In the beginning, Rick Farley asked a good question.  I think he got more in the 
way of answers than he expected.  Lots of good stuff here...  
Email addresses removed to protect us all from spammers.

From:	"Ricky Farley" 
To:	corvaircraft
Date:	Wed, 30 Mar 2005 00:42:52 -0600
Subject:	CorvAircraft> Painting Heads

What is the current school of thought on painting the heads with high 
temp
paint?  I've seen some of them painted with silver paint, but was 
wondering
if other colors are ok as well.  Is a layer of paint going to affect 
the
cooling of the head and if so, by how much?

Thanks,

Ricky Farley
KR2S, under construction
Corvair motor ready to put heads on.
San Angelo TX


Date:	 Wed, 30 Mar 2005 08:30:01 -0800 (PST)
From:	Rad Davis
Subject:	Re: CorvAircraft> Painting Heads
To:	corvaircraft

The *only* school of thought that I've ever been aware
of is never - ever - paint the heads with anything.

Paint is always and forever a barrier to convection,
which is the major way that heat is carried away from
the heads.

For this reason, incidentally, I'm a big fan of
painting the exhaust system, especially including the
stub stacks in the heads and the logs/short manifolds.

It is, in my opinion, probably OK to paint the
cylinders on a non-turbo corvair engine, because the
cylinders are a) ferrous--so are less sensitive to
extreme temperatures due to a lower coefficient of
thermal expansion and a much higher melting point, and
b) don't have to dissipate as much of the heat of
combustion as the heads do.

This does not, incidentally, mean that I *prefer* to
paint the cylinders on my Corvair engines, nor do I
recommend it.  Lately, I have been catalytically
derusting and then phosphate treating parts like this.
 But then my cylinders are underneath a decklid and a
fan shroud, so aren't as exposed to extremes of
weather as the typical Corvaircraft engine.

If I ever gave in to the (misguided) urge to paint the
heads, I'd be very careful to take temperature
readings from the hot spots on the head during
operation.  You're throwing an unknown variable (the
thermal conductivity of the paint) into the engine
cooling equation.

Conclusion:  If you want your heads to look pretty,
have them media blasted with plastic beads or walnut
shells...don't paint them.

::Disclaimer::  I am not an airplane guy.  All my
Corvair engines are in cars.  This post is not meant
to be indicative or suggestive that I have any
knowledge of what is acceptable or safe practice in
aviation.  You're on your own, so don't sue me when
the wings fall off your Corvaircraft.

Pity one has to say things like that, isn't it?

Rad Davis


Date:	 Wed, 30 Mar 2005 14:02:25 -0800 (PST)
From:	"John Brannen" 
Subject:	Re: CorvAircraft> Painting Heads
To:	"Corvair engines for homebuilt aircraft" <corvaircraft


First off, I'm not painting my heads simply because I
don't want to.  Also, I am not an expert on thermal
issues.  However, wrt the paint issue, I would have to
disagree unless you are talking about ceramic type
insulating paints.  Although all paint does have some
insulating properties, that's not the end of the
story.

All things have an associated thermal emissivity. This
is a measure of the items ability to absorb AND
radiate energy.  In simplified terms, the color of an
item has a strong influence on the thermal emissivity
of an item, and a thermal emissivity of 1.0 would be
ideal for transferring heat from a cylinder to the air
flowing through it.  An emissivity of 1.0 relates to
"true black".  Polished aluminum is bad at about 0.05
(Sorry Pat).  In general light colored shiny things
are bad while dark colored dull things are good.  This
is why car radiators are generally painted matte
black.

Aluminum is however a good conductor of heat, 3rd best
behind Silver and Copper. So the aluminum is good at
getting the heat from the combustion chamber to the
fins (Conduction), but then the best thing for getting
it from the fins to the air is to have a high
emissivity.

For our cylinder heads, black would be best, but as I
said before, there are some insulating properties
associated with the paint.  So you could anodize the
head which is the thinnest black coating you can get
to keep the insulating properties low while getting
that black color (Emissivity of black anodized Al is
about 0.80).  Btw the most effective aluminum heat
sinks are black anodized aluminum.  

What about paint though?  Conventional paint is an
insulator (bad), but a black enamel has an emissivity
of about 0.90 (really good).  the insulating
properties of "normal" paints are low, especially if
kept thin.  The emissivity issue is also minor since
most of the heat is taken away by conduction with only
a little by radiation.  

So my opinion is that if you keep the paint layer thin
and dark, you are OK. 

My 2 cents,

John Brannen



Date:	 Wed, 30 Mar 2005 15:06:06 -0800 (PST)
From:	Rad Davis
Subject:	Re: CorvAircraft> Painting Heads
To:	"Corvair engines for homebuilt aircraft" <corvaircraft

This is very, very well-tilled ground in the Corvair
community.  For that matter, I suspect there is an
NACA paper that covers this.  It's not a new problem,
and Mackerle covers it in detail in his monograph,
which was written around the time the Corvair ended
production.

