STF question

Ramy wrote:

This is all correct when you have enough altitude that all you need to
decide is how much higher you should climb to fly faster, a typical
contets scenario. I was thinking more of final glide at the end of the
day when you trying to stretch a marginal glide to try to make it home
against significant head wind (think final glide to Truckee from the
north end of the Pine Nuts or Airsailing). Normally you would put MC=0
when you want to maximize your glide, but if you have 20 knots head
wind a setting MC=1 will be better. Sounds like GPS-LOG is doing this.


Well... not exactly. You should always put MC=0 (setting it higher
would give you safety margin as you can slow down) Flight computer
should give you altitude and speed corrected for wind for the MC
setting you request. If you have the altitude, then it can calculate
maximum speed to reach destination and from speed MC. GPS_LOG
recalculates speed (and MC) in final glide continuously based on
altitude and you can display it if you wish, or you can display other
types of calculated speeds.

Just remember that all instruments lie, and the ones that predict
something lie the most.

Cheers,
Henryk

On Mar 17, 10:27*pm, Henryk Birecki
Well... not exactly. You should always put MC=0 (setting it higher
would give you safety margin as you can slow down)

Sorry, I don't acept that.

As I said in an earlier post - with a head wind present, increasing
the MC from zero will show reducing altitude required until it reaches
a minimum and then increases again.

Try it in simulator mode on any PDA based glide computer. If it
doesn't do that there something wrong with the implementation.
Computers as old as the MNAV did this right.


Andy

Andy wrote:

On Mar 17, 10:27*pm, Henryk Birecki
Well... not exactly. You should always put MC=0 (setting it higher
would give you safety margin as you can slow down)

Sorry, I don't acept that.

As I said in an earlier post - with a head wind present, increasing
the MC from zero will show reducing altitude required until it reaches
a minimum and then increases again.

Try it in simulator mode on any PDA based glide computer. If it
doesn't do that there something wrong with the implementation.
Computers as old as the MNAV did this right.


Yes, but you are talking about a different computation

Andy

On Mar 18, 5:09*pm, Henryk Birecki wrote:
Yes, but you are talking about a different computation

My understanding is that Ramy concluded that to get best glide
distance into a headwind he should use a MC setting greater than
zero. Your reply was that was wrong and he should use zero MC
setting. I then disagreed and said any computer should show less alt
required at a MC greater than zero.

What different computations are being discussed here?

thanks

Andy

OK Andy, I'll bite. First of all Ramy was making a guess about what
and how GPS_LOG computes, not how to use MNAV.... Now for the lesson:

Let's start with what is MC setting. MC number is essentially expected
lift that you will find in thermals (assuming you reach them) Since
you are going to find lift you can trade it off for airspeed and
increase your XC speed. In classic cross country speed maximization
theory the speed to fly to maximize XC speed does not depend on wind,
so MC and airspeed that you fly at are in a one to one relationship.
Just use table lookup, or memorise it, no need for any compute power.

Computation that you are talking about is done by computationally
challenged (for good reasons) instrumentation, and computer programs
that emulate it. In that case YOU are telling the computer what speed
you are going to fly at by selecting MC setting. The MC in this case
has no relationship to the lift you will encounter in flight, but an
easy way to enter speed. Computer then calculates the easy part
(altitude you need). GPS_LOG is not computationally challenged. You
tell it what lift you expect and it gives you both altitude AND speed
optimised for maximum distance. In a thermal, altitude is reported
assuming that you will NOT find lift (nor sink) along the way, so as
you increase MC, altitude follows. During flight, both speed and
altitude are recomputed depending on Vz of airmass are flying through.
There are also other speed displays and altitude computation modes.

One of the GPS_LOG displays gives you maximum speed to fly to reach
destination given altitude you have, wind... It also displays
equivalent MC. This display is of particular use in the last thermal
before final glide. As long as you are climbing faster than the
displayed MC, you stay in the thermal. Once you no longer climb as
fast as MC indicates, you should leave.

Cheers,
Henryk

Andy wrote:

On Mar 18, 5:09*pm, Henryk Birecki wrote:
Yes, but you are talking about a different computation

My understanding is that Ramy concluded that to get best glide
distance into a headwind he should use a MC setting greater than
zero. Your reply was that was wrong and he should use zero MC
setting. I then disagreed and said any computer should show less alt
required at a MC greater than zero.

What different computations are being discussed here?

thanks

Andy

Henryk,

Nice job with the lesson. For our club, I put together a basic cross-
country course. After reading your post, I am going to rewrite that
part of my lesson because your post makes it easier to understand.

Raul Boerner
LS6-b DM

wrote:

Henryk,

Nice job with the lesson. For our club, I put together a basic cross-
country course. After reading your post, I am going to rewrite that
part of my lesson because your post makes it easier to understand.

Raul Boerner
LS6-b DM

If you feel like sharing course materials on line I can probably host
them on my site.

Cheers,
Henryk

On Mar 19, 11:05*am, Henryk Birecki
wrote:
OK *Andy, I'll bite. *First of all Ramy was making a guess about what
and how GPS_LOG computes, not how to use MNAV.... Now for the lesson:

Thanks for the explanation. I was not clear to me that you and Ramy
were discussing a feature of GPS_LOG.

