MacCready in the Mountains

You have proved my point by having to use an example of a climb that is cut
off due to cloudbase, and by assuming that the next thermal is in an
entirely different airmass with very weak lift.

In the more general case (the one we normally talk about), you are going to
try to match the climb rate at the top of the current thermal with the climb
rate at the bottom of the next one. That matching exercise will depend on
the nature of both the current thermal and the next one. So the cruise
speed depends on the nature of both thermals. Neither is more important
than the other in determining cruise speed.



"Todd Pattist" wrote in message
...
"Greg Arnold" wrote:

Don't you want your climb rate at the top of the current thermal to equal
your expected climb rate at the bottom of the next thermal, and won't
those
climb rates determine your speed between the thermals?

Yes, but the climb rate in this thermal only affects when
you leave it, not how fast you cruise, while the climb rate
in the next thermal controls how fast you cruise. If you
hit cloudbase while climbing at 10 knots, but can only get
to the bottom of a 2 knot thermal, then you fly M=2 to that
2-knotter, not M=10.

Todd Pattist - "WH" Ventus C
(Remove DONTSPAMME from address to email reply.)

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
(Remove DONTSPAMME from address to email reply.)

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
(Remove DONTSPAMME from address to email reply.)

Todd:

My final comment on this matter -- you are talking about exceptions to the
general rule, not about the general rule. Exceptions to the general rule
don't disprove the general rule.

Greg


"Todd Pattist" wrote in message
...
"Greg Arnold" wrote:

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?

You don't always have the option to stay longer or leave
earlier. Cloudbase can cut the top off. If all you can get
to from cloudbase is a weak thermal, then you go slow, even
if you would have preferred to climb longer. A ridge you
need to cross could require you to climb higher in a weak
thermal, even if would have preferred to leave earlier and
even if you can get to the strong thermal at high speed once
you are high enough to cross. Moreover, sometimes you just
change your mind about how strong the next one will be. The
bottom line is it's the next thermal I can get to that
controls the speed I fly to get there.

Todd Pattist - "WH" Ventus C
(Remove DONTSPAMME from address to email reply.)

Todd Pattist wrote:

"Greg Arnold" wrote:

My final comment on this matter -- you are talking about exceptions to the
general rule, not about the general rule. Exceptions to the general rule
don't disprove the general rule.

OK, but the general rule is that the rate of climb in the
next thermal has to equal the rate of climb in the current
thermal for the current thermal to affect your cruise speed
to the next thermal. As soon as that's not true, the next
thermal and it's an "exception." Thus we agree on
what the pilot does, even if we don't agree on how to
describe it :-)


And anyway, even if both are equivalent, it is simpler to
state "the next thermal controls (with no exception)", than
"the climb rate at the top of the last thermal, which should
be the same at the bottom of the next one, controls, with
some exceptions"

I still don't think you guys get it. Yesterday, while flying over the Rocky
Mountains in my Nimbus 2C I was seeing 5 M/S on the averager, yet if I set
the M number to 5, the speed command would ask for 200+ MPH. Given the level
of turbulence associated with 5 M/S lift and the fact that I was flying dry,
I stayed in the green arc.

The other thing that no one has mentioned is that, at the high altitudes
required over mountains, the True Airspeed calculation has a larger effect
on average XC speed than the McCready calculation so flying slow and staying
high gets you a higher real speed.

In the mountains, structural limits, safe landing areas and terrain
clearance set maximum speed. McCready numbers are academic.

Bill Daniels



"Robert Ehrlich" wrote in message
...
Todd Pattist wrote:

"Greg Arnold" wrote:

My final comment on this matter -- you are talking about exceptions to
the
general rule, not about the general rule. Exceptions to the general
rule
don't disprove the general rule.

OK, but the general rule is that the rate of climb in the
next thermal has to equal the rate of climb in the current
thermal for the current thermal to affect your cruise speed
to the next thermal. As soon as that's not true, the next
thermal and it's an "exception." Thus we agree on
what the pilot does, even if we don't agree on how to
describe it :-)


And anyway, even if both are equivalent, it is simpler to
state "the next thermal controls (with no exception)", than
"the climb rate at the top of the last thermal, which should
be the same at the bottom of the next one, controls, with
some exceptions"

"Marc Ramsey" wrote in message
m...
"Bill Daniels" wrote...
I still don't think you guys get it. Yesterday, while flying over the
Rocky
Mountains in my Nimbus 2C I was seeing 5 M/S on the averager, yet if I
set
the M number to 5, the speed command would ask for 200+ MPH. Given the
level
of turbulence associated with 5 M/S lift and the fact that I was flying
dry,
I stayed in the green arc.

I would look carefully at the polar you are using and/or the instrument,
it's
calling for a lot more speed than I would expect. On the other hand, I
almost
always set my MC to half of what I'm expecting the next climb to be, and
it
seems to work pretty well.

