Total Pressure Sensor
Bernie
MKIV vario and Director I now have does not use a TE probe or flask, so
the TE probe socket on the verticle fin is currently used for the
Airspeed pitot probe. If I use the SB-8 it will need to be hooked up to
the TE probe as well as an airspeed (total pressure?) source. The
Libelle has only the one socket on the fin for the TE probe and I have
been told there is or can be a Pitot sensor in the chamber behind the
nose hook. I have checked mine and find no airspeed source in the nose.
Does anyone have an SB-8 or other set up that needs both TE and pitot
sources in a Libelle? And if so how do you accomplish it?
LL
--
Bllewis
Sent via Deja.com http://www.deja.com/
Before you buy.
Robert W. Cunningham
just realized that there is no need for a TE probe if the vario has 3-axis
accelerometers to measure changes in speed and direction. Analog Devices
currently sells two packages that would do the job: A dual-axis (X-Y)
sensor, and a single-axis Z sensor. Given how poorly TE probes do their
job, even low-resolution accelerometer readings should suffice.
-BobC
ChrisAtUpw
job
>>?? If your TE Probe isn't doing it's job very well, either there is something
wrong with your system (leaks!) or with the probe or with the mounting.
Commercial TE probes now do a remarkably good job.
>>?? So why do you say that it isn't doing a very good job? What is it doing
that you think it shouldn't be doing? What do you think it should be doing that
it isn't doing?
Greg
>To a TE probe, a climb (due to control input, not lift) and a hard turn
can't be
>distinguished (equivalent airflow changes and similar KE/PE changes).
While this
>may be less of a problem for something as massive as a sailplane, it is an
>insurmountable problem for paragliders, and a major problem for hang
gliders.
In a climb, the plane is slowing down. In a turn it is at a constant speed.
So why can't they be distinguished?
>
>Can you side-slip a sailplane? What does the TE probe (or, more precisely,
the
>result of the TE calculation) indicate then?
If you are in conditions where you need a slip, why would you care what the
vario says?
>-BobC
>
>
David Leonard
>
> ChrisAtUpw wrote:
>
> To a TE probe, a climb (due to control input, not lift) and a hard turn can't be
> distinguished (equivalent airflow changes and similar KE/PE changes). While this
> may be less of a problem for something as massive as a sailplane, it is an
> insurmountable problem for paragliders, and a major problem for hang gliders.
>
lightweights, but for sailplanes a turn and a pull-up are very different
for a TE probe. The pneumatic probes are designed to supply a pressure
proportional to v^2, so changes in velocity are proportional to changes
in kinetic energy. There's no potential energy change in a level
altitude turn, but there is a drop in kinetic energy. A gentle pullup
from 110 kts cruise to just a few degrees up gives a very large change
in potential energy , enough to peg the vario without TE compensation.
Unless you pull hard, the change in TE during the pull is small.
> Can you side-slip a sailplane? What does the TE probe (or, more precisely, the
> result of the TE calculation) indicate then?
>
Yes, thats part of the beauty of the modern TE probe: for slips like you
see in a reasonable thermaling turn, they're pretty insensitive for
practical purposes.
> Also, the TE calculation is fairly sensitive to the difference between the "real"
> atmosphere and the "standard" atmosphere modeled in most algorithms. Without an
> accurate humidity reading to correct the atmospheric model, the TE value can be
> off by up to 50% (tropical conditions, near sea level), since it is not a linear
> function, and it depends very much on absolute density. (Where pressure altitude
> depends or relative density.)
>
Another beauty of the TE probe: its insensitive to absolute air density,
at least over the range we normally fly thermals in. A simple pneumatic
probe hooked to the static port of the vario works very well. The same
density is used for static air pressure measurement and TE probe V^2
measurement, so it drops out of the equation. I'm sure the math is the
same for lower velocity / mass machines, although which factors are
ignored as insignificant may change. Amazingly simple, yet very
effective.
> Gee, I hope I got all that right. The actual math is in a MathCad worksheet I
> don't have access to right now.
>
-Dave Leonard
LS-6B ZL
carbonhole?
Robert W. Cunningham
To a TE probe, a climb (due to control input, not lift) and a hard turn can't be
distinguished (equivalent airflow changes and similar KE/PE changes). While this
may be less of a problem for something as massive as a sailplane, it is an
insurmountable problem for paragliders, and a major problem for hang gliders.
Can you side-slip a sailplane? What does the TE probe (or, more precisely, the
result of the TE calculation) indicate then?
Also, the TE calculation is fairly sensitive to the difference between the "real"
atmosphere and the "standard" atmosphere modeled in most algorithms. Without an
accurate humidity reading to correct the atmospheric model, the TE value can be
off by up to 50% (tropical conditions, near sea level), since it is not a linear
function, and it depends very much on absolute density. (Where pressure altitude
depends or relative density.)
Gee, I hope I got all that right. The actual math is in a MathCad worksheet I
don't have access to right now.
-BobC
ChrisAtUpw
>To a TE probe, a climb (due to control input, not lift) and a hard turn can't
be
distinguished (equivalent airflow changes and similar KE/PE changes). While
this
may be less of a problem for something as massive as a sailplane, it is an
insurmountable problem for paragliders, and a major problem for hang gliders.
an initial down when you pull. In a turn, you just get constant but increased
sink. Also, the horizon seems to go in different ways.
