Performance loss versus yaw string deflection

[Eric Greenwell]

I don't recall ever seeing an estimate of the performance loss in a
glider as a function of yaw string deflection. Does anyone know what it is?

Ultimately, I'm wondering how much attention should be paid to
deflections of, say, 1/8", 1/4", 1/2", etc., on the usual 4"-5" long yaw
string.

I suppose a professionally manufactured and tested yaw string, like the
Mk IV, comes with a performance correction card (much like compass has),
but I just have a blemished one I got an outlet store - no card.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* Sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

[sisu1a]

> I don't recall ever seeing an estimate of the performance loss in a
> glider as a function of yaw string deflection. Does anyone know what it is?
>
> Ultimately, I'm wondering how much attention should be paid to
> deflections of, say, 1/8", 1/4", 1/2", etc., on the usual 4"-5" long yaw
> string.

Without actually 'calibrating' your yawstring with a slip/skid ball
for your particular ship and installation first, I don't think such
specifically meaningful data can't really be derived, and it will vary
somewhat from ship to ship (at least type to type...) so would not be
universally applicable.

Since a straight yawstring in a turn does not mean perfect
coordination in all ships, and in fact indicates a skidding turn on
some due to crossflow effects on canopies as well as the position of
the string being forward of the C/G ( wait are you just being funny
and I took the bait, hmmm ;) the yawstring calibration card would
be nice to have, even if it's just committed to memory.

Reread 'Circling The Holighaus Way' by Johnson for a much better
explanation than mine...
http://www.owp.us/Johnson/CirclingTheHolighausWay.pdf

-Paul

[bumper]

"Eric Greenwell" > wrote in message
...
>I don't recall ever seeing an estimate of the performance loss in a glider
>as a function of yaw string deflection. Does anyone know what it is?
>
> Ultimately, I'm wondering how much attention should be paid to deflections
> of, say, 1/8", 1/4", 1/2", etc., on the usual 4"-5" long yaw string.
>
> I suppose a professionally manufactured and tested yaw string, like the Mk
> IV, comes with a performance correction card (much like compass has), but
> I just have a blemished one I got an outlet store - no card.
>
> --
> Eric Greenwell - Washington State, USA
> * Change "netto" to "net" to email me directly
>

Eric,

I take this yaw string business seriously, though admittedly making MKIV's
is a whole different thing than being able to fly with them straight. No
card is provided with the MKIV, as we have no control over which side of the
equator the MKIV is to be used. And, as I'm sure you are aware, coriolis
effect can wreck havoc upon yaw strings if they are not properly degaussed.

Without glider specific flight testing, we may still assume that in straight
flight and with the yaw string straight back, we have unity or 100%.
With the yaw string 90 degrees off to the side or anything forward of that,
we have a problem, or for our purposes, 0%.

By extrapolation . . . Ummm, what was the original question?

bumper
proudly churning out MKIV and QV's for the unwashed masses.

[Eric Greenwell]

sisu1a wrote:
>> I don't recall ever seeing an estimate of the performance loss in a
>> glider as a function of yaw string deflection. Does anyone know what it is?
>>
>> Ultimately, I'm wondering how much attention should be paid to
>> deflections of, say, 1/8", 1/4", 1/2", etc., on the usual 4"-5" long yaw
>> string.
>
> Without actually 'calibrating' your yawstring with a slip/skid ball
> for your particular ship and installation first, I don't think such
> specifically meaningful data can't really be derived, and it will vary
> somewhat from ship to ship (at least type to type...) so would not be
> universally applicable.
>
> Since a straight yawstring in a turn does not mean perfect
> coordination in all ships, and in fact indicates a skidding turn on
> some due to crossflow effects on canopies as well as the position of
> the string being forward of the C/G ( wait are you just being funny
> and I took the bait, hmmm ;) the yawstring calibration card would
> be nice to have, even if it's just committed to memory.

Mostly, I was thinking of straight flight. Circling is more complicated,
and aerodynamic efficiency is generally a lower priority to proper
positioning.

> Reread 'Circling The Holighaus Way' by Johnson for a much better
> explanation than mine...
> http://www.owp.us/Johnson/CirclingTheHolighausWay.pdf

Unfortunately, the article doesn't offer any estimate of performance
loss. As far as avoiding stalling or spinning (one of the reasons given
for a slipping turn while circling), I have to fight with my ASH 26 E to
make it do either in a turn; in fact, at my normal 35 to 45 degree range
of bank, it won't. So, maybe the newer gliders don't respond as well to
a slipping turn as the older gliders. The flight manual doesn't mention
doing it, either. Perhaps, being a flapped glider, where the flaps work
differentially with the ailerons, that eliminates the advantage of the
slipping turn.

