Question
Hi All,
>
>I have heard some convincing reasons to heat skins before riveting.
ie.
>tighter skins, less "oil can" noises etc.
>
>However, I purchased a copy of "14 years of the RVator" through
Avery, and
>there was a pretty good article on the subject. It basically
said that if
>the skins are heated unevenly you can cause wrinkles and distortion.
It
>left me with the impression that unless the skins can be set
out in the sun
>or heated in a oven before riveting, it probably wouldn't be
advisable to
>heat the skins with a heat gun or other device.
>
>So, I would like to receive some "net wisdom" on the subject.
How many
>folks are/did apply heat to the skins before riveting? What
were the
>results?
>
>thanks,
>John
Compiled Answers
If you do it, it's only necessary on the large flat skins.
In your case, just the wing skins and the flat aft fuselage skins.
If the
skins have much curvature, heating can make things worse, since
any slight
shrinking will 'highlight' the bulkheads the skins are curved
over.
I didn't do it, but if I did it again, I don't think I would heat,
but I would arrange a 'natural' way of doing it.
Work on the final riveting in the morning, and take the skins
outside to warm up in the sun (a darker primer color would help
here), but
your frame is still 'cold-soaked' from the cooler temperatures
in the
garage/shop during the night.
Of course, I did it the other way around, and riveted in the evening
:^(
... this is probably all you need to do .... Gil Alexander
gil@rassp.hac.com
Don't heat the skins, heat the room your skinning in. I skinned
my wings in
January in New england, and heated my shop up to 95F prior to
skinning. Felt
goo working in shorts and "T" shirts in the middle of the winter!
Fred Stucklen RV-6A N925RV
wstucklen@aol.com
Re heating wing skins before riveting, Dave Barnhart wrote:
> Guess what: unless you also *drilled all the rivet holes* while
> the skins are hot and the skeleton is cold, aren't the rivet
holes off a
> little bit?
When at the RV forum in Fulton, NY in September, Ron Jones and
Tom
Wood gave a seminar on wing construction, and said that the skins
should be drilled at room temperature, and heated for riveting.
I
asked the same question, and they answered that the skin expands
so
little during heating that it is well within the tolerance of
the
rivet holes. But when the rivets are driven, their extrusion
'glues'
the skins into the expanded position, and causes them to tighten
up on
cooling.
> And tell me, how can you keep that temperature differential
> while you are riveting? Won't the skins cool down and the skeleton
heat
> up (due to thermal transfer)?
They recommended painting the skins flat black with cheapo spray
paint, and suspending ceramic or quartz space heaters over the
skins
while riveting. Later, you remove the paint with acetone.
-Brian
byablon@lanart.com
>Lets assume that just prior to riveting you set the skins out
in the sun
>untill they are nice and hot. The skeleton sits in a nice, cool
hanger.
>The skins expand a bit, the skeleton does not. You've checked
the
>temperature of the skins by frying one grade A large egg on
their
>surface. Guess what: unless you also *drilled all the rivet
holes* while
>the skins are hot and the skeleton is cold, aren't the rivet
holes off a
>little bit? And tell me, how can you keep that temperature differential
>while you are riveting? Won't the skins cool down and the skeleton
heat
>up (due to thermal transfer)?
I agree with you here. Speaking of heat transfer and the described
method,
you would have to rivet the entire skin in just a few seconds.
We all know
how quickly aluminum transfers heat. Somehow you would need to
heat the
skins while you fit, drill the holes and rivet. And at the same
time keep
the ribs cool. Althought I'm not so sure the rib temperature
is critical.
The majority of the dimensional changes will happen in the skin
and the
deformation of the ribs wouldn't really be a factor. (I think)
So actually,
heating your shop to 90 isn't a bad idea. Just making sure the
skins aren't
cold when fitting, drilling and riveting is probably good enough.
-Steve Day
sday@pharmcomp.com
(CK ID - RV6a RV for short)
REGARDING Hot Skins
Geeze...what's real and what isn't? You got glider trailers in
New Zealand
and your got "official" RV seminars in Fulton.
The glider trailer most likely has a steel tube frame and aluminum
skin. The
skin ADSORBS/CONDUCTS the heat 4X faster than the steel tube
frame and EXPANDS
2X greater! No wonder it oil-cans in the sun.
