Directional control after touchdown...

Newps wrote:


Matt Whiting wrote:

Newps wrote:



Peter Dohm wrote:




I am pretty sure that there is also more wind six feet from the
graound than
there is three feet from the ground.


You're kidding.




Presumably there are plenty of statistics on the subject,



You'll never find that as it isn't true.


It absolutely is true. Wind velocity drops dramatically as it
approaches a surface.

I better get my digital handheld windmeter recalibrated becxause at 4
feet, the top of the wing on my Bo and six feet, the top of the wing on
a typical Cessna the wind is the same.

I would say so. The following article has a graph shows the
relationship on average between wind speed and height. The difference
between the wind at 1m (about the average height of your Bo wing) and at
2m (about the average height of a 182 wing) is about 10%.

http://www.rpc.com.au/products/windt.../wind_faq.html

Not a huge difference, but not no difference either.

Matt

Matt Whiting wrote:


I would say so. The following article has a graph shows the
relationship on average between wind speed and height. The difference
between the wind at 1m (about the average height of your Bo wing) and at
2m (about the average height of a 182 wing) is about 10%.

My Bo wing is 4'2" at the tip. I know that because I installed a
furnace in my hangar last month. The code calls for the furnace to be a
minimum 10 feet above the wing. I'm 6'2" and coud not walk under the
wing of my 182 as it sat on its big tires. So I would say the top of
the wing tip there would be at most 6'6". Even according to your
generalized graph an 18 meter difference in height brings a 5 kph
difference in windspeed, the difference between about two feet is
trivial and certainly not a factor at all for the subject at hand which
was stability of the airplane because one is a high wing and one is a
low wing.

Newps wrote:


Matt Whiting wrote:


I would say so. The following article has a graph shows the
relationship on average between wind speed and height. The difference
between the wind at 1m (about the average height of your Bo wing) and
at 2m (about the average height of a 182 wing) is about 10%.

My Bo wing is 4'2" at the tip. I know that because I installed a
furnace in my hangar last month. The code calls for the furnace to be a
minimum 10 feet above the wing. I'm 6'2" and coud not walk under the
wing of my 182 as it sat on its big tires. So I would say the top of
the wing tip there would be at most 6'6". Even according to your
generalized graph an 18 meter difference in height brings a 5 kph
difference in windspeed, the difference between about two feet is
trivial and certainly not a factor at all for the subject at hand which
was stability of the airplane because one is a high wing and one is a
low wing.


That is why I said average height of the Bo wing. Most Bonanzas I've
seen have a fair bit of dihedral and the wing isn't anywhere near 4'2"
at the root. I don't know what the average height is, but is sure isn't
4'2".

I don't see how you get a 5 kph difference with 18m altitude difference.
The chart expresses the difference as a factor relative to 1 as it
can't give an absolute wind velocity difference without knowing the base
velocity at 1m which is where the chart starts.

It states that the relationship is to the 1/7th power of the altitude.
Since it takes 1m as the reference altitude for a wind increase factor
of 1.0, an altitude of 18m has an increase factor of 18 to the 1/7th
power. This is 1.51 which indicates that the wind at 18m is 51% faster
than at 1m. Comparing 1m to 2m which is approximately correct to
compare a Bo wing to 182 wing, the increase factor is 2 to the 1/7th
power which is 1.1. Thus the wind velocity at 2m is 10% greater than at
1m. So, if the wind at 1m is 20 MPH then the wind at 2m would be
expected to be 22 MPH. If the wind at 1m is 40 MPH it would be expected
to be 44 at 2m.

As I said, 10% may or may not be significant in any particular
situation, but to say that the wind doesn't vary with altitude above the
ground as you claimed initially is simply false. I realize that you
never admit you are wrong and won't this time, but that doesn't change
the fact that you are wrong and it doesn't change the physics of wind
flow at various altitudes above the ground.


Matt

Recently, Matt Whiting posted:

I don't see how you get a 5 kph difference with 18m altitude
difference. The chart expresses the difference as a factor relative
to 1 as it can't give an absolute wind velocity difference without
knowing the base velocity at 1m which is where the chart starts.

[...]

