steel priming/painting state of the art?
Steven Estergreen
fuselage and steel wing fittings for a plans-built group project. None
of us have built a plane of this type before (Christavia MkI tube, wood
and fabric). What is the "state of the art" for corrosion-proofing?
Assume we'll cover with polyester fabric, finish material TBD. What will
keep the fuselage and fittings free from corrosion in Oregon, allow use
of any popular fabric finish, be light weight, allow inspection of
weldments for cracks, look good, etc.?
Bruce A. Frank
For the outside of the tube: sand blast, zinc rich wash followed by
urethane powder coating.
For the inside of the tube: Linseed oil, boiled or raw.(installed after
the powder coating).
For the wood wings: Sprayed two part clear urethane paint.
For the fabric: Polyesther coated with either Stitts (Polyfiber) or
(perfectly good also) Randolph non-tauting dope---there may be others
that are equal but few, if any, finishes that are any better.(Not trying
to start a fight here- you can choose a water based system, a urethane
based system, or an acrylic based system-- the good ones are as-good-as,
the bad ones will have you crying in a few years)
THere are other choices but, IMHO this IS state of the art.
--
Bruce A. Frank, Editor "Ford 3.8/4.2L Engine and V-6 STOL
BAF...@worldnet.att.net Homebuilt Aircraft Newsletter"
| Publishing interesting material|
| on all aspects of alternative |
| engines and homebuilt aircraft.|
*------------------------------**----*
\(-o-)/ AIRCRAFT PROJECTS CO.
\___/ Manufacturing parts & pieces
/ \ for homebuilt aircraft,
0 0 TIG welding
While trying to find the time to finish mine.
jkahn
>
> We're nearing the rust protection stage for the welded steel tube
> fuselage and steel wing fittings for a plans-built group project. None
> of us have built a plane of this type before (Christavia MkI tube, wood
> and fabric). What is the "state of the art" for corrosion-proofing?
> Assume we'll cover with polyester fabric, finish material TBD. What will
> keep the fuselage and fittings free from corrosion in Oregon, allow use
> of any popular fabric finish, be light weight, allow inspection of
> weldments for cracks, look good, etc.?
Sandblast.
Linseed oil for the tube. Do not use a brass screws to plug
the holes. Make a steel ones so that are compatible with the
steel tubing. Corrosion sometimes happens because of dissimilar
metal corrosion where the brass plug is installed at the bottom
of the rudder post, which lets oxygen into the interior of the
tubing. One of the oldtimer gurus that staffs the welding
shelter at Oshkosh talked about this. His point was that if
all potential for oxygen access to the tubing is eliminated,
you will not get significant corrosion because there is not
enough free oxygen in the tubing to do much damage. Then the
linseed oil is just insurance.
If you really want to get fancy, you can pressurize the fueselage with
nitrogen through a schrader valve on the end of the longeron and
have a pressure gauge that warns of a crack by the drop in
pressure that would occur. Aerospatiale helicopters do this.
For the outside, epoxy zinc chromate primer, with polyurethane
colour coat in visible areas. This is what transport aircraft
use. You can even apply the primer with one of those fine
sable hair artist's brushes (a one inch one or so) and get a
reasonable finish and save a LOT of epoxy by not painting
the surrounding floors and walls as well. (time consuming, but
we all have more time than money don't we). Epoxy primer is
very hard.
Anyway, that's what I'm planning for my Pegazair.
For the wood, Stits makes an epoxy varnish for wood aircraft.
Read up on the wooden boat business. Epoxy varnish is king.
Wooden structures finished in epoxy varnish last indefinitely.
JohnK
Bombardier Aerospace
Regional Aircraft
David MacKenzie
>If you really want to get fancy, you can pressurize the fueselage with
>nitrogen through a schrader valve on the end of the longeron and
>have a pressure gauge that warns of a crack by the drop in
>pressure that would occur. Aerospatiale helicopters do this.
Didn't Zlin do this for spars on aerobatic aircraft? Also, ISTR
Porsche doing something similar for 917's back in the '70s.
>Anyway, that's what I'm planning for my Pegazair.
Neat, but will it tow a glider? Have you started building yet?
