Constant speed or constant attitude?

I would like to hear what other glider pilots have found to be helpful
in trying to center thermals.

It is common in gliding books to read that a constant diameter circle,
as a product of flying a constant airspeed and a constant bank, is
important in centering thermals. If the airspeed and / or bank is
allowed to vary significantly the thermal circle becomes an inconstant
oval, which can make locating and centering a thermal more difficult.

I understand this, and I think it helps in staying in contact with a
thermal once I have identified the stronger area in a thermal and am
more or less centered in it.

But while I am exploring a thermal, while I am trying to get an idea
of how the lift is varying around the circle I'm flying, while I'm
working my way towards the core of the thermal, I find it more
informative to try to fly a constant attitude and bank and allow the
airspeed to rise and fall as the lift comes and goes, and not lower
and raise the nose in response to the airspeed changes resulting from
the lift changes. I find that flying a constant attitude rather than
a constant airspeed in this task greatly simplifies the task of
locating the stronger lift and moving to it.

If I do try to keep the airspeed constant by raising and lowering the
nose as I fly through the changing lift around the thermal I usually
end up behind the changes in lift and confused about the thermal's
structure.

By keeping a constant attitude while investigating a thermal I seem to
be better able to use the lift-created airspeed changes as markers of
the thermal's structure.

Am I goofing-up my thermalling this way?

"Jim" wrote in message
...
I would like to hear what other glider pilots have found to be helpful
in trying to center thermals.

It is common in gliding books to read that a constant diameter circle,
as a product of flying a constant airspeed and a constant bank, is
important in centering thermals. If the airspeed and / or bank is
allowed to vary significantly the thermal circle becomes an inconstant
oval, which can make locating and centering a thermal more difficult.

I understand this, and I think it helps in staying in contact with a
thermal once I have identified the stronger area in a thermal and am
more or less centered in it.

But while I am exploring a thermal, while I am trying to get an idea
of how the lift is varying around the circle I'm flying, while I'm
working my way towards the core of the thermal, I find it more
informative to try to fly a constant attitude and bank and allow the
airspeed to rise and fall as the lift comes and goes, and not lower
and raise the nose in response to the airspeed changes resulting from
the lift changes. I find that flying a constant attitude rather than
a constant airspeed in this task greatly simplifies the task of
locating the stronger lift and moving to it.

If I do try to keep the airspeed constant by raising and lowering the
nose as I fly through the changing lift around the thermal I usually
end up behind the changes in lift and confused about the thermal's
structure.

By keeping a constant attitude while investigating a thermal I seem to
be better able to use the lift-created airspeed changes as markers of
the thermal's structure.

Am I goofing-up my thermalling this way?

That's pretty much what I do. Keeping a stable attitude does seem to help
visualize the location of the thermal core. However, small speed changes
don't affect the turn diameter nearly as much as small bank changes. See:
http://home.twcny.rr.com/ghernandez/turn_rad.htm

I try to carefully adjust the bank angle to move the center of my turn
towards the stronger lift. Using the typical bank angles and airspeeds, a
15 degree change in bank will either double or halve the turn radius. I
will reduce the bank 15 degrees at the weakest point in the turn and after
an interval of about 8 second have elapsed, steepen the bank 15 degrees.
Plotting this to scale shows that it moves the circle about one diameter
toward the lift. This is not my idea, I got it from someone else.

Bill Daniels

When I thermal, there generally seems to be wind,
and since the glider doesn't climb quite as
fast as the thermal, I seem to do better slipping
or changing bank angle to fly a little into the
headwind during each turn (kind of like
turns around a point in power flying).

I started using slips in thermals after I flew
with a competition pilot and noticed he did this
a little when thermalling in steeper banks. I
also sometimes slip a little "into" a thermal.
I suspect this happens mostly because I'm
getting fatigued and my coordination is
getting worse and I'd rather slip into
it than skid. But there may be some
aerodynamic reason for slipping a little.
It seems that at the extreme (in a 90 degree bank),
a slip is better than coordinated flight since it
exposes more fusealage area to the thermal when
C.G. is forward.

There is also some coriolis(SP?) effect, so I notice
on .igc traces of extended thermalling that the
thermal circles a little as it rises.

I also fly by the rule that the center third
altitudes of a thermal often provide the best lift.
Sometimes I fly in the upper third if I expect
to cross a sink area, but the middle third
has been pretty good for me.

I definitely trim for the thermal, and I've
never had a consistent thermal greater than 8 knots,
and I haven't found constant banks greater than 50
degrees useful.

