MacCready in the Mountains

Martin Hellman wrote:

Something that is rarely said when MacCready speed to fly is discussed
(including texts on soaring!) is that there is a major assumption in
the math behind it: that you can stop at any point and immediately
find a thermal of the strength indicated in the MacCready window.
Wouldn't that make soaring easy -- and probably a bit boring.

There was an interesting article in Soaring Magazine about a year ago
which talked about related issues.


The best and most complete discussion of this issue is still Helut
Reichmann's Book "Cross Country Soaring".

Stefan

On Mon, 29 Sep 2003 19:51:14 +0200, Stefan
wrote:

Martin Hellman wrote:

Something that is rarely said when MacCready speed to fly is discussed
(including texts on soaring!) is that there is a major assumption in
the math behind it: that you can stop at any point and immediately
find a thermal of the strength indicated in the MacCready window.
Wouldn't that make soaring easy -- and probably a bit boring.

There was an interesting article in Soaring Magazine about a year ago
which talked about related issues.


The best and most complete discussion of this issue is still Helut
Reichmann's Book "Cross Country Soaring".

Stefan

However we will no doubt reinvent the wheel here.
The problem isn't whether to fly Macready speed, it is what to set the
"expected lift strength" to and Reichmann covers this nicely.

Mike Borgelt

"Martin Hellman" wrote in message

Snip a lot of good stuff________________

There was an interesting article in Soaring Magazine about a year ago
which talked about related issues. Sorry I don't have the date, but if
memory serves me it's by a Biz School prof from Chicago. He argued
that you should use a lower setting when close to the ground because
you have less time to find a thermal and will almost surely have to
settle for a weaker one. Once you're high, you have more time to be
picky and can use a setting close to or even equal to the maximal
thermal strength of the day.


Snip some more good stuff_____________________

The prof is John Cochrane and his articles are he
http://www-gsb.uchicago.edu/fac/john...s/soaring.html

I like his point that the M value is basically an optimism setting. If you
are optimistic about your situation and the weather ahead, set a high M
value. If you are less optimistic, set a lower M value. That pretty much
takes care of all situations.

Bill Daniels

A few points that might be worth adding:

1) The application of MacCready theory optimizes your speed over a
cross-country course if you fly at the speed indicated for the expected rate
of climb

2) MacCready flight optimizes time at the expense of height. If you are low
or trying to clear a mountain ridge, it isn't appropriate to fly as fast as
indicated. If you slow down, you will lose less height (often even when in
sink). John Cochrane has given a good description of the trade-off betweeen
speed and height.

3) It probably doesn't pay to follow the guidance of an audio speed-to-fly
vario too aggressively - flying at a more constant speed may often be more
efficient.

and, most important......

4) My experience has shown that people grossly overestimate the achieved
climb rate on a cross-country flight. Here in Arizona, days with memorable
8 to 10 knot thermals rarely produce more than a 5-knot flight average and
2.5 to 4 knot days are more common. If you climb in a rare 10-knot
thermal on a day with a 4-knot average and set your MacCready setting to 10,
you'll be in trouble in a hurry! A good flight computer that will give you
the climb average for a flight will keep you honest!

Mike the Strike

ASW 20 WA

"JJ Sinclair" wrote in message
...
I fly a lot in the mountains and find MacCready speed-to-fly information,
completely worthless. Let me explain; Before crossing a ridge, I will fly
slower (below MC), so that I'm assured of making the next ridge. After
crossing
the ridge, I may fly faster than MacCready. If I set the proper MC setting
in
my computer. I am constantly bombarded with WRONG information coming from
the
computer audio.

For the last few flights, I have tried something new. I turned OFF the
computer
audio (B-100) and turned on the back-up vario (B-40), Now I have audio
only
when going UP and nothing when going down. I locked the computer in cruse
mode
and if I need to know what MacCready thinks about the speed I should be
flying,
all I have to do is look at the speed-to-fly needle.

One more tid-bit, I had radio interference (breaking squelch) when I
installed
the B-100 in my Genesis. I found that the cable to the LCD display was
real
sensitive to triggering the interference. When I would bring the radio
antenna
cable close to the LCD cable, It would break squelch. I completely solved
the
problem by keeping these two cables 6 inches apart. I also found the NMEA
cable
from the Cambridge GPS would trigger the radio squelch, if it was closer
than
6 inches to the LCD cable.
Cheers,
JJ Sinclair

On Mon, 29 Sep 2003 21:18:08 GMT, "Michael Stringfellow"
wrote:

A few points that might be worth adding:

1) The application of MacCready theory optimizes your speed over a
cross-country course if you fly at the speed indicated for the expected rate
of climb

"expected" rate of climb says a lot. It isn't necessarily the rate of
climb of the whole next thermal.

