Boxing the wake cracks Pawnee tail tubings or long term fatigue?

On Wednesday, January 21, 2015 at 3:49:48 AM UTC-8, wrote:
Why teach students how to move from one undesirable out of position scenario
directly to all the other possible out of position scenarios ? When is that going
to be useful in real flying? It makes no sense to me.

The 4 corners of the box are where one will be to initiate a steering turn in high and low tow.

Flying a smooth box with a pause at each corner allows the student to practice and demonstrate mastery of flying on tow.

Drop into the wake then move to the side until the ride smooths out. Turns out it's not very far and puts minimal strain on the tow pilot's foot.

Again, boxing the wake is to be a demonstration of precise control of the sailplane on tow, it's not something to be rushed.

-Tom

At 09:43 21 January 2015, Ian Strachan wrote:
On Wednesday, 14 January 2015 15:58:40 UTC, wrote:
On Wednesday, January 14, 2015 at 9:35:43 AM UTC-5, mt wrote:
Our 235 hp Pawnee has again some cracks in the tail tubing. One of
them=
is on the bottom of the vertical member where rudder hinges are welded.
T=
he crack is between the lowest and the middle hinges.
=20
As a glider instructor and tow pilot (over 35 years) I don't see why
we=
should keep "boxing the wake", as part of glider pilot training. Having
sa=
id that, contrary to some opinions in my club, I don't believe aggressive
b=
oxing the wake is the only contributing factor in our costly maintenance
is=
sues. FYI, our Pawnee has been hangared at least in the past 20 years!
=20
What are your experiences:
1- with Pawnee tail fatigues
2- cause of repeated cracks, method of usage
3- methods of fixing them better next time
4- what contributtes to more stress on the tail tubing: hitting
rudder
=
stops in everyday operations, boxing the wake, hard landings, or rope
break=
s (weak link about 1250 lb).=20
Many Thanks,
C1
=20
Boxing the wake is essential to=20
learning how properly control a glider on tow. =20
I box the wake on virtually every tow, just to keep my skills sharp.=20

Reply from Ian Strachan, UK glider and power pilot.

I was a glider and power (civil and military) instructor for 40 years,
now
=
retired from instructing but still flying gliders and tow planes at
Lasham
=
in the UK. In my gliding career I was Chief Instructor at two UK clubs
and
=
in the military instructed on single, twin and four engined types.

When I started instructing in the 1960s there was no such thing as
"boxing
=
the wake". Before sending someone solo on aerotowing you ensured that
the
=
guy could get back to the central tow position from being out of position
l=
eft and right, up and down.

This was done by the instructor flying gradually and smoothly to the
out-of=
-position position and handing over to the student to recover to the
"centr=
al" tow position. The emphasis was on smooth, gradual and safe use of
the
=
controls, not rough and unneccessary control applications. The dangers of
o=
vercontrol leading to overshooting the central two position and the risk
of=
oscillation were discussed and demonstrated. In the UK the standard tow
p=
osition is "high tow" above the wake and part of pre-solo instruction was
t=
o show the wake and make sure that the pupil could get back into high tow
p=
osition from a wake encounter. We did not deliberately teach the "low
tow"=
position itself, at least until a guy had already been solo on aero
tow.

When the fashion started for so-called "boxing the wake", I was horrified
b=
ecause many instructors took it as a licence to be rough with the
controls
=
and IMHO took it too far. As a tug pilot I looked at what was happening
be=
hind me as a demonstration of poor airmanship and probably off-putting to
m=
any pupils. A long-term instructor often forgets that some pupils are
quite=
nervous. Pilots who have been instructors for many years need to be
remin=
ded of the basic principles such as gradual and sympathetic demonstration
a=
nd then student practice of the various skills, without taking things too
f=
ar because the instructor has lost the ability to put him/herself in the
pl=
ace of the student.

When instructing, I refused to Box the Wake but continued as I had done
bef=
ore. I don't think my aero tow students had any problem when they went
sol=
o on tow.

Clearly C1's claim that boxing is "essential", is not right. Unless you
re=
gard what I describe above as "gentle boxing".

I guess it all turns on what you mean by "boxing". =20

The problem was that once it started, there was far to much "aggressive
box=
ing" by instructors who maybe were trying to show how clever they were
(and=
annoying tow pilots like me) and had IMHO fogotten one of the basic
tenets=
of instruction which is "not to do the advanced course before the
basic",
=
or not to demonstrate running before the pupil can walk, if you see what
I
=
mean. I have seen instructors take it too far, for instance breaking the
r=
ope/weak link or even arriving just off the tug wing tip with a huge loop
i=
n the rope.

