Sharing a thermal

"Nyal Williams" wrote in message
...
How much of this depends on the reaction time of the
vario?


A lot.

Most vario's have a time constant of about 2-3 seconds or more. I put a
bunch of different varios on a test bench last winter and found some were as
bad as 11 seconds. The average 45 degree bank thermalling turn takes about
13 - 14 seconds so a 3 second delay can be a quarter turn. Some vario's
respond more quickly with the onset of lift but can take much longer to
return to zero after the lift ends.

FWIW, Mike Borgelts varios were the best of the lot on my test bench.

This means that you have to pay attention to the "whack it the back" (or
"whump in the rump" in the case of a 2-33) feel as you enter a thermal. If
you must watch the vario, figure a 60 degree of turn correction for vario
lag.

All this points up the need for an "inertial vario" that integrates the
vertical acceleration to display rate-of-climb. An inertial vario would
have no lag, no noise since it is damped by the mass of the glider and
should be dead accurate. UAV MEMS AHRS (Attitude Heading Reference Systems)
with three orthogonal gyros and accelerometers are in the $1000 range.
They'd make a fabulous vario. I'd love to have an instrument with a 1:1
correlation with the seat-of-the pants feel.

Bill Daniels

Bill Daniels wrote:


Most vario's have a time constant of about 2-3 seconds or more. I put a
bunch of different varios on a test bench last winter and found some were as
bad as 11 seconds. The average 45 degree bank thermalling turn takes about
13 - 14 seconds

Yikes! I don't know what you are flying, but it takes my ASH 26 about 27
seconds to make one turn (50 kts IAS, 8000' msl, 8.2 lb/sq ft wing
loading). That's measured from a flight trace. My ASW 20 was a little
quicker, flying at 7.5 lb/sq ft, but nothing like 14 seconds.

so a 3 second delay can be a quarter turn. Some vario's
respond more quickly with the onset of lift but can take much longer to
return to zero after the lift ends.

FWIW, Mike Borgelts varios were the best of the lot on my test bench.

How did the 302 compare, what did you have the time constants set at,
and where you judging by the needle or the audio?


This means that you have to pay attention to the "whack it the back" (or
"whump in the rump" in the case of a 2-33) feel as you enter a thermal. If
you must watch the vario, figure a 60 degree of turn correction for vario
lag.

At least with my glider and my vario, it seems to be less than 20
degrees, and I can ignore it, since the glider doesn't respond quickly
enough. Flying small gliders (11 meter SparrowHawk, 12 meter Russia,
etc.) will surely change the equation.


All this points up the need for an "inertial vario" that integrates the
vertical acceleration to display rate-of-climb. An inertial vario would
have no lag, no noise since it is damped by the mass of the glider and
should be dead accurate. UAV MEMS AHRS (Attitude Heading Reference Systems)
with three orthogonal gyros and accelerometers are in the $1000 range.
They'd make a fabulous vario. I'd love to have an instrument with a 1:1
correlation with the seat-of-the pants feel.

I'm guessing the hard part for an inertial vario would be ignoring stick
thermals.


--
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Eric Greenwell
Washington State
USA

"Eric Greenwell" wrote in message
...

Yikes! I don't know what you are flying, but it takes my ASH 26 about 27
seconds to make one turn (50 kts IAS, 8000' msl, 8.2 lb/sq ft wing
loading). That's measured from a flight trace. My ASW 20 was a little
quicker, flying at 7.5 lb/sq ft, but nothing like 14 seconds.

You might re-check your bank angle and your static port position error. 27
seconds seems too long.

I fly a Nimbus 2C and I do turn tight. At 6lbs PSF I usually thermal at
45-55 degree bank at 40 MPH. I checked the elapsed time with the sweep
second hand on the panel clock. At 11 PSF the 45 degree bank Vmin sink goes
up to 55mph or so. (One of these days, I'll get an ASI in knots.)

Although, I did look at some flight recorder traces of the Nimbus thermaling
at 17,999 feet on a hot summer day with the ballast tanks full. The turns
were 2000 feet in diameter.

OTOH, a good 1-26 driver can rip off a 360 in 10 seconds.

How did the 302 compare, what did you have the time constants set at,
and where you judging by the needle or the audio?

I didn't check the 302.


I'm guessing the hard part for an inertial vario would be ignoring stick
thermals.

TE compensation can be done by the same computer that integrates the
acceleration.

Bill Daniels

Bill Daniels wrote:

"Eric Greenwell" wrote in message
...


Yikes! I don't know what you are flying, but it takes my ASH 26 about 27
seconds to make one turn (50 kts IAS, 8000' msl, 8.2 lb/sq ft wing
loading). That's measured from a flight trace. My ASW 20 was a little
quicker, flying at 7.5 lb/sq ft, but nothing like 14 seconds.


You might re-check your bank angle and your static port position error. 27
seconds seems too long.

Using Frank's calculator, 55 knots @ 35 degrees gives about the right
diameter and time. I don't know the density altitude that day, but the
speed sounds a little low. The IAS was likely 50 knots, where I usually
thermal when empty.