I'm quite familiar with albedos and black-body
radiation.  Radiation is a very effective thermal
transfer method, providing that your cylinder head is
an efficient infrared emitter.

You should be able to reject a significant amount of
the heat of combustion at about the time your aluminum
cylinder heads have transitioned to a completely
liquid state.

Fortunately or unfortunately, The Corvair is designed
for an operating range of cylinder head temperatures
between 250 and 500 F.  Radiative emission is a small
portion of total heat rejection required in this
temperature range.

So yes, making the heads a black body helps radiative
emission.  That's the good news.  The bad news is that
you lose more cooling from the insulating quality of
the paint than you gain from the improvement in
emissivity.

Here's a nice page with all the formulas if you are
interested in working out Planck's law of radiation
for yourself.

http://www.electro-optical.com/bb_rad/bb_rad.htm

By using their handy-dandy emissivity calculator, I
find that you can get rid of 0.0122 Watts per square
centimeter of radiating surface at 250 F (394 K) or a
whopping 0.0553 W/cm^2 at 500 F (533 K).  I'd eyeball
the effective radiating surface area of a corvair head
to be around 200 cm^2, since the fins would mostly be
radiating to each other.  Let's assume you like to
live dangerously and run the heads at an average 500
F.  That gives us a flux of around 11 Watts per head. 


A typical otto-cycle engine has a thermal efficiency
30-40%  Let's assume you've built a miserly 40%
efficient engine.  110 HP is about 82 KW, so you're
generating 205,000 Watts of energy, and you have to
dissipate 123,000 of that as waste heat.  

So, under these extremely optimistic conditions, I
find that thermal radiation from the cylinder heads
accounts for 0.0179% of the engine's entire cooling
system heat budget.  If  you want to take out the ~30%
of heat that is leaving the engine through the exhaust
ports, you're up to 0.0298% of cooling system budget.

As I said, clean, bare aluminum... 

It's not an accident that GM used terne-coated steel
for the oil pan and valve covers.  It added expense,
but meant that they had a thermally-conductive
anticorrosion coating on these parts instead of paint.
 See SAE paper #140C for a discussion of the cooling
budget of the Corvair engine.

If you're really fascinated with a black cylinder
head, consider black anodizing, in which the head is
microscopically pitted and then dyed black.  It's the
thinnest way I know of to get a black surface.

(insert standard aviation disclaimer)

--Rad Davis


From:	"simpkinsxx" 
To:	corvaircraft
Date:	Wed, 30 Mar 2005 21:00:50 -0600
Subject:	CorvAircraft> Painting heads

Leaping back in history to WW2 the Wright Whirlwind radials had glossy 
black paint on the cylinders. Conversely the Pratt and Whitney's I flew 
in B24's had shinny sprayed aluminum coatings on the jugs. Which was 
best; I don't have a clue to this day. 82 and still flying. Ray


Date:	 Wed, 30 Mar 2005 20:39:44 -0800 (PST)
From:	Rad Davis
Subject:	Re: CorvAircraft> Painting heads
To:	"Corvair engines for homebuilt aircraft" <corvaircraft

This raises the point that I made in the first post: 
Depending on how your engine is shrouded and how you
park your Corvaircraft, you might need to paint engine
parts to avoid corrosion.

The radials used in WWII were generally quite out in
the weather, so paint was a very practical addition. 
GM designed the Corvair engine to be inside a closed
compartment and underneath a fan shroud.  Unless you
park one on damp ground for an extended period you
just don't get much corrosion on the finned surfaces.

It's probably worth considering just how your airplane
will live when deciding whether or not to paint.  An
open-cowled engine on a Cub clone or a Piet probably
needs more protection from the weather than the
cowled-in engine on a fast sportplane.

I'll reiterate that there's no thermal advantage to
painting the finned parts, and there may well be a
disadvantage.  All the engineering resources I've run
across are against paint unless you're likely to get
corrosion, since this also is an efficient thermal
insulator.

Note that by corrosion I mean a built-up layer.  The
usual brownish hue of the bare cast iron of the
cylinders is not thick enough to insulate the fins
from airflow, which is why GM didn't bother painting
them.

-Rad Davis


Date:	 Thu, 31 Mar 2005 06:19:33 -0800 (PST)
From:	"John Brannen" <j.brannen
Subject:	Re: CorvAircraft> Painting heads
To:	"Corvair engines for homebuilt aircraft" <corvaircraft

OK, I said I wasn't going to post on this anymore, but
this topic has come up in the past and I felt it
needed to be addressed with facts instead of
conjecture on both sides of the fence.