--
mac
Bill Berle
>
> Steven Estergreen wrote:
> >
> > We're nearing the rust protection stage for the welded steel tube
> > fuselage and steel wing fittings for a plans-built group project. None
> > of us have built a plane of this type before (Christavia MkI tube, wood
> > and fabric). What is the "state of the art" for corrosion-proofing?
> > Assume we'll cover with polyester fabric, finish material TBD. What will
> > keep the fuselage and fittings free from corrosion in Oregon, allow use
> > of any popular fabric finish, be light weight, allow inspection of
> > weldments for cracks, look good, etc.?
I attended the "Cub Doctor" seminar with Clyde Smith, for older Piper
aircraft. By the way, this seminar is worth five times the price I paid,
and should be mandatory for all tube and fabric airplane owners. He
showed us an old 1940 film of the Piper aircraft factory. They were
building Cubs and Cub Coupes. The method of applying the zinc chromate
primer to the fuselages was a garden hose! I thought this was really
awful until I realized the following:
The garden hose squirted the zinc primer into ALL the little crevasses
and clusters where spray would not have reached.
There was no toxic mist to inhale.
There was no OSHA or EPA environment spray booth to set up, and nothing
more than a drain on the floor to catch the runoff and probbaly recycle
it.
The Cubs (and other similar airplanes) have lasted 50+ years with a
remarkable record of servicability compared to any other machinery.
I know there is some great powder coating stuff out there, and some nice
shiny coatings to spend your money on. However, if I were building a
steel tube airplane, I would use a 1" epoxy brush to manually force the
2 part epoxy primer into the joints and brush it onto the tubes, drips
and all. Then I would perhaps "detail spray" another coat matching the
exterior on the cockpit area tubes.
If you are _very_ worried about water and corrosion, make it so the
fabric does not wrap around the lower longerons directly across the
bottom of the structure. If you can add something that makes the fabric
hang about an inch or two below the lower longerons, then trapped water,
wet grass, mouse shit, and birds nests will be below the steel tubes.
If the plane will sit outside, the Stits "Aerothane" final paint will
last longer than most anything. You should make the effort to get the
plane under a hangar or shade/rain cover if you can, otherwise you will
also definitely need to invest in a good set of aircraft covers. Having
a place to work on the airplane, keep your stuff, keep out critters and
vandals is very well worth the money to me.
Bill Berle
Steven Estergreen
other than a cleanroom environment and with common painting equipment
(brushes, compressor-driven spray guns, etc.). Powder coat doesn't fit, but I
appreciate the other suggestions.
Bruce A. Frank wrote:
> Steven Estergreen wrote:
> >
> > We're nearing the rust protection stage for the welded steel tube
> > fuselage and steel wing fittings for a plans-built group project. None
> > of us have built a plane of this type before (Christavia MkI tube, wood
> > and fabric). What is the "state of the art" for corrosion-proofing?
> > Assume we'll cover with polyester fabric, finish material TBD. What will
> > keep the fuselage and fittings free from corrosion in Oregon, allow use
> > of any popular fabric finish, be light weight, allow inspection of
> > weldments for cracks, look good, etc.?
>
Bruce A. Frank
>
> Thanks. I see I missed one criterion: application by amateurs in something
> other than a cleanroom environment and with common painting equipment
> (brushes, compressor-driven spray guns, etc.). Powder coat doesn't fit, but I
> appreciate the other suggestions.
Steve,
I have all the equipment to spray paint my fuselage; a five horse
compressor with a good quality Binks spray gun. I also have a reasonably
good HVLP spray system to cut down on the overspray if I want to go that
way. BUT, even though I have a small sandblast unit it is not practical
to sandblast it myself.
A local sandblaster quoted me $300 to $400(D.I.Y. with rented equip runs
about the same) to sandblast my V-6 STOL fuselage. The primer, paint and
other prep materials needed will run close to $250. A local powder
coater in San Jose quoted me $750 total for sandblast and urethane
powder coat. I have done the full process before; sandblast(with a good
commercial power blast unit with a fresh air mask and 700 lbs of 00
sand), prime and shoot 2 to 3 coats of two part urethane. All of which
required over 25 hours of my time invested (I will not ever sandblast my
own fuselage again). The powder coat is the only option I see as viable
for me.
