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Author Topic: What stops the blades of a propeller-driven aircraft from icing?  (Read 6621 times)

Offline chris

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Jet engined aircraft, I'm reliably informed, have various ice-abatement systems built in to direct warm air from the engine over the fan blades and also over the leading edges of the wings, both of which are potential icing sites.

But what do propeller-driven aircraft do?

Chris


 

Offline krool1969

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Nothing. They do ice up. There is a noticble loss of power but if the prop is icing up, usually the wings are as well which will cause a loss of lift.

Anti-icing on airplane wings is done in one of two ways. In tubine powered planes hot air from the engine is pumped into the wings to melt the ice. In piston powered planes they use a rubber boot that expands to break off ice. On propellers alcohol is usually used to melt the ice.

Most light airplanes like Pipers and Cessnas there is nothing to prevent icing. The piplot needs to be awear of the weather and avoid icing conditions.
« Last Edit: 02/02/2012 11:53:20 by krool1969 »
 

Offline RD

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Propeller driven aircraft canít fly at the high altitudes of jet planes.
so they do not encounter the extremely cold temperatures at high altitude which jets do.
« Last Edit: 02/02/2012 12:47:41 by RD »
 

Offline graham.d

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I don't think icing is a problem that occurs at very high altitudes, RD, though I may be wrong.

On light aircraft, as Krool says, there are usually no counter measures to icing on wings or props but there is usually a way of feeding warm air into the carburettor (on aircraft with this method of aspirating their engine). It is usually one of the pre-take-off checks to verify there is no icing occurring. If warm air is used the revs drop but if there is icing they gradually increase. It is on warm humid days that carb icing is most likely and it can be very dangerous as the engine can cut out. This is especially not good on take off!!
 

Offline RD

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Quote
ice generally accumulates slowly at high altitudes, anti-icing equipment usually eliminates any serious problems.
http://www.aviationweather.ws/083_Icing.php
« Last Edit: 02/02/2012 15:28:33 by RD »
 

Offline graham.d

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Thanks RD. So it can occur at altitude then, though not often. Aircraft usually avoid Cumulo-nimbus clouds because there can be huge wind shears , updrafts, downdrafts etc, not to mention ice.
 

Offline MikeS

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I would have thought centrifugal (centripetal?) force would help to keep the prop free of ice.  Ice builds up, a lump breaks off, the props out of balance and vibration sets in and more breaks off. ??? Just a thought.
 

Offline chris

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A pilot who contacted a radio programme I was on last week agreed with me that icing at higher altitudes is a lower risk because the lower temperatures (-40C) mean that the air is already very dry; most of the problems, he contended, are experienced during lower altitude flying and during take-off and landing when it's necessary to pass through zones that will promote icing.
 

Offline krool1969

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For water's phase to change from liquid to soilid (ice) there needs to be someting to seed the first crystal. Droplets of water in clouds can cool to below zero degrees and remain liquid. When something like an airplane wing strikes the droplet it will freeze to ice. Billions of these droplets string the airplane build up ice that destroys the lift normaly generated. (the propeller generates lift to pull the plane forward). Icing is only a problem in the clouds AND below freezing. If you have freezing rain you should not be flying anyway. Light aircraft are VERY unforgiving in poor weather.

So long as the outside air temapture is above zero icing will not happen. This same thing happens on the ground if it's foggy and below freezing. Beautiful layers of ice form on everything.

Carborator icing is an entirely diffrent problem. As air enters the carborator it passes over a small hole which is desinged to suck in fule vapor and mix it properly for burning. To do this the air pressure is lowered inside the carborater which lowers it's tempature. Cooler air can not hold as much water vapor as warm so there may be some condisation. If it's cool enough the water can feeze starving the engine of fule. This usually happens when the engine is set to high power, which is usually only done for take off. You can see the problem here....
« Last Edit: 08/02/2012 12:24:45 by krool1969 »
 

Offline CliffordK

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There was a discussion earlier about air friction heating. 

Windchill is only a factor if one has wind flowing over something warmer than ambient temperature.

Assuming a non-heated propeller, the propeller would be at ambient temperature.  Movement, and air friction would then tend to increase the temperature of the propeller. 

How much?  A couple of degrees might help.
 

Offline Cheese2001

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First, each aircraft will have different systems for anti-ice (prevention of ice formation) or de-ice (removal of formed ice) in-flight.  By each aircraft, I don't mean different power-plants.  I mean each different aircraft type.  A common issue for all types of power plants is to ensure no ice is ingested due to foreign object damage (FOD) concerns.

I happen to have quite a bit experience flying propeller driven aircraft.  My particular airplane happens to be all-weather capable.  When icing conditions are encountered in flight, engine anti-ice valves are opened.  These valves open a tap of hot air from the last stage of the compressor section to flow towards the engine intake.  This heats the inlets around the engine air intake, and intake vanes.  The heat ensures no ice forms within the air intake of the motor.  Immediately after the intake is the compressor section.  As the air compresses, it heats, and serves to keep the water in vapor form.

The procedures for my airplane call for using wing and propeller de-ice once 1/4 to 1/2 inch of ice has formed on the wing.  The same hot air that heats the leading edge of the wing.  As the skin heats, the ice flakes off and is blown back over the wing.  The wing de-ice is secured until the more ice forms. 

Lastly, the propeller is de-iced electrically.  My props are about 8 feet in radius and spin at just over 1,000 rpm.  As you can imagine the outer section of the prop doesn't collect ice, as it is spinning too fast, and simply sheds any accumulated ice outward.  The inner-section of the prop is coated in a heavy plastic with heating elements called the "prop boot."  Of the four blades on each prop, the each prop boot is heated serially for approximately 20 seconds.  Each boot is heated on a 25% duty cycle, and sheds the ice radially.  The central hub of the prop is heated continuously. 

It is a very old airplane, but works exceptionally well and in all weather conditions. 
« Last Edit: 04/03/2012 23:47:59 by Cheese2001 »
 

Offline Cheese2001

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There was a discussion earlier about air friction heating. 

Windchill is only a factor if one has wind flowing over something warmer than ambient temperature.

Assuming a non-heated propeller, the propeller would be at ambient temperature.  Movement, and air friction would then tend to increase the temperature of the propeller. 

How much?  A couple of degrees might help.

There is a well defined function based on absolute temperature and mach number which dictates the heat rise.  Wikipedia displays it nicely here:  http://en.wikipedia.org/wiki/Total_air_temperature [nofollow]

Based on a given airspeed, a heat rise calculator is here:  http://www.luizmonteiro.com/Altimetry.aspx#TemperatureRise [nofollow]

It is interesting to know see that you get 10 deg C heat rise at 270 knots, but 20 deg C heat rise at 380 knots.

There are stories of the SR-71 Blackbird leaking fuel on the ground, but not in flight, because the skin heating due to friction caused the joints on the airframe to expand and seal airborne.  I don't know if the stories are apocryphal, but they're fun.
 

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