Re: Why does a prop ice up so apparently readily?

[Brian Whatcott]

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

[Brian Whatcott]

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

[Mike Rapoport]

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.