>Can you side-slip a sailplane?
>What does the TE probe (or, more precisely, the result of the TE calculation)
indicate then?
>Also, the TE calculation is fairly sensitive to the difference between the
"real"
atmosphere and the "standard" atmosphere modeled in most algorithms.
rho, the actual air density. The whole point about glider TE Probes is that
they work at ANY height.
BUT they don't work in hang gliders or paragliders due to the very low
speeds. I can design you one that will, if you like.
HG Total Energy is usually provided by a propellor link, which could well
involve density corrections - you sense something akin to true speed through
the air.
Steve Bralla
(ChrisAtUpw) writes:
>The whole point about glider TE Probes is that
>they work at ANY height.
> BUT they don't work in hang gliders or paragliders due to the very low
>speeds. I can design you one that will, if you like.
>HG Total Energy is usually provided by a propellor link, which could well
>involve density corrections - you sense something akin to true speed through
>the air.
I made a TE probe for my HG Ball vario several years ago that worked just fine.
Steve Bralla
Eric Greenwell
In article <38487EB0...@acm.org>, rcun...@acm.org says...
=
= To a TE probe, a climb (due to control input, not lift) and a hard turn can't be
= distinguished (equivalent airflow changes and similar KE/PE changes). While this
= may be less of a problem for something as massive as a sailplane, it is an
= insurmountable problem for paragliders, and a major problem for hang gliders.
The TE probe has no need to "distinguish" between these situations, but
only to supply the vario the appropriate pressure change. For the climb,
this is a steady pressure; for the hard turn, this is a slowly
increasing pressure.
I think airflow changes are different in a climb and hard turn, but
don't effect a good TE probe in any case. The KE/PE changes are very
different:
Climb: KE decreasing rapidly, PE increasing rapidly
Hard turn: KE constant, PE slowly decreasing
= Can you side-slip a sailplane? What does the TE probe (or, more precisely, the
= result of the TE calculation) indicate then?
We don't intentionally side-slip sailplanes except for glide-path
control during landing. A good TE probe is not affected by small,
unintentional slide slips during cruise or climbing.
= Also, the TE calculation is fairly sensitive to the difference between the "real"
= atmosphere and the "standard" atmosphere modeled in most algorithms.
Both the sailplane and the probe respond only to the air pressures
produced by speed and density, and it doesn't matter if the atmosphere
is "standard" or not. This is why an air speed indicator is so useful to
a pilot, even though it doesn't give the true air speed.
= Without an
= accurate humidity reading to correct the atmospheric model, the TE value can be
= off by up to 50% (tropical conditions, near sea level), since it is not a linear
= function, and it depends very much on absolute density. (Where pressure altitude
= depends or relative density.)
This doesn't match my experience flying sailplanes all over the USA in
all kinds of conditions. My TE probes have compensated my varios
correctly in dry Nevada desert conditions and humid Florida conditions.
Also the compensation is good from the bottom of the thermal (low
humidity) to just below cloud base (high humidity).
= Gee, I hope I got all that right. The actual math is in a MathCad worksheet I
= don't have access to right now.
Given the wide-spread use of TE probes in low performance, low-speed
gliders on up to high speed, high performance gliders, and by the
majority of vario manufacturers (for sailplanes, certainly) you might
want to review the assumptions and physics in that worksheet!
--
>>Delete the "REMOVE" from my e-mail address to reply by e-mail<<
Eric Greenwell
Larry Goddard
<snip>
> = Can you side-slip a sailplane? What does the TE probe (or, more precisely, the
> = result of the TE calculation) indicate then?
>
> We don't intentionally side-slip sailplanes except for glide-path
> control during landing. A good TE probe is not affected by small,
> unintentional slide slips during cruise or climbing.
>
Actually, some of us do intentionally slip sailplanes at times in strong thermals to
increase climb rates.
Larry Goddard
"01" USA
Bernie
the original question I submitted regarding how to hook up the SB-8 to a
pitot source and a TE probe.
LL
In article <384AA4C6...@siriusimages.com>,
--
Eric Greenwell
In article <384AA4C6...@siriusimages.com>, la...@siriusimages.com
says...
= Eric Greenwell wrote:
=
= <snip>
=
= > = Can you side-slip a sailplane? What does the TE probe (or, more precisely, the
= > = result of the TE calculation) indicate then?
= >
= > We don't intentionally side-slip sailplanes except for glide-path
= > control during landing. A good TE probe is not affected by small,
= > unintentional slide slips during cruise or climbing.
= >
=
= Actually, some of us do intentionally slip sailplanes at times in strong thermals to
= increase climb rates.
I was trying to keep it simple for someone who clearly wasn't familiar
with sailplanes. In the case you mention, the slip isn't enough to
affect the TE probe, the major point I wanted to get across.
I am curious, though: why only strong thermals? Wouldn't the technique
be even more useful in weak thermals? My experience in strong thermals
is small differences in centering or altitude (separation from another
glider) make a significant difference in climb, so I'm wondering how you
tell the difference caused by such a subtle change as a small amount of
slip?
I know George Moffat wrote about using slip a long time ago (for a
Standard Cirrus, I think), but I've never been able to distinguish any
effect.
