Engineering tradeoffs

I've been going to the Reno Air Races for quite a few years. While I'm not a pilot, I get quite a kick out of the extreme engineering that the competitors engage in. This year, there was an exciting twist: a new entrant that made a very surprising tradeoff. At the right is a picture I took of a stock P51 mustang during a race. These are planes that one would normally think of as museum pieces, but they get rolled out and flown at speed. There's something of a dichotomy in the group of people that own these old planes: there's the crowd that goes for authenticity, and there's the crowd that says "to hell with authenticity, I want to go fast". This plane is pretty authentic. One of the most visible features of the P51 is the lump hanging down underneath the belly: this is the radiator and the airscoop for it. As in so many areas of engineering, one of the biggest problems in racing airplanes is dealing with heat. The normal cooling system is meant to cope with a stock Rolls-Royce Merlin engine that generates around 2000HP.

It isn't just mustangs that are raced. All kinds of planes compete. One of the more exciting types of plane are the Grumman F7F Tigercats that have two huge engines. At speed, these beasts make quite a thunder.

This airplane is Strega, a very highly modified P51 that has won many times (for a cooler paint job, take a look at VooDoo). These are very highly modified planes with chopped wings and somewhere in the neighborhood of 4000 hp coming from their hot-rodded engines. The cooling system is highly modified too. One of the most interesting modifications that is often done is to reshape the interior of the radiator chamber. Theses engines generate a lot of heat. This gets dumped out through the radiator, which causes the air passing through it to heat up and expand. The chamber is often reshaped to act like the bell in a rocket engine: Yes, they generate thrust from the waste heat. Pretty clever.

This year a surprise entrant showed up named Galloping Ghost. It was a newly modified plane that had sat alone in a hanger somewhere for many years. They really went to town modifying this plane. Chopped wings, seriously hot-rodded engine, and a shockingly cool piece of lateral thinking in the modification to the cooling system. If you look at the photo, you'll see that they sawed off the radiator. This is aesthetic sacrilege and (one would think) engineering suicide. What happens to the phenomenal amount of heat that the engine generates? Traditionally a cooling system is a closed loop with heat generated in the engine and transported by a fluid running through it to the radiator, where the heat is dumped and the fluid recirculates. In this beast, they left out the radiator and they just vent the fluid (mostly water). They've got a small tank of water that's big enough to let the plane fly the race before the water runs out. In this plane, water is a consumable, just like fuel. It's a steam-powered airplane. There's one interesting subtlety: in a normal heat exchanger, it's important that the fluid not boil. This would create a lot of pressure and probably blow a hole in something. So the fluid is usually something more interesting than water. But in galloping ghost, since the cooling system isn't a closed loop, they can let the water boil. If you remember your physics lessons, the heat required to do the liquid-to-gas phase change is immense. This means that the cooling system uses less water than you'd expect.

Galloping Ghost was doing surprisingly well in the preliminary races. Everyone was keyed up for the final race where all the top contenders would race together and drive their planes to the limit. This freakishly weird piece of upstart engineering had a shot. But no, at the last moment the final race was cancelled because the winds got too high and it was pretty clear that they weren't going to die down. In the second-to-last race, which was also affected by high winds, one of the planes had to declare a mayday and land without an engine. But when it tried to land, the wind caused it to roll and the plane was totalled (read the article, the picture is cool). So the folks running the race exercised some caution :-) Oh well, next year....

It isn't all about racing. They have the usual air show stuff. Lots of performers doing tricks with airplanes. This year the Snowbirds came.

I'm all psyched for next year. I'm having fun on my time off. As I type this I'm at the JAOO conference in Aarhus (It's been renamed "GOTO" for next year). More geek fun with a good crowd of interesting smart people.

October 4, 2010