Then the black wing program heats the skins and rivets the whole
mess together
in a couple of minutes. Can you do it that fast? If you take
any longer
everything will be the SAME temperature and all shrink benefit
is lost because
of aluminum's tremendous thermal conductivity.
If you have one piece skins (13ft?) and a 40deg F delta your MISalignment
on
the farthest hole will be 0.081". Is that a lot? Just move the
ribs -you
say. Sure, you can move each of them into position easily. If
they move so
easily then where is all of this great shrink-TENSION comming
from to hold
everything tight?
Like all great ideas...they sound good but the implementation
is hosed! Be
careful of those cockpit exhaust leaks!
To do it right you would have to (1) Determine the CTE and then
factor this in
as a dimensional change for EACH hole location (2) pre-drill
all skin holes
using Numerical Control (NC can do No.1 easily). (3) continously
chill all
structural parts during the assembly time. (4) continously heat
the skins
during the assembly. (5) Now the great fun begins! Stand back
and watch the
whole assembly come up to temperature! Opps! you forgot to do
finite element
analysis - looks like just a tad too much dihederal (or warp)!
Opps, the spar
is now constantly STRESSED! Wonder what's the new G limit!
CTE= ( 6X10e-6) (degF)(in/in)
Aluminum (6061-T6) = 13.5
Steel (AISI1020) = 8.4
Thermal Conductivity of:
Silver 242
Copper 232
Gold 196
Aluminum 130
Iron, pure 41.5
Steel(1%c) 26.2
Elon
In a message dated 95-11-02 16:02:35 EST, you write:
> thought the idea was to heat the skins but NOT the ribs and
spar.
>It was explained to me that the contraction of the cooling skin
over
>the unchanging dimensions of the ribs and spar is what makes
the skin
>taught. Doesn't heating the structural components defeat this?
Please
>clarify...
My conception of the process is that the skins, when cooled and
riveted in
place, are as tight as possible. Maybe heating the structure
does defeat some
benifits of the process, but its sure a lot easier to heat everthing
rather
than just the skin itself.
The end result sure looks good. I've seen many a Piper Warrior
and Cessna
that has a lot more tin canning than my RV-6A. I don't see any
deformation in
the my skin shape during flight on hot summer days. I can't say
that when
flying a Piper or Cessna.........
Fred Stucklen RV-6A N925RV
wstucklen@aol.com
My two cents worth.
1) If the plane is setting in the hot sun, the top wing skin
will
be the hottest and expand the most. The color will have a
big affect on this as well.
Dupont prints a graph. I think Black can get as hot as around
210 degrees F. A white skin will be about 150 to 160 F.
(this is from memory, I looked at these a week or so ago so
don't hold me to the exact numbers).
2) The bottom skin should be much cooler and this will help
keep the internal structure from expanding as much as well.
3) The structure will probably not expand as much as the top
skin
as it will take it a much longer time to heat up due
A) to the greater mass of the spar.
B) primer on the skins will insulate the metal some and
the AL will not transfer the heat to the underlying structure
as fast as would bare metal to bare metal.
C) The bottom skins being cooler should help radiate the heat
away from the structure that is being fed in at the top.
4) The rib spacing on most Cessna/Pipers is wider than on a RV.
That may make it look worse.
So, I think there is some heating affect/oil-canning.
It may be minor. It is difficult to work with heated skins.
I agree with all that has been said about the heated skins cooling
when applied to the structure. Some use heat lamps to feed in
heat while riviting.
The simplest would be to ensure the room is warm (not cold) when
doing this work.
Herman Dierks
mail: dierks@austin.ibm.com
As I was ready to rivet the skins on my wings I asked around of
the RVers
with flying machines in the area, the reply from several was
to heat the
surface. One of the replies was from an RV4, he did not heat
first wing, but
did the second, he claims he can tell the difference. So I heated
the
surface using electric heaters and followed the guidelines from
the articles
in the collection from the Rviators. I have both top wing skins
riveted on,
they look good hanging in the garage. Have ailerons and flaps
built, now
hanging these before riveting on bottom skins. Tom
TommyLewis@AOL.COM
RV6A project, wings almost done
fuselage on order
N967RV reserved
Go back to Hovan's Home Page
Comments or feedback, please send to jhovan@swbell.net