Robert W. Cunningham
when I started this branch in the thread. Your original subject line is
still valid, but it seems nobody wants to discuss that topic. You might
want to try again with "SB-8" in your subject header, just to make sure you
attract those who might have the information you seek, instead of using a
non-specific subject line.
-BobC
Bernie wrote:
> These are all great discussions about vario theory, but don't address
> the original question I submitted regarding how to hook up the SB-8 to a
> pitot source and a TE probe.
>
> LL
>
> In article <384AA4C6...@siriusimages.com>,
> Larry Goddard <la...@siriusimages.com> wrote:
> > Eric Greenwell wrote:
> >
> > <snip>
> >
> > > = Can you side-slip a sailplane? What does the TE probe (or, more
> precisely, the
> > > = result of the TE calculation) indicate then?
> > >
> > > We don't intentionally side-slip sailplanes except for glide-path
> > > control during landing. A good TE probe is not affected by small,
> > > unintentional slide slips during cruise or climbing.
> > >
> >
> > Actually, some of us do intentionally slip sailplanes at times in
> strong thermals to
Robert W. Cunningham
NG and email. Bottom line, sailplanes are such different beasts from paragliders (PG)
and hang gliders (HG) that the instrumentation overlap seems to be vastly smaller than I
had hoped. In particular, TE probes are just about useless on PGs, and of only moderate
use on HGs. For these craft, my plan to use accelerometers to calculate changes in TE
should yield significant improvements.
Since the sailplane is a far more stable platform than other soaring craft, TE probes can
operate very close to the limit of the ideal theoretical description. In this case, it
would take an exceptional accelerometer implementation to replace the TE probe, though
the accelerometers will have other uses in their own right (such as dead reckoning
navigation or a simple inertial navigation system).
Finally, my attempts to simplify the instrument by eliminating the need TE probe were
complicated by another sailplane characteristic: Cockpit pressure seldom exactly matches
ambient, requiring a separate line to sense true ambient pressure. Certainly not a
requirement for PG and HG!
So now I'm looking for a different tact for the sailplane instrument: Are there
sailplane varios that output digital data in real time? It doesn't matter what the data
format is (NMEA would be nice). If so, it would allow me to still produce an instrument
that is self-contained for PG and HG pilots, while taking advantage of the installed
systems on a sailplane to provide all the same benefits.
While the question above is most important to me, I do have another of a more speculative
nature: How hard would it be to install 2"x2" GPS antennas near the end of each wing?
The antennas would have to face up, but could be buried inside the wing or wing tip. A
small (3/64") coax cable would need to be run from each wing back to the cockpit. The
reason I ask is that I recently came across a VERY inexpensive way to obtain true
attitude information from paired antennae, and also obtain improved absolute GPS
positional accuracy and even INSTANTANEOUS HEADING data. Remember, normal GPS receivers
do not indicate heading other than by taking the difference of sequential positions. So
the GPS heading is independent of aircraft orientation. It would be the same if you were
flying backward. With dual antennae, I actually get more than twice the amount of data,
allowing me to provide true heading and a useful artificial horizon.
I'm not certain if this will work on HGs, and PGs look almost hopeless (how to mount on
ripstop nylon?), but it may be of great use aboard sailplanes. Plus, I've always wanted
to build a GPS receiver.
-BobC
franknfurter
Robert W. Cunningham <rcun...@acm.org> wrote in article
<384C1A70...@acm.org>...
[snippage].
>
> While the question above is most important to me, I do have another of a
more speculative
> nature: How hard would it be to install 2"x2" GPS antennas near the end
of each wing?
> The antennas would have to face up, but could be buried inside the wing
or wing tip. A
> small (3/64") coax cable would need to be run from each wing back to the
cockpit. The
> reason I ask is that I recently came across a VERY inexpensive way to
obtain true
> attitude information from paired antennae, and also obtain improved
absolute GPS
> positional accuracy and even INSTANTANEOUS HEADING data. Remember,
normal GPS receivers
> do not indicate heading other than by taking the difference of sequential
positions. So
> the GPS heading is independent of aircraft orientation. It would be the
same if you were
> flying backward. With dual antennae, I actually get more than twice the
amount of data,
> allowing me to provide true heading and a useful artificial horizon.
>
Bob you should be aware that this area is covered by some existing patents.
Sorry, I dont know the numbers, you will have to do a search. The basic
idea of doing some processing on multiple antenna feeds is sound,
unfortunately others have had the same thoughts.
Frank Herzog
email: fherzog {at} escape {dot} ca
John Iacobucci
indicating 15-20 degrees off center. Many good pilots have remarked about
this phenomenon yet no one seems to have, to my knowledge, an adequate
explaination for this.
Eric Greenwell wrote in message ...
Armand A. Medeiros
"A little slip in a turn is required for best climb performance for two
reasons. 1st, the BEST path around a circle when banked requires a little
slip to more evenly distribute the wing loading due to the different
circumferences (hence air speed) traveled by each wing which reduces induced
drag even though overall parasitic drag may increase slightly. This slip
requirement increases with increased bank angle to about 45 degrees then
decreases with from 45 to 90 degrees. At 0 and 90 degrees bank,
theoretically, both wings would be traveling the same speed distance and no
slip would be required...in practice, at 90 degrees, the airplane would fall
out of the sky due to lack of a vertical lift component. But you get the
point...which brings up the 2nd reason a slip is good. The slight increase
in lift on the inner wing segment when slipping decreases the over-banking
tendency of a glider which means less anti-roll aileron input is required,
which further decreases drag. In addition, extra lift means less elevator
required further increasing performance. The sum of all these components is
favorably improved when compared to a non-slipping turn."
