The Balance between "% Circling" and "MacCready Speed to Fly"

I have been told over the years two consistent things: when trying to
achieve the fastest possible cross country speeds, 1. Try to minimize
circling as much as possible and 2. Fly the true maccready airspeeds
between thermals.


But these two things contradict. The first couple days I flew at
Perry this year, I flew true maccready airspeeds between thermals.
The lift was strong, 5-7 kt thermals to 6-7,000 feet. So I flew about
70-80 between thermals. My average speed was then in the upper 40's.
Then for the later part of the contest, in the same weather
conditions, I delibertly did everything I could to reduce my %
circling. I flew 50-70 kts between thermals and drastically reduced
my circling. I had one flight with 15% circling and my resulting
average speed was 52 mph. So by slowing down I had a faster speed.

I understand that thermal density plays a big part in deciding what
speed to cruise at. If the thermal density is lower, it may be best
to stretch out the inter thermal glides to stay in the altitude power
band. I fly faster when the lift sources are obvious, gaggle, clouds,
birds, smoke. But when the lift is invisible, I slow it down to
stretch out my glide. I also slow waaay down if there's no good
landable fields insight....for safety.

So is there a balance between % circling and true maccready speed to
fly? What are some things that can flex your true maccready speeds up
or down?

On Jul 4, 8:29*am, Scott Alexander
wrote:
I have been told over the years two consistent things: when trying to
achieve the fastest possible cross country speeds, *1. Try to minimize
circling as much as possible and 2. Fly the true maccready airspeeds
between thermals.

But these two things contradict. *The first couple days I flew at
Perry this year, I flew true maccready airspeeds between thermals.
The lift was strong, 5-7 kt thermals to 6-7,000 feet. *So I flew about
70-80 between thermals. *My average speed was then in the upper 40's.
Then for the later part of the contest, in the same weather
conditions, I delibertly did everything I could to reduce my %
circling. *I flew 50-70 kts between thermals and drastically reduced
my circling. *I had one flight with 15% circling and my resulting
average speed was 52 mph. *So by slowing down I had a faster speed.

I understand that thermal density plays a big part in deciding what
speed to cruise at. *If the thermal density is lower, it may be best
to stretch out the inter thermal glides to stay in the altitude power
band. *I fly faster when the lift sources are obvious, gaggle, clouds,
birds, smoke. *But when the lift is invisible, I slow it down to
stretch out my glide. *I also slow waaay down if there's no good
landable fields insight....for safety.

So is there a balance between % circling and true maccready speed to
fly? *What are some things that can flex your true maccready speeds up
or down?

Must read classic:

http://www.betsybyars.com/guy/soarin...a/72price.html

On Jul 4, 7:56*am, T8 wrote:
On Jul 4, 8:29*am, Scott Alexander
wrote:



I have been told over the years two consistent things: when trying to
achieve the fastest possible cross country speeds, *1. Try to minimize
circling as much as possible and 2. Fly the true maccready airspeeds
between thermals.

But these two things contradict. *The first couple days I flew at
Perry this year, I flew true maccready airspeeds between thermals.
The lift was strong, 5-7 kt thermals to 6-7,000 feet. *So I flew about
70-80 between thermals. *My average speed was then in the upper 40's.
Then for the later part of the contest, in the same weather
conditions, I delibertly did everything I could to reduce my %
circling. *I flew 50-70 kts between thermals and drastically reduced
my circling. *I had one flight with 15% circling and my resulting
average speed was 52 mph. *So by slowing down I had a faster speed.

I understand that thermal density plays a big part in deciding what
speed to cruise at. *If the thermal density is lower, it may be best
to stretch out the inter thermal glides to stay in the altitude power
band. *I fly faster when the lift sources are obvious, gaggle, clouds,
birds, smoke. *But when the lift is invisible, I slow it down to
stretch out my glide. *I also slow waaay down if there's no good
landable fields insight....for safety.

So is there a balance between % circling and true maccready speed to
fly? *What are some things that can flex your true maccready speeds up
or down?

Must read classic:

http://www.betsybyars.com/guy/soarin...a/72price.html

I recall a talk a few years ago at the convention by Carl Herold
titled something like "If you fly MacCready you will lose". Pretty
rough title that drew a lot of people to the talk. The main idea was
that you have to base your speed to fly on your actual climb rate, not
the vario reading or the average of the vario reading but what is
actually happening to the glider. Also, by staying higher for longer
you take advantage of true airspeed benefits of flying in thinner
air. At the time Carl was flying pretty impressive cross countries in
his Nimbus while circling something like 10% of the time, or less.