Looking at a contest flight, the thermal I measured then plugged into
the calculator gave 65 knots @ 40 degrees to get the right diameter and
time (27 seconds). That was a 9800' msl on a warm summer day, so the
speed seems about right; also, I was flying with water.

Next time I thermal, which might be 5 months from now, I'll have to
concentrate on bank angle more and redo the measurements. I generally
find myself flying at steeper bank angles than most pilots, except at
contests with good pilots, where it's about the same. Maybe the
instrument mounting holes on our panels aren't quite at 45 degrees...

But, what I was getting around to saying is your comment -

The average 45 degree bank thermalling turn takes about
13 - 14 seconds

may be true for you, but not most pilots, based on my observations in
sport and contest flying, where 22+ seconds is typical. Maybe you meant
your average turn?


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Eric Greenwell
Washington State
USA

"Eric Greenwell" wrote in message
...

But, what I was getting around to saying is your comment -

The average 45 degree bank thermalling turn takes about
13 - 14 seconds

may be true for you, but not most pilots, based on my observations in
sport and contest flying, where 22+ seconds is typical. Maybe you meant
your average turn?


Nope, I meant my standard thermalling turn in the Nimbus is 14 seconds, 45
degrees and 45 MPH. If the core is small, I'll tighten up to a 60 degree
bank and 48mph. Ballast will, of course, bump those numbers up a little.

Eric, I think your turns are way too shallow and fast to make use of the
strong cores.

Bill Daniels

But, what I was getting around to saying is your comment -

The average 45 degree bank thermalling turn takes about
13 - 14 seconds

may be true for you, but not most pilots, based on my observations in
sport and contest flying, where 22+ seconds is typical. Maybe you meant
your average turn?


Nope, I meant my standard thermalling turn in the Nimbus is 14 seconds, 45
degrees and 45 MPH. If the core is small, I'll tighten up to a 60 degree
bank and 48mph. Ballast will, of course, bump those numbers up a little.

Eric, I think your turns are way too shallow and fast to make use of the
strong cores.

Bill Daniels

Eric &Bill
If a stall speed of a glider is 36 kt at a wing loading of 6 lb/sq/ft in a
45 deg.
it will be about 44 kt and at a 60 deg bank it will 50kt.
With a loading of 9lb/sq/ft the stall speed will be 44kt at a 45 deg bank it
will be 53kt and at a 60 deg bank it will be 61 kt.
I do not know what your respective wing loading are but both of you could be
right. It is more beneficial to turn tighter with a lighter wing loading ,
then it is
with a heavier wing loading.
Regards
Udo

Udo Rumpf wrote:


Eric, I think your turns are way too shallow and fast to make use of the
strong cores.

Bill Daniels

You might be right, but I'm not routinely outclimbed by other pilots
wherever I fly, even by US World team members, so experience suggests
I'm not too far off the mark. I do try tighter banking occasionally, and
sometimes it helps, but it usually not, so the 40 degree bank is what I
normally end up using.

Perhaps you are circling more tightly than needed, your thermals are
much smaller than the ones I encounter, or as Udo suggests, it's the
wing loading difference. Or all items...

It would be interesting to look at the circling times of good contest
pilots using contest flight files, or perhaps the OLC files. Circle time
(our primary parameter in the discussion) and circle diameter can be
obtained directly from the flight trace, the true airspeed can be
determined by noting the highest and lowest ground speeds during the
circle and taking the average. These can be plugged into Frank's web
calculator to get the bank angle.


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Eric Greenwell
Washington State
USA

"Eric Greenwell" wrote in message
...
Bill Daniels wrote:


Most vario's have a time constant of about 2-3 seconds or more. I put a
bunch of different varios on a test bench last winter and found some
were as
bad as 11 seconds. The average 45 degree bank thermalling turn takes
about
13 - 14 seconds

Yikes! I don't know what you are flying, but it takes my ASH 26 about 27
seconds to make one turn (50 kts IAS, 8000' msl, 8.2 lb/sq ft wing
loading). That's measured from a flight trace. My ASW 20 was a little
quicker, flying at 7.5 lb/sq ft, but nothing like 14 seconds.

Should be 17secs.

Try your flavor.
http://www.soarcsa.org/thinking_page...calculator.htm

Frank Whiteley

Try your flavor.

http://www.soarcsa.org/thinking_page...urn_radius_cal
culator.htm


Frank, the CSA website is truly impressive! Well done to you and your team.
Question though, where is the link to the Thinking Pages section? TIA.

Jim Vincent
N483SZ
illspam

"Jim Vincent" wrote in message
...
Try your flavor.

http://www.soarcsa.org/thinking_page...urn_radius_cal
culator.htm


Frank, the CSA website is truly impressive! Well done to you and your
team.
Question though, where is the link to the Thinking Pages section? TIA.

Jim Vincent
N483SZ
illspam

No such link as it wasn't quite fleshed out. We'll add one though.

Frank