2) MacCready flight optimizes time at the expense of height. If you are low
or trying to clear a mountain ridge, it isn't appropriate to fly as fast as
indicated. If you slow down, you will lose less height (often even when in
sink). John Cochrane has given a good description of the trade-off betweeen
speed and height.


Speed to fly theory assumes you *will* get the next thermal before you
hit the ground.

3) It probably doesn't pay to follow the guidance of an audio speed-to-fly
vario too aggressively - flying at a more constant speed may often be more
efficient.

Which is why the speed to fly command should be damped considerably
compared to the vario. This is in line with now 20 year old German
research.

It has also been shown that it isn't necessary to fly at the exact
speed to get most of the benefit.

Severe dolphining is detrimental and likely to make you sick.

and, most important......

4) My experience has shown that people grossly overestimate the achieved
climb rate on a cross-country flight. Here in Arizona, days with memorable
8 to 10 knot thermals rarely produce more than a 5-knot flight average and
2.5 to 4 knot days are more common. If you climb in a rare 10-knot
thermal on a day with a 4-knot average and set your MacCready setting to 10,
you'll be in trouble in a hurry! A good flight computer that will give you
the climb average for a flight will keep you honest!

Mike the Strike


Anyone who has been around soaring for a while will know that when the
pilot says" it was a ten knot thermal" he means "the vario once
indicated 10 knots during the climb".


This is extended to the day: "there were ten knot thermals that day"
means that "in one thermal that day the vario momentarily read 10
knots"

Maybe we aren't so different from sport fishermen.

The things that John Cochrane mentions in his Macready paper were all
discussed at greater length 30 or more years ago by Anthony Edwards -
the" Armchair Pilot" in Sailplane and Gliding. We are in danger of
reinventing the wheel.

Mike Borgelt

"Michael Stringfellow" wrote in message
ink.net...
2) MacCready flight optimizes time at the expense of height. If you are
low
or trying to clear a mountain ridge, it isn't appropriate to fly as fast
as
indicated. If you slow down, you will lose less height (often even when
in
sink). John Cochrane has given a good description of the trade-off
betweeen
speed and height.

Clearing a mountain ridge is simply a final glide exercise. You should fly
as fast as indicated assuming you have set yourself up for a proper final
glide with appropriate risk factors applied. And if the air is lively, MC
speed to fly works quite nicely.

I was always amazed at how many pilots in competitions could not or would
not perform a reasonable final glide calculation, and then fly it. I
started doing it with a homemade whiz wheel and found I could beat 50% of
regional competitors on *any* final glide (not to say the same the 50%
wouldn't beat me during almost any other contest task element).

When the GPS computers came online, I could consistently hit the airport on
a final glide flying strict MC and GPS calc'd final glide by just giving
myself a 500' cushion at the airport. That seemed to beyond the capability
of at least 80% of regional competitors and a shocking number of national's
competitors. Amazing!

Yes, a lot of reinventing the wheel going on. Just read Reichmann for a
good foundation.

Here is another fly to throw into the ointment. It
is not uncommon in the Rocky Mountains to be forced
to leave strong thermals due to FL180. So how would
you set a MC when you are departing thermals before
they weaken?



At 01:42 30 September 2003, Greg Arnold wrote:

'Mike Borgelt' wrote in message
.. .
On Mon, 29 Sep 2003 16:40:28 -0700, 'Greg Arnold'
wrote:

Further, I believe that you want to use the average
climb rate at the
point
where you leave the thermal, not the average climb
rate for the entire
thermal.


Only because that should be the same as the expected
average at the
bottom of the *next* thermal.


Right, the point being that the climb rate in the middle
of a thermal is
irrelevant.



The only time you don't base Speed to fly settings
on the *next*
thermal is when leaving the final glide thermal -
then you should use
the rate of climb in the last circle or so.

Mike Borgelt

Obviously I'm not as good as some people here at knowing
exactly how strong the next thermal's going to be...

;-)

At 21:06 02 October 2003, Greg Arnold wrote:



Neither is more important
than the other in determining cruise speed.

Unless they are different, in which case the next
one
controls cruise speed. Of course if they are the
same, I
might as well say the next one controls instead of
the
current one. If you want to assign some control over
cruise
speed to the current thermal provided it's the same
as the
next one, I don't see any reason to argue, as we agree
on
what the pilot does, but it seems odd to me to put
it that
way. I think of it as leaving the current thermal
when I
can get to something as good or better, and cruising
at the
M-speed that matches the climb rate of the next one.
If
it's stronger, I run faster.


In that case, haven't you stayed in the current thermal
too long?


If I think it will be weaker,
I run slower.


In that case, shouldn't you stay in the current thermal
longer?



Todd Pattist - 'WH' Ventus C
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