I found a bunch of NACA reports where they tested
exactly what we are talking about.  The most relevant
that I found (I didn't read them all) was NACA Report
676 found here: 

http://naca.larc.nasa.gov/reports/1939/naca-report-676/

In this one, they basically tested cylinder
configurations to see different airstream cooling
arrangements and how they performed.  They have a test
that shows the effect of an enameled sample.  Page 5
of the report states:

"The results of the tests on the smooth-enameled
cylinder are shown in figure 4 for wind-tunnel tests
at a constant air speed of 52 miles per hour.  These
tests show a slight improvement in q with enamel
thicknesses up to 0.002 inch; beyond this thickness,
the heat transfer decreases and, with thicknesses
greater than 0.005 inch, it becomes less than it is
without enamel.  The highest heat transfer is
presumably obtained with just sufficient enamel to
produce a smooth, black surface; any further increase
in thickness results in a lowered heat transfer caused
by decreased thermal conductivity."

So in effect we are both right depending on how thick
you put on the paint. 

There are many other reports, but looking at this one,
it is clear to me that at best, a thin black enameled
surface is slightly better than an un-enameled
surface, and at worst (0.007" enamel thickness from
the graph) it is 5% worse.  At 0.005" it is exactly
the same as the non-enameled sample.

A paint thickness of 0.007" is pretty darn thick, and
since we are talking at worst only 5% and most of the
flight reports I have seen don't show a cooling
problem wrt the corvair, I still think that if you
want to paint your heads, cylinders, case, whatever,
go ahead.  I would still keep the paint thin and dark
though on the heads and cylinders.  As a matter of
fact, most of the flight reports I have seen show very
cool CHT's.  In a few cases too cool. 

Now everyone can decide on their own whether or not to
paint their own heads, cylinder heads that is ;-)

John Brannen


Date:	 Thu, 31 Mar 2005 08:10:29 -0800 (PST)
From:	Rad Davis
Subject:	Re: CorvAircraft> Painting heads
To:	"Corvair engines for homebuilt aircraft" <corvaircraft

An interesting paper, and exactly what I was hoping
was in the NACA archives.  Some items of note:

"The enamel was a black baking japan applied by
spraying and was baked for 1 hour at 400 F" (p653)

The paint used was a "Japan black" which is a baked-on
high solid content lacquer that was popular at the
time for automotive and industrial use.  here's the
dictionary definition:

     Japan black (Chem.), a quickly drying black
lacquer or varnish, consisting essentially of
asphaltum dissolved in naphtha or turpentine, and used
for coating ironwork; --  called also Brunswick black,
Japan lacquer, or simply Japan.

Source: Webster's Revised Unabridged Dictionary (1913)

This concoction is a long way from any modern paint. 
See 

http://www.mtfca.com/encyclo/P-R.htm

for more than you ever wanted to know about the
industrial paints of the 1920s and 30s.  This
reference confirms that the paint was probably
Gilsonite asphalt dissolved in linseed oil with some
japan dryer (light aromatic solvents) added.

Cooking this down at 400 F would result in a very
dense coating of asphalt and solid linseed oil on the
cylinder.  I suspect its thermal conductivity is
significantly better than ye olde spray can rustoleum.

One question I couldn't answer was what was the
working temperature or delta t used in the experiment?
 It would relate directly to the ability of the
surface to radiate heat.

here's their conclusion on surface coating:

"The highest heat transfer is presumably obtained with
just sufficient enamel to produce a smooth, black
surface; any further increase in thickness results in
a lowered heat transfer caused by decreased thermal
conductivity."

Something just dawned on me here.  How much cooling do
you suppose resulted from the decrease in drag over
the fins caused by the shiny, black paint job versus
machined metal?

In any case, you won't go wrong with black anodizing,
since the coating thickness is much less than .001",
and anodizing is an effective anticorrosive treatement
for aluminum.

I could not, in good conscience, recommend Japan
black.  It's pretty toxic, if nothing else.  OTOH,
you'd definitely get a "vintage airplane smell" from
the stuff cooking off during takeoff.

BTW, I agree completely with you about the flight
reports showing excessively *low* CHT numbers.
Consider cowl flaps, folks.  Your CHT should be at
least 300 F in steady state operation.  This ensures
that you cook all the crud out of the oil, get good
combustion efficiency (read better power and fuel
economy), and avoid deposits building up in the intake
tract and combustion chambers.  Hotter is better up to
about 425 F.  It's worth mentioning that the Corvair
engine as installed in the car has a fairly efficient
thermostat system designed to keep the CHT above about
300 F in normal operation.

And no, higher CHT will not strongly relate to higher
oil temperatures.  There's not a direct relationship
between oil temperature (which is affected mostly by
BMEP due to piston and cylinder heating) and CHT
(which is mostly related to head cooling air flow and
power generated.

(insert standard disclaimer)

--Rad Davis

Copyright belongs to the respective authors.