Some of this comes from discussions with Serge Serfaty,
a fellow glider pilot. But he wasn't a big fan of
any uncoordinated flight :P

I've also had to use shallower banks or leave thermals
because I was getting dizzy or tired or hot or couldn't
track a fellow glider. I think these are other
factors that vary based on the pilot.

fly the airspeed.

That way you take advantage of the gusts as you fly through stronger or
weaker lift.

Al

"Jim" wrote in message
...
I would like to hear what other glider pilots have found to be helpful
in trying to center thermals.

It is common in gliding books to read that a constant diameter circle,
as a product of flying a constant airspeed and a constant bank, is
important in centering thermals. If the airspeed and / or bank is
allowed to vary significantly the thermal circle becomes an inconstant
oval, which can make locating and centering a thermal more difficult.

I understand this, and I think it helps in staying in contact with a
thermal once I have identified the stronger area in a thermal and am
more or less centered in it.

But while I am exploring a thermal, while I am trying to get an idea
of how the lift is varying around the circle I'm flying, while I'm
working my way towards the core of the thermal, I find it more
informative to try to fly a constant attitude and bank and allow the
airspeed to rise and fall as the lift comes and goes, and not lower
and raise the nose in response to the airspeed changes resulting from
the lift changes. I find that flying a constant attitude rather than
a constant airspeed in this task greatly simplifies the task of
locating the stronger lift and moving to it.

If I do try to keep the airspeed constant by raising and lowering the
nose as I fly through the changing lift around the thermal I usually
end up behind the changes in lift and confused about the thermal's
structure.

By keeping a constant attitude while investigating a thermal I seem to
be better able to use the lift-created airspeed changes as markers of
the thermal's structure.

Am I goofing-up my thermalling this way?

I am looking for a radio for my LS-4A and would like some
recommendations. Reliability, good features, power consumption, etc.
I might also be interested in a used radio if available. A 2.25" Ø
would be preferred. Thanks

What you guys are discussing is the 'Yates effect'
as described by Derek Piggot in 'Understanding Gliding'
Appendix A and also published in Gliding magazine in
1951 by Dr A.H. Yates.

John Galloway

At 21:42 15 August 2003, Jim wrote:
On Fri, 15 Aug 2003 13:25:09 -0700, Jim wrote:

On 15 Aug 2003 12:36:05 -0800,
(Mark James Boyd)
wrote:

With regards to vertical gusts...

A stable aircraft will react to this by pitching down
and increasing its airspeed

I thought a stable aircraft has the C.G. forward of
the center
of lift. If this is so, and this effect only happens
if
the aircraft is stable, then C.G. is important, right?

If the C.G. and center of lift coincide, does this
effect
still occur? If the C.G. is behind the center of lift
(my
understanding of 'unstable') does this occur?



My guess, and it sure is only a guess, is that the
changes in
the indicated airspeed as a result of the glider flying
into lift or
sink WOULD occur regardless of the stability or instability
of
the aircraft. I'm guessing this is so because I'm
also guessing
that THESE changes in the indicated airspeed are not
the
result of instaneous pitch changes in the aircraft's
attitude, but
rather are changes in dynamic and/or static pressure
directly
created by the changes in lift and sink themselves.

I suppose another way to say this is that the changes
in indicated
airspeed may be due to angle of attack changes that
are not due
to changes in the aircraft's attitude, but rather due
to changes to
the direction of the airflow (which are felt as changes
in lift and
sink.

I dunno. This is absolutely wonderful stuff, but it
leaves me
really wanting a wind tunnel so I could test these
things.



I think I only further muddled this by my saying 'actual
airspeed' may
not be changing. This is not at all the way to look
at things.
Indicated airspeed DOES change as a glider flies into
lift and sink.
Period. What I wanted to describe is a situation in
which the changes
in indicated airspeed are reflective of changes in
the airflow over
the glider created by the changed lift and sink, not
of accelerations
of the glider itself.

Phooey. This probably only made it worse. I know
what I want to say,
I just can't find the right way to say it.

On 15 Aug 2003 22:58:24 GMT, John Galloway
wrote:

What you guys are discussing is the 'Yates effect'
as described by Derek Piggot in 'Understanding Gliding'
Appendix A and also published in Gliding magazine in
1951 by Dr A.H. Yates.

John Galloway

At least someone gets it.

Also mentioned by Doug Haluza in an article in "Soaring" a few years
ago.

As you enter lift the glider accelerates forward due to the lift
vector tilting forward in the flight direction. Entering sink the
reverse effect occurs. This is a short lived effect for sharp edged
gusts with time constants of the order of 0 .15 to 0.5 seconds for
typical glider airspeeds and wing loadings.
It also has interesting effects on TE varios and is one of the reasons
that TE varios seem much quicker or more "nervous"in response than
uncompensated varios connected to static sources. The other is the
sensitivity of the TE vario to horizontal airmass changes"horizontal
gusts".
There is an article on our website about this.

