Tom Knauff's newsletter

At 03:26 02 July 2008, Bill Daniels wrote:

Examples of the questions include:

snip

During a steep, continuous left hand turn, how are the controls held?

A. Left Aileron, right rudder, back stick
B. Left aileron, left rudder, back stick
C. Right aileron, right rudder, back stick
D. Right aileron, left rudder, back stick


Q#2: This one needs a some qualification since it depends on the glider.

Obviously, into-the-turn aileron (left in a left turn) would never be
used

in a continuous turn so the choice is between C and D.

I find many gliders, once stabilized in a turn, will track nicely with
the

string centered with my feet off the pedals indicating no rudder is
needed

at all. Only opposite aileron (right in this case) is used to hold off
the
overbanking tendency - this right aileron provides all the left yaw
(adverse
yaw) needed to center the string. Gliders with less adverse yaw will
need

some into-the-turn rudder (Answer: D). Gliders with a lot of adverse
yaw

may need a little out-of-the-turn rudder (right) (Answer: C).

One must add that the Dick Johnson technique of using a slight slip also

works nicely and adds a little to the performance. Dick holds
out-of-the-turn rudder to oppose overbanking and keeps the stick
centered.

In my experience, this works best on gliders with generous dihedral and
without winglets or polyhedral.

snip

Bill Daniels

I notice from the back seat in a left turn that the yaw string for the
front cockpit is slightly more to the right than the one in the back; I
believe this to be because the front one is farther from the center of
lift -- or the tangent of the turning radius.

This leads me to believe that the rudder, also displaced from the center
of lift would likely align itself with its own relative wind and would
thus be slightly to the left if unattended.

I do not think of this as holding bottom rudder. I was taught to hold a
little top rudder and it might just be that a bit of pressure on the top
rudder would place it somewhere between the place it would naturally seek
and perfect alignment with the fuselage if not actually to the outside.

I do note that if the nose drifts too low in a turn it is much easier to
move it back to the proper position by applying top rudder than it is to
add more back pressure on the stick. I believe this is also much safer --
a little bit of slip instead of lowering the airspeed with the stick while
holding top aileron and bottom rudder, the classic setup for spin entry.

React, Bill?

Nyal Williams wrote:

I notice from the back seat in a left turn that the yaw string for the
front cockpit is slightly more to the right than the one in the back; I
believe this to be because the front one is farther from the center of
lift -- or the tangent of the turning radius.


I suspect not. Assuming the following: 1. on a 2 seater, the front yaw
string is about 6 ft (2m) ahead of the center of lift. 2. According to
the American Soaring handbook, a 45 degree banked turn at 60 mph (52
kts) has a radius of 240 feet (73m).

High school level geometry and trigonometry (I've been out of high
school for a looong time :-) ) shows that this results in an error of
only 1.4 degrees - small enough to be ignored for all practical purposes.

I suspect that a yaw string, typically taped to the canopy, may have
errors because the string is in the boundary layer.

Tony V.

I suspect that a yaw string, typically taped to the canopy, may have
errors because the string is in the boundary layer.

Tony V.

Closer than the OP, but still doesn't seem quite right. The front yaw
string is much more subject to 'crossflow effect' (on most ships) than
the back one due to canopy shape and the string's relative placement
on a compound curve that is sometimes splitting air striking it
(besides, both strings are in the boundary layer...)
With this in mind, it is not a bad idea to "calibrate" ones yawstring
by comparing it's position relative to a panel mounted slip-skid ball
when flying a new ship, perhaps even marking the coordinated positions
(at 45 deg banks?) with pinstripe tape if they are far off center.
Jonson also covers this in the previously mentioned article about
mildly slipping during thermalling turns (named "Circling the
Holighaus Way" http://www.owp.us/Johnson/CirclingTheHolighausWay.pdf
). He explains it much more elegantly than I and it is worth reading
if you have not already seen it.

-Paul

sisu1a wrote:

With this in mind, it is not a bad idea to "calibrate" ones yawstring
by comparing it's position relative to a panel mounted slip-skid ball....