Now, my memory may be fuzzy and maybe there is an error in how I relate
this...apologies before hand for any mistakes, but this is how I understand
the slip improves performance while thermalling. In practice, it works for
me!
Armand
John Iacobucci <jjb...@iserv.net> wrote in message
news:g58a4.18337$Ym1.8...@tw11.nn.bcandid.com...
Larry Goddard
Larry Goddard
"01" USA
mil...@cgpp.com
"John Iacobucci" <jjb...@iserv.net> writes:
> I do notice an improvement in climb rate when thermaling with the yaw string
> indicating 15-20 degrees off center. Many good pilots have remarked about
> this phenomenon yet no one seems to have, to my knowledge, an adequate
> explaination for this.
A while back E. E. Larrabee published an analysis showing that a
reduction in overall drag was possible in a slipping turn due to
reduced rudder and aileron deflections. The math isn't hard to follow:
Larrabee, E. E., Lateral Control and Sailplane Design Considerations
to Optimize Altitude Gain While Thermalling, AIAA Paper No. 74-1004,
In Proceedings of the AIAA/MIT/SSA 2nd International Symposium on
the Technology and Science of Low Speed and Motorless Flight,
Cambridge, Mass., September, 1974.
J.
--
Judah Milgram mil...@cgpp.com
College Park Press http://www.cgpp.com
P.O. Box 143, College Park MD, USA 20741 +001 (301) 422-4626 (-3047 fax)
John Cochrane
> A while back E. E. Larrabee published an analysis showing that a
> reduction in overall drag was possible in a slipping turn due to
> reduced rudder and aileron deflections. The math isn't hard to follow:
I used to do this with noticeable advantage until I got winglets. I'm not so sure
winglets like slip. Does this analysis say anything on the subject?
John Cochrane
mil...@cgpp.com
John Cochrane <John.STOPS...@GSB.UChicago.EDU> writes:
Hi John -
no, this was before winglets (on sailplanes). But it's interesting to
read to read of your experience, and makes sense in a way. Might be
equivalent to setting the winglets at the wrong angles. So you could
lose both on the induced drag and the winglet airfoil coming out of its
operating range, and maybe local separation to boot (all of this is
just speculation on my part).
Jud
John H. Campbell
especially for big span ships. This as claimed observed in flight (not
theorized) by Klaus Holighaus, George Moffat and others... --JHC
Sean
Sean
--
se...@direct.ca
John H. Campbell <jh...@greeleynet.com> wrote in article
<eoea4.1887$G3.192...@news.frii.net>...
Bo Brunsgaard
blew...@gateway.net wrote:
> the TE probe as well as an airspeed (total pressure?) source. The
> Libelle has only the one socket on the fin for the TE probe and I have
> been told there is or can be a Pitot sensor in the chamber behind the
> nose hook. I have checked mine and find no airspeed source in the
nose
Don't know if this helps, but, yes, there is room for an airspeed
source in the nose of the Libelle (at least on the 201b - don't know
about the 301).
On our Libelle dynamic pressure for the airspeed is taken through a
narrow metal tube that is lead through an opening in the housing of the
nose hook. If you take the nose hook out, you shold notice a
flat "groove" in top of it with a little horizontal bar at the front.
The tube lies in that groove and is fastened by being led under the bar.
Works quite well, and it sits so far out that it is surprisingly
insensitive to sideslipping. Only problem is that it makes taking the
nose hook out (and especially putting it back in) a bit of of a
threehand puzzle. And it is kind of hard to fit two people and a
spanner into the nose of a Libelle ;-)
Bo Brunsgaard
Std. Libelle OY-XKB
Ian Johnston
: Wouldn't a bit of slip also move you closer to the core ?
I used to have a share in a Slingsby Swallow. This is a 13m span wooden
glider with not much by way of performance - about 22:1 or so. Because I
am large-ish, I was at the top end of the weight range - no problem in
level flight but in turns I ran out of elevator at about 30 degrees of
bank. Some experiment showed that in very tight thermals I could get a
significant advantage by slipping into the core a bit. Note that this
depended on a slow thermalling speed and poor performance in the first
place. It's a matter of balancing the slipping losses with the thermalling
gains - in a glass ship I'd get nearer the core by turning tighter...
Ian
Donald Ingraham
> no, this was before winglets (on sailplanes). But it's interesting to
> read to read of your experience, and makes sense in a way. Might be
> equivalent to setting the winglets at the wrong angles. So you could
> lose both on the induced drag and the winglet airfoil coming out of its
> operating range, and maybe local separation to boot (all of this is
> just speculation on my part).
John/Judah,
I have thought about this a lot (mainly during climbs) since I got
winglets. In general, it seems logical that the penalty for *all*
uncoordinated flight is higher with winglets and I now pay a little
closer attention to the yaw string on those long "accuracy" final
glides. But I still feel that a little indicated slip helps the climb
(Discus CS).
A little *indicated* slip.
