Weather and ground effect

[Kevin Chandler]

I was wondering if someone can shed some light on a phenomon I keep
experiencing. I took a Mooney up to warm it up prior to an oil change.
There was 70 degree crosswind at 20 gusting 28. Needless to say, there was
moderate turbulance below 8K. As expected, I got beat up pretty good just
briefly flying out of the pattern to do a full systems check. The plane was
all over the place during the approach. I could keep the plane in an
imaginary box but inside that box, I was at the mercy of the elements. Once
I got the plane safely into ground effect, everything smooths out for the
touchdown and rollout. This is not the first time I have experienced this.
Nearly everytime where there is turbulance and gusting winds, it seems that
if I can safely make it into ground effect, the turbulance and gusts either
go away or have little effect. It makes for beautiful landings after an
uncomfortable approach. Why is this?

Thanks in advance,
Kevin Chandler

[Paul Tomblin]

In a previous article, "Kevin Chandler" > said:
>go away or have little effect. It makes for beautiful landings after an
>uncomfortable approach. Why is this?

What you're calling "ground effect" is actually the zone where the wind is
in the "boundary layer". Right at the bottom, turbulent air will hit the
gound and "stick", then each successive "layer" of air above it will slow
down a bit relative to the one below it, but also stop moving across the
layers. So near the ground, you get lower wind speeds but also more
laminar flow.

That's kind of a layman's summary of the science behind it. Pedants will
now point out every little simplification I made as an error.


--
Paul Tomblin > http://xcski.com/blogs/pt/
There is no substitute for good manners, except, perhaps, fast reflexes.

[John T Lowry]

Simply due to your airplane having a low wing and short "legs."

John.
--
John T Lowry, PhD
Flight Physics
5217 Old Spicewood Springs Rd, #312
Austin, Texas 78731
(512) 231-9391

"Kevin Chandler" > wrote in message
...
> I was wondering if someone can shed some light on a phenomon I keep
> experiencing. I took a Mooney up to warm it up prior to an oil change.
> There was 70 degree crosswind at 20 gusting 28. Needless to say, there
was
> moderate turbulance below 8K. As expected, I got beat up pretty good just
> briefly flying out of the pattern to do a full systems check. The plane
was
> all over the place during the approach. I could keep the plane in an
> imaginary box but inside that box, I was at the mercy of the elements.
Once
> I got the plane safely into ground effect, everything smooths out for the
> touchdown and rollout. This is not the first time I have experienced
this.
> Nearly everytime where there is turbulance and gusting winds, it seems
that
> if I can safely make it into ground effect, the turbulance and gusts
either
> go away or have little effect. It makes for beautiful landings after an
> uncomfortable approach. Why is this?
>
> Thanks in advance,
> Kevin Chandler
>
>

[Maule Driver]

"Paul Tomblin" > wrote in message
> What you're calling "ground effect" is actually the zone where the wind is
> in the "boundary layer". Right at the bottom, turbulent air will hit the
> gound and "stick", then each successive "layer" of air above it will slow
> down a bit relative to the one below it, but also stop moving across the
> layers. So near the ground, you get lower wind speeds but also more
> laminar flow.
>
That 'boundary layer' effect which includes 'wind gradient' is also what
allows one to land in seemingly impossible xwinds. I've now twice
experienced x-winds too high to maintain a proper slip (wing low, fuselage
aligned) during final but once within a wingspan or so of the ground, the
xwind is reduced just enough to let me 'assume the position'.

Are there any other effects that might be at work here. Would flying in
'ground effect' somehow smooth out the bumps? Would this effect be
significantly different in Maule versus a Mooney because of wing location?
(I wrestle with my Maule in the flare but found once the Mooney (201) was in
ground effect, a smooth landing was assured.) Just thinking out loud here.

[Andrew Sarangan]

"Kevin Chandler" > wrote in message >...
> I was wondering if someone can shed some light on a phenomon I keep
> experiencing. I took a Mooney up to warm it up prior to an oil change.
> There was 70 degree crosswind at 20 gusting 28. Needless to say, there was
> moderate turbulance below 8K. As expected, I got beat up pretty good just
> briefly flying out of the pattern to do a full systems check. The plane was
> all over the place during the approach. I could keep the plane in an
> imaginary box but inside that box, I was at the mercy of the elements. Once
> I got the plane safely into ground effect, everything smooths out for the
> touchdown and rollout. This is not the first time I have experienced this.
> Nearly everytime where there is turbulance and gusting winds, it seems that
> if I can safely make it into ground effect, the turbulance and gusts either
> go away or have little effect. It makes for beautiful landings after an
> uncomfortable approach. Why is this?
>
> Thanks in advance,
> Kevin Chandler

I have noticed this too, and IMHO, the most dominant effect is the
diminished wind as you descend below the treeline. An 8-knot gust
factor is not much to worry about. However, if the surface wind is
20G28, the winds at 500' was probably like 30G50 which is enough to
kick you around. As you descend, you will not only notice a drop in
gust, but also a drop in total wind, which will cause a drop in
airspeed and an increased sink rate. This is something to be cautious
about on windy days at airports that are surrounded by tall trees.

I don't believe that the boundary layer has a significant effect here,
as it is too thin (compared to the aircraft size) to be noticeable.