But the 2 instruments measure different things. The yawstring (in
theory) measures airflow angle at the fuselage. The inclinometer
measures the difference between gravity and the opposite centripetal
force - when in balance, the ball is centered.

The question is, which is a better measure of coordination? Or, IOW,
which is more important? Because of the difference in drag at the
wingtips during the turn, the nose may yaw away from the center of the
turn. The yaw string would show that, while at the same time, the
inclinometer would be centered. I understand that some believe that
that's the way it should be. Others believe that you need to hold rudder
into the turn to minimize the drag.

I should experiment in my LS6. Lord knows that I'm tired of looking up
at the gaggle :-).

Tony V.

But the 2 instruments measure different things. The yawstring (in
theory) measures airflow angle at the fuselage. The inclinometer
measures the difference between gravity and the opposite centripetal
force - when in balance, the ball is centered.

True they measure different forces, but they derive appx the same info
for the pilot- status of turn coordination.

The question is, which is a better measure of coordination?

BOTH! A good ball is more accurate, but it's responses are more
damped. The string gives you a good enough approximation for gaggling
and such, but no reason not to have a ball as well (works better in
icing too...) since they are available as 'space miser' instruments as
well (screwed in under an existing instrument by sharing two bottom
holes) and don't hog panel.

Because of the difference in drag at the
wingtips during the turn, the nose may yaw away from the center of the
turn. The yaw string would show that, while at the same time, the
inclinometer would be centered.

Again, the damped response of the ball vs the string. This effect is
amplified by using a low quality ball and a high quality string ;-)
It is recommended by some (Helmut Reichman was amongst this crowd)
that believe that a normal ball in fluid is too slow for soaring, and
instead used an inverted curved glass tube with a bubble of air. He
also used a string of course...

I should experiment in my LS6. Lord knows that I'm tired of looking
up
at the gaggle :-).

We really do appreciate the warning though ;-)

-Paul

I should experiment in my LS6. Lord knows that I'm tired of looking up
at the gaggle :-).

Tony V.

Tony, I've found that my LS6 prefers to thermal with several degrees
of apparent slip as indicated by the yaw string - if I try to center
the yaw string with the rudder, I have to use much more aileron to
control the bankangle and prevent overbanking.

Once established and trimmed, hands off, she will thermal on her own
just fine, maintaining that slight slip.

Pretty much agrees with what Dick Johnson wrote.

Kirk
66

By now, most people are bored with this thread, so I expect few will
get to this post. I generally avoid posting to RAS because of the
hostile responses / postings.

In this case, the demonstrable thesis that most glider pilots do not
have the knowledge to fly safely is generally supported by the
postings to date.

There is only one bottom line to aviation knowledge, and that is the
demonstrated safety record. In our case, there are no reliable
statistics save one: the fatality rate. Glider fatalities are reported
most of the time (not all.) Simply dividing the number of fatalities
into the membership gives a fatality rate that can be compared with
other activities. The NTSB has web sites listing comparisons of
fatality rates for many activities. Gliding is the worst of all. There
will be several postings to quibble with this last statement, but it
is true if you care to look it up.

It is also true that we have been teaching people to fly for 44 years,
and no one we taught as died. Last year we had an experienced power
pilot who we transitioned to gliders who had a seizure and we did do
his transition training.

So we are doing something right. That something, is to teach
everything stated in the FARs, to the standard implied by the PTS.
Nothing more, nothing less.

When it comes to specifics, the legal system punishes those who do not
comply with the standard, so we ensure everything we teach is correct.
We have been successful and we have a lot to lose if we do it wrong.

In the early days, we enlisted famous people like Wolfgang
Langeweische and Derek Piggott to help us. Both came to our gliderport
several times. We will long remember the discussions. Many other very
experienced instructors also helped, and of course we used every
reference to establish standards still used today.