Andy

Henryk Birecki wrote:
OK Andy, I'll bite. First of all Ramy was making a guess about what
and how GPS_LOG computes, not how to use MNAV.... Now for the lesson:

Let's start with what is MC setting. MC number is essentially expected
lift that you will find in thermals (assuming you reach them) Since
you are going to find lift you can trade it off for airspeed and
increase your XC speed. In classic cross country speed maximization
theory the speed to fly to maximize XC speed does not depend on wind,
so MC and airspeed that you fly at are in a one to one relationship.
Just use table lookup, or memorise it, no need for any compute power.

Computation that you are talking about is done by computationally
challenged (for good reasons) instrumentation, and computer programs
that emulate it. In that case YOU are telling the computer what speed
you are going to fly at by selecting MC setting. The MC in this case
has no relationship to the lift you will encounter in flight, but an
easy way to enter speed. Computer then calculates the easy part
(altitude you need). GPS_LOG is not computationally challenged. You
tell it what lift you expect and it gives you both altitude AND speed
optimised for maximum distance. In a thermal, altitude is reported
assuming that you will NOT find lift (nor sink) along the way, so as
you increase MC, altitude follows. During flight, both speed and
altitude are recomputed depending on Vz of airmass are flying through.
There are also other speed displays and altitude computation modes.

One of the GPS_LOG displays gives you maximum speed to fly to reach
destination given altitude you have, wind... It also displays
equivalent MC. This display is of particular use in the last thermal
before final glide. As long as you are climbing faster than the
displayed MC, you stay in the thermal. Once you no longer climb as
fast as MC indicates, you should leave.

Here's an example of why I like my computer to show me the classic MC
setting and speed to fly:

Final glide into a stiff wind, racing sunset. I have just enough
altitude to make the glide. Speed up i won't make it, slow down I won't
make it. I'll also get to the goal right at sunset, but cut very close.
To get my computer to show the right speed to fly to just make it in
this wind, I had to dial in an MC = 2 kt setting. If I understand right,
some think the computer should be set to MC=0 kts in this situation,
since I want the best L/D I can achieve. Sounds a bit simpler than
twiddling the MC knob to minimize altitude required. But both give the
same commanded speed to fly, we get there at the same time either way.

Now I hit a sweet little unexpected 1.5 kt thermal. It will drift me
backwards, but will get me higher so I can glide faster. What do I do? I
know from studying Reichman's Cross-Country soaring book, and John
Cochrane's lectures, that classic MC setting just happens to be the cost
of altitude where I am right now. That means if I can get altitude at a
rate greater than my MC setting, its a bargain and I should take some.
My net time will be reduced. If the lift is less than my MC setting, its
a bad deal, and I will loose time by stopping.

So since my computer is set to 2kts and thats enough to finish, I should
pass up 1.5 kts. It will just slow me down since it is less than my
current MC setting for this glide. The optimal action is to press on at
the classic MC=2 kts speed to fly.

But if my PDA is set to a wind adjusted MC=0, there is no easy, correct
way to make the decision. The wind adjusted MC does not have the same
meaning. It actually gives the cost of altitude assuming I can get it
without drifting with the wind. But this thermal is going to drift. If I
stop to climb until the new wind adjusted MC=1.5 gives me just enough to
get home at that setting, I will get there with a steeper, faster glide,
but it will be after sunset because I wasted some time climbing in lift
and drifting backwards such that my average speed through the air and
over the ground drops. No badge for me.

That does not mean the wind adjusted approach is wrong or won't work,
but its not as obvious what the correct strategy should be. Thats really
why most flight computers use classic MC speed to fly and don't try to
do wind corrections.

Now a computer that twiddles the MC for me on final glide might have
some value. I actually find it more intuitive to watch the altitude
required go up and down than the auto MC run up and down. But I want the
indicated MC to be the classic one, so I know what it means and what to
do with it.


ZL - wannabe ultimate competitor

I just did a little experiment; from Williams, I put in Redding, CA
which is due north. My B-100 read 93.3 s/m (Discus 1 polar), M/C 0.0 =
11,286

Then I dialed in a good headwind, 360/25 and started increasing the M/
C, the lowest altitude required occurred at M/C = 0.8 (18,035)

Then I dialed in 360/15 and the least altitude required occurred at M/
C = 0.4 (14710)

I believe this shows that we should not fly M/C zero except in very
light wind. Also I have found the Nimbus-3 and ASH-25 wont make the
glide that M/C zero says they should. Probably because of over
optimistic polar information from the manufacturer.

Another issue I found with the SN-10 happened on the last day at
Parowan, last year. We had a stiff S/W wind at 25 knots with 3 turn
areas. I has excess altitude as I entered the last turn area, which
was down wind from Parowan. I penetrated until the computer said I was
1000 feet over final glide home and then made my turn. As soon as I
finished the turn and centered up on the final leg, the computer said
I would just make it! I lost my 1000 foot cushion in the turn (wind
remained the same). What happened?

I believe the program in the SN-10 doesn't take into account the drift
correction angle which was considerable with the 25 knot cross-wind I
had on my final leg home. I was holding a good 10 degrees heading up-
wind as the ship tracked Parowan. This gave me much more of a head-
wind component and I believe the comuuter didn't compute on the 'real'
heading that would be required to track the destination into a stiff
cross-wind.
What do you thing?
JJ