The reason, revived here from a past r.a.s. discussion, may be that
the yaw string indicates a slip during "perceived efficient thermaling"
because the glider really is actually closer to coordinated flight than
with the yaw string straight back. This is because the yaw string is
*forward* of the center of lift, especially on long nosed two-place
ships, and therefore on the "outside" of your true circle. This will
cause it to show some slip.
Another factor influencing the yaw string to show slip when there
is none, is the fact that we are sinking downward at a few knots (not
outward...just look at the cows directly below you as they get bigger)
*and* the fuselage is "tipped". The airflow components over the fuselage
are 45-50 knots [from just off to the inside of] your nose and 3-6 knots
[depending on bank angle, ship, etc] from the lowest point of the
tipped fuselage. The yaw string will show this vector as "slip".
So maybe it is actually more efficient to thermal with a little
*indicated* slip, because then you are really flying closer
to coordinated.
Don
__________________________________________________________________
If Barbie is so popular, why do you have to buy her friends?
___________________________________________________________________
Eric Greenwell
In article <g58a4.18337$Ym1.8...@tw11.nn.bcandid.com>,
jjb...@iserv.net says...
= I do notice an improvement in climb rate when thermaling with the yaw string
= indicating 15-20 degrees off center. Many good pilots have remarked about
= this phenomenon yet no one seems to have, to my knowledge, an adequate
= explaination for this.
Are you sure of the 15-20 degrees? That would have a 3" long yaw
string about 0.75" to 1.0" off of center, which seems like a lot!
I'm still curious about how people have decided slipping while
thermalling is an asset, what tests they've done, and what gliders
they are doing it in. I'm a bit skeptical about the claims, for
several reasons:
- I don't recognize the people advocating it as Nationals winners,
World team members, or World record holders
- Except for the off-hand comment by Moffat over 25 years ago, I
haven't heard any top pilot talk about it, even at talks and seminars
devoted to flying techniques, nor is it mentioned in books like
Reichmann's
- None of the flight manuals for the gliders I've owned (Ka-6e, Std.
Cirrus, H301, ASW 20 C, ASH 26 E, Blanik, ASK 13) or flown (about six
other types) have mentioned the technique
- When I thermal with other pilots, sometimes I out-climb them and
sometimes they out-climb me, even if I fly the same way each time.
Slipping doesn't seem to change the results, either.
Robert Ehrlich
>
> Another factor influencing the yaw string to show slip when there
> is none, is the fact that we are sinking downward at a few knots (not
> outward...just look at the cows directly below you as they get bigger)
> *and* the fuselage is "tipped". The airflow components over the fuselage
> are 45-50 knots [from just off to the inside of] your nose and 3-6 knots
> [depending on bank angle, ship, etc] from the lowest point of the
> tipped fuselage. The yaw string will show this vector as "slip".
As opposite to the first cause you mentionned, this one is a "real" slip and
as such should be avoided. No matter if due to composition of sink speed with
forward speed, if the result is some sideways component of the speed of the
airflow relative to the fusealage, it will add some drag and should be avoided.
This question comes back periodically in this newsgroup and some possible
explanations were already given. One of them is as you mentionned the apparent
slip due to the different direction of the relative airflow at the position
of yaw string and at the widest point of the fuselage where is probably the
highest sensitivity to slip in term of drag. Another explanation that was
suggested is that the aerodynamic force generated by the slip has a lift
component. This argument is wrong in my opinion. Anyway the total of all
vertical components of the aerodynamic forces must be equal to the weight
of the glider. If the fuselage provides part of it, the contribution of the wings
will decrease by exactly the same amount. You pay the increase of lift given
by the fuselage by an increase of drag. You gain a decrease of drag due to
the decrease of lift from the wings. But as the fuselage is a lot less efficient
than the wings in providing lift, your loss far exceeds your gain.
A last explanation that I may suggest involves the fact that at the tail as well
as at the yaw string location, the direction of the relative airflow is not the
same as at the middle of the fuselage. The airflow at the rudder comes slightly
sideways from the outside of the turn, and in order to avoid that this turns the
nose out of the turn you need to have some inside rudder. There are other reasons
for which some inside rudder may be needed, but let's forget them now and consider
just this one. In this case the amount of rudder needed is what gives you a vertical
surface of fin + rudder with zero lift in the slipping local airflow. In this
position giving an angular cambered airfoil to the vertical tail, the drag is higher
than in the neutral position. So by decreasing the angular camber of this airfoil
you may decrease its drag. You can do that by a little slip toward the inside of the
turn, this will decrease the angle of the airflow at the tail and so decrease the
amount of rudder needed. Of cours you will pay the decrease in drag at the tail by
the drag generated by the slip and you may ask if you are winning or loosing at end.
The answer is that for a sufficiently small slip you certainly will win. This is
because at zero slip the drag of the fuselage is at its minimal value while the
drag of the vertical tail is not. The variation of the drag of the fuselage is
then of second order compared to the variation of the drag of the vertical tail, so
for sufficientlly small varitions the last one exceeds the first one in magnitude.
Sorry for this long post, this was my English exercise of the day, since you may have
guessed that my native language is French.
Donald Ingraham
>
> Donald Ingraham wrote:
> >
> > Another factor influencing the yaw string to show slip when there
> > is none, is the fact that we are sinking downward at a few knots (not
> > outward...just look at the cows directly below you as they get bigger)
> > *and* the fuselage is "tipped". The airflow components over the fuselage
> > are 45-50 knots [from just off to the inside of] your nose and 3-6 knots
> > [depending on bank angle, ship, etc] from the lowest point of the
> > tipped fuselage. The yaw string will show this vector as "slip".
