Inter-thermal cruise speeds?

On Feb 10, 5:35*am, wrote:
On 9 fév, 12:54, wrote:





On Feb 8, 8:45*pm, wrote:

Anyone else here see Carl Herolds talk at the convention a couple
years ago titled, If you fly Mcdready you will lose"? *Actually, he
said that was a title just to get attention but that the real title
was Fly Slower to Fly Faster, or something like that. *It was
fascinating to see all this graphs and flight traces. *It was very
convincing to see his data that indicated staying high and not
circling was ultimately faster. *I think there may be a threshold L/D
value particular to specific conditions in which his technique
worked. *Regardless, I now circle as little as possible.

MM

As had been already mentioned, there are a bunch of reasons why flying
slower than McCready theory makes sense. Some are consistent across
flying conditions, others are situation-specific.

Fist, in my experience, your perceived climb rate may not be your
actual climb rate - even using you vario or computer averager,
depending on how it calculates average. I consistently find average
climb rates looking at SeeYou to be a knot or more slower than was my
perception in the air. This is mostly because pilots (and perhaps some
instruments) don't adequately count the time centering a thermal with
no climb or include "trys", thermals that don't pan out. These two
effects reduce your realistic expected climb rate. Maybe your computer
properly adjusts for this maybe it doesn't, only some experimentation
can tell you for sure.

Flying slower keeps you higher, which has a number of direct and
indirect benefits that I've tried to quantify through the following
example. *Imagine a flight where the lift band is 10,000' to 17,500',
the average (achieved) climb is 5 knots, the distance between climbs
is 35 miles and there are cu present. For my glider the theory gives
an expected cruise speed of 98 knots (dry) and an altitude loss
between thermals of 7,100'. *If I slow down and fly 15 knots slower
(83 knots) instead, I end up with an altitude loss between thermals of
5,600' and an average achieved cross-country speed that is about 1.7
mph slower. So why fly slower? *By staying higher my average cruise
altitude is 14,700 rather than 13,900 so I gain back about 1.5 mph in
true airspeed difference. *You only need to find a 0.04 knot better
climb to close the remaining cross-country speed gap, or a 0.4 knot
faster climb if you ignore the TAS effect. Since we are flying higher
on average it is reasonable to expect you'll be able to do this under
the described conditions for several reasons. *You will be closer to
the clouds and will have a slightly better change of aligining on them
to find lift. You will also be higher in the lift band so less likely
to fall into weaker lift or will be less inclined to accept weaker
lift as you get lower. You will have a greater search distance to find
better lift. If I fly McCready in this scenario I can go about 35
miles between thermals before I get out of the lift band. If I fly 15
knots slower I can fly 45 miles for the same altitude range. Lastly, I
have found that I have a somewhat harder time sensing and successfully
pulling up into and quickly centering thermals if I am cruising at 100
knots versus 85 knots. In the extreme case, flying faster ups you risk
of getting stuck down low and having to take a sub-standard thermal to
get back up or even landing out. Individual flying style will
determine which of these effects matters most for any individual
pilot.

How you think about this varies with the conditions of the day. If it
is blue with a very wide lift band, large, closely-spaced thermals
with very consistent thermal strengths you won't get as much benefit
from slowing down. The TAS effect is also reduced for lower altitude
lift bands. If the thermal strengths are lower overall, you actually
have to find a thermal that is more significanly above average (on a %
basis) to make up the cruise speed difference.

If I change the example to flying 20 or 25 knots slower than McCready
it gets harder to see the benefits because the incremental climb rate
you need to achieve to make up for the sub-optimal cruise speed goes
up substantially.

9B

Hi,
Here's Ingo Renner rules to achieve fast x/c speed flying a Duo-
Discus.

-Ignore MaCready and fly one of three speed;
55 for thermaling, 70-80 kts for low weaker condition
90-110 kts for strong condition
-Fly straight to your goal with very minor deviation for lift.
-Slow down gently in lift and centre thermal in one circle or keep
going,
no second chance
-Leave as soon as climb falls off,
-Fly carefully with smooth control movement, no abrupt pull up or push
over.
-Always fly with the yaw string straight and centered.

Taken from a text entitle "Soaring with the master", by Ian Sutcliffe
in Free Flight,the Canadian magazine on soaring.
S6- Hide quoted text -

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Ingo is a great pilot so I wouldn't quibble that his technique works
for him in the conditions he was thinking of when he gave this advice.

That said, I can say from personal experience that I have seen world-
class pilots pursue very different strategies with great success under
conditions that may or may not be similar to what Ingo was thinking
of. In particular I see a lot of very good pilots in the high desert
of the western US make significant deviations from courseline to seek
out lift. This may have something to do with the topography of the
site and the lift distribution on any given day.

With respect to speeds, it all depends on what Ingo means by "low and
weak" versus "strong". His speed ranges cover the gamut and in the end
the basic idea is right - don't fly the speed director.

9B