Charles K. Scott
jkahn <jk...@mail2.planeteer.com> writes:
> If you really want to get fancy, you can pressurize the fueselage with
> nitrogen through a schrader valve on the end of the longeron and
> have a pressure gauge that warns of a crack by the drop in
> pressure that would occur. Aerospatiale helicopters do this.
I've heard of this being done but wonder about the consequences. One
sage retorically asked; "let's suppose you do this and successfully
pressurize the entire tube fuselage. You paint the thing and cover it
with fabric and assemble the airplane and go flying. A year later you
notice the guage has dropped off to zero . . . what do you do now? Do
you strip down the airplane and strip off all the fabric and
re-pressurize the frame and spray it with soapy water to find the tiny
leak?"
Man that sounds like a lot of work. I think I'd rather just use the
latest tubing interior coating and forget about it till I'm too old to
fly it anymore. ;-)
Corky Scott
highflyer
>
> We're nearing the rust protection stage for the welded steel tube
> fuselage and steel wing fittings for a plans-built group project. None
> of us have built a plane of this type before (Christavia MkI tube, wood
> and fabric). What is the "state of the art" for corrosion-proofing?
> Assume we'll cover with polyester fabric, finish material TBD. What will
> keep the fuselage and fittings free from corrosion in Oregon, allow use
> of any popular fabric finish, be light weight, allow inspection of
> weldments for cracks, look good, etc.?
My suggestion is a two part system.
1. Treat the INSIDE of the tubing with a flush coat of RAW linseed
oil. It is available in gallon cans at your local paint store,
around ten or fifteen dollars a gallon, last time I checked. A
gallon will do several aircraft. I usually expect to leave
about a quart inside the tubing.
2. Clean the outside of all of the tubing with a potent solvent,
such as MEK or one of the more environmentally friendly, but
less effective replacement solvents. No oil or finger prints
on the tubes.
Coat the outside of all tubes evenly and thoroughly with a good
two part epoxy zinc chromate primer, such as the one from Stitts
or Randolph. A smooth even coat using the light colored primer
makes crack detection easier.
Of course, weld normalization as the fuselage is assembled will
go far to make weld cracks an unlikely eventuality. Most all of
the cracked welds I have seen were on aircraft that were welded
electrically and not normalized afterwards.
An alternative outside system that is excellent, is a "powder coat"
process with an epoxy bases primer with a chromate component. That
is a little more expensive to apply and requires a power coater in
you neighborhood with a large oven. We have one here in southern
Illinois who can do small fuselages, and we have seen them do things
like engine mounts and gear legs. A good light colored power coat
finish is baked on and almost translucent and crack detection is as
easy as on an uncoated part.
HF
highflyer
>
> Sandblast.
>
> Linseed oil for the tube. Do not use a brass screws to plug
> the holes. Make a steel ones so that are compatible with the
> steel tubing. Corrosion sometimes happens because of dissimilar
> metal corrosion where the brass plug is installed at the bottom
> of the rudder post, which lets oxygen into the interior of the
> tubing. One of the oldtimer gurus that staffs the welding
> shelter at Oshkosh talked about this. His point was that if
> all potential for oxygen access to the tubing is eliminated,
> you will not get significant corrosion because there is not
> enough free oxygen in the tubing to do much damage. Then the
> linseed oil is just insurance.
>
that I used to introduce the linseed oil to the inside of the
tubing. I then touch up the weld with a bit of epoxy primer and
leakage or disimilar metal corrosion is never a problem. The
linseed oil flows over the spot where the weld dried off the metal
inside the tubing soon and maintains the corrosion protection inside.
> If you really want to get fancy, you can pressurize the fueselage with
> nitrogen through a schrader valve on the end of the longeron and
> have a pressure gauge that warns of a crack by the drop in
> pressure that would occur. Aerospatiale helicopters do this.
>
Filling the tubing with nitrogen also slows the oxidation of the oil.
However, many fuselages are NOT gas tight and weld pinholes can allow
the pressure to bleed off, even though the weld is structurally sound.