>
> As opposite to the first cause you mentionned, this one is a "real" slip and
> as such should be avoided. No matter if due to composition of sink speed with
> forward speed, if the result is some sideways component of the speed of the
> airflow relative to the fusealage, it will add some drag and should be avoided.
Point taken. It is real slip. But this is essentially your
"thermaling sink rate", and as such, is hard to avoid without
helium! ;-) Thanks for your observations. And I don't think you
need any help with your English!
Don
_______________________________________________________
Don Ingraham
SGI Friends help you move.
d...@sgi.com Real friends help you move bodies.
Donald Ingraham
> jjb...@iserv.net says...
> = I do notice an improvement in climb rate when thermaling with the yaw string
> = indicating 15-20 degrees off center. Many good pilots have remarked about
> = this phenomenon yet no one seems to have, to my knowledge, an adequate
> = explaination for this.
15-20 degrees is a lot. To clarify, I was thinking a 3" to 4"
yaw string might show about a pencil's width of slip (maybe
5-7 degrees?).
Don
Gary Osoba
Slipping certain configurations in the spiral flight mode can be more
efficient than coordinated flight. The main factors are the magnitude of
the dynamic pressure differential across the span, the effective
dihedral, and the modified lift distribution.
All other things being equal, gliders with more span, less wing loading,
higher wing CsubL's, and lower effective diehedral will tend to benefit
more.
All of these comparisons result in a larger dynamic pressure
differential due to larger differences in the tip speeds of the inside
and outside wings. This is what contributes to what most pilots identify
as overbanking tendencies. Although there is a sideslip component
present in even coordinated spiral flight, introducing more in
uncoordinated flight works along with effective dihedral, vertical tail
surfaces and other factors to counteract the overbanking tendency.
Winglets are especially effective in this regard because they are
coupled through such a long moment arm, which accounts for one posters
observation of what many pilots have found...winglets seem to improve
circling performance and handling. For the same reasons, poyhedral
configurations such as the new Discus and gliders of large, heavily
loaded and flexible span attain to the benefit as the wings assume a
highly non-planar form in the additional g's of circling flight...adding
to the effective dihedral while at the same time affecting the wake
roll-up through vertical displacment of the tips and attendant vortices.
These conditions reduce the induced drag of particularly the inside wing
which will be flying at a high CsubL.
The benefits may not accrue to every design, due to the first mentioned
points and some other factors as well. Many open class designs have
utilized stepped flaps across the span, optimized for circling flight so
that the amount of aileron on the inside tip which is required to hold-
off an overbanking tendency blends with the desired flap setting in
order to minimize the total induced drag on that wing.
To actually be perfectly coordinated, some slip will have to be
indicated due to the fact that the yaw string location is displaced
forward of the aerodynamic center, about which your spiral flight
occurs. Whereas most yaw strings on high performance ships are affixed
directly to the canopy, it should be recognized that an indicated slip
angle of 15-20 degrees will be exxagerated greatly in the local flow and
may be some 5-6 times that of the actual yaw angle in the freestream
velocity. The 1/2 of 1 percent or less penalty from yaw is more than
overcome in designs which combat high dynamic pressure differential.
Eric Greenwell, who has much experience to his credit, is reporting his
own experiences acurately, I'm sure. However, designs which he has not
flown and which may not be as well behaved or be more extreme in
configuration may benefit from the slipping techniques. Only when one
flies a design which really stretches the design limits do some of these
things become quite demonstrably apparent. A notable example would be
the Carbon Dragon, which flies at a wing loading of around 2 lbs/sq.
ft., a span of 44 feet, and 55 sq. ft. of full span flaps which result
in a very high wing CsubL. In a tight turn of 40' radius, there is a
dynamic pressure differential across the span which is several hundred
percent higher than that experienced in a typical sailplane. I can tell
you absolutely that not only is slipping circling flight more efficient
in this design, but that the effect is so dramatic you can not attain to
an efficient 45 degree banked turn without it, but rather have to circle
at a lower bank.
At the other end of the spectrum, both the Gemini and Sigma Open Class
variable geometry designs fly at a high wing loading but attain to
nearly double the circling CsubL's as typical high performance gliders.
These designs also benefit from the slipping approach, but not as
dramatically as the Carbon Dragon. The Gemini requires more than the
Sigma due to much more effective dihedral and because Dr. Marsden, among
his other excellent modifications, optimized the deflection of the
slotted flap in concert with the amount required by the inside aileron
in order to produce the necessary amount of hold-off in the bank.
As a point of interest, the Woodstock, another light glider (albeit
heavy in comparison to the Carbon Dragon) does not appear to benefit
from the slip much. The ailerons have high differential and there is not
the depth of camber line at the tip that is seen on the Carbon Dragon.
The inside tip CsubL is subsequently much lower, and this, in
combination with the shorter span, higher wing loading, and higher
circling speeds than the Carbon Dragon effectively cause the wing to
stall before the dynamic pressure differential reaches a point where
additional slipping seems to benefit the design.
This topic was discussed a few years ago, and I posted some more
detailed info at that time. There were several very good posts from
several sources.
