Tuesday, March 29, 2011

Geared TurboFans (GTF)

This concept is intriguing.  Any observations or comments about it would be most welcome.


Because the idea of gearing the fan came up many years ago in a seminar held by some people from Rolls-Royce plc.  The question arose from the floor and so the answer given, it was stressed, was not R-R official policy but more the opinion of the engineer representing Rolls-Royce.

The seminar was not on the subject of single stage fans  -  nor of high by-pass fans in general, but it was something that the person posing the question, on behalf of an operator, clearly thought strongly about.

The pros and cons stated were as follows:

Pros were that the current situation where the turbine and fan have to be a compromise in design, in order to maximise the power produced by the turbine and the power consumed effectively by the fan with minimum losses, could be avoided by having both the fan and the turbine rotating at their individual optimum rate.  This is bound to give an increase, even if only a small one, in the overall efficiency of the engine.  This leads to a decrease in overall sfc that is going to please the operator.

The cons are that the introduction of a gearbox does two things:
1.  It increases the overall weight of the engine unless savings can be made elsewhere in the construction.  On a twin spool (co-axial) engine, or a triple spool, the length of the LP spool must be increased to accommodate the gearbox.  This increase needs a corresponding increase in the case, carcass and fairing (cowl) which will add to the overall weight and this does not yet include the weight of the gearbox itself.  The alternative might be to snug up the gearbox into the nose cowl which will prevent the additional weight of the external parts but will not avoid the mass generated by the gearbox itself.

2.  No matter how this is done, and, one suspects, a single stage or two stage spur epicyclic will be used, the gearbox will need to be rotated.  Rotating any mass, however small, will result in power being used to do it.  Is it possible that the optimisation of the turbine and fan speed will produce sufficient benefit to both to increase the efficiency to the point that the residue from driving the gearbox will still give increased performance and improved sfc?

Intriguing.  We await figures from Pratt & Whitney and your opinions.

Monday, March 28, 2011


Aircraft on the, almost treeless, Shetlands only need about twenty minutes, or so, to build nests in the rudders. Birds don't see aeroplanes - they see trees or nest-building opportunities. Snakes and other creatures see caves for warmth and sanctuary. Rodents on aircraft - as well as bugs (see 'British Airways' bed-bug problem out of Thailand and Berjaya Air's German Cockroach infestation) are an ongoing and insidious problem. Rodents are especially problematical because they are inclined to chew through wires and cables.

"Airbus charged over Rio tragedy
March 18, 2011

JET builder Airbus has been accused of involuntary manslaughter over the deaths of 228 people in the Rio-Paris crash in 2009.

Preliminary charges have been laid by a judge to start a formal investigation into the crash of the Air France flight.

Airbus said there was an “absence of facts supporting the charge” and chief executive Thomas Enders said it was premature. He added it would be better to focus on finding the cause of the crash and making sure it never happened again.

Investigators found automatic messages from the Airbus A330-200’s flight computers indicating an electrical fault. The pilots may have been receiving false speed readings from sensors.

Air safety authorities ordered the pitot tube sensors to be replaced on other aircraft after the disaster, although the problem had been known about since 2002.

Franco-German company Airbus says that only finding the black box flight data recorders will give answers to what happened when Flight 447 crashed into the Atlantic during a storm.

A fourth phase of searching for the black box is due to begin this weekend, with Air France and Airbus paying the seven million euro cost. It will use a mini-submarine searching in the south Atlantic crevices, which can be as deep as 13,000ft.

Just three per cent of the plane has been recovered, including a large part of the tailfin. Fifty bodies have been found."

What does this have to do with wildlife?


The pitot tube problem cropped up much earlier in Birgenair Flight 301, in 1996. The reason for the faulty pitot tubes. Investigators suspected that some kind of insect could have created a nest inside the pitot tube. The prime suspect is a species called the Black and yellow mud dauber wasp, well-known by pilots flying in the Dominican Republic. The aircraft had not flown in 25 days during which time the pitot tubes were not covered, giving the wasps an opportunity to build nests in the tubes.

(NB:  Final comment courtesy of Charles Thomas)

ICING - Part 2

A Word (or two) About Icing.  Part 2

We need to get rid of ice.

Ideally we get rid of ice on the ground before the aeroplane leaps into the air.  Sometimes the ice will form after the aeroplane leaps into the air and that presents us with a whole new set of problems.

There are lots of different forms of icing but we will just think:
 ‘ice’ = ‘hard water’ on ‘aeroplane’.

Very often ground crews will de-ice aeroplanes on the ground before take-off.  They will do this so that the aeroplane is not damaged by ice (or snow) building up on the aeroplane and weighing it down.
The ice or snow can be removed by sweeping or spraying with a de-icing fluid.

Aeroplanes with a small wheel at the front (a nosewheel known as a ‘tricycle undercarriage) will be de-iced starting at the back because de-icing the front first can cause the aeroplane to tip onto its tail!

