Thursday, June 21, 2012

You Must be Choking!

Sorry about that.  Some things are just naturally irresistible!

The point of it is that I want to talk about ‘stall’ and ‘surge’.
Because there seems to be some confusion in the ranks. There have been questions posed that would lead me to believe that these are two aspects of a compressor that are not fully understood.

Firstly, are we talking about axial flow compressors or centrifugal flow compressors?
Both, really, but we will come to the centrifugal flow compressors later since they are somewhat different in their ‘cause and effect’.

Shall we take out an axial flow compressor blade and take a peek at it?
Hold it so that you are looking down from the tip to the root.
What do you see?
Yes, several things:
1.     It has an aerofoil shape, the same as a wing.
2.     It is ‘twisted’.
If we take out two blades and put them together, by holding the base of each blade firmly abutting (touching) each other, then we see something else.
That the gap between the blades, in the direction of airflow, is divergent – it gets bigger as the gas leaves it.

These designs in the blades are there to do one thing – to ‘do work’ on the air.
The turbine supplies energy to the compressor and the compressor uses that energy to bunch up the air.

Now we have to be very careful.
The flow of air through the compressor has to be (almost) uniform.
Not in terms of velocity but in terms of mass flow.
If we have 500 kgs per second going in the front of the compressor then we absolutely must have 500 kgs per second coming out of the back into the combustion section.
That is vital.
The velocity of the gas is, more or less, irrelevant and will depend on the rpm of the compressor and not (as many seem to believe) the speed of the aeroplane through the air.

Note that I have just suggested that the rpm of the engine is, for most engines, a variable.
Note that the cross-section of the blade is an aerofoil.
Aerofoils do well at an optimum angle of attack of four degrees (4°). This will only happen at one rpm.
That means that there is a compromise somewhere.

Listen to a jet engine starting up.  At some point in the start sequence you will, very likely, hear a rasping noise from deep within it.
This is a ‘stall’ occurring. It will always do this. You will never get a nice airflow over all the blades all the time to create that lovely angle of attack.
There are going to be times when the airflow through the engine does not match the speed of the blades (rpm). Start-up is one of those times. Every time.

If the angle of attack of the air on a blade is bad – naughty air! It may produce turbulence down the back of the blade.
Turbulence means that the blade is no longer ‘doing work’ on the air. But the turbine is still sending energy down the shaft to the compressor (remember that!).
It is unlikely that only one blade will stall.
It is more likely that a whole stage (row) of blades will stall. It may be the ones in the middle, the little ones at the back or, even, those hulking great big ones at the front.
If the conditions that created that breakdown of airflow over that stage are restored to normal, then the blades will continue to work.
If conditions do not return to normal there is a danger that other stages will also stall.
Because the stage that has stalled is doing no work on the air, which means that the air being received by the next stage is not at the best condition for it.
Similarly, the air leaving the stage in front of the stalled row is not being accepted by the stalled stage – slowing it down.
Air that changes speed is not at the same angle of attack that it was previously. This is why we cannot suddenly change the rpm of the engine because it will rapidly change the angle of attack on the blades.

We now have an engine that is in a state of stall.

The compressor is still receiving lots of energy to do the work with but it is doing less work. More stages stalling will reduce the workload even more.
How do we know that this stall is occurring?
Because the exhaust temperature will be going up, the engine is likely to be vibrating (because the gas is turbulent) and it is very likely that you will hear it.

What to do?
Close the throttle and watch the instruments. If the engine slows to idle rpm and the temperature comes down then run the engine for three minutes at cooling rpm (not necessarily idle – the RR Viper engine cools at a slightly higher rpm than ground idle), and shut it down to investigate the cause.
It may be that the temperature continues to climb (burning fuel with less load on the compressor and reduced air) and the rpm stays where it is.  Now all you can do is shut-off the fuel and hope that the bearings don’t suffer too much.
This last is what is called a ‘locked in’ stall.

But what if all the compressor stages stall?
Then the compressor is doing no work on the air at all. The high pressure air at the back can now come to the low pressure at the front of the compressor. It will do this suddenly and, almost, explosively.
You will hear a loud bang. If you are outside the aeroplane it will feel as if you have been punched all over by a giant fist. If you are very close to the engine it may knock you over.
If you are inside the aeroplane at the controls, you may see the exhaust temperature and rpm begin to rise. The rpm will only go up for a moment but the temperature will keep going up. This is a really good time to shut the fuel supply off. Of course, it may just be a ‘pop’ surge where engine ‘clears its throat’, as it were. Things may go back to normal immediately but it is still worth investigation.