Mike Borgelt

Borgelt Instruments
www.borgeltinstruments.com

I
On 15 Aug 2003 22:58:24 GMT, John Galloway
wrote:

What you guys are discussing is the 'Yates effect'
as described by Derek Piggot in 'Understanding Gliding'
Appendix A and also published in Gliding magazine in
1951 by Dr A.H. Yates.


One day I'll get round to reading that.

As you enter lift the glider accelerates forward due to the lift
vector tilting forward in the flight direction. Entering sink the
reverse effect occurs. This is a short lived effect for sharp edged
gusts with time constants of the order of 0 .15 to 0.5 seconds for
typical glider airspeeds and wing loadings.
It also has interesting effects on TE varios and is one of the reasons
that TE varios seem much quicker or more "nervous"in response than
uncompensated varios connected to static sources. The other is the
sensitivity of the TE vario to horizontal airmass changes"horizontal
gusts".
There is an article on our website about this.

Mike,
That is the clearest reason for it happening that I have ever seen.
When you sketch out the lift and drag vectors and then see what happens
when extra lift is added and removed it's obvious.

Thanks

Robin
Mike Borgelt

Borgelt Instruments
www.borgeltinstruments.com





--
Robin Birch

On Sun, 17 Aug 2003 11:04:23 +0100, Robin Birch
wrote:


I
On 15 Aug 2003 22:58:24 GMT, John Galloway
wrote:

What you guys are discussing is the 'Yates effect'
as described by Derek Piggot in 'Understanding Gliding'
Appendix A and also published in Gliding magazine in
1951 by Dr A.H. Yates.


One day I'll get round to reading that.

As you enter lift the glider accelerates forward due to the lift
vector tilting forward in the flight direction. Entering sink the
reverse effect occurs. This is a short lived effect for sharp edged
gusts with time constants of the order of 0 .15 to 0.5 seconds for
typical glider airspeeds and wing loadings.
It also has interesting effects on TE varios and is one of the reasons
that TE varios seem much quicker or more "nervous"in response than
uncompensated varios connected to static sources. The other is the
sensitivity of the TE vario to horizontal airmass changes"horizontal
gusts".
There is an article on our website about this.

Mike,
That is the clearest reason for it happening that I have ever seen.
When you sketch out the lift and drag vectors and then see what happens
when extra lift is added and removed it's obvious.

Agreed. I must try to consciously use constant attitude cruise. I'm
slowly approaching that way of flying in any case, but still have the
habit of clicking on 10 knots if the vario groans.... and am probably
going too fast as a result.


--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

Martin,=20

I just can't resist a good argument.

Sorry, but your vector diagram as described gives AOA with sign =
reversed.

On encountering a rising airmass from stabilized flight, the =
instantaneous effect is an increase in AOA - the relative wind that was =
flowing from straight ahead is now flowing from ahead and below. This =
increase in AOA gives added lift at the wing and reduced pushdown at the =
tail. Since the CG is ahead of both CL and tail, both give a nosedown =
moment. The aircraft attitude is disturbed and it pitches nosedown.

Now the inherent stability starts to work through a series of =
transients. The pitch down reduces the AOA and the aircraft accelerates =
because the gravity vector is closer to where the nose is now pointing. =
The reduced AOA and the increased drag gradually restore the aircraft to =
its original stable attitude [maybe after a few oscillations, it is far =
from a deadbeat system] and sink rate through the airmass. The final =
result is an aircraft flying at exactly the same attitude, speed, L/D =
etc in the new airmass, but with a sink rate relative to the ground =
equal to the old value less the upward velocity of the new airmass.

Agree with your conclusion, just can't help nitpicking the argument..

Going back to the original question, whether to fly constant speed or =
constant attitude, both are difficult to achieve when hit by a gust. =
But if you are skilful enough to stop the nose drop then your attitude =
will remain constant and your speed will also be constant if the gust is =
vertical. If the gust is horizontal, there wil be relatively little =
change in AOA but an increase/decrease in total energy will be reflected =
in the vario and your airspeed will change. Holding attitude will let =
the aircraft stabilize itself, but trying to regain airspeed is a better =
bet. If the gust gives a speed increase, a little nose up will turn =
your inertia into more altitude sooner, and if the gust gives a speed =
decrease you might want to nose down a little to avoid a wind-shear =
stall situation.

In my case, gusts are always some unknown combination of vertical and =
horizontal, and my reactions are too slow to hold anything truly =
constant...

Ian