Please don't take offence, all I want to do is clarify the science involved
Your reply convinces me that you do not understand any of the mechanisms of
heat transfer applicable to keeping the cockpit relatively cool when the
airplane is left in the sun. That is forgivable for folks that are not
educated in the science of heat transfer, but what is unforgivable is the
manufacture's mistaken belief that UV has something to do with heat, and
that conduction is a significant contributor to increasing the temperature
in the cockpit. UV is at the opposite end of the electromagnetic spectrum
---From infrared, and while all of the spectrum contributes something to
heating the further we get from infrared the less the contribution.
Briefly, there are three means of heat transfer, radiant, conductive, and
convective.
Radiant heat transfer predominates when the temperature of the source, in
this case the sun, is very high. This is because the radiant energy is
proportional to the absolute temperature (Kelvin or Rankine scale) raised to
the fourth power. We experience radiant heat transfer from everyday things
like the heating elements on an electric range, from a flame, or even a
tungsten light bulb, all of which are sources at relatively high
temperatures.
Conductive heat transfer predominates in solid materials such as the cockpit
covers in question, and at lower temperatures. Conduction is what makes the
handle of a pot on the stove get hot after the pot is heated by radiation
---From the burner and heat is transferred (conducted) through the aluminum,
cast iron, or copper pot, to the handle
Convection, either natural or forced, transfers heat by moving a fluid
(either a gas or liquid) from a higher temperature location to a lower
temperature location. We use this method to cool both liquid and air cooled
engines.
So, getting back to my original point:
The best way to keep the cockpit cool is with a material that reflects the
sun's radiant energy. A layer of aluminum foil will do this quite well.
Gold foil will be better still but a bit more expensive (check out the
amount of gold you see in pictures of satellites). Since the interior
surface of the cover in question is a few degrees warmer than ambient, and
nowhere near the temperature of a light bulb for example, there is virtually
no radiation from the interior of the cover to the cockpit, hence my
assertion that the measurements you made are meaningless. If anything, the
cover with the highest interior temperature might actually help reduce the
temperature in the cockpit because it is a demonstrably poor insulator which
means that once the cockpit is hotter than the ambient air the heat can be
transferred by conduction to the surface of the windows and from there
convection will remove heat to the air.
Further comments below.
Best regards,
Rob Housman
Irvine, California
Europa XS
Rotax 914
S/N A070
Airframe complete
Avionics soon
From: owner-europa-list-server@matronics.com
[mailto:owner-europa-list-server@matronics.com] On Behalf Of Sidsel & Svein
Johnsen
Sent: Wednesday, July 18, 2012 9:44 AM
Subject: SV: Europa-List: Cockpit cover - Any colour as long as it's white
Rob,
With respect, I wish to comment some of your statements:
>What these measurements do not tell us is how much of the sun's energy is
reflected from the outer surface of the cockpit covers, only what heat is
transferred by conduction through the cover.<
What is of interest is not what is reflected (per se), but what is
transferred by conduction. No - see above. The temperature on the underside
(shadow side) of the material was measured, and this gives indeed a good
indication of the heat transferred - the higher the temperature, the more
heat is transferred. True as far as it goes. Whether the heat that reaches
the underside is a result of low reflection from the outside surface or high
conductivity (low insulation) in the material is irrelevant. Sort of true
but it is indeed heated by conduction through the cover as a result of
minimal reflection.
>The metallic grey material is probably a very good reflector of radiant
energy in the part of the spectrum most important for reducing the
temperature within the cockpit.<
If this is the case, the material must have a very high conductivity/low
insulation, since it gets really warm on the underside, meaning that the
energy not reflected passes easily through the material. The cover
manufacturer stated the following in an e-mail to me yesterday: "These
covers will reflect UV radiation, but this is not the only way that heat is
tranferred. Conduction is likely to be a major factor. No - see above We
have not designed a product to protect and insulate from the heat of the sun
(insulation would surely be needed outside the glass)." Did they really mean
that the cover is useless for keeping the cockpit cool? Again they
demonstrate their ignorance by recommending insulation outside of the glass
(sic) because the polycarbonate-cover "system" would work the same whether
the insulation is outside or inside.