Apart from discovering the source of the problem, what are we looking for after a surge?
Check the oil (SOAP) and MCD’s (Magnetic Chip Detectors) as well as the oil pressure filter and scavenge strainers in case the bearings have been damaged.
(The SOAP sample results will have to be compared with the previous SOAP results to see if there is a sharp up swing in the contaminant level.
NB: SOAP = Spectrometric Oil Analysis Programme. This is done by a lab unless you work for a very big major airline that has it’s own kit!)
Do a borescope check – especially on the rear stages where possible.
Check the turbine for ‘spatter’ – it will look like the sky at night with little ‘stars’ of melted metal all over the turbine blades and, often, the exhaust.
Naturally, look down the intake. Several times we needed to go no further than a quick peek past the first stage on co-axial (twin spool or dual-axial) engines.

So what about centrifugal compressors?  Do they stall? Yes. They do.
Try and stay with this because it gets a bit tricky.
Between the rotating part of a centrifugal compressor and the static part there is a gap.  Not a big gap but it is there.
The reason for the gap is that the air coming off the impeller vanes is quite thick, it has been compressed, it is hot and it is moving quite fast.
As each impeller vane on the rotating bit goes past a diffuser vane on the static bit there will be a ‘thud’ as a lump of compressed gas gets caught between the two. The air behind the impeller vane is not quite so ‘thick’ so the sudden increase in viscosity is really noticed by the diffuser vanes.
(Listen, if you get the opportunity, to a RR Derwent engine on, say, a Gloster Meteor going overhead at low level; it sounds like a V16 piston engine because of all the ‘thuds’ as each impeller vane’s air hits each of the diffuser vanes.)
The gap has to be wide enough – remember the gap? For the air to ‘spread out’ a bit before going into the diffuser to lessen this ‘thud’ because too much ‘thud’ creates too much drag – wasted energy. Too small a gap = too big in the ‘thud’ department.
As the aeroplane goes up into the clouds, and beyond, the air gets thinner.
At some point the air will be so thin that, even with the impeller working very hard to compress it, it will slip around the gap. If air is going around in circles it will block off the air trying to get out of the impeller.
The compressor will now stall.
It is unlikely to surge (but it can happen) but the conditions of the stall are exactly the same as for an axial flow compressor.
So what causes a centrifugal compressor to stall?
Low air density - altitude.

There you have it.

Surge and stall made easy. I hope.

Now my brain has stalled, my temperature is going up and work is ceasing.  Time for bed after a quick refuel!

Thursday, June 14, 2012


Before I write anything else about anything else, there needs to be a clearing up of confusion.

Just recently there have been a few misconceptions posted here and there. 

For instance, there is a person on ‘E News’ who has taken to using the expression ‘Ell-Beez’. This is usually in conjunction with the revelation that a certain celebrity (who is, invariably, completely unknown to me!) has gained, or lost, a few ‘Ell-Beez’.

Let us all, with one accord as distinguished Jet Engine Professionals, put these people into the light of knowledge.

The plural of ‘lb’, the accepted abbreviation for ‘pound weight’, is ‘lb’

It comes from the Latin ‘Libra’ meaning ‘pound’; the plural of this is ‘Librae’. Observe – no ‘s’.

Ergo, said ‘celebrity’ has gained, or lost, a few ‘Ell-Bee’. Not, perhaps, quite so catchy but much more accurate. It also avoids pitying looks from the intelligentsia who watch the show.

Now heat.

Heat and temperature are not the same thing.

Heat is a form of energy. We get ours from the Sun. Exclusively. Try very hard not to tell me that we can get heat from burning coal or wood. The energy that gave us both these fuels came from the Sun so they are ‘second-hand’ solar heat.

Cold is not energy. Cold is an absence of heat in the same way as there is no such thing as ‘dark’. Dark is an absence of light (which is another form of heat energy.

Temperature is a measurement of how much heat is present in any particular volume of… well… anything.

Let’s take an example.

10 lb of dry air at ISA (International Standard Atmosphere – 14.7 psi at 15°C) takes up the space of a sphere approximately 6.28’ in diameter.

If we pass this volume of air through the compressor of a jet engine, the compressor will make it smaller. We shall say that the compressor gives a CR (Compression Ratio) of 10:1.

Ten to one? Time for lunch – see you later.

Ah! Good food.

Where were we? Oh, yes. We are going to reduce the volume of air by a factor of ten.

It will now be 0.628’ in diameter or, if we convert that, 7.536”. Sit in the hand just nicely; just under an octave

Of course, you would not want it sitting in your hand because we have also compressed the heat in it by a factor of ten.

