nuckolls.bob(at)aeroelect
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 Posted: Fri Aug 07, 2009 8:12 pm Post subject: Pitot tube heater requirements . . . |
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| Quote: | Question: I read your article on pitot tube heaters...absolutely
right...as far as it went.
What I was trying to find out was...how much power is required to
maintain the temperature on the surface and in the ram air (Pt)
throat above freezing in the face of a mach .82 blast at 35,000 ft
pressure and -80 deg C???
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Actually, it's really easy to keep it hot. I have
data collected from pitot tubes in flight and it's
not uncommon to see 100C surface temperatures at
max cruise, 41,000 feet altitude and in clear air.
| Quote: | This will be complicated by the fact that a thermally conductive
pitot tube is mounted to a very cold metal heat sink (aka the
airframe). Then there is the fact that the air pressure is very
low...basicly a cold near vacuum!!!
My experience with high altitude cold is that hot things get hotter,
and cold things get colder. The shape of the pitot head
matters...imagine it had fins!!
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The ability of air to carry away heat isn't the issue.
The ability of super-cooled WATER to carry away heat
IS the issue. This is why pitot tube temperatures in normal
conditions have little significance for operation under
icing conditions. This is why virtually every new pitot tube
installation is qualified with ICING TUNNEL tests to confirm
their ability to shed ice accumulated as a result of
super-cooled water impacts of so many grams/second/square
CM. Now, any icing tunnel worth its cost of operation
should be capable of overwhelming about any pitot tube
so that folks are aware of the tube's capabilities.
The program I worked about ten years ago was to investigate
CLEAR AIR lost of pitot data on both sides of a
biz jet at 41,000 feet. Data that returned as soon as the
airplane descended to lower, warmer altitudes. These were
pitot tubes previously qualified in a icing tunnel. The tubes
were modified based on second set of tests. However, some
airplanes (fewer) still experience the event.
So your right, the problem is messy and the answers
are not really intuitive. I'm obligated to tell you that
nobody can give you good advice on wattage, normal conditions
temperatures, etc. The physics by which heated pitot tubes
ultimately meet requirements are about 50% calculation, 40%
experience in the field and tunnel testing and 10% black art.
Bob . . .
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( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
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