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PGA
1st Dec 2006, 02:34
Just a quick one guys:

Say you`re flying around in a cloud, all things being equal, would one pick up more ice at say 200 knots or 300 knots?

Have been chatting about it with various people and got various answers, all inpunt welcome! :ok:

barit1
1st Dec 2006, 03:09
At 300 knots your Mach is 50% higher, thus TAT is more likely to be above freezing.:cool:

Clandestino
1st Dec 2006, 22:48
200 kTAS gives 5.5 C TAT rise and 300 kTAS gives 12.3C TAT rise. It seems that at 300 kt you'd pick less ice as your TAT would go below zero as SAT goes below -13C and by then there shouldn't be much supercooled water in clouds.

PEI_3721
2nd Dec 2006, 16:48
Be aware that any temperature rise from speed increase may encourage runback ice to form on unprotected surfaces and particularly near controls.
Many factors determine how and where ice will form on a particular aircraft. Remember that the tailplane could be more prone to icing (relatively thinner section), and thus as speed changes the tail could accumulate more ice due to local AOA change from the change in downwash / prop swirl.
IIRC, where a surface is prone to icing, then the rate icing could be higher with increasing speed due to the rate at which water droplets are encountered (comments?). Also, the larger (denser) droplets may penetrate the relative airflow more easily hitting the airframe surface forming ice? Thus in these conditions the aircraft would have less time to exit the conditions. Conversely flying faster could exit the conditions quicker, but don’t forget that you can change altitude, which is often the most effective way to exit icing.
Most of the variables depend on the aircraft type; check the manufacturer’s recommendations particularly for holding and approach speeds. Often a recommended speed increase is to maintain the stall margin and not for the ammount of ice accumulation reasons.
Whilst the originating question might be hypothetical, a more appropriate one would be to ask which speed is the best to reduce the threat of ice accumulation.

Contract Con
2nd Dec 2006, 22:18
my 2 cents,

I believe the speed is not a huge factor, unless you do fly fast enough to increase the TAT to above freezing.

There are charts produced by flight planning offices that give you the CAS required to avoid icing at a certain level. I have seen as high as 390kts required at 10000' in my part of the world.


Cheers,

Con:ok:

bookworm
3rd Dec 2006, 09:35
I believe the speed is not a huge factor, unless you do fly fast enough to increase the TAT to above freezing.

Speed is a huge factor, because in the important cases you do fly fast enough to increase the TAT to above freezing.

The worst icing conditions occur with high concentrations of supercooled water, and that occurs with SATs just below zero. Increasing TAT by 5 degC above SAT helps. Increasing TAT by 10 degC above SAT helps lots. Increasing TAT by 20 degC above SAT pretty much eliminates ice.

Figure in the quadratic dependence of TAT rise on speed (1% higher speed => 2 % more TAT rise etc.) and speed becomes a very important factor.

extreme P
3rd Dec 2006, 09:44
"The faster the speed of the aircraft the less chance the droplets have to be carried around the airfoil in the airstream so the greater the collection efficiency". More ice at high speed.

Contract Con
3rd Dec 2006, 10:41
Bookworm,

"unless you do fly fast enough to increase the TAT to above freezing."

You made my point in a more eloquent fashion for me, thanks:ok:

Cheers,

Con:ok:

Edit: In both posts I am refering to achieving a TAT of at least +10degC, in line with what most manufacturers consider "Icing Conditions". Cheers