I would hope that we could agree that "aggressive boxing" during pre-solo
a=
ero tow instruction is unneccessary.=20

IMHO it is a poor instructional technique which is not necessary before
saf=
ely sending a student solo on tow. Even on post-solo check rides, I see
no=
need for it and suggest that a less "agricultural" approach should be
used=
.. An aero tow is not an aerobatic and controls should be used
gradually,
n=
ot over-used when it is not necessary.

Ian Strachan
Lasham Gliding Centre, UK

I agree completely with Ian, over my 50 years of instructing I have seen
some crazy ideas introduced and "boxing" has to be one of them. The
procedure is not required to teach recovery from out of position. I have
never yet flown with a student who did not get out of position naturally
giving the opportunity to learn the skill of recovery.
We are now starting to use motor gliders as tugs and my club uses a Rotax
Falke. The Rotax Falke is a standard Falke airframe fitted with a more
powerful engine and a "proper" undercarriage. It was never ever designed to
be used to tow other aircraft, which is of course the case for most of the
aircraft used as tugs. Who knows what stresses and strains out of position
flying puts on the rear fuselage of a Falke, I suspect we will only find
out when a failure occurs, which will be of more than academic interest to
the tug pilot.

Do you know the history of your Pawnee? Was it ever used for aerial application of agricultural chemicals? I am told many of them are hygroscopic (attract water) and so contribute heavily to rusting of the tubular frame members. Of course if yours was built and used exclusively for banner towing, that is not the problem. But do check to see what its history was.

On Wednesday, January 14, 2015 at 9:35:43 AM UTC-5, mt wrote:
Our 235 hp Pawnee has again some cracks in the tail tubing. One of them is on the bottom of the vertical member where rudder hinges are welded. The crack is between the lowest and the middle hinges.

As a glider instructor and tow pilot (over 35 years) I don't see why we should keep "boxing the wake", as part of glider pilot training. Having said that, contrary to some opinions in my club, I don't believe aggressive boxing the wake is the only contributing factor in our costly maintenance issues. FYI, our Pawnee has been hangared at least in the past 20 years!

What are your experiences:
1- with Pawnee tail fatigues
2- cause of repeated cracks, method of usage
3- methods of fixing them better next time
4- what contributtes to more stress on the tail tubing: hitting rudder stops in everyday operations, boxing the wake, hard landings, or rope breaks (weak link about 1250 lb).
Many Thanks,
C1

Thanks for sharing this info. I imagine these cracks are present on many other Pawnee glider tugs without the knowledge of the operators...

I'm a little confused about the location of one of the cracks. For the crack that's between the bottom and middle rudder hinges, is the crack next to the bottom hinge weld or directly in the middle of the tube between the two hinges? Note, I'm assuming it's in the vertical stab tube that the hinges attach to, correct me if I'm wrong...

Also, is the crack opening facing aft, port, starboard, etc?

How did you guys discover the crack? I'm hoping it's easy to spot even with the covering on.

Where are some of the other cracks too? I'm getting at the ones not near the rudder hinges.

We don't have these cracks as far as we know. Although, we've seen failure in tailwheel bolts. One of them being the bolt that goes through the leaf spring. Another was nearer to where the leaf spring attaches to the fuselage. I'm having a hard time picturing the exact bolts at the moment...

P.S. I think it's unlikely that loads on the tow hook are contributing to the crack considering how the hook attaches to the fuselage and how small tow loads are when compared to other loads, like you've mentioned. Unless the wake boxing is done very aggressively... Worst case scenario is maxing out the weak link to 2500lb with a 30deg rope angle causes a 1250lb side load at the hook which should not happen during normal boxing... Most of that is quickly absorbed by rotational inertia and the sideslip angle quickly reducing from that maximum value. For a more reasonable steady state case of a 15/1 L/D and 1250lb glider banking 15deg and a 20deg rope angle would give about 354lbs.
1250/15*tan(20deg) +1250*sin(15deg)

Aggressive or sloppy? Some students are barely noticeable when boxing
the wake while others yank the tail around so hard they exceed the tug's
rudder (and elevator) authority. Of course, they're the same ones who
swing high and wide on every turn. What CFIs need to teach is that
minor corrections will get the glider into position much more quickly
than using large control inputs.


On 1/14/2015 10:15 AM, Andrew wrote:
On Wednesday, January 14, 2015 at 9:35:43 AM UTC-5, mt wrote:
Our 235 hp Pawnee has again some cracks in the tail tubing. One of them is on the bottom of the vertical member where rudder hinges are welded. The crack is between the lowest and the middle hinges.

