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. 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.
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.
>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. We have not designed
a product to protect and insulate from the heat of the sun (insulation would
surely be needed outside the glass)."
>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? 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.
>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. 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). 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
or different conductivity through the fabric, 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.
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). Yellow, green, grey, red, blue: Much hotter. 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. 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. I am not saying that the
"metallic" grey cover has the same properties as silver metallic paint, but
grey is still not white.
Regards,
Svein
LN-SKJ
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