Jul 15, 2008

Make it go go...

A whole whack of short to medium range jet transports, designed in the 1980s and earlier, are currently in service around the world. If a sharp increase in the price of jet fuel, suddenly rendered them uneconomical, then a newly developed aircraft with significantly better fuel economy could become an instant success. Enter the C-Series jet transport, Bombardier's new contender in the 100 to 130 seat airliner division.

Using composite materials, fly-by-wire technology and improved aerodynamics along with new engine technology, the Bombardier C is promising to reduce fuel consumption by 20% compared to current competitors. The ability to deliver on this promise mainly depends upon the Pratt and Whitney geared turbo fan (GTF) engine.

So, what's the secret ingredient? Haven't we already sucked as much energy as we can get out of a kilo of jet fuel? Apparently not. Remember that a jet engine is a compromise and in fact our transport aircraft don't really use pure jet engines at all. They use what are called turbo fans.

A pure jet directs all the air it inhales through the hot turbine section where it is combined with fuel, and ingited to produce thrust. The fan acts like a really big turbo-charger. Thrust is created by giving a large acceleration to a relatively small mass of air. (F = MA).

At lower altitudes where jets spend much of their time for one reason or another, the engine becomes more effective when it bypasses some of the inhaled air along the outside of the engine - like a shrouded propeller. This ducted fan imparts a relatively lower acceleration to a much greater mass of air. With each generation of jet engines fuel costs have risen and the fan section has grown.

Compare an old technology JT8D engine, used on the B727 and early B737:
... with a newer CFM56 - note the front fan section difference:

Increasing the fan section diameter causes a new problem. The larger blades need to spin slowly to stay efficient. As the blade tips approach the speed of sound, shock waves will destroy their aerodynamic efficiency. Up to now, these fan blades have been powered by a shaft connected directly to a turbine wheel in the engines "hot" section. And turbines with small diameter need a higher rpm range for greatest efficiency. A compromise is created trying to play off the efficiencies of one vs. the other.

So, the key ingredient in the new technology engines is the use of a planetary gear system allowing each unit to spin at its ideal rate. Here's an illustration of how a typical planetary gear works - push a button at the top left of the animation to see three different formats that can be used. The trade off (there's always a trade-off) is that the planetary gearing adds weight and complexity to the system.

As with every new aircraft, the C-Series' success or failure will be intricately tied to that of its engines. Many a great plane has struggled or actually failed to become economical because the engines didn't deliver. So you can bet that Bombardier will be working closely with Pratt and Whitney to ensure that the new PW1000G delivers all that it promises.

1 comment:

Ryan said...

That looks like a cross between an Embraer 190 and an Airbus. Speaking of engines from your post, remember the unducted fan that GE flew on the MD-81 testbed? I always thought that was a great idea, but I think it was too noisy or caused too much vibration.?