>It is erroneous to conclude that "the metallic-grey material is totally
unsuitable for sun heat protection" because cockpit air temperature is not a
function of the temperature of the heat shield, and cockpit air temperature
was not measured. At the temperatures involved, radiant heat transfer is
inconsequential so therefore the measured temperature on the interior side
of the cover material is insignificant and irrelevant<
How can cockpit air temperature NOT be a function of the temperature on the
inside of the cover? See above. Consider the following - case 1: The
entire aircraft is covered, and the temperature on the inside of the total
cover is 58.7 degr. C - constant over time. When the cockpit air temp has
stabilized, it will be 58.7 degr C. Case 2: The inside of the total cover
is 38.7 degr C. When the cockpit air temp has stabilized, it will be 38.7
degr C. In the real case, where parts of the structure surrounding the
cockpit are not covered, the inside air temperature will be different from
the temperature on the inside of the cover. However, the higher the cover
inside temperature is, the higher the cockpit temperature gets. Further,
the structure (e.g. fuselage skin) is in direct contact with the underside
of the cover, and I can assure you that the white skin under the cover was
significantly hotter than the white skin exposed to the sun, which felt cool
in comparison. Perhaps I should not have said "insignificant" in my
original message. That temperature would be a significant indicator of
cockpit temperature when at equilibrium but your report of the original
measurements did not indicate that equilibrium had been attained. The white
skin under the cover is an indication of the temperature of the cover after
the cockpit has already been heated by radiation, and the cover's interior
heated by conduction, and the white skin of the aircraft is cooler because
it has efficiently reflected the radiant energy of the sun.
>Now, having said that the conclusion was erroneous I should point out that
the cover in question may indeed be a rather poor means of reducing the
cockpit temperature. We simply do not know from the available information.
As with academic studies, here we must conclude that further research is
necessary. <
Ref. the above quote from the manufacturer with respect to the cover not
being suitable for heat reduction. I've already pointed out that the
manufacturer is simply ignorant of heat transfer mechanisms. There is,
however, another manufacturer who claims to have better suited material
(ref. Gerry's posting on Monday). I have asked for a sample of that
material, and will make a comparison of the underside temp of that and the
metallic grey that I presently have.
The best practical heat protection of the cockpit is to cover the windows on
the inside by auto heat shield (the foam layer offers effective insulation
low conductivity). See my comments above about the relative contribution by
conduction. The problem arises when you also need to cover the cockpit and
engine top (ref. air intake location for 912 UL/ 912 ULS) against rain.
If the white cover shows lower temperature on the underside than the
metallic-grey cover, this is either due to different radiant heat reflection
definitely or different conductivity through the fabric minor contribution ,
or a combination of both. How much is actually caused by one or the other
is beyond the scope of my interest in this subject - I only wish to find a
lightweight, water tight cover that is noticeably better than the
metallic-grey that I now have. White colour is in all likelihood better
than metallic grey. Definitely, and something metallic is better still.
Reflectivity in the infrared is a function of the material alone, specular
or diffuse. Aluminum is better for reflectivity than painted anything, but
I don't want a corroding airplane.
On a sunny day, check on cars in a parking lot: White is definitely coolest
to the touch (most heat reflection = least soaking up of heat into the car
body metal). Correct Yellow, green, grey, red, blue: Much hotter. Correct
again. Silver metallic: Almost as bad as any of the darker colours.
Reason according to paint manufacturer: The special "sheen" of metallic
paint is created by particles reflecting light in different, arbitrary
directions - also down towards the surface on which it is painted. We don't
care where the reflected energy goes as long as it leaves the surface we
want to keep cool, and since the angle of reflection equals the angle of
incidence there is no way for the energy to be reflected below the surface.
This is why our aircraft made of epoxy must not be painted silver metallic,
while aircraft (and cars) made of polyester may have this type of paint but
that's because of the glass transition temperature, and entirely different
concern. I am not saying that the "metallic" grey cover has the same
properties as silver metallic paint, but grey is still not white. Yup, any
color as long as it's white.
Regards,
Svein
LN-SKJ
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