As a glider instructor and tow pilot (over 35 years) I don't see why we should keep "boxing the wake", as part of glider pilot training. Having said that, contrary to some opinions in my club, I don't believe aggressive boxing the wake is the only contributing factor in our costly maintenance issues. FYI, our Pawnee has been hangared at least in the past 20 years!

What are your experiences:
1- with Pawnee tail fatigues
2- cause of repeated cracks, method of usage
3- methods of fixing them better next time
4- what contributtes to more stress on the tail tubing: hitting rudder stops in everyday operations, boxing the wake, hard landings, or rope breaks (weak link about 1250 lb).
Many Thanks,
C1
Thanks for sharing this info. I imagine these cracks are present on many other Pawnee glider tugs without the knowledge of the operators...

I'm a little confused about the location of one of the cracks. For the crack that's between the bottom and middle rudder hinges, is the crack next to the bottom hinge weld or directly in the middle of the tube between the two hinges? Note, I'm assuming it's in the vertical stab tube that the hinges attach to, correct me if I'm wrong...

Also, is the crack opening facing aft, port, starboard, etc?

How did you guys discover the crack? I'm hoping it's easy to spot even with the covering on.

Where are some of the other cracks too? I'm getting at the ones not near the rudder hinges.

We don't have these cracks as far as we know. Although, we've seen failure in tailwheel bolts. One of them being the bolt that goes through the leaf spring. Another was nearer to where the leaf spring attaches to the fuselage. I'm having a hard time picturing the exact bolts at the moment...

P.S. I think it's unlikely that loads on the tow hook are contributing to the crack considering how the hook attaches to the fuselage and how small tow loads are when compared to other loads, like you've mentioned. Unless the wake boxing is done very aggressively... Worst case scenario is maxing out the weak link to 2500lb with a 30deg rope angle causes a 1250lb side load at the hook which should not happen during normal boxing... Most of that is quickly absorbed by rotational inertia and the sideslip angle quickly reducing from that maximum value. For a more reasonable steady state case of a 15/1 L/D and 1250lb glider banking 15deg and a 20deg rope angle would give about 354lbs.
1250/15*tan(20deg) +1250*sin(15deg)

--
Dan Marotta

teach them to hold the stick 6-8" below the top, and explain to them what "milking the mouse" means

On Wednesday, January 14, 2015 at 12:32:47 PM UTC-5, Dan Marotta wrote:
Aggressive or sloppy?* Some students are barely noticeable when
boxing the wake while others yank the tail around so hard they
exceed the tug's rudder (and elevator) authority.* Of course,
they're the same ones who swing high and wide on every turn.* What
CFIs need to teach is that minor corrections will get the glider
into position much more quickly than using large control inputs.






On 1/14/2015 10:15 AM, Andrew wrote:



On Wednesday, January 14, 2015 at 9:35:43 AM UTC-5, mt wrote:


Our 235 hp Pawnee has again some cracks in the tail tubing. One of them is on the bottom of the vertical member where rudder hinges are welded. The crack is between the lowest and the middle hinges.

As a glider instructor and tow pilot (over 35 years) I don't see why we should keep "boxing the wake", as part of glider pilot training. Having said that, contrary to some opinions in my club, I don't believe aggressive boxing the wake is the only contributing factor in our costly maintenance issues. FYI, our Pawnee has been hangared at least in the past 20 years!

What are your experiences:
1- with Pawnee tail fatigues
2- cause of repeated cracks, method of usage
3- methods of fixing them better next time
4- what contributtes to more stress on the tail tubing: hitting rudder stops in everyday operations, boxing the wake, hard landings, or rope breaks (weak link about 1250 lb).
Many Thanks,
C1


Thanks for sharing this info. I imagine these cracks are present on many other Pawnee glider tugs without the knowledge of the operators...

I'm a little confused about the location of one of the cracks. For the crack that's between the bottom and middle rudder hinges, is the crack next to the bottom hinge weld or directly in the middle of the tube between the two hinges? Note, I'm assuming it's in the vertical stab tube that the hinges attach to, correct me if I'm wrong...

Also, is the crack opening facing aft, port, starboard, etc?

How did you guys discover the crack? I'm hoping it's easy to spot even with the covering on.

Where are some of the other cracks too? I'm getting at the ones not near the rudder hinges.

We don't have these cracks as far as we know. Although, we've seen failure in tailwheel bolts. One of them being the bolt that goes through the leaf spring. Another was nearer to where the leaf spring attaches to the fuselage. I'm having a hard time picturing the exact bolts at the moment...

