Tuesday, March 29, 2011
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
"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)
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.
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
AT THE POINT WHERE HEAT IS ADDED TO THE SYSTEM.
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.
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'