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

On Tue, 08 Nov 2005 08:46:43 +0000, Peter
wrote:


Brian Whatcott wrote:

Did I miss something?

Yes, I was talking about *aerodynamic heating* of the prop and
suggesting that - over most of its length - it ought to be sufficient
to prevent icing.


....and did I not account for aero heating, using the figures given by
you?

Brian Whatcott Altus OK

You are assuming that you are getting your calculated aero heating even
though the air is full of supercooled water.

Mike
MU-2

"Peter" wrote in message
...

Brian Whatcott wrote

...and did I not account for aero heating, using the figures given by
you?

Yes, but I don't think icing works that way. The reason is that
supercooled water won't (generally) exist below about -15C to start
with, so if the aerodynamic heating increment exceeds 15C one doesn't
get ice.

So, if e.g. the SAT is -25C and the aero heating is 20C, the airframe
part will be at -5C but it won't ice up because there wasn't liquid
water in the -25C air to start with. Any water would have been in
solid (frozen) form, as in e.g. cirrus clouds, but a -5C airframe
flying through that won't accumulate ice because the crystals don't
have time to melt and convert themselves into drops of liquid which
could stick.

That's my understanding, anyway.