The other thing that no one has mentioned is that, at the high altitudes
required over mountains, the True Airspeed calculation has a larger
effect
on average XC speed than the McCready calculation so flying slow and
staying
high gets you a higher real speed.

Be careful, though, polars are based on "indicated" speeds. Most modern
electronic variometers show something approximating a "true" climb rate.
Some,
but not all, glide computers assume you are setting the MC to a "true"
climb
rate, and actually convert it to an "indicated" (lower) equivalent climb
rate
before applying the speed to fly calculation.

In the mountains, structural limits, safe landing areas and terrain
clearance set maximum speed. McCready numbers are academic.

Perhaps, but I think the main problem is that many pilots fly far too fast
for a
given climb rate, due to instrumentation problems, incorrect polars, and
incorrect understanding of what the MC numbers actually mean.

I 've carefully verified the speed to fly calculations and corrections in
the
software I use in my LAK-17, and at 15K+ feet with full water and MC 5
(knots),
cruise speed to fly is normally less than 120 knots.

Marc

5 meter/second = 9.7 knots. I was setting M in meters per second. The
speed command just says push or pull. I think my computer is pretty well
set up. I think we agree on this: Most pilots fly too fast.

The point is that in mountain flying, total reliance on McCready speeds is
misleading and possibly dangerous if it induces a pilot to fly too fast.
Certainly, you don't want to base your strategic or possibly even your
tactical decisions on the McCready speed to fly. Be aware of it and factor
it in, but don't be a slave to it.

In a wide area of homogeneous airmass characteristics where thermals are
uniform in strength, diameter and spacing, the McCready speed to fly is a
major determinator of flying technique.

Flying in high mountain country is just the opposite. You had better be
ready for a complete weather change every few minutes. Thermic conditions
will cycle very fast and you may see blue sky, towering cumulus,
overdevelopment and back to blue in a single interthermal glide.

Bill Daniels

"Bill Daniels" wrote...
5 meter/second = 9.7 knots. I was setting M in meters per second. The
speed command just says push or pull. I think my computer is pretty well
set up. I think we agree on this: Most pilots fly too fast.

I was aware that you were talking in m/sec. Here's what you said:

I still don't think you guys get it. Yesterday, while flying over the Rocky
Mountains in my Nimbus 2C I was seeing 5 M/S on the averager, yet if I set
the M number to 5, the speed command would ask for 200+ MPH. Given the
level of turbulence associated with 5 M/S lift and the fact that I was flying
dry,
I stayed in the green arc.

If I plug MC 10 knots into my speed to fly spreadsheet for my LAK-17A 18M dry
(which is likely to have a higher STF than your IIC for the same MC setting), I
get 148 mph (130 knots) for 0 airmass movement. In order to get an STF of 200
mph, I would need to be in 10 knots of sink.

Your computer may be well set up, but it appears to be giving bogus STF...

The point is that in mountain flying, total reliance on McCready speeds is
misleading and possibly dangerous if it induces a pilot to fly too fast.
Certainly, you don't want to base your strategic or possibly even your
tactical decisions on the McCready speed to fly. Be aware of it and factor
it in, but don't be a slave to it.

In a wide area of homogeneous airmass characteristics where thermals are
uniform in strength, diameter and spacing, the McCready speed to fly is a
major determinator of flying technique.

Flying in high mountain country is just the opposite. You had better be
ready for a complete weather change every few minutes. Thermic conditions
will cycle very fast and you may see blue sky, towering cumulus,
overdevelopment and back to blue in a single interthermal glide.

I fly in high mountain country, just like you. I also spend a lot of time
cruising. For those times when I have a decent idea of what the next climb
rate is going to be, I'd want a computer which isn't telling me to fly 50+ MPH
faster than I should be...

Marc

In article ,
"Marc Ramsey" wrote:

If I plug MC 10 knots into my speed to fly spreadsheet for my LAK-17A
18M dry (which is likely to have a higher STF than your IIC for the
same MC setting), I get 148 mph (130 knots) for 0 airmass movement.
In order to get an STF of 200 mph, I would need to be in 10 knots of
sink.

Which is not at all impossible in the mountains!

-- Bruce

"Bruce Hoult" wrote...
In article ,
"Marc Ramsey" wrote:

If I plug MC 10 knots into my speed to fly spreadsheet for my LAK-17A
18M dry (which is likely to have a higher STF than your IIC for the
same MC setting), I get 148 mph (130 knots) for 0 airmass movement.
In order to get an STF of 200 mph, I would need to be in 10 knots of
sink.

Which is not at all impossible in the mountains!

He didn't say he was in 10 knots of sink. The implication was that STF called
for a "cruise" speed of 200+ MPH. It does not.

Marc