Reengining

I've learned in the last few weeks that a simple design decision can make your life much more problematic. At the moment there is some discussion around the next generation of the 737 and the A320. Both are cash-cows of their firms (787 two years late, 747-8i isn't really a blockbuster, A380 still problematic) but both system are aging designs. The 737 even more than the A320. At the moment they both think to put new engines on the planes instead of designing totally new aircrafts.

What's the design decision that could be problematic? It's the length of the landing gear. The 737 was designed in an age, when the jet engines of the aircraft were rather slim, but long designs. The original P&W JT8D has just a fan diameter of 1.25 meters. Thus it didn't needed such a large clearance between the the ground and the wings. At the time, the A320 was designed large turbofans with a large bypass ratio were normal, thus you needed more space between the wings and the ground. The A320's CFM56-5B1 has a fan diameter of 1.74m. At some time in the past the 737 was reengined, the fan got larger (but not as large at the A320 one) and nowadays you could joke that a 737 almost scrubs the ground with its engine. The 737 uses an CFM56-3B1 with 1.54 meters today.

Now there is another reengining imminent: For example there is some discussion around so called geared turbofans, where the turbine and the fans are coupled with a gear box allowing both to work at their most efficient speed.

The problem is that most new types of jet engines are substantially larger than the old ones.And there the design decision and the age of the design haunts you. You can't get the engine under the wings of an 737. The A320 has the room for it, as the landing gear is much longer. It was developed in a different time.

Even with the current engines this led to some design changes. When you look at an 737 and an A320 you will recognize that the 737 haven't a round inlet. It had to be done this way because they had to move equipment from the bottom of the engine to the left and right from the lowest most point.

However an P&W geared turbo fan with the same trust as an CFM56-3B1 has a fan diameter of 1.8 meters. The alternative CFM Leap-X is said to have almost the same diameter. I once read that the engine to ground distance is just 48 cm with the current CFM ... putting an engine with an 30 cm larger fan on it would cut it down to 15 cm. I'm not sure about the point if this is really sufficient

It pretty much looks like the 737 needs substantially longer legs. But a longer gear leads to structural changes to the fuselage, as you have to retract the gear into the fuselage. Perhaps they have even change the track width of the landing gears because of the center of gravity.

Why did i write about it - it's a good example i've learned in IT business, too: No matter how much headroom you design in your technical stuff, the moment of the end the headroom will reach you. And every design decision will haunt you at some point in time. And in the case you had planed for 2 generations ahead, someone will cut down your headroom And so you reach a time where you have really make big changes. So fight for every centimeter of headroom.