Best Regards,
Gary Osoba
Robert Ehrlich
>
> ... The Gemini requires more than the Sigma due to much more effective dihedral ...
I am probably missing some point, but as far as I understand these things I would
think that the amount of slip needed to cancel the overbanking tendency decrease
with increasing dihedral. My understanding is that the effect of dihedral is that
when slipping toward the inside, the angle of attack of the inside wing increase
and the angle of attack of the outside wing decrease, both increase and decrease being
proportional (in first order at least) to the dihedral and the slip angle. So to
obtain the change in angle of attack which counterbalance the overbanking tendency
you would need a high slip angle with a low dihedral and a little slip angle with
a high dihedral. And with no dihedral at all, no slip angle would help. Where
am I wrong ?
Chris O'Callaghan
right, but I do appreciate the effort that went into some of the funnier
replies.
Your glider flies around its CG. Your yaw string is located some distance
from the cg and therefore outside the circle that your CG is tracing during
a turn. Therefore, for the wings to be flying normal to the circle you
trace, the yaw string must show a SMALL slip. This same effect is notable
with the need to trim your rudder in a turn, since the tailboom is also
displaced from the CG.
If you do the math, you'll see that the angle of slip the yaw string should
show is small, on the order of several degrees.
As for differences between weak and strong thermals, poor airmanship is more
pronounced in weak thermals... in a strong thermal you can get away with
lazy piloting and still go up like a rocket. That is, until you compare
yourself with someone who is making best use of the thermal.
Chris O'Callaghan
on the use of the rudder in a turn.
----------
In article <g58a4.18337$Ym1.8...@tw11.nn.bcandid.com>, "John Iacobucci"
<jjb...@iserv.net> wrote:
>I do notice an improvement in climb rate when thermaling with the yaw string
>indicating 15-20 degrees off center. Many good pilots have remarked about
>this phenomenon yet no one seems to have, to my knowledge, an adequate
Gary Osoba
Yes,Robert is correct. As I stated toward the beginning of the post "All
other things being equal, gliders with more span, less wing loading,
higher wing CsubL's, and lower effective diehedral will tend to benefit
more."
I should have added a phrase to Gemini/Sigma comment to make it more
clear. I'll try again.
The Gemini requires more than the Sigma due to much more effective
dihedral *in the Sigma*...
Thanks, Robert.
"Reculer Pour Mieux Sauter".
Gary Osoba
Ian Johnston
: If you do the math, you'll see that the angle of slip the yaw string should
: show is small, on the order of several degrees.
I did this a few years back, but what the heck. 15m glider (Pirat!) thermalling
at 30 degree bank and 45 kt (23 m/s).
Radius of turn = v^2 / g tan theta = 100m, near as dammit.
Offset angle of yaw string in horizontal plane
= arctan (distance from c of g / radius)
= arctan (1 / 100)
= 0.6 degrees
Offset angle in the plane of the glider
= 0.6 / cos theta
= 0.66 degrees.
Offset distance of aft end of 2" yaw string
= 0.02"
Make the case more extreme (larger bank angle, longer fuselage) and you might
get up to a degree or two, but that will still be impossible to spot with
a yaw string. So if there is a visible yaw in a good thermalling turn, it
does not come from the yaw string offset from the C of G.
: As for differences between weak and strong thermals, poor airmanship is more
: pronounced in weak thermals... in a strong thermal you can get away with
: lazy piloting and still go up like a rocket. That is, until you compare
: yourself with someone who is making best use of the thermal.
Very true. The thermal profile is also important - some tight thermals
justify almost any measures taken (within reason) to get to the core.
(For glass glider pilots: the "core" is the bit the wooden gliders are
using to pass you...)
Ian
Shaber CJ
coordinated or in a skid?
Larry Goddard
> In article <g58a4.18337$Ym1.8...@tw11.nn.bcandid.com>,
> jjb...@iserv.net says...
> = I do notice an improvement in climb rate when thermaling with the yaw string
> = indicating 15-20 degrees off center. Many good pilots have remarked about
> = this phenomenon yet no one seems to have, to my knowledge, an adequate
> = explaination for this.
>
> Are you sure of the 15-20 degrees? That would have a 3" long yaw
> string about 0.75" to 1.0" off of center, which seems like a lot!
>
> I'm still curious about how people have decided slipping while
> thermalling is an asset, what tests they've done, and what gliders
> they are doing it in. I'm a bit skeptical about the claims, for
> several reasons:
>
<snip>
Centered up in big, strong thermals in Uvalde in August. Flying an LS-3a with full
water. When I fly coordinated (Yaw string straight back) the vario goes
BEEP......... BEEP.........BEEP. When I fly with some top rudder (a bit of a
slip) the vario goes BEEP..BEEP..BEEP. I know which one I am going to use.
Larry Goddard
"01" USA
Eric Greenwell
In article <386BE6...@fn.net>, go...@fn.net says...
= Eric Greenwell, who has much experience to his credit, is reporting his
= own experiences acurately, I'm sure. However, designs which he has not
= flown and which may not be as well behaved or be more extreme in
= configuration may benefit from the slipping techniques. Only when one
= flies a design which really stretches the design limits do some of these
= things become quite demonstrably apparent. A notable example would be
= the Carbon Dragon, which flies at a wing loading of around 2 lbs/sq.