P.S. I think it's unlikely that loads on the tow hook are contributing to the crack considering how the hook attaches to the fuselage and how small tow loads are when compared to other loads, like you've mentioned. Unless the wake boxing is done very aggressively... Worst case scenario is maxing out the weak link to 2500lb with a 30deg rope angle causes a 1250lb side load at the hook which should not happen during normal boxing... Most of that is quickly absorbed by rotational inertia and the sideslip angle quickly reducing from that maximum value. For a more reasonable steady state case of a 15/1 L/D and 1250lb glider banking 15deg and a 20deg rope angle would give about 354lbs.
1250/15*tan(20deg) +1250*sin(15deg)





--

Dan Marotta

On Wednesday, January 14, 2015 at 10:15:02 AM UTC-7, Andrew wrote:

Thanks for sharing this info. I imagine these cracks are present on many other Pawnee glider tugs without the knowledge of the operators...

No doubt. These are VERY old airplanes and maintenance requirements tend to rise exponentially with age. All steel tube airframes suffer rust, corrosion and fatigue with the lowest areas suffering most. If your AI doesn't know to check this, get another AI.

I once saw an old Piper (PA-20, I IIRC) siting in the weeds with its nose pointing higher than it should. It appeared the tubes forward of the tail wheel had failed from the weight of the parked airplane.

Ueli,

slipping is not part of the instruction syllabus in Switzerland, except for FCL aerobatics.
However, it is still a very useful skill and could have saved the odd DuoDiscus during outlandings.

On Wednesday, January 14, 2015 at 9:35:43 AM UTC-5, mt wrote:
Our 235 hp Pawnee has again some cracks in the tail tubing. One of them is on the bottom of the vertical member where rudder hinges are welded. The crack is between the lowest and the middle hinges.

As a glider instructor and tow pilot (over 35 years) I don't see why we should keep "boxing the wake", as part of glider pilot training. Having said that, contrary to some opinions in my club, I don't believe aggressive boxing the wake is the only contributing factor in our costly maintenance issues. FYI, our Pawnee has been hangared at least in the past 20 years!

What are your experiences:
1- with Pawnee tail fatigues
2- cause of repeated cracks, method of usage
3- methods of fixing them better next time
4- what contributtes to more stress on the tail tubing: hitting rudder stops in everyday operations, boxing the wake, hard landings, or rope breaks (weak link about 1250 lb).
Many Thanks,
C1

I highly doubt that boxing the wake has anything to do with the problem you are describing. The rope loads are not that high as to cause additional load.
I'm not sure why one would ever bang the rudder hard against the stops. Seems like a poor technique.
The most likely cause, in my experience would be the beating the structure takes doing 7-8 landings an hour.
Rather than repair, it may pay to replace the rudder post with new.
Boxing the wake is a useful training exercise and should not be reduced or eliminated because of a tail post issue.
42 years instructor and tow pilot.
UH

On Wednesday, January 14, 2015 at 12:36:01 PM UTC-5, wrote:
On Wednesday, January 14, 2015 at 9:35:43 AM UTC-5, mt wrote:
Our 235 hp Pawnee has again some cracks in the tail tubing. One of them is on the bottom of the vertical member where rudder hinges are welded. The crack is between the lowest and the middle hinges.

As a glider instructor and tow pilot (over 35 years) I don't see why we should keep "boxing the wake", as part of glider pilot training. Having said that, contrary to some opinions in my club, I don't believe aggressive boxing the wake is the only contributing factor in our costly maintenance issues. FYI, our Pawnee has been hangared at least in the past 20 years!

What are your experiences:
1- with Pawnee tail fatigues
2- cause of repeated cracks, method of usage
3- methods of fixing them better next time
4- what contributtes to more stress on the tail tubing: hitting rudder stops in everyday operations, boxing the wake, hard landings, or rope breaks (weak link about 1250 lb).
Many Thanks,
C1

I highly doubt that boxing the wake has anything to do with the problem you are describing. The rope loads are not that high as to cause additional load.
I'm not sure why one would ever bang the rudder hard against the stops. Seems like a poor technique.
The most likely cause, in my experience would be the beating the structure takes doing 7-8 landings an hour.
Rather than repair, it may pay to replace the rudder post with new.
Boxing the wake is a useful training exercise and should not be reduced or eliminated because of a tail post issue.
42 years instructor and tow pilot.
UH

I agree, I can't see very high loads from "boxing the wake" as you will turn the towplane 1st.
I CAN see the abuse from the tail hitting the ground (on landing), rough surface banging the tailwheel, tight "lock" on the tailwheel putting bending/twisting loads when turning on the ground, etc.

As to whether or not to teach "boxing the wake", I also think it's worthwhile to do. Better in "training" than "learn as you go" if you get out of position.