[Andrew Sarangan]

"Maule Driver" > wrote in
om:

> "Paul Tomblin" > wrote in message
>> What you're calling "ground effect" is actually the zone where the
>> wind is in the "boundary layer". Right at the bottom, turbulent air
>> will hit the gound and "stick", then each successive "layer" of air
>> above it will slow down a bit relative to the one below it, but also
>> stop moving across the layers. So near the ground, you get lower
>> wind speeds but also more laminar flow.
>>
> That 'boundary layer' effect which includes 'wind gradient' is also
> what allows one to land in seemingly impossible xwinds. I've now
> twice experienced x-winds too high to maintain a proper slip (wing
> low, fuselage aligned) during final but once within a wingspan or so
> of the ground, the xwind is reduced just enough to let me 'assume the
> position'.

Which is one of the reasons I teach transitioning to the slip in the
flare rather than before. At 200agl you may not have enough rudder to
maintain runway alignment, but it may be just fine once you get down into
the flare.

[H. Adam Stevens]

"Andrew Sarangan" > wrote in message
. 158...
> "Maule Driver" > wrote in
> om:
>
> > "Paul Tomblin" > wrote in message
> >> What you're calling "ground effect" is actually the zone where the
> >> wind is in the "boundary layer". Right at the bottom, turbulent air
> >> will hit the gound and "stick", then each successive "layer" of air
> >> above it will slow down a bit relative to the one below it, but also
> >> stop moving across the layers. So near the ground, you get lower
> >> wind speeds but also more laminar flow.
> >>
> > That 'boundary layer' effect which includes 'wind gradient' is also
> > what allows one to land in seemingly impossible xwinds. I've now
> > twice experienced x-winds too high to maintain a proper slip (wing
> > low, fuselage aligned) during final but once within a wingspan or so
> > of the ground, the xwind is reduced just enough to let me 'assume the
> > position'.
>
> Which is one of the reasons I teach transitioning to the slip in the
> flare rather than before. At 200agl you may not have enough rudder to
> maintain runway alignment, but it may be just fine once you get down into
> the flare.
>
>
>
Boundary layers are tiny.
Close to the ground the vertical component of air movement is constrained.
Maintain coordinated flight until the flare; then slip.
Keep the momentum vector nailed down the centerline.
That keeps you going straight in the transition to slip in the flare.
Use the correct airspeed for your craft; no more, no less.
Blue skies.
H.
CP AS&MEL IA

[Marty]

"Paul Tomblin" > wrote in message
...
> In a previous article, "Kevin Chandler" >
said:
> >go away or have little effect. It makes for beautiful landings after an
> >uncomfortable approach. Why is this?
>
> What you're calling "ground effect" is actually the zone where the wind is
> in the "boundary layer". Right at the bottom, turbulent air will hit the
> gound and "stick", then each successive "layer" of air above it will slow
> down a bit relative to the one below it, but also stop moving across the
> layers. So near the ground, you get lower wind speeds but also more
> laminar flow.
>
> That's kind of a layman's summary of the science behind it. Pedants will
> now point out every little simplification I made as an error.
>
>
> --
> Paul Tomblin > http://xcski.com/blogs/pt/
> There is no substitute for good manners, except, perhaps, fast reflexes.

Thanks Paul,
All these years I just assumed it was simply surface friction. Your summary
makes better sense. If there were no other forces present,(another
assumption) surface friction would only make matters worse?

MG

[Greg Esres]

<<I don't believe that the boundary layer has a significant effect
here, as it is too thin (compared to the aircraft size) to be
noticeable.>>

According to "Meteorology for Scientists and Engineers", by Roland B.
Stull, the boundary layer is about 2 km in height.

[Greg Esres]

<<Would flying in 'ground effect' somehow smooth out the bumps?>>

If anything, it would make it worse. There is a "lift slope curve"
associated with each aircraft which determines how much lift comes
from a particular increase in AOA. When the curve is shallow, then a
large increase in AOA only produces a small increase in lift. This is
associated with low aspect ratio wings, such as a delta wing.

In ground effect, for any aircraft, the lift slope gets steeper.
Therefore, a small increase in AOA, such as produced by a gust will
produce a larger increment in lift, i.e., a bigger bump.

[Kevin Chandler]

> This is something to be cautious
> about on windy days at airports that are surrounded by tall trees.
>

Andrew,

I did not mention this in the original posting but the wind was blowing over
the tree at the approach end of Rwy 2 @ MGY.

Kevin

[Andrew Sarangan]

Greg Esres > wrote in
:

> <<I don't believe that the boundary layer has a significant effect
> here, as it is too thin (compared to the aircraft size) to be
> noticeable.>>
>
> According to "Meteorology for Scientists and Engineers", by Roland B.
> Stull, the boundary layer is about 2 km in height.
>
>
>
>


Hmm.. that must be a different boundary layer from what I am thinking of. A
boundary layer to me is the transition layer between a free flowing air and
the staionary air next to the surface. On a flying surface this is just a
few millimeters thick.

[Greg Esres]

<<staionary air next to the surface. On a flying surface this is just
a few millimeters thick.>>

Even the boundary layer on an a/c isn't completely motionless. That's
only true very near the surface. The boundary layer ends when the
speed of the flow is 99% of the freestream velocity.

The boundary layer on an a/c thickens the further it goes; the wind on
the planet Earth flows for hundreds of miles, not just a few feet.
Seems like the difference between the Earth's boundary layer and that
of an a/c is a matter of degree, rather than kind. But I'm only
guessing. ;-)