The linseed oil does help to block these pinholes, since it tends to
oxidize and harden when exposed to air. If you welding is of a high
standard and you do not have any pinholes, this technique is an
excellent way to check the entire fuselage structure for cracks.
It is not usually used on something like a ChristAvia, which is
built a bit on the "stout" side to start with, but can be invaluable
on a competition aerobatic machine.
> For the outside, epoxy zinc chromate primer, with polyurethane
> colour coat in visible areas. This is what transport aircraft
> use. You can even apply the primer with one of those fine
> sable hair artist's brushes (a one inch one or so) and get a
> reasonable finish and save a LOT of epoxy by not painting
> the surrounding floors and walls as well. (time consuming, but
> we all have more time than money don't we). Epoxy primer is
> very hard.
>
> Anyway, that's what I'm planning for my Pegazair.
>
> For the wood, Stits makes an epoxy varnish for wood aircraft.
> Read up on the wooden boat business. Epoxy varnish is king.
> Wooden structures finished in epoxy varnish last indefinitely.
>
The epoxy varnishs are excellent for the wood and are generally
"dope proof" so that you do not have to worry about the dope
dissolving the varnish. A good grade of Urethane varnish also
works well. I have used urethane varnishs on my wooden airplane
structures. Make sure that all metal fittings are bedded in a
thick coat of varnish or some other material to isolate the metal
from the wood and prevent air and moisture from trapping under
the metal. Fittings bolted on without a good bedding will corrode
underneath and also rot the wood away around the bolts. That is
also why it is so important to thoroughly finish the INSIDE of
bolt holes. Important bolts in wood should usually be surrounded
with a phenolic bushing of the correct length to keep the bolt
from contacting the wood and keep it from crushing the wood
fibers when it is tightened.
Good ventilation of all structure is absolutely required. I usually
try to use "seaplane" grommets on the underside of the wings and
the fuselage. Their little builtin fairing tends to positively
draw air through the grommet in the air, and it also tends to keep
fluids splashed up from a wet runway from making their way inside.
Always check all grommets on inspections to be sure they are not
plugged with debris and will easily flow air.
There are a lot of now fabric finishs available now for polyester
fabric. Over the years I have found two that go on easily and give
a superior job that is durable and attractive. These are the
Stitts Process and the Ceconite Process. Both processes specify
exactly what to use and how to apply part of the finish and each
step of the process. DO NOT MODIFY THESE PROCESSES. These exact
processes are known to work. Every time I have seen the process
modified in some way, to save a few dollars, or to get a shinier
finish, the result has been a finish that fails disasterously after
a relatively short time. On the other hand, I have seen both of
these finishs las for over thirty years when they were properly
applied according to the STC directions and specifications, even
though the airplanes were flown regularly and parked outside.
Understand that is not to give a guarantee of thirty years for
these finishs, or even to say that a thirty year cover life is
even GOOD. Personally, I like to take a look at the structure
at least every twenty or twentyfive years! :-) My own airplane
was recovered using the Ceconite process with dopes from Certified
Coatings in 1991. The finish shows no signs of aging and no finish
problems are apparent. Most of this time, but not all of this time,
it was hangared. I do have an awful time finding hangars with a
door wide enough to get my wings inside! I finally had to build a
hangar with fifty foot doors.
HF
highflyer
>
> In article <36B373...@mail2.planeteer.com>
> jkahn <jk...@mail2.planeteer.com> writes:
>
> > nitrogen through a schrader valve on the end of the longeron and
> > have a pressure gauge that warns of a crack by the drop in
> > pressure that would occur. Aerospatiale helicopters do this.
>
> sage retorically asked; "let's suppose you do this and successfully
> pressurize the entire tube fuselage. You paint the thing and cover it
> with fabric and assemble the airplane and go flying. A year later you
> notice the guage has dropped off to zero . . . what do you do now? Do
> you strip down the airplane and strip off all the fabric and
> re-pressurize the frame and spray it with soapy water to find the tiny
> leak?"
