Monday, March 28, 2011

ICING - Part 1

Given the Weather in Europe recently  -

A Word (or two) About Icing.  Part 1

Icing is a terrible thing to happen to an aeroplane.  It is also a terrible thing to happen to a jet engine and the propellers that some of them drive.

Let’s consider what happens.

Firstly, ice is water.  Water is heavy.  Hard water is also heavy and it sticks to things very well indeed.

Lift a bucket of water, feel the weight of it and now imagine that bucket of water smeared thinly all over an aeroplane.  Not a very thick layer, is it?  That amount of ice will not do terrible things to an aeroplane.

Now imagine a layer of ice two inches (five centimetres) thick.  All over the aeroplane. How many buckets do you think that will fill up?  You are right  -  lots.  Now we have a lot of very heavy hard water.

The weight of the ice on the aeroplane can cause damage to the structure of the aircraft by pushing down on it.
Can you imagine how hard it is to get an aeroplane off the ground when it is covered in ice?

The four forces on an aeroplane are:
Drag.  This pushes against an aeroplane in flight trying to slow it down.  The faster you go the more drag you get.
Thrust.  The engines push the aeroplane forward against the drag of the air flowing over it.
Gravity.  The aeroplane is being pulled down towards the ground by gravity.  The heavier the aeroplane is the harder gravity will pull at it.
Lift.  The wings generate lift.  Lift works against gravity.  To get lift you need the aircraft to be going forward fast enough for the wings to give lift.

Ice works with gravity to stop the aeroplane going upwards.  Weight is the enemy of aeroplanes.  More weight?  You need more lift.  To get more lift you need to go faster.  To go faster you need more power (thrust) from the engines.  To get more power from the engines you need to burn more fuel.  Burning more fuel makes the engines hotter.
Ice wears out the engines faster.


That was only ‘Firstly’!

Secondly, the ice breaks up the airflow over the aeroplane.  Instead of a nice, smooth surface there is now a rough surface that is not the right shape to make a smooth airflow.

Turbulent airflow makes for a lack of lift that no amount of thrust from the engines will overcome.

Ice forming in the intakes and around the engine is not only heavy but also affects the airflow going into the engine in two ways:
1.             It creates turbulence so that the smooth flow of air into the engine is now rough.  The engine doesn’t like this and may well ‘cough’ causing it to break. The ‘cough’ is what we call a ‘surge’; this is when the       airflow in the engine decides to suddenly (very suddenly) change direction and go from back to front.
2.             The diameter of the intake may be reduced by the thickness of the ice.  This reduces the amount of air going into the engine.  The engine needs air to burn the fuel, less air going in means that there must be less fuel being burnt.  Less fuel?  Less thrust.  This is at a time when the aeroplane really, really needs more thrust  -  not less!!
We need to get rid of that terrible ice.


Watch out for the next episode.

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