Low approaches in ground effect

An additional concern to those who dive to go into ground effect is that you
are giving up the ability to choose a touchdown point. Once in ground
effect, the pilot loses the ability to turn and must touch down straight
ahead. Altitude, however, gives the pilot the option of turning slightly to
avoid something hard.

If you cannot make the airport, pilots must select the best option as soon
as possible, using the aircraft as a bargaining chip. Walking away is the
concern rather than making the airport. From that standpoint, altitude
gives us the most options for the longest time.

Colin N12HS


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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

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!

FWIW Earlier I said this effect worked better with
low wing gliders. Bill Daniels queried low wing, Bill
I stand corrected!

My only experiences are with Ask 4's and Ask7(high
wing) many years ago and later ASK13, Grob and Puchacz.
As Bill correctly says these later ones are mid wing
rather than low wing, however the effect appears more
marked with the mid wing gliders than the higher ones.

Gethard, for a better explanation see

http://www.se-technology.com/wig/index.php

where there is both a description and pictures.


At 11:06 19 October 2004, 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.

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.

The Hughes HK-1 Flying Boat (commonly and mistakenly called the Spruce Goose
since those closer to the project called it the Birch Bitch) was designed to
fly the entire flight in ground effect. It is displayed a short distance
from my home. When first acquired by Evergreen Museum, consideration was
given to fly it to the new home. However, the local fire marshall had
required a fire retardant to be sprayed on the aircraft before it was
originally put on display and that forever destroyed the ability to fly.

Now that would have been a sight to behold - the HK-1 soaring the Sierra
Nevadas in a Wave.

Colin N12HS


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"COLIN LAMB" wrote in message
nk.net...
The Hughes HK-1 Flying Boat (commonly and mistakenly called the Spruce
Goose
since those closer to the project called it the Birch Bitch) was designed
to
fly the entire flight in ground effect. It is displayed a short distance
from my home. When first acquired by Evergreen Museum, consideration was
given to fly it to the new home. However, the local fire marshall had
required a fire retardant to be sprayed on the aircraft before it was
originally put on display and that forever destroyed the ability to fly.

Now that would have been a sight to behold - the HK-1 soaring the Sierra
Nevadas in a Wave.

Colin N12HS



Actually, Hughes wanted the HK-1 to use 8 Lycoming R-7755's but was forced
by the government to use much less powerful Pratt & Whitney R-4360's. With
8 7000 HP Lyc's the HK-1 would have cruised in the stratosphere at a very
respectable speed for the time. With the Pratts, all it could do was fly in
ground effect.

The Northrop B-35 flying wing was also supposed to get the R-7755 but the
then Secretary of War owned a huge block of stock in P&W.

The Smithsonian Air & Space Museum has placed the only remaining R-7755 on
prominent display at Udvar-Hazy Center without political comment. The
Lycoming engineers were confident that the R-7755 could be developed to
produce 10,000 HP.

Bill Daniels

Bill Daniels

Bill,
If memory serves me right the 4360 had 4 rows of 9
cylinders for a total of 36 jugs. The aft rows were
spiraled to allow cooling to the rear rows, but even
so, the fourth row would run hotter. (KC-97F --circa
1952)
What was the configuration of the 7755?



At 13:48 19 October 2004, Bill
Lycoming engineers were confident that the R-7755 could
be developed to
produce 10,000 HP.

Uh-Oh!


We've just discovered this winter's discussion topic.
For what it's worth, Gale Craig, a physicist in Anderson,
IN, has written a book titled, I believe, Why Airplanes
Fly, in which he analyzes ground effect. He derives
his theories from Newton rather than from Bernouilli.
It is an interesting read.

A second edition had the title changed to something
like, Don't Abuse Bernouilli. The book is published
privately and has been reviewed on the web. I can
supply an address and telephone number to anyone interested.


At 11:06 19 October 2004, 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.

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!

John Sinclair wrote in message ...
Bill,
If memory serves me right the 4360 had 4 rows of 9
cylinders for a total of 36 jugs. The aft rows were
spiraled to allow cooling to the rear rows, but even
so, the fourth row would run hotter. (KC-97F --circa
1952)
What was the configuration of the 7755?


http://www.aviation-history.com/engines/xr-7755.html

-Dan