But, if anyone has estimates of the performance loss/gain versus yaw
string defection in circling flight, I'd love to see those, too.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* Sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

[Eric Greenwell]

bumper wrote:
> "Eric Greenwell" > wrote in message
> ...
>> I don't recall ever seeing an estimate of the performance loss in a glider
>> as a function of yaw string deflection. Does anyone know what it is?
>>
>> Ultimately, I'm wondering how much attention should be paid to deflections
>> of, say, 1/8", 1/4", 1/2", etc., on the usual 4"-5" long yaw string.
>>
>> I suppose a professionally manufactured and tested yaw string, like the Mk
>> IV, comes with a performance correction card (much like compass has), but
>> I just have a blemished one I got an outlet store - no card.
>>
>> --
>> Eric Greenwell - Washington State, USA
>> * Change "netto" to "net" to email me directly
>>
>
> Eric,
>
> I take this yaw string business seriously, though admittedly making MKIV's
> is a whole different thing than being able to fly with them straight. No
> card is provided with the MKIV, as we have no control over which side of the
> equator the MKIV is to be used. And, as I'm sure you are aware, coriolis
> effect can wreck havoc upon yaw strings if they are not properly degaussed.
>
> Without glider specific flight testing, we may still assume that in straight
> flight and with the yaw string straight back, we have unity or 100%.
> With the yaw string 90 degrees off to the side or anything forward of that,
> we have a problem, or for our purposes, 0%.
>
> By extrapolation . . . Ummm, what was the original question?
>
> bumper
> proudly churning out MKIV and QV's for the unwashed masses.

I've just read on aviationblather.com the FAA is cooking up a new NPRM
to require all MKIV yaw string installations to display a 2" square
placard within 6" of the yaw string. The placard supposedly shows a yaw
string at right angles to the canopy, with a red circle/slash over it.
Not only would this raise the cost of the MKIV, but it would reduce the
pilot's view of the sky and increase the chance of collisions. Another
example of bureaucratic meddling!

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* Sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org

[Andy[_1_]]

On Jul 13, 7:47*pm, Eric Greenwell > wrote:
> I don't recall ever seeing an estimate of the performance loss in a
> glider as a function of yaw string deflection. Does anyone know what it is?


Actually I have pondered this quite a bit. My ASW 28 has a very
annoying loud noise after being cold soaked at altitude and if flown
at greater than 90 kts. I found that slight rudder pedal pressure in
either direction would kill the noise. The higher the speed the more
pedal input is needed required and the greater the yaw string
deflection.

I have long suspected that the noise came from the rudder fairing
mylar strips. On the 28 the mylars attach to the fins skins aft of
the the fin rear spar. The skins are much thinner at the top of the
fin than in the same area of the ASW 27 and ASG 29 and on my glider
have warped outward so that there is a slight gap between the mylars
and the rudder at the top of the mylar strips. The area of warping is
above the upper rudder hinge.

I did an experiment that seems to confirm the noise is caused by a
mylar seal resonance. I attached an additional mylar seal as an
extension of the width of the existing seal and at the top 6 inches or
so on each side. The additional width and the curvature keeps the
strips in contact over full rudder deflection whereas before there was
seal separation even with the rudder centered. I found that the noise
frequency was reduced and the onset airspeed was increased.

Until I find out why the 28 uses thinner skins, and get a factory
appoval to stiffen this area, I'll have to keep flying sideways or
wear earplugs. So I need the yaw string calibration chart.

Andy
28048

[mike]

Andy,

A turbulator 2" forward of the rudder hinge line may also help.

HP sailplanes using mylar seals for the large ruddervator gap have had
some success in preventing noise from the vibrating mylars at higher
speed. On my RS-15, I gained another 15 mph or so before the mylar
began to resonate.

Mike


> I did an experiment that seems to confirm the noise is caused by a
> mylar seal resonance. *I attached an additional mylar seal as an
> extension of the width of the existing seal and at the top 6 inches or
> so on each side. *The additional width and the curvature keeps the
> strips in contact over full rudder deflection whereas before there was
> seal separation even with the rudder centered. *I found that the noise
> frequency was reduced and the onset airspeed was increased.
>
> Until I find out why the 28 uses thinner skins, and get a factory
> appoval to stiffen this area, I'll have to keep flying sideways or
> wear earplugs. *So I need the yaw string calibration chart.
>
> Andy
> 28048

[Uncle Fuzzy]

On Jul 13, 10:21*pm, "bumper" > wrote:
> "Eric Greenwell" > wrote in message
>
> ...
>
> >I don't recall ever seeing an estimate of the performance loss in a glider
> >as a function of yaw string deflection. Does anyone know what it is?
>
> > Ultimately, I'm wondering how much attention should be paid to deflections
> > of, say, 1/8", 1/4", 1/2", etc., on the usual 4"-5" long yaw string.
>
> > I suppose a professionally manufactured and tested yaw string, like the Mk
> > IV, comes with a performance correction card (much like compass has), but
> > I just have a blemished one I got an outlet store - no card.
>
> > --
> > Eric Greenwell - Washington State, USA
> > * Change "netto" to "net" to email me directly
>
> Eric,
>
> I take this yaw string business seriously, though admittedly making MKIV's
> is a whole different thing than being able to fly with them straight. No
> card is provided with the MKIV, as we have no control over which side of the
> equator the MKIV is to be used. And, as I'm sure you are aware, coriolis
> effect can wreck havoc upon yaw strings if they are not properly degaussed.
>
> Without glider specific flight testing, we may still assume that in straight
> flight and with the yaw string straight back, we have unity or 100%.
> With the yaw string 90 degrees off to the side or anything forward of that,
> we have a problem, or for our purposes, 0%.
>
> By extrapolation . . . Ummm, what was the original question?
>
> bumper
> proudly churning out MKIV and QV's for the unwashed masses.

I degaussed the MKIII, then starched it and used a 'Zero Stat' gun to
eliminate residue charges. The starch keeps it nice and straight,
drastically improving the performance of my old Speed Astir. I'm
considering ordering several dozen to place in various locations of
the airframe for further performance gains.