Typically in the Cherokee I set the ring at about half of what the
Vario is saying and figure that is a good number. I think that % of
time circling is more important than speed between thermals. No
forward movement is going to kill average speed a lot more than being
off by 5 or 10 mph for a little while. I've been working a lot more
on minimizing my circling time on recent flights.

I noticed that on your second day at Perry your average climb was
actually down in the 250 fpm range but your interthermal speeds seemed
about the same as the day before when the average climb was over 450.
I'm no expert but I'd guess that is why you spent 40% of the time
circling on day 2 vs 25% on day 1.

Funny, the guy flying probably the slowest glider on RAS trying to
talk about how to go fast.

Scott,

You need to read Johnnie Cochran's article:

http://faculty.chicagobooth.edu/john...search/Papers/

Down at the bottom of the page titled "Just a little faster Please"

A very good description of the ideas promoted by many excellent
pilots.

Mike

On Jul 4, 3:55*pm, SoaringXCellence wrote:
Scott,

You need to read Johnnie Cochran's article:

http://faculty.chicagobooth.edu/john...search/Papers/

Down at the bottom of the page titled *"Just a little faster Please"

A very good description of the ideas promoted by many excellent
pilots.

Mike


Generally you should not try to minimize your % circling directly by
reducing your cruise speed - it will slow you down overall. Unless of
course you are flying too fast for the conditions in the first place -
and that is the rub.

Overall, you need to set your cruise based on McCready for your
expected ACTUAL climb rate, including centering time and other
considerations such as changes in climb rate at the top or bottom of
the climb (Cochrane explains this pretty well in his papers) -
different pilots estimate this in different ways, some computers give
bottom-to-top averages as well. The net effect is slower climb rates
than you might otherwise estimate based on staring at you 30-second
averager.

Next, you need to adjust your speed to optimize the tradeoff between
theoretical cross-country speed (as estimated above) and the odds that
you might have to take a weaker than expected thermal because you got
low before you found a good one. You might do this for the whole
flight (imagine a blue day with a lot of distance between good
thermals and a not very tall lift band). This is basically trading
off optimal cruise speed for a higher probability of getting a good
climb. You will see experienced pilots often "topping up" before
heading out into suspected soft areas or pressing low and passing up
weaker lift because they know they are likely to hit stronger lift in
a few miles. It is a big exercise in estimating odds.

The last thing to remember is that the biggest contributor to speed is
to find lift lines you can follow. This can be cloud or blue streets,
convergence lines, storm shelves, wave, ridge - all allow you to make
time without going backwards. You don't make this happen by slowing
down, you make it happen by picking your path well. If thermals are
hard to center and/or if you can make sustained climbs straight ahead,
you may elect to slow down to climb straight in lift, but only under
circumstances that are supported by the "adjusted" theory described
above (accounting for circling and centering losses, probability of
"false positives", etc.) In a modern ship on a day with clouds and
some modest streeting you can cruise at 85 knots and have achieved
cruise L/Ds in the 50-60 range and % climbing in the low teens.

Lastly, keep in mind that your achieved cross country speed has very
little to do with the cruise speed you pick - within certain bounds.
If you fly 15 knots slower than McCready optimal for the entire flight
it costs you 2-3% on cross-country speed. Taking a single 3 knot climb
instead of a 6 knot climb for 2500' costs you about the same amount.

9B

On Jul 4, 10:06*pm, Nine Bravo Ground wrote:


Overall, you need to set your cruise based on McCready for your
expected ACTUAL climb rate, including centering time and other
considerations such as changes in climb rate at the top or bottom of
the climb (Cochrane explains this pretty well in his papers) -
different pilots estimate this in different ways, some computers give
bottom-to-top averages as well. *The net effect is slower climb rates
than you might otherwise estimate based on staring at you 30-second
averager.


9B

Just a quick note on this point. I've been informally checking with
pilots for several years after flights on our local DIY contest here
to calibrate the actual conditions against my weather forecasts.
Often times, I'll hear that it was a "great day - I was hitting
5-6kts". Post flight analysis of several traces reveal that achieved
climbs were more like 3-4kts at best. It's very clear that we don't
do a great job of accounting for our centering losses and hanging in
for too long once the lift tails off. By the way, in the good old
days before flight recorders, it seems that lift was a lot stronger.
Maybe it's weight of the FRs that's slowing things down :-)