Low approaches in ground effect

"CV" wrote in message
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Dave Martin wrote:
Don't know whether this answers your question!

No bearing on the question, really, though it is
slightly related, and interesting as well.

Ground effect does work with a low wing glider, other

I don't know of any low-wing gliders. Most are mid-wing
and some older types are high-wing, but that difference
is so small I wouldn't expect it to matter. In the
situation I asked about we'd want a safety margin
of perhaps 5 feet or so off the ground in any case.

AFAIK ground effect starts being significant from
approx. half a wingspan off the ground, the effect
being to increase performance, as if you had greater
span.

lived. A few hundred yards rather than miles, but then
I have never tried to go miles.

A few hundred yards would be sufficient for what I had
in mind, provided that the other factors balanced out
and resulted in a net gain in how far you can glide.

Cheers CV

The rule I learned was that ground effect became measurable at one wingspan
above the ground and near the ground could double the L/D. Running in
ground effect is a lot of fun but you'd better be very smooth on the
elevator since the pitch control gets 'twitchy'.

(I strongly suspect that unanticipated elevator sensitivity in ground effect
is a secondary cause of some of the G103 "PIO" accidents.)

The best glide stretching technique is to approach the ground at slightly
better than best L/D speed leveling off a couple of feet above the ground.
Diving to the ground is dangerous and wastes energy that could better be
spent at best glide. Maintaining the usual approach speed seems to work
best.

Of course, all this assumes that the approach and runway under-run are
completely free of obstacles like wires or fences.

Bill Daniels

Bill Daniels wrote:

The rule I learned was that ground effect became measurable at one wingspan
above the ground and near the ground could double the L/D. Running in
ground effect is a lot of fun but you'd better be very smooth on the
elevator since the pitch control gets 'twitchy'.

(I strongly suspect that unanticipated elevator sensitivity in ground effect
is a secondary cause of some of the G103 "PIO" accidents.)

Interesting observation. Flying in ground effect places the center of
pressure of the wing at about mid-chord, while out of ground effect the
center of pressure is about 1/4 chord. So in short, ground effect
usually has a stabilizing effect by essentially shifting the CG forward
with respect to the center of lift.

"nafod40" wrote in message
...
Bill Daniels wrote:

The rule I learned was that ground effect became measurable at one
wingspan
above the ground and near the ground could double the L/D. Running in
ground effect is a lot of fun but you'd better be very smooth on the
elevator since the pitch control gets 'twitchy'.

(I strongly suspect that unanticipated elevator sensitivity in ground
effect
is a secondary cause of some of the G103 "PIO" accidents.)

Interesting observation. Flying in ground effect places the center of
pressure of the wing at about mid-chord, while out of ground effect the
center of pressure is about 1/4 chord. So in short, ground effect
usually has a stabilizing effect by essentially shifting the CG forward
with respect to the center of lift.

Al I can say is put a G103 in ground effect and see for yourself.
Obviously, flying qualities in ground effect deals with the whole glider and
not just the wing. No doubt the flow over the tail is involved as well.

Bill Daniels

Bill Daniels wrote:
"nafod40" wrote in message

Interesting observation. Flying in ground effect places the center of
pressure of the wing at about mid-chord, while out of ground effect the
center of pressure is about 1/4 chord. So in short, ground effect
usually has a stabilizing effect by essentially shifting the CG forward
with respect to the center of lift.


Al I can say is put a G103 in ground effect and see for yourself.
Obviously, flying qualities in ground effect deals with the whole glider and
not just the wing. No doubt the flow over the tail is involved as well.

I have, and i don't remember any decrease in longitudinal stab. Having
the ground that close certainly tends to "up the gain" in the soft mushy
grey matter autpilot though.

nafod40 wrote:
Interesting observation. Flying in ground effect places the center of
pressure of the wing at about mid-chord, while out of ground effect the

Are you sure? Note that a forward CG implies a pitch down moment which
would have to be compensated by negative lift on the tail. Hence
performance degradtation, contrary to what ground effect is supposed to
create.

I'm still searching for a good explanation of ground effect :-)

Cheers
-Gerhard
--
Gerhard Wesp o o Tel.: +41 (0) 43 5347636
Bachtobelstrasse 56 | http://www.cosy.sbg.ac.at/~gwesp/
CH-8045 Zuerich \_/ See homepage for email address!

Gerhard Wesp wrote:
nafod40 wrote:

Interesting observation. Flying in ground effect places the center of
pressure of the wing at about mid-chord, while out of ground effect the


Are you sure? Note that a forward CG implies a pitch down moment which
would have to be compensated by negative lift on the tail. Hence
performance degradtation, contrary to what ground effect is supposed to
create.

That part would be minor, more than compensated for by decrease in
induced drag and increase in lift.

I'm still searching for a good explanation of ground effect :-)

Here's a killer page describing the aerodynamics of it. Follwoing is a
short excerpt from the page.
http://www.se-technology.com/wig/htm...en=aero&code=0

Two phenomena are involved when a wing approaches the ground. Ground
effect is one name for both effects which is sometimes confusing. The
two phenomena are sometimes referred to as span dominated and chord
dominated ground effect. The former results in a reduction of induced
drag (D) and the latter in an increase of lift (L). The overall effect
is an increase of the L/D ratio. This ratio is a measure for the
efficiency of an aircraft which can be expressed as the amount of power
(thrust) that is required to propell an aircraft of a certain weight.
Since thrust is equal to drag and weight is equal to lift in stationary
flight this efficiency can be expressed as the L/D ratio. As the L/D of
a wing increases with decreasing ground clearance the craft becomes more
efficient in ground effect.