Low approaches in ground effect

First a disclaimer: I understand the security issues involved
in the following and would not encourage anyone to try this
at home, but I am interested in the theoretical side of it.

Imagine you get things wrong and are caught out low on final,
still a fair distance out, and it looks marginal whether you
are going to reach the runway or not.

One technique I have sometimes heard described is to dive for
the deck and complete the remaining distance in ground effect.
For the sake of the argument we can assume fairly flat ground,
free of obstacles, though not necessarily landable.

The advantages claimed are usually better glide performance in
ground effect and less headwind and absence of downdrafts close
to the ground.

On the other hand you'll be travelling at higher than optimal
airspeed for most of the distance.

I am wondering how much truth there actually is to this
technique. Would it significantly increase your range and
improve your chances of reaching the field or not ?

Would it perhaps work better against a strong wind gradient
(as I suspect it might), and maybe not help a lot in calm
conditions ?

I'd be interested in any hard data/analysis or otherwise
enlightening comments on this.

Please note though, that I am not talking about high-speed
competition finishes, rounded off with a beatup and a sharp
pullup and all the dangers and other issues involved in that.

Cheers CV

Don't know whether this answers your question!

A Russian airforce pilot perfected this technique for
getting home on minimum fuel using ground effect to
prolong his flights. His ideas were developed into
ground effect vehicles which the Russians built in
a variety of forms.

The largest being the size of 747's and were named
the Caspian Sea Monsters. The intended development
was for military use. the end of the cold war stopped
production of the big ones. Some smaller craft have
been produced.

Briefly the idea is that you force air under the craft
with huge forward mounted jet engines, which provide
both the air cushion and thrust. Not quite the same
as just using ground effect to prolong an approach!

Ground effect does work with a low wing glider, other
than using the effect to float along way across flat
airfields I have never used it to prolong a final glide
over rough ground. But practice suggest that you have
to be very low to get the best effect and is only short
lived. A few hundred yards rather than miles, but then
I have never tried to go miles.

For Kranoplan information see

See http://aeroweb.lucia.it/~agretch/RAFAQ/WIG.html

First a disclaimer: I understand the security issues
involved
in the following and would not encourage anyone to
try this
at home, but I am interested in the theoretical side
of it.

Imagine you get things wrong and are caught out low
on final,
still a fair distance out, and it looks marginal whether
you
are going to reach the runway or not.

One technique I have sometimes heard described is to
dive for
the deck and complete the remaining distance in ground
effect.
For the sake of the argument we can assume fairly flat
ground,
free of obstacles, though not necessarily landable.

The advantages claimed are usually better glide performance
in
ground effect and less headwind and absence of downdrafts
close
to the ground.

On the other hand you'll be travelling at higher than
optimal
airspeed for most of the distance.

I am wondering how much truth there actually is to
this
technique. Would it significantly increase your range
and
improve your chances of reaching the field or not ?

Would it perhaps work better against a strong wind
gradient
(as I suspect it might), and maybe not help a lot in
calm
conditions ?

I'd be interested in any hard data/analysis or otherwise
enlightening comments on this.

Please note though, that I am not talking about high-speed
competition finishes, rounded off with a beatup and
a sharp
pullup and all the dangers and other issues involved
in that.

Cheers CV

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

"CV" wrote in message
news

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

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!

On Mon, 18 Oct 2004 16:13:41 +0200, CV wrote:


First a disclaimer: I understand the security issues involved
in the following and would not encourage anyone to try this
at home, but I am interested in the theoretical side of it.

Imagine you get things wrong and are caught out low on final,
still a fair distance out, and it looks marginal whether you
are going to reach the runway or not.

One technique I have sometimes heard described is to dive for
the deck and complete the remaining distance in ground effect.
For the sake of the argument we can assume fairly flat ground,
free of obstacles, though not necessarily landable.

The advantages claimed are usually better glide performance in
ground effect and less headwind and absence of downdrafts close
to the ground.

On the other hand you'll be travelling at higher than optimal
airspeed for most of the distance.

I am wondering how much truth there actually is to this
technique. Would it significantly increase your range and
improve your chances of reaching the field or not ?