You can see the differences of opinions in the details of this thread.
If you ask specific questions on even the simplest subjects, you will
get many firmly held opinions.

So, anyway, I don’t have the time to get involved with the discussions
in this newsgroup. Most everything I know, I have written in books and
articles, and made presentations at SSA conventions, seminars and FAA
Glider CFI Revalidation Clinics, which I started many years ago.

For those of you who made it this far, I figure you deserve answers to
the three questions.

During a steep left turn, how are the controls held?

Right aileron to counteract the overbanking tendency.
Left Rudder in the direction of the turn. (A paragraph or two is
required to explain this, but the right wing in this case is creating
more drag and the tail needs to be slightly outside the arc of the
turn as the CG is exactly on the arc.)
Back stick pressure. The glider weighs more in a turn, and the center
of mass is ahead of the center of lift.

During a left turn on aerotow, the glider pilot should see the left
side of the towplane’s fuselage. The suggested analogy of sitting in
an aft car of a railroad train during a turn is a good mental image.

Finally, what color field is most desirable on an off field landing?

The answer as so often happens is – “It depends.”

But, if you had to pick one answer, it would be the color of the
earth. There is an adage stated as, “You won’t get hurt if you land it
dirt.”

However, this does not mean a freshly plowed field, which has deep
furrows, rocks and giant clods of hard earth.

You want a freshly cultivated field that has been plowed, harrowed and
the rocks have been removed for generations.

In some parts of the planet, this is not possible. It never happens,
so you must go to plan B.

Freshly harvested fields usually have one major problem. Time has gone
by and animals have dug landing gear burrows in the ground just
waiting for the next glider landing gear. Erosion can also be a
problem.

Some crops are planted in nice earth color, but on top of prepared
mounds. You must be familiar with the farming practices of your local
area to choose wisely.

In the NE USA, farmers have an adage, “Knee high by the 4th of July.”
This means corn should be this high by the 4th if the crop is to be a
good one. For glider pilots, it means you generally can no longer land
in a fresh corn field. The good news, is the alfalfa crop is being
harvested about this same time, by then animal holes are a high risk.

Well, there is a lot more to this subject and Doris is taking me to
see fireworks. I suppose I omitted something important in the above,
and I am equally sure there will be some who will blast away as that
seems to be the tenure of the medium. I probably won’t respond.

Frankly, everything you need to know to fly safely has been written. A
small library would include excellent books by Derek Piggott, Wolfgang
Langeweische, Anne & Lorne Welch and others. For those of you who
abhor self-promotion in this forum, I’ll avoid telling you my books
have everything required by the FAA flight training requirements, have
been described as “the best training manuals of any kind,” and are
easy to read and understand. I also won’t mention you can find them on
our web site: www.eglider.org

At the bottom of our web site you will find links to several flight
safety newsletters I have written over recent years. They are free!

Happy fourth of July!!!

Tom Knauff
http://www.eglider.org

[posted and mailed]

wrote:
Simply dividing the number of fatalities
into the membership gives a fatality rate that can be compared with
other activities. The NTSB has web sites listing comparisons of
fatality rates for many activities. Gliding is the worst of all.

I've done an analogous computation and your assertion above appears to
be surprisingly incorrect (or the source of my data or methodology is
highly corrupted - you decide.) By my calculations the metric you
mention actually shows aviation accidents of all types to be
approximately _over three times greater then gliding_. The single worst
accident rate in aviation appears to be helicopters, which is seven
times worse than gliding!

My own conclusion is that gliding, by your suggested metric, is actually
safer than aviation in general!

I'm afraid I can't reconcile my results with your assertion, since you
haven't provided specifics on your methodology or data. But for the
record (and critique) here's the methodology I used (numeric results are
at bullet point (7) for the impatient):

(1) I used the NTSB accident query database as my data source for
accident counts: http://www.ntsb.gov/ntsb/query.asp

(2) I used the FAA Civil Airman Statistics web page to get an estimate
of the number of all active airmen certificates, those holding rotocraft
class, and those holding glider class (Tables 1, 7, and 8 at this site):

http://www.faa.gov/data_statistics/a...atistics/2007/

(3) In the NTSB web site I selected a large enough range that a single
bad or good year wouldn't cause unexpected outliers to dominate, but not
so large as to encompass long term trends. So I selected the 8 year
range of 2000-1-1 to 2007-12-31.

