Low approaches in ground effect

CV wrote in message ...

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

Actually, no. If I remember correctly, the test concluded that in
order to achieve any noticeable benefit from ground effect, the glider
had to be flown extremely precisely at VERY low altitude - only a
couple of feet above the ground. None of this "half a wingspan" - so
unless you are stetching your glide over the Bonneville salt flats,
just keeping out of the usual bushes, fences, stray airport dogs, etc
would eliminate any ground effect benefit.

If the wind gradient is that strong, the turbulence at ground level
would make any precise low flying sporting and inefficient, anyway.

I think pilots confuse the distance a glider will "float in ground
effect" (with the dive brakes closed) with the simple low drag glide
angle as the glider slows down - and if over the runway this may be
helped a little by ground effect, but remember this "float to the end
of the runway" is usually started at a relatively low speed, so the
drag penalty of diving down to a high speed is not felt.

Try a constant altitude decelleration from Vne to Vstall at altitude
some time; it's amazing how long and far you go! (helps when you get
above the MSH...yeah I know you're not supposed to do it!)

Kirk

Kirk Stant wrote:
CV wrote in message ...

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.

Actually, no. If I remember correctly, the test concluded that in
order to achieve any noticeable benefit from ground effect, the glider
had to be flown extremely precisely at VERY low altitude - only a

I should have worded that differently.

What makes the difference there is less headwind close to
the ground. The ground effect would of course not increase
on account of the wind gradient.

couple of feet above the ground. None of this "half a wingspan" - so
unless you are stetching your glide over the Bonneville salt flats,
just keeping out of the usual bushes, fences, stray airport dogs, etc
would eliminate any ground effect benefit.

With bushes and fences it's not on. I was thinking more along the
lines of a big plowed field, perhaps with some ditches crossing
and maybe just a low fence between the field and the runway.

If there is some ground-effect benefit at 2 ft though, it won't
be magically "eliminated" at 2,5 ft. The effect will decrease
gradually with height.

If the wind gradient is that strong, the turbulence at ground level
would make any precise low flying sporting and inefficient, anyway.

It doesn't have to be very turbulent there. We are assuming
flat ground. When the gradient is strong the surface wind is
much weaker than winds aloft. That is the meaning of "gradient".

I think pilots confuse the distance a glider will "float in ground
effect" (with the dive brakes closed) with the simple low drag glide
angle as the glider slows down - and if over the runway this may be
helped a little by ground effect,

The slowing down factor is a valid point. And ground effect may
help a little, or more than a little, if it is true that it can
doube the L/D as someone mentioned.

but remember this "float to the end
of the runway" is usually started at a relatively low speed, so the
drag penalty of diving down to a high speed is not felt.

Try a constant altitude decelleration from Vne to Vstall at altitude
some time; it's amazing how long and far you go! (helps when you get
above the MSH...yeah I know you're not supposed to do it!)

I suspect at altitude I wouldn't be able to appreciate the exact
distance covered and fail to be amazed.

CV

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.


A few years ago Soaring Magazine reported experiments on this
undertaken (I believe) by the airforce glider program. They used
Blaniks and a very long runway. The conclusion was that it was better
to stay at max glide and not dive for the ground. I forget though how
much wind they had -- there is some headwind (50 mph) and gradient (to
zero at ground level) where it has to be better to dive. And Blaniks
are pretty bad at high speed -- though will have more induced drag at
any speed in ground effect too.

John Cochrane BB

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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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.


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.


USAF Test Pilot School Candidates did this as a student project 10 yrs
+/- ago. Used a G103 and found that one was definitely better off
flying speed-to-fly u\into ground effect and then leveling off.
Probably diving down would be better in a strong wind gradient but
determining the gradient and what speed-to-fly through it would be
operationally very difficult.

You could probably get a copy of the report from USAF TPS at Edwards,
CA.

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
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CH-8045 Zuerich \_/ See homepage for
email address!

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.

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