15°C now becomes 150°C – considerably more than the boiling point of water (at ISA).

It is exactly the same amount of HEAT but it is now occupying a smaller VOLUME of, in this case, AIR.

Of course, this is ignoring the work done on it to compress it. Since we are less than efficient, the work done to do the compressing will be mostly wasted in heat energy transferred into our bundle of gas and so the heat within our sphere will be much greater than it was at the start.

In other words, the temperature will be much higher. Much.

An axial flow compressor giving a CR of about 11:1 will give us an exit gas temperature of around 550°C. If you yearn to know how that feels then go to a friend who longs to kill him/herself with noxious gases. Ask them to light the cigarette and request that they suck long and hard on it.

When the tip glows brightly that will equate to around 550°C. Try to avoid the temptation to put your finger on it.

The HEAT in the cigarette end will be quite small but the TEMPERATURE will be very high.

Tuesday, May 1, 2012



A small, simple word. How devastating it can be.

We have already discussed the difference between anti-icing and de-icing on these pages. Now let’s have a tiny peek at the effect of ice.

There are 6.229 Imperial Gallons (7.48 US Gallons) contained in one (1) cubic foot of water. Get your rulers out and construct a cube with each side measuring one foot (1’).
Not very big, is it?
That much water, irrespective of whether it is the tiny US Gallon or the proper sized Imperial Gallon, weighs 62.288 lb (27.76 kgs). Heavy, yes?
One cubic foot is 1,728 cubic inches so a one-inch thick layer will cover 144 square feet (12’ x 12’)
(Sorry about all of this math – just trying to make a point here!)
The surface area of a small jet airliner – like a Boeing 737-200 is 6,500 square feet (compare with your home). This is equivalent, using the above data, to 281.17 Imperial Gallons of water at one inch thick.
That is 2811.7 lb (1253 kgs) of water. One and a quarter tonnes.
Want to consider the top of the fuselage and the tailplane? Hmm. Thought not.
Well, we could more than double that weight of water if we look at those other areas.
Is a one-inch thick layer of ice outlandish – an exaggeration? Not really. This is a perfectly feasible thickness. But, even a quarter inch thick layer of ice over the whole aircraft will come to nearly a tonne Actually, around 785 kgs). This is, you will recall, only a small aeroplane we are considering. We could, if you have time, do the calculations on something a shade bigger – like a B747?
Very well. We’ve made the point.

Ice is very heavy.

The enemy of aircraft is weight. The greater the mass you have the more power/thrust you need to lift it into the air. If the total mass of aeroplane, fuel, passengers and luggage plus the ice is too great then the aircraft will not fly. The limit is in the books as the Maximum Permissible Take-Off Weight (MTOW). Of course, we cannot measure the weight of the ice which is why it is vitally important to get rid of it before take-off; it is also vitally important to get rid of it as close to take-off as possible to prevent a further build up if it is still snowing or there is freezing rain.

Then there are aerodynamics.

This hits the aeroplane in two distinct ways.

Firstly, the ice changes the shape of the wing and also it can create rough surfaces on the wing.
This is critical because the wing is designed specifically to produce sufficient lift at a certain forward speed to get 20,000lb of B737 (in our example) off the ground safely and smoothly.
Note: ‘safely’.
Changing the shape of the wing changes the lift characteristics of the aerofoil (cross-sectional shape of the wing) to the point where, in extreme cases, all lift is lost.
Certainly there is a major move of the airflow towards stalling (the point where the work done to lift the aircraft into the air reduces to the point where no work at all is being done.
While the stalling point is being approached the centre of lift – this is the point under the wing where the air is pushing the aeroplane up, is on the move. It is moving forwards.
Eventually it moves so far forward that the nose of the aircraft pitches up and airspeed reduces dramatically reducing lift even more and the aircraft now possesses the flight characteristics of, say, a cat.

The second consideration is that an ice build-up in the engines, especially, but not solely, in the intake, increases the mass of the aircraft and disrupts the flow of air into the engines.
Jet engines really, really, like a smooth flow of air going in. Anything that ripples or is in any way turbulent tends to disagree with the first bit of the engine which will now stall.
This is a bad thing.
The engine now produces less thrust at a time when the main thing that the aeroplane really needs is? Thrust!

The result of not de-icing a ‘plane before take-off and not switching on the anti-icing and de-icing systems is that the ‘Air Florida’ B737 lands in the Potomac River after crushing vehicles on the 14th Street Bridge.


A small word but a big force.