= ft., a span of 44 feet, and 55 sq. ft. of full span flaps which result
= in a very high wing CsubL.
This was always my belief, so I'm glad someone of Gary's experience
confirms it. My suspicion was (and still is) the postings from pilots
(now with the exception of Gary) claiming discernible improvements
from slipping aren't flying any of these extreme designs, but most
likely are flying 15 meter sized gliders in the 7 to 10 lbs/sq.ft
range.
Armand A. Medeiros
not only CofG, but also specific aircraft types fly differently for best
results. This isn't wholly unexpected...
Has anyone has installed a turn indicator and correlated the "ball" to what
is seen on the yaw string?
I might try this in my next sailplane...
Armand
Eric Greenwell
In article <386D34D1...@siriusimages.com>,snip
= > I'm still curious about how people have decided slipping while
= > thermalling is an asset, what tests they've done, and what gliders
= > they are doing it in. I'm a bit skeptical about the claims, for
= > several reasons:
= >
=
= <snip>
=
= Centered up in big, strong thermals in Uvalde in August. Flying an LS-3a with full
= water. When I fly coordinated (Yaw string straight back) the vario goes
= BEEP......... BEEP.........BEEP. When I fly with some top rudder (a bit of a
= slip) the vario goes BEEP..BEEP..BEEP. I know which one I am going to use.
Do you get the same improvement in weak thermals (I know Uvalde has a
few - I found them!)? Are you sure you actually have top rudder, or
are you assuming this because the yaw string indicates a slip? How far
forward of the leading edge is your yaw string? How long is the
string, and how much deflection is needed?
I'm beginning to suspect that yaw string position might be at least a
partial explanation for the different experiences pilot have with
slipping while thermalling. My yaw string is almost overhead at 27" in
front of the leading edge.
I'm going to add one at the front of the canopy, about 58" ahead of
the leading edge, generally a position much more sensitive to slip
angle (I'll also try to mark the rudder cables so I can see if I have
top or bottom rudder at any time). Perhaps the forward string will
show a slip, while the rearward one does not; if so, I may already be
flying at the slip angle needed to optimize the climb, but just not
realizing it due to the string location.
For many years, I've mounted the yaw string as far back as possible
and still keep it visible. Someone (Waibel, I think) suggested this to
maintain laminar flow over the canopy for the greatest distance.
John M. Morgan
> Has anyone has installed a turn indicator and correlated the "ball" to
what
> is seen on the yaw string?
>
> I might try this in my next sailplane...
>
> Armand
>
>
I have a "needle and ball" in my Stemme. While gliding they pretty much
agree with each other, which is not to imply that I'm good at keeping them
in the middle.
--
John "Bumper" Morgan <bump...@castles.com> S10-VT N50ZZ
To REPLY please remove aviation part of address.
"Eagles & hawks are self launchers."
Hannu Niemi
with full
of a
Just to be sure... Are you going UP faster, or might it just be, that vario
reacts to different pressure because of the slip. Not trying to say that it
is so, but it might be...
Not to say I am not trying next Summer - just 4-5 months to go :( - myself
;)
regards
hannu
Jan Smit
> the vario goes BEEP......... BEEP.........BEEP. When I fly with some top rudder (a
> bit of a
> slip) the vario goes BEEP..BEEP..BEEP. I know which one I am going to use.
I believe the LS-3a uses a Venturi (with a ring at the end) compensated vario. It
could be anglesensitive. What did your altimeter say?
Larry Goddard
> could be anglesensitive. <snip>
My LS-3a is equipped with a Cambridge L-NAV with vertical stabilizer mounted L-shaped (2
hole version) total energy probe. It is very UN-sensitive to yaw and slip. I am
confident that (in this plane, in these conditions) it helps to put a little 'high side'
rudder into the equation.
All that said, the most important thing in my opinion is to fly for effect. Do whatever
makes you go up faster.
Larry Goddard
"01" USA
Eric Greenwell
In article <m2ln6e6...@tcraft.cgpp.com>, mil...@cgpp.com says...
= A while back E. E. Larrabee published an analysis showing that a
= reduction in overall drag was possible in a slipping turn due to
= reduced rudder and aileron deflections. The math isn't hard to follow:
=
= Larrabee, E. E., Lateral Control and Sailplane Design Considerations
= to Optimize Altitude Gain While Thermalling, AIAA Paper No. 74-1004,
= In Proceedings of the AIAA/MIT/SSA 2nd International Symposium on
= the Technology and Science of Low Speed and Motorless Flight,
= Cambridge, Mass., September, 1974.
I spoke with E.E. Larrabee last night. Summarizing what he told me:
"The more dihedral a glider has, the more likely it is to benefit from
slipping while thermalling. Winglets have an effect similar to
dihedral as long as they are above the CG. Gliders with polyhedral
(like the Ventus 2) will benefit even more than those with dihedral.
The more dihedral or polyhedral, the less slip required to allow the
ailerons to be centralized."
"The benefit will be of very little value if the thermals are bubble-
like, as all the gliders will soon be going up at the rate of the
bubbles ascent."
His remarks suggest that large gliders like the Nimbus 4 and ASW 22
that have a lot of wing bending (and hence more dihedral) while
circling will benefit more than a small stiff wing glider, and the
Ventus 2 will get some benefit but the Ventus B (without winglets)
probably won't.