>
> Man that sounds like a lot of work. I think I'd rather just use the
> latest tubing interior coating and forget about it till I'm too old to
> fly it anymore. ;-)
>
> Corky Scott
That is a definate problem. When you use some conplicated system to
warn you there may be a crack in the fuselage somewhere and it TELLS
you it found something what do you do. It is either a false alarm
and your Schraeder valve leaked ( never happens :-) or it is a real
alarm and you have a crack or a pinhole somewhere.
Suppose it is a real pinhole! It really isn't a problem, and the only
real problem you have is that the fuselage won't hold pressure. It
has not lost any strength. Real alarm but no real problem.
Suppose it is a real broken tube! Now there IS a real problem and
you had BEST find it, whereever it is, and fix it before you kill
yourself! In this case, any effort, such as removing all of your
nice new fabric to fix the problem, is not inappropriate.
Which of these possibilities is the real one? Well ... Hmmm. I
think some kind of investigation to determine the actual cause is
not inappropriate. Is the whole excercise with the pressurized
nitrogen worthwhile? Maybe. Likely for the Aerospatiale or for
an unlimited aerobatics contender. Not so likely for a puddle
jumper that is not normally subjected to extreme forces or loads.
Will a standard visual inspection detect a cracked or broken fuselage
tube? Probably.
You pays your money and you takes your chance!
HF
jkahn
Well I was about to concede defeat and admit that maybe that wasn't
such a good idea, then I thought... waitaminnit... if the pressure
in the gauge is down, I just hook up some 80lb shop air to the
schrader valve and go over the airplane with my *ear* listening
for the telltale hiss. HA! If you get a hiss loud enough to hear
with your ear around the outside of the fuselage, you have a crack.
Now, you'd need a hangar that's REAL quiet to do it... shhhhhhhhhh...
I know, so maybe it's overkill...
JohnK
jkahn
>. Clean the outside of all of the tubing with a potent solvent,
> such as MEK or one of the more environmentally friendly, but
> less effective replacement solvents. No oil or finger prints
> on the tubes.
>
> Coat the outside of all tubes evenly and thoroughly with a good
> two part epoxy zinc chromate primer, such as the one from Stitts
> or Randolph. A smooth even coat using the light colored primer
> makes crack detection easier.
>
> Of course, weld normalization as the fuselage is assembled will
> go far to make weld cracks an unlikely eventuality. Most all of
> the cracked welds I have seen were on aircraft that were welded
> electrically and not normalized afterwards.
>
> An alternative outside system that is excellent, is a "powder coat"
> process with an epoxy bases primer with a chromate component. That
> is a little more expensive to apply and requires a power coater in
> you neighborhood with a large oven. We have one here in southern
> Illinois who can do small fuselages, and we have seen them do things
> like engine mounts and gear legs. A good light colored power coat
> finish is baked on and almost translucent and crack detection is as
> easy as on an uncoated part.
>
> HF
Are there any practical alternatives to sandblasting for a fuselage
with light amounts of rust?
JohnK
James Thursby
I used a Poly-fiber product called tube seal. Poly-fiber claims it has all
the corrosion protection qualities of linseed oil but adds a "creeping"
element, and will seek out voids and fill them over time, congealing in
them. I used this in my Spacewalker 2 fuselage. As a test, I coated a small
sheet of 4130 with it and nailed it to my fence in 1995. I live in Florida
and this thing is showing no signs of rusting yet. Not very scientific, but
it works.
Jim Thursby
Europa N814AT
jkahn wrote in message <36B90A...@mail2.planeteer.com>...
dion.m...@pobox.com
jk...@mail2.planeteer.com wrote:
...
> Well I was about to concede defeat and admit that maybe that wasn't
> such a good idea, then I thought... waitaminnit... if the pressure
> in the gauge is down, I just hook up some 80lb shop air to the
> schrader valve and go over the airplane with my *ear* listening
> for the telltale hiss. HA! If you get a hiss loud enough to hear
> with your ear around the outside of the fuselage, you have a crack.
> Now, you'd need a hangar that's REAL quiet to do it... shhhhhhhhhh...
>
> I know, so maybe it's overkill...
