft gliding without power will have a dramatically increased
    glide ratio when it gets near the ground.

In general, texts define "near the ground" as one wingspan or less.

In order to understand ground effect, you first need to understand a little
bit about what a wing does [no, I'm not going to bring up Bernoulli once
more].

To make a long story short, a wing generates lift by throwing air downward.
The lift generated by the wing is exactly the product of the mass of the
air being thrown downward times the acceleration being imparted to that
air.

During unaccelerated flight, the lift generated is exactly equal to the
weight of the plane.  Thus, the mass of the air being accelerated times
the acceleration being imparted to it equals the weight of the plane.

There are various ways that this total lift can come out properly.  For
planes with long wings, like gliders, the total air being affected by the
wing is quite large; the acceleration imparted to any piece of the air is
correspondingly small.  For planes with short wings, the total air being
affected by the wing is relatively small; therefore the acceleration needs
to be large.

Up to some limit [after which you stall], you can increase a wing's lift
by increasing the angle of attack.  This is true because increasing the
angle of attack causes the wing to impart more acceleration to each unit
of air that it encounters [it throws the air downward harder].

You can also increase your lift by increasing your airspeed.  As the
airspeed
increases,, the quantity of air being affected per unit time increases
linearly.  If you keep the wing's angle of attack constant, the
acceleration
imparted to each unit of air also increases linearly.  This is why, at
constant angle of attack, a wing's lift is proportional to the square of
the airspeed.

Accelerating this air takes energy.  This energy is proportional to the
mass of air being accelerated (per unit time), and the square of the
velocity to which the air is accelerated.  If we accelerate a lot of air
a little bit, we use some energy.  If we get the equivalent lift by
accelerating half as much air by twice as much, we use twice as much
energy.  [Hang on, this will become relevant shortly.]  This energy that
we lose accelerating air downward is the cause of "induced drag".


Now, let's get back to ground effect.

When a wing is flying near the ground, it is still throwing air downward.
However, this air is hitting the earth's surface, and is effectively
pushing the entire earth downward a tiny bit.  The lift generated by
the wing is equal to the mass of the air being thrown times the
acceleration
imparted to that air, plus the mass of the earth times the acceleration
being imparted to the earth.  Of course, the mass of the earth is *huge*,
and the acceleration imparted to the earth is *tiny*.  The product is
nevertheless significant.  So, a wing flying near the earth's surface needs
to throw less air downward less hard, because it's also throwing the earth
downward.  This explains phoenomenon 1 above.

The energy taken to keep the plane flying is proportional to the mass of
the air being accelerated times the square of the velocity to which it is
accelerated, plus the mass of the earth times the square of the velocity
to which it is accelerated.  Since this second term involves squaring an
extremely tiny number before multiplying by the mass of the earth, it
comes out to essentially zero.  Thus, the energy required to keep a
plane flying in ground effect is a function only of the reduced mass and
acceleration of the air that the wing encounters.

Stated another way, it takes less energy to fly in ground effect.  For an
unpowered aircraft, this energy comes from a decrease in altitude of the
plane.  Thus, the plane will lose altitude more slowly in ground effect
than otherwise.  This explains phenonenon 2 above.

NOTE FOR PURISTS:

Above, I've talked about an airplane pushing the earth underneath it away.
Do our everyday aviation activities really cause the earth to move?
Not really.  The reason we have wings do all of this stuff is that gravity
is pulling our plane down toward the earth.  However, that same gravity is
also pulling the earth up toward the plane.  By the time you add up all of
the forces involved in the entire earth/atmosphere/airplane system, you
discover that the combined center or mass of the earth, atmosphere and
airplane change not at all as you fly.  This does imply, however, that
when you take a 300 ton 747 up to 30,000 feet, the center of the earth
(excluding the plane) *does* move a little bit in the opposite direction.
It does, however, come back to where it started from when the plane lands.

-- Jim Lyon                          "... where the work is NonStop,
   Lyon_Jim@Tandem.Com                    and we do everything twice."
Newsgroups: rec.aviation