(4) I elected to count only the number of fatal accident records, rather
than attempt to add up the fatalities in all the matching records. There
were simply too many accidents in the entire period to manually do the
arithmetic for this exercise (maybe some other time.) In any case, since
glider accidents rarely involve more than 1 fatality per accident, this
method actually should work against the gliders since other aviation
accidents, on average, involve more fatalities per accident.

(5) As of 2007, the FAA estimates 590,349 active airmen certificates of
all classes. It estimates 30,853 pilots with rotocraft and 29,513 with
glider. Of those 29,513 with glider, 14,955 hold nothing but glider.

(6) In the 8 year range, selecting only for "Fatal" accident severity,
then subselecting for "Category: All" I get 3407 accident records. For
"Category: Helicopter" I get 356 accident records. For "Category:
Glider" I get 46 accident records.

(7) Using the above sources and method, my results a

(A) For all aviation accidents:
(3407 fatal accidents/8 years)/590,349 pilots =

0.00072 fatal accidents/(pilot-year)

(R) For rotorcraft accidents:
(356 fatal accidents/8 years)/30,853 =

0.00144 fatal accidents/(pilot-year)

(G) For glider accidents:
(46 fatal accidents/8 years)/29,513 =

0.00020 fatal accidents/(pilot-year)

(G.1) If all the glider accidents were due only to those who could only
fly gliders:
(46 fatal accidents/8 years)/14,955 =

0.00038 fatal accidents/(pilot-year)

While my computations seem to imply gliders are actually safer than
other methods of flying, the metric is of course highly suspect.

Lastly, for the record, an Australian cross modal safety comparison
found that motorcycling was probably the least safe mode of transport:

http://www.atsb.gov.au/publications/...omparisons.pdf

(Unfortunately gliders weren't broken out into their own category.)

This is certainly what makes hand launched gliders work. For a right
handed person they are built for a left turn and thrown into the air in a
climbing right turn, which, if done correctly will round out at the top
and begin a left turn just as the extra energy is dissipated.

To build one it is necessary to put in a little left rudder and a little
right aileron to stabilize the turn. Done properly, they will fly away in
a thermal and probably not be found again.


At 22:10 04 July 2008, wrote:

snip

During a steep left turn, how are the controls held?

Right aileron to counteract the overbanking tendency.
Left Rudder in the direction of the turn. (A paragraph or two is
required to explain this, but the right wing in this case is creating
more drag and the tail needs to be slightly outside the arc of the
turn as the CG is exactly on the arc.)
Back stick pressure. The glider weighs more in a turn, and the center
of mass is ahead of the center of lift.

snip

Happy fourth of July!!!

Tom Knauff
http://www.eglider.org

OK, I don't do trig. How much difference would it make if the speed were
45kts at a 45 deg bank?

At 23:41 03 July 2008, Tony Verhulst wrote:
Nyal Williams wrote:

I notice from the back seat in a left turn that the yaw string for the
front cockpit is slightly more to the right than the one in the back;
I
believe this to be because the front one is farther from the center of
lift -- or the tangent of the turning radius.


I suspect not. Assuming the following: 1. on a 2 seater, the front yaw
string is about 6 ft (2m) ahead of the center of lift. 2. According to
the American Soaring handbook, a 45 degree banked turn at 60 mph (52
kts) has a radius of 240 feet (73m).

High school level geometry and trigonometry (I've been out of high
school for a looong time :-) ) shows that this results in an error of
only 1.4 degrees - small enough to be ignored for all practical
purposes.

I suspect that a yaw string, typically taped to the canopy, may have
errors because the string is in the boundary layer.

Tony V.