>
> JohnK
>
I was contemplating making a snide and nasty suggestion (I often contemplate
them, but rarely make them) when I realized that it just might work. Suppose
you do as above, but instead of 80 lb. shop air, you use a gas for which a
detector is readily available, like carbon monoxide. It doesn't support
combustion, so it won't increase corrosion. Use a standard CO detector to
find the leak, fix it and repressurixe with N2. Of course you would want to
use an outside air source, just like when painting.
Now all we need is a good reason to go to so much trouble.
Dion
-----------== Posted via Deja News, The Discussion Network ==----------
http://www.dejanews.com/ Search, Read, Discuss, or Start Your Own
JStricker
Use R134. You can even get it in 12 oz cans and a leak would show up with a
detector pretty quick.
John Stricker
--
Remove the "nosp..........." Oh hell, you folks know what to do and
why I had to put it in. If one of you real humans wants to contact me:
"I didn't spend all these years getting to the top of the food chain
just to become a vegetarian"
dion.m...@pobox.com wrote in message
<79cg2q$n6a$1...@nnrp1.dejanews.com>...
highflyer
> > HF
>
> Well I was about to concede defeat and admit that maybe that wasn't
> such a good idea, then I thought... waitaminnit... if the pressure
> in the gauge is down, I just hook up some 80lb shop air to the
> schrader valve and go over the airplane with my *ear* listening
> for the telltale hiss. HA! If you get a hiss loud enough to hear
> with your ear around the outside of the fuselage, you have a crack.
> Now, you'd need a hangar that's REAL quiet to do it... shhhhhhhhhh...
>
> I know, so maybe it's overkill...
>
> JohnK
Spray some fluorescent ink into the intake of the air compressor
and turn on the black lights in your hangar! The glowing spot is
where the crack, pinhole, or whatever, has to be! Of course, it is
easy enough to check the Schrader Valve for leaks before you get
too carried away. :-)
HF
highflyer
Sure. I often use sandpaper and elbow grease. Use a fine grit wet
or dry paper. It doesn't hurt to carry a squirt can of WD40 or some
other light oil to lubricate the sandpaper and carry off the rust
removed. Then clean well with solvent and prime. That also works
well for spot repairs when light surface rust is localized.
Sandblasting is a better choice though if time is critical and if a
whole fuselage is to be done.
You can get spot sandblasters for doing small areas inside a cluster
where sand paper doesn't work well.
HF
jkahn
>
> In article <36B909...@mail2.planeteer.com>,
> jk...@mail2.planeteer.com wrote:
> ...
> > such a good idea, then I thought... waitaminnit... if the pressure
> > in the gauge is down, I just hook up some 80lb shop air to the
> > schrader valve and go over the airplane with my *ear* listening
> > for the telltale hiss. HA! If you get a hiss loud enough to hear
> > with your ear around the outside of the fuselage, you have a crack.
> > Now, you'd need a hangar that's REAL quiet to do it... shhhhhhhhhh...
> >
> > I know, so maybe it's overkill...
> >
> > JohnK
> >
>
> them, but rarely make them) when I realized that it just might work. Suppose
> you do as above, but instead of 80 lb. shop air, you use a gas for which a
> detector is readily available, like carbon monoxide. It doesn't support
> combustion, so it won't increase corrosion. Use a standard CO detector to
> find the leak, fix it and repressurixe with N2. Of course you would want to
> use an outside air source, just like when painting.
>
> Now all we need is a good reason to go to so much trouble.
>
> Dion
>
> -----------== Posted via Deja News, The Discussion Network ==----------
> http://www.dejanews.com/ Search, Read, Discuss, or Start Your Own
I've modified the idea a little, so it will be very little trouble.
All you're doing is putting a schrader valve in the end of the longeron.
About 10 grams maybe. No silly gases or anything.
When the fuselage is first welded I'd pressure test it then to find
pinholes, maybe gaps in the welding and reweld those.
Once it was established that it was reasonably airtight, or even
if it wasn't, at least establish
a leak down rate. Then I would simply hook up shop air at each
annual and see if it leaks down substantially more quickly than it
should. If it did there would be an obvious problem, which you
could locate by ear and soap bottle.
An easy way to detect cracks, for the cost of a tire valve.
What could be simpler?