The conditions in the original post is that the winds were 10 kts.
*reported by ATIS*. I don't know how one would translate that into wind
speed at the point of touchdown with any accuracy, but it's a safe bet
that the height of the wing doesn't much matter in this scenario. One can
be factually correct, yet irrelevant.

Neil

In article ,
"Neil Gould" wrote:

Recently, Matt Whiting posted:

I don't see how you get a 5 kph difference with 18m altitude
difference. The chart expresses the difference as a factor relative
to 1 as it can't give an absolute wind velocity difference without
knowing the base velocity at 1m which is where the chart starts.

[...]

The conditions in the original post is that the winds were 10 kts.
*reported by ATIS*. I don't know how one would translate that into wind
speed at the point of touchdown with any accuracy, but it's a safe bet
that the height of the wing doesn't much matter in this scenario. One can
be factually correct, yet irrelevant.

Neil

It all boils down to:

1. ATIS is only a guide about landing conditions.

2. Use every control available to correct for drift and to keep the
plane straight on landing.

3. Use flaps as necessary (you may not want flaps with high crosswinds).

4. You, the pilot, are responsible for controlling the plane until it is
parked and the engine shut down -- not the air traffic controller, not
the weather observer, not the FBO.

Matt Whiting wrote:




That is why I said average height of the Bo wing. Most Bonanzas I've
seen have a fair bit of dihedral and the wing isn't anywhere near 4'2"
at the root. I don't know what the average height is, but is sure isn't
4'2".

Cessna's have dihedral too. May not be exactly the same but it's there.



I don't see how you get a 5 kph difference with 18m altitude difference.


Did I read that wrong? The chart said the wind went from 15 kmh to 20 kmh.





It states that the relationship is to the 1/7th power of the altitude.
Since it takes 1m as the reference altitude for a wind increase factor
of 1.0, an altitude of 18m has an increase factor of 18 to the 1/7th
power. This is 1.51 which indicates that the wind at 18m is 51% faster
than at 1m. Comparing 1m to 2m which is approximately correct to
compare a Bo wing to 182 wing, the increase factor is 2 to the 1/7th
power which is 1.1. Thus the wind velocity at 2m is 10% greater than at
1m. So, if the wind at 1m is 20 MPH then the wind at 2m would be
expected to be 22 MPH. If the wind at 1m is 40 MPH it would be expected
to be 44 at 2m.

That's a lot of words to prove that the difference is irrelevant for the
purpose of this discussion. No way anybody tells the difference from 20
- 22 mph or 40 - 44 mph.

Newps wrote:

Cessna's have dihedral too. May not be exactly the same but it's there.

Yes, but very little. High-wings have more inherent stability since the
CG is below the CP.


I don't see how you get a 5 kph difference with 18m altitude difference.


Did I read that wrong? The chart said the wind went from 15 kmh to 20 kmh.

That was a representative example, but didn't apply as we are taking
from about 1m to about 2m, not from 2m to 18m. It is the general
formula that I was looking at which can be used for any wind speed and
altitude difference of interest.


It states that the relationship is to the 1/7th power of the altitude.
Since it takes 1m as the reference altitude for a wind increase factor
of 1.0, an altitude of 18m has an increase factor of 18 to the 1/7th
power. This is 1.51 which indicates that the wind at 18m is 51%
faster than at 1m. Comparing 1m to 2m which is approximately correct
to compare a Bo wing to 182 wing, the increase factor is 2 to the
1/7th power which is 1.1. Thus the wind velocity at 2m is 10% greater
than at 1m. So, if the wind at 1m is 20 MPH then the wind at 2m would
be expected to be 22 MPH. If the wind at 1m is 40 MPH it would be
expected to be 44 at 2m.

That's a lot of words to prove that the difference is irrelevant for the
purpose of this discussion. No way anybody tells the difference from 20
- 22 mph or 40 - 44 mph.

4 MPH is the difference between stalling at not stalling if you are
already flying only 2 MPH above the stall.

Whether that is relevant depends on the circumstances, no doubt. I
never said anything different. I'm simply correcting your statement
that wind speed is constant with height above the ground - which it isn't.

Having a 10% higher headwind when touchdown in a 182 vs. a Bo or Piper
is significant with respect to landing and takeoff distance. Check your
POH if you don't think a 10% difference in wind makes a performance
difference.

Matt