Small propellers can be ‘greased’ with a de-icing compound that prevents ice from sticking to the blades.

Once an aeroplane has been de-iced it must be got off the ground fairly quickly or the process will have to be repeated to prevent further build up of ice or snow.

The problem now is the build up of ice on the aeroplane, and in the jet engines, in flight.

Why does ice form on the aeroplane?

Because the way an aeroplane gets ‘lift’ from the wings is to create a low pressure on top of the wings and a higher pressure under the wings.  The aeroplane is ‘sucked’ into the air!  Reduce the pressure of gas and the temperature drops, if it drops sufficiently any water in the air will freeze and stick to the wing.

The engine air intake also has a low pressure in it when the engine is on the ground and the aeroplane is not moving forward.  Ice can (and does) form inside the intake ‘lip’.

Propellers act very much like an aerofoil (wing) and so ice can form on the propeller blades, too.

How to get rid of ice in the air?
Two methods:
1.             Anti-icing.
2.             De-icing.

Anti-icing prevents ice from forming on a surface and de-icing allows the ice to form and then gets rid of it – usually by using it’s own weight to help.

Anti-icing and de-icing use several ways of working.  Sometimes heat is used.  Heat is obtained from two main sources:
1.             Hot air from the engine compressor (High pressure air is hot)
2.             Electrical heater pads

It is also possible to use low pressure air from the compressor to inflate rubber balloons -  called ‘boots’, that will cause ice to crack up and flake off from wings, tailplanes, fins and engine intakes.  This pressure is usually very low and rarely goes much above 20 psi.  The boot is then ‘sucked shut’ to conform to the shape of the surface it is protecting.  The valve that controls the inflation and closing of the boot needs to be heated so that it will not freeze and cease to work.

Electrical de-icing heater pads use a quick burst of electricity to heat up.  The heat needs to be applied at high temperature very quickly so that the minimum amount of ice is melted.  The ice will then blow off or be flung off, on a propeller blade, and the airflow will then cool the heater.  Not too much ice should be melted because water will flow back from the heater element and cause ‘run-back’ icing that cannot now be removed!  These heater elements should not be used on the ground when there is no cooling airflow over them or they may well burn out.

Hot air from the compressor is applied to surfaces to keep them free of ice.  A small bleed of air at low pressure  -  possibly around 30 psi, is allowed to flow out from small holes around the area that needs to be kept free from ice.  Sometimes it is discharged gently from a pipe that has many tiny holes in it called a ‘piccolo tube’, the air sprays onto the inside of a metal skin to keep it just above freezing point so that no ice forms. This is commonly used on engine air intakes and spinner fairings in the middle of the intake.

The other area that needs de-icing is the windshield.  This is usually electrically heated.

The contents of windscreen washer bottles can be kept liquid by electrical heater elements or by using waste air from fans cooling the electronic equipment.

Electrical elements are rarely used for anti-icing because the load on the electrical system would be too high.  Much better to use short bursts of electricity every so often.

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.

Power v. Thrust

There was a bold statement that said "Piston engines develop power but jet engines (gas turbines) develop thrust"

This is, broadly true.


Power, however it is measured, is the RATE at which WORK is done.  Work can be described as torque.

Torque is a twisting moment about a shaft.  If someone grabs your hand and rotates it there is a twisting moment in your forearm.  Work is being done on your arm.

Now imagine a mangle.  Oh.  You're not old enough to know what a mangle is!  Hmm.  Two sets of rollers between which you pass clothes and sheets, etc, to squeeze most of the water out.  The rollers are operated by a handle that cranks them around.
By turning the handle you are putting torque on the rollers.  If you apply sufficient torque to overcome the resistance of the rollers - and the wet clothes between the rollers, then they will begin to rotate.  The speed at which they rotate will depend on the effort you wish to put into the job.
The torque that you apply is the work done.  The rotation is the rate at which the work is done.

You are applying Horse Power to the mangle.

A piston engine applies torque to the wheels of a car or a propeller on an aeroplane.  In both cases the resistance to motion has to be overcome by the torque to get movement, after that the speed of rotation (rpm  =  revolutions per minute) is the rate at which the work is done.

Power can be high rpm and low torque or, for the same power, low rpm and high torque.

Power = RPM x WORK.

Jet engines apply a reaction force to the engine by the hot gases rushing out of the back of the engine.  This is Newton's Third Law  -  to every action there is an equal and opposite reaction.

The reaction felt by the engine is an applied force just like someone pushing you in the back but doing it continuously.  This force is described in pounds (lbf) or in kN (kiloNewtons).

1000 lbf of thrust  =  4.45 kN (Actually: 4.4482216 kN)

1 kN  =  224.81 lbf    (Actually: 224.8089431 lbf)

On a jet engine  -  or a rocket, there is only the 'push', there is no 'rate' at which this is done so a jet engine does not develop power.