JohnK
highflyer
Now you are getting somewhere. Cheap, easy, and a first rate check
at annual time, or after any hard landing! Ain't it amazing what
comes out when you get a bunch of idiots brainstorming together?
HF
Charles K. Scott
jkahn <jk...@mail2.planeteer.com> writes:
> Once it was established that it was reasonably airtight, or even
> if it wasn't, at least establish
> a leak down rate. Then I would simply hook up shop air at each
> annual and see if it leaks down substantially more quickly than it
> should. If it did there would be an obvious problem, which you
> could locate by ear and soap bottle.
>
> An easy way to detect cracks, for the cost of a tire valve.
> What could be simpler?
>
> JohnK
How much of the tubing is accessible to spray with a soap bottle
without slicing all the fabric off? I mean the seat and instrument
panel is in the way. Yikes, just thinking about that job makes me
shudder.
Corky Scott
Corky Scott
J Kahn
Charles K. Scott <Charles.K.Scott@**NOSPAM**.dartmouth.edu> wrote in
article <79f068$g9s$1...@dartvax.dartmouth.edu>...
Corky, if I had a previously airtight fuselage truss that was all of a
sudden
leaking down, I'd do whatever it takes to find out why, wouldn't
you?
Forget about soap...
If you put 80-100 psi into it and go around when it's quiet like at
night, you will hear it. Or if your hearing's not up to par find a kid
to go around listening. I can easily locate a tire leak by ear with
only 35 pounds if it's quiet, and if you triple that pressure the
noise is quite loud.
If I can hear air pissing out of the tubing down by the tailwheel,
that wasn't doing that the last time, you can bet I'll have the knife
out....
I used to be in a flying club in the mid 70s that had a Breezy, CF-BPE,
and one of the longerons at the base of the vertical fin
developed a crack right at the weld. I discovered it on a
walk around, since there was no fabric. Really made me shudder
since the tail boom only has *3* longerons!
JohnK
Charles K. Scott
"J Kahn" <jk...@planeteer.com> writes:
> Corky, if I had a previously airtight fuselage truss that was all of a
> sudden
> leaking down, I'd do whatever it takes to find out why, wouldn't
> you?
John, I'm welding up a fuselage right now. It's a bit overdesigned as
I've mentioned several times (Christavia Mk 4). Even if I did nothing
to the interior of the tubes I'm positive the thing will not rust out
from the inside out in the time I have left on earth. And I'm going to
really carefully paint the outside after blasting and properly prepping
it.
The welding is going real well. I'll be pressure testing for leaks
before I paint it. If I find any, I'll fix em. When I'm done, the
entire fuselage should be air tight. Barring literal cracks from ham
handed landings (who me?), I'm hard put to figure out how moisture
could get in to begin rusting.
Maybe what I should to is tap some tube for a Schrader valve and
pressure test the frame each year. I guess that amounts to the same
thing as the constantly pressurized system but cheaper. Ironically,
having the Schrader valve there makes possible the introduction of air,
with moisture; exactly the thing we were trying to prevent.
So my opinion is if you do the welding properly and don't drill holes
in the tubing for this or that and have treated the inside of the tubes
with something, that's enough.
Corky Scott
highflyer
It is enough to prevent corrosion. However, it doesn't give you the
simple test at annual time for broken tubes or welds. I will admit
that with a decently gas welded and normalized Christavia, you will
NOT be likely to EVER have a tubing or weld crack. Then again, after
a really hard landing, it might be nice to KNOW everything was ok.
I would weld a short length of tubing, about an inch long, into the
blind cap that closes off the rudderpost at the bottom. Tap the
inside of this tube for a schrader valve. Polish the end of the tube
flat and shiny and screw a short machine screw with a rubber gasket/
washer to seal it. YOu can even locktite it with the easier locktite
stuff.
Then, at annuals, or after hard landings when you are concerned about
the structure, take the screw out, put a schraeder valve IN, and blow
it up to about 100 psig with your aircompressor, or, if you are
worried about corrosion, with CO2 or Nitrogen. Leave it overnight,
while listening in the dark for air leaks. Check the pressure in the
morning, and put your mind at rest! Then take out the schraeder valve
and locktite the screw back in!
HF