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#1
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Why does a prop ice up so apparently readily?
The potential for icing is based on the amount of supercooling more than the
theoretical or actual skin temp. It seems doubtfulthat the skin heats up as much as would be predicted if the air has droplets in it that are actually impacting the surface. These two things combined with the fact that the prop has a high collection efficiency because of its thin cross section mean that the prop will usually ice worse than anything else. I have an in-flight picture of a prop iced up while flying through a thunderstorm. The amount of ice is surprising, and this is with a heated prop. Mike MU-2 "Peter" wrote in message ... 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. -- Return address is invalid to help stop junk mail. E-mail replies to but remove the X and the Y. Please do NOT copy usenet posts to email - it is NOT necessary. |
#2
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Why does a prop ice up so apparently readily?
"Peter" wrote in message
... These two things combined with the fact that the prop has a high collection efficiency because of its thin cross section This I don't see - as it happens, the SOP for this turboprop when in icing conditions is to go max revs, the reason being that the less the AOA the less ice will be picked up. They also go 10-15-deg flaps for the same reason - it reduces the wing AOA. Changing the AOA doesn't alter the cross-section of the prop. Thinner airfoils DO accrete ice faster than thicker ones, even as airfoils at higher AOA accrete ice faster than airfoils at lower AOA. Pete |
#3
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Why does a prop ice up so apparently readily?
"Peter" wrote in message ... "Mike Rapoport" wrote These two things combined with the fact that the prop has a high collection efficiency because of its thin cross section This I don't see - as it happens, the SOP for this turboprop when in icing conditions is to go max revs, the reason being that the less the AOA the less ice will be picked up. They also go 10-15-deg flaps for the same reason - it reduces the wing AOA. I donn't see the conflict between what I said and what you are saying. BTW does the flight manual say that the reason for going to higher rpm is to reduce AOA? Mike MU-2 |
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