Rolls-Royce RB211 Trent 900 Series

Following the failure of an engine on a QANTAS Flight out of Changi, Singapore, which caused airframe (wing) damage and the aircraft to turn back, Rolls-Royce have really pulled out all the stops and got to the bottom of it very quickly to the extent that there has been a EASA EAD (Emergency Airworthiness Directive)  -  courtesy of RR and the UK CAA, issued already.

EASA EAD 2010-0236-E: RB211 Trent 900 series engines: Engine - High Pressure / Intermediate Pressure (HP/IP) Structure – Inspections

Details of the EAD have been copied onto the following site:


for those who wish to see the details of the action required.  It is in a picture album titled "RB211 Trent 900 Series".

Rolls-Royce are to be highly commended on their swift and positive response to this situation.

If there is more news on this we will let you know.

Why does air get bigger when it gets hotter?

Why does air get bigger when it gets hotter?

This happens anywhere and not just in a jet engine but it concerns us because we need to add energy to the engine to get thrust (energy), or power (energy) out.

(Note that piston engines develop power but jet engines develop thrust.  We shall look at that later.)

When energy is added to air, or anything else, the energy (normally 'heat') spreads through the air causing all the molecules in the air to get excited.  Watch a football crowd, when they get excited they start to jump around and wave their arms.  They are, individually, taking up more space.  Molecules do that.  Their mass (weight) remains the same but they take up more room  -  they expand.  They become less dense (less 'crowded together').  If they are not allowed to expand they will press against one another, they will have increased pressure.

In the jet engine we allow them, the molecules of air, to have more space by keeping the pressure (nearly) constant.  They will spread out and become bigger; to escape they will need to accelerate.


The engine is a tube.

Air goes into the front and comes out the back.

Let's forget 'velocity' for a moment and think in terms of the weight of air.

Suppose that, over a one second period of time, one pound of air goes into the front of the engine.

We now have a "Mass Flow" (Wf) of 1 lb/sec.  We should reasonably expect that we should get 1 lb/sec of air out of the back of the jet engine.  If we only get half a pound/sec where has the other half a pound gone?  If we get 2 lb/sec then we have miraculously created 1 lb of air, every second, within the engine.  Wow!  That would be something wonderful!!

No.  what goes in the front comes out the back.  But the air coming out of the back is a different size.  It is bigger  -  much bigger.  To maintain a mass flow of 1 lb/sec it has to come out MUCH faster.

Momentum = Mass x Velocity

Same mass going in as coming out but the velocity has changed because we have added energy to it.

More velocity?  More momentum.

More momentum?  More reaction  -  more thrust.

See how simple Jet Engines are?

Jet Engines

Let's start from the very beginning:

There are two types of heat engines  -  External Combustion Engines and Internal Combustion Engines.

An external combustion engine is a steam engine like they have on trains and ships (paddle steamers out on the Mississippi of the Old West; these are cool.).

An internal combustion engine comes in two basic forms:
The Piston Engine - or reciprocating engine
The Jet Engine  -  or Gas Turbine Engine.

The similarities are that they both burn a fuel that transfers the heat energy to air.  The effect on the air is what drives the engine.
This means that, in both cases, AIR is the working fluid.

The differences are that the piston engine has lots of moving parts which means lots of bits to break and lots of friction and lots of power needed to drive each part  -  power that does not get to propel whatever it is that you want to push long.
Jet engines have one moving part (for a 'basic model') and thus it becomes more efficient because less energy is wasted in driving different bits.

The MAIN Difference is that the Piston Engine is an (Modified) 'Otto' Cycle machine and a Gas Turbine is a 'Brayton' Cycle.
What does that mean?
An 'Otto' Cycle is a constant VOLUME engine.
A 'Brayton' Cycle is a constant PRESSURE engine

Now you are going to say that the piston in a piston engine is going up and down so the volume in the cylinder constantly changes.
That's true.
Imagine any very small moment; the piston will appear, in that very small moment, to be in one place.  The volume will appear to be fixed.  The air is getting hooter  -  it is expanding with nowhere to go.  What happens?  The pressure builds up and pushes the piston down to a new "fixed" volume.
So it is the increase in pressure at a fixed volume that drives the piston down  (or across if you are in a Beetle!).

In a jet engine the pressure is held constant (it doesn't work in practice  -  we shall look at that later, 'almost constant' is good).  The volume increases and, in a confined and fixed space, the only way for it to get out of the machine is to increase in velocity.  The increase in velocity on a mass of air means that the momentum is increased.
If you run faster your momentum increase  -  try walking slowly into a wall and then running as fast as you can into that same wall.  See the difference?
Isaac Newton said (Third Law, you know):  To every action there is an equal and opposite reaction.

Increase the momentum of the escaping air and the reaction against the engine increases.

You now have a 'Jet Engine'