Why does a prop ice up so apparently readily?

The potential for icing is based on the amount of supercooling more than the
theoretical or actual skin temp. It seems doubtfulthat the skin heats up as
much as would be predicted if the air has droplets in it that are actually
impacting the surface. These two things combined with the fact that the
prop has a high collection efficiency because of its thin cross section mean
that the prop will usually ice worse than anything else. I have an
in-flight picture of a prop iced up while flying through a thunderstorm.
The amount of ice is surprising, and this is with a heated prop.

Mike
MU-2




"Peter" wrote in message
...
The other day I was talking to a commercial pilot of a big twin
passenger turboprop. He has been iced up a few times and recently was
down to 200fpm climbing flat out through FL150; looking out of the
window he saw a bit of ice on the wings but enough on the prop for it
to be visible while the prop was rotating.

He has rubber boots, and the props are electrically heated.

Now, I know a bit about mach heating and I can work out the
temperature rise over SAT (i.e. the TAT) using the Jepp CR-5 circular
slide rule. At 200kt IAS at FL150 his airframe temperature should be
SAT+9C. At 300kt TAS the TAT should be SAT+12C which nearly puts him
out of the stratiform cloud icing range of 0C to -15C or so.

So he can get ice on the airframe especially in slow flight, and
especially if there are local mach numbers where the airflow slows
down.

What puzzles me is the prop. Assuming a SOP of max revs if icing is
likely, much of the prop is going at between mach 0.5 and mach 0.8,
with a temp rise of 15C to 30C, so even on a slow piston aircraft only
the innermost part should ever ice up.

Is this true?

I haven't been able to test this myself because I have a TKS de-iced
prop (TB20) and always have the deicing on if in IMC below 0C. I've
had up to 1cm of ice on the wings but never noticed any performance
drop so presumably the prop was doing OK.

Peter.
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