Would it perhaps work better against a strong wind gradient
(as I suspect it might), and maybe not help a lot in calm
conditions ?

I'd be interested in any hard data/analysis or otherwise
enlightening comments on this.

Please note though, that I am not talking about high-speed
competition finishes, rounded off with a beatup and a sharp
pullup and all the dangers and other issues involved in that.

Cheers CV


I demonstrate to my students ground effect by approaching a 4200 foot
runway in a Blanik L-13 at about 55-60 knots. I would be at the
threshold a foot above ground. I could float over 3500 feet before I
would settle down and use the wheel brake to avoid running off the end
of the field. It is very effective. (There was little or no wind
when I did this.)

I once had to use this technique when I allowed myself to be
distracted on base leg. When I turned final I realized I was too low
to make the field but had plenty of speed. I dove toward the ground,
and put the glider in ground effect and made the field. (There were no
obstructions.)

A fellow instructor referred to ground effect as the last refuge of
the scoundrel glider pilot.

First a disclaimer: I understand the security issues involved
in the following and would not encourage anyone to try this
at home, but I am interested in the theoretical side of it.

Imagine you get things wrong and are caught out low on final,
still a fair distance out, and it looks marginal whether you
are going to reach the runway or not.

One technique I have sometimes heard described is to dive for
the deck and complete the remaining distance in ground effect.
For the sake of the argument we can assume fairly flat ground,
free of obstacles, though not necessarily landable.

The advantages claimed are usually better glide performance in
ground effect and less headwind and absence of downdrafts close
to the ground.

On the other hand you'll be travelling at higher than optimal
airspeed for most of the distance.

I am wondering how much truth there actually is to this
technique. Would it significantly increase your range and
improve your chances of reaching the field or not ?

Would it perhaps work better against a strong wind gradient
(as I suspect it might), and maybe not help a lot in calm
conditions ?

I'd be interested in any hard data/analysis or otherwise
enlightening comments on this.

Please note though, that I am not talking about high-speed
competition finishes, rounded off with a beatup and a sharp
pullup and all the dangers and other issues involved in that.

Cheers CV


CV, a group of test pilot trainees at Edwards AFB did an exhaustive
test on ground effect versus distance as a project during their
course; it was reported in the Feb 1990 SOARING magazine. IIRC, they
found that one had to fly a very precise profile - 0.95g push followed
by 1.05g pull, to a precise height - to see any measurable effect, and
concluded that it was better for the casual flier to fly best
lift/drag speed instead... I think they used a G103. The notation
from the index is:
Hadfield, Chris; with Chuck Louie, Ken Green, Rick Husband and Nate
Jones Is Ground Effect Worth It? [Aerodynamics], February, page 33

Chris Hadfield was a Canadian mission specialist on the Shuttle; two
flights, two spacewalks; top test pilot of his class at Edwards; and
US Navy Test Pilot of the Year for a F/A-18 out of control recovery
test program; Rick Husband was the crew commander on Columbia which
was destroyed on re-entry in Feb 2003, his second shuttle flight.

It might be worth getting the article reprint if you're interested in
the theory, or e-mail Edwards Test Pilot School for the report; I bet
it's swimming in math! IF you do, let us know how good my memory held
up.
DD

Daniel wrote:
CV, a group of test pilot trainees at Edwards AFB did an exhaustive
test on ground effect versus distance as a project during their
course; it was reported in the Feb 1990 SOARING magazine. IIRC, they
found that one had to fly a very precise profile - 0.95g push followed
by 1.05g pull, to a precise height - to see any measurable effect, and
concluded that it was better for the casual flier to fly best
lift/drag speed instead... I think they used a G103. The notation

I see. So if the results as reported here apply to calm conditions
it would mean the ground-effect technique wins out whenever there
is any significant wind gradient.

Where there is wind there is usually a wind gradient, certainly
in strong winds, meaning the ground-effect technique would normally
win against a headwind.

As I said before I totally agree about the security issues. If
you end up in a position where you need this something is already
badly wrong. And if you have the option it would be better to
pick a field you are certain you can reach rather than rely on
these effects.

CV