Why does a prop ice up so apparently readily?

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.

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.

Well, I certainly have a lot of experience with ice...I flew the
Argus for 8 years. Almost all of that time was spent below 1000
feet over the North Atlantic, summer and winter. The Argus is a
four engined ASW a/c (big piston engines) and spends most of it's
life below 200 kts, down to about 160 knots for anywhere from 18
to 26 hours at a crack.

You can believe that we've come across some fearful icing
conditions. The airframe has a formidable anti and deicing
arsenal...each wing has it's own 600 BTU gasoline fired heater
and the tail also has one. A 200 BTU heater is supplied for cabin
heating. the props are electrically deiced and various intakes
have matt electrical heating pads. The engine carburetors have
hot air 'Carb heat' manually selected for anti-icing.

I've seen upwards of a foot of an ice 'pipe' build up on the carb
air intakes until vibration or an attitude change dislodges it.

The prop deicing cycles and quite regularly you can hear chunks
of ice hitting the 'ice shields' on the fuselage opposite the
inboard engines.

I've seen the odd amount of ice collect on the prop blade root
cuffs inboard of the heated portions but not much...we certainly
didn't worry too much about ice...didn't increase RPM to fight it
for sure...if it got really bad we might climb above it if it was
convenient...the racket that it made clattering on the fuselage
would keep the younger crewmembers who were off duty from
sleeping sometimes...
--

-Gord.
(use gordon in email)