Why does a prop ice up so apparently readily?

On Mon, 07 Nov 2005 22:56:42 +0000, Peter
wrote:

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.

/// 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.
///
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?
//
Peter.

Let's run a sanity check.
Lapse rate 2 degC/1000ft (can be less)

Airframe

Cold day = 5 degC MSL
at FL150 = 15 X 2 = 30 degC drop from 5 deg = -25 degC
SAT + 9 to SAT + 12 = -16 degC to -13 degC

Std day = 15 degC MSL
at FL 150 = -6degC to -3 degC

Prop
Cold day -25degC + 15 to 30 degC = -10degC to +5 degC

Std Day -15degC + 15 to 30 degC = 0 to 15 degC

One is warned that glaze can accrete very, very fast.

I oversimplified with the lapse rate, but I conclude that it's easy to
be in an accretion phase at FL150.
Did I miss something?

Brian Whatcott Altus OK